*2.2.1.7 Moon*

*Solar System Planets and Exoplanets*

tion of sunlight on surface water molecules.

surface with few observable craters [7].

of iron with a diameter exceeding 900 km.

surface temperature is about 93 K [7].

knowledge of Triton and its characteristics.

carbon precipitation [7].

resemble smog [11].

*2.2.1.6 Triton*

Europa, Io has minimal water.

*2.2.1.5 Titan*

porting evidence.

*2.2.1.4 Io*

existence of this ocean has caused considerable speculation regarding the presence of possible life forms. At the present time, this is merely speculation without sup-

The surface of Europa is smooth that suggests a relative young origin given the dearth of craters. Its surface is also uniquely colored with interesting patterns.

The atmosphere of Europa is limited and contains oxygen. In a manner previously addressed in the Ganymede discussion, oxygen is liberated from the interac-

Io is about the same size as the Moon, is the innermost of Jupiter's satellites, and has one of the largest densities of any Solar System moon [15]. It has some of the most active volcanoes in the Solar System. These volcanic eruptions are likely attributed to gravitational tidal stresses caused by Jupiter's mass. Io's orbit and its relationship to Europa, Callisto, and Ganymede also contribute to the tidal stresses [7]. Voyager 1 observed a number of Io's volcanic eruptions. Io's volcanoes vigorously eject sulfur compounds into the atmosphere. A portion of this material produces a ring around Jupiter. The degree of volcanic activity creates a relatively smooth

Io has a number of unique surface features that are enhanced by the volcanism. It has liquid sulfur lakes, mountains, and sulfur lava flows. Io's unique and varied coloration is attributed to the various sulfur compounds. In contract to Callisto and

The Galileo probe provided data regarding Io's core. The core is likely composed

Saturn's moon Titan with a diameter of about 5200 km is the second largest moon in the Solar System [15]. Titan is larger than Earth's moon. It has a heavy atmosphere with thick clouds. The atmosphere is composed primarily of nitrogen with some methane and trace gases with a pressure of about 1.5 atm [7, 15]. The

The Cassini spacecraft and its Huygens lander were able to penetrate Titan's thick clouds. Additional data were provided by the Hubble Space Telescope. The surface appears to be relatively flat, and impact craters could be filled with hydro-

NASA's Cassini probe observed indications of erosion on Titan caused by a flowing liquid. It is likely that the liquid is methane and there are liquid methane lakes near Titan's poles [11]. The European probe Huygens was released from Cassini, traversed its dense atmosphere, and landed on the surface of Titan. Huygens data suggests that Titan has an orange color, and its atmosphere contains clouds that

Neptune's largest moon Triton has a diameter of about 2700 km and it orbits the planet in a retrograde manner [15]. It is possible that Triton's orbital characteristics are a consequence of its gravitational capture by Neptune [7]. Voyager 2 extended

Neither of these unique features has been satisfactorily explained [7].

**12**

The Moon is Earth's only satellite. It has a diameter about 25% of Earth's, a mass about 0.01 times the mass of Earth, and its surface gravity is about one sixth g (~1.6 m/s2 ). The Moon's orbit is tidally locked, and the same side faces the Earth [7].

The second densest planetary satellite in the Solar System is Earth's moon [7]. It has a thin crust, thick mantle, and an iron core that is estimated to be less than 400 km in diameter. The Moon's surface is inactive with no apparent volcanic or tectonic activity [7]. In addition, there is a minimal magnetic field approximately 100 times weaker than Earth's [7].

The Moon's surface contains numerous craters, dark areas that appear to be flat (*mare*), and basins that were created by impacts that subsequently filled with lava that has solidified [7]. The bright regions are mostly mountains and other elevated areas. The Moon's far side has minimal *mare* areas [7]. Its surface has a thin layer of fine particles of powered surface rock (regolith). NASA's Apollo missions provided considerable Lunar data including the return of sample materials to Earth.

The Moon's has essentially no atmosphere. Some material (e.g., radon gas) is released from the surface through radioactive decay. Other material is generated through micrometeorite impact and sunlight interactions with surface materials. However, much of the atmospheric constituents are swept from the Moon by the action of the Solar wind [7].
