The Cassini Space probe revealed Saturn's moon Enceladus as one of the most promising places in the Solar System to find life beyond Earth. Among the many mysteries about this 500 kilometer-wide (310 mile) ball of ice with a watery heart is why it is the only place we know to have a set of four “tiger stripes” across its south pole. These stripes are no mere curiosity, they're what gives us a chance to discover if life really does exist deep inside.
When water was found to be bursting forth from Enceladus' geysers a previously obscure and insignificant moon became a subject of intense interest. Liquid water is thought to be the underlying requirement of life, and we've since discovered Enceladus has some of the others as well.
The reason water survives within this moon when surface temperatures are -200º C (-325º F) was quickly established – the changing gravitational influence of Saturn and nearby moons causes internal heating, similar to what occurs within Jupiter's moons Io and Europa. However, planetary scientists were still puzzled as to how this water was reaching the surface, and ultimately space. A new paper in Nature Astronomy provides a likely answer.
The tiger stripes are around 135 kilometers (90 miles) long and quite thin, spaced around 35 kilometers (22 miles) apart. “What makes them especially interesting is that they are continually erupting with water ice, even as we speak. No other icy planets or moons have anything quite like them," said Dr Doug Hemingway of the Carnegie Institute in a statement.
Hemingway has shown the deformation caused by the play of other objects' gravity on Enceladus is strongest at the poles, making the outer ice sheet thinnest there. The paper proposes the expansion of water at the top of the inner ocean as it freezes puts pressure on the icy shell, making an eventual split inevitable. The thinness at the poles makes them the point of vulnerability. Hemingway and co-authors don't think there is anything special about the south pole – it just happened to split before the north, and in the process released the moon-wide pressure.
Once the fissure named Bagdad that directly crosses the South pole opened, ocean water sloshed into it, bringing enough warmth to prevent refreezing. Ever since, sub-ice jets have created plumes of spray, sometimes escaping Enceladus' weak gravity entirely to form Saturn's E ring. Most, however, falls back onto the ice, building up until its pressure causes new breaks to form in parallel with the original.
Before the Voyager mission astronomers assumed the biggest moons would be the most interesting, but if Enceladus was much larger its gravity would be too great to prevent the fractures opening. If so, instead of future probes being able to sample the plumes we would have needed to drill through the ice to discover if life thrives underneath. Thankfully, it's already done that part for us.