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Climate is thought to affect the structure and evolution of drainage basins, but it is not clear how climate impacts the power law scaling between channel length and drainage area. Since climate controls runoff, streamflow, and erosion regimes, we looked for dependency of drainage basin morphometrics on climate within a near‐global data set. We show that increasingly arid regions have longer channels and narrower drainage basins, and power law scaling between channel length and basin area (Hack's Law) increases monotonically with aridity. We suggest these results arise due to downstream channel extension by rare large floods that erode channels into previously unchanneled terrain, yielding a morphometric signature in drylands that is preserved over long timescales due to a lack of subsequent topographic smoothing. This new understanding of drainage basin morphometrics on Earth may be used to inform interpretations of past climates on our planet and other solar system bodies. Plain Language Summary: The development and structure of river basins is of great interest to various research disciplines, and it has long been assumed that climate plays an important role in drainage basin characteristics. We leveraged a new global database of drainage basin length and shape to assess how these metrics vary with climate. We show that multiple drainage basin metrics change with the degree of aridity, suggesting that longer channels in narrower basins are more common in progressively drier regions. These results are relevant to the understanding of how rivers will respond to climate change and for interpreting drainage basin histories on other planetary bodies. Key Points: Our near‐global analysis of 254,951 basins shows climate dependence in Hack's LawDrainage basins are systematically longer and narrower in drier regionsThese findings suggest that arid channels extend downstream during extreme events [ABSTRACT FROM AUTHOR]

Ismenius Cavus (33.76° N, 17.05° W) is a large Martian basin that has been interpreted as an ancient paleolake. It is situated in the middle of the path Mamers Valles traces from the Cerulli Crater to the Borealis Basin. This ancient lake served as a key deposition area (or depocenter) for multiple lake chains originating in the southern highlands. The collected water was then transported downstream to the Borealis Basin. The ancient hydrologic activity of this zone is evidenced in the widespread appearance of valley networks and fretted channels, but especially in the presence of deltaic deposits converging in Ismenius Cavus. We made a hydrological and geomorphological analysis of two of these deltas, which compose a three-crater lake chain system. It was interpreted that both deltas, although being close to each other, were created by different processes and at different times. The Aracataca Delta was deposited during the Noachian by a valley network system. The Ariguani Delta, in turn, was the result of the discharge of a fretted channel carved by groundwater sapping. The transformations of the hydrologic systems in the Ismenius Cavus region were established by analyzing the change in the deltaic pulses over time. It was determined that the first hydrologic stage was dominated by widespread valley networks that probably represent subglacial hydrologic systems, a second stage started when these systems were replaced by groundwater activity; and finally, when the water sources were exhausted, a final stage of glacial processes prevailed until the present. [ABSTRACT FROM AUTHOR]

This paper performs an analysis of aeolian activity on Herschel crater on Mars from two dune fields in different topographical settings. At East field, between small simple craters, and at West the dune field is located inside a large impact crater, near to uplifted central peak. As source of data will be employed high resolution images (spatial resolution 0.25 m/pixel) and digital terrain models (DTM), with 1m / pixel spatial resolution, generated by HiRISE sensor of orbital probe Mars Reconnaissance Orbiter (MRO). Were measured horizontal parameters (Area, Azimuth length, width, and length/width ratio) and vertical parameters (height of crest (HC), height of slipface (HS), slipface slope, curvature) of 10 dunes in each field. In Field Herschel East, it is characterized by the unimodal wind regime. They have fat and free barchans on the uphill. The evolution is for axial barchanoids, with the presence of transverse dunes. It consists of a megabarchan interrupted by an impact crater 947m in diameter. The wind shadow generated by the main impact crater stands out as a phenomenon. The dunes have little azimuth variation. Ripple marks occur in the interdune region influenced by night wind and topography. Similar conditions were identified in similar terrestrial environments such as White Sands (New Mexico), Wolfe Creek (Australia) and Lagoa da Conceição (Santa Catarina). In Herschel Field West Acute Bimodal. They present normal and thin sloping downhill dunes, some anchored in Yardangs, with lots of azimuth variation. As a terrestrial analogue, we have the Liwa Oasis (United Arab Emirates / Saudi Arabia). [ABSTRACT FROM AUTHOR]

Araneiforms are enigmatic dendritic negative topography features native to Mars. Found across a variety of substrates and exhibiting a range of scales, morphologies, and activity level, they are hypothesized to form via insolation‐induced basal sublimation of seasonal CO2 ice. With no direct Earth analog, araneiforms are an example of how our understanding of extant surface features can evolve through a multipronged approach using high resolution change‐detection imaging, conceptual and numerical modeling, and analog laboratory work. This review offers a primer on the current state of knowledge of Martian araneiforms. We outline the development of their driving conceptual hypothesis and the various methodologies used to study their formation. We furthermore present open questions and identify future laboratory and modeling work and mission objectives that may address these questions. Finally, this review highlights how the study of araneiforms may be used as a proxy for local conditions and perhaps even past seasonal dynamics on Mars. We also reflect on the lessons learnt from studying them and opportunities for comparative planetology that can be harnessed in understanding unusual features on icy worlds that have no Earth analog. Plain Language Summary: Araneiforms, more colloquially coined "spiders," are strange branched networks of troughs that are carved in the Martian regolith within the south polar regions, poleward of ∼70°. They have been proposed to form in spring, when sunlight passes through and heats the Martian seasonal CO2 ice layer, causing gas to build up beneath it and crack the ice, scouring squiggly and branched troughs on the surface and depositing the eroded material in the form of a plume. Such a process does not occur on Earth, so since their original detection, scientists have used creative approaches to understand the formation of araneiforms; comprising computational mathematical modeling, small‐scale experiments in planetary chambers to recreate the process, and even citizen science campaigns, where planetary enthusiasts have helped to map their locations on Mars. We review the work that has been conducted to understand the formation of these beautiful and puzzling surface features and discuss how they may help us to understand seasonal change on Mars in the present‐day and even the past. We discuss how our understanding of araneiforms can be applied to other icy planetary surfaces and finally present gaps in our knowledge and ways to address them. Key Points: We review and synthesize the body of literature surrounding the study of Martian araneiforms over the last ∼two decadesWe provide discussions on observations made through remote sensing of araneiforms, as well as laboratory analog and numerical modeling workWe present open questions and offer ways in which they can be addressed to advance the field moving forward [ABSTRACT FROM AUTHOR]

Multiple layers of lava flows and channels characterize the region adjacent to the eastern slope of Olympus Mons, the largest volcano on Mars. We have mapped this volcanic region to survey and classify individual channel systems and determine their formative processes. As a final output of this mapping effort, we have produced a 1:1.9 million scale channel map that is first published in this paper in both GIS and static formats.

We have produced a 1:650,000 scale geomorphological map of the southwest Sinus Sabaeus, a region of Mars approximately centered at 25.0°S and 6.5°E and located in the topographic transition between Arabia Terra and Noachis Terra, in the Martian highlands. This heavily cratered region, subjected to extensive surface erosion, shows a complex valley network system known as Marikh Vallis. In this work, we study the history and role of water in and around Marikh Vallis, focusing on the modification and evolution of this area during the earliest Martian times, the Noachian period. The map described in this paper was produced through the analysis of a combination of available imagery data, topography, and thermal inertia, which together allow defining different geomorphological units in this area. This new map provides a basis for identifying the ancient presence of water in the region, both in the liquid state and in the ice phase.

We use multi-source remote sensing data to identify the details of a mascon south-east of the lunar Copernicus crater. Studies of the topography, gravity, geochronology and mineral are combined to prove that the mascon is a buried peak-ring basin with diameters of about 130 km and 260 km. The underground structure is covered by 890 m thick mare basalts, as determined by analyzing the spectral features of the impact crater, Copernicus H. The determination of the crater size–frequency distribution (CSFD) suggests that the impact that created the C-H basin occurred earlier than 3.9 Ga. Then, a Hawaiian-style eruption in the late Imbrian period formed the Sinus Aestuum-I dark mantling deposit (DMD). Soon, mare basalts covered the basin several times from 3.8 Ga. Finally, the ejecta from the Copernicus impact event at about 800 Ma, and the weathering processes caused the disappearance of the C-H basin rim from the lunar surface. [ABSTRACT FROM AUTHOR]

Wind-driven sand transport generates atmospheric dust, forms dunes, and sculpts landscapes. However, it remains unclear how the flux of particles in aeolian saltation-the wind-driven transport of sand in hopping trajectories-scales with wind speed, largely because models do not agree on how particle speeds and trajectories change with wind shear velocity. We present comprehensive measurements, from three new field sites and three published studies, showing that characteristic saltation layer heights remain approximately constant with shear velocity, in agreement with recent wind tunnel studies. These results support the assumption of constant particle speeds in recent models predicting linear scaling of saltation flux with shear stress. In contrast, our results refute widely used older models that assume that particle speed increases with shear velocity, thereby predicting nonlinear 3/2 stress-flux scaling. This conclusion is further supported by direct field measurements of saltation flux versus shear stress. Our results thus argue for adoption of linear saltation flux laws and constant saltation trajectories for modeling saltation-driven aeolian processes on Earth, Mars, and other planetary surfaces.

Multiple layers of lava flows and channels characterize the region adjacent to the eastern slope of Olympus Mons, the largest volcano on Mars. We have mapped this volcanic region to survey and classify individual channel systems and determine their formative processes. As a final output of this mapping effort, we have produced a 1:1.9 million scale channel map that is first published in this paper in both GIS and static formats. [ABSTRACT FROM AUTHOR]

The angle of repose--i.e., the angle θr between the sloping side of a heap of particles and the horizontal--provides one of the most important observables characterizing the packing and flowability of a granular material. However, this angle is determined by still poorly understood particle-scale processes, as the interactions between particles in the heap cause resistance to roll and slide under the action of gravity. A theoretical expression that predicts θr as a function of particle size and gravity would have impact in the engineering, environmental, and planetary sciences. Here we present such an expression, which we have derived from particle-based numerical simulations that account for both sliding and rolling resistance, as well as for nonbonded attractive particle-particle interactions (van der Waals). Our expression is simple and reproduces the angle of repose of experimental conical heaps as a function of particle size, as well as θr obtained from our simulations with gravity from 0.06 to 100 times that of Earth. Furthermore, we find that heaps undergo a transition from conical to irregular shape when the cohesive to gravitational force ratio exceeds a critical value, thus providing a proxy for particle-scale interactions from heap morphology. [ABSTRACT FROM AUTHOR]

We have produced a 1:650,000 scale geomorphological map of the southwest Sinus Sabaeus, a region of Mars approximately centered at 25.0°S and 6.5°E and located in the topographic transition between Arabia Terra and Noachis Terra, in the Martian highlands. This heavily cratered region, subjected to extensive surface erosion, shows a complex valley network system known as Marikh Vallis. In this work, we study the history and role of water in and around Marikh Vallis, focusing on the modification and evolution of this area during the earliest Martian times, the Noachian period. The map described in this paper was produced through the analysis of a combination of available imagery data, topography, and thermal inertia, which together allow defining different geomorphological units in this area. This new map provides a basis for identifying the ancient presence of water in the region, both in the liquid state and in the ice phase. [ABSTRACT FROM AUTHOR]

The Navua Valles are a system of channels and valleys on the inner rim of Hellas Basin. The aim of this mapping study was to determine the geologic history of the Navua Valles region; and the relationships between the basement, flow, and channel units along the northeastern slope of Hellas Basin. We have produced a 1:1 million scale geologic map of the Navua Valles region, utilizing standard USGS geologic mapping procedures, but not within a regular USGS mapping project. We selected the mapping area boundaries specifically to cover the Navua Valles drainage systems. The primary base of this mapping effort was a mosaic of 161 Mars Reconnaissance Orbiter Context Camera images, at approximately 6 m/pixel. This paper is part of a double publication, one paper describing the geology of this area, and this paper presenting the geologic map produced during the investigation.

• Overlapping crater populations constrain the total era of martian valley formation. • Martian valleys may have formed intermittently over hundreds of millions of years. • Long-term erosion rates would have been similar to the modern Atacama Desert. A number of studies have constrained the cessation of martian valley network activity to around the Noachian-Hesperian boundary, but the total time period over which these features formed remains poorly constrained. Most attempts to ascertain valley network formative timescales rely on an intermittency factor that varies widely for terrestrial rivers and is unknown for Mars. In this contribution, superposition relationships are used to identify crater populations that pre- and postdate valley network incision, resulting in maximum cumulative valley formation times of ∼108 years. These timescales correspond with long-term erosion rates of ∼0.25 m/Myr, similar to those calculated for the Middle to Late Noachian as well as the central Atacama Desert since the Late Miocene. Assuming sediment transport rates previously calculated for other martian valley networks, this implies a minimum fluvial intermittency of ∼10−5, several orders of magnitude lower than modern day Earth rivers but similar to values independently calculated for some martian fluvial deposits. This low intermittency suggests that martian valley incision may have been significantly hindered by erosion into existing bedrock or by boulder armoring of the valley floor. Alternatively, unlike river valleys on Earth, runoff events driving martian valley network activity may have been non-seasonal, Milankovitch-like cycles, with long eras of quiescence between periods of intensified fluvial activity. [ABSTRACT FROM AUTHOR]

The Navua Valles are a system of channels and valleys on the inner rim of Hellas Basin. The aim of this mapping study was to determine the geologic history of the Navua Valles region; and the relationships between the basement, flow, and channel units along the northeastern slope of Hellas Basin. We have produced a 1:1 million scale geologic map of the Navua Valles region, utilizing standard USGS geologic mapping procedures, but not within a regular USGS mapping project. We selected the mapping area boundaries specifically to cover the Navua Valles drainage systems. The primary base of this mapping effort was a mosaic of 161 Mars Reconnaissance Orbiter Context Camera images, at approximately 6 m/pixel. This paper is part of a double publication, one paper describing the geology of this area, and this paper presenting the geologic map produced during the investigation. [ABSTRACT FROM AUTHOR]

This study explores validating and calibrating the wind regime predicted by Global Circulation Models (GCM) on Earth and other planets using optical remote sensing of dune dynamics. We use Spot-5 images to track the migration of 64 Barchan dunes in Qatar using the COSI-Corr technique. We estimate the volume of the dunes using a scaling law calibrated from one particular dune, which was surveyed in the field. Using volume and migration rate, we determine the sand flux from a single dune, Q D u n e s , and scale this estimate to the whole dune field. We compare the measured sand flux with those derived from wind velocity measurements at a local meteorological station as well as with those predicted from ERA-Interim (a Global Circulation Model). The comparison revealed that the wind velocity predicted by ERA-Interim is inappropriate to calculate the sand flux. This is due to the 6-h sampling rate and to systematic bias revealed by a comparison with the local wind data. We describe a simple procedure to correct for these effects. With the proposed correction, similar sand flux are predicted using the local and ERA-Interim data, independently of the value of the value of the shear velocity threshold, u ⁎ t . The predicted sand flux is about 65% of Q D u n e s . The agreement is best assuming the value u ⁎ t = 0.244 m/s , which is only slightly larger than the value of u ⁎ t = 0.2612 m/s estimated based in the sand granulometry measured from field samples. The influence of the dune topography on the wind velocity field could explain the underestimation. In any case, the study demonstrates the possibility of validating GCM model and calibrating aeolian sand transport laws using remote sensing measurements of dune dynamics and highlights the caveats associated to such an approach. [ABSTRACT FROM AUTHOR]

Martian gullies resemble gullies carved by water on Earth, yet are thought to have formed in an extremely cold ( < − 50 °C ) and dry (humidity <100 precipitable micrometers) surface environment (cf. Mellon et al., 2004 ). Despite more than a decade of observations, no consensus has emerged as to whether liquid water is required to form martian gullies, with some recent studies favouring dry CO 2 -driven processes. That this argument persists demonstrates the limitations of morphological interpretations made from 2D images, especially when similar-looking landforms can form by very different processes. To overcome this we have devised a parameterization scheme, based on statistical discriminant analysis and hydrological terrain analysis of meter-scale digital topography data, which can distinguish between dry and wet surface processes acting on a landscape. Applying this approach to new meter-scale topographic datasets of Earth, the Moon and Mars, we demonstrate that martian gullied slopes are dissimilar to dry, gullied slopes on Earth and the Moon, but are similar to both terrestrial debris flows and fluvial gullies. We conclude that liquid water was integral to the process by which martian gullies formed. Finally, our work shows that quantitative 3D analyses of landscape have great potential as a tool in planetary science, enabling remote assessment of processes acting on planetary surfaces. [ABSTRACT FROM AUTHOR]

Morphological evidence for ancient channelized flows (fluvial and fluvial-like landforms) exists on the surfaces of all of the inner planets and on some of the satellites of the Solar System. In some cases, the relevant fluid flows are related to a planetary evolution that involves the global cycling of a volatile component (water for Earth and Mars; methane for Saturn's moon Titan). In other cases, as on Mercury, Venus, Earth's moon, and Jupiter's moon Io, the flows were of highly fluid lava. The discovery, in 1972, of what are now known to be fluvial channels and valleys on Mars sparked a major controversy over the role of water in shaping the surface of that planet. The recognition of the fluvial character of these features has opened unresolved fundamental questions about the geological history of water on Mars, including the presence of an ancient ocean and the operation of a hydrological cycle during the earliest phases of planetary history. Other fundamental questions posed by fluvial and fluvial-like features on planetary bodies include the possible erosive action of large-scale outpourings of very fluid lavas, such as those that may have produced the remarkable canali forms on Venus; the ability of exotic fluids, such as methane, to create fluvial-like landforms, as observed on Saturn's moon, Titan; and the nature of sedimentation and erosion under different conditions of planetary surface gravity. Planetary fluvial geomorphology also illustrates fundamental epistemological and methodological issues, including the role of analogy in geomorphological/geological inquiry. [ABSTRACT FROM AUTHOR]

Impact cratering is the one geologic process which is common to all solar system objects. Impact craters form by the resulting explosion between a solar system body and hypervelocity objects. Comparison with craters formed by chemical and nuclear explosions reveals that crater diameter is related to other morphometric characteristics of the crater, such as depth and rim height. These relationships allow scientists to use impact craters to probe the subsurface structure within the upper few kilometer of a planetary surface and to estimate the amounts and types of degradational processes which have affected the planet since crater formation. Crater size–frequency distribution analysis provides the primary mechanism for determining ages of planetary terrains and constraining the timing of resurfacing episodes. Thus, impact craters provide many important insights into the evolution of planetary surfaces. [ABSTRACT FROM AUTHOR]

Three broad themes from the history of planetary geomorphology provide lessons in regard to the logic (valid reasoning processes) for the doing of that science. The long controversy over the origin of lunar craters, which was dominated for three centuries by the volcanic hypothesis, provides examples of reasoning on the basis of authority and a priori presumptions. Percival Lowell's controversy with geologists over the nature of linear markings on the surface of Mars illustrates the role of tenacity in regard to the beliefs of some individual scientists. Finally, modern controversies over the role of water in shaping the surface of Mars illustrate how the a priori method, i.e., belief produced according to reason, can seductively cloud the scientific openness to the importance of brute facts that deviate from a prevailing paradigm. [ABSTRACT FROM AUTHOR]

Planetary geomorphology is the study of extraterrestrial landscapes. In recognition of the promise for productive interaction between terrestrial and planetary geomorphologists, the 45th annual Binghamton Geomorphology Symposium (BGS) focused on Planetary Geomorphology. The aim of the symposium was to bring planetary and terrestrial geomorphologists together for symbiotic and synthetic interactions that would enrich both subdisciplines. In acknowledgment of the crucial role of terrestrial field work in planetary geomorphology and of the BGS tradition, the symposium began with a field trip to the Appalachian Mountains, followed by a dinner talk of recent results from the Mars Surface Laboratory. On Saturday and Sunday, the symposium was organized around major themes in planetary geomorphology, starting with the geomorphic processes that are most common in our Solar System—impact cratering, tectonism, volcanism—to set the stage for other geomorphic processes, including aeolian, fluvial, lacustrine, and glacial/polar. On Saturday evening, the banquet talk provided an historical overview of planetary geomorphology, including its roots in the terrestrial geosciences. The symposium concluded with a full-afternoon tutorial on planetary geomorphologic datasets. This special issue of Geomorphology consists of papers by invited authors from the 2014 BGS, and this introduction provides some context for these papers. [ABSTRACT FROM AUTHOR]

Aeolian dune fields have been described on Earth, Mars, Venus, and Titan. The amount and fidelity of data being returned from orbiting spacecraft and landers have enabled a new era in aeolian studies. This progress report presents an overview of the latest planetary geomorphic studies characterizing aeolian processes on extraterrestrial surfaces. Our understanding of aeolian processes on other planetary surfaces comes largely from Earth analog studies, along with wind tunnel experiments and theoretical modeling. However, an important difference is that unlike terrestrial dunes most dunes on Venus and Mars are composed primarily of basaltic particles. Additional research is needed to understand how basaltic particles weather both physically and chemically so that it will be possible to apply traditional sedimentological concepts, such as sediment maturity, to understanding aeolian processes on Venus and Mars. It may also be possible to characterize sediment maturity and provenance through remote sensing data once we have a better understanding of basaltic sediments. Although there have been a variety of dune forms identified on the surfaces of the other terrestrial planets, the only dune form found on all of them is linear dunes. Even though linear dunes are the most common dune forms on Earth, we currently have a poor understanding as to how they are formed, and additional work is needed to understand these features. [ABSTRACT FROM AUTHOR]

Abstract: In a study area spanning the martian surface poleward of 50° S., 1190 dune fields have been identified, mapped, and categorized based on dune field morphology. Dune fields in the study area span ∼ 116400km2, leading to a global dune field coverage estimate of ∼904000km2, far less than that found on Earth. Based on distinct morphological features, the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding. The six dune field classes occur in specific latitude zones, with a sequence of reduced activity and degradation progressing poleward. In particular, the first signs of stabilization appear at ∼60° S., which broadly corresponds to the edge of high concentrations of water-equivalent hydrogen content (observed by the Neutron Spectrometer) that have been interpreted as ground ice. This near-surface ground ice likely acts to reduce sand availability in the present climate state on Mars, stabilizing high latitude dunes and allowing erosional processes to change their morphology. As a result, climatic changes in the content of near-surface ground ice are likely to influence the level of dune activity. Spatial variation of dune field classes with longitude is significant, suggesting that local conditions play a major role in determining dune field activity level. Dune fields on the south polar layered terrain, for example, appear either potentially active or inactive, indicating that at least two generations of dune building have occurred on this surface. Many dune fields show signs of degradation mixed with crisp-brinked dunes, also suggesting that more than one generation of dune building has occurred since they originally formed. Dune fields superposed on early and late Amazonian surfaces provide potential upper age limits of ∼100My on the south polar layered deposits and ∼3Ga elsewhere at high latitudes. No craters are present on any identifiable dune fields, which can provide a lower age limit through crater counting: assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age, their estimated crater retention age is <∼10000years. An average-sized uncratered dune field (94km2) has a crater retention age <∼8My. This apparent youth suggests that present-day climate conditions are responsible for the observed degradation and reduced level of aeolian activity. A lack of observed transport pathways and the absence of large dune fields in the largest basins (Hellas and Argyre Planitiae) are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on Mars. [Copyright &y& Elsevier]

Abstract: We investigate the geomorphology and chronology of catastrophic flooding in a major martian outflow channel, Ares Vallis. We use recently acquired stereo and colour images and derived topographic data with grid-spacing of 50m from the High Resolution Stereo Camera (HRSC) onboard Mars Express to constrain the detailed flood geomorphology of proximal Ares Vallis. We identify morphologic evidence for 6 distinct erosion surfaces that were likely formed by multiple episodes of catastrophic flooding. These surfaces occur at distinct topographic levels, exhibit multiple sets of flood grooves, and contain distinct post-flood crater populations. Crater count statistics on the flood-eroded surfaces from MRO Context Camera (CTX) and Thermal Emission Imaging System (THEMIS) visible images indicate that flooding was initiated during the Late Noachian/Early Hesperian at ∼3.6Ga. The morphology data confirm that the oldest flooding events occurred on the topographically highest erosion surfaces while subsequent floods abandoned the upper surfaces and continued to modify craters in the topographically lowest, main channel of Ares Vallis into the Late Hesperian/Early Amazonian (∼2.5Ga). Our results suggest that release of water to Ares Vallis likely occurred at repeated intervals over a long period of geological time, thus supporting recent numerical modelling studies that suggest a recharging water source. [Copyright &y& Elsevier]

Abstract: We present evidence, drawn from new 25 cm/pixel “HiRISE” images of Mars, of landforms indicative of geologically-recent thaw degradation of ice-rich terrain near the Martian equator. We have focused our study on the head-region of Athabasca Vallis, a catastrophic flood-carved outflow channel thought to have been active within the last 2–8 Ma. The geomorphology of the study area is characterised by a series of polygonally-patterned surfaces that are bounded by erosional scarps. The scarps are indented with cirque-shaped niches that appear to be controlled in scale and form by the pre-existing polygonal sculpture. Shallow basins, defined by niche-indented scarps, are also seen and contain pitted “mound and cone” structures that are similar in form to terrestrial pingos. Associated with many of these scarps are dendritic channels and gullies, blocky debris and hummocky terminal deposits. Together, these correspond morphologically to terrestrial “retrogressive thaw slumps” which are landforms typical of thermokarst environments. We have also observed an evolutionary sequence of morphologies beginning with subsidence of polygonised surfaces and ending with scarp bounded basins fed by inter-polygon channels. This is consistent with the former presence of surficial liquid water released by ground-ice thaw and is again indicative of thermokarst. Moreover, the observations of epigenetic polygons and channels, and of polycyclic retrogressive thaw slumps, suggest that thaw and standing water persisted for many seasonal cycles. The assemblage of landforms, the occurrence of polygons and pingo-like forms within erosional basins, and observations of fluvial-like channels cross-cutting pre-existing surface features are consistent with a surface that has evolved significantly since it was originally emplaced. This precludes this terrain being a volcanic (i.e., primary) landscape, as has been suggested in the past. The formation of this landform assemblage requires that there was thaw of ground ice and persistent liquid water at the surface. Given the geologically young flood deposits in which this assemblage occurs, our observations point to a recent period in which Mars’ climate supported thaw conditions in this region. [Copyright &y& Elsevier]

The study of how rivers cross obstructing mountains, once popular in the early twentieth century, has seen a dramatic resurgence in the last decade. Since Hutton's scholarly introduction to a possible cause for transverse drainage, however, no single study has compiled all of the various criteria that can be used to discriminate among the four possible mechanisms of antecedence, superimposition, overflow, and piracy. This paper presents the first such compilation and related methodology to apply these criteria both in tabular and graphical formats, as well as an online interactive tool in the data repository. Combining nominal and ordinal data sources, this methodology generates an objective, reproducible assessment for the mechanism most likely to have established the transverse drainage at five ordinal levels of confidence. When applied to southwestern U.S. sites, randomly selected through an objective spatial procedure, four general observations emerged on the relationship between the development of transverse drainage and landscape evolution. (1) Streams persisting through lengthy periods of extension develop antecedent canyons. (2) In order to reestablish through-flowing channels, streams overflow closed basins as active extension wanes. (3) Following a drop in base level related to the newly developed trunk channels, streams tributary to the trunk channels incise into basin-fill deposits--sometimes leading to the development of superimposed drainages. (4) Tributaries eroding headward, in response to the integration of two or more closed basins, can capture and redirect drainage; this permits transverse drainage through both piracy and superimposition upstream of the capture event. Because extant criteria use nominal and ordinal data almost entirely, considerable potential exists to refine this approach through future strategies that incorporate interval data. Future use of a criteria-based method has the potential to inform on prior geomorphic studies by providing a new perspective with which to study how basins evolve in active tectonic regions, the analysis of related basin sedimentation, the hydrological and biological aspects of drainage evolution, and transverse drainage found in Martian crater fields. [ABSTRACT FROM AUTHOR]

Abstract: Aeolian processes are probably the dominant ongoing surface process on Mars; Large Dark Dunes (LDDs), particularly common aeolian landforms, were first recognized in the early 1970s. Recent, higher resolution images have revealed another, morphologically distinct, large population of smaller, ripple-like aeolian bedforms that have been termed “Transverse Aeolian Ridges” (TARs) as it is unknown whether they formed as large ripples or small dunes. We have begun a new study of TARs that examines their distribution, orientation, and morphology using >10,000 high-resolution Mars Orbiter Camera (1.5 to 8 m/pixel resolution) images in a 45° longitude wide, pole-to-pole survey. The aim of this study is to assess whether TARs are active, to identify possible sediment sources and pathways, and to determine the volumes of sediment that they comprise. We present results from the first half of this study, in which we examine the northern hemisphere, and describe a new three-part classification scheme used to aid the survey. Our results show that TARs are abundant but not ubiquitous: preferentially forming proximal to friable, layered terrains such as those found in Terra Meridiani — the location of the ongoing Mars Exploration Rover “Opportunity” mission. TAR distribution in the northern hemisphere shows a strong latitudinal dependence with very few TARs being found north of ∼30° N. We also find that in most cases TARs are less mobile than LDDs, a conclusion possibly explained by Mars Exploration Rover Opportunity observations that show TAR-like ripples to have a core of fine material armored by a monolayer of granule-sized particles. This could disallow significant bedform movement under the current wind regime. That TARs are essentially inactive is confirmed by superposition relations with slope streaks and LDDs and by observations of superposed impact craters. We suggest that observations made by the Opportunity Rover in Terra Meridiani indicate that the small aeolian bedforms common here are ripples and not small dunes. Farther south, these bedforms transition into larger features indistinguishable from TARs, suggesting that TARs (in the Meridiani area at least) are ripples and not dunes. [Copyright &y& Elsevier]

In deserts, the interplay between occasional fluvial events and persistent aeolian erosion can form composite modern and relict surfaces, especially on the distal portion of alluvial fans. There, relief inversion of alluvial deposits by differential erosion can form longitudinal ridges. We identified two distinct ridge types formed by relief inversion on converging alluvial fans in the hyperarid Chilean Atacama Desert. Although they are co-located and similar in scale, the ridge types have different ages and formation histories that apparently correspond to minor paleoclimate variations. Gravel-armored ridges are remnants of deflated alluvial deposits with a bimodal sediment distribution (gravel and sand) dated to a minor pluvial phase at the end of the Late Pleistocene (~12 kyr). In contrast, younger (~9 kyr) sulfate-capped ridges formed during a minor arid phase with evaporite deposition in a pre-existing channel that armored the underlying deposits. Collectively, inverted channels at Salar de Llamara resulted from multiple episodes of surface overland flow and standing water spanning several thousand years. Based on ridge relief and age, the minimum long-term deflation rate is 0.1–0.2 m/kyr, driven primarily by wind erosion. This case study is an example of the equifinality concept whereby different processes lead to similar landforms. The complex history of the two ridge types can only be generally constrained in remotely sensed data. In situ observations are required to discern the specifics of the aqueous history, including the flow type, magnitude, sequence, and paleoenvironment. These findings have relevance for interpreting similar landforms on Mars. • Two inverted channel types co-located on a bajada have different histories. • Wind erosion of gravel-bearing mudflows formed a clast-armored inverted channel. • Evaporites deposited in pre-existing channels were eroded to sulfate-capped ridges. • Key events that led to relief inversion may link to minor paleoclimate variations. [ABSTRACT FROM AUTHOR]

The Puna Plateau of northwestern Argentina is a promising region for analogue investigations of Martian aeolian processes owing to its high altitude, low atmospheric pressure, aridity, geology, and widespread granular and bedrock aeolian landforms. Despite a growing body of research about the region's enigmatic aeolian landforms, little is known about the contemporary aeolian environment, processes, and landscape dynamics. To address this gap, we used a combination of modelled weather data, satellite imagery, and field measurements to characterize the wind regime, aeolian sediment transport, and allied surface changes. The study focused on the Campo de Piedra Pómez (CPP) – a volcanic landscape with a variety of Mars analogue landforms: gravel-mantled megaripples, yardangs, ventifacts, periodic bedrock ridges, and aeolian rat-tails. Modelled weather data from the European Center for Medium-Range Forecasting's ERA-Interim reanalysis dataset confirm the contemporary dominance of northwesterly winds in this region, consistent with the primary orientation of yardangs. These data also indicate the presence of less frequent southeasterly winds, which may explain why re-entrant profiles are observed on the southeast flanks of a small portion of the yardang population, and why some aeolian rat-tails point to the northwest. Observations from multi-temporal satellite imagery indicate that deposition and displacement of small sediment patches overlying ignimbrite bedrock is relatively common across the region. Samples retrieved from sediment traps deployed over one year in the region suggest a broad range of particle densities are mobilized by wind, which implies a wide range of wind speed for fluid entrainment. Conventional aeolian transport models do not consider the effects of particle density variations on transport processes. However, in areas with multiple density sediment present, our results point towards a need for aeolian models to consider density when quantifying and qualifying sediment transport. • Remote sensing, in situ data, and climate reanalyses are used to characterize the aeolian regime at a Mars analogue site; • Change detection attests to widespread sediment transport despite low modelled wind speeds; • Thermally-driven winds caused by albedo may be responsible for sediment transport from non-dominant directions; • Documented transport of particles with a wide range of sizes and densities; • In multimodal density particle areas careful consideration of sediment transport equations are needed. [ABSTRACT FROM AUTHOR]