Persistent or repeated surface habitability on Mars during the Late Hesperian - Amazonian

Large alluvial fan deposits on Mars record relatively recent habitable surface conditions (\(\lesssim\)3.5 Ga, Late Hesperian - Amazonian). We find net sedimentation rate 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.