Moons, Rings, and Pluto: Small Worlds Among Giants

The Large Moons

The Galilean Moons of Jupiter: A “Miniature Solar System” Four large moons travel on nearly circular, prograde orbits in Jupiter’s equatorial plane. Moving outward from the planet, they are Io, Europa, Ganymede, and Callisto. Like a miniature solar system, their densities decrease with increasing distance from Jupiter.

A “Miniature Solar System” Io and Europa have large, iron-rich cores surrounded by thick mantles of rocky composition. Europa has a water / ice outer shell between 100 and 200 km thick. Ganymede and Callisto are of more lightweight overall composition (low-density materials like water ice). Ganymede has a small metallic core topped by a rocky mantle and a thick icy outer shell. Callisto is an undifferentiated mixture of rock and ice.

Io: The Most Active Moon Most geologically active object in entire solar system. Similar to Earth’s moon in mass and size. Uncratered surface is collage of oranges, yellows, and blackish browns. Has active volcanoes. Io’s smooth surface is apparently the result of molten matter constantly filling in any dents or cracks. Thin atmosphere made up mainly of sulfur dioxide, produced by volcanic activity and temporarily retained by the moon’s gravity.

Io: The Most Active Moon More than 80 active volcanoes have been identified on Io. Largest volcano, named Loki, is larger than the state of Maryland and emits more energy than all of Earth’s volcanoes combined. Io is too small to have all of this activity - it should have died long ago like our moon. The source of Io’s energy is external - Jupiter’s gravity, combined with that of the other moons. If no other moons were present, it would have suffered the same fate as our moon. Due to the gravity of the other moons, Io is constantly “pulled” back and forth creating tidal stresses that continually flex and squeeze the interior. The large amount of heat generated within Io causes huge jets of gas and molten rock to squirt out of the surface. Likely much of the interior is soft or molten, with only a relatively thin solid crust overlying it.

Europa: Liquid Water Locked in Ice Relatively few craters on its surface. Surface displays vast network of lines crisscrossing bright, clear fields of water ice. Ice may be several km thick, with oceans below up to 100 km deep. Again Jupiter’s gravity and the pull of the other moons is the source of energy, but the effects here are less extreme due to increased distance from the planet.

Europa: Liquid Water Locked in Ice Icy equivalent of lava flows on Earth - water erupted through the surface and flowed for many km before solidifying (also called ice volcanism). Has a weak magnetic field that constantly changes strength and direction (direct evidence for the saltwater ocean, which would conduct electricity and thus establish a magnetic field). May contain more liquid water than exists on Earth. While still a hostile environment, the large presence of water oceans presents the possibility of life forming on Europa.

Ganymede and Callisto: Fraternal Twins Ganymede is largest moon in solar system, also larger than both Mercury and Pluto. Darker regions on Ganymede are original icy surface, heavily cratered. Lighter regions are smoother - liquid water upwelled from the interior, flooding the impact regions before solidifying. Ganymede has a system of grooves and ridges indicative of plate tectonics occurring in the moon’s past - ceased 3 billion years ago as the crust became too thick.

Ganymede and Callisto: Fraternal Twins Callisto is more heavily cratered with fewer fault lines than Ganymede. Callisto’s most obvious feature - series of concentric rings surrounding two large basins. Formed from an impact - upthrust ice partially melted and solidified before ripples subsided. Callisto froze before plate tectonics or other activity could start. From cratering rate, Callisto’s surface is maybe 4 billion years old.

Ganymede and Callisto: Fraternal Twins Ganymede’s differentiation indicates it was largely molten at some point, while Callisto is undifferentiated and apparently never molten. Ganymede has a weak magnetic field, suggesting the presence of a liquid or “slushy” water below its surface. Can’t yet explain all the features of these moons.

Titan: A Moon with an Atmosphere Largest moon of Saturn. Orange color comes from presence of an atmosphere. A thick, uniform haze layer completely covers the moon. Surface has icy plateaus smeared with hydrocarbon tar.

Titan: A Moon with an Atmosphere Ridges and cracks on surface suggest geological activity in the form of “titanquakes” may be common. Evidence for some sort of erosion occurring (possibly wind or volcanic activity). Fewer craters than expected also suggests geological activity (resurfacing is occurring). Atmosphere is thicker and denser than Earth’s and more substantial than any other moon. Atmospheric composition: –90% Nitrogen –Up to 10% Argon –Few % Methane –Trace amounts of other gases. Surface temperature is a frigid 94 K. Water ice plays the role of rock on Earth, and liquid water the role of lava (as on Ganymede and Callisto). At the temperatures typical of the lower atmosphere, methane and ethane behave rather like water on Earth - possibly methane rain, snow, fog, and even rivers and oceans.

Titan: A Moon with an Atmosphere Atmosphere acts like a gigantic chemical factory. Scientists are interested in it as the same reactions may have preceded the existence of life on Earth. These reactions would have been necessary for Earth to have been hospitable for life. Huygens probe in 2005 entered Titan’s atmosphere - images show a network of drainage channels leading to a shoreline. It landed on solid ground and transmitted images for an hour. Titan’s low temperature allows it to retain an atmosphere easier than other moons like Ganymede and Callisto. Don’t yet know if Titan’s differentiated or not.

Triton: Captured from the Kuiper Belt? Neptune’s large moon, about half the mass of Europa. Icy surface that reflects much of the sunlight reaching it - temperature of only 37 K. Thin nitrogen atmosphere and solid frozen surface probably composed of water ice. Nitrogen frost forms and evaporates seasonally over the polar caps. Surface activity has erased most of the impact craters. Large fissures on the surface (like Ganymede). Numerous frozen lakes of water ice, thought to be volcanic in origin.

Triton: Captured from the Kuiper Belt? Voyager 2 detected great jets of nitrogen gas erupting several km into the sky. Scientists speculate such nitrogen geysers on Titan are common and perhaps responsible for the thin atmosphere. Triton is the only large moon in the solar system to orbit its planet in retrograde. Only large jovian moon to not orbit in the parent planet’s equatorial plane (i.e. orbit is tilted compared to others). Due to odd orbital behavior, it is thought that Triton was captured by Neptune rather than forming along with Neptune. Triton is spiraling in towards Neptune due to retrograde orbit. (Our moon is slowly spiraling away from Earth.) Triton is doomed to be torn apart by Neptune’s gravitational field, probably within 100 million years due to its inward motion.

The Medium Moons

The Medium-Sized Jovian Moons Radii between 200 and 800 km. Densities suggest that all are composed mostly of rock and water ice. All move on nearly circular orbits and are tidally locked by their parent planet’s gravity into synchronous rotation.

The Medium-Sized Jovian Moons Most show no signs of significant geological activity, with a few exceptions. Saturn’s Dione has bright ice cliffs created by tectonic fractures - surface cracked and buckled as moon cooled. Ice volcanism appears to have erased older craters. Saturn’s Iapetus has a two-faced appearance - one hemisphere is very dark and the other very light. Don’t know if dark material is coming from inside or outside the moon. Saturn’s Tethys and Uranus’s Ariel both have extensive cracks on their surfaces, which are most likely the result of meteoritic bombardment. Saturn’s Enceladus has ongoing geological activity - possibly coated with ice crystals, the result of water volcanoes. Much of surface is devoid of craters, which would have been erased by “lava flows” of water liquefied during internal upheavals and now again frozen. Don’t know why such a small moon has so much activity.

The Medium-Sized Jovian Moons Most show heavy cratering. Saturn’s Mimas and Uranus’s Miranda show clear evidence of violent meteoritic impacts. The impact that caused the large crater on Mimas must have come very close to shattering the moon. Some speculate Miranda’s varied terrains are the result of multiple major impacts, with the pieces falling back together in a chaotic, jumbled way.