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OCR for page 14
he early solar system was a chaot-
ic and violent place. Debris from
the formation of the solar system
rained down onto the surfaces of the
forming planets much more frequent-
ly than it does in the solar system
today. Indeed, an early, key advance
in the understanding of shaping of
planetary environments was the real-
ization that these impacts are not
merely catastrophic accidents but
instead constitute a fundamental
process in planetary formation and
evolution. For example, a variety of
data strongly supports the idea that a
Mars-size object struck the primordial
Earth, resulting in the formation of
the Moon and setting Earth on a dis-
tinctive evolutionary path.
of planetary atmospheres was also
affected by impacts. Impacting objects
are believed to have been the source of
much of the gases that made up Earth's
early atmosphere as well as most of the
terrestrial water. Learning when these
impacts occurred will offer tremendous
insight into planetary evolution.
Impact research also has implications
for the origin of life because living
organisms could not gain a toehold on
Earth, and perhaps elsewhere in the
solar system, until the era of planet-
wide, sterilizing impacts was at an end.
By studying the patterns of visible
craters on such bodies as the Moon
and Mercury, researchers have devel-
oped some theories on the timing of
the impacts. There is considerable
The region around the Moon's South Pole as seen by the Clementine orbiter.
The additional heat resulting from uncertainty in this system of interpreta-
impacts likely caused partial, or even tion, however, and two differing mod-
global, melting of Earth and other ter- els have been proposed, though both
restrial planets, leading to the formation have considerable uncertainty. In one,
of magma oceans and the differentia- the rate of impacts has decreased expo-
tion of their interiors. The development nentially since the beginning of the
Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
Quebec's 70-km-diameter Manicouagan
impact structure is one of Earth's largest.
solar system. In the other, the number
of impacts peaked around 4 billion
years ago, before dropping off to their
present level. To test these theories (or
develop totally new ones) requires
knowing the actual ages of the craters,
which can be determined only by dat-
ing the surface materials an objective
whose accomplishment depends in
turn on obtaining samples from known
locations on a number of bodies.
At this time, only lunar samples
have been returned to Earth, by the
Apollo and Luna missions. As a result,
the Moon is the only planetary body
whose surface age is known with any
confidence, and thus all attempts to
date other bodies stem from a compari-
son with the Moon. While incredibly
valuable, the data set from the Apollo
missions is very small. Gathering a
wider variety of samples from the lunar
surface is an important step in more
accurately determining ages for objects
throughout the solar system and there-
by better understanding the evolution
of our celestial neighborhood.
After the initial formation of plane-
tary bodies and the conclusion of the
period of heavy bombardment, the
internal structure of the planets shaped
their history. Key issues in under-
standing a terrestrial body's evolution
include the dissipation of internal
heat, core formation and the associat-
ed magnetic field, distribution of heat-
producing radioactive elements, and
styles and extent of volcanism. For
example, Earth's crust is the product of
differentiation and a few billion years
of recycling through the movement of
continental plates. Based on analysis
OCR for page 15
-
S~ ~ ~ ~ ~ ~ 1
outn Jo e-lIt (en asin amp e ~elurn 11 1.
i 3 3 3 3 i i
Profile
South Po~e-Ailken Basin
Sample Return
Mission Type: Sample Return
Cost Class: Medium
Priority Measurements:
Measure elemental and mineralogical
surface compositions.
Determine interior (mantle)
compositions.
Study compositional variations and
the evolution of crusts and mantles.
Quantify the large-impactor flux in
the early solar system and calibrate
the lunar impact record.
Investigate how major impacts early
in a planet's history can alter its
evolution and orbital dynamics.
of the Apollo lunar samples, scientists
believe that the Moon began hot, with
an ocean of magma some 400 kilome-
ters deep, and that its crust rose to the
surface as the low-density component
during solidification of the magma
ocean. Knowledge of the internal
structure of the Moon is constrained
by the small set of Apollo samples,
limited geophysical measurements on
the surface, and observations from
orbit. Remote sensing data show that
the Moon has a strong hemispheric
asymmetry the side facing Earth and
the farside differ significantly.
Although its cause is not known, the
asymmetry likely influenced the
amount and location of subsequent
volcanic activity on the Moon.
The goal of the South Pole-Aitken
Basin Sample Return mission is to
understand the nature of the Moon's
internal structure and tie down the
history of early impacts by returning
samples from the Moon's South Pole-
Aitken Basin. The largest known basin
in the solar system and the oldest and
deepest impact structure preserved on
the Moon, this giant excavation pene-
trates the lunar crust and, unlike any
other location in the solar system,
allows access to materials from the
upper mantle. Data from the South
Artist's impression of the South Pole-Aithen Basin Sample Return. The largest lunar
impact structure, this basin occupies the lower two-thirds of the image of the Moon's
southern polar region shown on page 14.
Pole-Aitken Basin may have a substan- Another benefit of the South Pole-
tial effect on our understanding of the Aitken Basin Sample Return mission is
evolution of planetary interiors.
Absolute dating of returned samples,
which will include both soil and
diverse rock chips, could also change
our understanding of the timing and
that it gives scientists and engineers
the opportunity to try out sample-
return techniques and strategies on a
relatively easy target before moving
on to more challenging and expensive
intensity of the bombardment suffered sample-return missions in the rest of
by both early Earth and the Moon. the solar system.
Guiding Themes Addressed Important Planetary Science Questions Addressed
Representative terms from entire chapter:
basin sample
