Descriptive Astronomy 101


Normally the eighth planet from the Sun
Neptune is the most distant of the gas giants and smallest of the gas giants
Discovered in 1846 by Johann Galle and Louis d'Arrest following predictions by Urbain Leverrier

Interior Structure

Blue-green color, similar to Uranus, due to the absorption of red light by a haze of methane in its atmosphere.
Rocky core covered by a icy crust,
Two thirds of Neptune is thought to be composed of a mixture of molten rock, water, liquid ammonia and methane
Outer third is a mixture of hydrogen (85%), helium (13%), water, methane, and ammonia gas.


Neptune shows very faint bands of cloud cover.
Caused by large-scale convection driven by an internal heat source
Giant storm called the Great Dark Spot.
The size, shape, and location of the spot vary greatly over time, and it has been known occasionally to disappear and reappear.
Winds near spot have a speed of 2,400 kph.

Magnetic Field

Strongly tilted relative to its rotational axis, by 47°.
Offset by about 13,500 km from planet's physical center.


Neptune has 4 faint rings.
Unknown composition and curiously clumpy structure
There is evidence that some of the rings are incomplete arcs.


Neptune has 13 known moons.
Triton: 2,700 km diameter; Retrograde orbit
Tidal interactions between Neptune and Triton are causing its orbit to decay.
Axis of rotation is tilted 157° with respect to Neptune's axis.
Results in radical seasonal changes.


Composed of water ice (25%) and rock material.
Very tenuous atmosphere composed mostly of nitrogen with a small amount of methane.
Few craters are visible
indicates a relatively young surface.
Southern hemisphere is covered with an ice cap of frozen nitrogen and methane
Extensive ridges and valleys known as cantaloupe terrain

Triton Volcanoes


Volcanic Mechanisms

Earth, Venus, Io and Triton are known to be volcanically active.
On Earth, Venus, Mars, Mercury and the Moon eruptions are of:
Io's eruptions of:
Triton's eruptions are of:

Neptune: Other Facts

Distance from Sun 30.1 AU
Distance from Sun 4,488 million km
Equatorial diameter 49,538 km
Mass (Earth=1) 17.2
Mean density 1.7 g/cm3
Axial period 19.1 hours
Axial inclination 28.8°
Orbital period 164.8 years
Orbital inclination 1.8°
Orbital eccentricity 0.009
Gravity at cloud-tops (Earth=1) 1.2
Escape velocity 23.9 km/s
Albedo 0.35