What is #34: Io?

The last post in this What Is series took a look at Europa, an icy moon orbiting Jupiter that has an ocean of liquid water underneath its icy surface. Today we look at Io, another moon orbiting Jupiter. Io is a very large moon that is the third largest of the Galilean moons. These are the four large moons that Galileo discovered orbiting Jupiter when he viewed Jupiter with a telescope. Io is larger than Europa and a bit larger than our own moon.

NASA / JPL / University of Arizona [Public domain]

Io is very, very different from Europa. Io is not covered in ice like Europa. Io is very volcanic. In fact, it is the most volcanically active object in the entire Solar System! This is seen in the image above. The varied colors and constantly changing atmosphere are evidence of Io's very active surface. We've even seen active plumes in real time extending from the surface of Io, as seen in the image below.

NASA's Galileo spacecraft [Public domain]

Why is Io so active? Although a large moon, an object this size is expected to be geologically dead, as our Moon is no longer geologically active. Io's proximity to Jupiter, however, creates strong tidal forces that heat its interior, resulting in a moon that is still active and very active at that!

When it comes to moon's in the Solar System, Io, despite the lack of a water, is very, very interesting!