It strikes me that a number of planets* in the solar system get a disproportionately low amount of attention. There have been a number of discussions at great length about Europa, Mars and Titan (and of course, I find these worlds as fascinating as anyone else). But what about all the other worlds out there?
Personally, one such forgotten planet I find particularly interesting is Jupiter’s giant moon, Ganymede (topically enough for me at the moment). Here are a few reasons why…
A mysterious place, with a radius approximately 2 fifths that of Earth, Ganymede is the largest moon in the Solar system, larger than Titan and significantly larger than our own Moon (shown conveniently to scale against a few other rocky planets in this image). But this isn’t it’s only interesting feature. Ganymede is the only Moon known to posess a fully fledged magnetosphere — a powerful internally generated magnetic field. Many planets have “fossil” magnetic fields, effectively frozen into them as they cooled and solidified, and some even have induced magnetic fields, courtesy of the solar wind. Ganymede’s is just too strong for either of these explanations to hold — it even causes Ganymedian aurorae at the poles, just like the aurorae on Earth! The most likely reason for a magnetosphere would be a core of liquid iron. Even Mars doesn’t have an intrinsic magnetic field like this (Mars too, is fossilised). Some have even argued in the past that a magnetic field is a good indicator of whether a planet is “alive” (in the sense that Earth is) or not.
The bizarre similarity with Earth doesn’t end there, however. It’s surface (shown in the top image which, by the way, is computer generated) shows an obvious dichotomy of light and dark regions. One theory as to why this is, is that Ganymede had (or maybe still has) tectonic plates. If true, this would make it the only other body in the solar system to show tectonic activity (neither Mars nor Venus have any obvious tectonics).
EDIT– Interestingly since writing this, some other evidence of tectonic activity has been found in a rather surprising location — on Saturn’s tiny moon Enceladus. Surprising, because, by radius, Enceladus is only one fifth the size of Titan!
Considering all of this, Ganymede would probably be a very good location for a human colony in the outer solar system. It’s magnetosphere would eliminate the need to shield a colony from solar radiation (as well as the radiation from Jupiter which blights Europa!), while the plentiful water ice and even oxygen found on the moon would be extremely helpful in supporting a colony.
Incidentally, that oxygen shouldn’t get any hopes up for life. It can be created quite easily from water by solar UV. This is a ready supply for Ganymede’s (practically nonexistant) atmosphere of oxygen and ozone. Though the other things on the surface do hint at interesting chemistry. Carbon dioxide, sulfur dioxide, cyanogen, and various organics. If some ammonia were there too, it would be not unlike the combination of chemicals used by Stanley Miller to create basic biomolecules in ice. Same temperature too.
Also detected on Ganymede’s surface are sulfate salts, causing some to suggest something interesting — Ganymede could have a subsurface ocean! In fact, it’s estimated at around 200km below the surface. If Europa could, it seems fair that Ganymede could too. They most likely formed from the same material, and are both believed to have a mantle composed primarily of water ice (Earth’s mantle, for comparison, is hot silicate, which makes it’s presence known at the surface as magma). Anomalies detected in Ganymede’s magnetic field could also hint at an ocean, and theories have been devised to explain how tectonic activity on Ganymede could circulate nutrients to a potential subsurface ocean, supporting a biosphere. So could there really be a hidden biosphere in this subsurface ocean? Could Ganymede be just as good a target for astrobiology missions as Europa?
For now, I guess we’ll just have to wait until the human race eventually goes back to Jupiter. If all goes according to plan, it looks like that could actually happen quite soon. It’s funny how space exploration is the only time when 7-8 years can be considered as “soon”.
* And yes, throughout this article, by “planet” I mean anything round that’s smaller than a star and larger than an asteroid. It’s just easier that way, ok?
Disclaimer: The title, “Autumn on Ganymede” is more artistic than factual. Technically, with virtually no axial tilt, Ganymede has no seasons! However, it is currently Autumn for half of Saturn, given it’s current appearance from Earth — with rings nearly edge-on.