Tuonela Planitia

Tuonela Planitia is a long, narrow plain on Neptune's moon Triton, likely formed from a frozen lake of cryolava. It lies in Triton's northern hemisphere, in a region called Monad Regio, and sits on top of part of the moon's distinctive cantaloupe terrain. It is one of the youngest features on Triton's surface.

Discovery and naming

The plain was first seen by the Voyager 2 spacecraft when it flew past Neptune on August 25, 1989. The International Astronomical Union approved the name in 1991, taking it from the Finnish underworld of the dead.

Geology

Tuonela Planitia is the largest of Triton's four major walled plains. While the nearby Ruach Planitia is roughly circular, Tuonela is stretched out. Its longest axis runs about 400 kilometers from north to south, nearly three times its short axis of about 150 kilometers. This north-south alignment might be due to tectonic forces, though similar features are usually found closer to Triton's equator.

The floor is mostly flat but may be slightly warped; the poor quality of Voyager 2's elevation data makes it unclear if this warping is real or a data glitch. The plain's depressed floor is surrounded by a single, scalloped scarp with bays, alcoves, and "islands" that rise 100 to 250 meters above the plain.

Like other walled plains, Tuonela has a cluster of pits, but here they are concentrated in its northern lobe. The largest pit has a low dome in its center, rising about 200 meters above a surrounding moat. The plain cuts into the older cantaloupe terrain, confirming it is among the youngest features on Triton.

Scientists think Tuonela Planitia, like Ruach Planitia, is a huge basin filled with cryolava—water-based erupted material, similar to silicate lava on Earth. The pit clusters may be volcanic vents or drainage pits where material erupted.

The plain's eruptive history might have happened in stages: first, explosive eruptions of high-viscosity or high-volatility material cleared a caldera, then effusive eruptions filled the basin with cryolava. The shoreline-like walls suggest they were eroded by fluid sitting in the basin for a long time, and the surrounding smooth terrain could be viscous cryolava flows or cryoclastic deposits. However, the walls do not look like typical caldera collapse scarps.

Both Tuonela and Ruach Planitia have bounding scarps that were later modified by erosion, carving channels and valleys into them.