11. Uranus and Neptune
- Uranus and Neptune were unknown
to ancient astronomers, and were not discovered until after the invention of
- Uranus is just barely visible to
the unaided eye, and Neptune requires a telescope to be seen.
- The blue-green, turquoise color
of Uranus and the blue, indigo color of Neptune come from methane in their clouds.
- Uranus and Neptune have a similar
size, mass, and bulk composition and they are both much smaller, less massive,
and denser than the other two giant planets, Jupiter and Saturn.
- In contrast to all the other
planets in the solar system, Uranus is tipped sideways so its rotation axis
lies nearly within the planetís orbital plane, leading to extreme seasonal
variations in solar heating of the planetís polar regions.
- Although Uranus apparently has no
strong internal source of heat, Neptune radiates 2.7 times the energy it
absorbs from the Sun. The source of Neptuneís excess energy is most likely
heat leftover from the planetís formation.
- The cloud bands and winds on
Uranus blow parallel to the planetís equator, apparently controlled by the
planetís rapid spin rather than by direct heating from the Sun.
- Despite receiving a relatively
small amount of sunlight, compared to the other major planets, Neptuneís
atmosphere is surprisingly active and dynamic, with large storm systems and
high-speed winds that may be driven by internal heat.
- As the southern hemisphere of
Neptune turned slowly toward the Sunís faint heat, the temperature of the
planetís south polar region increased by just 10 kelvin and methane storm
clouds became more frequent in the southern hemisphere.
- Unlike Jupiter and Saturn, there
is no liquid metallic hydrogen inside Uranus and Neptune, but they both contain
deep atmospheres of molecular hydrogen.
- The internal structure of Uranus
and Neptune includes vast internal oceans of water, methane and ammonia ďicesĒ,
melted at the high temperatures inside.
- The magnetic fields of Uranus and
Neptune are askew, tilted from their rotation axes; rotation-driven currents in
internal shells of ionized water could generate these magnetic fields.
- The austere, skeletal rings of
Uranus are very narrow and widely spaced from each other, and made of very dark
- Shepherd satellites are most
likely responsible for the narrowness of the rings of Uranus and Neptune.
- The rings around Uranus are not
quite circular, do not lie exactly in Uranusís equatorial plane, and vary in
width; these irregularities are attributed to the gravitational interaction of
ring particles with small nearby moons.
- The material in one narrow ring
around Neptune has been concentrated into three clumps, probably by the gravity
of a nearby moon or moons.
- The sparse rings around Neptune
contain no more material than that found in a single small moon only a
- Most of Neptuneís rings that we
see now will probably be ground into dust by collisions and meteoritic
bombardment in a few hundred million years, eventually being consumed by their
central planet and vanishing from sight. But the rings can easily be replaced
by debris blasted off small moons already embedded in them.
- The five major moons of Uranus
are dark and dense, made up predominantly of rock and water ice.
- Miranda, the innermost mid-sized
satellite of Uranus, exhibits a bizarre variety of surface features that
suggest repeated violent impacts in the past. It may have been shattered by
catastrophic collisions and reassembled, or it became frozen in an embryonic
stage of differentiation.
- Uranusís large moons all revolve
in the planetís equatorial plane, almost perpendicular to the planetís orbital
plane, in circular synchronous satellite orbits with the same side always
- Neptuneís largest satellite,
Triton, revolves about the planet in a direction opposite to that in which
- Triton has a very tenuous,
nitrogen-rich atmosphere, bright polar caps of nitrogen and methane ice, frozen
lakes of water flooded by past volcanoes of water ice, and towering geysers
that may now be erupting on its surface.
- Triton may have formed in
orbit around the Sun and was subsequently captured by Neptune, whose tidal
forces kept Triton molten for much of its early history. These tides are now
pulling the satellite toward a future collision with the planet.
Copyright 2010, Professor Kenneth R. Lang, Tufts University