— Geysers may erupt from subsurface reservoirs of liquid water on Saturn's moon Enceladus (Image: NASA/JPL/Space Science Institute)
In a daring act of space-obatics, the Cassini probe is to buzz one of Saturn's stranger moons. If NASA opts for the most precarious of the trajectories under consideration, Cassini will approach within just 30 kilometres of Enceladus before flying through a plume of steam emanating from its south pole.
The aim of this exceptionally close encounter is to find out what is powering that plume. A dozen or so jets appear to be blasting water vapour and ice crystals out of fissures near the south pole (watch a sequence of four images showing the jets).
Small ice moons don't usually have such volcanic activity. "How can a body only 500 km in size have enough internal heat to drive a spewing plume of water into space to distances much greater than its own size?" wonders James Green, director of NASA's Planetary Division.
Planetary scientists think the heat must have something to do with the fact that Saturn's gravity squeezes the moon, but they still don't know why it is focused at the south pole. Nor do they know whether it is piped into hot-water geysers like those on Earth, or whether it merely warms up surface ice, making it evaporate.
The Cassini spacecraft was scheduled to pass by Enceladus in March 2008 at a distance of around 1000 kilometres, but now engineers at NASA's Jet Propulsion Laboratory in Pasadena, California, US, are calculating whether it's safe to go nearer.
"Cassini was never designed to fly this close" says Green. "But the people at JPL have really worked this out carefully and believe they can hit the mark within a few kilometres, so we have plenty of margin for error."
One risk is that ice crystals in the plume could damage Cassini's sensitive optics, so the plan is to skim close to the equator of Enceladus heading south. Then Cassini's instruments can be pointed at the south pole while being shielded from plume particles by the body of the spacecraft.
As well as giving the cameras a closer look at the fissures to see where the jets of steam are actually coming from, this manoeuvre should allow Cassini's onboard chemical analyser a chance to taste the plume. It will find out whether the water is tainted with ammonia, methane and other substances, as scientists suspect from earlier measurements. That chemical makeup could reveal the nature of Enceladus's remarkable plume.
A feature article investigating the fountains of Enceladus will appear in New Scientist on 25 August. You can also keep up-to-date with the Cassini mission in our special report.