AOSC 637 Lesson 24

Uranus

Has been visited by Voyager 2 in Plane spins on an axis almost parallel to the ecliptic plane. Polar regions can point directly at the sun.iu Uranus is however hotter at the equator than at the poles. Not known why. Bulk of planet composed largely of rock and various ices. Has about 15% hydrogen and a little helium. Has very dark rings. Composed of fairly large particles. (up to 10 meters in diameter) and fine dust. Has a magnetic field – probably generated by motions at shallow depths.

Uranus

Blue color is the result of absorption of red light by methane in the upper atmosphere. Like the other gaseous planets, Uranus has bands of clouds that blow around rapidly. They are extremely faint. Pictures from the Hubble Space Telescope (HST) show an apparent seasonal effect

Uranus

Neptune

Voyager 2, Bulk composition similar to Uranus – various ices and rocks. About 15% hydrogen and a little helium. Blue coloration is the result of absorption of the red light from the sun by methane. Like Jupiter and Saturn, Neptune has an internal heat source – it radiates more radiation than it receives from the sun. At the time of the Voyager encounter, the most prominent feature on Neptune was the Great dark Spot. Neptunes winds blew the spot westward at 300 meters per second. However, HST observations in 1994 showed that the Dark Spot had disappeared. Later on a new dark spot was seen.

Neptune

Triton

Voyager 2 on Aug 25, Largest of Neptune’s satellites. It has a retrograde orbit about Neptune. It is believed that Triton was formes elsewhere and the captured by Neptune. Triton’s density (2.0) is slightly greater then that of Saturn’s icy moons. Triton is probably about 24% water ice, with the remainder rocky material. Few craters – young surface. Almost the entire Southern hemisphere is covered with an ‘ice cap’ of frozen nitrogen and methane Extensive ridges and valleys – freezing and thawing cycles Ice volcanoes

Triton

Pluto

No spaecraft flybye. HST can only resolve largest features. Surface temperature varies from about 38 to 63 K. Bulk is probably a mixture of 70% rock, and 30% water ice. Bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of solid methane, ethane and CO

Atmospheric Chemistry Composition of the atmospheres of Uranus and Neptune is closer to that presumed for the sun, than that of Jupiter and Saturn. Neptune has winds that approach the speed of sound. The presence of H 2 had been measured from Earth. Airglow and auroral emissions from Voyager have also shown the presence of atomic and molecular hydrogen up to high altitudes. Excitation is either from impact of low energy electrons fron the Sun, or from Rayleigh and resonance scattering of solar radiation.

Atmospheric Chemistry Spectra show strong absorption bands of methane. As noted before the absorption in the red leads to the color of the planets. Voyager detected clouds of methane in both Uranus and Neptune. C2H2 and C2H6 is produced in a similar manner to that on Jupiter and Saturn. Synthesis of higher hydrocarbons is likely and aerosols may be formed. Changes in the brightness of Neptune are in antiphase with changes in solar activity – suggesting that changes in solar radiation is connected withy changes in aerosol production – which then changes the planetary albedo.

Atmospheric Chemistry Condensation removes most of the ammonia on Uranus and Neptune in contrast to Jupiter and Saturn. The presence of CO in the atmosphere of Neptune is a puzzle. The observed mixing ratio is about three orders of magnitude larger than on Jupiter and Saturn, and 40 times larger than on Uranus. Measurements from HST show an almost constant mixing ratio throughout the troposphere and stratosphere, which indicates a source close to the surface. Neptune has a large internal heat source, which could provide the dynamical forcing for the mixing. By contrast Uranus does not have such an internal heat source.

Atmospheric Chemistry Triton and Pluto have tenuous atmospheres according to ground-based measurements. Voyager fly-by of Triton shows strong airglow features from neutral N 2 and ionized oxygen – similar to Titan. Triton atmosphere made up predominately of N 2. Methane is also present in the lower atmosphere. In addition further measurements have detected CO, CO 2 and H. The H must be produced from methane, as on Titan, Uranus and Neptune. The observed surface temperature and pressure are consistent with a saturation equilibrium of N 2 in the atmosphere. Concentration of methane is some 30 times less than equilibrium, indicating that it is removed by photochemical processes. Because nitrogen is present, this photochemistry should be similar to Titan’s.

Atmospheric Chemistry The pressure at the surface of Pluto has been estimated to be 3 microbars. The experimental information comes mainly from ground-based instruments and IRAS (InfraRed Astronomical Satellite) Near IR spectrum dominated by bands from solid methane. Other measurements indicate a system with a surface frost. This surface ice is inferred to consist of solid N 2 (98%) with methane (1.5%) and CO (0.5%).