Artist’s impression of the plumes. Credit: J. KAUFMANN

NASA’s Cassini spacecraft is set to plunge into an intriguing plume of gas spouting from Enceladus, one of Saturn’s moons.

Cassini stumbled upon the astonishing discovery — a geyser of ice and water vapour spewing out of cracks in the tiny satellite’s south pole — during a fly-by in 2005. But the spacecraft’s probe barely touched the plume. This week, Cassini’s instruments will get their first true taste of the tantalizing plume, when they try to measure the size and speed of the gas and ice particles, and their chemical composition.

“Last time, we just skimmed the edge,” says Cassini scientist John Spencer at the Southwest Research Institute in Boulder, Colorado. At its closest point in the fly-by, the probe will come within 200 kilometres of the plume’s source (see graphic).

A key question is, will it find ammonia? The gas is an antifreeze, allowing water to remain liquid at lower temperatures. But during the last fly-by, ammonia was strangely absent. If it remains undetected, this would suggest that the moon’s interior is hot enough to host liquid water.

Just how hot it is inside and how much liquid water exists are much debated. The mechanism for how the plume is produced is also poorly understood, but is known to be driven by Saturn’s gravity, which kneads the moon and causes internal shearing and heating. Some scientists maintain that the water comes from large oceans deep under the surface that erupt like volcanoes, whereas others argue that it exists in cracks closer to the surface. Either way, astrobiologists are excited, says Spencer. “Any kind of ocean is going to be hot by Saturn standards, and that’s plenty warm enough for astrobiology.”

Susan Kieffer, a geologist at the University of Illinois at Urbana–Champaign, proposes a third model that doesn’t require liquid water. Mixed in with the plume’s water vapour are other gases, including nitrogen, carbon dioxide and methane — far too much, Kieffer argues, to be dissolved in liquid water in Enceladus’ weak-pressure environment. Those gases, she says, exist more comfortably if trapped in ice-cage structures called clathrates, which could erupt into the plume and immediately form water vapour.

Enceladus is only 500 kilometres in diameter, so the fly-by will be over in minutes. But as Cassini departs, it will turn its thermal imager back for a glance. At that time, Saturn will be casting a shadow onto the moon, presenting a ripe opportunity for accurate, high-resolution readings of the surface temperature.

The thermal imager has so far detected temperatures only as high as 145 K from the south pole of the moon. But Spencer hopes it will record much higher values as the imager zeroes in on the ‘tiger stripes’ — the four long, thin cracks on the moon’s surface from which the gas plume emanates.

Cassini will return to Enceladus in August, for the second of eight further fly-bys in its extended mission. Each time, it will inch closer to the plume’s centre. Scientists want to test the waters slowly — any encounters with particles bigger than one millimetre could damage the spacecraft.

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