THE seemingly bleak icy surfaces of Europa, Enceladus and Triton are in fact among the most active landscapes in the solar system. They may even contain cosy habitats for living creatures.
Jupiter鈥檚 moon Europa is covered by a cracked icy crust which resembles the Arctic floes of Earth. Its rocky core, however, is warmed by tidal heating, a result of the changing gravitational pull from Jupiter that arises from the moon鈥檚 slightly elliptical orbit (see 鈥淚cy Inferno鈥�). This probably generates enough heat to maintain a watery ocean beneath Europa鈥檚 frozen surface.
If this ocean stretches right down to the moon鈥檚 core, hydrothermal vents on the dark seabed could supply nutrients that could support micro-organisms, and perhaps even shrimp-sized predators.
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Saturn鈥檚 snowball, Enceladus, is more violent. A set of geysers near its south pole blasts out jets of water vapour and ice crystals. Some of this tumbles back down to Enceladus鈥檚 surface as snow, giving it a bright wintry coat that makes it the whitest object in the solar system. The rest escapes to form a foggy ring around Saturn.
The geysers may be rooted in an interior ocean beneath the moon鈥檚 south pole. If so, traces of any microbes that might be scratching out a living there would be blasted out too, where they could be picked up by a passing probe. Life on Enceladus would be much easier to detect than any imprisoned creatures on Europa.
Living on Enceladus would be no easy ride, however. All the moon鈥檚 activity is probably caused by tidal heating 鈥� unless there is something genuinely weird in there pumping out a lot of heat 鈥� and it seems that over hundreds of millions of years Enceladus wobbles in and out of its eccentric orbit, putting it in an uncomfortable cycle of climate change. Life would be doomed if the sea freezes completely during the coldest epochs.
Even chilly Europa and Enceladus, with mean surface temperatures of around 100 kelvin and 75 K, are balmy paradises compared with Neptune鈥檚 largest moon, Triton, where the temperature hovers around 40 K (below -230 掳C). Triton鈥檚 surface is frosted with various exotic ices, including blends of water, nitrogen and methane.
And yet this frozen world is surprisingly lively. Geysers erupt when sunlight evaporates volatile deposits of nitrogen, and a thin atmosphere of nitrogen holds tenuous clouds in weather patterns that change with Triton鈥檚 seasons.
Like Europa and Enceladus, Triton has a flat landscape with very few impact craters. Such a smooth complexion implies that the surface is very young 鈥� probably less than 10 million years old, a tiny fraction of the moon鈥檚 4-billion-year age. Triton鈥檚 fountains of youth are thought to be volcanoes that erupt a cold lava of water and liquid ammonia, which freeze to cover the surface with fresh ice and erase the signs of age.
Triton may once have been a dwarf planet like Pluto, orbiting the sun independently of Neptune. Indeed, Triton is about the same size as Pluto and has a similar composition, suggesting a similar origin. But the clincher is that it orbits Neptune backwards, in the opposite direction to Neptune鈥檚 rotation, impossible if it formed from the same rotating cloud of gas and dust as its planet. Instead, Triton was probably captured by Neptune.
Capturing such a large object is no mean feat. It may be that Triton smashed into an existing moon of Neptune, which slowed it down sufficiently for the planet鈥檚 gravity to snare it. A more likely theory is that it started life in a binary pair of dwarf planets, one of which was flung away at high speed when the pair encountered Neptune鈥檚 gravity, leaving Triton behind.
As well as being a remarkable moon in its own right, Triton may be giving us a hazy picture of all the unexplored dwarf planets 鈥� not just Pluto, but also Eris, Makemake, Haumea and probably dozens more that wander in the outer darkness of the solar system.
Read more: Weird worlds: The solar system鈥檚 10 strangest moons