A fragile record of fleeting water on Mars

The light-toned sedimentary layers that outcrop widely throughout Mars’ Southern Highlandshave long been an enigma in uncovering the climatic history of Mars. Although theseunits seem to contain unique records of fluctuating surface conditions, the role of water intheir formation is debated. A distinctive property of many such deposits is their elevatedthermal inertia relative to that of surrounding materials. This temperature-controlling trait isoften interpreted to indicate induration resulting from aqueous processes. However, prevalenterosional landforms suggest that the deposits host much weaker materials than neighboringunits. We address this apparent contradiction by disentangling the relationships betweenthermal inertia, mineralogy, and erosion susceptibility and by quantifying the cohesion oflayered deposits in the Arabia Terra region. We demonstrate that variations in dust coverassociated with relative erodibility and eolian abrasion can be inconspicuous controls on apparentthermal inertia. We ultimately find that these deposits are not as cohesive as would beexpected from a high water-to-rock ratio setting either during or after deposition. If waterrichsurface conditions existed in the region after the Noachian, these deposits suggest theymay have only been intermittent and fleeting.