Another pretty film-noir image from space.
Our moon is relatively large compared to the planet it orbits. A number of outer solar system moons are of significant size. Compared to these huge planets, they seem rather puny. Take Rhea, Saturn’s second-largest moon, for instance, in this image from the Cassini space probe.
Rhea is about half the diameter of our own moon, but when viewed with Saturn, seems relatively insignificant in size. In this image, you can just make out some brighter swaths against the dirt-gray ice. Rhea features ice cliffs, fractures in the frozen water crust. Scientists are still unsure why the much smaller Enceladus has active water and ice geysers and Rhea does not. Before space probes glimpsed these moons closely, volcanism, even ice volcanism, wasn’t much of a thought. And when we saw those first volcanoes on Jupiter’s moon Io, the conventional thinking was that larger bodies had more of a chance of heated interiors. Yet Rhea looks dead. Why?