Hadriaca Patera’s heavy footprint

On the northeast edge of the giant Hellas impact basin, the thick stack of lava in the volcano Hadriaca Patera weighed so much it depressed the Martian surface immediately around it. This bent and cracked the crust on the volcano’s eastern and southern sides.

GROUNDWATER EMERGED inside a trough on the southeast side of Hadriaca Patera volcano. As the water flowed southwest into the Hellas basin, it eroded the outflow channels of Dao and Niger Valles. (Taken from Figure 5 in the paper.)

Groundwater in the area, already under pressure, burst to the surface and ran down into the Hellas basin. On its way, the water eroded two large valleys — Dao Vallis and Niger Vallis.

That’s the outline scenario developed by Stefanie Musiol (Freie Universität Berlin) and colleagues, and published in the Journal of Geophysical Research. They used both topographic elevation data from the High Resolution Stereo Camera (HRSC) on the Mars Express orbiter and computer modeling to investigate the geophysical forces and their effects.

The Hellas impact occurred around 4 billion years ago, and scientists think that Hadriaca was erupting off and on between 3.9 and 3.5 billion years ago. (Hadriaca, with a diameter of about 450 kilometers, is part of a group of volcanos flanking Hellas that are the oldest known on Mars.)

According to the researchers, the weight of Hadriaca’s lava alone wasn’t sufficient to cause an outburst of groundwater. They found that a pressurized aquifer that could have developed in a broad trough reaching from Hellas northeast into Hesperia is also required. The downwarping of crust due to the increasing load as Hadriaca grew cracked the crust and released the pressurized groundwater on the volcano’s southeast side. As the water flowed toward the Hellas basin, it eroded Dao and Niger Valles.

But the flow, the researchers explain, probably wasn’t a single gigantic flood. “We suggest that Dao and Niger Valles source regions originated as flowing wells.” The valley floors do not show the telltale high-energy features — scouring, giant ripple marks — seen in many other outflow channels.

The team concludes, “One or more progressive outflow events involving several source regions, rather than a catastrophic flooding event from a single source, can explain Dao and Niger Valles.”

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