Johns Hopkins Gazette: January 9, 1995

Zuber, Clementine Discover Moon's Mysteries

By Emil Venere

     At a time when many scientists are nervously competing for
scarce research grants, Maria Zuber is not exactly looking for
more work: she's juggling projects for three major space
missions, including the high-profile Clementine moon project.
     "It's either feast or famine," said the Hopkins
geophysicist, leading a team of scientists who wrote a paper on
the latest Clementine findings, published Dec. 16 in the journal
     Clementine was launched Jan. 25, 1994, and orbited the moon
for two months, collecting a wealth of information that has
enabled scientists to make the first accurate map of lunar
topography. Data from Clementine will keep scientists busy for a
decade and have already revealed surprising mysteries about
Earth's lifelong companion, said Dr. Zuber, an associate
professor in the Department of Earth and Planetary Sciences, who
also is a senior research scientist at NASA's Goddard Space
Flight Center.
     "These data are telling us that we don't know as much about
the shape and evolution of the moon as we thought we did," she
     But Clementine represents just one of three big science
projects on her agenda. Dr. Zuber's schedule is so dense these
days that she is working on two missions set for launch the same
year--a Mars space probe and the Near Earth Asteroid Rendezvous
(NEAR), both planned for 1996.
     NEAR will be the first spacecraft to orbit an asteroid. It
is also the first spacecraft in NASA's Discovery program of
small-scale, inexpensive probes and is to be built and operated
by the Applied Physics Laboratory. Dr. Zuber leads a team of
scientists who will use a laser aboard NEAR to study the
structure and shape of an asteroid called 433 Eros. 
     At the same time, she is deputy principal investigator for a
science team that will design and operate a laser on an upcoming
Mars spacecraft.
     "I am busy," she said, with more than a hint of
     And she's been busy since Clementine reached lunar orbit
last February. For two and a half months, as the spacecraft
circled the moon, she found herself dividing time between
teaching at Hopkins, working at Goddard, caring for her two sons
and spending late nights at the "Batcave," a Naval Research
Laboratory facility in Alexandria, Va.
     The nondescript warehouse-like building concealed a mission
operations center buzzing with excitement.
     But science or no science, she wasn't about to sacrifice
quality time with her sons, 5-year-old Jack and 2-year-old
     "I get to see my kids from about 5 o'clock to about 9
o'clock, and I wasn't willing to give that up, even for the
     So each night the geophysicist put the kids to bed, made the
30-mile drive from her home in Silver Spring to the Batcave,
where she arrived around 10 o'clock. After checking the data
beamed down from Clementine that day, she would program the
instrument for the next day's work.
     "It was a pretty wild spring," said Dr. Zuber, 36, a member
of a research team that includes geophysicists David E. Smith and
Frank G. Lemoine, at Goddard, and Gregory Neumann, a postdoctoral
fellow at Johns Hopkins. "I used to get home at 2 or 3 in the
morning and then have to get up at 6 and come back in here and do
my lectures and that sort of stuff. 
     "I look back on those days fondly because we got good data,
but I hope we never have to do that again."
     Good data, indeed.
     Information from Clementine is revealing that the moon's
evolution was much more complex than previously thought, leading
to a central mystery: Why would parts of the moon have quickly
cooled off early in its development, preserving craters and peaks
that are much deeper and higher than scientists had expected? 
     Throughout its early life the moon generated heat by the
decay of radioactive elements in its interior. It also was
pummeled by space-borne debris in collisions that produced
intense heat, melting its surface and forming deep craters and
high peaks. But then parts of the moon apparently cooled off
quickly enough to freeze many surface features in place. Evidence
of rapid cooling runs counter to the traditional view of lunar
evolution, which theorizes that lunar basins caused by meteor
impacts should have relaxed into a smoother surface shape. This
slow cooling would have permitted the peaks and deep impact
basins to flow like molasses into a natural state of equilibrium,
or low stress, since the moon's mass would have settled evenly
around the lunar globe.
     But Clementine is painting a different picture, a scenario
of uneven strength of the moon's outer shell and an uneven
distribution of mass in the interior.
     "It was thought that the moon was much like a billiard ball,
not a lot of topography, that originally deep impact basins would
have shallowed," said Dr. Zuber.
     It turns out that the moon isn't so smooth, after all. The
topographic map, which can resolve changes in height as small as
10 meters, revealed that the moon's variation from the highest
peaks to the deepest craters is much greater than previously
thought. That overall range of topography was found to be 16
kilometers, or almost 10 miles--about 30 percent greater than
scientists had expected.
     The findings suggest that the moon cooled off quickly_a time
spanning perhaps tens of millions of years. "This may sound like
a lot of time, but remember that we are cooling something as big
as a planet, which takes a long time," Dr. Zuber said.
     It's a sharp departure from previous ideas of lunar
evolution, based largely on  moon samples brought to Earth by the
Apollo astronauts.
     Laser light was bounced from Clementine to the lunar
surface, providing precise measurements for the topographic map.
Another crucial tool is a record of the moon's gravity field,
which was determined by tracking the spacecraft's speed and
altitude above the lunar surface.  
     So far the research has resulted in several major
discoveries, including confirmation that the moon harbors the
biggest crater in the solar system, the South Pole-Aitken Basin.
The massive structure on the moon's far side, first identified by
images taken in previous lunar research, is 7.4 miles deep and
1,550 miles wide. On Earth it would stretch from the East Coast
to the Rocky Mountains, said Dr. Zuber, whose lunar research fits
the same motif as the upcoming asteroid and Mars probes.
     All three projects strive to understand the physics of solar
system bodies.
     NEAR's goal is to answer fundamental questions about the
nature and composition of "near Earth" asteroids, those that
cross Earth's orbital path around the Sun. The spacecraft will
rendezvous with Eros, a rocky cigar-shaped mass that represents a
tantalizing time capsule for scientists trying to learn more
about the early solar system.
     Asteroids are at least as old as the moon. But the moon has
been struck by so much debris that its crust has been melted many
times, producing a complex history of chemical change. Asteroids,
however, are much smaller targets. They have not been bombarded
by so many objects and are less tainted by the effects of time.
     "What it allows us to do is to study the properties of an
object that was formed at the beginning of the solar system," Dr.
Zuber said. 
     NEAR will observe Eros for at least a year, getting as close
as 22 miles and giving scientists the first long-term, close-up
view of an asteroid. It is scheduled for a February 1996 launch
and for a December 1998 rendezvous with Eros.
     NASA plans to launch the Mars Global Surveyor in November
1996. It will be the first mission to the red planet since the
failed Mars Observer, a $1 billion spacecraft that has not been
heard from since August 1993.
     The probe mysteriously fell silent on approach to Mars, and
scientists believe the most likely culprit was a ruptured fuel
line. Dr. Zuber helped design a laser altimeter on the Mars
Observer, and she also has a hand in designing the new probe's
laser, which will enable scientists to measure the shape of Mars.
     Mars is a more exciting research subject than the moon: it
has an atmosphere and seasons, and it possibly possessed a global
ocean at one time, said Dr. Zuber, who holds the Second Decade
Society chair and has recently won a Hopkins Oraculum Award for
Excellence in Teaching.

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