About 30 years ago, we Earthlings sent a probe to check out Mercury, the tiny planet closest to the sun, and concluded that it was just a big hot rock. But after poking around on the moon and Mars for a few decades, we decided to take another look at Mercury. Messenger, the probe that has now passed Mercury twice, has sent back a collection of photos that shake up our earlier assumptions about the planet. For example: There are a lot more volcanoes there than we originally thought — many more than on the moon, which we used to think was a pretty close approximation.
