Moon of the Day (3)
Today’s pick comes from the Jovian system:
Image via: Wikipedia and the Galileo orbiter.
Ganymede is hard to miss. It’s sufficiently prominent that it was originally discovered by Galileo on the 7th of January, 1610 – in fact, it may well have been the first object ever discovered via a telescope. (We just don’t know precisely which of the Galilean Moons Galileo saw first; we do know that he saw all four of them on the same night!) Ganymede is actually fairly bright, for an outer-system object. At magnitude 4.5ish, it would actually be a naked-eye object, except for the glare from Jupiter.
Ganymede is so relatively-bright because it’s big. In fact, it’s the biggest moon in the solar system. At 5,268 Km in diameter, it’s actually wider than the planet Mercury, and it’s only about 1500 Km smaller than Mars. The total mass of Ganymede is slightly more than twice that of our Moon. This means that it’s also first in terms of mass as well as radius. (Second place goes to Titan, in both lists.)
Ganymede is believed to have a fully-differentiated structure. Here’s another picture, via Wikipedia, to illustrate that:
In this sense Ganymede is much more similar to the inner, terrestrial planets than to the more icy moons of Saturn, Uranus and Neptune. It has a solid, rocky core, surrounded by a silicate mantle. Over this is believed to be a water mantle. The crust-material at the moon’s surface appears to be predominantly water ice.
Ganymede’s surface terrain has two distinct portions. The darker, crater-saturated regions are believed to be the oldest, with an estimated age of around four billion years. The lighter regions are thought to be younger – partly indicated by their comparatively-low crater numbers – and it is thought these were originally tectonic features. Whether these processes are still active today is unknown, as far as I’m aware, but it generally seems to be believed that they aren’t.
Ganymede has an additional weird aspect; it’s the only known moon with a reasonably-strong magnetic field. This is thought to be due to the presence of that silicate-iron core, deep inside the moon. In fact, it’s magentic field is about three times stronger than that of Mercury (Ganymede does seem to like spiking Mercury’s guns, doesn’t it?).
Ganymede’s large mass allows it to affect other bodies significantly, even at some distance. The timing sequences of its orbit and that of Europa combine to ‘pump’ the orbit of Io, helping it maintain its eccentricity. This eccentricity is why Io is such a volcanically-active object (I have a previous post on the subject of Io, in fact).
Ganymede also has a connection to another inner moon; like Europa, it’s believed to possess a subsurface ocean. This does raise the possibility that there may even be something living down there, although I suspect it’s unlikely that we’ll ever know for a fact!
One more thing is worth noting; when Ganymede first formed, it would presumably have had surface water. This would be due to Jupiter; Jupiter’s formation would have released a lot of heat and for at least a few million years, Jupiter would have behaved much more like a very small brown dwarf than like the planet we’re used to. (It still emits more heat than it receives from the Sun, even to this day – see this post of mine if you’d like to see how Jupiter’s IR spectrum compares to that of a brown dwarf.) Whether this would have been long enough for any biological activity to occur, of course, is a question I can’t answer. But who knows?
Whatever else, Ganymede certainly remains an interesting (and rather large) object.