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earth's moon

courtesy NASA, view from International Space Station


Moons (or satellites) orbit their planets and are held in place by the gravitational pull of the larger body they orbit. They are at the exact distance necessary to orbit. Too close and they would be pulled into the planet, too far away and they would drift out of orbit and into space.

Earth's Moon

Earth's moon is 1/4 the diameter of earth. It is a heavily cratered rock with no atmosphere, no water, and no wind. Therefore, there is no erosion. There is no tectonic or volcanic activity because the moon's interior is cold and inactive. The gravitational force on the surface of the moon is one-sixth of that on the Earth. This low gravity results in the lack of atmosphere on the moon. This means there is no blanket of air to hold in heat at night so the moon gets very cold at night. The lack of atmosphere on the moon decreases visual acuity and depth perception. Add to this the fact that daytime and nighttime on the moon are each two weeks long!
Conseqently there is a lot of time for the daytime side of the moon to heat up during the day and for the nighttime side of the moon to cool off at night. The result is an enormous day-to-night variation in temperature on the moon of about 250° C. This is a difference of 450° F, compared to an average day-to-night difference of 36° F on the earth. All of these factors made walking and driving space vehicles on the lunar surface far more dangerous than driving on Earth. Nonetheless, carefully placed footprints and memorabilia from astronauts' visits during the 60s and 70s sit on the moon as reminders of the excitement of initial explorations, and foreshadow potential future endeavors on Earth's Moon.

The Moons of Jupiter

The four largest moons of Jupiter are Io, Europa, Ganymede, and Callisto.

Io (pronounced eye-oh) is the only moon in the solar system known to have active volcanoes. They spew out sodium (Na) and sulfur dioxide (SO2). Sulfur is the predominant element in Io's atmosphere. Sulfur compounds are also found in hot lava lakes on its surface.

Europa appears to have an icy surface containing many fractures, and few impact craters. The lack of craters indicates that its surface is in transition. This appearance could be explained if Europa is warm inside but cooling. Detailed images from the Galileo spacecraft (taken in December 1997) show evidence of recent freezing. It may be that Jupiter's tidal forces are also keeping Europa warm. If so, there could be a liquid water ocean beneath Europa's icy crust.

Ganymede is the largest moon in the solar system. It is slightly larger than the planet Mercury. Like the Moon, Ganymede's surface has a lot of craters and grooved terrain. Ganymede's density of 1.9 implies that this satellite is about half water and half rock. The Galileo probe detected a magnetic field from Ganymede, indicating that the interior of this moon is heated by tidal friction from Jupiter's gravity.

Callisto is even more heavily cratered with shallow craters that imply a soft surface, probably part ice and part rock. Callisto has a huge impact region called Valhalla, which is surrounded by concentric mountain ranges. Valhalla has few craters, indicating that it formed after most of the intense bombardment of planetary and satellite surfaces had ceased.

The Moons of Saturn

Similar to Earth's Moon, most of these moons keep the same side toward Saturn all the time. Saturn's satellites can be placed into three groups, based on their size. The three groups are:
Titan - the largest and in a class of it's own.
Six large, icy moons - Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus.
Many small moons - Phoebe, Hyperion, Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Telesto, Calypso, and Helene are the ones that have been officially named.

Titan is the second largest moon in the solar system. Its diameter is about 3,200 miles (5120 km) and its density is 1.9, indicating that, similar to Jupiter's Ganymede, it's about half rock and half ice. Titan has a substantial atmosphere, with an atmospheric pressure of about 1.5 atm. The chemical composition of Titan's atmosphere is 99 percent N2, 1 percent CH4, some traces of Argon, and of various hydrocarbons. Scientists find Titan especially interesting because it contains organic compounds, i.e. those containing the element Carbon, which makes Titan a potential candidate for (most likely very simple) forms of life.

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Satellites of Uranus

Before the Voyager spacecraft visited Uranus, we knew of five of its moons: Miranda, Ariel, Umbriel, Titania, and Oberon. These range in size from 320 km (200 mi.) to 1600 km (1000 mi.). They're composed of a mixture of rock and ice, with masses ranging from 0.22 to 0.81 times that of the Earth's Moon.
The Voyager spacecraft revealed a very complex surface on Miranda. Topographic features including grooved regions, earthquake faults, 3-mile-high cliffs, and many craters. Its topography is so heterogeneous that some scientists think it must have been shattered by an impact and then subsequently reformed.

Neptune's Moons

Before the Voyager fly-by, we knew of only two moons revolving around Neptune: Triton and Nereid. Triton is 2,720 km (1,700 mi.) in diameter. It is somewhat unusual in that it revolves around Neptune backward, or retrograde. For this reason, many scientists think it is a captured moon, as a moon would not have formed that way naturally. It is not uncommon for the gravitational pull of planets to capture asteroids. However, Triton is interesting as it is not a mere stray asteroid, but a large, spherical moon. The direction that Triton was moving when it was captured resulted in its retrograde orbital motion. Triton is massive enough (that is, it has enough gravity) to have an atmosphere. It's a very thin, low-pressure atmosphere, though, with a pressure of only 10-15 atm. The atmosphere is made of molecular nitrogen (N2) and methane (CH4).

Voyager revealed six new (previously unknown) moons. They all have rugged, dark surfaces. Voyager also provided much more detailed information about Triton. Its temperature is only 37 K (-393°F or -236° C), and it appears to have frozen ice lakes that look as though there have been repeated episodes of melting and re-freezing. Interestingly, Triton has geysers that spew out liquid nitrogen. Apparently there is some heating in Triton caused by tidal friction from Neptune.

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