GULLY FORMATION AND SEASONAL FLOWS ON MARS: REVISITING WATER FLOW PROCESSES AS A FORMATION MECHANISM ON RECENT AND CURRENT MARS

The discoveries of Martian gullies (Malin and Edgett, 2000) and seasonal slope flows (McEwen et al., 2011) active on Mars today have re-ignited the debate over various channel, valley, and gully formation mechanisms on Mars. The controversy over whether liquid water was the primary formation mechanism, harkens back to the contentious debate over the cataclysmic flooding of the Channeled Scablands advocated by J Harlen Bretz. Likewise, during the mid-1970s to early 2000s, catastrophic flooding and ground-water sapping processes advocated by Vic Baker and others were strongly debated along with other mechanisms as the primary processes responsible for outflow channel and valley network formation on Mars. During the last few years, the value of multiple working hypotheses is again becoming apparent, this time in understanding Martian gullies and Recurring Slope Lineae. Formation mechanisms put forth to explain these fluvial-like landforms include liquid H₂O/ice erosion, CO₂ ice/frost sublimation processes, CO₂ice block sliding, salt deliquescence, and dry granular flows, among others.

In this study, we used recent images returned by NASA’s Mars Reconnaissance Orbiter’s (MRO) HiRISE camera along with other data to evaluate the potential role of water processes in forming the gullies and other similar features. We constructed drainage maps and carried out detailed morphological and morphometric studies of gully systems in various environmental settings. Using HiRISE digital terrain models, we generated detailed longitudinal and cross-sectional profiles of numerous gully systems and derived volume estimates for both the gullies and their debris aprons. Several gully networks formed highly integrated patterns similar in morphology to fluvial systems. We find that the highly integrated systems generally have eroded volumes significantly larger than their apron volumes, suggesting the missing volumes (~40-60% or more) were likely volatiles involved in gully formation. Surface temperature data suggest that the volatile component may be more consistent with H₂O although CO₂ cannot be ruled out. Other less integrated systems have apron volumes that equal or significantly exceed the eroded gully volumes suggesting that dry flows, avalanching, gully infill, or other dry processes may have been more important.