Ancient Mars Had Ice-free Lakes in Gale Crater

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Symmetrical wave ripples identified with NASA’s Curiosity rover in ancient lake deposits at Gale crater provide a key paleoclimate constraint for early Mars: At the time of ripple formation, climate conditions must have supported ice-free liquid water on the surface of Mars. These features are the most definitive examples of wave ripples on another planet. The ripples occur in two stratigraphic intervals within the orbitally defined Layered Sulfate Unit: a thin but laterally extensive unit at the base of the Amapari member of the Mirador formation, and a sandstone lens within the Contigo member of the Mirador formation. In both locations, the ripples have an average wavelength of ~4.5 centimeters. Internal laminae and ripple morphology show an architecture common in wave-influenced environments where wind-generated surface gravity waves mobilize bottom sediment in oscillatory flows. Their presence suggests formation in a shallow-water (<2 meters) setting that was open to the atmosphere, which requires atmospheric conditions that allow stable surface water. The Curiosity rover discovered evidence for long-lived ancient lakes in 2014, and now 10 years later, Curiosity has discovered ancient lakes that were free of ice, offering an important insight into Mar's early climate. Above – Symmetric ripple marks in the AMB outcrop.
Symmetric ripple marks (A) are observed within the AMB outcrop. Ripple crests are identified with yellow arrows (A) and are aligned near-vertical (white arrow) in successive ripple layers. Internal laminae can be traced continuously through the ripple troughs. In plan view (B), the ripple crests are linear with occasional tuning-fork bifurcation, oriented consistently NW/SE. The AMB ripple unit is laterally extensive, of consistent thickness, and is conformably overlain by a unit of planar laminae [(C): contact is covered here—dotted line is inferred contact]. The AMB ripple unit at the Amapari location is ~15 cm thick and is composed of five resistant beds containing symmetric ripple marks. Image credit: NASA/JPL-Caltech/MSSS.

The ripples within the AMB resistant beds are composed of parallel millimeter-scale laminae that can be traced continuously through the ripple troughs and crests. Individual resistant beds are 1 to 2 cm thick. Internal laminae within the recessive beds (where visible) appear to uniformly drape the rounded ripple crests and define upward building of the ripples. The ripple crests in successive resistant beds are aligned near-vertically with no observed truncation of crests or laminae, indicating ripples formed by vertical accretion with little translation during higher sediment fallout. The rapid sedimentation during ripple formation and the darker tone of the AMB outcrop relative to the underlying stratigraphy suggest that the ripple layers are not formed from locally reworked lakebed sediments. The sediment was most likely sourced from further away, either as wind-blown sediment or in surface runoff from a location beyond the Amapari outcrop location.

The formation of the centimeter-scale wave ripples described here requires the activity of oscillatory flows and the transmission of surface wave energy to a shallow lake bed. The sedimentologic evidence for ripples created by wind-water interaction leads us to conclude that the lakes in which the ripples formed were free of ice and open to the atmosphere. Our modeling indicates that, under a range of relevant parameters, the wave-generated ripples could form at the observed scale. The occurrence of the wave ripples within Gale crater stratigraphy suggests that shallow lakes formed multiple times at successive stratigraphic intervals. Collectively, the ripples preserved in the Mirador formation provide strong evidence for a climate warm and humid enough to permit recurring open-water lakes during this 110-m aeolian depositional interval in Gale crater.

They assumed a median grain size of 100 μm (very fine sand). They used bedform stability constraints to find the water depth and wind-speed combinations where ripples are stable

Debate About Early Mars

Debate about the paleoclimate conditions on early Mars ranges from cold and wet scenarios, with abundant glaciogenic processes and limited standing water free of an ice cover to warm and wet scenarios, with abundant standing water free of ice. While there is geomorphic evidence for surface water activity across Mars, the climate models lack consensus on the precise conditions, timing, and duration of surface water, specifically whether standing water would be stable in early Mars atmospheric conditions. Detailed documentation of sedimentary structures preserved in ancient martian strata is an essential element for resolving this debate because they are capable of recording processes that preserve precise paleoenvironmental conditions, which are critical for constraining and validating climate models. Specifically, features such as wave ripples that are formed by the interaction of atmosphere, water, and sediment can be used to constrain both aqueous and atmospheric conditions.

Both Earth and Mars have (and had) a well-defined atmosphere, hydrosphere, cryosphere, and lithosphere. Processes within these systems generate currents of air, liquid water, and ice, which transport sediments from sites of weathering and erosion where they are formed to sites of deposition where they accumulate to create a record of sedimentary rocks