From: LARRY KLAES (ljk4@msn.com)
Date: Tue May 14 2002 - 12:11:43 PDT
Moon's Dust Hides a Throbbing Heart
May 14, 2002
By WILLIAM J. BROAD
It takes two and a half seconds for a laser beam to flash
from Earth to the Moon and back again. But it has taken 33
years of doing such experiments for scientists to glimpse
what may be the Moon's greatest secret: that far beneath
its cold craters and rocky landscape lies a heart that is
warm and yielding. The discovery, if confirmed, could boost
a theory that the Moon was born in the aftermath of a
violent collision between early Earth and a speeding cosmic
wanderer.
The suspicion that a sizable zone of molten rock lies
hidden beneath the craggy lunar surface comes from laser
ranging experiments that have increased steadily in
accuracy since Apollo astronauts first put laser reflectors
on the Moon in 1969. For decades, scientists have been
analyzing the round-trip travel time of the flashes to make
increasingly exact measures of the Moon's shape, wobbles,
distance from Earth, and physical characteristics.
At first, the measurements were good to a precision of
about 10 inches, then 5 inches, then 1. Today, they track
changes of a little less than an inch, and the readings may
improve still more if plans are approved for a more
sensitive laser observatory.
Scientists at the Jet Propulsion Laboratory in Pasadena,
Calif., who have long analyzed readings from the laser
reflectors, recently concluded that the Moon's surface
moves in and out as much as four inches in response to
shifting gravitational tugs of Earth. This elasticity, the
scientists say, suggests that its interior is pliable and
partly melted.
"Finding out what's inside the Moon isn't simple," said Dr.
James G. Williams, who directs the research. "We have to
use indirect methods. In this case, we were able to use
tidal distortion."If the Moon was made of solid rock, he
added, the bulging would be less pronounced and detectable.
Science is seldom about poetry or romantic imagery. But in
this case, it comes close. The technical term for a
planetary tidal measurement is a Love number, after the
Oxford mathematician Augustus E. H. Love, who pioneered
theories of elasticity more than a century ago.
When they wrote a news release about the findings, public
affairs officials at the Jet Propulsion Laboratory could
not resist the metaphor: "Moon's Heart Melted, Say Lunar
Love Numbers."
The scientists who did the research put the finding more
prosaically - and more tentatively. The laser beams, they
say, appear to suggest that much of the Moon's interior is
molten slush. But they are quick to emphasize that no one
knows for sure.
"The uncertainties in the determinations are sizable," Dr.
Williams said in an interview, adding that his team's
finding of a melted heart still had a good chance of being
confirmed in the years ahead.
Remarkably, given the long service of the system of laser
reflectors and observatories, it promises to shed more
light on this topic and others if it continues to improve
in precision. For instance, scientists hope that greater
accuracy will help explain the mysterious outward rush of
the universe.
"It's exciting," Dr. Thomas W. Murphy Jr., a physicist at
the University of Washington, said of planning for a new
laser observatory to make such measurements. "The same
force responsible for the acceleration of the universe may
show up in the Earth-Moon system."
The laser reflectors are the last vestiges of the Apollo
missions that put a dozen Americans on the Moon from 1969
to 1972. Unlike other scientific experiments left there,
the reflectors require no power and are still working
decades after their deployment.
Reflectors were set up during Apollo 11 in July 1969,
Apollo 14 in February 1971 and Apollo 15 in July 1971. On
unpiloted lunar craft, the Soviets set up French-built
units in November 1970 and January 1973, though testing
showed that the first one apparently failed to reach proper
alignment.
Each reflector is a glassy checkerboard of silica cubes a
little more than an inch wide. The cubes were cut and
polished to reflect a beam of light directly back toward
its point of origin - the same way the tiny elements of
reflectors do on cars, trucks and bicycles.
Laser beams are the only way to bounce light off the Moon
because only they are concentrated enough to remain tightly
focused during the round trip of roughly half a million
miles. Even so, the beam spreads slowly so that it is more
than 4 miles wide at the Moon and 12 miles wide upon
return. Because returning signals are so weak, detectors
must be extremely sensitive, even if relatively small.
A large telescope sends the burst of light and receives it,
too. In the transmitting phase, the telescope takes the
concentrated laser beam, focuses it and fires it toward a
particular reflector on the Moon. Then, anywhere from 2.2
seconds to 2.7 seconds later, depending on whether the Moon
is high overhead or close to the horizon, it tries to
gather up the returning signal.
Since Apollo days, the main American site for lunar
laser-ranging experiments has been the McDonald Observatory
of the University of Texas. Located in the Davis Mountains
450 miles west of Austin, the observatory today uses a
reflecting telescope whose main mirror is 30 inches wide
and a laser that sends out short bursts of up to one
billion watts of power.
In theory, the flash could blind humans.
"We have a
radar," said Dr. Jerry R. Wiant, an engineer who has done
lunar laser ranging at the McDonald Observatory for 32
years. "If it finds a plane, it automatically shuts the
system down."
He added that, weather permitting, the observatory fired
laser beams at the Moon up to 20 days a month. "When it's a
full moon it's too bright," he said, adding that the Moon's
own light in that case would overwhelm the detectors.
Dr. Wiant said the Texas system had improved over the
decades as its mix of lasers, optics, refrigerators,
detectors, computers, filters and timing devices had slowly
gained in precision. He added that he had recently helped
Dr. Murphy of the University of Washington to refine his
plans for a new laser observatory.
"He hopes to beat us," Dr. Wiant said, adding that he was
working hard to advance that goal.
It is the McDonald Observatory that has played the dominant
role in uncovering the Moon's inner secrets, its ranging
work complemented by that of an observatory in France.
Ever since Apollo days, scientists have suspected that
Earth's closest neighbor may harbor a core or zone of
melted rock. The main initial evidence came not from lasers
but seismometers, which detected weak moonquakes
originating at depths of 500 to 600 miles, about halfway to
the Moon's center.
But the seismometers, deployed by the astronauts,
eventually ran out of power and the mystery lingered.
Moreover, if a core did in fact exist, it was unclear if it
was solid or molten.
Scientists were intrigued by the topic because the question
bore strongly on the question of lunar origins. A molten
metallic core was seen as bolstering the idea that a cosmic
wanderer and the early Earth collided to produce a ring of
debris that coalesced to form the Moon. An alternative
theory had a fast-rotating early Earth spinning off the
Moon.
Today, the increasingly accurate lasers have put scientists
closer to a firm answer on the core topic. The discoveries
made by laser ranging include not only the large Love
numbers but surprising perturbations in the Moon's orbit.
Dr. Williams and four colleagues at the Jet Propulsion
Laboratory reported last Nov. 25 in The Journal of
Geophysical Research that analysis of the wobbles suggested
the presence of a small molten core with a radius up to 230
miles.
More recently, the team's research on lunar Love numbers
has deepened the portrait. Not only is there apparently a
small molten core, but a large layer of partly melted rock
around it. That, Dr. Williams and his colleagues say, is
the best explanation for the periodic bulging of the Moon's
surface. The core itself, even if molten, they say, is too
small to account for the Moon's outer elasticity.
"Lunar interior models may need to become more complex,"
the team concluded in a March presentation to the Lunar and
Planetary Science conference, an annual meeting sponsored
by National Aeronautics and Space Administration and the
Lunar and Planetary Institute in Houston.
The four-inch rises, Dr. Williams said in an interview,
occur in 27-day cycles - the time it takes the Moon to make
one full orbit of Earth, known as the sidereal month. In
contrast, the period from one new moon to the next, or the
synodical month, is 29 days because Earth orbits the Sun in
the same direction as the Moon.
The gravitational tugs on the Moon tend to be greatest -
and the bulging greatest - when it moves closest to Earth,
the scientists said. The Moon's orbit is slightly
elliptical. So over 27 days, it moves near and far, though
these changes are small enough that most admirers of the
night sky would never notice.
Dr. Williams said hints of a sizable molten lunar interior
had been suggested independently by NASA's Lunar Prospector
spacecraft, which cost $63 million and recently mapped the
Moon's gravity field. It too, he said, found a high Love
number. Both team's measurements, he added, are higher than
physical models predict for a Moon with a small core.
For Dr. Murphy of the University of Washington, bouncing
laser beams off the Moon promises to tell much about the
wider universe. His dream is to build a laser observatory
that is more precise than the ones in Texas and France.
Laser ranging, Dr. Murphy wrote recently with eight
colleagues, "is poised to take a dramatic step forward."
His chosen site is Apache Point, a mountain in southern New
Mexico nearly two miles high. Many telescopes already crowd
its ridges. His plan is to adapt the biggest one, whose
138-inch (about 12 feet) primary mirror is huge compared
with the 30-inch one in Texas and many times better at
gathering light.
If a big laser and new detectors are added, the large
telescope will be able to do laser ranging measurements in
full daylight and in the light of the full Moon. Thus, Dr.
Murphy and his colleagues wrote, it could "fully sample the
lunar cycle." It would also measure the distances between
Earth and the Moon with remarkable precision - down to
millimeters instead of centimeters.
Dr. Murphy and his colleagues are now seeking financing to
adapt the Apache Point telescope, which is run by a
consortium of universities led by the University of
Washington. NASA has paid for preliminary studies of the
plan.
A major objective, Dr. Murphy said, will be to subject
Einstein's theory of general relativity to more stringent
testing as a way to improve the understanding of the nature
of gravity. If the new observatory found subtle violations
of predictions of Einstein's theory, the findings could
bolster alternative ideas and help explain why the universe
is accelerating.
In recent years, astronomers have become increasingly
convinced that galaxies in the last five billion or six
billion years have speeded up their expansion - as if some
mysterious repulsive force contrary to gravity was pushing
them apart and outward.
In theory, the new observatory could investigate this
expansion by letting scientists look for local effects in
the solar system. For instance, it could see if the Sun's
gravitational field causes Earth and the Moon to accelerate
differently in minute ways.
"It's like Galileo's free-fall experiment," Dr. Murphy
said. "Instead of a ball and feather, we're dropping the
Earth and the Moon toward the Sun all the time, and we'll
have the opportunity to very carefully measure the distance
between them to see if there are any differences in
acceleration."
Such experiments with the reflectors on the Moon, he said,
would give new life to the field of gravitational physics
and provide decades of new challenges for laser ranging.
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