# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1812.11722