MARS OBLIQUITY THROUGH DEEP TIME: NEW CONSTRAINTS FROM THE BOMBARDMENT COMPASS (Invited Presentation)

The dynamics of Mars' obliquity are believed to be chaotic, and the historical ~3.5 Gyr (late-Hesperian onward) obliquity probability density function (PDF) is highly uncertain and cannot be inferred from direct simulation alone. Obliquity is also a strong control on post-Noachian Martian climate, enhancing the potential for equatorial ice/snow melting and runoff at high obliquities (> 40^o) and enhancing the potential for desiccation of deep aquifers at low obliquities (< 25^o). We developed a new technique using the "bombardment compass" provided by the orientations of elliptic impact craters to constrain the true late-Hesperian onward obliquity PDF. To do so, we developed a forward model of the effect of obliquity on elliptic crater orientations using ensembles of simulated Mars impactors and ~3.5 Gyr long Mars obliquity simulations. In our model, the inclinations and speeds of Mars crossing objects bias the preferred orientation of elliptic craters, which are formed by low-angle impacts. Comparison of our simulation predictions with a validated database of elliptic crater orientations allowed us to invert for the best-fitting obliquity history. We found that since the onset of the late-Hesperian, Mars' mean obliquity was likely low, between ~10^o and ~30^o, and the fraction of time spent at high obliquities > 40^o was likely < 20%.