Wave ripples formed in ancient, ice-free lakes in Gale crater, Mars

Abstract

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.

Fig. 1.. Location of the Amapari Marker Band and Prow outcrops.

Along the Curiosity traverse [(A), gray line] through the Mirador formation, the rover observed symmetric ripple marks within the Prow outcrop near the base of Mirador butte at elevation −3950 m (B) and the Amapari Marker Band (AMB) outcrop along the west side of Marker Band Valley (MBV) at elevation −3860 m (C). Red stars indicate the locations of the ripple-containing outcrops within the stratigraphic column (D) along the MSL traverse (A). The Prow is one of the multiple lenticular outcrops within the Contigo member. The AMB outcrops are nearly continuously around MBV [(C): solid orange line], often forming flat, erosionally resistant benches (inner edge traced in dashed orange line). The basemap used in (A) to (C) is a Mars Reconnaissance Orbiter HiRISE mosaic generated by Calef et al. (61, 62) and accessed through the NASA Planetary Data System.

Fig. 2.. 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.

Fig. 3.. Symmetric ripple marks in the Prow outcrop.

The Prow is an 18-m-long, lenticular outcrop (A) composed of a lower unit of unidirectional cross-strata and an upper unit of flaser bedding (B) containing symmetric ripple marks [(C and D): ripple crests indicated by yellow arrows]. The vertical accumulation of ripple marks within the flaser bedding creates a complex patter of interwoven cross-lamination [as in (34), figure 8, ex. 9], preserving both symmetric ripple troughs and crests at various locations. High-resolution imaging by the MAHLI taken at 1 cm standoff from the outcrop (E) shows observable sand grains making up the ripple layers, draped by fine-grained material with no observable grains (grain size <180 μm, below the MAHLI limit of resolution). See fig. S4 for location context for (E). Image credit: NASA/JPL-Caltech/MSSS.

Fig. 4.. Modeled water depth and wind speed of AMB ripples.

Conditions to produce orbital wave ripples with 3 to 6 cm wavelength (blue shaded zone) with a fetch of 500 m and for 100 μm of sand (text S3). Wind speeds are calculated for modern Earth (ρ_e = 1.2 kg/m³) and Mars (ρ_m = 0.02 kg/m³) atmospheric densities. Larger depths under similar wind speeds would produce boundary shear stresses below the requirement to form ripples according to Myrow et al. (54), producing lower plane bed or no sediment motion. Faster wind speeds would produce larger ripples (gray shaded zone) or breaking waves that do not produce orbital ripples. Changing the fetch from 200 to 5000 m results in small changes to the predicted wind speeds to produce the observed ripples, and requires similarly small water depths (fig. S6).