Tethys is Saturn’s major moon with a density less than that of water.
Tethys has a mean density of about 0.98 grams per cubic centimeter, compared with roughly 1.00 gram per cubic centimeter for liquid water. That low value indicates that the moon is composed overwhelmingly of water ice, with a smaller fraction of rocky material. It does not mean Tethys is literally floating in an ocean; the comparison describes average density, not surface conditions.
Giovanni Domenico Cassini discovered Tethys in 1684 using a large aerial telescope near the Paris Observatory. He found it at the same time as Dione, and both moons were later named after figures from Greek mythology. Tethys is about 1,060 kilometers across and is Saturn’s fifth-largest moon.
The moon’s icy surface carries two especially striking features. Odysseus is a huge impact crater roughly 445 kilometers wide, while Ithaca Chasma is a canyon system extending for about 2,000 kilometers. Tethys is also part of a co-orbital relationship with Telesto and Calypso, which occupy stable Lagrange points around Saturn.