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Volcanism has played a major role in modifying the Martian surface. The Tharsis volcanic province dominates the western hemisphere of the planet with numerous effusive volcanic constructs and deposits. Here, we present the results of an in‐depth study aimed at characterizing and modeling the emplacement conditions of 40 lava flows in the Tharsis volcanic province. These lava flows display a range of lengths (∼15–310 km), widths (∼0.5–29 km), and thicknesses (∼11–91 m). The volumes and flow masses range from ∼1 to 440 km3 and ∼1011 to 1014 kg, respectively. Using three different models, we calculated a range of eruption rates (0.3–3.5 × 104 m3/s), viscosities (104–107 Pa s), yield strengths (800–104 Pa), and emplacement times (8 h–11 years). While the flow lengths and volumes are typically larger than terrestrial lava flows by an order of magnitude, rheologies and eruption rates are similar based on our findings. Emplacement times suggest that eruptions were active for long periods of time, which implies the presence and persistence of open subsurface pathways. Differences in flow morphology and emplacement conditions across localities within Tharsis highlight different pathways and volumes of available material between the central volcanoes and the plains. The scale of the eruptions suggests there could have been eruption‐driven local, regional, and perhaps, global impacts on the Martian climate. The relatively recent age of the eruptions implies that Mars has retained the capability of producing significant localized volcanism. Plain Language Summary: Volcanoes have resurfaced a majority of the Martian surface. Understanding the volcanic history of Mars is critical to understanding the evolution of the planet. Lava flows are one of the most common volcanic features on Mars, and numerous examples are present in the Tharsis volcanic province on shield volcanoes and in vast volcanic plains. Using three different approaches, we characterized the eruption conditions of 40 lava flows. In addition, we measured their dimensions and calculated volumes and masses. We observed flow dimensions (e.g., lengths and volumes) larger than typical terrestrial lava flows. Viscosities and yield strengths were similar to terrestrial values; however, effusion rates were generally at the higher end of terrestrial rates. These results indicate that lava flows erupted on Mars did not require exotic compositions or exceptionally high eruption rates to be emplaced. However, this does imply longer eruption times, which requires the existence of long‐lived subsurface conduits capable of transporting magma to the surface. Key Points: The large thick lava flows in Tharsis are explained by basaltic and basaltic‐andesite compositions without resorting to exotic compositionsThe observed lava flows likely required longer‐lived eruptive conditions and larger active conduits than their terrestrial counterpartsSeemingly high eruption rates for some solitary flows are reasonable extrapolations of terrestrial conditions with higher supply dimensions [ABSTRACT FROM AUTHOR]

Recent acquisition of high‐resolution satellite imagery of the Martian surface has permitted landslides to be studied on a global scale on Mars for the first time. We apply the Scoops3D software package to compute slope stability for select regions of the Martian surface, combining calculations of slope stability with number of observed landslides (Crosta, Frattini, et al., 2018; Crosta, De Blasio, et al., 2018), as reported in a recently published inventory of Martian landslides, to understand controls on the global distribution of landslides on Mars. We find that the distribution of landslides does not simply follow the distribution of unstable slopes. In particular, there is an abundance of landslides around Tharsis, and especially in Valles Marineris and Noctis Labyrinthus, which is not explained by an abundance of unstable topography alone. We analyzed for but did not find a clear large‐scale lithologic or stratigraphic control on landslide occurrence from subsurface heterogeneities. Other possibilities to explain the increased occurrence of landslides in Tharsis include (1) thin weak unit(s) that is regionally widespread and at multiple stratigraphic levels, such as from interbedded ashes; (2) seismic activity related to the Tharsis's geological activity, and (3) possible groundwater near Valles Marineris into the Amazonian. Given the apparently young ages of many landslide deposits in Valles Marineris (Quantin et al., 2004), continued modern day analysis of lithologies in Valles Marineris and observations of Martian seismicity may act to strengthen or rebut the first two hypotheses. Plain Language Summary: New satellite images of Mars's surface are helping scientists study landslides on the planet on a bigger scale than ever. In particular, the first database of landslides on Mars mapped globally across the planet's whole surface was published recently. We use this data in combination with new analysis of existing datasets of Mars's surface to investigate the factors setting the spatial distribution of Mars's landslides. In particular, we apply landslide modeling software (Scoops3D) to determine the relative vulnerability to landsliding of large parts of Mars's surface. We then compare equally vulnerable parts of Mars's surface across different regions of the planet, and see whether they have similar numbers of landslides. We find that landslides are concentrated more than expected in a region of giant volcanoes and canyons known as Tharsis. Possible contributions to landsliding in this area include layers of weak rocks near the bottom of Tharsis, shaking from "Marsquakes" triggered by volcanic activity and faults around Tharsis, and groundwater that may have persisted for longer around Tharsis than elsewhere on Mars. Continued research to investigate the rock types and modern seismic activity on Mars may help strengthen or rebut these ideas. Key Points: The concentration of landsliding in the Tharsis Rise region is not explained solely by the abundance of steep topography in TharsisFuture study of Martian seismicity and lithology may help to illuminate which factors are driving the elevated landsliding near Tharsis [ABSTRACT FROM AUTHOR]

We report a previously unnoticed annually repeating phenomenon consisting of the daily formation of an extremely elongated cloud extending as far as 1,800 km westward from Arsia Mons. It takes place in the solar longitude (Ls) range of ∼220°–320°, around the Southern solstice. We study this Arsia Mons Elongated Cloud (AMEC) using images from different orbiters, including ESA Mars Express, NASA MAVEN, Viking 2, MRO, and ISRO Mars Orbiter Mission (MOM). We study the AMEC in detail in Martian year (MY) 34 in terms of local time and Ls and find that it exhibits a very rapid daily cycle: the cloud growth starts before sunrise on the western slope of the volcano, followed by a westward expansion that lasts 2.5 h with a velocity of around 170 m/s in the mesosphere (∼45 km over the areoid). The cloud formation then ceases, detaches from its formation point, and continues moving westward until it evaporates before the afternoon, when most sun‐synchronous orbiters make observations. Moreover, we comparatively study observations from different years (i.e., MYs 29–34) in search of interannual variations and find that in MY33 the cloud exhibits lower activity, while in MY34 the beginning of its formation was delayed compared with other years, most likely due to the Global Dust Storm. This phenomenon takes place in a season known for the general lack of clouds on Mars. In this paper we focus on observations, and a theoretical interpretation will be the subject of a separate paper. Plain Language Summary: In September and October 2018, the Visual Monitoring Camera onboard Mars Express observed a spectacular water ice cloud extending as far as 1,800 km westward from the Arsia Mons Volcano on Mars. This curious extremely elongated cloud caught the attention of the public (http://www.esa.int/Science%5fExploration/Space%5fScience/Mars%5fExpress/Mars%5fExpress%5fkeeps%5fan%5feye%5fon%5fcurious%5fcloud). We study this Arsia Mons Elongated Cloud (AMEC) with the aid of several instruments orbiting Mars. We find that the AMEC repeated regularly each morning for a number of months, and that it is an annually repeating phenomenon that takes place every Martian year around the Southern Hemisphere during spring and summer. The AMEC follows a rapid daily cycle: it starts to expand from Arsia Mons at dawn at an altitude of about ∼45 km, and for ∼2.5 h it expands westward as fast as 170 m/s (around 600 km/h). The cloud then detaches from Arsia Mons and evaporates before noon. In previous Martian years, few observations of this phenomenon are available because most cameras orbiting Mars are placed in orbits where they can only observe during the afternoon, whereas this cloud takes place in the early morning, when the observational coverage is much lower. Key Points: We report a new phenomenon consisting of an extremely elongated water ice cloud (up to 1,800 km) extending westward from the Arsia Mons volcanoThe cloud reaches the mesosphere (45 km), and expands at a velocity of around 170 m/s in Martian Year 34.This cloud repeatedly forms in the early mornings, and repeats in a daily cycle between Ls 220° and 320° every Martian year [ABSTRACT FROM AUTHOR]

In August 2015, the Curiosity Mars rover discovered tridymite, a high‐temperature silica polymorph, in Gale crater. The existing model for its occurrence suggests erosion and detrital sedimentation from silicic volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicic volcanic material. Using data from Curiosity, including chemical composition from the Alpha Particle X‐ray Spectrometer, mineralogy from the CheMin instrument, and evolved gas and isotopic analyses from the Sample Analysis at Mars instrument, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silica‐rich alteration halos which crosscut the stratigraphy. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification. Plain Language Summary: In August 2015, the Curiosity Mars rover discovered tridymite, an unexpected mineral phase, in Gale crater. The existing model for its occurrence suggests erosion and deposition from silicon‐rich volcanic rocks in the crater rim or central peak. The chemistry and mineralogy of the tridymite‐bearing rocks, however, are not consistent with silicon‐rich volcanic material. Using data from Curiosity's instrument suite, we show that the tridymite‐bearing rocks exhibit similar chemical patterns with silicon‐rich alteration zones which crosscut the layered sediments. We infer that the tridymite formed in‐place through hydrothermal processes and show additional chemical and mineralogical results from Gale crater consistent with hydrothermal activity occurring after sediment deposition and lithification. Key Points: Chemical and mineralogical data from the Curiosity Mars rover suggest a history of hydrothermal alteration within Gale craterSilica‐rich alteration halos and tridymite‐bearing deposits exhibit similar chemical signatures, suggesting related formation processesWe propose the in situ formation of tridymite through hydrothermal processes as an alternative to a detrital origin [ABSTRACT FROM AUTHOR]

The extensive Medusae Fossae Formation (MFF) along the dichotomy boundary on Mars has geologic features indicative of wind erosion of low‐density material. There is evidence suggesting a water ice component, but with considerable uncertainty linked to the unknown MFF porosity and compaction behavior. We use SHARAD radar sounder data to estimate the real permittivity and loss tangent of MFF deposits, and compare these to a model for sediment compaction and to the properties of ice in mid‐latitude glaciers. In areas along the margins of Eumenides Dorsum, between Gordii Dorsum and Amazonis Mensa, and in northwest Zephyria Planum, the loss tangent is about 0.001 at 170 m and plateaus at about 0.003 for 310–550 m thickness. The real dielectric constant across the study areas ranges from 2 to 3. We propose that the MFF is a two‐layer deposit, with 300–600 m of fine‐grained, self‐compacting material above up to 2 km of minimally compacting, low‐loss material. The lower unit could be ice‐free and very coarse‐grained, but we see no evidence of extensive sand exposed by erosion. The lower layer might instead be ice‐rich and protected from sublimation by the dry cover. The volume of cover relative to a high ice content in the lower layer implies hybrid MFF formation as glacial or polar layered deposits capped by a dry, perhaps pyroclastic ash, component. Plain Language Summary: The Medusae Fossae Formation (MFF) on Mars covers a vast area along the boundary between the rugged southern highlands and the smooth northern plains. While the MFF appears to be thick sediments or volcanic ash slowly eroding in the martian winds, how this material was emplaced remains mysterious. Most intriguing is evidence suggesting that some areas of the MFF may contain water ice. In this work we use sounding radar data from the SHARAD instrument on the Mars Reconnaissance Orbiter to probe up to 600 m below the surface and measure the electrical properties of the MFF material. The results suggest that the shallow parts of the MFF deposits are very porous and compress readily under their own weight. To match deeper probing by the Mars Advanced Radar for Subsurface and Ionosphere Sounding instrument on Mars Express requires a second layer of either vast porous deposits or ice‐rich material protected from sublimation by the dry sediments. Key Points: The Medusae Fossae Formation is a 5,000‐km band of thick material along the edge of the martian highlandsSHARAD measurements show an increase in electrical loss as the first few hundred meters of material compactDeposits of ice buried by sediments may be required to explain the lack of further increase in dielectric loss beyond 300–600 m thickness [ABSTRACT FROM AUTHOR]

Distributed‐style volcanism is an end member of terrestrial volcanism that produces clusters of small volcanoes when isolated magma bodies ascend from broad magma source regions. Volcano clusters can develop over millions of years, one volcano at a time, and can be used to infer unobserved geologic phenomena, including subsurface stresses and cracks during eruption periods. The Tharsis Volcanic Province covers approximately one‐quarter of the martian surface and hosts a large concentration of small volcanoes that formed from distributed volcanism. We present a catalog of 1,106 small volcanic vents identified within Tharsis Volcanic Province. This catalog includes morphologic measurements for each cataloged vent. Vent lengths range from 71 m to 51 km, widths range from 40 m to 3.1 km, and 90% of vents have lengths at least 1.5 times their widths. Additionally, 90% of edifices associated with vents have topographic prominences <100 m. Vents are found throughout Tharsis, though they generally form clusters near large volcanoes or among large graben sets. Older regions with volcanic eruption ages of >1 Ga are found at the Tharsis periphery in the Tempe‐Mareotis region and Syria Planum. Vents in the Tharsis interior have reported ages <500 Ma. Regional trends in vent orientation and intervent alignment are dependent on nearby central volcanoes and fossae. We use these findings to hypothesize that within the most recent 500 Ma, magma was present under and to the east of the Tharsis Montes and that some of this magma erupted and built hundreds of small volcanoes in this region. Plain Language Summary: Clusters of small volcanoes are formed over long periods of time (hundreds of thousands of years to tens of millions of years). They form when magma is present underground but is not voluminous or concentrated enough to form a single magma chamber when it ascends that would otherwise create a large, central volcano. At Mars, a large region called the Tharsis Volcanic Province has both very large volcanoes and many small volcanoes. We have used images taken in orbit around Mars to map 1,106 small volcanic vents. We used images and topography data to measure the sizes of volcanic vents. Most vents are significantly longer (up to 51 km) than they are narrow, while only 10% are circular. Most small volcanoes are short: less than 100 m tall. Vents are as young as a few million years, while some are over 3 billion years old. Their arrangement is also dependent on the neighboring large volcanoes and fractures. We use these findings to hypothesize that within the most recent 500 million years, magma was present under the three large volcanoes in Tharsis, the Tharsis Montes, and that this magma created hundreds of small volcanoes in the center of the study region. Key Points: We present a catalog of 1,106 small volcanic vents identified within Tharsis Volcanic ProvinceDistributed‐style volcanism has been common throughout Tharsis history and has been affected by large volcanoes and regional fossaeRecent (≤500 Ma) magmatism near the Tharsis Montes created volcanic fields to the east and rift apron lavas between the large shields [ABSTRACT FROM AUTHOR]

Decades of study of the igneous martian crust concluded that it was primarily basaltic, but a range of new investigations find evidence of evolved compositions. Foremost of these is a highly feldspathic unit within the Nili Patera caldera of Syrtis Major, the only detection with preserved volcanic context but which nonetheless remains ambiguous in exact composition and formation. We conduct compositional mapping of this feldspathic unit via near‐infrared spectroscopy from the Compact Reconnaissance Imaging Spectrometer for Mars instrument and find that the unit occupies at minimum 104 km2 at high confidence and an additional 41 km2 at moderately high confidence, meaning the unit is locally significant. We compare our mapping with that inferred from geomorphology and find that while texture and albedo are useful proxies, they are not perfectly reliable as substitutes for thorough compositional investigation. Study of the boundary between the feldspathic unit and surrounding mafic rock indicates the former formed early and may extend locally in the subsurface. We consider what compositional mixtures could explain the conflicting interpretations derived from visible/near‐infrared and thermal infrared spectroscopy, concluding it is likely due to thermophysical differences between the light‐toned feldspathic unit and the infilling dark mafic sand. We discuss proposed plutonic and volcanic formation scenarios for the feldspathic unit, considering Earth analogs and implications for the parent magmatic system, and offer observations in rock texture and composition that would clarify. Plain Language Summary: For many years, the surface of Mars derived from cooled magma or lava was thought to be predominantly a low‐silica rock called basalt. Recent observations have produced evidence of high‐silica rock that is rich in a mineral called feldspar (termed feldspathic), which was long thought not to occur on Mars. For our study, we focus on a detection site in Nili Patera, a crater at the summit of the Syrtis Major volcano. Using the Compact Reconnaissance Imaging Spectrometer for Mars instrument, we study how reflected sunlight interacts with the martian surface to deduce rock composition at the surface and thereby map the extent of this feldspathic rock. We find that the unique rock covers 104 km2 at high confidence and an additional 41 km2 at moderately high confidence. This indicates that the unit is locally significant and, as this is the only detection site with a preserved volcanic context, worth further study to understand its formation. We conclude with discussion of potential sources for the disagreement in compositional interpretation of the feldspathic unit and how potential formation mechanisms might be distinguished with further observations or laboratory measurements. Key Points: Nili Patera is the only feldspathic site on Mars with its volcanic context intactCompositional mapping of the unit reveals it is locally significant and occupies 145 km2 at minimumConsidering all data sets holistically optimizes the interpretation of this remotely sensed composition and its magmatic history [ABSTRACT FROM AUTHOR]

Hephaestus Fossae are a system of northwest trending troughs west of the Elysium Mons volcanic construct on Mars. Pitted cones present in the fossae region have previously been interpreted as both igneous and mud volcanism products based on morphometry and morphology observations. We take advantage of multiple orbital remote sensing datasets to assess the validity of these hypotheses. Newly examined visible and near‐infrared spectroscopy observations from the Compact Reconnaissance Imaging Spectrometer for Mars provide the strongest line of evidence for an igneous origin for fossae cones based on the presence of olivine signatures on cone rims. We report an expanded set of morphometric parameter values for 253 cones. Morphometry data from High Resolution Imaging Science Experiment and Context Camera (CTX) digital terrain models reveal that fossae cones appear most similar to terrestrial and putative Martian rootless cones but also exhibit similarities with mud volcanoes on Earth and Mars. Nine hundred and twenty‐six mapped cones in our study region exhibit a clustering pattern, as revealed by newly incorporated geospatial analysis, indicating that a nonrandom process controlled feature distribution and individual cones formed near preexisting cones. Cone‐fossae relationship constraints are revealed by CTX images, which show individual fossae cutting across preexisting cones. Fossae region cones lack associated flows that characterize mud volcanoes on Earth and Mars. The study region physiographic setting also does not support a sedimentary depocenter mud volcanism formation scenario. Our analysis of cones in the Hephaestus Fossae region adds to the studied diversity of pitted cones on Mars. Plain Language Summary: Both igneous and mud volcanism have been proposed as formation mechanisms for the origin of pitted cones on Mars. Hephaestus Fossae, a system of troughs in the northern hemisphere of Mars, is an ideal location to test these hypotheses because this region consists of a field of pitted cones where previous workers have proposed each of these formation mechanisms to explain cone formation. We incorporate new and expanded remote sensing datasets, acquired from instruments currently orbiting Mars, to test whether fossae cones are the result of igneous‐ or mud‐related activity. Compositional analysis provides the strongest line of evidence for an igneous formation scenario. Morphometric analysis reveals that fossae cones appear most similar to rootless cones on Earth and these same proposed features on Mars, while some morphometric parameters also reveal similarities with mud volcanoes on both terrestrial planets. A preferential, nonrandom cone distribution pattern is revealed by spatial analysis. Morphologically, pitted cones lack associated flow units characteristic of mud volcanoes on Earth and Mars. The cones also do not appear to be located in a sedimentary basin‐like formation environment typical of proposed Martian mud volcanoes. Overall, our study highlights the diversity of pitted cones encountered on the Martian surface. Key Points: Visible and near‐infrared spectroscopy observations of pitted cones in the Hephaestus Fossae region are consistent with an igneous originHephaestus Fossae region cones are typically hundreds of meters in diameter and morphologically similar to pitted cones in Isidis BasinHephaestus Fossae region cones are spatially clustered indicating cone distribution control by a nonrandom process [ABSTRACT FROM AUTHOR]

The origin(s) of the chassignites and nakhlites, closely related martian olivine and augite cumulates, respectively, are much debated. Northwest Africa (NWA) 8694 is the third chassignite to be discovered and the most ferroan, containing 85% olivine (Fo 54). Its O-isotope compositions (δ18O ∼ 4.4‰, Δ17O ∼ 0.30‰) are typical of other martian meteorites. It has adcumulate texture and contains cumulus chromite, poikilitic pigeonite (En 56 Fs 37 Wo 7) and mesostasis (trapped interstitial liquid). The latter contains pyroxene and plagioclase (An 23 Ab 70 Or 8) plus rare K- feldspar (Or 74), and has a trachyandesitic to trachytic bulk composition. Melt inclusions in olivine contain a variety of phases including biotite and rare amphibole. Olivine, chromite, and pigeonite compositions are intermediate between those of the other chassignites and those of the nakhlites. Augite, which appears to mantle pigeonite, has a composition overlapping that in nakhlite NWA 998 and some other nakhlites at (En 41-40 Wo 38-39). The augite lamellae in pigeonite 1–2 μm in apparent width, and the survival of Ca zoning in olivine, suggest a near-surface cooling environment. The bulk-rock REE concentrations in the three chassignites do not correlate with Mg# but depend on the abundance of trapped liquid. The form of REE patterns calculated for olivine subtraction is very like those of nakhlite mesostases, but the observed concentrations of LREE in NWA 8694 trapped liquid have a very steep slope. This is explained by undersampling of baddeleyite and zirconolite that occur near olivine contacts with mesostasis. Though pyroxene is unzoned, its trace element variations indicate fractional crystallization. The range of olivine compositions in the three chassignites (Fo 79-54) is too large to result from the crystallization sequential growth of olivine from a single magma undergoing fractional crystallization. The Ge/Si ratios show degassing of NWA 8694 which sets this chassignite apart from other chassignites and nakhlites, implying a unique batch of magma for its genesis. Many potential parent liquids are capable of generating the NWA 8694 olivine composition, though not its alkaline mesostasis. We calculated that Nakhla parent liquid NA01a (Stockstill et al., 2005) with 10% Nakhla core olivine added would produce both olivine crystals and alkaline daughter liquids with compositions matching those of NWA 8694. This meteorite is a chassignite cumulate containing nakhlitic mesostasis, a direct link between the chassignites and the nakhlites and the association of dunitic to trachytic compositions is reminiscent of terrestrial shield volcanoes. Chassignites and nakhlites were possibly formed when solidification fronts on chamber walls were disrupted, mainly as side eruptions of olivine-charged magmas from the deeper zones, and augite-charged fractionated magmas from nearer the summit of a volcano resembling Piton de la Fournaise on Earth. [ABSTRACT FROM AUTHOR]

Highlights • Kasei Valles hosts long, turbulently-emplaced flood lavas. • These lavas descended from the Tharsis rise. • Large Martian flood lavas display distal inflation as well as medial drainage or volume loss. Abstract Both the northern and southern arms of Kasei Valles are occupied by platy-ridged flood lavas. We have mapped these flows and examined their morphology to better understand their emplacement. The lavas were emplaced as high-flux, turbulent flows (exceeding 106 m3 s−1). Lava in southern Kasei Valles can be traced back up onto the Tharsis rise, which is also the likely source of lavas in the northern arm. These eruptions were similar to, but somewhat smaller than, the Athabasca Valles flood lava in Elysium Planitia, with estimated volumes of > 1200 km3 here and 5000 km3 in Athabasca Valles. The flood lavas in both Kasei and Athabasca Valles have evidence for distal inflation as well as widespread drainage or volume loss in medial areas; this may be an important characteristic of many large, recent Martian eruptions. Despite their great size and flux, the Kasei Valles flood lavas are only a late modification to the valley system capable of only modest local erosion. The more vigorous Athabasca Valles lava may have been capable of somewhat more erosion in its smaller valley system. [ABSTRACT FROM AUTHOR]

Abstract Ravi Vallis is a Martian outflow channel that is widely interpreted as a product of one or more catastrophic outbursts of groundwater from adjacent Aromatum Chaos. However, solar system analogs are unknown for formation of large channels by outbursts from aquifers, and the Ravi Vallis region lacks the obvious sedimentological and mineralogical signatures expected of aqueous origins. Instead, the basic nature of this channel system is consistent with volcanic origins involving voluminous effusions of low-viscosity lavas. On bedrock slopes of only 0.2°, 50-m-deep lava flows with viscosities of 1 Pa s are predicted to have been characterized by discharge rates of ∼3 × 107 m3/s for 25-km-wide flows, with associated mechanical incision rates of ∼4 m/day and thermal incision rates of ∼2 m/day. Formation of Aromatum Chaos and the preserved 215-km-long extent of Ravi Vallis is conservatively estimated on the basis of thermal considerations to have required eruption of a minimum of ∼64,000 km3 of lava, but greater volumes would have been necessary to form the original (unknown) length of this channel system. A volcanic origin for Ravi Vallis is consistent with development of all Martian outflow channels by igneous processes, and with the broader premise that early development of immense volcanic channel systems is typical of all large rocky bodies. Highlights • Ravi Vallis has the properties expected of volcanic rather than aqueous origins. • Eruptions at Aromatum Chaos are likely to have been fed by deeply-rooted intrusions. • System development required eruption of at least ∼64,000 km3 of lava. [ABSTRACT FROM AUTHOR]

Abstract The investigation of the Martian surface through remote sensing allowed the identification of mound-like topographically positive features that, based on geomorphological observations, have been ascribed to different phenomena. New observations will be performed in the forthcoming future to look for possible methane sources, hence discriminating morphologically similar features is a key objective to efficiently investigate and target the Martian surface. Here, we performed fractal analysis on five Martian fields of mound-like features that have been interpreted respectively as mud volcanoes, pingos, tumuli, rootless cones and monogenetic volcanic vents successfully validating the major interpretations of these features and in turn, the applicability of the fractal clustering method for discriminating among such features. Indeed, this technique is able to assess if the analyzed features are directly related to underlying systems of connected percolating fracture networks and estimates their extension in the subsurface. Accordingly, volcanic vents and mud volcanoes appeared to be connected to percolating systems involving several kilometers of crust, pingos and tumuli resulted to be unequivocally unrelated to active percolating fractures while rootless cones outputted weak relationship with shallow active fracture systems. Highlights Through the development of this work we aim to provide the scientific community with an innovative tool for the planetary surfaces investigation. The successful outcomes herein presented can be summarized as follows: • The analysis of mounds' spatial distribution allows to spot percolating fracture networks • The method allows to locate the depth of the fluid source where present Such goals make possible: • The discrimination of morphologically convergent features on the surface • Tracing the fluid circulation in the subsurface [ABSTRACT FROM AUTHOR]

Hebes Chasma is an 8‐km deep, 126 by 314 km, isolated basin that is partially filled with massive deposits of water‐altered strata called interior layered deposits (ILDs). By analyzing the ILD's structure, stratigraphy, and mineralogy, a depositional history of Hebes Chasma is interpreted. Three distinct ILD units were found and are informally referred to as the Lower, Upper, and Late ILD. These units are distinguished by their layer thicknesses, layer attitudes, mineralogies, and erosional landforms. The Lower and Upper ILDs comprise the chasma's 7.5‐km tall, 120 by 43 km, central mound, and the Late ILD is located in the valley between the central mound and the chasma's northern wall. A horizontal unconformity separates the Lower and Upper ILDs, and layer attitudes revealed large‐scale shallow folding within the Lower ILD. All ILDs are characterized by both monohydrated and polyhydrated sulfates signatures. Erosional landforms such as hummocks, polygons, and debris flows suggest past glacial activity within the chasma. A scenario involving several ash fall events during various stages of chasma formation is proposed as the dominant setting throughout Hebes' geologic history. Plain Language Summary: On the planet Mars there is a large 4,000 km‐long system of 12 canyons called Valles Marineris. On Mars these canyons are called chasmata or individually a chasma. Some are linked together, while some are isolated, but all are considered to share the same origins of formation. Within the interiors of several chasmata are large layered mounds up to 8‐km thick. The origins of these layered mounds are elusive, but there are several contending theories that typically consider them to have been made after or during the chasmata formation. Detections from orbital instruments show that these mounds also contain abundant hydrated minerals that could have only been formed in the presence of water. One particular chasma called Hebes is the topic of this paper and its large layered mound has been measured and described using several different methods to help constrain the geologic history of Valles Marineris. The mound within Hebes was discovered to have three units formed in three separate depositions. These units differ in their mineralogies, layer thicknesses, layer attitudes, and erosional features. A geologic map of Hebes Chasma was created, and stages of deposition are described. Key Points: Three separate ILD units and an unconformable contact were identifiedThe ILD units have unique mineralogies, layer thicknesses, and layer attitudesNew geologic map, cross section, and depositional historyis presented [ABSTRACT FROM AUTHOR]

In a previous paper (Chassefière et al. ), we have shown that most volcanic sulfur released to the early Mars atmosphere could have been trapped in the upper cryosphere under the form of CO2-SO2 clathrates. Huge amounts of sulfur, up to the equivalent of an ~1 bar atmosphere of SO2, would have been stored in the Noachian upper cryosphere, then massively released to the atmosphere during the Hesperian due to rapidly decreasing CO2 pressure. It could have resulted in the formation of the large sulfate deposits observed mainly in Hesperian terrains, whereas no or little sulfates are found at the Noachian. In the present paper, we first clarify some aspects of our previous work. We discuss the possibility of a smaller cooling effect of sulfur particles, or even of a net warming effect. We point out the fact that CO2-SO2 clathrates formed through a progressive enrichment of a pre-existing reservoir of CO2 clathrates and discuss processes potentially involved in the slow formation of a SO2-rich upper cryosphere. We show that episodes of sudden destabilization at the Hesperian may generate 1000 ppmv of SO2 in the atmosphere and contribute to maintaining the surface temperature above the water freezing point. [ABSTRACT FROM AUTHOR]

The Hellas basin on Mars is the second-largest topographically well-defined impact structure in the Solar System and has repeatedly been interpreted as a major sink of volcanic, glacio-fluvial and eolian materials. Based on established guidelines for planetary mapping, we compiled a comprehensive photogeological map of Hellas Planitia, i.e., the Hellas basin floor (1:2,000,000; ∼1.8 × 10 6 km 2 ; see Supplementary online material ), using the Thermal Emission Imaging System (THEMIS-IR) day-time mosaic as basemap (supplemented by several other datasets). We identified 33 units, which were categorized into a “Rim Assemblage”, containing “Dissected units”, a “Layered rim sequence”, and “Other basin rim units”, as well as a “Floor Assemblage”, containing the “Honeycomb formation”, an “Interior formation”, and a “Plains sequence”. Relative dating of units was performed wherever contacts revealed stratigraphic relationships and was complemented by absolute model ages (AMAs) of all units that lend themselves to reliable crater-size frequency distribution (CSFDs) measurements. On the basis of our results, as well as AMAs of circum-Hellas volcanic provinces by previous authors, we compiled a chronostratigraphic model of the Hellas basin floor. The northern basin rim shows evidence (vast layered, hydrous mineral-bearing deposits containing meandering, channel-like valleys), that the early history of the basin until ∼3.8 Ga ago experienced extended periods of low-energy fluvial, and possibly lacustrine, activity. Superposing the layered rim sequence, the majority of the Hellas basin infill (∼1.5–1.7 × 10 6 km 3 ) consists of volcanic material (a lower and an upper wrinkle-ridged plains unit), which was shortened by a compressive stress field relatively soon after its emplacement. Based on their ages and stratigraphic considerations, we identified Malea Patera (and possibly also Tyrrhena Patera) as a suitable source for the older, lower plains (∼3.8 Ga), and Hadriaca and/or Amphitrites Paterae for the younger, upper plains (∼3.7 Ga). Shortly after these volcanic episodes, the entire basin floor was covered by large amounts of deposits (>10 6 km 3 ) containing aqueously altered mafic materials. We interpret the deposits to have originated from Hesperia Planum and the Hesperia-Hellas trough, where they might have been removed by glacio-fluvial processes, which also sculpted the Hellas rim sections along northern Promethei Terra and Malea Planum around the same time. Such an apparent correlation between nearby volcanic activity and increased erosion/deposition is in agreement with previous models, which suggest outgassing by a maximum in global volcanism at that time caused warmer episodes enabling repeated subaerial water run-off. After the emplacement of these deposits, intense erosion, likely deflation along a rim-parallel annulus, began to exhume the outer parts of the wrinkle-ridged plains again, forming an interior formation of more elevated erosional remnants including the Alpheus Colles. Since ∼3.7 Ga ago, fluvial activity of Dao/Niger and Harmakhis Valles dissected the eastern plains and emplaced pancake-shaped deposits in the basin center. Later, deflation and abrasion by katabatic winds moving through the Hellas basin in a clock-wise direction parallel to the basin floor outline carved out the northwestern Hellas Planitia trough (called Peneus Palus), possibly within a time span of few hundreds of Ma or less within the Amazonian. Being consistent with erosional patterns we observed within the basin, circulation models predict such winds to be ongoing today and might, thus, have recently exhumed the so called “honeycomb” formation on parts of Peneus Palus. This enigmatic, unique terrain was possibly formed by ductile deformation of pre-3.8 Ga material and appears to be partially covered by another unique landform, the “banded terrain”. Although we cannot rule out that it is the surface expression of a thicker unit, the banded terrain might represent flows of volatile-rich airfall deposits emplaced along the edges of the Hellas wind alley by turbulent, marginal wind currents, which currently/recently prevail on both sides of the Hellas Planitia trough. In summary, our investigations enabled us to draw an updated, comprehensive and self-consistent picture of the geologic evolution of the Hellas basin floor, including volcanic, (peri-)glacial, fluvial and eolian processes, their possible interactions, and the implications on the climatic and geologic development of the circum-Hellas region and the entire planet. [ABSTRACT FROM AUTHOR]

In this study, the impact event that produced the Martian Isidis basin (0–20°N and 70–100°E) was modeled using the iSALE hydrocode, and the current lithospheric structure was obtained comparing the calculated admittance from topography and gravity data, with the admittance from top-load and bottom-load flexure models. The best-fit admittance model was obtained using a bottom-load flexure with a thinned crust of 10 km, likely formed by cooling of the surficial melt pool after the impact, and an elastic thickness of 36 km. We found that the impact produced crustal excavation and thickening, surficial melt pool and temperatures >1500 K beneath the area where the free-air maxima is observed. The geometry resulting from the impact modeling is compatible with the radius of the Isidis basin. Derived thermal gradient and heat flux are in agreement with an early-Noachian epoch for the Isidis impact event and with the intense volcanic activity that followed the impact. [ABSTRACT FROM AUTHOR]

Abstract: While most of Tharsis rise was in place by end of the Noachian period, at least one volcano on Tharsis swell (Arsia Mons) has been active within the last 2 Ma. This places an important constraint on mantle convection and on the thermal evolution of Mars. The existence of recent volcanism on Mars implies that adiabatic decompression melting and, hence, upwelling convective flow in the mantle remains important on Mars at present. The thermal history on Mars can be constrained by the history of melt production, specifically generating sufficient melt in the first billion years of the planets history to produce Tharsis rise as well as present day melt to explain recent volcanism. In this work, mantle convection simulations were performed using finite element code CitcomS in a 3D sphere starting from a uniformly hot mantle and integrating forward in time for the age of the solar system. We implement constant and decaying radioactive heat sources; and vary the partitioning of heat sources between the crust and mantle, and consider decreasing core–mantle boundary temperature and latent heat of melting. The constant heat source calculations produce sufficient melt to create Tharsis early in Martian history and continue to produce significant melt to the present. Calculations with decaying radioactive heat sources generate excessive melt in the past, except when all the radiogenic elements are in the crust, and none produce melt after 2 Gyr. Producing a degree-1 or degree-2 structure may not be pivotal to explain the Tharsis rise: we present multi-plume models where not every plume produces melt. The Rayleigh number controls the timing of the first peak of volcanism while late-stage volcanism is controlled more by internal mantle heating. Decreasing the Rayleigh number increases the lithosphere thickness (i.e., depth), and increasing lithosphere thickness increases the mean mantle temperature. Increasing pressure reduces melt production while increasing temperature increases melt production; hence predicting melt production from convection parameters is not straightforward. Generating enough melt in the mantle to create Tharsis early on and also to explain recent volcanism may require other mechanisms such as small-scale convection or lowering the thermal conductivity of the crust. [Copyright &y& Elsevier]

Highlights: [•] Examines bedforms over a wide elevation range on martian volcanos. [•] High resolution imaging resolves small bedforms ∼1–5m across. [•] Bedforms exhibit systematic size trend, inversely proportional to air density. [•] This proportionality is expected from several models. [ABSTRACT FROM AUTHOR]

Abstract: A planetary global circulation model developed by the Laboratoire de Météorologie Dynamique (LMD) was used to simulate explosive eruptions of ancient martian volcanoes into paleo-atmospheres with higher atmospheric pressures than that of present-day Mars. Atmospheric pressures in the model were varied between 50mbar and 2bars. In this way it was possible to investigate the sensitivity of the volcanic plume dispersal model to atmospheric pressure. It was determined that the model has a sensitivity to pressure that is similar to its sensitivity to other atmospheric parameters such as planetary obliquity and season of eruption. Higher pressure atmospheres allow volcanic plumes to convect to higher levels, meaning that volcanic pyroclasts have further to fall through the atmosphere. Changes in atmospheric circulation due to pressure cause pyroclasts to be dispersed in narrower latitudinal bands compared with pyroclasts in a modern atmosphere. Atmospheric winds are generally slower under higher pressure regimes; however, the final distance traveled by the pyroclasts depends greatly on the location of the volcano and can either increase or decrease with pressure. The directionality of the pyroclast transport, however, remains dominantly east or west along lines of latitude. Augmentation of the atmospheric pressure improves the fit between possible ash sources Arsia and Pavonis Mons and the Medusae Fossae Formation, a hypothesized ash deposit. [Copyright &y& Elsevier]

Gravitational deformation strongly influences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Réunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie. [ABSTRACT FROM AUTHOR]

Abstract: A field of small (10s of km in diameter) volcanoes in the Syria Planum region of Mars is mapped to determine abundance, distribution, and alignments of vents. These data are used to assess possible variations in eruption style across space and time. Each eruption site is assigned a point location. Nearest neighbor and two-point azimuth analyses are conducted to assess the spacing and orientations between vents across the study area. Two vent fields are identified as unique volcanic units along with the previously identified Syria Mons volcano. Superposition relationships and crater retention rates indicate that these three volcanic episodes span ~900Ma, beginning in the early Hesperian and ending in the Early Amazonian. No clear hiatus in eruptive activity is identified between these events, although a progression from eruptions at Syria Mons, to regionally distributed eruptions that form the bulk of the Syria Planum plains, to a final migration of dispersed eruptions to Syria''s northwest is identified. Nearest neighbor analyses suggest a non-random distribution among the entire population of Syria Planum, which is interpreted as resulting from the interaction of independent magma bodies ascending through the crust during different stress regimes throughout the region''s eruptive history. Two-point azimuth results identify three orientations of enhanced alignments, which match well with radial extensions of three major tectonic centers to the south, east, and northwest of the study area. As such, Syria Planum volcanism evolved from a central vent volcano to dispersed shield field development over several hundred million years, during which the independent magma bodies related to each small volcano interacted to some extent with one or more of at least three buried tectonic patterns in the older crust. These results show a strong relationship between independent mapping efforts of tectonic and volcanic features. Continued integration of volcano-tectonic mapping should provide direct constraints for future geodynamic models of magma production and thermal evolution of the Tharsis province. [Copyright &y& Elsevier]

Abstract: Based on theoretical grounds, explosive basaltic volcanism should be common on Mars, yet the available morphological evidence is sparse. We test this hypothesis by investigating a unique unnamed volcanic field north of the shield volcanoes Biblis Patera and Ulysses Patera on Mars, where we observe several small conical edifices and associated lava flows. Twenty-nine volcanic cones are identified and the morphometry of many of these edifices is determined using established morphometric parameters such as basal width, crater width, height, slope, and their respective ratios. Their morphology, morphometry, and a comparison to terrestrial analogues suggest that they are martian equivalents of terrestrial pyroclastic cones, the most common volcanoes on Earth. The cones are tentatively interpreted as monogenetic volcanoes. According to absolute model age determinations, they formed in the Amazonian period. Our results indicate that these pyroclastic cones were formed by explosive activity. The cone field is superposed on an old, elevated window of fractured crust which survived flooding by younger lava flows. It seems possible that a more explosive eruption style was common in the past, and that wide-spread effusive plain-style volcanism in the Late Amazonian has buried much of its morphological evidence in Tharsis. [Copyright &y& Elsevier]

Abstract: A multidisciplinary approach involving various remote sensing instruments is used to investigate Apollinaris Mons, a prominent volcano on Mars, as well as the surrounding plains for signs of prolonged hydrologic and volcanic, and possibly hydrothermal activity. The main findings include (1) evidence from laser altimetry indicating the large thickness (1.5–2km at some locations) of the fan deposits draping the southern flank contrary to previous estimates, coupled with possible layering which point to a significant emplacement phase at Apollinaris Mons, (2) corroboration of Robinson et al. (Robinson, M.S., Mouginis-Mark, P.J., Zimbelman, J.R., Wu, S.S.C., Ablin, K.K., Howington-Kraus, A.E. [1993]. Icarus 104, 301–323) hypothesis regarding the formation of incised valleys on the western flanks by density current erosion which would indicate magma–water interaction or, alternatively, volatile-rich magmas early in the volcano’s history, (3) mounds of diverse geometric shapes, many of which display summit depressions and occur among faults and fractures, possibly marking venting, (4) strong indicators on the flanks of the volcano for lahar events, and possibly, a caldera lake, (5) ubiquitous presence of impact craters displaying fluidized ejecta in both shield-forming (flank and caldera) materials and materials that surround the volcano that are indicative of water-rich target materials at the time of impact, (6) long-term complex association in time among shield-forming materials and Medusae Fossae Formation. The findings point to a site of extensive volcanic and hydrologic activity with possibly a period of magma–water interaction and hydrothermal activity. Finally, we propose that the mound structures around Apollinaris should be prime targets for further in situ exploration and search for possible exobiological signatures. [Copyright &y& Elsevier]

Abstract: The Medusae Fossae Formation (MFF) has long been thought to be of Amazonian age, but recent studies propose that a significant part of its emplacement occurred in the Hesperian and that many of the Amazonian ages represent modification (erosional and redepositional) ages. On the basis of the new formational age, we assess the hypothesis that explosive eruptions from Apollinaris Patera might have been the source of the Medusae Fossae Formation. In order to assess the likelihood of this hypothesis, we examine stratigraphic relationships between Apollinaris Patera and the MFF and analyze the relief of the MFF using topographic data. We predict the areal distribution of tephra erupted from Apollinaris Patera using a Mars Global Circulation Model (GCM) combined with a semi-analytical explosive eruption model for Mars, and compare this with the distribution of the MFF. We conclude that Apollinaris Patera could have been responsible for the emplacement of the Medusae Fossae Formation. [Copyright &y& Elsevier]

Abstract: The Tharsis bulge is the largest magmatic/volcanic center on Mars and in the solar system, having a volume of ~3×108 km3, with the majority of its mass emplaced more than 3.7billion years ago. The igneous history of Tharsis has been implicated in the generation of an atmosphere and hydrosphere capable of clement conditions on early Mars. It has been proposed that an early plume migration from the southern highlands of Mars relative to a one-plate lithosphere led to the development of the Tharsis bulge in its current location along the Martian crustal dichotomy. We used geologic mapping, crustal magnetic data, crater age-dating and crater morphometry to detail a previously-unidentified path of putative plume migration. Our results indicate that extensive volcanic resurfacing occurred from a location near the present south pole to the equator around 3.8billion years ago, obliterating older cratered terrains. The resurfacing path is manifest as smooth volcanic plains embaying ancient massifs and infilling large impact craters as well as a lack of a magnetic signature in this portion of the crust. Our results have significant ramifications for mantle dynamics and the early geologic history of Mars. [Copyright &y& Elsevier]

Abstract: The outflow channels of Mars are widely believed to have formed through aqueous outbursts from aquifers, implying the past existence of large near-surface water reservoirs and the past operation of a vigorous hydrological cycle. However, accepted mechanisms of channel development suffer from numerous weaknesses, including (1) reliance upon implausible hydrological assumptions; (2) requirement of water abundances that are orders of magnitude greater than can be justified by geochemical considerations; (3) reliance upon long-term conditions that may be incongruous with the preserved mineralogical record; (4) limited correspondence between expected and observed channel properties; and (5) reliance upon exotic aqueous processes for which there are no known solar system analogs. In contrast, channel characteristics are consistent with volcanic origins involving low viscosity lava flows and associated processes of thermal or mechanical erosion. The volcanic hypothesis is founded upon the existence of analog landforms on multiple bodies of the inner solar system and on corresponding analog processes that are variations on familiar terrestrial volcanic themes. Volcanic channel origins are compatible with available mineralogical and geochemical data and are consistent with the nature of preserved channel landforms. The volcanic hypothesis fits within a wider geological framework that economically accounts for the existence and nature of large outflow systems located on the Moon, Venus, and Mars. A volcanic origin reduces the probability that extensive aqueous environments existed during the Hesperian and Amazonian along the outflow channels and in associated terminal basins, and it narrows the possible range of Martian environments once hospitable to life. [Copyright &y& Elsevier]

Abstract: Tharsis Tholus, a more than 3.9Ga old composite shield volcano to the east of the major Tharsis Montes, has experienced a complex history of growth and destruction. On the basis of new high resolution images we analysed the morphology as well as the tectonic structures of the Tharsis Tholus volcano in detail. From morphological data, cross-cutting relations of the surface structures, and crater modelling ages we propose a chronostratigraphy for the volcano-tectonic history of Tharsis Tholus. The strongly faulted volcano reveals two large-scale landslide events followed by two subsequent shield re-growth phases between 3.8 and 1.7Ga and two caldera collapses. Tharsis Tholus was also affected by regional extensional tectonics between 1.7Ga and 0.4Ga recorded by sub-parallel sets of NE trending graben structures. The steep and up to 5.4km high landslide scarps on Tharsis Tholus suggest deep faulting of the edifice. In order to confirm this hypothesis we used analogue sand box models in which we demonstrated that gravitational flank movement on top of weak basal substrata may have produced the deformation structures as observed on Tharsis Tholus. [Copyright &y& Elsevier]

Abstract: We report the detection of mound-like landforms interpreted as mud volcanoes within Firsoff crater, in the Arabia Terra region of Mars. The mounds stratigraphically overlie layered deposits within the crater, which consist of high albedo well-bedded deposits disrupted in meter-sized polygons. Hundreds of mounds are observed on the crater floor, as isolated or composite subcircular cones 100–500m in diameter and tens of meters high. More than one third of the mounds (35%) have subcircular depressions 5–39m in diameter at their apices that we interpret as vents. The mounds consist of meter-sized boulders of high albedo, embedded in an apparently finer-grained unit; we interpret these deposits as mud volcanic breccia, containing boulders of reworked light-toned layered deposits carried upward from deeper portions of the succession. The mounds are mainly located near the rim-bordering faults and in places aligned with fractures, suggesting subsurface pathways for fluid migration. The mounds are interpreted as fossil mud volcanoes, recording the upward migration of overpressured fluids through the crater infill from deeper crustal sources, localized by impact-related faults. The geological conditions and processes that might have controlled sedimentary expulsion on Mars are discussed. Mud volcanism has most of its controls in the subsurface. As a consequence, subsurface fluid flow processes could be expected in other craters within Arabia Terra and are less likely in higher, adjacent highland terrains. [Copyright &y& Elsevier]

Abstract: A lava channel identified on the wall of an Elysium Planitia impact crater is investigated to identify the dominant erosion mechanism, mechanical vs. thermal, acting during channel formation. Observations of channel morphology are used to supplement analytical models of lava channel formation in order to calculate the duration of channel formation, the velocity of the lava flowing through the channel, and the erosion rate in each erosion regime considered. Results demonstrate that the channel observed in the Elysium Planitia impact crater formed primarily due to mechanical erosion. In a more general sense, results of this study suggest that lava channels can form primarily due to thermal erosion in the presence of more gradual slopes and more consolidated substrates whereas lava channels can form primarily due to mechanical erosion in the presence of more energetic flows on steeper slopes and more poorly consolidated substrates. Therefore, both erosion regimes must be considered when analyzing origins of eroded lava channels that cut through strata of different strengths. [ABSTRACT FROM AUTHOR]

Abstract: Ascraeus Mons was one of the first of the Martian volcanoes to be imaged by the High Resolution Stereo Camera (HRSC) experiment onboard the ESA Mars Express spacecraft. These images show much of the volcano at a higher resolution than previously, and details of its lava flows, sinuous rilles, flank vents and tectonic features indicate an unexpected origin for some of these features. We establish the time-stratigraphic sequence for these features, and use a numerical model on HRSC stereo DTMs of the sinuous rilles, and conclude that they were formed by water erosion. Terrestrial analogues for such features are found at Réunion Island and other volcanoes. We then examine the overall structure of the volcano, which is dissimilar to that of large terrestrial volcanoes in important respects, and perform laboratory analogue experiments of its deformation, concluding that the tectonic features were formed by sinking of the volcano into a substratum that was much weaker than the volcanic edifice. An ice-rich substratum melted by a combination of pressure melting and magmatic heating seems the most likely mechanism. Analogous water-escape structures in a similar volcanic situation have been identified at Mt Haddington in the Antarctic. The possible role of a hydrological cycle and a hydrothermal system within the volcano are discussed. Based on field evidence, we propose that much of the broad aprons of lobate flows issuing from the NE and SSW foot of Ascraeus Mons are composed of mudflows rather than lava flows. These different approaches are linked into a coherent history of this volcano. The similarity of Ascraeus Mons to Pavonis Mons and Arsia Mons (though Ascraeus is younger) suggests that some of our conclusions may apply to these volcanoes too. [Copyright &y& Elsevier]

Abstract: We study the thermal fields over Olympus Mons separating seasons (northern spring and summer against southern spring and summer) and local time observations (day side versus night side). Temperature vertical profiles retrieved from Planetary Fourier Spectrometer on board Mars Express (PFS–MEX) data have been used. In many orbits (running north to south along a meridian) passing over the top of the volcano there is evidence of a hot air on top of the volcano, of two enhancement of the air temperature both north and south of it and in between a collar of air that is colder than nearby at low altitudes, and warmer than nearby at high altitudes. Mapping together many orbits passing over or near the volcano we find that the hot air has the tendency to form an hot ring around it. This hot structure occurs mostly between LT=10.00 and 15.00 and during the northern summer. Distance of the hot structure from the top of the volcano is about 600km (10° of latitude). The hot atmospheric region is 300–420km (5–7°) wide. Hot ring temperature contrasts of about 40K occur at 2km above the surface and decrease to 20K at 5km and to 10K at 10km. The atmospheric circulation over an area of 40°×40° (latitudes and longitudes) is affected by the topography and the presence of Olympus Mons (−133°W,18°N). We discuss also the thermal stability of the atmosphere over the selected area using the potential temperatures. The temperature field over the top of the volcano shows unstable atmosphere within 10km from the surface. Finally, we interpret the hot temperatures around volcano as an adiabatic compression of down-welling branch coming from over the top of volcano. Different air temperature profiles are observed in the same seasons during the night, or in different seasons. In northern spring–summer during the night the isothermal contours do not show the presence of the volcano until we reach close to the surface very much, where a thermal inversion is observed. The surface temperature shows higher values (by 10K) in correspondence of the scarp (an abrupt altimetry variation of roughly 5km) on south (6°N) and north (30°N) sides of volcano. During the southern spring–summer, on the contrary the isothermal curves run parallel to the surface even on top the volcano, just like the GCM have predicted. [Copyright &y& Elsevier]

Abstract: We provide an overview of features indicative of the interaction between water and lava and/or magma on Mars as seen by the High Resolution Imaging Science Experiment (HiRISE) camera during the Primary Science Phase of the Mars Reconnaissance Orbiter (MRO) mission. The ability to confidently resolve meter-scale features from orbit has been extremely useful in the study of the most pristine examples. In particular, HiRISE has allowed the documentation of previously undescribed features associated with phreatovolcanic cones (formed by the interaction of lava and groundwater) on rapidly emplaced flood lavas. These include “moats” and “wakes” that indicate that the lava crust was thin and mobile, respectively [Jaeger, W.L., Keszthelyi, L.P., McEwen, A.S., Dundas, C.M., Russel, P.S., 2007. Science 317, 1709–1711]. HiRISE has also discovered entablature-style jointing in lavas that is indicative of water-cooling [Milazzo, M.P., Keszthelyi, L.P., Jaeger, W.L., Rosiek, M., Mattson, S., Verba, C., Beyer, R.A., Geissler, P.E., McEwen, A.S., and the HiRISE Team, 2009. Geology 37, 171–174]. Other observations strongly support the idea of extensive volcanic mudflows (lahars). Evidence for other forms of hydrovolcanism, including glaciovolcanic interactions, is more equivocal. This is largely because most older and high-latitude terrains have been extensively modified, masking any earlier 1–10m scale features. Much like terrestrial fieldwork, the prerequisite for making full use of HiRISE’s capabilities is finding good outcrops. [Copyright &y& Elsevier]

Abstract: HiRISE images of Mars with ground sampling down to 25cm/pixel show that the dust-rich mantle covering the surfaces of the Tharsis Montes is organized into ridges whose form and distribution are consistent with formation by aeolian saltation. Other dusty areas near the volcanoes and elsewhere on the planet exhibit a similar morphology. The material composing these “reticulate” bedforms is constrained by their remote sensing properties and the threshold curve combined with the saltation/suspension boundary, both of which vary as a function of elevation (atmospheric pressure), particle size, and particle composition. Considering all of these factors, dust aggregates are the most likely material composing these bedforms. We propose that airfall dust on and near the volcanoes aggregates in situ over time, maybe due to electrostatic charging followed by cementation by salts. The aggregates eventually reach a particle size at which saltation is possible. Aggregates on the flanks are transported downslope by katabatic winds and form linear and “accordion” morphologies. Materials within the calderas and other depressions remain trapped and are subjected to multidirectional winds, forming an interlinked “honeycomb” texture. In many places on and near the volcanoes, light-toned, low thermal inertia yardangs and indurated surfaces are present. These may represent “duststone” formed when aggregates reach a particle size below the threshold curve, such that they become stabilized and subsequently undergo cementation. [Copyright &y& Elsevier]

Abstract: In order to assess the age relations between astra/novae (features with extensive radial fracture-graben systems) and their surroundings, and to determine the duration of their activity, we undertook a photogeologic analysis of Magellan images of 78 astra, 49 dark-parabola craters and 114 clear-halo craters. For seven of these 78 astra it was found that the astrum-forming faulting started before or close to the time of emplacement of regional plains and extended into the second part of post-regional-plains time. Because the mean age of the regional plains is close to the mean surface age of Venus (which is estimated to be ∼750m.y), this means that the duration of activity of these seven astra was several hundred millions of years. This is longer than the duration of activity of ongoing mantle plumes on Earth, but shorter than the duration of activity of the plume feeding the martian volcano Olympus Mons. The basic morphologic characteristics of these seven astra, as well as their age relations with other geologic units, are similar to those of the majority of other astra; therefore, such a long duration could also be typical of some other astra. We confirm the two-phase (pre- and post-regional-plains) evolution of astrum-forming faulting suggested in previous studies. For the first phase we show evidence for purely tectonic faulting caused by the diapiric rise of a mantle plume. For the second phase we find evidence supporting the interpretation of other studies that the observed faults resulted from subsurface dike intrusions produced by magmatism associated with the plume. We also found that faulting during the second phase was not instantaneous but distributed over a long period of time. [Copyright &y& Elsevier]

Abstract: We examine the consequences of pyroclastic deposits being emplaced onto ice layers on Mars, both those in the polar caps and those forming glaciers on the flanks of some of the large shield volcanoes. We show that layers of pyroclasts greater than a few meters in thickness, whether emplaced cold (as fall deposits) or hot (as pyroclastic density current deposits) act almost exclusively to protect ice layers beneath them from sublimation, irrespective of whether they are emplaced at high or low elevations or high or low latitudes. Layers less than about 2 m thick, on the other hand, can cause significant ice loss by raising the surface temperature due to their low albedo and then transmitting that increased temperature to the underlying ice, especially on a diurnal time scale. This can have a significant bearing on the emplacement history of polar water ice and on the survival time of glacial ice on shield volcano flanks. A key factor in the latter case is the timing of the episodic volcanic activity relative to the cycles of climate change driven by Mars'' obliquity and eccentricity variations. [Copyright &y& Elsevier]

Abstract: As a part of a consortium study, we analyzed the Martian meteorite Yamato (Y) 000097 by prompt gamma-ray analysis, instrumental neutron activation analysis, and instrumental photon activation analysis. For comparison, we also analyzed Allan Hills (ALH) 77005 using the same methods. The data confirm that Y000097 belongs to lherzolitic shergottites in terms of chemical composition. Although there exist slight differences in elemental abundances among lherzolitic shergottites due to differences in the modal abundances of constituent minerals, they have essentially the same chemical compositions, suggesting they are genetically related and experienced similar formation histories. Zr/Hf ratios obtained for Y000097 and ALH 77005 are subchondritic, consistent with values reported for other lherzolitic shergottites and olivine-phyric shergottites. Such fractionation can be explained by invoking clinopyroxene, ilmenite, or majorite in the petrogenesis of the shergottites'' source material. CI-normalized Hf/Sm ratios obtained for Y000097 and ALH 77005 are 1.52 and 1.37, respectively, consistent with superchondritic Hf/Sm ratios reported for shergottites. Based on experimentally derived partition coefficients, majorite is the best candidate mineral for the fractionation of Hf and Sm in shergottites. [Copyright &y& Elsevier]

Abstract: A sharp crustal magnetic field contrast of almost two orders of magnitude at 185 km altitude, as determined by electron reflection (ER) magnetometry, exists between the nonmagnetic bulk of the Tharsis province and its relatively strongly magnetized southwestern region. The 3 nT magnetic field contour passes west of Ulysses Patera, south of Arsia Mons, through Thaumasia Planum and appears largely unmodified by impact craters, suggesting a post-Noachian origin. This sharp magnetic boundary is most easily explained by thermal demagnetization caused by pervasive magmatic intrusion throughout the upper crust on its nonmagnetic side. Using a best guess range of assumptions, we model these intrusions and their demagnetizing effects on preexisting crustal magnetization distributions and fit the resulting model magnetic field magnitudes to ER magnetic profiles across the boundary. Within the framework of our assumptions, we find that the magmatic boundary may not be as sharp as its magnetic counterpart, extending over 0–600 km, and that a minimum of ∼10–35 km average accumulated thickness of intrusions are required to completely demagnetize the crust on the northeast side of the boundary. The best-fit modeled intrusions extend horizontally between ∼120 km and 220 km beyond the magnetic boundary to the southwest for most of its length, with the inferred intrusion thickness and penetration distance being larger for minerals with higher magnetic blocking temperatures (magnetite vs. pyrrhotite). Such thicknesses of intrusion are consistent with magma production rates similar to those at Hawaii, if we allow accumulation over 0.1 to 1 Ga. If the volume of intrusion is representative of most of Tharsis, these thicknesses imply average intrusive–extrusive ratios higher than previously estimated, and in closer agreement with previous magma production estimates based on heat flow. Mapped fields of late Amazonian small volcanic vents, with diverse morphologies and a wide spatial distribution, may represent the latest stages of volcanism and record some of the polybaric processes that likely have occurred as magma intruded multiple levels of the crust. Also, intrusions are inferred to extend beneath most of the length of the upper southwest rift apron of Arsia Mons, implying that localized extrusion may be responsible, along with volcanism from the rift zone, for the apron''s plateau shape. Lastly, within our model, the maximum pre-intrusion lateral magnetization coherence scale in this region is found to be less than ∼200 km. [Copyright &y& Elsevier]

Abstract: This paper focuses on the shape of 31 shield volcanoes of small to intermediate size (a few tens of kilometers in diameter) in 5 regions of Mars: Tempe Terra, Syria Planum, Pavonis Mons, Arsia Mons and central Elysium Planitia. A model for the shape of these shield volcanoes is applied, based upon the concept of porous flow of an unconfined aquifer proposed by Lacey et al. [Lacey, A., Ockendon, J.R., Turcotte, D.L., 1981. On the geometrical form of volcanoes, Earth Planet. Sci. Lett., 54(1), 139–143] and developed by Turcotte and Schubert [Turcotte, D., Schubert, G., 2002. Geodynamics. Cambridge University Press, second edition]. The agreement between the topographic profiles and the theoretical shape suggests that the model is appropriate and can be used to derive the product of flow rate and viscosity (Qµ). Estimates of Qµ are found to be homogeneous within a given volcanic region, but show large differences from one region to another, values generally decreasing with age (from oldest to youngest). Intrinsically, the method cannot separate the flow rate from the viscosity, but independent observations on individual lava flows in Syria Planum and central Elysium Planitia suggest that this evolution may be explained by decreasing magma viscosity with age. Independent constraints on the viscosity are used to interpret Q variations between volcanic regions. Except for Tempe Terra, and within error bars, relative effusion rates are essentially independent of volcano volumes, suggesting that the size of a shield volcano is principally controlled by the duration of volcanic activity. The specificity of central Elysium Planitia is emphasized, with highest flow rates associated with lowest lava viscosity. However, the lower viscosities at central Elysium Planitia cannot totally explain low Qµ values. Considering the equation of magma flow through a dike from a magma chamber at depth, to the shield, we suggest that lower volatile contents and/or deeper magma chambers may contribute to the decrease of Qµ values with time. Finally, the emplacement times of the 31 shield volcanoes were calculated using a calibration based upon terrestrial volcanoes Mauna Loa, Eldborgir, and Skjalbreiður. Emplacement times of martian volcanoes calculated in this way range from a few hundred thousand years (Tempe Terra) to a few tens of thousands of years (central Elysium Planitia). Despite the simplicity of the model, the quality of the fits and the internal consistency of the results are reasonable, encouraging development of more advanced models linking shield volcano shape to rheological properties of lavas and their characteristic time-scales of emplacement. [Copyright &y& Elsevier]

Abstract: Building on previous studies of volcanoes around the Hellas basin with new studies of imaging (High-Resolution Stereo Camera (HRSC), Thermal Emission Imaging System (THEMIS), Mars Orbiter Camera (MOC), High-Resolution Imaging Science Experiment (HiRISE), Context Imager (CTX)), multispectral (HRSC, Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA)), topographic (Mars Orbiter Laser Altimeter (MOLA)) and gravity data, we define a new Martian volcanic province as the Circum-Hellas Volcanic Province (CHVP). With an area of >2.1millionkm2, it contains the six oldest central vent volcanoes on Mars, which formed after the Hellas impact basin, between 4.0 and 3.6Ga. These volcanoes mark a transition from the flood volcanism that formed Malea Planum ∼3.8Ga, to localized edifice-building eruptions. The CHVP volcanoes have two general morphologies: (1) shield-like edifices (Tyrrhena, Hadriaca, and Amphitrites Paterae), and (2) caldera-like depressions surrounded by ridged plains (Peneus, Malea, and Pityusa Paterae). Positive gravity anomalies are found at Tyrrhena, Hadriaca, and Amphitrites, perhaps indicative of dense magma bodies below the surface. The lack of positive-relief edifices and weak gravity anomalies at Peneus, Malea, and Pityusa suggest a fundamental difference in their formation, styles of eruption, and/or compositions. The northernmost volcanoes, the ∼3.7–3.9Ga Tyrrhena and Hadriaca Paterae, have low slopes, well-channeled flanks, and smooth caldera floors (at tens of meters/pixel scale), indicative of volcanoes formed from poorly consolidated pyroclastic deposits that have been modified by fluvial and aeolian erosion and deposition. The ∼3.6Ga Amphitrites Patera also has a well-channeled flank, but it and the ∼3.8Ga Peneus Patera are dominated by scalloped and pitted terrain, pedestal and ejecta flow craters, and a general ‘softened’ appearance. This morphology is indicative not only of surface materials subjected to periglacial processes involving water ice, but also of a surface composed of easily eroded materials such as ash and dust. The southernmost volcanoes, the ∼3.8Ga Malea and Pityusa Paterae, have no channeled flanks, no scalloped and pitted terrain, and lack the ‘softened’ appearance of their surfaces, but they do contain pedestal and ejecta flow craters and large, smooth, bright plateaus in their central depressions. This morphology is indicative of a surface with not only a high water ice content, but also a more consolidated material that is less susceptible to degradation (relative to the other four volcanoes). We suggest that Malea and Pityusa (and possibly Peneus) Paterae are Martian equivalents to Earth''s giant calderas (e.g., Yellowstone, Long Valley) that erupted large volumes of volcanic materials, and that Malea and Pityusa are probably composed of either lava flows or ignimbrites. HRSC and OMEGA spectral data indicate that dark gray to slightly red materials (often represented as blue or black pixels in HRSC color images), found in the patera floors and topographic lows throughout the CHVP, have a basaltic composition. A key issue is whether this dark material represents concentrations of underlying basaltic material eroded by various processes and exposed by aeolian winnowing, or if the material was transported from elsewhere on Mars by regional winds. Understanding the provenance of these dark materials may be the key to understanding the volcanic diversity of the Circum-Hellas Volcanic Province. [Copyright &y& Elsevier]

Abstract: Chlorine (Cl) is abundant, to percent levels, on Mars, in Mars basalts, and in some Earth basalts. Yet, little is known about the effects of Cl on basalt phase equilibria, which provide crucial constraints on melting temperatures, melt compositions, and melt production. To explore the effects of Cl on basalt equilibria, we located the liquidus (at high P and T) for a Martian basalt composition (the rock Humphrey, Gusev Crater, Mars) with 0.7% Cl-added, and obtained mineral and melt compositions from the experimental charges. Addition of Cl produces an unexpectedly large change in the liquidus—it is shifted to lower temperature by ~50 °C, and the field of pigeonite stability is enlarged so that the point of liquidus cosaturation in olivine & pigeonite is shifted down pressure by ~4 kbar (from 12.5 kbar and 1355 °C in the Cl-free composition to 8.5 kbar and 1305 °C). This temperature shift is comparable to that produced by addition of ~0.8% H2O; so, on a molar basis, Cl is twice as effective as H2O in reducing this basalt''s liquidus. Conceptually, the shift in liquidus temperature and enlargement of the field of pigeonite+melt is consistent with formation of complexes between Cl and network-modifying cations (e.g., Mg, Fe, Ca), which would make the latter unavailable to modify (depolymerize) the silicate network. However, the actual effect of Cl is larger than predicted by this simple model, even allowing up to 8 cations complexed per Cl. For Mars, these results suggest that Cl may play a crucial role in its basalts'' generation and evolution. While, some aspects of Martian basalt petrogenesis have been consistent with experimental results on water-rich systems (e.g., to ~2% H2O in basalt magma); the work here suggests that similar experimental results would be obtained for Cl-rich, H2O-poor systems, and that Martian basalts would contain little water. For Earth, the work here suggests that Cl must be considered explicitly in the petrogenesis of Cl-rich magmas, like those of subduction zones. [Copyright &y& Elsevier]

Abstract: Wudalianchi volcanic field, located in northeast China, consists of 14 Quaternary volcanoes with each volcano as a steep-sided scoria cone surrounded by gently sloping lava flows. Each cone is topped with a bowl-shaped or funnel-shaped crater. The volcanic cones are constructed by the accumulation of tephra and other ejecta. In this paper, their geologic features have been investigated and compared with some Martian volcanic features at Ascraeus Mons volcanoes observed on images obtained from High-Resolution Imaging Science Experiments (HiRISE), Mars Orbiter Camera (MOC), Context Imager (CTX) and Thermal Emission Imaging System (THEMIS). The results show that both Wudalianchi and Ascraeus Mons volcanoes are basaltic, share similar eruptive and geomorphologic features and eruptive styles, and have experienced multiple eruptive phases, in spite of the significant differences in their dimension and size. Both also show a variety of eruptive styles, such as fissure and central venting, tube-fed and channel-fed lava flows, and probably pyroclastic deposits. Three volcanic events are recognized at Ascraeus Mons, including an early phase of shield construction, a middle eruptive phase forming a low lava shield, and the last stage with aprons mantling both NE and SW flanks. We suggest that magma generation at both Wudalianchi and Ascraeus Mons might have been facilitated by an upwelling mantle plume or upwelling of asthenospheric mantle, and a deep-seated fault zone might have controlled magma emplacement and subsequent eruptions in Ascraeus Mons as observed in the Wudalianchi field, where the volcanoes are constructed along the northeast-striking faults. Fumarolic cones produced by water/magma interaction at the Wudalianchi volcanic field may also serve as an analogue for the pseudocraters identified at Isidis and Cerberus Planitia on Mars, suggesting existence of frozen water in the ground on Mars during Martian volcanic eruptions. [Copyright &y& Elsevier]

Abstract: The areal centroids of the youngest polar deposits on Mars are offset from those of adjacent paleopolar deposits by 5–10°. We test the hypothesis that the offset is the result of True Polar Wander (TPW), the motion of the solid surface with respect to the spin axis, caused by a mass redistribution within or on the surface of Mars. In particular, we consider the possibility that TPW is driven by late-stage volcanism during the Late Hesperian to Amazonian. There is observational and qualitative support for this hypothesis: in both north and south, observed offsets lie close to a great circle 90° from Tharsis, as expected for polar wander after Tharsis formed. We calculate the magnitude and direction of TPW produced by mapped late-stage lavas for a range of lithospheric thicknesses, lava thicknesses, eruption histories, and prior polar wander events. We find that if Tharsis formed close to the equator, the stabilizing effect of a fossil rotational bulge located close to the equator leads to predicted TPW of <2°, too small to account for observed offsets. If, however, Tharsis formed far from the equator, late-stage TPW driven by low-latitude, late-stage volcanism would be 6–33°, similar to that inferred from the location of paleopolar deposits. A volume of 4.4±1.3×1019 kg of young erupted lava can account for the offset of the Dorsa Argentea Formation from the present-day south rotation pole. This volume is consistent with prior mapping-based estimates and would imply a mass release of CO2 by volcanic degassing similar to that in the atmosphere at the present time. The South Polar Layered Deposits are offset from the present rotation pole in a direction that is opposite to the other paleopolar deposits. This can be explained by either a sequential eruption of late-stage lavas, or an additional contribution from a plume beneath Elysium. We predict that significant volcanic activity occurred during the time interval represented by the Basal Unit/Planum Boreum unconformity; Planum Boreum postdates the Promethei Lingula Lobe; and that the north polar deposits span a substantial fraction of Solar System history. If the additional contribution to TPW from plumes is small, then we would also predict that Tharsis Montes Formation postdates the Promethei Lingula Lobe of the South Polar Layered Deposits. We conclude with a list of observational tests of the TPW hypothesis. [Copyright &y& Elsevier]

The Olympus Mons volcano on Mars is notable not only for its immense height and width, but also for substantial asymmetries in its structure. The gently sloped northwest flank extends to a much greater distance from the central caldera complex than the more steeply sloped southeast flank. Furthermore, the northwest flank exhibits lower-flank extensional faults, whereas the southeast shows upper-flank compressional terraces and lower-flank upthrust blocks. However, both the northwest and southeast flanks exhibit characteristic concave-upward profiles and steep bounding scarps, in contrast to other sectors. The NW-SE asymmetries are aligned with the regional slope from the Tharsis rise, but an understanding of the underlying causes has remained elusive. We use particle dynamics models of growing, spreading volcanoes to demonstrate that these flank structures could reflect the properties of the basement materials underlying Olympus Mons. We find that basal slopes alone are insufficient to produce the observed concave-upward slopes and asymmetries in flank extent and deformation style that are observed at Olympus Mons; instead, lateral variations in basal friction are required. These variations are most likely related to the presence of sediments, transported and preferentially accumulated downslope from the Tharsis rise. Such sediments likely correspond to ancient phyllosilicates (clays) recently discovered by the Mars Express mission. [ABSTRACT FROM AUTHOR]

Abstract: The Tractus Fossae region of Mars is a wide area dominated by grabens, normal faults and pit crater chains. In this work, based on previous studies on the area and the new interpretation of topographic data and morphological units based on images as a geologic framework we present a new insight on the origin of the graben structures as well as on the processes associated with the formation of the volcanic features. Here we propose a new model for this region, on the basis of new measurements of graben extension and geological interpretations, together with a reinterpretation of the stratigraphy and the geologic history of the area. Finally, it is postulated that this region underwent a tectonic regime analog to an asymmetric rift system on Earth. [Copyright &y& Elsevier]

Abstract: Fan-shaped deposits up to in area are found on the northwest flanks of the huge Tharsis Montes volcanoes in the tropics of Mars. Recent spacecraft data have confirmed earlier hypotheses that these lobate deposits are glacial in origin. Increased knowledge of polar-latitude terrestrial glacial analogs in the Antarctic Dry Valleys has been used to show that the lobate deposits are the remnants of cold-based glaciers that formed in the extremely cold, hyper-arid climate of Mars. Mars atmospheric general circulation models (GCM) show that these glaciers could form during periods of high obliquity when upwelling and adiabatic cooling of moist air favor deposition of snow on the northwest flanks of the Tharsis Montes. We present a simulation of the Tharsis Montes ice sheets produced by a static accumulation pattern based on the GCM results and compare this with the nature and extent of the geologic deposits. We use the fundamental differences between the atmospheric snow accumulation environments (mass balance) on Earth and Mars, geological observations and ice-sheet models to show that two equilibrium lines should characterize ice-sheet mass balance on Mars, and that glacial accumulation should be favored on the flanks of large volcanoes, not on their summits as seen on Earth. Predicted accumulation rates from such a parameterization, together with sample spin-axis obliquity histories, are used to show that obliquity in excess of 45° and multiple 120,000 year obliquity cycles are necessary to produce the observed deposits. Our results indicate that the formation of these deposits required multiple successive stages of advance and retreat before their full extent could be reached, and thus imply that spin-axis obliquity remained at these high values for millions of years during the Late Amazonian period of Mars history. Spin-axis obliquity is one of the main factors in the distribution and intensity of solar insolation, and thus in determining the climate history of Mars. Unfortunately, reconstruction of past climate history is inhibited by the fact that the chaotic nature of the solution makes the calculation of orbital histories unreliable prior to about 20 Ma ago. We show, however, that the geological record, combined with glacial modeling, can be used to provide insight into the nature of the spin-axis/orbital history of Mars in the Late Amazonian, and to begin to establish data points for the geologically based reconstruction of the climate and orbital history of Mars. [Copyright &y& Elsevier]