Tethys (moon)

• 1. Introduction

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  Tethys, the fifth-largest moon of Saturn, sits about 295,000 kilometers from the planet's center. Its surface is almost pure water ice, bright enough to rank as the second-most reflective moon in the entire Saturnian system. Yet beneath that gleaming exterior lies a world scarred by one of the largest impact craters in the Solar System, split by a canyon that stretches more than 2,000 kilometers, and shaped by forces that remain only partly understood. How did a moon made almost entirely of ice end up in orbit around Saturn? What cracked its surface so dramatically? And what does the story of Tethys tell us about the early history of the Solar System itself?

• 2. Discovery and Naming

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  Giovanni Domenico Cassini spotted Tethys through a large aerial telescope set up on the grounds of the Paris Observatory in 1684, the same night he also found Dione. He had already discovered Rhea and Iapetus in 1671-72. Rather than naming his finds individually at first, Cassini grouped them together as Sidera Lodoicea, meaning "the stars of Louis," in honor of King Louis XIV of France.

  For much of the seventeenth century, astronomers simply numbered the known Saturnian moons Saturn I through Saturn V. When William Herschel found Mimas and Enceladus in 1789, the sequence was bumped to Saturn VII. The discovery of Hyperion in 1848 pushed Iapetus to Saturn VIII, and the numbering finally stabilized.

  The name Tethys itself came from John Herschel, son of William. In his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, he proposed that the moons of Saturn be named after the Titans, sisters and brothers of Kronos, the Greek counterpart of Saturn. Tethys takes her name from the titaness Tethys. The moon is also formally designated Saturn III. Its name carries two accepted pronunciations, with either a long or short "e," and the adjective form is Tethyan.

• 3. An Ice World

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  A density of 0.98 grams per cubic centimeter makes Tethys the least dense of all the major moons in the Solar System. Water has a density of exactly 1 gram per cubic centimeter, so that figure tells the story plainly: Tethys is composed of almost nothing but water ice.

  Spectroscopic analysis of its surface confirms this picture. Strong water ice absorption bands appear at 1.25, 1.5, 2.0, and 3.0 micrometers in the near-infrared. No compound other than crystalline water ice has been unambiguously identified on the moon, though organics, ammonia, and carbon dioxide remain possible minor constituents.

  The surface is extraordinarily bright, with a visual albedo of 1.229, placing it among the most reflective surfaces in the Solar System. Much of that brightness comes from a steady supply of fresh ice particles. Saturn's faint E-ring, which is fed by geysers erupting from the south pole of nearby Enceladus, effectively sandblasts the leading hemisphere of Tethys and keeps it polished. The leading hemisphere runs 10-15% brighter than the trailing side.

  Despite all that ice, the icy regolith is far from a simple solid sheet. Radar observations by the Cassini spacecraft showed the surface layer has a porosity exceeding 95 percent, making it more like an extraordinarily fluffy frost than solid ice. The internal structure of Tethys remains uncertain. If a rocky core exists, it would not exceed 145 kilometers in radius and would account for less than 6 percent of the moon's total mass.

• 4. Odysseus and Ithaca Chasma

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  Odysseus crater occupies the western part of Tethys's leading hemisphere, and its 450 kilometer diameter amounts to nearly two-fifths of the moon's own diameter. An impact of that scale would have been catastrophic for a smaller or rockier body. On Tethys, the icy crust slowly adjusted: over geologic time, viscous relaxation caused the crater floor to flatten until it conforms to the moon's spherical shape. The rim still stands about 5 kilometers above the mean satellite radius, and the central complex features massifs elevated 6-9 kilometers above the crater floor.

  Running roughly three-quarters of the way around Tethys is Ithaca Chasma, a valley about 100 kilometers wide, 3 kilometers deep, and more than 2,000 kilometers long. It occupies roughly 10 percent of the moon's entire surface area. One pole of Ithaca Chasma lies only about 20 degrees from the center of Odysseus, raising the question of whether the two features share an origin.

  The prevailing explanation for Ithaca Chasma is internal expansion. As Tethys cooled after formation, a subsurface ocean of liquid water gradually froze. Water expands as it freezes, and that expansion cracked the surface to relieve the pressure. A competing theory held that the Odysseus impact itself sent shock waves around the moon, fracturing the brittle surface. However, crater-count dating from high-resolution Cassini images showed that Ithaca Chasma is older than Odysseus, making the impact hypothesis unlikely. The subsurface ocean itself may have resulted from tidal heating during an early 2:3 orbital resonance between Tethys and the neighboring moon Dione.

• 5. Color, Light, and Pac-Man

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  Tethys is not uniformly bright or uniformly colored. The trailing hemisphere grows progressively redder and darker toward the anti-apex of motion, driven by impacts from plasma co-rotating with Saturn's magnetosphere. The leading hemisphere reddens slightly near its apex without noticeable darkening. These two patterns together produce a bluish band running between the hemispheres in a great circle through the poles.

  On the leading hemisphere, spacecraft observations revealed a distinct dark bluish band stretching roughly 20 degrees north and south of the equator. It has an elliptical shape and narrows as it approaches the trailing side. A comparable band exists only on Mimas among the Saturnian moons.

  Energetic electrons from Saturn's magnetosphere, carrying energies greater than about 1 MeV, drift opposite to the planet's rotation and preferentially strike the equatorial region of the leading hemisphere. The absorbed energy alters the surface chemistry in that band. Temperature maps from Cassini revealed that the bluish region is cooler at midday than the surrounding surface, giving Tethys a distinctive appearance in mid-infrared wavelengths that mission scientists described as resembling "Pac-Man."

  The darker material mixed into the trailing hemisphere ice shares spectral properties with the surfaces of the dark Saturnian moons Iapetus and Hyperion. The most probable candidate for that dark material is nanophase iron or hematite.

• 6. Formation and Inner History

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  Tethys formed from the Saturnian sub-nebula, a disk of gas and dust that surrounded Saturn for some time after the planet itself took shape. The low temperature at Saturn's position in the early Solar System meant that water ice was the dominant solid available for moon-building. Other volatile compounds like ammonia and carbon dioxide were likely present but their exact proportions are not well established.

  Why Tethys ended up so extraordinarily rich in water ice, even compared to other outer Solar System bodies, is still unexplained. One proposed mechanism points to chemistry in the sub-nebula: conditions there likely promoted conversion of molecular nitrogen and carbon monoxide into ammonia and methane. Oxygen freed from that carbon monoxide would have reacted with hydrogen to form additional water. A more exotic hypothesis proposes that Saturn's rings and inner moons, including Tethys, assembled from the ice-rich crust that was tidally stripped from a Titan-like moon before that moon was swallowed by Saturn.

  Accretion probably ran for several thousand years. During that process, the heat from impacts raised the temperature in the outer layer to around 155 K at a depth of about 29 kilometers. After accretion ended, the surface layer cooled and contracted while the interior warmed and expanded. That stress reached an estimated 5.7 MPa, enough to crack the crust. Because Tethys contains so little rock, radioactive decay contributed negligible additional heat. Any significant melting in its history would have required tidal heating during a resonance passage, such as the proposed early resonance with Dione.

• 7. A Century of Flybys

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  Pioneer 11 made the first close pass at Saturn in 1979, approaching Tethys to within 329,197 km on the 1st of September that year. It offered little more than a distant glimpse.

  Voyager 1 followed on the 12th of November 1980, passing 415,670 km from Tethys. The images it returned had a resolution no better than 15 km, enough to detect Ithaca Chasma for the first time but not to study it in detail. Voyager 2 came far closer on the 26th of August 1981, passing only 93,010 km from the surface and returning images with a resolution as high as 2 km. That sharper view revealed that Ithaca Chasma runs a full 270 degrees around the moon, and it disclosed Odysseus crater for the first time. Tethys was the Saturnian satellite most completely imaged by either Voyager.

  The Cassini spacecraft entered Saturn orbit in 2004 and spent over a decade studying the system. On the 24th of September 2005, it executed one very close targeted flyby of Tethys at just 1,503 km. A later flyby on the 14th of August 2010 at 38,300 km captured detailed images of Penelope crater, the fourth-largest on Tethys at 207 km wide. Cassini's full campaign produced surface maps at a resolution of 0.29 km. Its observations also confirmed that Tethys is geologically inactive today, producing no new plasma in Saturn's magnetosphere.

  What comes after Cassini remains uncertain. The Titan Saturn System Mission has been proposed as one possible future expedition to this part of the Solar System.