The main question addressed here is how much of this atmosphere Mars lost to space and why? This is related to the question whether early Mars was potentially habitable.
Using a new atmospheric escape model, we find that most of the early dense atmosphere was lost due to strong and long-lasting impact erosion in the first several hundred Myr, rather than by Jeans escape processes. Jeans escape has been considered the main mechanism for the atmospheric escape of Mars to space, especially during the first 1 Gyr of its history. The Jeans escape theory predicts the escape rate is highly dependent on temperature, so that the hotter the atmosphere is, the faster it escapes.
It has been traditionally considered that the early Mars's atmosphere was warm and therefore easily escaped to space. However, there is increasing skepticism about the high temperature proposed by most previous studies. In this study we considered various conditions, even higher temperatures than most previous studies, but find that the total mass of the atmosphere loss by the Jeans escape process is small, even under extremely hot conditions.
To the contrary, we find that the impact erosion mechanism is highly effective in removing the early Mars atmosphere. We calculated the atmospheric loss due to impact erosion for various conditions, including the variations of the size distribution and the impact angles of impactors. We find that the erosion rate of the early atmosphere due to impact erosion was extremely high, and most of the early dense atmosphere (more than 99% of the initial amount) could have been lost to space by the impact erosion process in the first 500 Myr.
These findings support the argument that the early Mars's climate could be habitable only if the water supply was more than two orders of magnitude higher than that from Martian mantle outgassing.
