  Large alluvial fan deposits on Mars record relatively recent habitable
surface conditions ($\lesssim$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.
