Geology Reference
In-Depth Information
Table 1.2. Basic data for selected satellites
Satellite
name
Period
(days)
Diameter
(km)
Mass
(10
20
kg)
Density
(g/cm
3
)
Planet
Discovery
Surface material
Earth
Moon
-
27.32
3,476
735
3.3
Silicates
1 × 10
−
4
Mars
Phobos
Hall (1877)
0.32
27
2.2
Carbonaceous
2 × 10
−
5
Deimos
Hall (1877)
1.26
13
1.7
Carbonaceous
Jupiter
Io
Galileo (1610)
1.77
3,660
893
3.6
Sulfur, SO
2
Europa
Galileo (1610)
3.55
3,130
480
3.0
Ice
Ganymede Galileo (1610)
7.15
5,268
1,482
1.9
Dirty ice
Callisto
Galileo (1610)
16.69
4,806
1,076
1.8
Dirty ice
Saturn
Mimas
Herschel (1789)
0.94
396
0.376
1.2
Ice
Enceladus Herschel (1789)
1.37
504
0.74
1.10
Pure ice
Tethys
Cassini (1684)
1.89
1,048
6.27
1.0
Ice
Dione
Cassini (1684)
2.74
1,120
11
1.4
Ice
Rhea
Cassini (1672)
4.52
1,528
23
1.3
Ice
Titan
Huygens (1655)
15.95
5,150
1,346
1.9
Methane ice
8 × 10
−
3
?
Hyperion
Bond, Lassell (1848) 21.3
360
?
Dirty ice
Iapetus
Cassini (1671)
79.3
1,436
16
1.1
Ice/carbonaceous
550 (R
a
)
Phoebe
Pickering (1898)
220
0.004
?
Carbonaceous?
Uranus
Miranda
Kuiper (1948)
1.41
474
0.7
1.3
Dirty ice
Ariel
Lassell (1851)
2.52
1,159
14
1.6
Dirty ice
Umbriel
Lassell (1851)
4.14
1,170
12
1.4
Dirty ice
Titania
Herschel (1787)
8.71
1,578
35
1.6
Dirty ice
Oberon
Herschel (1787)
13.5
1,522
30
1.5
Dirty ice
5.88 (R
a
)
Neptune
Triton
Lassell (1846)
2,704
214
2.0
Methane ice
Pluto
Charon
Christy (1978)
6.39
1,186
16.2
?
Ice
a
R = retrograde.
these moons represent a myriad of objects of different
sizes, compositions, and geologic histories. They are clas-
si
ed as regular satellites (orbiting in the same direction
as the parent planet
'
s spin direction) or irregular satel-
lites (orbiting in the opposite direction) that are probably
captured objects. Jupiter
largest asteroids are hundreds of kilometers in diameter.
Comets consist of primordial material left over from the
early stages of Solar System formation. Most comets are
found in the Oort cloud and the Kuiper belt, both beyond
the orbit of Pluto. The Oort cloud forms a spherical zone
some 3 × 10
12
km from the Sun and is the apparent source
of long-period comets (those that take more than 200
years to complete an orbit around the Sun), while the
Kuiper belt is a disk-shaped region extending from
Neptune
'
is orbit to ~8 × 10
9
km from the Sun and is the
source for short-period comets (those that orbit the Sun in
less than 200 years). Just to make things a little more
complex, objects that are in orbit in this belt are referred
to as Kuiper belt objects, or KBOs. Some of the outer
planet satellites, such as Neptune
'
s Triton, could have
been captured from the Kuiper belt.
Often described as
s moons Ganymede and Callisto
are about the size of the planet Mercury. At least three
moons, Jupiter
'
s satellite Io, Saturn
'
s Enceladus, and
Neptune
'
s Triton, are currently volcanically active
-
in
fact, Io is the most geologically active object in the Solar
System (
Fig. 1.3
). Other outer planet satellites appear to
have remained relatively unaltered since their initial for-
mation. Many of the geologic processes that operate on
terrestrial planets are also seen on outer planet satellites;
however, because of their different compositions (mostly
ices, plus some silicates) and extremely cold environ-
ments, the outer planet satellites also display features
representing processes unique to the outer Solar System.
'
comets are com-
posed of dust grains and carbonaceous (carbon-rich)
materials embedded in a matrix of water-ice (
Fig. 1.4
).
Study of cometary material collected from Comet Wild 2
by NASA
“
dirty snowballs,
”
1.1.3 Small bodies, Pluto, and
“
dwarf planets
”
s Stardust mission (Brownlee et al.,
2006
) and
returned to Earth suggests that at least some comets are
composed of grains that were heated in the inner Solar
'
Asteroids, comets, and the smaller moons of the outer
planets are often called small bodies, even though the
