Lunar Data Supports Idea That Collision
Split
Earth, Moon
Analysis of data from NASA's
Lunar Prospector spacecraft has
confirmed that the Moon
has a small core, supporting the theory
that the bulk of the Moon
was ripped away from the early Earth when
an object the size of Mars
collided with the Earth.
Scientists presented this
result and other findings today in a series
of papers at the 30th Lunar
and Planetary Science Conference in
Houston, TX. Their data
show that the lunar core contains less than
four percent of the Moon's
total mass, with the probable value being
two percent or slightly
less. This is very small when compared with
the Earth, whose iron core
contains approximately 30 percent of the
planet's mass.
"This is a critical finding
in helping scientists determine how the Earth
and Moon formed," said Dr.
Alan Binder of the Lunar Research Institute,
Tucson, AZ, principal investigator
for Lunar Prospector.
Similarities in the mineral
composition of the Earth and the Moon
indicate that they share
a common origin. However, if they had simply
formed form the same cloud
of rocks and dust, the Moon would have a
core similar in proportion
to the Earth's. A third theory suggests that the
moon was captured fully
intact by the Earth's gravity.
Based on information first
gathered during the Apollo era, scientists
suggested that the Moon
was formed when a Mars-sized body hit the
Earth during its earliest
history. "This impact occurred after the Earth's
iron core had formed, ejecting
rocky, iron-poor material from the outer
shell into orbit," Binder
explained. "It was this material that collected to
form the Moon.
"Further analysis of Lunar
Prospector data to refine the exact size
of the lunar core and the
amounts of elements like gold, platinum and
iridium in lunar rocks --
all of which are concentrated with metallic iron
-- is required," Binder
added. "This will do much to pin down for good if
the 'giant impact' model
of the formation of the Moon is correct, or if the
Moon formed in a different
manner."
The current data come from
gravity measurements conducted by Dr. Alex
Konopliv of NASA's Jet Propulsion
Laboratory, Pasadena, CA. His results indicate that the Moon's core radius
is between 140 and 280 miles (220 and 450 kilometers). This result is consistent
with independent magnetic data, evaluated by Dr. Lon Hood of the University
of Arizona, Tucson, which suggest that the core radius is between 180 and
260 miles (300 and 425 km).
In other results from Lunar
Prospector, Dr. Robert Lin of the University of California at Berkeley,
Dr. Mario Acuna of NASA's Goddard Space Flight Center, Greenbelt, MD, and
Hood also found that a broad section of the southern far-side of the Moon
has large localized magnetic fields in its crust. These fields occur opposite
the large Crisium, Serenitatis and Imbrium basins -- three of the "seas"
that cover much of the Moon's near side. This result supports earlier evidence
linking strong magnetized concentrations on one side of the Moon with young,
large impact basins
on the other side.
Results of efforts to map
the composition of the lunar crust have surpassed the expectations of the
spectrometer team, led by Dr. William Feldman of the Department of Energy's
Los Alamos National Laboratory in New Mexico. Data obtained are so good
that the distribution of thorium has been mapped with a resolution of 36
miles (60 kilometers). At this amount of detail, scientists can detect
individual deposits rich in thorium and related elements. Their current
observations suggest that thorium was excavated by impacts of asteroids
and comets, and then distributed around craters, rather than being deposited
by volcanic activity.
Lunar Prospector conducted
its primary mapping mission at an altitude
of 63 miles (100 kilometers)
for almost one year after its arrival in lunar
orbit on Jan. 11, 1998.
In December and January, the spacecraft's altitude was lowered to approximately
15 miles by 23 miles (24 kilometers by 37 kilometers). Analyses of
data from the lower-altitude observations are expected to further improve
scientific understanding of the origin, evolution and physical resources
of the Moon.
The $63 million mission is
managed by NASA's Ames Research Center, Moffett Field, CA, and was
developed under NASA's Discovery Program
of lower-cost, highly focused
small scientific spacecraft.
Further information about
Lunar Prospector, its science data return,
and relevant charts and
graphics can be found on the project website
at: http://lunar.arc.nasa.gov
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