Uranus Neptune 1 Uranus Neptune basic data Semimajor

Uranus & Neptune 1

Uranus & Neptune basic data Semi-major axis Orbital period Rotation period Inclination Diameter Mass Density Temp at cloud tops Uranus 19. 2 AU 84 yrs 17 hrs 98° 4. 0 DEarth 15 Mearth 1318 kg/m 3 or 1. 3 g/cm 3 50 K Neptune 30. 1 AU 165 yrs 16 hrs 30° 3. 9 DEarth 17 MEarth 1638 kg/m 3 or 1. 6 g/cm 3 55 K 2

Discovery of Uranus & Neptune • Uranus: by chance – had been mapped many times as a star. William Herschel observed it through a telescope in 1781 – slightly fuzzy and moving relative to stars. • Neptune: scientific prediction - Uranus’s orbit was not a perfect ellipse. Either Newton was wrong or something was pulling on it. John Adams and Urbain Le Verrier in 1845/46 independently calculated where unseen planet might be. Johann Galle found it on the first night after receiving the predictions. 3

Spacecraft missions • Voyager 2 flew past Uranus & Neptune: – Uranus: January 1986 – Neptune: August 1989 • Both have been extensively studied using the Hubble Space Telescope: – Long-term monitoring of atmospheric weather patterns. – IR imaging of their atmospheres, rings, and moons. 4

Why the uniform blue appearance? 5

Uranus’ atmosphere • Mostly H 2, He 2. T=50 K. Temperatures do not indicate internal heat source – only Sun’s energy. • Colder than Jupiter or Saturn, ammonia and water have frozen and sunk deep down. • But methane remains in gas form. There is 5 -10 times as much methane in its atmosphere as in Jupiter or Saturn. Methane absorbs long wavelengths Uranus appears bluegreen. Also makes Neptune blue. • Methane haze high in atmosphere gives uniform appearance. Methane droplets only condense at high pressure, deeper down, so no clouds visible. 6

Uranus’ radical tilt causes interesting illumination conditions. 84 year orbit. 42 years of light or dark at the poles! Tilt is probably a result of a collision (or collisions) with a planet-sized object when Uranus and its moons were forming. 7

IR image from Hubble. Penetrates deeper into atmosphere, showing bands and storms. 8

Neptune • Nearly a twin to Uranus, with blue due to methane. But cloud patterns visible through haze. Clouds are frozen methane at high altitudes. • Great Dark Spot found by Voyager 2 in 1989. Had disappeared by 1994 when HST observed it. • T=55 K in upper atmosphere, bit higher than Uranus. But further from Sun. Emits more radiation than is received. Probably still contracting? Extra internal heat may drive convection, causing high winds, clouds and storms. 9

Interiors are similar • Earth-mass rocky core, surrounded by liquid/icy water and ammonia, then liquid molecular hydrogen and helium, then gaseous atmosphere. 10 Interior pressure not sufficient to form liquid metallic hydrogen. Magnetic fields probably due to ionized liquid ammonia. Upper atmosphere: hydrogen, helium, ~2% methane (almost no ammonia or water)

Both Uranus and Neptune have higher densities and lower H and He abundances than Jupiter and Saturn. So they have higher abundances of heavy elements. Why? Possible explanation: Formed closer to the Sun, at 10 -15 AU (possibly with Neptune forming closer to Sun than Uranus), then moved outwards due to gravitational interaction with Jupiter or Saturn or just planetesimals before they could accrete much? In fact, at 20 -30 AU, simulations indicate accretion process too inefficient. Planets orbit slowly, and not much material to accrete. Still a puzzle. “Nice model” 11

Uranus' satellites (27) • 5 large, icy moons • ~13 smaller ones cluttered in Uranus’ rings • Plus a number of smaller, outer moons with retrograde rotation (captured asteroids) 12

Miranda – two very different types of terrain. One cratered, dating to > 4 billion years ago. Other suggests some upheaval. Reasons not clear. Disruptive event or tidal heating in the past? 13

20 km high cliff 14

Neptune's satellites (14) Triton 2700 km • Triton much larger than the rest • Highly inclined, retrograde orbit => captured, formed elsewhere 15

Tenuous nitrogen and methane atmosphere. Despite cold (38 K), sunlight can still evaporate some ices from surface. Triton Cantaloupe terrain thought to be due to volcanism and rifting of the surface. Age only 6 million years from crater counts. Southern part is large polar nitrogen ice cap. <50 million years old. Nitrogen plumes in south, leaving surface deposits => Sub -surface solar-heated greenhouse effect causing geysers, or internal heat source? 16

• Tectonic activity is likely related to its capture. • Initial, highly elliptical orbit would become circularized by tidal forces. Varying tidal stresses may have led to internal heat. • Tidal force, along with retrograde orbit, causing Triton to spiral in toward Neptune. • It should fall inside of Roche limit in 108 years spectacular rings? 17

Rings • Both have thin, dark ring systems, discovered from Earth during stellar occultations. • Uranus’s ring particle sizes are 10 cm to 10 meters. Not as reflective as Saturn’s. • Neptune’s are narrower. Particles range from few m to several meters. • Both much darker than Saturn’s. Note: most rings are within Roche limit for planet. 18

Uranus’ rings discovered accidentally during “occultation” experiment in 1977 to determine Uranus’ size. Star flickered before and after. 19

Uranus’ rings seen in silhouette against planet 9 discovered from Earth, 2 by Voyager 2, 2 by HST. Neptune’s rings (Voyager 2). Also faint “disk” ending between brightest rings. 20

• Origin: possibly fragments of moons chipped off by meteoroid impacts • Why are they so narrow? Confined by “shepherding” satellites (also true of Saturn’s narrow rings). Some have been discovered, others may be too small to be seen. • Why are these rings darker than Saturn’s when moons are icier? Rings cold enough to retain methane ice, not just water ice. When hit by electrons in the magnetosphere, methane converted into dark carbon compounds. • Neptune’s rings are getting fainter since discovery. One may vanish within 100 years. Implies frequent, episodic replenishment? 21

Voyager 2: brightest Epsilon ring is shepherded by two satellites, Cordelia and Ophelia. 22

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Two outer rings, outermost fed by dust kicked out from impacts on a newly discovered satellite Mab. Outside Roche Limit. 24

Where are the larger satellites? 25

Magnetic fields • All Jovian planets have B-fields (Jupiter's field is the strongest; Saturn, Uranus & Neptune's fields are similar) • Jupiter and Saturn: liquid metallic hydrogen. Uranus and Neptune: not dense enough for that, but expect ammonia liquid to be ionized, ammonia/water layer can carry current. Reasons for misalignment and off-centering of Uranus and Neptune 26 fields not clear

Oberon: old surface, heavily cratered. Ariel: youngest surface, only small craters. Flat-floored valleys. Umbriel: another old surface. Titania. Also young surface 27

Generic phase diagram Uranus atmosphere is in methane gas region, further down is methane ice and maybe liquid . Titan surface is close to methane solid/liquid boundary CH 4 0. 11 atm 28 CH 4 91 K

Extreme seasons • Tilt is probably a result of a collision (or collisions) with a planet-sized object when Uranus and its moons were forming. • Poles are in darkness for extremely long periods, but the temperatures over the whole of the planet are almost constant. Heat must therefore be efficiently transferred away from Sunfacing pole. • This heat transport may be the reason for the lack of features in the atmosphere – perpendicular to wind flow, homogenized atmosphere? • Temperatures do not indicate internal heat source – only Sun’s 29 energy.