neptune. discovery john couch adams, cambridge mathematician, predicted the existence of an unseen...
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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”
Neptune
Discovered 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
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
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
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.
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.