Tethys

Tethys is Saturn's fifth-largest moon and one of the whitest, cleanest worlds in the solar system — a roughly 1,060-km ball of almost pure water ice orbiting deep inside Saturn's family of icy satellites. With a density of just 0.98 g/cm³, barely more than liquid water, it is one of the least rocky large bodies known: crack Tethys open and you would find little but ice most of the way down.

For its modest size it carries two of the most spectacular scars in the Saturn system — a crater almost as wide as the moon is deep, and a fracture that runs most of the way around its circumference. Both are frozen records of a violent early history on a world that has barely changed since.

A world made of ice

Tethys is about 531 km in mean radius, roughly the width of the American state of Arizona rolled into a sphere. What sets it apart is its composition: its density of about 0.98 times that of water is so low that the moon must be almost entirely water ice, with only a few percent rock mixed in. That makes it one of the purest ice bodies of its size anywhere in the solar system.

That icy makeup shows on the surface. Tethys is strikingly bright, reflecting more sunlight than fresh snow — a sign that its terrain is clean, fine-grained ice, kept fresh in part by a faint 'snow' of ice grains drifting in from the geysers of neighbouring Enceladus. Under that brilliant surface the temperature sits near −187 °C, cold enough that water ice behaves like solid rock and holds craters and canyons intact for billions of years.

Odysseus: an impact that nearly split it apart

The surface of Tethys is dominated by Odysseus, a colossal impact crater about 400 km across — close to two-fifths of the entire moon's diameter. An impact of that scale into a body this small should, by rights, have shattered it; that Tethys survived is itself a clue to how it is built.

Unlike the sharp bowls of younger craters, Odysseus is unusually flat and shallow, its rim slumped and its central peak collapsed back down toward the surrounding terrain. Planetary scientists read that softened, relaxed shape as evidence that the moon's ice was warm and pliable when the crater formed, slowly flowing back toward a sphere over the ages that followed.

Ithaca Chasma: a canyon wrapped around a moon

Following a great circle that runs through the poles is Ithaca Chasma, one of the longest canyon systems in the solar system relative to its host: a trench about 100 km wide and 3 to 5 km deep that stretches more than 2,000 km — wrapping roughly three-quarters of the way around Tethys. Fittingly, it is named for Ithaca, the island kingdom ruled by Odysseus, tying the moon's two great landmarks together in Homeric myth.

Its origin is still debated. The leading idea holds that Ithaca Chasma formed when a former subsurface ocean froze solid: water expands as it turns to ice, and that swelling would have stretched and split the brittle crust above. An older suggestion tied it directly to the Odysseus impact, but crater counts indicate Ithaca Chasma is older than Odysseus, which makes a direct link unlikely. Either way, it is a fossil of an era when the interior of Tethys was more active than the frozen, quiet world we see today.

Mysterious red arcs

In April 2015, as the changing seasons finally lit its northern latitudes, NASA's Cassini spacecraft photographed something entirely unexpected: a set of narrow, arc-shaped reddish streaks running for hundreds of kilometres across the otherwise pale, icy surface. Only a few kilometres wide, these red arcs are among the most unusual colour features seen on any of Saturn's moons.

Their cause remains an open question. Cassini scientists have proposed exposed ice laced with chemical impurities, or material outgassing from the interior along cracks, but no explanation is settled. The arcs are a reminder that even a body long assumed to be geologically dead can still hold genuine mysteries.

Locked in Saturn's dance

Tethys circles Saturn once every 1.9 days at a distance of about 295,000 km, keeping the same face toward the planet just as our Moon does toward Earth. It is caught in an inclination resonance with the smaller inner moon Mimas — though because Tethys is far more massive, it is Mimas whose orbit is mostly affected, its tilt raised while Tethys is barely nudged.

More curious still, Tethys shares its orbit with two tiny companion moons. Telesto rides ahead of it and Calypso trails behind, each parked near one of the stable Lagrange points 60 degrees around the orbit — the same gravitational sweet spots that hold Jupiter's Trojan asteroids in place. Tethys is one of only a handful of worlds known to shepherd such co-orbital 'Trojan' moons.

Discovery and exploration

Tethys was discovered on 21 March 1684 by the Italian-French astronomer Giovanni Cassini, who spotted it and Dione the same night and grouped his Saturnian finds as the 'Sidera Lodoicea', the Stars of Louis, in honour of King Louis XIV. The mythological name we use today came later, when John Herschel proposed naming Saturn's moons after the Titans and giants of Greek legend; Tethys was a sea Titaness, daughter of Uranus and Gaia.

The moon stayed a mere point of light until the Voyager 1 and 2 probes swept through the system in 1980 and 1981, returning the first images of Odysseus and Ithaca Chasma. The definitive survey came from NASA and ESA's Cassini orbiter, which studied Tethys repeatedly between 2004 and 2017, making its closest targeted approach of 1,503 km on 24 September 2005 and mapping the moon in the detail we know it by today.

How to spot it

Tethys is well beyond naked-eye reach, but it is within grasp of a good backyard telescope. Under dark skies, an instrument of around 150 mm (6 inches) or larger will show it as a faint speck near Saturn, strung out along the same line as the planet's rings together with its brighter siblings Dione, Rhea and Titan. The challenge is Saturn's glare rather than the moon's distance: nudging the bright planet just outside the field of view makes the tiny points of its inner moons far easier to pick out.