Decameter-scale pedestal craters in the tropics of Mars: Evidence for the recent presence of very young regional ice deposits in Tharsis

a r t i c l e i n f o Global climate models predict that ice will be deposited in tropical regions during obliquity excursions from the current mean obliquity of ~25° to ~35°, but no geological evidence for such deposits has been reported. We document the presence of very small (decameter scale) pedestal craters in the tropics of Mars (the Daedalia Planum– Tharsis region) that are superposed on an impact crater dated to ~12.5 Ma ago. The characteristics, abundance, and distribution of these small pedestal craters provide geological evidence that meters-thick ice accumulations existed in the tropical Tharsis region of Mars in the last few million years when mean obliquity was ~35° (~5– 15 Ma) before it transitioned to a mean of ~25° (~0–3 Ma). A reconnaissance survey reveals similar small pedestal crater examples superposed on the older Amazonian Arsia Mons tropical mountain glacier deposit, suggesting that ice can accumulate in these tropical regions without initiating large-scale glacial conditions. These results support the predictions of general circulation models that ice can migrate to the equatorial regions during periods of moderate obliquity and then serve as a source for mid-latitude deposits. Currently, Mars is a hyperarid, hypothermal desert and the largest reservoir of surficial water ice on Mars resides at the poles. It is known, however, that variations in spin-axis/orbital parameters (obliquity, eccentricity, and precession) (Laskar et al., 2004) can cause mobilization of water ice, transport in the atmosphere, and redeposition at lower latitudes. Evidence has been presented that a recent, meters-thick, ice-rich mantle was emplaced from the poles down to about 30° N and S latitude in the last several million years during an " ice age, " and that it has been undergoing modification in the 30°–50° latitude region in the last few hundred thousand years, as Mars' obliquity amplitude decreased and ice returned to the poles (Head et al., 2003). During earlier periods of higher obliquity (mean obliquity of ~35°) in the Late Amazonian, ice was deposited in the mid-latitudes, and formed widespread valley and plateau glacial land systems (Head et al., 2010; Madeleine et al., 2009). Somewhat earlier in the Amazonian, during periods when the mean obliquity was thought to have been ~45°, vast tropical mountain glaciers formed on the Unknown, however, has been the exact pathway and residence time of volatiles during transitions from one regime to another. Geological evidence, for example, has suggested …