A Multitude of Moons
Numerous natural satellites of the planets are bigger than Pluto and Mercury - so they're not always just lumps of rock. This page is dedicated to the more interesting discoveries about the moons in the solar system, with direct links to sites with more information about them.
The Moon:Deserving a special mention, the Earth's sole companion is a magnificent site in the sky - both during the day and night (depending on where it is in its orbit). It is on the Moon where part of humankind's future lies.
Phobos:Thought for years just to be an 'ordinary' captured asteroid,in September 1998 the Mars Global Surveyor showed the surface of this small body to have been pounded by eons of meteroid impacts. Measurements show that the surface is compsed largely of finely ground powder at least a metre thick.
Temperature measurements of the day and night sides show extreme variations: the sunlit side rivalling a winter day in London (-4°C), while a few kilometers away, on the dark side, the climate is more harsh than a night in Antarctica (-112°C). Phobos rotates every 7 hours, and such extreme heat loss can only be explained by a thick dust layer.
Deimos:Deimos is the smallest of Mars' moons, orbiting at about the same rate that Mars rotates.
Callisto:Until October 1998, most scientists thought Jupiter's moon Callisto was dead and boring, an unchanging piece of rock and ice. Data reported in Nature could change all that. It appears that Callisto, like another of Jupiter's moons Europa, may have an underground liquid ocean and at least some of the basic ingredients for life.
Ganymede:If it orbited the Sun on its own then it would undoubtedly be a planet. Ganymede, the largest moon in the solar system, has a diameter of 5,260 km, is larger than the planets Mercury and Pluto and just over three quarters the size of Mars.
Ganymede's craters are remarkably smooth. They seem to have become flattened as the moon's icy surface slowly relaxes. Its surface shows a mixture of old, dark, cratered terrain and lighter, younger, regions streaked with grooves and ridges.
Another moon of Jupiter, Europa, has captured most of the attention in recent years. This is because of the possibility of an ocean beneath its frozen surface.
Io:The only body other than the Earth known to have active volcanoes. Io is hell.
The image right shows Io in front of Jupiter as well as its shadow cast onto the clouds by the Sun's light. Click the image for a larger version.
Titan:Titan is larger than Mercury, and may be the only body in the solar system, other than Earth, that may have oceans and rainfall on its surface, albeit oceans and rain of ethane-methane rather than water. The movie on the right was made (using Intergif) from H.S.T. images. Click for a larger movie (80K), or for original images - (42K).
Titan will be observed directly in 2004 when the Cassini probe launches Huygens to land on its surface! There will be enough battery power for up to 30 minutes' data collection. For more information, visit the UK Cassini-Huygens homepage.
Cassini will spend four years in orbit around Saturn. The plan allows for 44 close flybys of Titan, a prime target for the Cassini/Huygens mission.
Each flyby of Titan will allow Cassini's to closely study its atmosphere, interior, surface and interactions with Saturn's magnetic field. The Titan radar mapper, will use RADAR to "see" through Titan's opaque, brown-orange atmosphere.
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