Volcanism on Mars: True story.

Andrea Thompson, for space.com

A NASA spacecraft's first flyby of Mercury has yielded a wealth of information about the inner-most planet, some of which confirms volcanism occurred there, settling a longstanding debate.

MESSENGER (short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging) made its debut flyby of Mercury on Jan. 14, passing about 124 miles (200 kilometers) over the planet's surface. The spacecraft's instruments took a closer look at the areas seen by the Mariner 10 mission in 1974 and 1975, which imaged about 45 percent of the planet's surface, as well as an additional 21 percent of the surface never before seen by a spacecraft.

Volcanism or impact melt?
Volcanism has long been thought to be a major force in shaping the rocky, terrestrial planets. Volcanoes still ravage Earth. On Mars, subdued volcanism may still be alive. Venus is riddled with old volcanoes.

Images of Mercury from the Mariner 10 mission showed areas of smooth plains covering parts of the planet's surface. Scientists speculated that these could be volcanic deposits, similar to the basaltic maria (seas) on the Moon. But unlike the maria, these plains were lighter, not darker, than the surrounding landscape, and Apollo 16 astronauts had just discovered that similarly light plains on the Moon were actually impact breccia, or rock that was smashed apart and then re-welded together again.

MESSENGER images of the Caloris basin, the youngest known impact basin on Mercury, showed smaller craters within the impact basin that had been in-filled with material, "and if you had impact melt [as with the lunar breccia], that wouldn't happen," explained Scott Murchie, a co-investigator for the Mercury Dual Imaging System (MDIS), of Johns Hopkins University.

The small craters likely were the result of impacts in the basin long after it was formed and later still, volcanic eruptions spewed lava across the basin, all but erasing the smaller craters. Head says this is "clear evidence that you're looking at lava flows."