Diagenetic layers in the upper walls of Valles Marineris, Mars: Evidence for drastic climate change since the mid‐Hesperian

A packet of relatively resistant layers, totaling ∼400 m thickness, is present at the tops of the chasma walls throughout Valles Marineris. The packet consists of an upper dark layer (∼50 m thick), a central bright layer (∼250 m thick), and a lower dark layer (∼100 m thick). The packet appears continuous and of nearly constant thickness and depth below ground surface over the whole Valles system (4000 km E‐W, 800 km N‐S), independent of elevation (3–10 km) and age of plateau surface (Noachian through upper Hesperian). The packet continues undisturbed beneath the boundary between surface units of Noachian and Hesperian ages, and continues undisturbed beneath impact craters transected by chasma walls. These attributes are not consistent with layer formation by volcanic or sedimentary deposition, and are consistent with layer formation in situ, i.e., by diagenesis, during or after upper Hesperian time. Diagenesis seems to require the action of aqueous solutions in the near subsurface, which are not now stable in the Valles Marineris area. To permit the stability of aqueous solutions, Mars must have had a fairly dense atmosphere, ≥1 bar CO 2 , when the layers formed. Obliquity variations appear to be incapable of producing such a massive atmosphere so late in Mars' history.