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Saturn’s Moon Tethys

Saturn's moon Tethys, imaged by Cassini on April 14, 2012.

Saturn’s moon Tethys, imaged by Cassini on April 14, 2012. Credit: NASA/JPL/SSI

Thanks the Voyager missions and the more recent flybys conducted by the Cassini space probe, Saturn’s system of moons have become a major source of interest in recent years. From water ice and interior oceans, and some interesting surface features caused by impact craters and geological forces, Saturn’s moons have proven to be a treasure trove of discoveries.

This is particularly true of Saturn’s moon of Tethys, also known as the “Death Star” moon because of the massive crater that marks its surface. In addition to closely resembling the space station out of Star Wars lore, it boasts the largest valleys in the Solar System and is composed mainly of water ice.

Originally discovered by Giovanni Cassini in 1684, Tethys is one of four moons discovered by the great Italian mathematician, astronomer, astrologer and engineer between the years of 1671 and 1684. These include Rhea and Iapetus, which he discovered in 1671-72, and Dione which he discovered alongside Tethys.

An engraving of the Paris Observatory during Cassini's time.  Credit: Public Domain

An engraving of the Paris Observatory during Cassini’s time. Credit: Public Domain

Cassini observed all of these moons using a large aerial telescope he set up on the grounds of the Paris Observatory. At the time of their discovery, he named the four new moons “Sider Lodoicea” (“the stars of Louis”) in honor of king Louis XIV of France.

The modern names of all seven satellites of Saturn come from John Herschel (son of William Herschel, discoverer of Mimas and Enceladus). In his 1847 treatise Results of Astronomical Observations made at the Cape of Good Hope, he suggested that all should be named after the Titans – the brothers and sisters of Kronos – from Greek mythology.

Tethys is the third furthest large moon from Saturn and the 13th most distant moon over all, with an orbital altitude of 295,000 km above the center of Saturn. At 1066 km across, it is the 16th-largest moon in the Solar System, and is more massive than all known moons smaller than itself combined.

The density of Tethys is 0.97 grams per cubic centimeter. Since water is 1 g/cm3, this means that Tethys is comprised almost entirely of water ice. This means that if the moon were brought closer to the Sun, the vast majority of it would evaporate away.

The Odysseus Crater, a Credit: NASA/JPL/SSI

The Odysseus Crater, the 400 km surface feature that gives Tethys it’s “Death Star” appearance. Credit: NASA/JPL/SSI

It is not currently known whether Tethys is differentiated into a rocky core and ice mantle. However, given the fact that rock accounts for less 6% of it mass, the radius of the core would not exceed 145 km. This ice is also very reflective, which makes Tethys the second-brightest of the moons of Saturn, after Enceladus.

There are two different regions of terrain on Tethys. One portion is ancient, with densely packed craters, while the other parts are darker and have less cratering. The surface is also marked by numerous large faults or graben.

The western hemisphere of Tethys is dominated by a huge, shallow crater called Odysseus. At 400 km across, it is the largest crater on the surface, and roughly 2/5th the size of Tethys itself. Due to its position, shape, and the fact that a section in the middle is raised, this crater is also responsible for lending the moon it’s “Death Star” appearance.

The largest graben, Ithaca Chasma, is about 100 km wide and more than 2000 km long, making it the second longest valley in the Solar System. Named after the island of Ithaca in Greece, this valley runs approximately three-quarters of the way around Tethys’ circumference. It is also approximately concentric with Odysseus crater, which has led some astronomers to theorize that the two features might be related.

Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.

Cassini closeup of the southern end of Ithaca Chasma. Credit: NASA/JPL/Space Science Institute.

Scientists also think that Tethys was once internally active and that cryovolcanism caused some parts of surface to resurface. This is due to the fact that a small part of the surface is covered by smooth plains, which are devoid of craters and graben that cover much of the planet. The most likely explanation is that subsurface volcanoes deposited fresh material on the surface and smoothed out its features.

Like all other regular moons of Saturn, Tethys was formed from the Saturnian sub-nebula – a disk of gas and dust that surrounded Saturn soon after its formation. As this dust and gas coalesced, it formed Tethys and its two co-orbital moons: Telesto and Calypso. These two moons have been captured into Tethy’s Lagrangian points, with one orbiting ahead of Tethys and the other following behind.

Tethys has been approached by several space probes including Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), and multiple times by Cassini since 2004. By 2014, all of the images taken by Cassini allowed for a series of enhanced-color maps that detailed the surface of the entire planet (shown below).

This set of global, color mosaics of Saturn's moon Tethys was produced from images taken by NASA's Cassini spacecraft during its first ten years exploring the Saturn system. Credit: NASA / JPL-Caltech / Space Science Institute / Lunar and Planetary Institute

Global, color mosaics of Saturn’s moon Tethys, as produced from images taken by NASA’s Cassini spacecraft between 2004-2014. Credit: NASA/JPL-Caltech/Space Science Institute/ Lunar and Planetary Institute

The color and brightness of Tethys are also sources of interest. On the leading hemisphere of the moon, spacecraft have found a dark bluish band spanning 20° to the south and north from the equator. The band has an elliptical shape getting narrower as it approaches the trailing hemisphere, which is similar to the one found on Mimas.

The band is likely caused by the influence of energetic electrons from Saturn’s magnetosphere, which drift in the direction opposite to the rotation of the planet and impact areas on the leading hemisphere close to the equator. Temperature maps of Tethys obtained by Cassini have shown this bluish region to be cooler at midday than surrounding areas.

At present, Tethys’ water-rich composition remains unexplained. One of the most interesting explanations proposed is that the rings and inner moons accreted from the ice-rich crust of a much larger, Titan-sized moon before it was swallowed up by Saturn. This, and other mysteries, will likely be addressed by future space probe missions.

We have many great articles about Tethys here at Universe Today. Here’s one about the story about Tethys, with a photograph taken by NASA’s Cassini spacecraft, and another about a feature on the surface of Tethys called Ithaca Chasma.

Want more info on Tethys? Check out this article from Solar Views, and this one from Nine Planets.

We have recorded two episodes of Astronomy Cast just about Saturn. The first is Episode 59: Saturn, and the second is Episode 61: Saturn’s Moons.


Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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