Large alluvial fan deposits on Mars record relatively recent habitable surface conditions (≲3.5 Ga, Late Hesperian - Amazonian). We find net sedimentation rate <(4-8) {\mu}m/yr in the alluvial-fan deposits, using the frequency of craters that are interbedded with alluvial-fan deposits as a fluvial-process chronometer. Considering only the observed interbedded craters sets a lower bound of >20 Myr on the total time interval spanned by alluvial-fan aggradation, >10^3-fold longer than previous lower limits. A more realistic approach that corrects for craters fully entombed in the fan deposits raises the lower bound to >(100-300) Myr. Several factors not included in our calculations would further increase the lower bound. The lower bound rules out fan-formation by a brief climate anomaly. Therefore, during the Late Hesperian - Amazonian on Mars, persistent or repeated processes permitted habitable surface conditions.
@article{arxiv.1703.06386,
title = {Persistent or repeated surface habitability on Mars during the Late Hesperian - Amazonian},
author = {Edwin S. Kite and Jonathan Sneed and David P. Mayer and Sharon A. Wilson},
journal= {arXiv preprint arXiv:1703.06386},
year = {2018}
}