Comparing Monte Carlo Models of Impact Alteration of Planetary Atmospheres

Abstract

One process that affects atmospheric surface pressure is impact bombardment. The evolution of a planet's atmosphere under impact bombardment is an open question. We use a Monte Carlo method to evolve a range (0.006–92.5 bar) of initial atmospheres at Mars, Earth, and Venus under bombardment of $5\times 10^6$ impactors using seven individual models. Since these seven models are best suited for specific impactor size regimes, we also combine these models into a composite model and compare it to other existing composites. Alterations to the existing models are required to apply to broad initial conditions. If we use each component model for every impactor, starting from present-day atmospheric pressure, we find about two or three orders of magnitude spread in the final atmospheric pressure for low initial pressures. For high initial pressures, we see a spread of about fourteen orders of magnitude. Given these differences, we suggest that the use of any one model to determine atmospheric change due to impact bombardment is risky. Most models and starting parameters result in net growth between $\Delta P=+0.01$ and $+100$ bar for low initial pressures. For high initial pressures, the net change in atmospheric pressure can vary between about ΔP = −100 and +100 bar. Our composite model shows that the atmospheres of Venus, Earth, and Mars tend to grow under bombardment, with Earth's atmosphere growing most quickly. For an early Martian ($P_0=1$ bar) and an early terrestrial (with an initial pressure of $P_0=0.25$ bar) atmosphere, Earth’s atmosphere tends to grow under bombardment, but Mars’s could grow or shrink. The results suggested here, where the models are universally applied, suggest that impact bombardment was likely a significant source of volatiles in the early Solar System. Additional work and careful consideration of how impact events affect the evolution of planetary atmospheres is needed.