The first clue that Arabia Terra was formed via landslide is that the relatively flat region has steep slopes at both its northern and southern edges—like a giant step—Andrews-Hanna said.
Similar features, he said, occur in other large impact craters, many of which have bull's-eye patterns—concentric circles or ellipses of steep ridges separated by gently sloping plateaus. The similarity of Arabia Terra to these other craters indicates that it too might have been created by an impact.
Another clue is that, at Arabia Terra, the inner rim of the Borealis Basin doesn't line up with its inner rim elsewhere on the planet. Instead, the rim juts northward by about 300 kilometers—as if a landslide had smudged the clean break seen in areas to the west and east.
Most landslides occur on the surface. This one, Andrews-Hanna said, appears to have been deeper. After the asteroid impact, "these rocks in the lower crust want to flow inwards into the basin, dragging the rocks at the surface with them. It appears as though the crust has flowed hundreds of kilometers into this basin right after it formed," he said.
How quickly did it move? Andrews-Hanna isn't sure, but in geological terms it was quite rapid.
In fact, in the best studied (though much smaller) Borealis-like basin on the moon, a similar landslide seems to have occurred before material blasted out of the impact crater even had time to hit the ground.
"On the other hand, things might move more slowly with a giant basin like Borealis on Mars," he said.
Chicago-based astronomer Mark Hammergren hails the new idea as a "natural extension" of the Borealis asteroid-impact hypothesis.
"At the same time, a terrace is something we might expect to see in some form around the Borealis basin, so this hypothesis is not uncomfortably 'shoehorned' into a Borealis impact 'theory of everything,'" he said via email.
Brown University geophysicist James Head III also finds it "pretty plausible" that Arabia Terra was created in conjunction with the Borealis Basin impact.
But Head isn't sure whether a deep landslide did the deed. Rather, he said by email, the force of the impact may have twisted blocks of the planetary crust outside the basin beneath what is now Arabia Terra, causing its surface to drop to its current midlevel position.
Study author Andrews-Hanna notes that the thin crust beneath Arabia Terra requires that something stretched the crust, with the likely candidate being this landslide deep beneath the surface. However, he suggests that a combination of the two mechanisms may have been responsible, with a landslide of material deep beneath the surface being the cause of the twisting nearer to the surface.
No matter how the giant asteroid impact may have formed Arabia Terra, Andrews-Hanna joked, "it was certainly not a good time to be a Martian!"
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