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Cassini eyes a giant's footprint in Saturn's atmosphere

Morgan Rehnberg, Correspondent
Jul 31, 2015, 20:24 UTC

Sen—Saturn's moon Enceladus may orbit nearly a quarter of a million kilometers away, but it can have a very real impact on the planet’s surface. This week, NASA’s Cassini spacecraft undertook an observing campaign to study that effect. Enceladus, named after a giant of Greek mythology, is among the Solar System’s most fascinating objects.

Cassini, in orbit about Saturn for more than 11 years, has revealed the moon as one of the most dynamic ever observed. Jets of nearly pure water blast out of its warm south polar region and create the enormous E ring in which the moon is embedded.

Molecules of water do not just form the E ring, however. Some probably drift onto the surfaces of other nearby moons like Tethys, while others pick up an electric charge in Saturn’s magnetosphere. It is this final category that Cassini investigated this week.

When an atom or molecule becomes electrically-charged (called an ion), it suddenly begins to react to Saturn’s magnetic field. The field pulls the particle along with the planet’s rotation, but each particle also creates a little bit of drag on the field. The cloud of ions surrounding Enceladus creates a noticeably-large alteration in the magnetic field, which sends a stream of charged atoms and molecules whizzing towards the planet.

As this current of ions penetrates Saturn’s atmosphere, collisions with molecules there cause a spot to glow faintly in the ultraviolet, similar to an aurora. Cassini’s Ultraviolet Imaging Spectrograph can search for this spot, while other instruments monitor the source region around the spacecraft.

Enceladus was not the first world known to create such an “auroral footprint” on its host planet; The Hubble Space Telescope has shown that Io, Europa, and Ganymede all create such spots in the atmosphere of Jupiter.

The Cassini-Huygens mission is a collaborative effort between NASA, ESA, and the Italian Space Agency. Launched in 1997, it reached Saturn in 2004 and has since been studying the planet, its moons, and its rings. In 2005, the Huygens probe made the first landing on Titan, Saturn’s largest moon. After completing its second mission extension in 2017, Cassini will make a series of close passes to the planet and then end its time at Saturn by plunging into the planet’s atmosphere.