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Thin layer of oxygen found on Dione may change search for alien life

After the Cassini probe's flyby over Saturn's frozen moon, Dione, scientists saw evidence of ionized oxygen molecules in its atmosphere. This could have interesting implications for astrobiology.

Saturn's moon Dione appears against the globe of Saturn as Cassini approached the icy moon for its close rendezvous in 2004. Data collected from a Cassini flyby of Dione last year reveals the presence of oxygen in the moon's atmosphere.

Space Science Institute/NASA/JPL/AP

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Data streaming home from NASA's Cassini probe suggests that one of Saturn's largest moons, Dione, has a thin glaze of oxygen in its upper atmosphere.

In April of 2011, the probe did a close pass of Dione to confirm scientists' suspicions that the gelid moon had an atmosphere. The layer of oxygen ions is inconceivably thin, detectable only by Cassini's sensitive equipment. You'd have to travel 300 miles above the Earth's surface to find such scarce levels. (By comparison, world records in parachuting are from about 20 miles up.)

The lorn surface of Dione is pocked with craters and ancient tectonic scarring. A third the size of our own moon, Dione is an orb of frozen water compacted around a rock core. As it swings around the ringed gas giant, it is bombarded by charged particles from Saturn's bustling magnetic field. These spry particles crash into Dione's surface, upheaving ionized molecular oxygen into its thin atmosphere. Scientists call this process "sputtering."

Cassini team leader Robert Tokar of Los Alamos labs recently said in a statement that the Cassini data demonstrates that oxygen "can come from a process that doesn't involve life."

This might require scientists to rethink how oxygen ends up in an atmosphere.

Most astrobiologists agree that oxygen strongly indicates the presence of life. Oxygen is extremely reactive. It comprises a mere fifth of our own atmosphere, yet is corrosive enough to rust metal. This reactivity drastically reduces the time that molecular oxygen can exist on its own without bonding to other molecules. The reason for its noticeable presence in Earth's atmosphere is that it is cyclically replenished by specific organisms, namely plants.

The reasoning behind using oxygen as an indicator of life is simple. Nearly all living things on Earth, with the exception of some bacteria and single-celled organisms – depend on it. But astrobiologists have long recognized that this view, though pragmatic, is also narrow.


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