Voyager 2 passed by Triton about 5 hours after skimming within 5000 kilometers (3000 miles) of the cloudtops of Neptune. Triton is only slightly smaller than Earth’s Moon and is one of the most unusual objects encountered during all the Voyager planetary flybys. This image is a digital mosaic of 12 individual images, with color information added from lower-resolution frames. The large south polar cap at the bottom of the image is a slowly evaporating layer of frozen nitrogen. The dark streaks on the polar cap are probably deposits resulting from the expulsion of frozen nitrogen that suddenly changed to the vapor phase, essentially a nitrogen eruption. Voyager data showed that Triton is extremely cold (daytime temperature of 37 K, or –400°F), extremely bright (reflecting nearly 100% of the sunlight incident upon it), and has a very tenuous atmosphere of nitrogen and methane (with a surface pressure 10 millionths of Earth’s atmosphere at sea level). The darker, slightly redder color beyond the polar cap may result from radiation effects on methane included within the ice. The ridges and depressions visible away from the polar cap are probably due to the deformation of water ice. Triton is essentially the same size and density as Pluto so it is possible that the surface of Pluto may look somewhat like Triton, but Pluto remains the only planet not yet visited by a spacecraft.
Mosaic of Voyager 2 images (Press Release P-34764).