Shadow on the Rings


ring shadows 2The changing length of Saturn’s shadow marks the passing of the planet’s seasons. As the planet nears its northern-hemisphere solstice in May, 2017, the shadow will get even shorter. At solstice, the shadow’s edge will be about 45,000 km from the planet’s surface, barely making it past the middle of the B ring.

The white speck in the lower left of the picture isn’t a dust mote on your monitor. It’s the moon Mimas, only a few pixels wide in this image.

Image Credit: NASA

A Great Divide


A Great DivideThe rings of Saturn are very thin, but they are very, very wide; the Cassini Division (seen here between the bright B ring and dimmer A ring) is almost as wide as the planet Mercury. The 4,800-km-wide division in Saturn’s rings is probably caused by the moon Mimas. Particles within the division orbit Saturn almost exactly twice for every time Mimas orbits. That results in a series of gravitational nudges from the moon which sculpt the outer edge of the B ring and keep its particles from drifting into the Cassini Division.

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.

Mimas and Atlas (If You Look Carefully)


converted PNM fileThe great eye of Saturn’s moon Mimas is a 130-km-wide impact crater called Herschel. It seems to be looking back at you in this picture taken by the Cassini spacecraft. The small moon Atlas is also visible (sort of) just outside the main rings. You’re probably mistaking it for a bit of dust on you monitor. Mimas is 397 km across; Atlas is 32 km across.

Image Credit: NASA

Crescent Mimas


Crescent Mimas

Death Star MimasCassini has seen back this crescent view of Saturn’s moon Mimas, the long shadows showing off its many craters, indicators of the moon’s violent history. The most famous evidence of a collision on Mimas (just under 400 km across) is the crater Herschel that gives Mimas its Death-Star-like appearance.

Image Credits: NASA