11. Uranus and Neptune

    • Uranus and Neptune were unknown to ancient astronomers, and were not discovered until after the invention of the telescope.

    • 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 material.

    • 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 kilometer across.

    • 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 facing Uranus.

    • Neptuneís largest satellite, Triton, revolves about the planet in a direction opposite to that in which Neptune rotates.

    • 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