Researchers also have found shield volcanoes, which can build to enormous heights through repeated eruptions of a more free-flowing form of magma. Mauna Loa is the largest shield volcano on Earth, rising just over 30,000 feet from its undersea base to its above-sea-level summit. Mars boasts the solar system's largest shield volcano, Olympus Mons, which vaults nearly 69,000 feet from its base – a base wide enough to cover most of France.
On Mars, however, evidence for supervolcanoes, which can alter climate as well as the landscape, has been missing.
Or maybe the evidence has just been misinterpreted, say researchers Joseph Michalski of the Planetary Science Institute in Tucson and Jacob Bleacher from NASA's Goddard Space Flight Center in Greenbelt, Md.
Exhibit A: a collection of what the pair interpret as calderas in Arabia Terras, a transition region between Mars’ southern highlands and northern lowlands.
Until now, the features that the duo analyzed were largely thought to be ancient impact craters that had long since filled in with sediment or were depressions that formed when ices beneath the surface melted and flowed away, causing the floor of the formations to slump.
The two researchers zeroed in on two in particular, Eden patera and Euphrates patera. Eden is a depression some 34 miles wide in one direction, 53 miles in the other, and about 1 mile deep at its deepest point.
Unlike impact craters, the researchers note, the suspected calderas have no elevated rims, central peaks on the crater floor, or noticeable rays of ejected material radiating out from the crater. Nor is there any surface evidence of sudden flows of water in the region to suggest ice melt.
Instead, the calderas appear in a patch of ancient crust surfaced with rock already known to have originated in volcanoes. The depressions are far larger than any known to have formed from melting deposits of underground ice. Perhaps more telling: The calderas display internal terraces, much like the interior of the far smaller Kilauea volcano on the big island of Hawai'i.