Lee Billings is a senior editor for space and physics at Scientific American. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Get smart. Sign up for our email newsletter. Sign Up. Support science journalism.
Knowledge awaits. See Subscription Options Already a subscriber? Mimas, the "Death Star moon ," is pretty much one big snowball. There doesn't seem to be much more to it than water ice. Yet a few unusual features hint at something weird on Mimas. The moon wobbles as it orbits Saturn, which indicates something unusual going on beneath the surface. The Cassini team says that it could be an ocean. But only maybe. The other chief possibility is that Mimas has a football-shaped core giving it the unusual tilt.
At around the size of Enceladus, the moon is too small to retain the heat from its formation, so any ocean on Mimas would have to have an outside force acting on it—possibly radioactive decay. Neptune's largest moon, Triton, looks a lot like Pluto. There's a reason for that. Its retrograde backwards orbit in comparison to the rest of the system suggests that Triton could be a captured Kuiper Belt object, and not something that formed alongside the planet.
The moon's surface seems to be a mix of methane and water ices, much like Pluto, and there's the outside chance of an internal ocean, provided there is enough heating or radioactive decay.
The moon likely has geysers, but instead of water they probably shoot nitrogen, giving the moon a thin atmosphere.
We just don't know much else about Triton because the only close-up imaging came from Voyager 2's flyby in The same thing goes for Uranian moons: We need a better look at them. But preliminary indications show that Titania and Oberon are likely ice and rocky materials.
Neither has, at the time, enough evidence to support liquid water hypotheses without an anti-freeze agent like ammonia. Umbriel, too, is largely composed of ice, but is even less likely to have an ocean.
It does, however, contain a bright spot of ice near one of its poles, likely the effect of a crater impact on the surface. There is also evidence of carbon dioxide gasses trapped under the surface. These moons of Saturn appear similarly frozen, though there's an outside chance of liquid water on Rhea.
These worlds are relatively inert, though Iapetus shows evidence of water sublimation moving directly from solid to gas on the surface.
While these moons may not be good candidates for liquid water, they demonstrate the sheer abundance of water in the outer solar system. There are hundreds of known objects out in the Kuiper Belt, where Pluto resides, many of them believed to be icy. The dwarf planets Eris and Haumea are believed to be similar to Pluto in composition, with water ice on the surface.
But these small worlds have been discovered only in the last decade. There are also a few dwarf planet candidates that are known to be icy in nature, including Varuna, Quaoar, and Orcus. The latter has some indication of cryovolcanism and could potentially have a liquid ocean. There are also a number of comets in the Kuiper Belt and beyond that are believed to be composed of water.
This includes the first identified member of the Oort Cloud, Sedna. Perhaps the most surprising place water has been detected in the solar system is Mercury, the closest planet to the sun. While the surface is scorching, the pole are often untouched by the sun's heat, leading to an area where ice can accumulate. The Moon and Mercury, they're really not so different in appearance. Both are airless, rocky worlds, and both, it seems, have accumulations of water ice at the poles.
Scientists had long suspected ice could be on the moon. India proved it in … by crashing the Chandrayaan-1 probe headlong into the ice and seeing the plumes it formed. While it's far from abundant, the water ice on the moon could help out moon colonization some day.
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