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.

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.

Changes in Martian fluvial geomorphology with time-stratigraphic age, including decreases in paleochannel widths, suggest waning paleodischarges through time. Where fluvial landforms do not preserve paleochannel widths (e.g., meander deposits), other landform dimensions (i.e., radius of curvature) may be used to estimate paleodischarges. In the Aeolis Dorsa region, topographically inverted and stacked fluvial deposits-wide meander point bars overlain by thin channel fills-preserve ostensible evidence of decreasing paleodischarges through time. However, a robust paleohydraulic analysis of these distinct deposits requires knowledge of the accuracy of a terrestrial-based empirical relationship that estimates channel width from point-bar radius of curvature. We assess the accuracy of this radius-width relationship by applying it to a well-studied terrestrial analog, the Quinn River, Nevada. We find that radii of curvature from the Quinn River exceed the values predicted from the empirical relationship. These anomalously high radii are associated with greater resistance in the channel cut banks, indicating that bank strength is a confounding factor in the radius-width relationship. Some deposits in the Aeolis Dorsa include irregular meander morphologies, suggesting variably resistant channel banks and overestimates of both paleochannel widths and paleodischarges. Furthermore, the morphometry of the overlying thin channel fills suggests their widths have been eroded, such that their paleodischarges are underestimates. These overestimates and underestimates, when considered together, suggest little change in paleodischarge during the stratigraphic transition from meander deposits to channel fills. This work demonstrates the importance of terrestrial analog studies for revealing confounding factors in Martian fluvial systems and cautions against simplistic interpretations of Martian fluvial history. The discovered inaccuracies of paleodischarge estimates expose sources of uncertainty in the extant paleodischarge data that bias inferences toward waning hydrologic activity through time. (C) 2017 The Authors. Published by Elsevier Inc.

Estimates of submarine channel discharges have been combined with an existing data set of measured fluvial discharges to produce generalized bankfull hydraulic geometry relationships. Previous analysis implies that generalized hydraulic geometry relationships are universal across channels formed under a range of gravity conditions. Here we use data from two different subaqueous channel systems formed by flows with distinct buoyancy characteristics to test whether generalized hydraulic geometry relationships apply in reduced gravity environments where channels tend to have relatively steep slopes. On this basis and assuming formative (bankfull) conditions, we derive estimates of Martian paleodischarges and provide better constrained estimates of Martian paleochannel slopes and flow depths, direct measurements of which are otherwise elusive due to structural deformation and erosion since formation. The steeper slopes in reduced gravity environments tend toward greater Froude numbers at formative discharge conditions; this has implications for bedform dynamics and preservation.

The Aeolis Dorsa region of Mars preserves many ancient and topographically inverted fluvial deposits, some of which represent analogs to pre-vegetated meandering fluvial deposits on Earth. The regional stratigraphy of the Aeolis Dorsa preserves a tradition from deposits of meandering fluvial channels to alluvial fans. On Earth, fluvial channel and alluvial fan environments have different hydrologic regimes, sedimentary conditions, and depositional slopes, implying that the Aeolis Dorsa region experienced significant changes in hydrology, sedimentology, and topography. Here, we map deposits and derive stratigraphic columns of four local areas-two in southeast Aeolis Dorsa and two in the northwest-to elucidate the local hydrologic processes and sedimentary conditions coincident with regional change. Formative processes and conditions are inferred from shared morphologic attributes between Martian and terrestrial analog deposits. Results suggest mutually consistent local histories; all areas show a transition from fluvial deposits to alluvial fan deposits. However, the hydrologic processes and sedimentary conditions were non-uniform. Specifically, local deposits form two dichotomies: (1) southeast Aeolis Dorsa preserves meandering fluvial deposits, whereas the northwest preserves only wide channel fills; (2) southeast Aeolis Dorsa includes alluvial fans with debris-flow deposits, but northwest fans include only sheetflood or channelized deposits. A relative abundance of cohesive, weathered sediment in southeast Aeolis Dorsa explains both the fluvial meandering channels and debris flows on alluvial fans in those localities. Greater weathering in southeast Aeolis Dorsa is consistent with previous theories of enhanced snowmelt or orographic precipitation in southeast Aeolis Dorsa.

Abstract Correlative width‐discharge relationships from the Missouri River Basin are commonly used to estimate fluvial paleodischarge on Mars. However, hydraulic geometry provides alternative, and causal, width‐discharge relationships derived from broader samples of channels, including those in reduced‐gravity (submarine) environments. Comparison of these relationships implies that causal relationships from hydraulic geometry should yield more accurate and more precise discharge estimates. Our remote analysis of a Martian‐terrestrial analog channel, combined with in situ discharge data, substantiates this implication. Applied to Martian features, these results imply that paleodischarges of interior channels of Noachian‐Hesperian (~3.7 Ga) valley networks have been underestimated by a factor of several, whereas paleodischarges for smaller fluvial deposits of the Late Hesperian‐Early Amazonian (~3.0 Ga) have been overestimated. Thus, these new paleodischarges significantly magnify the contrast between early and late Martian hydrologic activity. Width‐discharge relationships from hydraulic geometry represent validated tools for quantifying fluvial input near candidate landing sites of upcoming missions.