The origin and timing of fluvial activity at Eberswalde crater, Mars

The fan deposit in Eberswalde crater has been interpreted as strongevidence for sustained liquid water on early Mars with a paleolakeformed during the Noachian period (>3.7 Gy). This location became akey region for understanding the Mars paleo-environment. Eberswaldecrater is located 50 km north of the rim of the 150 km diameter craterHolden. Stratigraphic relationships and chronology obtained using recentMars Express High Resolution Stereo Camera and Mars ReconnaissanceOrbiter Context Camera images show that Eberswalde fluvial activitycrosscuts Holden ejecta and thus postdates Holden crater, whoseformation age is estimated from crater counts as Late Hesperian(˜3.5 Gy, depending on models). Fluvial modeling shows that shortterm activity (over several years to hundreds of years) involving denseflows (with sediment:water ratio between 0.01 and 0.3) may be as good anexplanation of the fluvial landforms as dilute flow over longerdurations. Modeling of the thermal effect of the Holden impact in theEberswalde watershed is used to evaluate its potential role in aqueousactivity. The relative timing of the Holden impact and Eberswalde's fanis a constraint for future studies about the origin of these landforms.Holden ejecta form a weak and porous substrate, which may be easy toerode by fluvial incision. In a cold climate scenario, impact heatingcould have produced runoff by melting snow or ground ice. Any attempt tomodel fluvial activity at Eberswalde should take into account that itmay have formed as late as in the Late Hesperian, after the greatmajority of valley network formation and aqueous mineralization on Mars.This suggests that hypotheses for fan formation at Eberswalde bytransient and/or localized processes (i.e. impact, volcanism, unusualorbital forcing) should be considered on a par with globally warmerclimate.