Neptune

1. Neptune

3. 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”

4. 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

5. 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.

6. Appearance Neptune shows color variation in bands Also, As season progresses, southern bands brighten (near IR)

7. 0 The Atmosphere of Neptune The “Great Dark Spot” 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. White cloud features of methane ice crystals

8. Composition 79% H2 18% He 3% CH4 Traces of N2, HCN, H2S, NH3, CO

9. Neptune’s Banded Structure

10. Keck AO Infrared image No more Great Dark Spot Dynamic weather patterns Wind speed up to 1500 km/h (half the speed of sound)

11. 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

12. 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

13. Neptune’s Clouds Much more cloud activity is seen on Neptune than on Uranus

14. High altitude clouds

15. 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

16. Great Dark Spot • Hole in the methane droplet cloud deck? • Like the ozone hole over Antarctica?

17. The Dark Spot

18. Internal Structure of Neptune 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

19. 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

20. 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

21. The magnetic field of Neptune The magnetic field of Neptune is oriented at an unusual angle The magnetic axis of Neptune is steeply inclined from its axis of rotation The magnetic and rotational axes of all the other planets are more nearly parallel The magnetic field of Neptune is also offset from the center of the planet

22. 0 The Rings of Neptune Interrupted between denser segments (arcs) Made of dark material, visible in forward-scattered light. Ring material must be regularly re-supplied by dust from meteorite impacts on the moons. Focused by small shepherd moons embedded in the ring structure.

23. Neptune has a system of thin, dark rings

24. Ring Arcs

25. 0 The Moons of Neptune Two 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).

26. Neptune’s Satellites Triton is a frigid, icy world with a young surface and a tenuous atmosphere 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

27. Triton Probably a captured Kuiper belt object: orbiting Neptune opposite Neptune’s direction of rotation. Smaller than Earth’s Moon, yet has recent geological activity.

28. 0 The Surface of Triton Very low temperature (34.5 K) → Triton can hold a tenuous atmosphere of nitrogen and some methane; 105 times less dense than Earth’s atmosphere. 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.

29. 0 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.