December 24 – Rhea

Saturn’s second-largest moon, Rhea, was discovered on this day in 1672 by Giovanni Domenico Cassini, who had presumably finished his Christmas shopping early.

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Rhea from the Cassini probe (image: NASA / JPL / SSI)

Like our own Moon, Rhea always keeps one side, the one you can’t see in this photo, facing towards the parent planet, and from the above view of Rhea’s cratered anti-Saturnian surface, this does look to be a place very much like the Moon we are familiar with down here.  But appearances can be deceptive.  For a start, it’s a lot smaller than the moon, and if it soft-landed on the Earth it would fit quite comfortably inside the borders of Zaïre (in this country people usually use Wales as the benchmark for large areas, but I felt like a change).  Secondly, although it looks nice and rocky, Rhea is thought to be comprised of as much as three-quarters ice, and only one quarter rock (studies of Rhea’s inertia suggest that it doesn’t even posses a rocky core).

Rhea is named after a Titaness of some standing, the daughter of Uranus and Gaia.  As mother of Zeus and Hera, she was grandmother or great-grandmother to almost all the Olympian gods and goddesses worth mentioning.

Rhea

Rhea

There are several suggestions doing the rounds concerning the origin of the name Rhea.  It may derive from the word for “flow”, or it could have a root in a much older word for “powerful”.  But the guy who named the large, flightless, South American bird after her was almost certainly thinking of a derivation of the Greek έρα, meaning “ground”.

Another Rhea

Another Rhea