Summary: Emily Lakdawalla from the planetary society describes one of the mysteries that's currently fascinating her. There are strange structures on Mercury which have been called "hollows". What might they be? "My name is Emily Lakdawalla - I'm the senior editor and planetary evangelist for the Planetary Society." What are the hollows on Mercury? "Well, the Messenger mission has been mapping and orbiting Mercury and they've discovered these new kinds of features that nobody had ever seen before that they were calling hollows - they're these pits in the ground, but they're not impact craters. They look like somewhere where something has evaporated. They look like Swiss cheese - like that terrain at the south pole of Mars where the carbon dioxide cap is evaporating. They look kind of the same, except that this is mercury - there's no carbon dioxide or water ice. It's all gotta be solid rock." "So what exactly is evaporating to create these holes and how it's disappearing from the ground is still a complete mystery. And the Messenger missions - they're following up on the high-resolution imaging, but they still don't have good info for what those hollows are." Are they similar to the caves on the Moon and Mars? "The caves that have been found on the moon and Mars are likely skylights into lava tubes. There's vulcanism all over the solar system - that's how most of the surfaces of most of the surfaces of most of the planets were built - with lava spewing out. And wherever you have lava, you've got often a solid surface on the top and then liquid lava running underneath the surface. Eventually that drains and you get a hollow tube, and then maybe an earthquake happens or something opens a skylight down into the tube. So we've found cave skylights on both the moon and Mars, and of course on Earth. I don't know that we've found any of these on Mercury - I don't think we have."
