Download Big Moons in the Outer Solar System

The Big Moons of the Outer Planets
Six large moons exist in the outer solar system, which are as large or larger than Earth's
Moon. Each displays unusual properties; no two are alike.
Jupiter  Io, Europa, Ganymede & Callisto
Saturn
 Titan
Neptune 
Triton
Their properties are defined by their distance from the Sun and by their respective
distances from their parent planet.
Jupiter:

Discovered by Galileo, the four large moons have approximately circular orbits in
Jupiter's equatorial plane. The average densities of these moons decrease with
increasing distance from Jupiter.

Io is the innermost large satellite of Jupiter, and is one of the oddest worlds in the solar
system
 Orbits Jupiter in 1.8 days
 Io has the appearance of a mildly rotten orange; the surface is a riot of red, yellow
and orange-brown features.
 Most geologically active object within the solar system.
 The surface is completely reworked by molten material ejected from many active
volcanoes found all over the moon. Io has the youngest surface in the solar
system.
 Due to its lack of an atmosphere, and low gravity, volcanic material spews tens of
kilometers above the surface, and rains back down over a wide area.
 The active geology of Io is driven by tidal stresses caused by Jupiter and Europa
as Io swings in its orbit around Jupiter. These stresses heat Io, and have probably
caused much of the interior to be molten.

Europa
 Displays an extremely smooth surface, with few craters, and is covered with
multitudes of crisscross “cracks”.
 The relative lack of impact craters suggests that the surface is young, and is
renewing itself.
 Europa is highly reflective, indicating that the surface is composed largely of water
ice. Darker material is visible across the equatorial regions. Europa has the
appearance of a dirty cueball.
 The Galileo probe has found evidence that Europa may conceal a subsurface H2O
ocean.
 Surface features can be explained by tidal stresses from Jupiter causing motion of
ice blocks, and the upwelling of fresh water. Europa undergoes ice plate
tectonics, in which blocks of ice float on a water “mantle”.
 The possibility of life in the European ocean is considered good, especially of
hydrothermal vents exist at the bottom of the ocean.

Ganymede: the largest moon in the solar system, larger than the planets Mercury and
Pluto.
 Has a substantially lower mean density (1.9 g/cm3) compared to Io (3.4) or Europa
(3.1). This lower density implies Ganymede contains a greater concentration of icy
material.
 Composed of a mixture of rock and ice with some settling of heavy material to the
center. The surface is composed of water ice, like Europa
 The surface is heavily cratered with light & dark regions, vaguely resembling lunar
features. There are more hills and valleys than seen on Io or Europa.
 The lighter, more reflective layers show long grooves, possibly stress fractures, and
appear to be younger than the darker regions.
 Shows evidence for early geological activity, but is now a dead world.

Callisto
 The surface appears dark, pockmarked with numerous bright craters.
 Similar to Ganymede but with almost 10 times the number of craters visible, and
less surface relief compared to Ganymede.
 Impact cratering churns up “fresh” ice, giving those craters a shinier appearance.
 Crater counts suggest an old surface, with little geological activity.
 Callisto appears to be a largely undifferentiated ball of rock and ice, essentially a
cratered iceball.
Saturn:
Titan: Second largest moon in solar system, larger than the planets Mercury and Pluto.
 Only moon with a substantial atmosphere.
 Atmospheric pressure at the surface is 1.6 times Earth's; the atmosphere extends
to 200 km.
 Composed of 90% nitrogen, 10% argon, traces of methane, carbon dioxide and
ethane.
 The surface is not visible from above; obscured by a photochemical haze (smog)
 Cold ! 94 K at the surface.
 Titan may possess oceans of liquid hydrocarbons, with continents of organic
sludge.
 Conditions possibly like the early Earth, except for temperature.
 The Huygens probe landed on Titan in January 2005, producing the first image
from the surface of a moon other than Earth’s.
Neptune:
Triton
 Smallest of the “big” moons, 10% smaller than Europa.
 Orbits in a retrograde direction (i.e., “backwards”) unlike every other large moon,
and its orbit is highly inclined to Neptune’s equator.
 No reasonable explanation for Triton’s orbital behavior exists; this moon was
almost certainly captured by Neptune, but how ?
 Really cold! – about 40 K on the surface.
 Extremely thin atmosphere discovered by Voyager, composed of nitrogen.
 The surface appears to be composed of water ice, with nitrogen frost.
 Voyager II detected nitrogen geysers, where jets of gas erupt onto the surface
 Impact craters as well as many ridges are visible on the surface, suggesting some
geologic activity. The sparseness of impact craters in many regions indicates that
some process(es) have obliterated older craters.
Summary:

The properties of the large moons depend primarily on their distance from the Sun,
which determined the temperature at which the moons formed.

Colder objects retain more of their light materials – ices at the cold temperatures in the
outer solar system. Therefore these moons are mixtures of ice and rock.

The Galilean moons of Jupiter form a mini - solar system in which the closer moons
are more rocky, with high density and the outer moons are more icy, with a low
density. The evolution of these moons was affected by their proximity to massive
Jupiter.

Geological activity decreases with distance from Jupiter, due to the decreasing tidal
friction from Jupiter affecting the interior of these moons. Their surfaces reflect this
range of geological activity. Io is the most active large object in the solar system, while
Callisto shows an ancient, unmodified surface.

Tidal forces arising from Jupiter have subsequently altered the evolution of Io and
Europa, adding internal heat to these moons.