The USGS has published a brand new map of the surface of Io, a moon of Jupiter and one of the most geologically active bodies in the entire solar system. The map shows hundreds of volcanoes, massive lava flows, and some of the most violently active surface features planetary scientists have ever seen.
Here's the latest map, which is a combination of data from the Voyager probes and the Galileo mission (click to enlarge):
You can go ahead and blame Jupiter for Io's bad attitude: the insides of the moon are heated tidally, which means that as Io orbits around Jupiter, Jupiter's gravity slowly pulls the moon's innards back and forth. All that stuff moving around creates lots of friction-induced heating, which is where the excessively nasty volcanism comes from.
The map also shows that Io's surface has no visible impact craters. Zero. This implies that Io hosts one of the most active surfaces anywhere in the solar system, with more volcanic activity and erosion than any other planet, including Earth.
Impact craters, you see, form on every single planetary body. Mars has lots of them, and so does the Moon. Earth has a bunch, too. The fresh ones are easy to spot, like Meteor Crater in Arizona. But older craters gradually get covered up over time. For example, underneath Chesapeake Bay is an impact crater about 90km in diameter. You can't see it anymore because over the last 35 million years it's been covered in sediment, but that's the way it works: new craters form, and geological processes gradually cover them over.
Since the rate of new crater formation on planets is constant, planetary scientists can count the number of craters on a surface and then estimate how old that surface is. Younger surfaces will have had less time to accumulate craters, and older surfaces will show more, larger craters. Earth, thanks to all our plants and water and stuff, has a very geologically active surface, while all except the very largest craters get erased quickly, you can still see some if you look. Io, on the other hand, has no craters at all, implying that it's surface is so volcanically active that every time a crater forms, it gets covered up in the blink of a geologic eye by fresh lava. Here's a picture of this actually happening, with a caption from NASA:
"NASA's Galileo spacecraft caught this volcanic eruption in action on Jupiter's moon Io on November 25, 1999. This mosaic shows Tvashtar Catena, a chain of calderas, in enhanced color. It combines low resolution (1.3 kilometers, or .8 miles, per picture element) color images of Io taken on July 3, 1999 with the much higher resolution (180 meters, or 197 yards, per picture element) black and white images taken in November. The molten lava was hot enough, and therefore bright enough, to saturate, or overexpose, Galileo's camera (original image is inset in lower right corner). The bright lava curtain (a chain of lava fountains) and surface flows shown in the color image were assembled as an interpretive drawing by Galileo scientists, based on their knowledge of how the camera behaves when saturated. The lava appears to be producing fountains to heights of up to 1.5 kilometers (5,000 feet) above the surface. Several other lava flows can be seen on the floors of the calderas. The darkest flows are probably the most recent."
And here's an image from New Horizons (en route to Pluto) showing another gigantic active volcano on Io (click to clear the fuzz):
Clearly, Io is a wild, wild place, arguably one of the most interesting things to explore in the entire solar system. Our next really good look at Jupiter (and its moons, including Io) will come in 2016, when Juno arrives and starts sending back some brand new pics.