Environmental Effects of Large Impacts on Mars

TLDR

Martian valley networks formed near the end of the heavy bombardment period (~3.5 Ga), when large impacts produced global blankets of hot ejecta and injected steam into the atmosphere. Simulations show that impact ejecta kept Mars’ surface above freezing for decades to millennia, melted shallow ice, and that 100–250 km asteroids injected 2–16 m of water into the atmosphere, producing ~2 m/yr rainfall that formed rivers and recharged aquifers.

Abstract

The martian valley networks formed near the end of the period of heavy bombardment of the inner solar system, about 3.5 billion years ago. The largest impacts produced global blankets of very hot ejecta, ranging in thickness from meters to hundreds of meters. Our simulations indicated that the ejecta warmed the surface, keeping it above the freezing point of water for periods ranging from decades to millennia, depending on impactor size, and caused shallow subsurface or polar ice to evaporate or melt. Large impacts also injected steam into the atmosphere from the craters or from water innate to the impactors. From all sources, a typical 100-, 200-, or 250-kilometers asteroid injected about 2, 9, or 16 meters, respectively, of precipitable water into the atmosphere, which eventually rained out at a rate of about 2 meters per year. The rains from a large impact formed rivers and contributed to recharging aquifers.