With Mars, Pluto and Ceres grabbing all the headlines at the moment, you’d be forgiven for having never heard of Enceladus, the sixth-largest moon of Saturn. Discovered in 1789 by the famous astronomer William Herschel, it remained unvisited by humanity until the intrepid Voyager spacecrafts paid it a visit at the beginning of the 1980s. This icy world has since been visited by the Cassini spacecraft in 2005, which revealed a host of incredible ice volcanoes dotted across its surface. Last month, NASA confirmed that Cassini had discovered strong evidence for a global ocean beneath its icy crust. Today, the little spacecraft begins a series of flybys of the Saturnian moon, due to take place over the next few months, which will no doubt reveal more about this exciting little icy globe around 1.4 billion kilometers (890 million miles) from home.
One of the flybys will pass extremely close, zooming by at only 49 kilometers (30 miles) above the surface of the moon’s south pole, gathering more data on the nature of the ice volcanism that has been observed there. More significantly, the probe’s final flyby in December will examine the level of heat being emitted from the moon’s interior.
Ice volcanoes aren’t actually too different from regular ones. In much the way magma is the molten equivalent to solid rock, water is the molten equivalent to solid ice, and in the cold distant depths of our Solar System ice volcanoes are thought to be in plentiful supply. The most prominent plumes of erupting water and water vapor seem to be concentrated on Enceladus’ southern polar region.
Cryovolcanism, as the process is known, is driven by two things: the escape of pressurized gases (volatiles) from beneath the ice into the ultra-thin atmosphere, and differences in temperature within the planet causing hotter, less dense material to rise to the surface.
The question is, where is the internal heat coming from on such a frigid world, where surface temperatures reach lows of -240 degrees Celsius (-400 degrees Fahrenheit)? The answer lies, counterintuitively, on Io, one of the innermost moons of Jupiter and the most volcanically active object we know of. This hellish moon – which has volcanic eruption plumes reaching heights of 60 Mount Everests and lava erupting at temperatures of up to 1,300 degrees Celsius (2,400 degrees Fahrenheit) – is in fact heated in the same way Enceladus is.
The massive pull of Jupiter’s gravity, along with the moon’s interaction with two other nearby moons, exerts a huge gravitational force on its innards, ripping and tearing apart solid rock and causing it to melt. This process is known as tidal heating. Within Enceladus, the heat comes from its interaction with Saturn.
This internal heating, causing the melting of the moon’s ice, has led to the generation of a global, subsurface ocean. Cassini detected that Enceladus wobbles more than it should as it orbits Saturn, indicating a lower density segment of the planet that can only be explained by an oceanic region. This ocean is almost certain to be feeding the cryovolcanic activity seen at the south pole.
"The global nature of Enceladus' ocean and the inference that hydrothermal systems might exist at the ocean's base strengthen the case that this small moon of Saturn may have environments similar to those at the bottom of our own ocean," said Jonathan Lunine, an interdisciplinary scientist on the Cassini mission at Cornell University in Ithaca, New York, in a statement. "It is therefore very tempting to imagine that life could exist in such a habitable realm, a billion miles from our home."
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