Here’s a Look at Saturn’s Most Tortured Moon

2 minute read

Tethys shouldn’t be alive—but it’s a lovely thing for the solar system that it is, as a recently released picture from the Cassini spacecraft makes evocatively clear. Merely one of 62 confirmed or provisional moons orbiting Saturn, Tethys is easily the one with the most compelling life story.

For one thing, it is a good sister to the other moons in the Saturnian brood. At 660 mi. (1,062 km) across, it’s the fifth largest of all of Saturn’s satellites and orbits at an altitude of 182,689 miles (294,009 km). But it does not fly alone. Its tiny siblings Telesto and Calypso—19 mi. and 16 mi. across (31 km and 26 km) respectively—fly with it, with Telesto in front Calypso in the rear, and Tethys herding them along gravitationally like a mama duck.

Orbiting lower than Tethys, at 147,572 miles (237,494 km) is the fanciful Enceladus. Squeezed by the gravity of both Saturn below and Tethys and other moons above, Enceladus emits sparkling, ice plume volcanos, which leave bright tendrils behind it and continually fall back down to dust the moon’s face. The result is a world that has been eternally battered by incoming meteorites but never shows the scars, since no sooner does one appear than it is covered up.

Tethys enjoys no such cosmetic advantages. Nearly every one of the uncounted hits it has taken in its 4-plus billion years of life is stamped in its face, giving the rocky, icy world an almost sponge-like appearance. On the moon’s eastern limb is the biggest scar of all, the crater Odysseus, which covers 18% of Tethys’s surface. On the far-larger Earth, that would be the equivalent of a crater the size of Africa.

A crack that runs nearly three-quarters of the way around the moon suggests that it almost didn’t survive the collision. Had the projectile that caused the crater been just a little bigger or moved just a little faster, it would have murdered Tethys outright.

There’s no telling how many other moons in Saturn’s litter did suffer that fate. It is a matter of cosmic history that Tethys didn’t. And it’s a matter of cosmic fact that we have reason to be grateful.

See the Astronomical League's Most Beautiful Photos from 2014

Rosette nebula, NGC 2237, taken from Waukesha, Wisc., on Jan. 6, 2014.
The Rosette nebula, also known as NGC 2237 or Caldwell 49, taken from Waukesha, Wisc., on Jan. 6, 2014.Dennis Roscoe, Ph.D.
The Elephant's Trunk Nebula, also known as IC 1396, on April 14, 2014.
The Elephant's Trunk Nebula, also known as IC 1396, on April 14, 2014.Eric Coles
The Pleiades, also known as M45 or the Seven Sisters, imaged from Fayetteville, Ark., on Jan. 25, 2014.
The Pleiades, also known as M45 or the Seven Sisters, imaged from Fayetteville, Ark., on Jan. 25, 2014.Joel Tonyan
Andromeda Galaxy (M31), imaged from Fayetteville, Ark., on Jan. 19, 2014,
Andromeda Galaxy (M31), imaged from Fayetteville, Ark., on Jan. 19, 2014, Joel Tonyan
A time lapse showing star trails above Big Lagoon State Park in Pensacola, Fla., on March 30, 2014.
A time lapse showing star trails above Big Lagoon State Park in Pensacola, Fla., on March 30, 2014.Kenny Alexander
Eta Aquarid meteors above Bryce Canyon in Utah, in May 2014.
Eta Aquarid meteors above Bryce Canyon in Utah, in May 2014.David Lane
The Orion Nebula, taken from Waukesha, Wisc., on Feb. 7, 2014.
The Orion Nebula, taken from Waukesha, Wisc., on Feb. 7, 2014.Dennis Roscoe, Ph.D.
A star-forming nebula in Gemini, in January 2014.
A star-forming nebula in Gemini, in January 2014.Brian Peterson
Propeller Nebula in Cygnus, also known as DWB111, on May 1, 2014.
Propeller Nebula in Cygnus, also known as DWB111, on May 1, 2014.Eric Coles
The Milky Way, taken from Pensacola Beach in Pensacola, Fla., on March 2, 2014.
The Milky Way, taken from Pensacola Beach in Pensacola, Fla., on March 2, 2014.Kenny Alexander
An aircraft turns over the night sky with the Milky Way in the background above Santa Rosa Beach, Fla., on Jan. 5, 2014.
An aircraft turns over the night sky with the Milky Way in the background above Santa Rosa Beach, Fla., on Jan. 5, 2014.Matt Jylha
Pelican Nebula, taken from Waukesha, Wisc., on Jan. 9, 2014.
Pelican Nebula, taken from Waukesha, Wisc., on Jan. 9, 2014.Dennis Roscoe, Ph.D.
The Horsehead nebula, also known as Barnard 33 in emission nebula IC 434, taken at Seneca and Oswego in Illinois, Feb.-March 2014
The Horsehead nebula, also known as Barnard 33 in emission nebula IC 434, taken at Seneca and Oswego in Illinois, Feb.-March 2014Martin Hrdlicka
A panorama of the Milky Way taken from Fall Creek Falls State Park during the Eta Aquarid meteor shower on May 4, 2014.
A panorama of the Milky Way taken from Fall Creek Falls State Park during the Eta Aquarid meteor shower on May 4, 2014.Matt Harbison
The Rosette nebula, also known as NGC 2237 or Caldwell 49, taken from Seneca, Ill., in March 2014.
The Rosette nebula, also known as NGC 2237 or Caldwell 49, taken from Seneca, Ill., in March 2014.Martin Hrdlicka
The Pinwheel Galaxy, also known as Messier 101, M101 or NGC 5457, taken at the Winter Star Party in the Florida Keys on March 1, 2014.
The Pinwheel Galaxy, also known as Messier 101, M101 or NGC 5457, taken at the Winter Star Party in the Florida Keys on March 1, 2014.Richard S. Wright Jr.
The Milky Way with Venus rising at Pensacola Beach in Pensacola, Fla., on March 2, 2014.
The Milky Way with Venus rising at Pensacola Beach in Pensacola, Fla., on March 2, 2014.Kenny Alexander
Waxing Moon over Winter Garden, Fla., on Feb. 10, 2014
Waxing Moon over Winter Garden, Fla., on Feb. 10, 2014Matt Jylha

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