Neptune

Discovery• John Couch Adams, Cambridge mathematician,

predicted the existence of an unseen planet. Uranus was being pulled slightly out of position in its orbit (1845).

• His work was ignored by the Royal observatory Greenwich

• Until a French mathematician, Urbain Leverrier, published a similar prediction.

• Search started in July 1846 lead by Prof. Challis at Cambridge university

• Leverrier with the help of Johann Galle at Berlin Observatory found the planet on the 23rd September 1846

• Leverrier and Adams are now jointly credited with their “discovery”

NeptuneDiscovered in

1846 at position predicted from gravitational

disturbances on Uranus’ orbit.

Blue-green color from methane in the atmosphere

4 times Earth’s diameter; 4 % smaller than Uranus

Neptune is a cold, bluish world with Jupiterlike atmospheric features

• No white ammonia clouds are seen on Neptune

• Presumably the low temperatures have caused almost all the ammonia to precipitate into the interiors of the planets

• All of these planets’ clouds are composed of methane

• • Much more cloud activity is seen on Neptune than on Uranus.

AppearanceNeptune shows color variation in bandsAlso, As season progresses, southern bands brighten (near

IR)

The Atmosphere of Neptune

Cloud-belt structure with high-velocity winds; origin not well understood.

Darker cyclonic disturbances, similar to Great Red Spot on Jupiter, but not long-lived.

The “Great Dark Spot”

White cloud features of methane ice crystals

Composition79% H2 18% He 3% CH4

Traces of N2, HCN, H2S, NH3, CO

Neptune’s Banded Structure

Keck AO Infrared image

No more Great Dark SpotDynamic weather patternsWind speed up to 1500 km/h(half the speed of sound)

Zonal Winds• Far fewer bands than on Jupiter and

Saturn• Neptune -300m/s(0o), 0(±50o),

225(±75o) • Less sunlight = less turbulence = flow less

disturbed

Clouds• methane ice (CH4) <1bar (limit of probing)

Probably have:• Methane droplets?• hydrogen Sulfide (H2S) 2-5bar

more H2S, so no NH3 clouds?• NH4 hydrosulfide (NH4SH) 15-30bar• water 15-50bar

Neptune’s CloudsMuch more cloud

activity is seen on Neptune than on Uranus

High altitude clouds

Great Dark Spot• Comparable in size to GRS• Storm? (1200mph,

counterclockwise rotation, anticyclone)

• Below the tropopause • Moved towards equator,

dissipated, new one formed in the northern hemisphere

Great Dark Spot

• Hole in the methane droplet cloud deck?

• Like the ozone hole over Antarctica?

The Dark Spot

Neptune contains a higher proportion of heavy elements than Jupiter and Saturn

Neptune may have a rocky core surrounded by a mantle of water and ammonia

Electric currents in the mantles may generate the magnetic fields of the planets

Internal Structure of Neptune

Interior Structure• Ice and rock in core (same size

than Earth–Not enough pressure to force

hydrogen into metallic hydrogen–Off-axis and offset magnetic

field probably due to polarized water or ammonia dissolved in water in outer core

Neptune’s Structure • Similar composition than Jupiter: 84% H

and 14% He, CH4 (3%) (blue color). • No NH3 since Tcondensation = 70 K and

Touttermostplanets ~58 K• Far more atmospheric activity than

Uranus b/c of Interior heat (3 times more energy) + less haze– Modest axial tilt may allow for more

atmospheric mixing– Storm activity seen and seems to change

The magnetic field of Neptune is oriented at an unusual angle

The magnetic axis of Neptune is steeply inclined from its axis of rotationThe magnetic and rotational axes of all the other planets are more nearly

parallelThe magnetic field of Neptune is also offset from the center of the

planet

The magnetic field of Neptune

The Rings of Neptune

Made of dark material, visible in forward-

scattered light.

Interrupted between denser segments (arcs)

Focused by small shepherd moons embedded in the

ring structure.

Ring material must be regularly re-supplied

by dust from meteorite impacts on the moons.

Neptune has a system of thin, dark rings

Ring Arcs

The Moons of NeptuneTwo moons (Triton and Nereid) visible from Earth;

6 more discovered by Voyager 2

Unusual orbits:

Triton: Only satellite in the solar system orbiting clockwise, i.e. “backward”.

Nereid: Highly eccentric orbit; very long orbital period (359.4 d).

Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter

Triton has a young, icy surface indicative of tectonic activity

The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit

Triton has a tenuous nitrogen atmosphere

Triton is a frigid, icy world with a young surface and a tenuous atmosphere

Neptune’s Satellites

TritonProbably a captured Kuiper

belt object: orbiting Neptune opposite Neptune’s direction of rotation.

Smaller than Earth’s Moon, yet has recent geological activity.

The Surface of Triton

→ Triton can hold a tenuous atmosphere of

nitrogen and some methane; 105 times less

dense than Earth’s atmosphere.

Very low temperature (34.5 K)

Surface composed of ices: nitrogen, methane, carbon monoxide, carbon dioxide.

Possibly cyclic nitrogen ice deposition and re-vaporizing on

Triton’s south pole, similar to CO2 ice polar cap cycles on Mars.

The Surface of Triton

Ongoing surface activity: Surface features probably not more than 100 million years old.

Large basins might have been flooded multiple times by liquids from the interior.