The further back in Earth’s history your mind wanders, the more work your imagination has to do. This is even more true for Mars. None of us have physically set foot on the current version of the planet, and the past was clearly very different from the present, with evidence pointing to flowing and standing water.
Among the remnants of the watery past are networks of valleys cut into the surface of Mars. The highlands of the Southern Hemisphere of the Red Planet are home to many valleys, which have largely been interpreted as being formed by rivers and groundwater sources. The source of water in rivers — whether from rain in a warm climate or simply melting glacial ice — has been a question mark.
Mars’ past is thought to have been quite cold in general, so a connection between the valleys and glacial ice is quite plausible. But how direct is that connection? We can identify the drainages in which water flows below ice sheets based on physical features of the backwater valleys. So a team led by Anna Grau Galofre in the state of Arizona set out to explore the valleys on Mars to see if one would be a better fit for a subglacial origin.
They worked with topographical data for more than 10,000 valleys in nearly 70 connected valley networks, each characterized by a number of statistics. That includes some simple things, such as the width of the valley, the number of tributaries upstream, and the angle between connected channels. Another stat that turned out to be important is whether the valley ripples up and down a bit, or instead drops down smoothly without interruption.
By combining all the stats, the valleys fall into different clusters. There is an indistinct area of overlap meaning that some valleys cannot be identified as one or the other, but the others fell in line. For example, the width of the valley is good at picking out valleys carved here on Earth by glacial ice. Simple channels with few tributaries indicate groundwater sources seeping down. And subglacial drainage valleys tend to meet at angles closer to 90 degrees and waves up and down — propelled by high water pressure beneath a glacier.
Of the 66 networks of valleys, 18 could not be identified and three appeared to be the result of groundwater sources. Only 14 were recognizable as formed by rivers. But nine valley networks had the characteristics of glacial erosion and 22 fell under the subglacial drainage category.
To see if that was true, the researchers compared the locations of the subglacial drainage valleys with a previously published ice sheet model. Glaciers and ice sheets have a higher, colder zone where snowfall accumulation dominates, and a lower, warmer zone where melting or sublimation losses dominate. In that lower zone, where meltwater is produced, subglacial drainages must be present. Shown on a map, those 22 valley networks are generally in the region where the ice sheet model predicts they should — between the edge of the ice sheet and the “balance line” separating the melt zone from the accumulation zone.
The researchers think this ice sheet was mostly frozen to the ground, but with localized areas where friction or geothermal heat from below melted the ice at the base. That would actually be a simple explanation for consistent sources of flowing water on a cold Mars, though it might paint a frostier picture than the one you had in your head.
“This image changes the current understanding that all Mars valley networks were created by surface runoff and provides geomorphological support for the presence of ice sheets on early Mars,” the researchers write, “reconciling climate predictions with observations from the geologic field.” record.”
Nature Geoscience, 2020. DOI: 10.1038/s41561-020-0618-x (About DOIs).