AOSC 637
Lesson 24
Uranus
Uranus
• Has been visited by Voyager 2 in 1966.
• 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
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
Neptune
• Voyager 2, 1989.
• 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
Neptune
Triton
Triton
• Voyager 2 on Aug 25, 1989.
• 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
Triton
Pluto
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 H2 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 N2 and ionized oxygen – similar to Titan.
• Triton atmosphere made up predominately of N2.
Methane is also present in the lower atmosphere. In
addition further measurements have detected CO, CO2
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 N2 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 N2 (98%) with
methane (1.5%) and CO (0.5%).