A Sustained Greenhouse Climate and Erosion Period on Mars Following an Impact Event

The existence of craters of size 200 km and greater proves that large (> 30 km diameter) impacts were abundant in the early history of Mars. Injected water from three sources (the impactor, water innate to the crater, and from melting of the polar caps) provides periods of rain following such impacts. Very hot, global debris blankets are another consequence of these large impacts, and these layers create a thermal pulse that propagates into the subsurface, melting additional water. The melted and precipitated water and debris blanket combine to produce a temporarily altered climate. The effects of moderate-sized (30-100 km diameter) impacts on Mars were studied using a 1-dimensional radiative-convective model. The model computes the evolution of temperature following an impact and includes: a subsurface model to compute the evolution of the ground temperature; a hydrological cycle to follow the evaporation, condensation and precipitation of injected and surface evaporated water; a radiative transfer code to compute greenhouse warming by CO2, water vapor, and water clouds; and an atmospheric thermodynamics module to compute the latent heating due to cloud formation/dissipation. We have found that parts of the Martian regolith may be kept above freezing for 95 days to decades by the modeled events. However if we include the radiative effects of water clouds, a sustained greenhouse climate is computed for impactors 50 km in size that could be centuries-long. The amount of water precipitated out of the atmosphere from vaporization of impactor, target, and polar caps, yields global rainfall totals ranging from 40 cm to 18 m depending on the size of the impactor and assumed background CO2 atmosphere. We also estimate the surface erosion following precipitation events and find that the total erosion done by all impactors in time is the same order of magnitude as the total erosion estimated [Golombek and Bridges, 2000] to have occurred on Early Mars.