Geologic Constraints on Early Mars Climate

Early Mars climate research has well-defined goals (MEPAG 2018). Achieving these goals requires geologists and climate modelers to coordinate. Coordination is easier if results are expressed in terms of well-defined parameters. Key parameters include the following quantitative geologic constraints. (1) Cumulative post-3.4 Ga precipitation-sourced water runoff in some places exceeded 1km$1~\mbox{km}$ column. (2) There is no single Early Mars climate problem: the traces of ≥2 river-forming periods are seen. Relative to rivers that formed earlier in Mars history, rivers that formed later in Mars history are found preferentially at lower elevations, and show a stronger dependence on latitude. (3) The duration of the longest individual river-forming climate was >(102–103)yr${>}(10^{2}\mbox{--}10^{3})~\mbox{yr}$, based on paleolake hydrology. (4) Peak runoff production was >0.1mm/hr${>}0.1~\mbox{mm}/\mbox{hr}$. However, (5) peak runoff production was intermittent, sustained (in a given catchment) for only <10% of the duration of river-forming climates. (6) The cumulative number of wet years during the valley-network-forming period was >105yr${>}10^{5}~\mbox{yr}$. (7) Post-Noachian light-toned, layered sedimentary rocks took >107yr${>}10^{7}~\mbox{yr}$ to accumulate. However, (8) an “average” place on Mars saw water for <107yr${<}10^{7}~\mbox{yr}$ after the Noachian, suggesting that the river-forming climates were interspersed with long globally-dry intervals. (9) Geologic proxies for Early Mars atmospheric pressure indicate pressure was not less than 0.012 bar but not much more than 1 bar. A truth table of these geologic constraints versus currently published climate models shows that the late persistence of river-forming climates, combined with the long duration of individual lake-forming climates, is a challenge for most models.