Search

Pluto's Sputnik basin is the only large impact basin in the outer Solar System that resembles basins in the inner Solar System in morphology and scale, with dimensions of ∼1800 × 1000 km and a depth of ∼2.5 km. Sputnik basin has been compared to giant impact basins without prominent topographic rings, such as the Hellas basin on Mars and South Pole-Aitken basin on the Moon. However, a north-south trending arcuate chain of water-ice mountain blocks within the western half of the basin resembles a peak-ring structure as found in impact basins on terrestrial bodies, suggesting that Sputnik basin may be more analogous to a peak-ring or multiring basin. Here, we perform a comparative analysis between Sputnik basin and 23 impact basins in the inner Solar System. We reconstruct Sputnik basin's topography prior to being partially filled with an N 2 -rich ice deposit. In our analyses, we measure key characteristics such as average radial topography, ring spacing and crustal/shell thinning beneath the basins. We find that Sputnik basin is morphologically, topographically, and structurally consistent with the peak−/multiring basins, having most similarities to Apollo and Humboldtianum basins on the Moon, rather than giant impact basins without prominent topographic rings. Sputnik's consistency with a peak−/multiring structure has implications for aspects of its formation, evolution, and structure. In particular, this new interpretation of the basin structure decreases the size of both the impactor and the possible mass anomaly contained within the basin by half relative to previous interpretations. • Sputnik basin is a large impact basin that resembles inner Solar System basins. • Newly reconstructed Sputnik pre-fill topography illuminates subsurface structure. • Sputnik is topographically and structurally consistent with peak−/multiring basins. • New interpretation decreases the predicted size of impactor and mass anomaly by half. [ABSTRACT FROM AUTHOR]

The Orientale basin is the youngest and best-preserved multiring impact basin on the Moon, having experienced only modest modification by subsequent impacts and volcanism. Orientale is often treated as the type example of a multiring basin, with three prominent rings outside of the inner depression: the Inner Rook Montes, the Outer Rook Montes, and the Cordillera. Here we use gravity data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission to reveal the subsurface structure of Orientale and its ring system. Gradients of the gravity data reveal a continuous ring dike intruded into the Outer Rook along the plane of the fault associated with the ring scarp. The volume of this ring dike is ∼18 times greater than the volume of all extrusive mare deposits associated with the basin. The gravity gradient signature of the Cordillera ring indicates an offset along the fault across a shallow density interface, interpreted to be the base of the low-density ejecta blanket. Both gravity gradients and crustal thickness models indicate that the edge of the central cavity is shifted inward relative to the equivalent Inner Rook ring at the surface. Models of the deep basin structure show inflections along the crust–mantle interface at both the Outer Rook and Cordillera rings, indicating that the basin ring faults extend from the surface to at least the base of the crust. Fault dips range from 13° to 22° for the Cordillera fault in the northeastern quadrant, to 90° for the Outer Rook in the northwestern quadrant. The fault dips for both outer rings are lowest in the northeast, possibly due to the effects of either the direction of projectile motion or regional gradients in pre-impact crustal thickness. Similar ring dikes and ring faults are observed around the majority of lunar basins. [ABSTRACT FROM AUTHOR]

Observations of lakes, fluvial dissection of the surface, rapid variations in cloud cover, and lake shoreline changes indicate that Saturn's moon Titan is hydrologically active, with a hydrocarbon-based hydrological cycle dominated by liquid methane. Here we use a numerical model to investigate the Titan hydrological cycle – including surface, subsurface, and atmospheric components – in order to investigate the underlying causes of the observed distribution and sizes of lakes in the north polar region. The hydrocarbon-based hydrological cycle is modeled using a numerical subsurface flow model and analytical runoff scheme, driven by a general circulation model with an active methane-cycle. This model is run on synthetically generated topography that matches the fractal character of the observed topography, without explicit representation of the effects of erosion and deposition. At the scale of individual basins, intermediate to high permeability (10 −8 –10 −6 cm 2 ) aquifers are required to reproduce the observed large stable lakes. However, at the scale of the entire north polar lake district, a high permeability aquifer results in the rapid flushing of methane through the aquifer from high polar latitudes to dry lower polar latitudes, where methane is removed by evaporation, preventing large lakes from forming. In contrast, an intermediate permeability aquifer slows the subsurface flow from high polar latitudes, allowing greater lake areas. The observed distribution of lakes is best matched by either a uniform intermediate permeability aquifer, or a combination of a high permeability cap at high latitudes surrounded by an intermediate permeability aquifer at lower latitudes, as could arise due to karstic processes at the north pole. The stability of Kraken Mare further requires reduction of the evaporation rate over the sea to 1% of the value predicted by the general circulation model, likely as a result of dissolved ethane, nitrogen, or organic solutes, and/or a climatic lake effect. These results reveal that subsurface flow through aquifers plays an important role in Titan's hydrological cycle, and exerts a strong influence over the distribution, size, and volatile budgets of Titan's lakes. [ABSTRACT FROM AUTHOR]

A large set of linear to arcuate gravity anomalies which are not associated with any surface expressions was revealed by lunar gravity data from the GRAIL mission. The anomalies can be categorized into two types: a set of large anomalies that border the Procellarum KREEP terrane (PKT) region, and the linear gravity anomalies (LGAs) that are scattered throughout the lunar highlands. In this study, we use band-passed gravity gradient maps and localized power spectral analyses to characterize the nature of and the differences between the two types of anomalies. The results show that the PKT border anomalies are primarily long wavelength features whose power spectra are not clearly distinguishable from the background spectrum of the surrounding region, while the linear gravity anomalies are short wavelength features whose power spectra in at least one case rises significantly above the background spectrum over a wide range of degrees. This difference in gravitational signature suggests a fundamental difference in the nature of the sources of the two types of anomalies. We then used a Markov chain Monte Carlo model to test different interpretations for the most prominent LGA by comparing the observed power spectrum to modeled spectra for elliptical, triangular, and T-shaped intrusive geometries. We find multiple geometries are able to fit the power spectrum of the LGA equally well. Analogs of comparable scale to the LGAs originate from extensional stress regimes, supporting an inferred period of global expansion in the early Moon, though the cause of that expansion remains uncertain. If the depth extent of the intrusions were governed by neutral buoyancy, we find that intrusions on the nearside similar to those on the farside would have been eruptive and may have contributed to the earliest mare volcanism. The total volume of the intrusions is ~20% of the mare volume, revealing a lower magma production rate on the farside than on the nearside. • Lunar linear gravity anomalies fundamentally different from PKT border anomalies in nature and origin. • Prominent northern farside linear gravity anomaly (~600 km length) has a best-fit top depth of ~10 km. • Inferred intrusions beneath large Martian collapse pits perhaps best analog to lunar linear gravity anomalies. [ABSTRACT FROM AUTHOR]

Highlights: [•] Bouguer gravity anomalies around Orientale may arise from faults beneath the rings. [•] We inverted the Bouguer gravity gradients for the properties of the faults and crust. [•] Gravity data supports a density stratified crust and a fault beneath the Cordillera. [•] The existence of a fault beneath the Outer Rook cannot be confirmed. [Copyright &y& Elsevier]

Abstract: Many lunar basins are characterized by prominent positive gravity anomalies over the basin interiors, referred to as mass concentrations or mascons. While a significant fraction of some near-side mascon anomalies can be explained as a result of the flexural support of the mare basalts within the basins, a number of basins, including Orientale, exhibit mascons in excess of those that can be plausibly ascribed to the mare. Some basins exhibit mascons but lack mare altogether. Lunar gravity and topography data are used to map the isostatic anomaly, or the height of the surface above or below its isostatic level. Orientale is representative of the majority of lunar basins, in which the super-isostatic basin center is surrounded by a sub-isostatic annulus of comparable magnitude but greater area. The basin structure as a whole is found to be strongly sub-isostatic. High-resolution crustal thickness models of Orientale confirm that it is surrounded by an annulus of thickened but sub-isostatic crust. It is proposed that the flexural uplift of the annulus causes the uplift and positive gravity anomalies within the basin center. Finite element models are used to examine the flexural uplift of the sub-isostatic annulus and the basin center for a range of lithosphere thicknesses both outside the basin and in the basin interior. The uplift of the basin center can exceed 2km, increasing the central gravity anomaly by ∼200mGal. This annular uplift explains a significant fraction of the Orientale mascon, and is likely a dominant cause of non-mare mascons globally. [Copyright &y& Elsevier]

Abstract: The sedimentary deposits of Meridiani Planum formed during the martian climate transition at the Noachian–Hesperian boundary between warm–wet and cold–dry conditions, and give valuable insight into how and when this transition occurred. We show that these deposits share characteristics with sedimentary outcrops across Arabia Terra. Despite near-ubiquitous dust cover across much of Arabia Terra, spectral signatures of polyhydrated sulfate minerals resembling those in Meridiani were identified in Schiaparelli and another unnamed crater. An erosional morphology analysis using both image and topographic data was then used to identify morphologies characteristic of Meridiani-type deposits and catalogue their occurrences throughout Arabia Terra. The occurrences of deposits with compositions and morphologies resembling the Meridiani deposits throughout Arabia Terra suggest that Meridiani-type sedimentary rocks were once more widespread. Elevations of the eroded remnants were used to reconstruct the pre-erosional paleo-surface of the deposits. Within this study area, these deposits once covered ∼2.5–3.6×106 km2 and represent an eroded volume of 0.9–1.7×106 km3 of sediment. Crater retention ages using craters of a range of preservation states and stratigraphic levels reveal that the deposits were laid down and subsequently eroded during a ∼270Myr period between ∼3.83 and 3.56Ga. The deposits formed following the transition from fluvial dissection to evaporite deposition at the end of the Noachian. The high erosion rates (∼3×10−6 m/yr) suggest that Mars may have maintained a thick atmosphere relative to today even as it dried out in the Early Hesperian. [Copyright &y& Elsevier]

The geodynamic history of Mars is written largely in the form of its tectonic structures. A global population of compressional structures records a history of interior cooling and contraction, in combination with volcanic loading and flexure of the lithosphere. The most common of these structures are wrinkle ridges, interpreted as having formed by folding of near-surface layered volcanic materials above blind thrust faults. Topographic profiles of wrinkle ridges are found to dominantly fall into three classes: symmetric ridges, arch-style ridges characterized by broad convex upward backlimbs, and double ridges characterized by a broad arch and narrow ridge. A Monte Carlo boundary element model was used to constrain the underlying geometry of faults associated with the different ridge types. Symmetric ridges are pop-up structures formed between quasi-symmetric thrust and backthrust faults, in some cases above a horizontal décollement or low angle fault at depth. Arch-style ridges formed above a dominant primary thrust fault of near constant dip, with a secondary backthrust playing a lesser role in generating subtle inflections in the backlimb. Double ridges require two backthrust faults to localize strain within the narrow ridge at the ridge crest and to generate the secondary antiform on the backlimb. In these ridges, both the upper primary thrust and backthrust are contained within the layered volcanic materials, while the lower portions of the primary thrust can extend deep into the crust or upper mantle. A shallower depth of fault penetration in areas of thicker crust is consistent with the depth of faulting being limited by the brittle-ductile transition. Dips vary widely, but are typically steep in the upper faults and quite gentle in the lower faults. Thus, while wrinkle ridges do provide important records of tectonic and geodynamic processes on Mars as well as other bodies, the associated tectonic structures are more complex and varied than previously considered. • Three classes of wrinkle ridges: symmetric, arch-style, and double ridges • Symmetric ridges are pop-up structures between symmetric thrusts and backthrusts. • Arch-style ridges formed above a dominant primary thrust of near constant dip. • Double ridges require two backthrust faults. • Depth of faulting is consistent with limitation by the brittle-ductile transition. [ABSTRACT FROM AUTHOR]

Radar images from the Magellan mission reveal compressional ridge belts transecting several low-lying plains on Venus, including Atalanta, Vinmara, Lavinia, and Rusalka/Llorona/Vellamo Planitiae. Vedma Dorsa is an ∼1800 km long ridge belt with 0.5–1 km of relief in Llorona/Vellamo Planitiae. We analyze sections of the ridge belt that have an asymmetric topographic cross section, indicative of deep thrust faulting and shallow folding as the dominant formation mechanisms. We perform elastic dislocation and Monte Carlo tectonic modeling of thrust faulting at several locations along Vedma Dorsa to constrain the fault displacement (D), depth of faulting (z), and fault dip (θ). We find that the models are consistent with blind thrust faulting and folding as the dominant mechanism for producing Vedma, with preferred fault parameters of D ∼ 1–2 km, z ∼ 10–25 km, and θ ∼ 25–30°. The results support an upper fault tip >3 km below the surface (blind fault), and a change in dip at depth. Our preferred models and the additional fault complexities supported at each site suggest that this section of Vedma is an intermediate structure between lobate scraps and fold and thrust belts, incorporating morphometric aspects from both classifications. Observations of terrestrial fault depths and the best-fitting depth of faulting from our models lead to an estimated heat flux of 16–39 mW m−2 at the time of Vedma Dorsa's formation, consistent with formation over a cold, downwelling region of the mantle. • Vedma Dorsa ridge belt topography is consistent with blind thrust faulting at depth. • Modeling suggests faults have dips of 25–30°, depths 10–25 km, and offsets of 1–2 km. • Faulted Layer thickness range leads to heat flux estimate of 16–39 mW m−2. • Vedma Dorsa may be an intermediate tectonic structure similar to a fold-and-thrust belt. [ABSTRACT FROM AUTHOR]

Volcanic activity on Mars peaked during the Noachian and Hesperian periods but has continued since then in isolated locales. Elysium Planitia hosts numerous young, fissure-fed flood lavas with ages ranging from approximately 500 to 2.5 million years (Ma). We present evidence for a fine-grained unit that is atypical of aeolian deposits in the region and may be the youngest volcanic deposit yet documented on Mars. The unit has a low albedo, high thermal inertia, includes high‑calcium pyroxene-rich material, and is distributed symmetrically around a segment of the Cerberus Fossae fissure system in Elysium Planitia. This deposit is superficially similar to features interpreted as pyroclastic deposits on the Moon and Mercury. Unlike previously documented lava flows in Elysium Planitia, this feature is morphologically consistent with a fissure-fed pyroclastic deposit, mantling the surrounding lava flows with a thickness on the order of tens of cm over most of the deposit and a volume of 1.1–2.8 × 107 m3. Thickness and volume estimates are consistent with tephra deposits on Earth. Stratigraphic relationships indicate a relative age younger than the surrounding volcanic plains and the Zunil impact crater (~0.1–1 Ma), with crater counting suggesting that the deposit has an absolute model age of 53 ± 7 to 210 ± 12 ka. This young age implies that if this deposit is volcanic then the Cerberus Fossae region may not be extinct and that Mars may still be volcanically active. This interpretation is consistent with the identification of seismicity in this region by the Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport (InSight) lander, and has additional implications for astrobiology. • We present evidence of the youngest volcanic product discovered to date on Mars. • The morphology and properties of the deposit suggest a pyroclastic origin. • The age of the deposit has implications for the seismicity observed by InSight. [ABSTRACT FROM AUTHOR]