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OCR for page 20
n its earliest youth the solar system
was inhabited by a huge swarm of
small rocky and icy bodies called
planetesimals, orbiting the Sun in a
giant disk. And, although the full
details are still being debated (see
the section, Formation of the Giant
Planets), it is clear that collisions
between planetesimals led to the for-
mation of increasingly larger objects,
with the end result being the planets
we know today. Many of the diverse
Above: Cometary material bombarding
early Earth, as illustrated in this artist's
concept, may have carried organic mate-
rials from which life arose. Right: A
wide-angle view of the twin tails of
Comet Hale-Bopp on the night of
March 10, 1997.
small bodies now seen in the solar sys-
tem are directly related to the primor-
dial population of planetesimals.
Beyond the orbit of Pluto, for
example, many small icy objects
remain in their archaic forms. For the
most part these so-called Kuiper Belt
objects and their relatives remain in
their orbits far from the Sun. But
every so often, one gets flung into a
new path taking it into the inner solar
system we call these objects comets.
As comets travel through the solar
system, their surfaces undergo a num-
ber of changes. The most obvious
example is the development of the
comet's characteristic tail as the Sun
warms the surface ices and causes
them to turn from a solid into a
vapor. Other physical and chemical
changes occur owing to the effects of
fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
radiation, micrometeorite impacts,
and other processes. By studying
these changes in a comet's surface
material, scientists glean a significant
amount of information about the his-
tory of the Sun and the solar system,
much as reading the rings of a tree
can teach us about the history of cli-
mate changes on Earth.
One change of particular interest to
researchers results from the discovery
that complex carbon compounds are
created when a comet's surface ices
(water, ammonia, carbon dioxide,
methane, and so on) are irradiated by
ultraviolet light, high-energy particles
from the Sun, or cosmic rays. Labora-
tory simulations indicate that, when
exposed to liquid water, such radiation-
processed material produces amino
acids and other organic molecules
found in living systems. Radiation on
the surfaces of planets and their satel-
lites can also create and destroy com-
plex organic molecules, but the details
of the required conditions and the bal-
ance between destruction and creation
are not known. Studying the differ-
ences between cometary and planetary
organic molecules can help scientists
understand this balance by showing
which molecules are likely to have
come from comets and which have
been formed by other processes. This
understanding will provide important
insights into the origin and evolution
of life on Earth.
The Comet Surface Sample Return
mission emphasizes the return to
Earth of a sample of cometary surface
material that will provide the first
direct set of data on cometary
OCR for page 21
Come! Surface Sample Return
Profile
Comet Surface Sample Return
Mission Type: Sample Return
Cost Class: Medium
Priority Measurements:
· Characterize the target comet.
· Select, document, and return mate-
rial collected at one or more sites,
preferably in or near an active vent.
processes and answer whether the
water in comets is very close to the
surface. Such a sample will also show
if the organic materials we believe to
exist there actually do. If they do,
then the sample will also give us our
first look at ancient organic mole-
cules similar to those first brought to
our planet by cometary impacts 4 bil-
lion years ago. Knowing the nature
of these molecules should provide
exciting new insights into the origin
of life on Earth.
The Comet Surface Sample Return
(CSSR) mission would address other
scientific interests as well. It would
provide the first direct data on the dif-
ference between a comet's nucleus and
the relatively well studied material in
the cometary tail. In addition, data
from this mission would enable scien-
tists to resolve questions about the
wealth of large-mass molecules and
fragments seen by spacecraft in the
cloud of gas and dust around Halley's
comet during its last visit to the inner
solar system in 1986.
Finally, the CSSR mission concept
would give scientists their first look
at how a comet is actually put
together. Is it really a dirty snowball,
as conceived in popular and scientif-
ic imagination? Is a comet a homo-
genous mixture of dust and ices, or
are there pockets of differing materi-
als scattered throughout its body? If
the latter is true, what holds these
different pieces together? Studying
the structure of the material returned
by CSSR should answer many of
these questions.
Artist's impression of the lander portion of the Comet Surface Sample Return space-
craft just after it has deployed its twin solar arrays.
Guiding Themes Addressed Important Planetary Science Questions Addressed
¢ ~ ~ ~ ~
. . ~
Representative terms from entire chapter:
surface sample
