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(meteorobs) [ASTRO] NEAR-Shoemaker Science Update - March 22, 2000
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From: Ron Baalke <BAALKE@KELVIN.JPL.NASAdot gov>
Date: Thu, 23 Mar 2000 0:43:38 GMT
Subject: [ASTRO] NEAR-Shoemaker Science Update - March 22, 2000
NEAR-Shoemaker Science Update
March 22, 2000
http://near.jhuapldot edu/news/sci_updates/00mar22.html
Last week the NEAR spacecraft was renamed "NEAR
Shoemaker," in honor of the late Eugene M. Shoemaker, a
pioneer in the study of asteroid and comet impacts on
Earth and other planets. It is largely because of Gene's
work that we know that the famous Meteor Crater in
Arizona (also called Barringer Crater) was indeed
created by the impact of a 15-meter iron-rich meteoroid
and not by a volcano. While many scientists suspected an
impact origin, including Barringer himself, it was
Shoemaker and his colleague Chao's discovery of the high
pressure silicate mineral coesite that finally convinced
skeptics.
Gene was also heavily involved in the Apollo missions
that resolved one of the leading scientific
controversies of our time, whether the craters on the
Moon were formed by volcanoes or by impacts. We now know
that the surface of the Moon is actually saturated with
impact craters and that even the largest features we
see, those that make up the "man in the moon", lie
within the scars of giant impacts. These features are
dark because, later in the moon's history, the giant
impact scars were filled with volcanic lavas. The Moon's
surface records a period of violent bombardment in the
early history of the solar system, a bombardment which
the Earth itself did not escape. Of course, on Earth the
record of ancient giant impacts has been mostly erased
by the actions of weather and plate tectonics. Gene was
among the first to recognize the importance of large
impacts for the geologic history of the Earth and for
the evolution of life on Earth.
Since the projectiles that bombarded the Moon, the
Earth, and other terrestrial planets were asteroids and
comets, Gene initiated telescopic observing programs to
search for such objects in orbits close to Earth's orbit
(that is, near-Earth asteroids and comets) as well as in
more distant orbits. He and David Levy discovered the
comet Shoemaker-Levy 9 that plunged into Jupiter in
1994, after splitting into more than 20 fragments,
temporarily creating dark spots in Jupiter's clouds
larger than the Earth.
NEAR Shoemaker is now orbiting one of the largest of the
near-Earth asteroids, 433 Eros. Gene always thought of
the near-Earth asteroids as "roadcuts in the heavens",
which would have fascinating and important stories to
tell about the formation of the planets. We all have the
experience of driving through a roadcut on the highway
and looking at exposed layers of sediment or rock on
either side, which would reveal something of the
geologic history of that particular site. How
appropriate it is that we now see fantastic systems of
linear features - ridges, grooves, and chains of craters
or pits - all around the surface of Eros. Were the
linear features formed by ancient geologic activity on
the parent body of Eros, making Eros possibly a
'roadcut' through the parent body from which it was
derived? Or were the linear features formed by later
processes? Our task is now to find the evidence that
would indicate which of these possibilities may be
correct.
One important line of evidence will come from NEAR's
x-ray and gamma-ray spectrometers, which will measure
the elemental compositions of surface materials. Last
week, NEAR recorded the first detections of x-ray
emissions from an asteroid. NEAR's x-ray spectrometer
was able to identify emission from the elements silicon,
aluminum, magnesium, calcium and iron during a large
solar flare on March 2, 2000. The flare bombarded the
asteroid's surface with an unusually high intensity of
x-rays, enabling NEAR's x-ray spectrometer to detect the
asteroid emissions at a range of more than 200
kilometers from the surface. From that one observation,
we were not able to determine quantitative abundances of
these elements, which is the information that might tell
us whether Eros is from a differentiated parent body
(one large enough to have melted in the interior and
separated into heavier and lighter constituents) or from
a more primitive parent body. We'll have to be patient
and await more observations, especially from the lower
orbits where the x-ray and gamma ray spectrometers were
designed to operate. Gene would have had difficulty with
the notion of being patient at Eros - he wanted to go
there and bang on it with a hammer - but he would have
understood.
Andrew Cheng
NEAR Project Scientist
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