Assessing the termination of valley networks at the proposed Noachian ocean shoreline on Mars

It has previously been suggested that an ocean occupied the martian northern lowlands during the Noachian, and paleo-shorelines have been proposed as evidence for this hypothesis (e.g., Clifford and Parker, 2001). The widespread presence of fluvial valley networks (VNs) indicate the presence of abundant flowing surface water on early Mars, potentially contemporaneous in time with the presence of the putative ocean. To date, it is not understood if and how the VNs were hydrologically connected to a lowlands ocean. Here we assess how VNs may have fed directly into a Noachian ocean through tributary flow. Specifically, we determine the populations of Noachian- and Hesperian-aged VNs that terminate at the proposed shoreline for a Noachian ocean (elevation of -1680 m) or near the proposed shoreline (elevation of -1680 m ±20 m), as defined by the shoreline elevation and its 1-sigma standard deviation analyzed by Head et al. (1999) with Mars Orbiter Laser Altimeter topographic data. Using the population of VNs that terminate at or near the proposed shoreline elevation, we then estimate how much water may have been delivered to the putative ocean by overland flow using the methods of Rosenberg et al. (2019). We compare our estimate to the total volume of water that is suggested to have once been enclosed by the Contact 1 shoreline (Carr and Head, 2003). We conclude by discussing implications for possible water sources contributing to a Noachian ocean on Mars. Understanding the relationship between surface water flow from VNs into the proposed Noachian ocean is an important component in analyzing the early climate of Mars and testing the hydrologic model discussed in Clifford and Parker (2001).

References:

Carr, M. H., Head, J. W. (2003). JGR 108, 5042. DOI: 10.1029/2002JE001963.

Clifford, S. M., Parker, T. J. (2001). Icarus 154, 40-79. DOI: 10.1006/icar.2001.6671.

Head, J. W., et al. (1999). Science 286, 2134-2137. DOI: 10.1126/science.286.5447.2134.

Rosenberg, E. N., et al. (2019). Icarus 317, 379-387. DOI: 10.1016/j.icarus.2018.07.017.