Impact-triggered greenhouses on Mars

I have modeled the effects of large (30--250 km diameter) impacts on Mars using a 1-dimensional radiative-convective model that I have developed. The model includes a radiative transfer code to calculate the evolution of the atmospheric temperature following impact, a subsurface model to calculate the evolution of the ground temperature, a hydrological cycle to follow the evaporation, condensation and precipitation of injected and surface evaporated water, a radiative code to calculate the radiative effects of water clouds, and an atmospheric module to calculate the latent heating due to cloud formation/dissipation. The specific purpose for the development and execution of this model is to understand the environmental effects of large impacts and how they might have contributed to the formation of the valley networks on the surface of Mars. I have found that parts of the Martian regolith may be kept above freezing for from 95 days to 200 years by the modeled events, and for millennia for even larger impacts. The length of time the planet is above 273 K is determined by how large the CO2 surface pressure is assumed to be, whether the radiative effects of clouds are considered, and of course, by the size of the event itself. The amount of water precipitated out of the atmosphere from vaporization of impactor, target, and polar caps, and melted below ground for different diameter asteroid impactors yields global water totals ranging from 40 cm to 50 m for modeled examples, and up to 200 m for the largest objects. By experimenting with the effects of smaller (<100 km diameter) impactors, I have found that their cumulative effects over geologic time can induce as much erosion as their larger counterparts. From my research emerges a picture of early Mars in which it was not "warm and wet" for any significant (geologically) period of time, but rather that it was mostly cold and dry as it is today, with brief periods of "warm and wet" following these impact events. I have also found that the equations describing the gray model of planetary atmospheres show that two simultaneous solutions for temperature may exist for a given flux. One solution is reached when a planet is cooling from a large temperature perturbation, such as an impact. This then shows that a planet may be pushed to the "hot" or "runaway" solution following a comet or asteroid impact event.