Saturn's triple crescents

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Saturn's triple crescents

January 19, 2016

Janus and Tethys demonstrate the main difference between small moons and large ones. It's all about the moon's shape.

Moons like Tethys (660 miles or 1,062 kilometers across) are large enough that their own gravity is sufficient to overcome the material strength of the substances they are made of (mostly ice in the case of Tethys) and mold them into spherical shapes. But small moons like Janus (111 miles or 179 kilometers across) are not massive enough for their gravity to form them into a sphere. Janus and its like are left as irregularly shaped bodies.

Saturn's narrow F ring and the outer edge of its A ring slice across the scene.

This view looks toward the unilluminated side of the rings from about 0.23 degrees below the ring plane. The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on Oct. 27, 2015.

(Photo via NASA/JPL-Caltech/Space Science Institute)

It is easy to forget just how large Saturn is, at around 10 times the diameter of Earth. And with a diameter of about 72,400 miles (116,500 kilometers), the planet simply dwarfs its retinue of moons. One of those satellites, Tethys (660 miles or 1,062 kilometers across), is seen here at lower right.

This view looks toward the sunlit side of the rings from about 8 degrees above the ring plane. The image was taken with the Cassini spacecraft wide-angle camera on March 7, 2015 using a spectral filter that preferentially admits wavelengths of near-infrared light centered at 752 nanometers.

Tethys has been brightened by a factor of 2 to increase its visibility.

(Photo via NASA/JPL-Caltech/Space Science Institute)

What looks like a pair of Saturnian satellites is actually a trio upon close inspection.

Here, Cassini has captured Enceladus (313 miles or 504 kilometers across) above the rings and Rhea (949 miles or 1,527 kilometers across) below.  The comparatively tiny speck of Atlas (19 miles or 30 kilometers across) can also be seen just above and to the left of Rhea, and just above the thin line of Saturn's F ring.

(Photo: NASA)

Although Enceladus and Saturn's rings are largely made up of water ice, they show very different characteristics. The small ring particles are too tiny to retain internal heat and have no way to get warm, so they are frozen and geologically dead. Enceladus, on the other hand, is subject to forces that heat its interior to this very day. This results in its famous south polar water jets, which are just visible above the moon’s dark, southern limb, along with a sub-surface ocean.
(Photo: NASA)

A single crescent moon is a familiar sight in Earth's sky, but with Saturn's many moons, you can see three or even more.

(Photo credit: NASA/JPL-Caltech/Space Science Institute)

Although the D ring of Saturn is so thin that it's barely noticeable compared to the rest of the ring system, it still displays structures seen in other Saturnian rings. Here the spiral structures in the D ring are on display.

(Photo Credit: NASA/JPL-Caltech/Space Science Institute)

NASA's Cassini imaging scientists processed this view of Saturn's moon Dione, taken during a close flyby on June 16, 2015.  This was Cassini's fourth targeted flyby of Dione and the spacecraft had a close approach altitude of 321 miles (516 kilometers) from Dione's surface.
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A single crescent moon is a familiar sight in Earth's starry sky, but with Saturn, you can see up to three moons or more.

On June 30, 2004, the Cassini spacecraft entered orbit around Saturn to begin the first in-depth, up-close study of the planet. The spacecraft is conducting a second extended mission called the Cassini Solstice Mission. So far its observations of Saturn's largest moon, Titan, have given scientists a glimpse of what Earth might have been like before life evolved.

Check out the stunning images above of Saturn's moons.
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