Atmospheric Implications Derived from the Small Crater Record from the Curiosity Rover, Gale Crater, Mars.

Abstract:

The obliquity of Mars is theorized to undergo semi-periodic fluctuations and to be tied to the atmospheric density through the deposits of carbon dioxide at the poles. As the obliquity angle increases, the poles are exposed to greater amounts of sunlight, causing the CO₂ to sublimate and contribute to the density of the atmosphere. At lower angles, the reverse occurs, freezing out more CO₂ from the atmosphere. The obliquity and corresponding atmospheric pressure fluctuations can be seen in geologic features such as the size and abundance of the smallest craters observed at the surface. Theoretical models predict that the smallest crater that can form under current Martian conditions is a crater of diameter, D ≈ 25cm. A survey of small craters was conducted for the Curiosity rover mission over the first 2300 sols (LPSC L, abstract #3147) and a total of 198 craters were cataloged. The smallest crater found was D = 0.33m. An estimate for the area surveyed, crater size frequency distribution statistics, and cratering models from Williams et al., (2018) were used to determine that the abundance of small craters is less than predicted. A lack of small craters possibly indicates a recently denser atmosphere that prevented small craters from forming, through ablation and declaration of projectiles, or from eroding small craters faster than the present atmosphere. Current estimates for erosion rates support a D = 1.0m crater surviving as long as 20 Ma. Therefore, the small crater record can provide insight into the obliquity and resulting atmospheric density for the past 20 Ma. According to Laskar et al., (2004), the Martian obliquity was closer to an average of 40 degrees approximately 5 to 20 Ma, supporting a period of higher atmospheric density and lower small crater production. Under current Martian atmospheric conditions, liquid water cannot survive long on the surface due to the low temperature and pressure, but periods of higher obliquity and resulting higher atmospheric density could provide a more suitable environment for liquid water on Mars. Further work is needed to refine atmospheric and crater production models that consider the total CO₂ availability of Mars and to expand the small crater survey to other regions or the planet’s surface.