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Cassini seeks out Saturn's shimmering aurorae

Morgan Rehnberg, Correspondent
May 22, 2015, 16:53 UTC

Sen—Few sights are more striking than the glimmer of an aurora, but these dancing light displays are not confined to Earth. This week the Cassini spacecraft had a front row seat as similar shimmering lights flickered across Saturn. Aurorae, though, aren’t just beautiful—they are powerful tools that enable scientists to uncover the unseen around other worlds.

This is not the first time that Cassini, in orbit about Saturn for nearly 11 years, has searched for the planet’s aurorae. Amongst other discoveries, past observations have helped shed light on how energy from the Sun heats Saturn’s atmosphere.

So what makes aurorae so useful? It all comes down to how they form. Like the Earth, many other planets have protective magnetic fields that shield them from the solar wind, a stream of electrically-charged particles emanating from the Sun. When these particles reach Saturn, most pass harmlessly by, but some strike Saturn's diffuse E ring and transfer their charge. These ring particles then follow paths guided by the field, which can direct them down into the planet’s atmosphere.

When a charged particle strikes the atmosphere, it transfers some of its energy to an atom or molecule contained within. Now unstable, the atom emits this energy as a burst of light, which Cassini’s detectors can pick up. Each color of light is characteristic of a particular element. Aurorae on Earth are red and green because our atmosphere is dominated by nitrogen and oxygen; the red-purple hue at Saturn indicates the presence of much hydrogen.

As on Earth, Saturn’s aurorae are primarily found near its poles, where the magnetic field enters the planet. By searching for small-scale variations in this structure, astronomers can better understand the intricacies of a phenomenon they will never see with their naked eyes.

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.

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Saturn's aurorae shimmer in this timelapse movie taken by NASA's Cassini spacecraft. The orange is a false color, added to this black and white movie for clarity before the true color of the aurorae was known. Credit: NASA/JPL/Space Science Institute