On Earth and Mars, aeolian transport causes sand grains to become abraded, resulting in mineralogic and textural changes. Understanding how sands evolve, or mature, with transport via experimental studies is important for understanding the origins, geologic history, and cycling of sediments, as well as dust production. Previous experimental works have used a variety of methods to simulate aeolian transport in the laboratory, but practical limitations and similitude concerns have limited such research. Here, we present and validate the Sand AbrasioN Device for Aeolian Research (SANDAR), a modified air mill that uses pressurized air to circulate sand around a small abrasion chamber, simulating the effects of aeolian transport. This device is re-circulating to simulate long-distance transport, and it allows for repeated analyses of well-constrained sediment samples, revealing their evolution over time. It is compatible with the grain sizes (74-500 mu m) and grain impact velocities (similar to 0.6-3.7 m/s) typically expected for natural aeolian environments, and is also adaptable for diverse applications simulating different wind conditions. We show that the SANDAR achieves similitude of kinetic energy with respect to saltating sand on both Earth and Mars. SEM and optical microscope imaging reveal that the SANDAR produces microtextures on the surfaces of sand grains similar to those found with natural aeolian transport, demonstrating that it effectively simulates the mechanical effects of aeolian processes. Thus, the SANDAR is a valid tool for use in experimental research to improve our understanding of sedimentary processes across the Solar System.

Mercury hosts widespread smooth plains that are concentrated in the Caloris impact basin, in an annulus surrounding the Caloris basin, and in the adjacent northern smooth plains. The origins of these smooth plains are uncertain, although prior work suggests these plains in the northwestern Caloris annulus might reflect volcanic activity, impact ejecta, or a combination of the two. Deciphering the timing and mode of emplacement of these plains would provide a critical constraint on regional late-stage volcanism or impact effects. In this work, the region northwest of Caloris was investigated using geomorphological and color-based mapping, crater counting techniques, and spectral analyses with the goal of placing constraints on the source of the observed units and identifying the primary emplacement mechanism. Mapping and spectral analyses confirm previous findings of two distinct, yet intermingled, units within these plains, each with similar crater count model ages that postdate the formation of the Caloris impact basin. Mapping, spectra analysis, ages, and the identification of potential flow pathways are more consistent with a predominantly volcanic origin for the smooth plains materials, although these data do not rule out contributions from impact ejecta or impact melt. We propose several hypothetical scenarios, including post-emplacement modification by near-surface volatiles, to explain these observations and clarify the emplacement mechanism for these specific smooth plains regions. Further observations from the BepiColombo mission should provide data to potentially address the outstanding questions from this work.

Grain sizes of Martian sand dunes are critical sedimentological data on sand provenance and transport pathways. Thermal inertia values are used to characterize the grain sizes of dune sand. Most early characterizations involved single dune fields. Recent work based on global data sets has provided more wide-spread dune sand locations, though these data sets include the non-sandy interdune areas. To provide a more accurate grain size characterization, we leverage a global thermal inertia data set, a global dune database and a global imaging mosaic to develop a freeware-based methodology for deriving grain sizes. This methodology involves delineation of sand-only areas within dune fields and collection of thermal inertia values from those areas. We consider a unimodal histogram of values with a mode <similar to 350 thermal inertia units (J m(-2) K-1 s(-1/2)) to imply an effective exclusion of non-sand surfaces. Application of this methodology to dune fields for which thermal inertia values have been previous derived shows our results fall within the envelope of those values. We apply our methodology to tropical dune fields on Mars for which Dust Cover Index data imply dust-free surfaces. Conversion of these thermal inertia values to sand grain sizes yields a range of sand classifications of fine sand to granules. Comparison of sand size classifications with geographic location shows grain size ranges that are distinctive by location, consistent with local sourcing. This work points toward geographically diverse sand formation mechanisms yielding diverse grain sizes, while providing a freeware-based and thus widely accessible method for expanding the derivation of these critical data.

Aeolian processes are a dominant cause of Martian surface modification today. Sand is pervasive, even while the contemporary transport of sand at high-threshold wind speeds implies sand breakdown. Thus, detecting the origin(s) of sand on Mars is an important open question in Mars science. One long-standing hypothesis for an origin of Martian sand is as volcaniclastic sediments, either epiclastic (from the breakdown of effusive volcanic rock) or pyroclastic (from explosive sedimentation). Recent analyses identified the most likely pyroclastic unit on Mars, the Medusae Fossae Formation (MFF) - mapped as the Hesperian and Amazonian-Hesperian transitional units (Tanaka et al., 2014) - as an origin of Martian sand, a genesis consistent with inferences from other work. On these bases, we continued in this work to test for a volcaniclastic origin of Martian sand elsewhere on Mars. We inspected the seven units from the global geologic map of Mars interpreted as possibly/partially volcaniclastic and six geographic terrains ascribed as likely volcanogenic in a more recent analysis. Of these units and geographic terrains, potential sand sources (PSSs) were detected only within two locales: 1) the remaining (central and eastern) extent of the MFF, and 2) Arabia Terra. Arabia Terra shows more numerous PSS, commonly characterized as having proximal dune fields. The explosive volcanic history of the region points towards a pyroclastic origin, though the geologic complexity of the region suggests complexity in the provenance of Arabia Terra PSSs. In contrast to PSS identifications in the western MFF, PSSs in the central and eastern MFF were clustered along the contact of the MFF with lava plains, indicating a potential epiclastic sand origin. Thus, these results indicate both pyroclastic and potentially epiclastic origins for sand on Mars. However, these sand origins as detected by this work are confined to limited locales and (for the MFF) in the amount of sand generated. To further evaluate the amount of sand production in these two study regions from volcanogenic processes, spectroscopic analysis of PSSs in comparison with sand deposits would be a means to test first whether the PSSs were indeed generating sand and secondly if that sand were of volcanic origin. Potential sand flux modeling would be a means to establish the source location(s) of the sand. The origin(s) of the pervasive sand on Mars remain(s) to be better understood, for which the work presented here provides a foundation.

Aeolian sand transport on Mars is active today and was likely so throughout its history. Widespread dune motion is theorized to comminute sand to sub-sand sizes, a process also implied by lab experiments. In view of this sand destruction, discovering the source(s) and origin(s) of Martian sand provides critical information for understanding Martian sediment cycling.Local sand sources have been discovered and considered to be consistent with the long-standing hypothesis for Martian sand as volcaniclastic in origin. A local source of Martian sand has recently been inferred in the western Medusae Fossae Formation (wMFF). Given the pyroclastic origin of the vast MFF, the new discovery of sand generation from that deposit substantiates a volcaniclastic origin of Martian sand.However, the wMFF is limited in extent and unlikely to constitute an origin for the globally distributed dune fields on Mars. Continued exploration for sand origins is needed to explain this widespread distribution.We examined the five global geological units interpreted as volcaniclastic, which yielded limited evidence of sand sourcing outside the wMFF. In these five units, sand sourcing was detected in visible-wavelength data in the Hesperian and Amazonian transitional units that comprise the central and eastern MFF and in the Noachian units of Arabia Terra. Investigation to characterize sand production from these units is revealing a variety of sand source outcrops.Tracing sand deposits back to their sources is another approach for determining sand origins, as was used in determining the source – and thereby the origin – of sand in and from the wMFF. Determining sources for the widespread sand on Mars requires determining sand survivability: how far could sand travel from their sources before being destroyed by comminution to sub sand sizes? Simulation of aeolian transport on Mars has shown different sand mineralogies comminuting at different rates, suggesting that the bulk mineralogy of a sediment may change with increased transport distance. Building on that previous experimental work, we are undertaking comminution of 14 different Mars-analog sands to more fully characterize the mineralogical and physical effects on sand of aeolian transport. The results will support using dune sand compositions and distances from possible source outcrops to test if these outcrops sourced the sand.Thermal inertia is used to characterize Martian sand, e.g., to estimate grain sizes. Available dune field mapping facilitates investigation into dune sand thermal inertia values, thereby providing data, e.g., on sand particle sizes and induration states. As available mapping incorporates non-sand substrate, we are remapping dune fields to include only visible sand and using the distributions of thermal inertia values to assess if non-sand substrate is still included in our mapping. Having completed remapping of tropical dune fields, we are beginning analysis of their thermal inertia values. The results will reveal any trends relative to geography, underlying geologic unit, elevation, and other factors.These three investigations – into the sources and origins, effects of transport, and thermal inertia values of Martian sand – will support improved understanding of Martian aeolian sand cycling, one of the most active geologic agents on Mars.

Across planetary surfaces, aeolian bedforms are unique and useful records of sustained interaction between wind and granular materials. The first extraterrestrial dunes were found on Mars and martian dunes studies have since yielded improved estimations of dune sand size(s) and saltation threshold wind speeds, refined dune morphology models that enable improved prediction of past/present winds, and present-day sediment flux rates that contribute to erosional processes. However, new data are needed to address critical questions, such as about the sources/ages of martian dune sand, estimation of the martian sediment budget, and the connections between surface wind distributions and sediment lofting/transport rates.

(1) Methods, Figures S1–S3, and captions for additional supplemental information, (2) Animation S1, (3) the ArcGIS geodatabase from this work, (4) CRISM browse products, and (5) the ArcGIS database with the locations of grain size derivations.

The discovery of impact crater paleolakes, like the discoveries of ancient fluvial valleys, has been instrumental in our developing understanding of the history of water on Mars. Numerous and varied inverted paleo-fluvial deposits have been found within the westernmost lobes (Aeolis and Zephyria Plana) of the Medusae Fossae Formation,-800 km east of Gale Crater (and its inferred paleolake) and just north of the dichotomy boundary. Within the topographic depression between these two plana are located five >10-km-diameter craters that either predate or formed coevally with this fluvial activity, as evidence by stratigraphic relationships. The emplacement of these craters before or during pervasive aqueous activity in the region and their low relative elevation provide the environmental context supportive of the formation of paleolakes in these interplana craters. In this work, we test the hypothesis that these five interplana craters contained lakes by mapping, describing, and interpreting these interior crater deposits. Our primary focus is to identify any potential lacustrine units. Results of this investigation are the identification of sedimentary units with characteristics in support predominately of aeolian and fluvial paleoenvironments, with lacustrine depositional paleoenvironments as an alternative possibility for some units. The lack of conclusive evidence for paleolakes in this environmental context could be due to an actual absence of lakes or alternatively suggests limitations in identifying paleolakes from orbit and the possible presence of more crater paleolakes than have been identified remotely on Mars.

Despite undeserved challenges to participation in the field of planetary geomorphology, women have made significant contributions in the fluvial, aeolian, and (cryo)volcanic subdisciplines. In this work, some women-in particular, women of color-are highlighted to show a part of these foundational contributions. We focused on women scientists who were working in the latter half of the 20th century, a revolutionary time for terrestrial geomorphology and the inception of the discipline of planetary geomorphology. We also focused on women working in our scientific subdisciplines so that we could provide proper context for their work. These contributions have occurred both as discoveries in terrestrial geomorphology leading to follow-on discoveries in planetary geomorphology and through serving as educators and role models. With women increasingly achieving positions of influence both in the geo- and planetary sciences as in American society, this research allows us to celebrate these contributions of women and particularly women of color while looking forward to a more complete record of their past contributions and greater future achievements.

A critical gap in planetary observations has been in situ characterization of extra-terrestrial, present-day atmospheric and surface environments and activity. While some surface activity has been observed and some in situ meteorological measurements have been collected by auxiliary instruments on Mars, existing information is insufficient to conclusively characterize the natural processes via concurrent and high-resolution measurement of environmental drivers and activity. Thus, many atmospheric, aeolian, and other surface processes models - which are used to generate key constraints on science and exploration in many areas of planetary investigation-such as surface exposure/erosion estimates, landscape interpretation, and modeling dust storm development-remain untested under non-Earth conditions. Analogous terrestrial processes are often studied intensively via numerical modeling that integrates empirical results from laboratory and/or field studies of process-response interactions between the atmosphere and relevant surface landforms. Incorporation of such in situ measurements into model development has significantly advanced our understanding of atmosphere-surface interactions and related geomorphic processes on Earth, and is poised to do so on other planets. However, to date, such testing and refinement have not been possible in other planetary environments, partially because investigations of this sort require new technologies, mission architectures, and operations designs (e.g., different from large rovers focused on geochemical investigations) to fully address the key gaps in our understanding while keeping cost and risk low. Fortunately, technological developments in the areas of surface access, instrumentation, and onboard processing/memory now enable small spacecraft to accommodate meteorological and aeolian instrumentation that could collect the needed measurements to fill this critical gap while remaining within typical small spacecraft resource budgets. Furthermore, maturity of our understanding of the broader geologic and atmospheric context on Mars provides a ready framework for ingestion of discrete ground truth measurements into our understanding of the broader and multi-scale martian natural systems and processes. These advancements make addressing key science questions with novel mission concepts feasible, promising results that would significantly advance our understanding of extraterrestrial surface-atmosphere interactions. This summary follows from a community-generated white paper for the ongoing Planetary Science/Astrobiology Decadal Survey, small spacecraft concept development at JPL, and numerous JPL and community discussions.

Introduction Cassini RADAR and Huygens DISR images reveal evidence of fluvial activity on Titan [1,2]. We have taken a tripartite approach to understanding the influence of this activity on Titan’s sediments and their resultant characteristics: (1) radar analog study in Death Valley, (2) laboratory simulation of icy clast comminution, and (3) analysis of radar backscatter from Titan’s fluvial features. Radar analogs in Death Valley reveal sedimentological influences We take ground truth measurements on alluvial fan deposits in Death Valley, California, to understand the surface properties that could be influencing radar backscatter in Titan’s coarse sediment deposits (e.g., Fig. 1A). Although desert alluvium is highly angular, for study of radar backscatter (as σ0), a hyper-arid environment like Death Valley has the advantage of isolating the effects of roughness from the otherwise strong dielectric influence of soil moisture that occurs at most terrestrial locales. Thirty study sites (e.g., Fig. 1B) were selected with a range of σ0 values, incorporating varying grain size, sorting, and shape. C-band (λ = 5.6 cm) and L-band (λ = 23.5 cm) radar backscatter statistics were measured using images from Sentinel-1 and ALOS PALSAR, respectively, to compare with hand measurements of sediment similar to that in past work (e.g., [3]) but with updated methods. We find a logarithmic increase in σ0 with median grain size (Fig. 1C) and possible small increases with roundness and sorting. 3-D surface models made using structure from motion photogrammetry also allow for quantification of roughness parameters like RMS height (s) and correlation length. Such parameters commonly feed into theoretical backscatter models like the Advanced Integral Equation Model (AIEM) and allow us to evaluate these models against our in-situ grain measurements in these unusually rough, coarse-grained conditions (e.g., 2πs/λ > 3). Titan Tumbler experiments constrain abrasion of sediments in fluvial transport In order to understand how sediment evolves during fluvial transport on Titan, we conducted experiments tumbling icy clasts in a custom liquid-nitrogen-cooled tumbling mill [5] (Fig. 2A). Clasts of varying size, shape, and constituent grain size were tumbled for 10s of kilometers and measured at regular intervals for changes to clast shape and size distribution. Cross-sectional roundness indices (R = 4πArea/Perimeter2) were comparable to the roundest clasts at the Huygens landing site (R > 0.9; Fig. 1A) after just a few kilometers of transport, indicating that rapid rounding is possible for water ice clasts on Titan. Clasts abraded primarily through chert-like fragmentation rather than gradual attrition of clast surfaces to produce fines. Thus, abrasion of sediments in fluvial transport may not be an efficient means of sand production on Titan, which hosts vast stores of sand-sized sediment. Exponential mass-loss rates for water ice clasts at Titan-like temperatures are comparable to those of relatively weak terrestrial materials (Fig. 2B; Ed of ~0.1 to 1 km−1 where Ed = ln(M0/M)/x). For sediment to survive the significant lengths of Titan’s fluvial features [7], material strength greater than water ice may be necessary, e.g., possibly that of expected clathrate hydrates. Cassini RADAR mapping and analysis of downstream trends Among the variety of fluvial features identified in Cassini RADAR images, some radar-bright fluvial features have been interpreted as coarse-grained, ephemeral streams [1,7]. Because their radar backscatter can be expected to change with ground properties that largely depend on surface alluvium, downstream trends may suggest changes in sediment properties. We mapped and measured the downstream changes in backscatter for >60 large radar-bright fluvial features across Titan’s surface (e.g., Fig. 3A). Despite the difficulty of interpreting these features near the limit of the radar resolution, clear trends often emerge (e.g., Fig 3B). Over a few hundred kilometers, σ0 trends can be fit with absolute linear slopes up to ~0.01 km-1 (i.e., ~0.04 dB/km). Interestingly, although decreasing backscatter would support a hypothesis of clast comminution, some features clearly show increasing σ0 in the downstream direction. Possible factors include fresh sediment input from tributaries, changes in clast composition / dielectric constant, or changes in flow dynamics leading to coarse-grained deposition. Conclusions Synthesizing field, lab, and spacecraft data helps us constrain clast properties and their evolution in Titan’s fluvial features and quantify the resultant granulometric changes. Although sediment abrasion can rapidly round water ice clasts into the efficient retroreflectors proposed by [1], they may also be comminuted over shorter distances than many terrestrial materials. The minimal effects of roundness and sorting on radar backscatter compared to grain size suggests more complicated contributions to the backscatter are at play, rather than simple downstream fining, in order to produce the observed variable and non-monotonic downstream trends. References [1] Le Gall, A., et al. (2010) Icarus, 207, 2, 948–958. [2] Tomasko, M.G., et al. (2005) Nature, 438, 765–778. [3] Schaber G., et al. (1976) GSAB, 87, 29–41. [4] Keller, H.U., et al. (2008) PSS, 56, 728–752. [5] Maue, A.D., et al. (2022) Icarus, 375, 114831. [6] Attal, M. and Lavé, J. (2009) JGR, 114, F04023. [7] Burr, D.M., et al. (2013) GSAB, 125, 27–41.

Dark, windblown (eolian) sand on Mars has produced significant geologic effects throughout Martian history. Although local and regional sand sources have been identified, a primary origin, or genesis, for Martian sand has not been demonstrated. This knowledge gap was recently heightened by the discovery of widespread sand motion, implying breakdown of grains to sub-sand sizes. To address the question of sand genesis, we investigated the source(s) of sand in Aeolis Dorsa (AD), the westernmost Medusae Fossae Formation, using comparisons to sand potentially sourced from multiple regions, each connoting a different sand genesis. Our methods included comparison of (1) AD sand mineralogies with those of possible sand source features, and (2) mapped AD sand deposits and inferred emplacement directions with modeled sand deposit locations and transport pathways. The results point to a time-transgressive unit, interpreted as pyroclastic, as a source of dark sand. High-resolution images of this unit reveal outcrops with dark sand weathering out of lithified bedrock. Given the extent of interpreted pyroclastic deposits on Mars, this sand genesis mechanism is likely widespread today and operated throughout Martian history. Whereas this work identified olivine-rich sand, a range of original pyroclastic lithologies would account for the mineralogic variability of dune fields on Mars. These findings can be tested through analyses of other pyroclastic deposits and potentially by data from the NASA Curiosity rover in nearby Gale crater.

Data in this article are related to the research article "Rapid rounding of icy clasts during simulated fluvial transport in the Titan Tumbler". Whereas that research focused on low-temperature ice abrasion in the context of Saturn's moon Titan, the full dataset on experiments testing the breakdown of water ice under a variety of tested conditions is reported in this article. Following the work of previous terrestrial studies, these experiments utilize tumblers that produce collisions to simulate some aspects of mechanical weathering during fluvial transport. Data files publicly available on Mendeley Data include measures of mass and roundness of clasts of specific grain sizes as well as raw images, videos, and the MATLAB script used for analysis. In this article, the varying conditions of temperature, initial clast size, shape, ice type, number of clasts for each of the 42 experiments are reported, along with best-fit models of abrasion typically applied in terrestrial tumbler studies. This text describes the methodology, including the development of icy clasts, operation of the tumblers, measurement of clast properties, calculation of derived parameters, and application of abrasion models. Exploration of various approaches to tumbler development and data acquisition are reported to benefit future researchers in this area. Experiments on the abrasion of different materials benefit from cross-comparison, which is also a fundamental aspect of planetary science.

The Importance of Venus Experimental Facilities Introduction: The planet Venus is the most Earth-like body in terms of its size, composition, and location in the solar system, and hosts some similar geologic processes, such as widespread basaltic volcanism and aeolian sand transport [1,2].At the same time, Venus offers what is likely the greatest set of technical challenges to exploration out of all terrestrial bodies [3].With an average surface temperature of ~460 ºC and pressure of ~92 bars (at mean planetary radius) [4], many types of instrumentation, including conventional electronics, are outside their standard operating conditions on Venus [5,6].On top of these factors, the Venusian atmosphere is a chemically reactive mixture of carbon and sulfur species, including SO2, CO2, H2S, and COS, in addition to other trace, but important, reactive species such as HCl and HF [4,7].This dense gas mixture will react with most structural and fabrication materials, aggressively corroding metals, and ruining electrical wiring [8,9].Nevertheless, the exploration of Venus is vital in our quest to understand the evolution of terrestrial planets, and can reveal why some sustain liquid water, plate tectonics, and a complex biosphere like Earth, while others develop hostile environments.The answers to these questions, along with numerous other vital science questions such as those presented in the VEXAG Scientific Goals, Objectives, and Investigations for Venus Exploration [10], can only be found through exploration of Venus.Crucial tools in the exploration of Venus are experimental facilities that can replicate the pressure, temperature, and chemical conditions of that world.Such facilities allow us to develop, test, and prove technologies to explore Venus, and understand how to manage the challenges caused by the Venus environment.These facilities also benefit the scientific study of Venus as they can be used as laboratories to investigate the natural processes that occur in this extreme environment.Because these facilities advance both science and technology, they contribute to all parts of the exploration cycle, including investigating environmental phenomena that missions will need to survive and measure, developing and testing of technology and mission hardware to send to Venus, and using laboratory investigations to understand past and future mission data.This capability to contribute to all parts of the Venus exploration cycle makes Venus experimental facilities critical to moving forward our understanding of this planet in the next decade.Current Capabilities: A "facility" for the purposes of this discussion consists of a chamber or reaction vessel and its accompanying heating system, gas control system, and data collection system.A list of current facilities with Venus capabilities is given in Table 1.A critical advancement in recent years is that the barriers to replication of the Venus environment have been overcome at multiple institutions; this represents a significant advancement in the development of Venus experimental facilities since the last Decadal Survey.Multiple facilities are now capable of replicating the temperature and pressure of the Venus surface.The main variations in most of these facilities are the experimental chamber volumes, and how the gas composition in the replicated environment is treated, both in terms of what species are supplied and how they are monitored.The listed facilities run the gamut ranging from supporting technology and instrumentation development, mission development, and varied science experimentation.It is important to remember that different investigations require different facility capabilities, thus diversity in capabilities is required.More complex and capable facilities are needed to provide testing for whole mission concepts, while simpler more limited systems can be used to test and develop individual subsystems or technologies, such as functionality of specific instruments or measurement techniques.Similarly, some scientific investigations may focus on select atmospheric gases, while others require a more complete replication of the atmospheric

First posted December 14, 2021 For additional information, contact: Contact Astrogeology Research Program staffAstrogeology Science CenterU.S. Geological Survey2255 N. Gemini Dr.Flagstaff, AZ 86001 The Aeolis Dorsa region of Mars, located just north of the global dichotomy boundary, includes the Aeolis and Zephyria Plana, and a depositional basin between them. This interplana region consists of extensive networks of ridges—the eponymous Aeolis Dorsa—and is interpreted as having formed by topographic inversion of fluvial and alluvial deposits. To the south is a nearly 1-km-deep trough (Aeolis Chaos) and the southern highlands. These elements of the map area compose a landscape of extensive erosional and depositional sedimentary processes. The plana are pervasively abraded into yardangs, and the interplana area shows scattered yardangs superposed on the underlying terrain. The Aeolis Dorsa fluvial deposits are concentrated within and around the margins of the interplana region, exposed and (or) inverted by this pervasive aeolian abrasion. During this period of extensive erosion and deposition, impacts have also redistributed material. The geologic mapping of this region, conducted at 1:500,000 scale to enable depiction of the fine-scale fluvial features, divides the landscape into six unit groups. The highlands units group comprises three units, located in the southwestern map area. These units, having the highest elevation in the map area, consist of mesas surrounded by more gently sloping terrain. The transitional units group, located northeast of the highlands units, includes the Aeolis Chaos, denoted as a chaos terrain unit, and two transitional units, differentiated on the basis of surface texture and relative elevation. The plana units group comprises five units. The oldest unit consists of mesas similar to those of the highlands mesas unit but located about 200 kilometers north of the highlands in Aeolis Planum. Three other plana units, located on the Aeolis and Zephyria Plana, are differentiated on the basis of yardang texture, crosscutting relations, and relative elevations. They are interpreted as abraded sedimentary and (or) volcaniclastic deposits. The fifth plana unit, which crops out in the north corners of the map area, is at low elevation, has numerous small craters, and is interpreted as cratered lava plains. The interplana units group hosts a hummocky unit and a mounds unit, differentiated on the basis of texture and relief. In the Aeolis Dorsa units group, four units are mapped on the basis of dorsa morphology and adjacent textures. Stratigraphic relations indicate a decrease in discharge over time. The crater units group includes a crater unit, found throughout the map area although concentrated within the interplana region, and a crater fill unit that is found within several craters in this interplana region. Based on this mapping, the interpretation of the regional geologic record begins with emplacement of the highlands terrain during the Noachian Period. Emplacement was followed during the Early Hesperian by erosion and redistribution of this high-standing terrain to form the transitional units, and Aeolis Chaos formed after deposition of the other transitional units. The high-standing plana were emplaced and eroded repeatedly throughout the Hesperian and Early Amazonian time, and impact cratering occurred at decreasing crater sizes. The fluvial and alluvial activity that gave rise to the Aeolis Dorsa also extended throughout this time, leaving their diagnostic signature on this region.

Cassini Synthetic Aperture Radar (SAR) images of Titan’s surface revealed river networks strikingly similar to those on Earth. However, Cassini SAR has low spatial resolution and image quality compared to data used to map channels on Earth, so traditional methods for characterizing river networks might not yield accurate results on Titan. We mapped terrestrial analog networks with varying resolutions and image qualities to determine which geomorphologic metrics were invariant with scale or resolution. We found that branching angle and drainage density varied significantly with image resolution, and we therefore expect the actual drainage density of Titan’s channel networks to be significantly higher than the values calculated from Cassini data. Calculated network geometry did not change predictably with resolution and would therefore not be an ideal metric for interpreting Titan’s channel networks. The measured channel width, basin length and width, and drainage area all behaved predictably as resolution varied, leading us to conclude that these metrics could be applied to Cassini data. We then mapped all observable fluvial features on Titan—excluding those in the highly incised labyrinth terrains—visible in the Cassini data set. In our new maps, we observe differences in basin shapes between Titan’s polar and equatorial regions and dichotomies in the relative channel density between the northern and southern midlatitudes and poles of Titan: channels are concentrated at the poles and southern midlatitudes. These patterns may reflect differences in bedrock material and/or different climate histories.