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Cassini's First Flyby of Dione.

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Dione orbit around Saturn.
Dione orbit around Saturn.

Just days after completing its second flyby of Titan, Saturn's largest moon, Cassini proceeded on to its next target: Dione, Saturn's 4th largest moon. The spacecraft came within just 81,000 kilometres (50,600 miles) of the Moon and sent back a stream of close up images of its strange surface, which are now being analyzed by scientists.

Dione is a small heavily cratered Moon 560 km (350 miles) in diameter orbiting Saturn once every 2.73 days, which is actually the same period as its axial rotation. The Moon lies at a distance of 377,400 kilometres (234,555 miles) from its parent planet. Dione is also known to share its orbit around Saturn with a small asteroid named Helene, that occupies a stable point ahead of it in orbit. The surface temperature of Dione is very similar to that of Titan at -186 degrees centigrade, and cryogenic activity is known to have helped shape the moon’s surface, although, unlike Titan, Dione has no known atmosphere.

The Cassini flyby this week has helped to confirm that some time in the moon’s past there were two episodes of cryo-volcanic flooding widely spaced in time that affected different regions. It is believed these episodes may be the result of tidal heating caused by the orbital interaction with another of Saturn’s moons named Enceladus.

As the Cassini Orbiter passed by Dione it took detailed images of an uncharted surface region of the area known as the 'Trailing Hemisphere,’ centered on Latitude 0, and Longitude 270 that is dominated by three craters, one large named Amata, with two smaller craters nearby named Catillus and Coras. Geologically speaking, this is the most interesting area of the Moon for planetary astronomers, because this region of Dione is marked by two distinct white ray systems.

The largest, Palatine Linea, streaks down towards the moon’s south polar region that ends with a large unnamed crater. The new Cassini images show this area to be composed of long linear groves, and rills, with intermittent small craters at varying distances.

The second white linear feature, named Padua Linea, is about half the size Palatine Linea, and is also crossed with linear rills that stretch from Dione’s equator at Longitude 240 down to the southeast, ending at Latitude -20. The largest crater dominating the area named Cassandra is shown prominently in the Cassini photographs.

Cassini also imaged the 'Leading Hemisphere’ for the first time between Longitude 180 – 145 degrees, at Latitude of around 40 degrees either side of the moon’s equator, and again it is shown to be covered in small impact craters.

All of the eight images returned by Cassini today show that much of Dione is heavily potholed with small craters with intermittent large impact craters at wide intervals, all of which are uncharted and unnamed. Therefore, Cassini has confirmed what planetary scientists had believed all along; that the resurfacing events on Dione must have taken place long before the resurfacing of Enceladus, because Dione’s least cratered areas have far more craters than those on Enceladus itself.

While the images of Dione returned today are the best ever close range images taken of this moon, the Cassini space craft is due to fly by Dione even closer next October, when it will pass just 500 Km (311 miles) above the moon’s surface.

The Moon Dione has a visual magnitude of +10.4 so that it's visible in medium sized telescopes, and amateur Astronomers can view the Moon over the next few months for themselves. Saturn is at opposition on 13 January, and lies in the zodiac constellation of Gemini (The Twins).

By Science Correspondent Richard Pearson

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