This week Cassini makes its second very close flyby of complicated Dione. It will search for signs of activity on this enigmatic icy satellite of Saturn. Although only 1100 km across, it is perhaps second to Enceladus in terms of complexity. Despite that, no obvious current activity has been detected. Way back in the 1980's, shortly after the Voyager flybys first mapped these moons, several investigators including Jeff Moore argued that the smooth plains on Dione's leading face were formed by some type of effusive volcanism involving water and water ice. At a Lunar and Planetary Science conference a few years ago, Jeff Moore and I further proposed that a set of oddly-shaped craters seen by Cassini near Dione's equator during its ongoing mapping efforts were in fact volcanic calderas, formed by more energetic styles of volcanism, including explosive and collapse volcanism. These pits are 30 to 40 km across and very shallow.
The views shown here include perspective views of these putative volcanic features, as well as relaxed craters, smooth plains and narrow sinuous canyons that could be volcanic flow channels. Hopefully Cassini will help answer some of these questions.
Dione: North Polar Region. The large relaxed impact crater is 150 km across. The narrow canyons originate near the north pole and could be lava channels.
Dione: Equatorial Smooth Plains. The irregular pits in the background are 30 to 40 km across and could be volcanic calderas.
Dione: Smooth Plains. The largest crater is 105 km across. Its large central peak towers 3 km above the plains and is a classic indicator of viscous relaxation and a sign that Dione's internal heat flow was once much higher than today.
THE VIDEO:
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