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 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.”

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