Thalassa, like Naiad, most likely formed from fragments of Neptune's original moons, which were smashed by the disturbances caused when the ice giant Neptune captured Triton. Thalassa is unusual for an irregular moon because it is roughly disk-shaped.
Also like Naiad, Thalassa circles the planet in the same direction as Neptune rotates, and remains close to Neptune's equatorial plane. Thalassa's orbit is slowly decaying due to tidal deceleration and may eventually crash into Neptune's atmosphere or be torn apart and form a planetary ring.
Thalassa was discovered in August 1989 in images taken by Voyager 2.
How Thalassa Got its Name:
Moons of Neptune are named for characters from Greek or Roman mythology associated with Neptune or Poseidon, or the oceans. Irregular satellites are named for the Nereids, daughters of Nereus and Doris and the attendants of Neptune.
Thalassa was named after a daughter of Aether and Hemera from Greek mythology. Thalassa is also the Greek word for sea. The moon was originally designated as S/1989 N 5.