Wet Streaks in the McMurdo Dry Valleys, Antarctica: Implications for Recurring Slope Lineae on Mars

Recurring Slope Lineae (RSL) are dark streaks that seasonally propagate and fade on Martian slopes. The morphology and dynamics of RSL suggest that they might be brine flows; however, more recent hypotheses propose that they are dry, granular flows. To determine if RSL are consistent with brine flows, we investigated Mars analog wet streaks in Wright Valley Antarctica using new chemical analyses of soils and waters, time-lapse photography, and satellite images. Results show that the source of water to wet streaks is primarily local snowmelt, which seasonally percolates downslope. Meltwater leaches hygroscopic CaCl2-rich salts from soils and underlying Ferrar dolerite bedrock. These salts accumulate at soil surfaces via evapoconcentration and deliquesce/effloresce in response to daily relative humidity changes, causing the surface brightness to fluctuate. Over several years, the active CaCl2 salt component leaches back into the subsurface and the streaks dry, leaving a residue of less hygroscopic salts at the surface. Applied to Mars, wet streaks are inconsistent with the surface expression and dynamics of RSL. Wet streaks propagate and fade over multiple years, drain onto low angled slopes, and have a characteristic pattern of dark downhill and lateral edges. In contrast, RSL are seasonal features, terminate on angle-of-repose slopes, and typically appear monochromatic. These inconsistencies provide evidence against brine flow hypotheses of RSL formation.