Geologic Constraints on Early Mars Climate

TL;DR

This paper synthesizes geologic constraints to better understand early Mars climate, highlighting the complexity and variability of water activity and climate duration, and challenges for existing climate models.

Contribution

It provides a comprehensive set of quantitative geologic constraints that refine the parameters for modeling early Mars climate.

Findings

Multiple river-forming periods with different characteristics

Long durations of individual lake-forming climates (>100-1000 years)

Intermittent peak runoff with short durations (<10% of climate periods)

Abstract

Early Mars climate research has well-defined goals (Mars Exploration Program Analysis Group 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 km column. (2) There is no single Early Mars climate problem: the traces of $\ge$2 river-forming periods are seen. Relative to rivers that formed earlier in Mars history, rivers that formed later in Mars history are found preferentially at lower elevations, and show a stronger dependence on latitude. (3) The duration of the longest individual river-forming climate was >(10$^2$-10$^3$) yr, based on paleolake hydrology. (4) Peak runoff production was >0.1 mm/hr. However, (5) peak runoff production was intermittent, sustained (in a given catchment) for only <10% of the duration of river-forming climates. (6) The cumulative number of wet years during the valley-network-forming period was >10$^5$ yr. (7) Post-Noachian light-toned, layered sedimentary rocks took >10$^7$ yr to accumulate. However, (8) an "average" place on Mars saw water for <10$^7$ 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.