Tuonela Planitia
Elongated plain and probable cryolava lake on Triton.
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.
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.
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.
Quick Facts
- Location
- Monad Regio, Triton
- Depth
- ~200 m
- Dimensions
- ~400 × 150 km
- Discoverer
- Voyager 2
- Eponym
- Tuonela
Facts from the source article.
Lore & Background
Tuonela Planitia was discovered alongside Triton's other surface features by the Voyager 2 spacecraft during its flyby of the Neptune system on 25 August 1989. The name Tuonela Planitia was approved by the International Astronomical Union (IAU) in 1991, originating from the underworld realm of the dead from Finnish mythology. Of the four observed major walled plains on Triton, Tuonela Planitia is by far the largest. In contrast to neighboring Ruach Planitia, which is roughly circular, Tuonela Planitia is significantly elongated, with its longest axis of roughly 400 km nearly thrice longer than its short axis of roughly 150 km, aligned roughly north to south. Its alignment may be tectonically controlled, although north–south trending tectonic features are usually found closer to Triton's equator. Its floor is broadly flat, though its surface may be slightly warped; the low quality of Voyager 2 elevation data makes it uncertain whether the warping is real or an artifact. The depressed floor is bound by a single scarp with a crenulated profile, superficially resembling coastlines on Earth with alcoves, bay-like features, and numerous 'islands' rising 100–250 meters above the floor. Tuonela Planitia hosts an unusual cluster of pits within its northern 'lobe'; the largest pit has a low dome centered within it, rising some 200 meters above its surrounding moat. The plain cuts into older cantaloupe terrain, indicating it is one of the youngest features on Triton's surface.
Reader's Guide
Tuonela Planitia is significant as the largest of the four major walled plains on Triton and as a probable cryolava lake, representing a massive basin infilled with water-dominated erupted material analogous to silicate-dominated lava. Its elongated shape, with a longest axis roughly 400 km and a short axis roughly 150 km, contrasts with the roughly circular Ruach Planitia, and its north–south alignment may be tectonically controlled. The pit clusters within the plain have been compared to volcanic vents or drainage pits, possibly marking sites of eruption. As with terrestrial calderas, the eruptive history may have occurred in multiple stages: an early stage of explosive eruptions clearing a caldera, later transitioning into effusive eruption filling the basin with cryolava. The shoreline-like profile of its walls suggests erosion by fluid sitting within the basin for extended periods, and surrounding smooth terrain may represent viscous cryolava flows or cryoclastic deposits. However, the walls do not appear to resemble collapse scarps of typical calderas. Both Tuonela Planitia and Ruach Planitia have bounding scarps modified by erosive processes, with Tuonela Planitia's eastern wall hosting a system of short, dense, narrow channels possibly carved by glacial activity. Nearby pitted cones, roughly conical hills with central depressions, superficially resemble terrestrial and lunar cinder cones. The low quality of Voyager 2 elevation data leaves uncertainty about whether the floor's slight warping is real or an artifact.
Did You Know?
- The name Tuonela Planitia comes from the underworld realm of the dead in Finnish mythology.
- The largest pit within Tuonela Planitia has a low dome rising some 200 meters above its surrounding moat.
Frequently Asked Questions
What is Tuonela Planitia?
Tuonela Planitia is an elongated, walled plain on Neptune's moon Triton measuring roughly 400 km along its long axis and about 150 km across. It is the largest of the four major walled plains identified on Triton and is widely interpreted as the frozen remains of an ancient cryolava lake.
How was Tuonela Planitia discovered?
The plain was first captured in imagery during Voyager 2's single flyby of the Neptune system on 25 August 1989. Because no other spacecraft has since visited Triton, those Voyager 2 frames remain the only close-up data available for the feature.
Where exactly is Tuonela Planitia located on Triton?
It lies in Triton's northern hemisphere within a region designated Monad Regio, and it sits atop a portion of the moon's distinctive cantaloupe terrain. Geologically, it ranks among the youngest surfaces mapped on Triton.
Why is it called Tuonela Planitia?
The International Astronomical Union approved the name in 1991, borrowing Tuonela from Finnish mythology, where it refers to the underworld realm of the dead. The name follows the broader IAU convention of assigning mythological names to Triton's surface features.
How did Tuonela Planitia form?
The leading explanation is that a pool of cryolava — superheated liquid nitrogen and methane — once filled the basin and later solidified into the flat, walled plain visible today. Its remarkably fresh, unweathered appearance points to a resurfacing event that is geologically recent on Triton's timescale.
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