Dione Close Up

Dione chasmsSome parts of the surface of Saturn’s moon Dione are covered by linear features, called chasmata, in dramatic contrast to the round impact craters that cover most moons. The bright network of fractures on Dione was seen in poor resolution Voyager images and was called “wispy terrain.” The actual nature of this terrain was unclear until Cassini photos showed we weren’t seeing something like surface deposits of frost but a pattern of bright icy cliffs among myriad fractures. This stress pattern may be related to Dione’s orbital evolution and the effect of tidal stresses over time.

Image Credit: NASA

Saturn in IR

DarknessDescendingInfrared images can help determine the altitude of clouds in the planet’s atmosphere.  Cassini’s wide-angle camera used an IR filter especially sensitive to the wavelengths that are absorbed by methane.  Methane is not a major component of Saturn’s atmosphere, but there’s enough there to make a difference in how much infrared light is reflected by different clouds. The darker areas reveal clouds that are lower in the atmosphere and, therefore, under more methane. Bright areas are higher altitude clouds. It’s likely that the lower-altitude clouds are in regions where “air” is descending while the higher-altitude clouds are in regions where “air” is rising. Thus, images like this one can help us determine vertical air movements on Saturn.

Image Credit: NASA

Dione and Mimas

Dione_MimasBecause of the illumination angle, Mimas (right) and Dione (left) appear to be staring up at Saturn looming in the background of this image captured by the Cassini spacecraft.

Although certainly large enough to be noticeable, moons like Mimas (396 km across) and Dione (1123 km across) are tiny compared to Saturn (120,700 km across). Even the enormous moon Titan (5,150 kilometers across, larger than the planet Mercury) would be dwarfed by the giant planet in such a picture.

Image Credit: NASA

Triple Crescent

Triple CrescentSaturn has many moons. The three shown here—Titan, Mimas, and Rhea—show marked contrasts in their surface features. Titan, Saturn’s largest moon and the largest moon in this image, appears fuzzy because we only see its clouds. Because Titan’s atmosphere refracts light around the moon, its crescent is wrapped just a little further around the moon than it would on an airless body. Rhea (upper left) appears rough because its icy surface is heavily cratered. A close inspection of Mimas, though difficult to see at this scale, would show surface irregularities because of its violent history.

Image Credit: NASA

One more thing … If it’s clear where you are this evening, go outside and look up in the western sky just after sunset. There’s a conjunction of Venus and Jupiter tonight. They will be separated by less than half the diameter of the Full Moon.

Tethys Eyes Saturn

Tethys and SaturnIn this picture taken by the Cassini spacecraft last April, the two large craters on Tethys near the line where day fades to night seem to be looking at Saturn. (Click the image to embiggen it.)

The shadowing on the craters caused by being near Tethys’ terminator throws their topography into sharp relief. The larger, southernmost of the two shows a more complex structure. Its central peak is  probably the result of the surface reacting to the violent post-impact excavation of the crater. The northern crater doesn’t have a similar feature. The impact was likely too small to form a central peak, or the composition of the material in the immediate vicinity couldn’t support the formation of a central peak.

Image Credit: NASA


Tethys with craterOdysseus is the name of the huge crater on Saturn’s moon Tethys. Tethys is a bit more than 1000 km in diameter, and the crater is roughy 450 km across. To put that into scale, a crater that covered the same percentage of the Earth’s surface would be about the size of Africa.

Image Credit: NASA