Introduction - Introduction to Planetary Geomorphology - page 5

Figure 1.3. One of the moons of Jupiter, Io, is the most
volcanically active object in the Solar System. This image was
taken by the New Horizons spacecraft in 2007 during its flyby of

the Jupiter system on the way to Pluto. The huge umbrella-
shaped plume at the top of the image is pyroclastic material
rising 290 km from the active volcano Tvashtar. Also visible (left
side) is a smaller (60 km high) plume erupted from the volcano,
Prometheus (NASA PIA09248).

System, were driven outward from the Sun, and then
coalesced to form some comets. This led to the reference
to some comets as “ icy dirt balls, ” a concept that was
supported in 2005 when the Deep Impact spacecraft
launched a roughly half-ton metal ball into Tempel 1, a
comet measuring 7.6 km by 4.9 km. The impact explosion
released a plume of icy dust, suggesting the properties of
freshly fallen, fluffy snow with dust. Images taken by
Deep Impact and those taken by the NASA NExT space-
craft in 2011 after the impact show that Tempel 1 ' s surface
has smooth terrains and areas that have been eroded.
Most asteroids are found in the zone between the orbits
of Mars and Jupiter, known as the main asteroid belt.
However, asteroids are also found in orbits of larger
planets and are called Trojan asteroids, while those in
orbits that come close to the Earth are called near-Earth
objects, or NEOs. Asteroids can be further classi ed in
terms of their spectral properties and comparisons with
meteorites, many of which were derived from asteroids.
Historically, asteroids were thought to be either remnants
of a former planet that broke apart or objects that never
accreted to form a planet early in Solar System history. As
with many ideas in planetary science, this was an over-
simpli cation. It is now fairly clear that some asteroids

Figure 1.4. This view of the 5 km in diameter Comet Wild 2 was
taken by the Stardust spacecraft in January 2004 (NASA Stardust
Project).

(and the corresponding meteorites) represent fragments of
a larger body that had been differentiated. Thus, “ metal-
lic ” objects are thought to represent the core of a planet,
while those having signatures of the mineral olivine
would represent a planetary mantle, and “ stony ” objects
would represent the crust. Other asteroids have the signa-
tures of carbon-rich materials and are considered to rep-
resent “ unprocessed, ” or primitive, planetary material. In
this regard, many planetologists suggest that some of
these types of asteroids are actually the rocky material
left over from comets that have lost most or all of their
volatile materials.
Numerous missions have flown past, orbited, and even
landed on asteroids, with one mission returning samples to
Earth. The first images of asteroids up-close were taken in