Climate simulations of early Mars with estimated precipitation, runoff, and erosion rates

TL;DR

This study uses advanced climate modeling to show that early Mars likely had a warm, semi-arid climate with a significant ocean, sufficient to form valley networks, challenging the cold climate hypothesis.

Contribution

It provides the first detailed energy balance model including a northern ocean, supporting a warm early Mars scenario consistent with geological evidence.

Findings

Mean surface temperatures near or above freezing were necessary for valley formation.

A large northern ocean was likely present on early Mars.

Episodic warming may not have been required for valley network formation.

Abstract

The debate over the early Martian climate is among the most intriguing in planetary science. Although the geologic evidence generally supports a warmer and wetter climate, climate models have had difficulty simulating such a scenario, leading some to suggest that the observed fluvial geology (e.g. valley networks, modified landscapes) on the Martian surface, could have formed in a cold climate instead. However, as we have originally predicted using a single-column radiative-convective climate model [Ramirez et al. 2014a], warming from CO2-H2 collision-induced absorption (CIA) on a volcanically active early Mars could have raised mean surface temperatures above the freezing point, with later calculations showing that this is achievable with hydrogen concentrations as low as ~1%. Nevertheless, these predictions should be tested against more complex models. Here, we use an advanced energy balance model that includes a northern lowlands ocean to show that mean surface temperatures near or slightly above the freezing point of water were necessary to carve the valley networks. Our scenario is consistent with a relatively large ocean as has been suggested. Valley network distributions would have been global prior to subsequent removal processes. At lower mean surface temperatures and smaller ocean sizes, precipitation and surface erosion efficiency diminish. The warm period may have been ~< 10 million years, perhaps suggesting that episodic warming mechanisms were not needed. Atmospheric collapse and permanently glaciated conditions occur once surface ice coverage exceeds a threshold depending on CIA assumptions. Our results support an early warm and semi-arid climate consistent with many geologic observations.