AbstractPotential Desiccation Polygons (PDPs), tens to hundreds of meters in size, have been observed in numerous regions on Mars, particularly in ancient (>3 Gyr old) terrains of inferred paleolacustrine/playa geologic setting, and in association with hydrous minerals such as smectites. Therefore, a better understanding of the conditions in which large desiccation polygons form could yield unique insight into the ancient climate on Mars. Many dried lakebeds/playas in western United States display large (>50 m wide) desiccation polygons, which we consider to be analogues for PDPs on Mars. Therefore, we have carried out fieldwork in seven of these dried lakes in San Bernardino and the Death Valley National Park regions complemented with laboratory and spectral analysis of collected samples. Our study shows that the investigated lacustrine/playa sediments have (a) a soil matrix containing ~40–75% clays and fine silt (by volume) where the clay minerals are dominated by illite/muscovite followed by smectite, (b) carbonaceous mineralogy with variable amounts of chloride and sulfate salts, and significantly, (c) roughly similar spectral signatures in the visible‐near‐infrared (VIS‐NIR) range. We conclude that the development of large desiccation fractures is consistent with water table retreat. In addition, the comparison of the mineralogical to the spectral observations further suggests that remote sensing VIS‐NIR spectroscopy has its limitations for detailed characterization of lacustrine/playa deposits. Finally, our results imply that the widespread distribution of PDPs on Mars indicates global or regional climatic transitions from wet conditions to more arid ones making them important candidate sites for future in situ missions.

AbstractThis paper sheds light on the hydrodynamic conditions of transport and sedimentary effects of wind‐induced water currents produced during strong windstorms in low gradient systems. Repeated field surveys were conducted in a playa lake in central Spain to determine the impacts of major winter storms on the bed form morphology in real time. The succession of storms that passed through the area from mid‐December 2013 to early February 2014 left behind a variety of sedimentary structures: mainly ripple marks showing complex patterns and erosional structures. The latter include obstacle scours, grooves and other tool marks. In situ observations revealed that strong storm events in almost flat, extremely shallow lakes (less than 5 cm) have enough hydraulic energy to erode and remove high volumes of sediments and may also lead to large stones sliding across the bed, thus creating long grooves.Sole marks found in ancient continental successions have been typically attributed to fluvial conditions. We suggest that shallow lake basins should not be discounted when storm‐generated structures are preserved in ancient rocks. The identification of such sedimentary structures provides valuable information for reconstructing hydrodynamic conditions and paleoclimatic conditions in semi‐arid environments. Copyright © 2014 John Wiley & Sons, Ltd.

Abstract. Trails in the usually-hard mud of Racetrack Playa in Death Valley National Park attest to the seemingly-improbable movement of massive rocks on an exceptionally flat surface. The movement of these rocks, previously described as "sliding stones", "playa scrapers", "sailing stones" etc., has been the subject of speculation for almost a century but is an exceptionally rare phenomenon and until now has not been directly observed. Here we report documentation of multiple rock movement and trail formation events in the winter of 2013–2014 by in situ observation, video, timelapse cameras, a dedicated meteorological station and GPS tracking of instrumented rocks. Movement involved dozens of rocks, forming fresh trails typically of 10s of meters length at speeds of ~5 cm s−1 and were caused by wind stress on a transient thin layer of floating ice. Fracture and local thinning of the ice decouples some rocks from the ice movement, such that only a subset of rocks move in a given event.