Why isn Pluto colder than Neptunes moon Triton, being farther from the sun?
Triton’s axis of rotation is also unusual, tilted 157 degrees with respect to Neptune’s axis, which is in turn inclined 30 degrees from the plane of Neptune’s orbit. The net result of these two axial tilts is that Triton’s rotational axis points almost directly toward the Sun twice per Neptunian year, much like Uranus’. As Neptune orbits the Sun, Triton’s polar regions take turns facing the Sun, probably resulting in radical seasonal changes as one pole then the other moves into the sunlight. During the Voyager 2 encounter, Triton’s south pole was facing the Sun. Almost the entire southern hemisphere is covered with an “ice cap” of frozen nitrogen and methane. Triton has a density of 2.0 g/cm3, and is probably about 25% water ice with the remainder being rocky material. It has a tenuous nitrogen atmosphere with small amounts of methane. Its atmospheric pressure is only about 0.01 millibar. Triton’s surface temperature is only 34.5 K, even colder than Pluto’s average temperature of 44K.
