Miércoles 13 de Agosto de 2008, Ip nº 242

Cassini to search for source of Saturn moon's plumes
Por Devin Powell

On Monday, the Cassini spacecraft will return to Saturn's icy moon Enceladus, passing within 50 kilometres of its south pole. NASA team members hope the flyby will provide evidence for subsurface liquid water containing the building blocks of life.

Previous encounters revealed huge plumes of ice and water vapour venting from blue-green fault lines, or "tiger stripes", that criss-cross the south pole. The source of these jets, which feed Saturn's rings, is hotly contested.

Gathering data about these features has been slow because only a few instruments can be used fully during each flyby. Early budget cuts to the mission in 1992 limited the ability of its detectors to move independently, so some are often on the wrong side of the spacecraft to be useful.

"Imagine gluing a digital camera to your car," says team member Bob Brown of the University of Arizona in Tucson. Monday's mission will showcase a high-resolution camera and a device that looks for surface heat.

The Visual and Infrared Mapping Spectrometer (VIMS) will detect infrared radiation, or heat, on the moon's surface. Monday's pass will bring VIMS closer to the surface than ever before, allowing it to spot small regions of higher temperatures.

Hot spots close to or above 0 °Celsius – the melting point of water – could indicate a pressurised reservoir of liquid water just under the surface that vaporises when it erupts into space. "Cold Faithful" would discharge water at a rate similar to its near-namesake on Earth.

Icy source?

But previous readings from a different instrument only found temperatures of -93 °C, favouring a "Frigid Faithful" model in which the water is locked in special ice compounds called clathrates. Sublimation of this ice from solid to vapour could feed the plumes.

"Cassini could also help us understand what is generating this heat inside of the moon," says Francis Nimmo of the University of California, Santa Cruz. Saturn's gravity may deform the moon, rubbing fault lines together to generate heat in a process called "tidal friction." In this scenario, each vent should have a different temperature based on its individual characteristics.

VIMS will also be looking for reflections from organic compounds, such as ammonia and hydrogen peroxide, in the ice, since these could hint at the presence of life. In March, one of Cassini's particle collectors discovered carbon dioxide, methane and other simple hydrocarbons like propane in an icy plume.

"We're looking for any type of signature of life. There's been no evidence of that so far," says VIMS team member Bonnie Burrati of NASA's Jet Propulsion Laboratory in Pasadena, California.

Photo op

As the probe closes to within 50 kilometres, the Imaging Science Subsystems (ISS) team will attempt to take high-resolution photos of the areas around these vents. But with Cassini traveling at 64,000 kilometres per hour, the cameras will not have time to lock on to these targets. "It's like trying to take snapshots of a fencepost on the side of a highway from your car," says Carolyn Porco, ISS team leader at the Space Science Institute in Boulder, Colorado.

Instead, the spacecraft will try an unusual manoeuvre. While approaching the surface, the camera will point just ahead of the moon. Enceladus will then race through the camera's field of view, and Cassini will snap a series of seven photos, one every 30 seconds, as the south pole passes in and out of view. "We're honing in on the sweet spots," says Porco, who hopes to capture three of the vents in these images.

Still, the gaps that these plumes come out of – which may only be half a metre wide – are probably too small to show up at this resolution of 8 metres per pixel. Instead, the team hopes these photos will reveal unusual features that might provide new insights into the geysers, like the arrangement of nearby boulders or irregularities in the ice.

The cameras will also take several lower-resolution pictures from farther out. Some of the images – snapped on Cassini's approach – will be used to create a 3D stereoscopic map of its heavily cratered north pole.

  08/08/2008. New Scientist Magazine.