Ruach Planitia

Ruach Planitia is a roughly circular, flat plain on Neptune’s moon Triton, likely a frozen cryolava lake. It lies in Triton’s northern hemisphere, within Monad Regio, bordering the cryovolcanic plains of Cipango Planum to the east and Tuonela Planitia to the west. It is among the youngest and flattest features seen on Triton, along with the moon’s three other walled plains.

Discovery and naming

The feature was first spotted by Voyager 2 during its flyby of the Neptune system on 25 August 1989. The International Astronomical Union approved the name Ruach Planitia in 1991, after the legendary French island of Ruach.

Geology

The plain is quasi-circular, about 175–180 km across. Its floor sits roughly 200 meters lower than the surrounding plains, enclosed by a scarp that varies from 40 to 250 meters in height. Most of the plain is ringed by a single scarp, but on the east side the scarp splits into two, creating a terraced profile. The floor is generally flat but may be subtly warped; whether these undulations are real or artifacts of Voyager 2’s limited elevation data is unclear.

The central area is more topographically varied, featuring a large central pit—Dilolo Patera—nearly 10 km wide and about 350 m deep. This pit is surrounded by a network of smaller pits, linear troughs, and scarps. This pit cluster resembles those found in Triton’s other three walled plains, each having a single cluster with the largest pit at the center. Both Ruach Planitia and nearby Tuonela Planitia cut into the surrounding cantaloupe terrain, and Ruach Planitia also cuts into the western parts of Cipango Planum.

The terraced appearance of Ruach Planitia is similar to that of large terrestrial volcanic calderas. As with such calderas, eruptions here may have occurred in stages: an early explosive phase involving high-viscosity or high-volatility cryolava (water-dominated material analogous to silicate lava) that formed the terraced walls, followed by a later effusive phase that created the flat floor.

However, the crenulated walls do not clearly indicate collapse; instead, they resemble eroded shorelines on Earth. The multiple terraces on the eastern side may result from compositional differences in Triton’s crustal layers, some richer in volatiles than others. The flat floor suggests fluid cryolava later filled the depression, with the pits around Dilolo Patera acting as central vents for eruptions.