Monday, May 31, 2010

Charon, the Moon of Pluto

If, having been discovered in 1930, Pluto was a late addition to our known solar system, its moon, Charon, is almost brand new, having been found in 1978. Named, fittingly, for the mythological ferryman who rowed the dead across the River Styx to the underworld ruled by Pluto, Charon is a little more than half the size of its parent: 806 miles (1,300 km) in diameter versus Pluto’s 1,426 miles (2,300 km). Orbiting 12,214 miles (19,700 km) from Pluto, it takes 6.4 Earth days to make one circuit. Pluto and Charon are tidally locked—forever facing one another; the orbital period and rotation period for both are synchronized at 6.4 days. Like Venus and Uranus, Pluto’s rotation is retrograde spinning on its axis in the opposite direction of most of the planets.

A Dozen More Moons in the Outer Solar System

Thanks to Voyager, the six medium-sized moons of Saturn have also been explored. All of these bodies are tidally locked with Saturn, their orbits synchronous, so that they show but one face to their parent planet. They are frozen worlds, mostly rock and water ice. The most distant from Saturn, Iapetus, orbits some 2,207,200 miles (3,560,000 km) from its parent. Because these moons orbit synchronously, astronomers speak of their leading faces and trailing faces. That one face always looks in the direction of the orbit and the other in the opposite direction has created asymmetrical surface features on some of these moons. The leading face of Iapetus, for example, is very dark, while the trailing face is quite light. While some astronomers suggest that the dark material covering this moon’s leading face is generated internally, others believe that Iapetus sweeps up the dust it encounters.
The innermost moon of Saturn, Mimas is 115,320 miles (186,000 km) out. It is also the smallest of Saturn’s moons, with a radius of just 124 miles (200 km). Mimas is very close to Saturn’s rings and seems to have been battered by material associated with them. Heavily cratered overall, this small moon has one enormous crater named for the astronomer William Herschel, which makes it resemble the “Death Star” commanded by Darth Vader in Star Wars. Whatever caused this impact probably came close to shattering Mimas. Indeed, some astronomers believe that similar impacts may have created some of the debris that formed Saturn’s great rings. The Cassini mission will add greatly to our knowledge of these moons and rings. The Cassini-Huygens Mission to Saturn and its mysterious moon Titan was launched on October 15, 1997.
The spacecraft will separate into two parts as it approaches Saturn, sending the Huygens Titan probe on a mission to the surface of the atmosphereenshrouded moon. The mission will study the magnetosphere of Saturn, the planet Saturn itself and its atmosphere and rings, the moon Titan, and finally the other icy moons that orbit the planet. If there were any worries about the performance of the spacecraft, or what it will do when it arrives at Saturn on July 1, 2004, they were substantially allayed in early 2001 when the Cassini-Huygens Mission sped past Jupiter, snapping pictures of the gas giant. You can check on the progress of the mission and view its photos of Jupiter at
The medium-sized moons of Uranus are Miranda, orbiting 80,600 miles (130,000 km) above the planet; Ariel, 118,400 miles (191,000 km) out; Umbriel, 164,900 miles (266,000 km) out; Titania, 270,300 miles (436,000 km) out; and Oberon, 361,500 miles (583,000 km) out. Of these, the most remarkable is Miranda, which, in contrast to the other moons, is extremely varied geographically, with ridges, valleys, and ovalshaped faults. To the camera of Voyager 2, it presented a chaotic, violently fractured, cobbled-together surface unlike that of any other moon in the solar system. Clearly, this moon had a violent past, though it is unclear whether the disruptions it suffered came from within, without, or both. Some astronomers believe that Miranda was virtually shattered, its pieces coming back together in a near-jumble.

Triton, Neptune’s Large Moon

Triton’s distinction among the jovian moons is a retrograde (backward) orbit—in the reverse direction of the other moons. Moreover, Triton is inclined on its axis about 20 degrees and is the only large jovian moon that does not orbit in the equatorial plane of its planet. Many astronomers believe that these peculiarities are the result of some violent event, perhaps a collision. Others suggest that Triton did not form as part of the Neptunian system of moons, but was captured later by the planet’s gravitational field.
Triton’s atmosphere is so thin that Voyager 2 had no trouble imaging the moon’s surface, finding vast lakes of water ice or water-ammonia mixtures there. Nitrogen frost, found at the polar caps, appears to retreat and reforms seasonally.