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Cassini clocks wind speeds on Saturn

Morgan Rehnberg, Correspondent
Sep 11, 2015, 22:58 UTC

Sen—For a gas giant like Saturn, it is all atmosphere, all the time. That has made understanding Saturn's cloud tops a critical goal of NASA’s Cassini spacecraft on its 11-years-and-counting mission to the sixth planet. The questions here are fundamental: which atoms and molecules make up the atmosphere, how are they distributed, and how do they move? It is this final question that had Cassini’s attention for ten hours this week.

Go outside on Earth and one of the very first atmospheric quantities you will notice is the speed of the wind. With a small device, such as a pinwheel, it is even trivial to begin to make an estimate of that velocity. Not so at Saturn. Where the atmosphere is dense enough that its movements can realistically be called wind is far too dangerous a location for a spacecraft to venture. Designed to glide through the vacuum of space, Cassini would quickly be dragged down and burn up like a meteoroid.

Instead, scientists must measure the speed of the wind from afar. Cassini does this, as it did this week, using its Imaging Science Subsystem. Over a series of hours, a camera onboard the spacecraft takes repeated images of the planet. Once these images are returned to Earth, scientists can track the movement of particularly unusual clouds—or spots on the visible surface—to estimate the speed of the wind around them, which can be up to 1,800 kph.

There’s a further complication, though. Unlike the other planets in the Solar System, we do not precisely know the speed at which Saturn rotates. Since we typically measure wind as relative to the motion of the planet as a whole, this has led to ambiguity on such basic questions as which direction it is even blowing. Fortunately, Cassini has been able to provide much more accurate answers than we have ever had before.

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.