24.1 Characteristics of Neptune
Neptune is the furthest planet from the Sun, orbiting at about 30 AU (where 1 AU is the distance between the Earth and the Sun).[1] Johann Galle discovered Neptune in 1846, following calculations made by Urbain Le Verrier.[2] William Lassell discovered its largest moon Triton two and a half weeks later.[3]
Figure 24.1 |
Neptune, an image taken by Voyager 2. |
Neptune is the third most massive planet in the Solar System after Jupiter and Saturn. It is the fourth-largest planet, having a slightly smaller diameter than Uranus. It takes over 160 years for Neptune to orbit the Sun and one day on Neptune is just over 16 hours long.[4] Like Mercury, Venus, Mars, Jupiter, and Saturn, but unlike Uranus, Neptune is named after a Roman god, the god of the sea.[5]
Neptune is an ice giant like Uranus, and its atmosphere is similar to Uranus’, composed mostly of hydrogen, helium, and methane.[6] Neptune’s core is mostly composed of ice and rock.[7]
Neptune Fact Sheet[1]
| Designation | = | Ice giant planet |
| Mass | = | 1.0×1026 kg (17 × the mass of Earth) |
| Radius | = | 24,764 km (3.9 × the radius of Earth) |
| Density | = | 1638 kg/m3 (29.7% density of Earth) |
| Length of Day | = | 16.1 hours |
| Length of year | = | 59,800 Earth-days (164 Earth-years) |
| Days per year | = | 89,143 days on Neptune per year on Neptune |
| Distance from the Sun | = | 4.5×109 km (30.0 AU) |
| Orbital Velocity | = | 5.4 km/s |
| Orbital Eccentricity | = | 0.011 |
| Obliquity (tilt) | = | 28.3° |
| Mean Temperature | = | -200 °C |
| Moons | = | 14 (including former Oort Cloud object, Triton) |
| Ring System | = | Yes |
24.2 Neptune’s moons
Neptune has a faint ring system and at least 14 moons. The largest of these is Triton, which contains over 90% of the mass of all the Neptunian moons and is the only Neptunian moon that’s spherical.[8]
24.2.1 Triton
Triton is the only moon in the Solar System to have a retrograde orbit. This means it orbits in the opposite direction to Neptune’s rotation.[9] It’s also the only known moon in the Solar System to have a surface made mainly of frozen nitrogen.[10]
Triton is thought to have once been a Kuiper Belt object, like Pluto (discussed in Chapter 26), but was captured by Neptune while the Solar System was still forming.[11]
Triton is composed of a core of rock and metal, with an icy mantle and active volcanoes.[9] It may also contain water.[12] Triton has a thin nitrogen atmosphere. This is the coldest atmosphere in the Solar System, at about -235 °C.[9]
24.3 Missions to Neptune
NASA’s Voyager 2 probe came within 5000 km of Neptune in 1989, before passing Triton and discovering six new moons.[13,14] There are currently no plans for a new mission to Neptune.
Figure 24.2 |
Triton, an image taken by Voyager 2. |
Figure 24.3 |
Clouds of Neptune, an image taken by Voyager 2. |