By Phil Plait
We've known for a long time that Mars has volcanoes; after all, Olympus Mons on the Martian Tharsis shield is the largest volcano in the solar system.
But there may have been a more powerful menace lurking just below the red planet's surface eons ago. A paper published last week in Nature (though the authors first reported about it in March at 44th Lunar and Planetary Science Conference) claims to have found evidence of supervolcanoes on Mars.
On Earth, a supervolcano is far larger than a regular volcano; they have huge chambers of magma that can be dozens of kilometers across (like under Yellowstone). They build slowly over hundreds of thousands of years, and erupt catastrophically, and can change the landscape and climate of nearly the entire planet.
In the northern hemisphere of Mars is a series of craters that have been thought to be from impacts. Mars is littered with impact craters, since it has little atmosphere and no running water to erode away ancient features. But the craters in question, most notably Eden Patera (85 by 55 kilometers in size, or 53 by 34 miles), are a bit odd. The entire area is actually lower than average, literally sunken, which is consistent with a supervolcano; once the magma under the surface erupts out, the ground around it collapses to fill the empty space.
Other features make it suspiciously un-impact-like. The crater isn't round, it has no rim, no ejecta (material blasted out from impact), and no central peak (which are common in large craters, like Tycho on the Moon). There are substantial flows of lava around the area, and more evidence of ancient volcanism. Also, benches, or flattened rings around the inside of the crater, are present, and these are common in volcanoes as lava lakes subside.
If Eden Patera (and several other craters seen in the Arabia Terra region of Mars) are in fact supervolcanoes, this changes how we see ancient Mars. Olympus Mons, for example, probably built itself up over eons with big eruptions, but not catastrophic enough that it blew itself apart. That's why it grew to such enormous size; 600 kilometers (370 miles) wide and 22 kilometers (14 miles) tall- Mars also lacks plate tectonics, so a single volcano can just sit in one spot and grow ever larger.
But supervolcanoes explode. Gas dissolved in the magma is pressurized underground, but once it hits the surface it expands violently, creating a massive explosion. This would have distributed volcanic output like ash for thousands of kilometers across the planet. In fact, fine-grained volcanic deposits have been found across the equatorial regions of Mars that are most likely from volcanoes, but no source volcano has ever been found. Moreover, the deposits are layered, which is expected as well from episodic supervolcano explosions. Similar features are seen on Earth.
It's not certain that these craters are in fact supervolcanoes, but it would explain a lot. As we investigate more of Mars we're sure to find features that were previously unknown, and our view of the red planet will no doubt continually evolve. Perhaps we'll find evidence to support this idea of supervolcanoes; perhaps we'll find contradictory data. Either way, the only way we'll know is to keep exploring, keep looking around, and keep learning.