Astrophile is Joshua Sokol 's monthly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

See those tiger stripes NASA/JPL-Caltech/SSI/Lunar and Planetary Institute, Paul Schenk (LPI, Houston)

Place yourself at the south pole of Enceladus, the icy moon of Saturn. You are standing on a ridge overlooking a trench a few hundred kilometres long and 2 kilometres wide, parallel to three similar trenches – a linear pattern that planetary scientists call tiger stripes.

Below, the ice is cracked and jagged. Plumes of gas, ice and organic compounds hiss out of metre-wide crevasses, rising from an ocean of liquid water below all the way up into space.

Without all this, Enceladus wouldn’t grab so many headlines. It’s not much of a world, really. You could fit almost seven Enceladuses end to end along the equator of Earth’s moon. Its gravity is so weak that a bullet shot from a gun could easily escape into space. And it’s colder than a tank of liquid nitrogen, even during the summer.


Enceladus does have an attractive ocean of liquid water sealed beneath a coat of ice. But so does Jupiter’s much bigger moon Europa, and half a dozen other bodies in the solar system, probably.

What Enceladus offers, however, is data about the contents of an alien sea right now. Just the other week, evidence was announced that hydrothermal vents at the bottom of its ocean are bubbling out hydrogen gas – a substance on which microbes on Earth like to feast.

All that information is thanks to the tiger stripes: a place where stuff made at the very bottom of the ocean is helpfully thrown all the way out into space, where we can sample it.

A leaky moon

First discovered by NASA’s Cassini spacecraft in 2005, the tiger stripes look like a set of claws raked across the surface, revealing the moon’s tender insides.

The surfaces of the trenches, which a lander might visit some day, are the hottest spot on not just Enceladus, but any of Saturn’s moons. That’s all relative, though: it’s still objectively about as cold as a cooler full of dry ice.

Lower down, there must be caverns filled with water vapour at higher pressure, says John Spencer at the Southwest Research Institute in Boulder, Colorado. And lower still, lapping at the bottom of those chambers, is that tantalising ocean.

Since 2005, Cassini has allowed planetary scientists to piece together a rough sense of what’s happening at the tiger stripes. “Things are starting to come together,” Spencer says.

The heat and spray rising from the stripes can be fully accounted for, he says. As Enceladus circles Saturn, its orbit is squished out into an ellipse by the neighbouring moon, Dione. Strong gravitational tides between Saturn and Enceladus then pull its path back into a circle, stretching the interior of Enceladus and heating it up. That heat rises through the tiger stripes, where liquid water meets the moon’s thin atmosphere and vaporises, carrying small particles up with it.

Lingering mysteries

Some mysteries still linger, though. For example, the water vapour that comes out of the fractures should turn solid when it hits the atmosphere, freezing the fractures shut in just a few years – yet they seem to have been active for at least half a century, and probably many millennia.

The plumes on Enceladus are waning too, at least in the short term. For reasons that are unclear, they are half as active now as when Cassini arrived.

Time is running out to observe this moon. In September, Cassini’s mission to the Saturn system will be over, leaving us squinting at Enceladus from afar. Cassini is almost out of fuel, and leaving it puttering around Saturn would risk letting it crash into a moon – and possibly bring microbial stowaways with it.

To avoid that, engineers chose to have Cassini throw itself into Saturn – protecting precious Enceladus, its tiger stripes, and whatever lurks below.