The history of rivers on Mars is an important constraint on Martian climate evolution. The timing of relatively young, alluvial fan-forming rivers is especially important, as Mars’s Amazonian atmosphere is thought to have been too thin to consistently support surface liquid water. Previous regional studies suggested that alluvial fans formed primarily between the Early Hesperian and the Early Amazonian. In this study, we describe how a combination of a global impact crater database, a global geologic map, a global alluvial fan database, and statistical models can be used to estimate the timing of alluvial fan formation across Mars. Using our global approach and improved statistical modeling, we find that alluvial fan formation likely persisted into the last ∼2.5 Gyr, well into the Amazonian period. However, the data we analyzed were insufficient to place constraints on the duration of alluvial fan formation. Going forward, more crater data will enable tighter constraints on the parameters estimated in our models and thus further inform our understanding of Mars’s climate evolution.

Highlights:• Obliquity and Mars crossers' inclinations control elliptic crater orientations• Late Hesperian and younger craters possess a ~3:2 N-S orientation preference• Inversion of Mars' obliquity history PDF reveals low ~3.5 Gyr mean obliquity Abstract. The dynamics of Mars' obliquity are chaotic, and thus the historical ~3.5 Gyr 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°) and enhancing the potential for desiccating deep aquifers at low obliquities (< 25°). We developed a new technique using the orientations of elliptical 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 tracks. 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%.

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