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Moons of our Solar System
Moons of our Solar System

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2.6 A look at Triton

The southern terrain is pinkish and hummocky (Figure 20). North of this is a greenish terrain with a texture similar to the skin of a cantaloupe melon, where the landscape may be the product of cryovolcanic eruptions. The pinkish material is a polar cap predominantly of frozen nitrogen. The green material is thought to be mostly methane or carbon dioxide-ice, which constitutes the upper part of Triton’s surface crust, with a veneer of nitrogen-frost coating the ground. Changes in atmospheric pressure have been detected on Triton as a result of the seasons, as nitrogen frost turns to vapour.

This is a Voyager 2 image showing Triton’s two main terrains.
Figure 20 Voyager 2 image showing Triton’s two main terrains.

The dark smears on the pinkish terrain (seen in Figure 21) are thought to be deposits from plumes from geysers that erupted from below the polar ice cap, some of them active during the Voyager 2 flyby. These may be solar-powered geysers. If that is correct, sunlight shines through the ice and warms the dark dusty substrate below. The absorbed heat vaporises the base of the nitrogen-ice, creating a pressured blister, which then bursts, ejecting nitrogen and dust.

This is an image of Plumes and wind streaks on Triton’s south polar cap.
Figure 21 Plumes and wind streaks on Triton’s south polar cap.

To many people’s surprise, when New Horizons flew past Pluto in 2015, its landscape (part of which was imaged in more detail than Voyager had managed at Triton) bore little resemblance to Triton, despite their broadly similar bulk compositions.

Described image
Figure 22 Voyager 2 view across the limb (edge) of Triton, showing a tenuous layer of nitrogen ‘cirrus’ cloud a few kilometres above the horizon. The version on the right has been processed to show the cloud layer more clearly.