FORMATION TIMESCALES OF THE MARTIAN VALLEY NETWORKS

The presence of valley networks across much of the ancient surface of Mars [e.g. 1] together with the locations and morphologies of the Martian deltas [2] and ancient paleolakes [e.g. 3], provides strong evidence that the Martian surface environment was once capable of sustaining long-lived flowing water [e.g. 4-6]. As many of the largest valley networks appear to have formed primarily from surface runoff of precipitated water approximately 3.6 to 3.8 Ga [7, 8], their formation can be modeled through sediment transport by bed load and suspended load processes.

Numerous sediment transport equations have been developed to describe flows on Earth [e.g. 9], many of which have been applied to Mars [e.g. 10, 11]. In this work, we investigate the formation timescales of the Martian valley networks through the use of the Darcy-Weisbach equation for average flow velocity, four different sediment transport models, and a variety of parameters to encompass a range of possible formation conditions.

The formation timescales for eight of the largest ancient valley networks investigated ranged from a few years to a few hundred million years, depending on the combination of parameters used. The continuous formation and deeper flow depth (25-40 m) scenario provides minimum formation timescales of 10 to 100,000 yrs. As the continuous formation scenario is perhaps ambitious for maintaining these flow depths continuously on early Mars, an intermittent formation scenario that is more typical of terrestrial river formation describes potentially more realistic formation conditions and is consistent with Martian valley network and surrounding crater geomorphology [e.g. 6]. For shallower flows (~5 m) occurring 5% of the time, the intermittent formation scenario produces timescales of 10,000 to 100 million yrs.

References: 1. Hynek et al. (2010) J. Geophys. Res., doi:10.1029/2009JE003548; 2. Di Achille and Hynek (2010) Nature Geoscience, doi:10.1038/ngeo891; 3. Irwin et al. (2005) J. Geophys. Res. 110; 4. Craddock and Howard (2002) J. Geophys. Res. 107; 5. Howard et al. (2005) J. Geophys. Res. 110; 6. Barnhart et al. (2009) J. Geophys. Res. 114; 7. Fassett and Head (2008) Icarus 195; 8. Hoke and Hynek (2009) J. Geophys. Res 114; 9. Gomez and Church (1989) Water Resources Res. 25; 10. Komar (1980) Icarus 42; 11. Kleinhans (2005) J. Geophys. Res. 110