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Cassini sees Titan exposed to the power of space weather

Jenny Winder, News Writer
Jan 30, 2015, 17:00 UTC

Sen—NASA's Cassini probe has observed Saturn's largest moon, Titan, exposed to the full onslaught of space weather from the Sun.

The data suggest that unmagnetized bodies like Titan, Venus, Mars or a comet, might interact with the solar wind in the same basic ways, regardless of their nature or how far they lie out in the Solar System.

Since arriving at Saturn in 2004 Cassini has made more than a hundred flybys of Titan, but always when it lay within Saturn's own protective magnetosphere.

One flyby made in 2013 but only detailed now was unique within Cassini's mission, as it was the only time the spacecraft has observed Titan outside the region of space dominated by Saturn's magnetic field. The spacecraft has not been able to detect a magnetic field coming from Titan itself.

"We observed that Titan interacts with the solar wind very much like Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of the Institute of Astronomy and Space Physics in Buenos Aires, who led the research with colleagues from the Cassini mission. "We thought Titan in this state would look different. We certainly were surprised," he said in a statement.

The solar wind of charged particles continually streams outward from the Sun, flowing around the planets. Studying the solar wind at other planets helps scientists understand how the Sun's activity affects their atmospheres, modifying their chemistry or causing their gradual loss into space.

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This diagram depicts conditions observed by NASA's Cassini spacecraft during a flyby in December 2013, when Saturn's magnetosphere was highly compressed, exposing Titan to the full force of the solar wind. Image credit: NASA/JPL-Caltech

For about 95 per cent of the time, Titan lies within Saturn's magnetosphere. But during the flyby on 1 December, 2013, the giant moon was on the sunward side of Saturn when a powerful outburst of solar activity reached the planet.

The surge in the solar wind compressed the Sun-facing side of Saturn's magnetosphere so that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to the stream of energetic solar particles.

Earth's powerful magnetic field acts as a shield against the solar wind, helping to protect our atmosphere from being stripped away. In the case of Venus, Mars and comets, none of which have a global magnetic field, the solar wind drapes around the objects themselves, interacting directly with their atmospheres (or in the comet's case, its coma). Cassini saw the same thing at Titan—the solar wind draped itself around Titan, creating a shockwave.

Previously, researchers had thought Titan would have a different sort of interaction with the solar wind due to the moon's complex atmospheric chemistry. Cassini's magnetometer, like a highly sensitive compass, was able to observe Titan as it interacted directly with the solar wind. The special circumstance allowed the team to study the shockwave that formed around Titan where the full-force solar wind slammed into the moon's atmosphere.

"This could mean we can use the same tools to study how vastly different worlds, in different parts of the solar system, interact with the wind from the Sun," Bertucci said. The list of similarly unmagnetized bodies might include the dwarf planet Pluto, which is being visited this year for the first time by NASA's New Horizons spacecraft.

"After nearly a decade in orbit, the Cassini mission has revealed once again that the Saturn system is full of surprises," said Michele Dougherty, principal investigator of the Cassini magnetometer at Imperial College, London. "After more than a hundred flybys, we have finally encountered Titan out in the solar wind, which will allow us to better understand how such moons maintain or lose their atmospheres."