Billions of years ago, Mercury was choked in unimaginable amounts of lava. About six percent of the entire planet was covered in the lava of a single volcanic maelstrom. Not bad considering we weren't even sure Mercury had volcanoes.
Technically speaking, all that lava didn't come from volcanoes, at least not the kind we're familiar with here on Earth. Instead, lava simply started pouring out of cracks in Mercury's surface somewhere between 3.5 and 4 billion years ago. The lava spread out throughout the north polar region, spewing out enough lava to cover 60% of the United States, or enough to bury Texas four miles deep in lava. (And no, that isn't a proposal.) It isn't just surface area we're talking about - the lava was a mile deep in places.
Astronomers have wondered about the presence of volcanic activity on Mercury for decades, but it's only now with the arrival of the MESSENGER spacecraft in orbit around the planet that we can know for sure. The evidence for volcanoes on Mercury is subtle at best - there aren't any big mountain-like volcanoes on the planet, and the lava-covered regions don't look all that different from other areas, unlike the significant darker volcano-shaped highlands on the Moon.
Indeed, the massive outpouring of lava created hundreds of square miles of almost completely smooth plains, making it difficult to detect any clear physical features, let alone those that would prove past volcanic activity. To find proof, Brown University researchers had to look 125 miles outside the volcanic zone to find a fissure vent, which is just like the cracks that spewed out lava all those billions of years ago - only this vent isn't buried underneath thousands of feet of preserved lava.
Lead researcher James Head says this is the first confirmed evidence of volcanic activity on Mercury, but this single event was so gigantic that it's pretty much impossible that it was an isolated event. Now the hope is that MESSENGER can find even more evidence of Mercury's newfound chaotic past.
Via Science. Image courtesy of James Head research group, Brown University.