Geologic Constraints on Early Mars Climate

Abstract

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 $1~\mbox{km}$ 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 ${>}(10^{2}\mbox{--}10^{3})~\mbox{yr}$, based on paleolake hydrology. (4) Peak runoff production was ${>}0.1~\mbox{mm}/\mbox{hr}$. 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 ${>}10^{5}~\mbox{yr}$. (7) Post-Noachian light-toned, layered sedimentary rocks took ${>}10^{7}~\mbox{yr}$ to accumulate. However, (8) an “average” place on Mars saw water for ${<}10^{7}~\mbox{yr}$ 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.

Appendix

See Table 3 and Fig. 12.

Table 3 Absolute date estimates used in this paper, reproduced from Table 1 of Michael (2013) which in turn follows the Hartmann (2005) chronology

Fig. 12

Latitude-elevation plots of climate-relevant geologic activity for the Late Noachian/Early Hesperian and the Hesperian/Early Amazonian. The black regions have no data, and the gray regions correspond to terrain that was geologically reset after the time slice in question. Vertical black lines highlight latitudes ±15° and ±30°. (a) Late Noachian/Early Hesperian time slice: Black dots correspond to individual valleys from the catalog of Hynek et al. (2010). Only every 10th valley is plotted, for legibility. The density of black dots reflects the nonuniform distribution of elevation as a function of latitude (for example, there is not much Noachian terrain S of $30\ {}^{\circ}\mbox{S}$ above +3 km elevation). To correct for this effect, and get the latitude-and-elevation dependent density of valleys, we used a kernel density estimator. The resulting blue zone corresponds to the latitude/elevation zones that have the highest density of valleys, and is drawn to contain $\frac{2}{3}$ of the valleys. Blue dashed line is the same, but for 9/10 of the valleys. (b) Late Hesperian/Amazonian time slice: Blue disks mark large alluvial fans (combining catalogs of Howard et al. 2005 and Kraal et al. 2008a). Pale blue stripes mark latitude range of Fresh Shallow Valleys (Wilson et al. 2016). Black dots correspond to the sedimentary rocks from the catalog of Malin et al. (2010). The density of black dots reflects the nonuniform distribution of elevation as a function of latitude. To correct for this effect, and get the latitude-and-elevation dependent density of sedimentary rocks from Malin et al.’s (2010) catalog, we used a kernel density estimator. The resulting orange zone corresponds to the latitude/elevation zones that have the highest density of sedimentary rocks, and is drawn to contain $\frac{2}{3}$ of the sedimentary rocks. Orange dashed line is the same, but for 9/10 of the sedimentary rocks