Lecture #28: Uranus. The Main Point. The Jovian Planets. Uranus: General properties. Atmosphere. Interior. Origin and evolution.

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1 Lecture #28: Uranus Uranus: General properties. Atmosphere. Interior. Origin and evolution. Reading: 7.1 (Uranus) and 11.1 The Main Point Uranus, the first planet discovered by telescopes, is a mid-sized and highly tilted Jovian class planet with a bland atmosphere and a very dark ring system. Astro 102/104 1 Astro 102/104 2 The Jovian Planets Astro 102/104 3 Astro 102/

2 Basic Properties of Uranus Average Distance from Sun: 2.87 billion km (a=19.2 AU). Orbital period: 84.1 years; eccentricity: Period of Spin around axis: ~17 hours and retrograde. Tilt of Uranus' spin axis: 98 (extreme seasons). Mass: 8.7x10 25 kg = 15 M E ; Radius: 25,600 km = 4.0 R E. Density = 1.3 g/cm 3 (data indicate mostly "rocky/icy"). No "surface" like the terrestrial planets: clouds/haze visible. Gravity = 9.0 m/sec 2 (0.92 times Earth's). Cloud-top temperature: 60 K (-213 C). Atmosphere: Mostly H, He, CH 4 --but only ~15% of planet. Moons: 27 presently known (12 discovered since late 1990s). Uranus has a small set of dark, dusty rings. Astro 102/104 5 Discovery First planet discovered by telescope: Uranus had been observed as early as 1690 but was catalogued as a star (very slow sky motion). Recognized as a planet in 1781 by the musician and amateur astronomer William Herschel ( ), after a systematic search of the sky with a 7 foot refractor. Herschel wanted to name the planet Georgium Sidus (George's planet) after his patron, King George III. But Uranus (pronounced YUR uh nus), the Greek god of the heavens, was proposed by others and the name stuck. Astro 102/104 6 Observations Uranus' disk spans only about 2 to 4 arcseconds. It can be barely visible to the naked eye, but you must have very dark skies and know exactly where to look. Uranus appears slightly blue and relatively bland through most ground based telescopes. More detail can be seen in higher resolution HST images, or using telescopes in the infrared. Features in Uranus' atmosphere are very small and subtle compared to those on Jupiter or Saturn. Astro 102/104 7 Space Missions Only one encounter: Voyager 2. Mission Date Goals and Results Voyager Flyby; high resolution imaging and other studies No plans have been made for any new space missions to Uranus. Astro 102/

3 Features in Uranus' Atmosphere Zones and belts weak or absent. Smaller clouds, storms do occur. Astro 102/104 9 Atmospheric Composition Determined by spectroscopy from telescopes and spacecraft. Observed (upper) composition very solar. The ratio of H2/He in Name Symbol % Volume Uranus' atmosphere is Hydrogen H 2 83 almost identical to that Helium He 15 in the Sun. Methane CH 4 2 Much more He and Water H 2O??? Carbon Monoxide CO ~1 x 10-5 CH4 than detected on? Jupiter or Saturn. Hydrogen Disulfide H 2S 1 x 10-6 Ammonia NH 3 2 x 10-7 Implies that Uranus C 2H 4 7 x 10-7 atmosphere still "well CH 3NH 2 3 x 10-7 mixed" (heavier He did not differentiate?) Astro 102/ Atmospheric Structure Uranus' Interior Clouds/hazes of methane gas give Uranus its distinctive blue/green color Altitude (km) Slightly higher density, higher Helium abundance, and lower internal pressures & temperatures argue that Uranus and Neptune have very different interior structures than Jupiter and Saturn. ice and rock are much larger fractions of Uranus and Neptune, where ice here means volatiles like CH 4, NH 3, H 2 O, and rock means silicate minerals like the terrestrial planets. T (K) Astro 102/ Astro 102/

4 Internal vs. External Heat Sources Again, examine the energy balance. For Uranus: Outgoing = Incoming. Unlike Jupiter & Saturn (& Neptune), Uranus doesn't generate its own internal heat. Why not??? Lack of internal heating may explain the rarity of clouds & other "weather" on Uranus. Strange Seasons! Uranus is tilted on its side, essentially "rolling" around the solar system! This is another factor that may influence differences between Uranus and the other gas giant planets. What caused this strange tilt? UNKNOWN Primordial? Giant Impact? Tides? ~2028 ~2007 ~1986 Astro 102/ year time steps Astro 102/ Uranus has a strong magnetic field But Uranus has no metallic H, so how is the field generated? Hypothesis: Field is generated in the core by high pressure ionization of ices/rock (making them conductive). Uranus' field is tilted and offset relative to its spin axis: supports the idea that the field is generated in the rocky/ice mantle? Satellites Uranus has 5 "regular" large satellites, discovered by telescopic observations between All are relatively dark and icy (based on spectroscopy). 10 smaller moons discovered by Voyager more tiny outer moons discovered in Naming scheme based on Shakespeare and Pope. Astro 102/ Astro 102/ More details: Lecture 30 4

5 Rings! Uranus has a system of 9 dark "main" rings. These rings are also surrounded by belts of fine-grained dust. Discovered in 1977 by watching a star "blink out" behind them. Made of organics+ices? Astro 102/ More details in Lecture Summary Uranus is a gas giant planet with a thick H, He, and CH 4 atmosphere. Uranus is tilted at 98 to the ecliptic: odd seasons. Uranus' atmosphere has many fewer clouds and other features than Jupiter's or Saturn's. No internal heat of its own. Uranus' interior consists mostly of "icy" and "rocky" materials at high pressure and temperature. Strong & offset magnetic field generated in the mantle. Uranus has 27 known satellites and a ring system. Astro 102/ Neptune: General properties. Atmosphere. Interior. Origin and evolution. Next Lecture... Reading: 7.1 (Neptune) and 11.1 Astro 102/