The Nature and Origin of Deposits in Uzboi Vallis on Mars

Uzboi Vallis in northwestern Noachis Terra is the uppermost segment of the Noachian to Hesperian Uzboi-Ladon-Morava mesoscale outflow system that dominates regional northward drainage through Noachis and Margaritifer Terrae. Recently acquired High Resolution Imaging Science Experiment (HiRISE) and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) data, as well as topography, help to characterize the nature and origin of a previously unrecognized deposit on the floor of Uzboi Vallis at the confluence of Nirgal Vallis. The Uzboi floor deposit consists of a lower unit (similar to 255 km(3)) and an upper unit (similar to 34 km(3)). The origin and age of the lower unit are less constrained; it may have formed by a combination of impact (e.g., ejecta from Luki crater), fluvial/alluvial, and (or) mass wasting processes. The superimposed upper unit is symmetrical relative to the mouth of Nirgal, fan-shaped in planform and layered. The morphology and location of the upper unit are consistent with alluvial (and possibly debris flow) deposition related to discharge from Nirgal Vallis. Crater Luki and the upper unit likely postdate the draining of the short-lived lake in Uzboi that formed after the mid- to Late Hesperian Holden crater blocked the northern end of Uzboi. Late-stage aqueous activity from Nirgal may have been sustained to some extent by groundwater discharge from aquifers along Nirgal that were recharged when Uzboi hosted a lake and (or) somewhat limited precipitation-fed surface runoff. The associated climate in the mid- to Late Hesperian was favorable for such aqueous processes, and it may have provided habitable surface conditions in this region relatively late in Martian history. Plain Language Summary One of the most impressive landscapes carved by liquid water on Mars occurs in northwestern Noachis Terra and southern Margaritifer Terra. Uzboi Vallis, Ladon Valles, and Morava Valles are a sequence of valleys that formed as water flowed into and out of depressions in the landscape, stretching from the southern highlands to the northern plains. Understanding the role of water in this region of Mars-in particular the period in which it occurred and the length of time it persisted on the surface-provides important information about past climate conditions. Recently acquired high-resolution images and topography were used to describe the nature and origin of a previously unrecognized mound of material on the floor of Uzboi Vallis. The mound of material in Uzboi is adjacent to the mouth of Nirgal Vallis, Uzboi's largest tributary. The lower part of the mound may have formed by a combination of material related to impact cratering, water-driven movement of sediment, or landslides. The upper part of the mound, however, is a fan-shaped deposit that formed as water flowed out of Nirgal and deposited sediment into Uzboi. Thus, the surface conditions in this region of Mars may have been hospitable for life relatively late in Martian history.