Natural Satellite - astronomy.

« Titan, Saturn’s largest moon, also has few impact craters.

Although icy volcanism from water and ammonia may occur along with some tectonic activity, Titan’s youngsurface is most likely the result of weather processes.

Titan’s dense, cold atmosphere precipitates particles of complex organic molecules that accumulate as dunes andmountains.

Methane rain erodes the surface and creates lakes at the moon’s poles.

Triton’s young surface also may result from processes in its atmosphere, as well aseruptions of nitrogen geysers from underground pockets. B Atmospheres and Auroras on Moons Most moons are too small for their gravity to retain an atmosphere.

Nonetheless, cold temperatures or geological activity allow a few moons to maintain detectableatmospheres. Titan has the densest atmosphere of any moon—60 percent denser than Earth’s atmosphere.

Titan’s atmosphere is made up of nitrogen and methane.

Triton has a thinnitrogen and methane atmosphere, formed largely from frozen gas on its surface heated by the Sun.

Some of Triton’s atmosphere freezes to create polar caps. Ganymede has a very thin oxygen atmosphere thought to come from charged particles breaking apart molecules of water ice on its surface.

Most of the chargedparticles that strike Ganymede come from volcanoes on the moon Io—the particles are trapped and energized in Jupiter’s giant magnetic field.

The charged particlesalso raise a spray of water molecules that falls back and freezes on the surface of Ganymede, giving its polar regions bright caps of frost.

Ganymede also has auroras atits poles, caused by charged particles striking its thin oxygen atmosphere.

Little Enceladus has a thin local atmosphere from water vapor released by geysers at onepole. Io has a thin atmosphere made of sulfur dioxide gas released by volcanic eruptions over many parts of its surface.

Some of this atmosphere freezes out as frost on themoon’s nightside, then becomes a gas again when sunlight returns.

Auroras occur on the nightside of Io at the moon’s equator. C Magnetic Fields on Moons Ganymede is the only moon that has its own intrinsic magnetic field, thought to be generated by a subsurface ocean circulating beneath its icy crust.

Europa has nomagnetic field of its own.

However, Europa orbits inside the giant magnetic field of Jupiter.

Measurements taken by the Galileo spacecraft show that Europa has aninduced magnetic field that changes as the moon orbits Jupiter.

The ability to pick up an outside magnetic field is strong evidence that Europa has a liquid ocean underits surface.

A similar fluctuating induced magnetic field was also detected around Callisto, suggesting that Callisto may have a subsurface ocean, as well. D Subsurface Oceans on Moons The subsurface oceans thought to exist on Jupiter’s moons Europa, Ganymede, and Callisto are likely made of water mixed with salts, allowing the oceans to conductelectricity and have magnetic properties.

Such an environment of water and chemicals also might allow life to exist.

The hidden ocean on Europa is the best candidatefor some kind of life because it is warmer and the moon’s interior may be geologically active. Scientists are analyzing data from the Cassini space probe to determine if Saturn’s moon Titan might have a subsurface ocean of water mixed with ammonia.

Sometheoretical works suggests other outer moons may have subsurface oceans containing a mix of water, ammonia, and methane.

Moons that are possible candidatesinclude Saturn’s Rhea, Uranus’s Titania and Oberon, and Neptune’s Triton. E Rings and Satellites The four giant planets (Jupiter, Saturn, Uranus, and Neptune) are each surrounded by rings in the plane of their equators.

Rings are made of rocky or icy material thatcan range in size from tiny particles to objects as large as houses.

Scientists think such rings are likely debris from broken-up satellites or from passing comets, KBOs,or asteroids torn apart by the planet’s gravity.

Another possibility is that rings are made of ancient, leftover material that never condensed into satellites. Complex interactions can occur between ring material and existing satellites.

Small so-called shepherd moons orbit along the edges of rings or within gaps betweenrings, keeping the ring material in place.

Small satellites may form out of ring material or be shattered by impacts or collisions to form new rings.

In some cases, ringsmay be made of material thrown into space off the surface of satellites by impacts or by geological activity. V SEARCH FOR EXTRATERRESTRIAL LIFE ON MOONS Beginning in the 1970s, space probes such as Voyager, Galileo, and Cassini have provided scientists with a wealth of information about moons in the outer solar system.During the same period, researchers discovered microorganisms and even complex life on Earth that can survive under extreme conditions once thought to be toohostile for life.

Such so-called extremophile organisms are found in superhot seafloor volcanic vents or in geyser pools, in rocks deep underground or under ice, andeven in toxic or corrosive chemical environments.

Some of these organisms thrive without oxygen or light, or in freezing cold or boiling heat. Astrobiologists (scientists who study the prospects for life elsewhere in the universe) have raised the possibility that life might exist on distant moons such as Europa,Enceladus, or Titan.

Underground liquid water and organic chemicals, combined with energy from geological heat, gravitational stresses, or charged particles, mightcreate the chemistry and processes needed for life.

Europa is seen as one of the most promising places to hunt for extraterrestrial life—it has a subsurface ocean andsigns of complex chemistry and active geology.

Space missions to send probes that can penetrate its icy crust and explore the hidden ocean beneath are underconsideration. In the mid-1990s scientists began finding planets around other stars—discoveries that greatly improve the odds that life might exist beyond our solar system.

Many ofthe extrasolar planets detected so far are extremely large and often have orbits very near their suns.

Although conditions on such planets appear hostile to life, theseplanets may have large satellites where life might be possible.

However, scientists may need many decades to develop technology that can detect and study satellitesorbiting such distant planets. VI MORE INFORMATION ABOUT SATELLITES Encarta has separate articles on most of the natural satellites that have been studied in detail.

These include the moons of Mars: Deimos and Phobos; the moons ofJupiter: Adrastea, Amalthea, Ananke, Callisto, Carme, Elara, Europa, Ganymede, Himalia, Io, Leda, Lysithea, Metis, Pasiphae, Sinope, and Thebe; the moons of Saturn:Atlas, Calypso, Dione, Enceladus, Epimetheus, Helene, Hyperion, Iapetus, Janus, Mimas, Pan, Pandora, Phoebe, Prometheus, Rhea, Telesto, Tethys, and Titan; themoons of Uranus: Ariel, Belinda, Bianca, Caliban, Cordelia, Cressida, Desdemona, Despina, Juliet, Miranda, Oberon, Ophelia, Portia, Puck, Rosalind, Sycorax, Titania, andUmbriel; and the moons of Neptune: Galatea, Larissa, Naiad, Nereid, Proteus, Thalassa, and Triton.

In addition, overviews of planets’ systems of moons and ringsappear in the planet articles Jupiter, Saturn (moons and rings), Uranus, and Neptune.. »