GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 86-1
Presentation Time: 8:00 AM


HOLO, Samuel J., Department of Geophysical Sciences, University of Chicago, 5734 S Ellis Ave, Chicago, IL 60637, KITE, Edwin S., Dept. Geophysical Sciences, University of Chicago, 5734 S. Ellis Avenue Chicago, Chicago, IL 60637 and ROBBINS, Stuart J., Southwest Research Institute, 1050 Walnut Street, Boulder, CO 80302

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 (> 40o) and enhancing the potential for desiccation of deep aquifers at low obliquities (< 25o). 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 ~10o and ~30o, and the fraction of time spent at high obliquities > 40o was likely < 20%.