Office of News and Information
212 Whitehead Hall / 3400 N. Charles Street
Baltimore, Maryland 21218-2692
Phone: (410) 516-7160 / Fax (410) 516-5251
May 25, 1994
FOR IMMEDIATE RELEASE
CONTACT: Emil Venere
Hopkins, Goddard Scientists Find Lunar
The moon has much deeper craters and higher peaks than
scientists had thought, data from the Clementine space probe
Surprises with Clementine
The overall range of topography, from the deepest to the
highest formations around the lunar globe, is greater than 20
kilometers, or about 12 and a half miles, said Maria Zuber, a
geophysicist at The Johns Hopkins University and Goddard Space
Flight Center. Scientists had previously thought the height
variation was only about half that.
The data suggest that the lunar surface was cooler, and
therefore stronger, earlier in its history than previously
believed. Features caused by impacts into the lunar crust were,
therefore, preserved to a higher degree than expected.
"If it was hot, all this high and low topography would have
flowed away like molasses," said Dr. Zuber, a member of a
research team that includes geophysicists David E. Smith and
Frank G. Lemoine, at Goddard, and Gregory Neumann, a
post-doctoral fellow at Johns Hopkins.
One example of lunar landscape extremities is the South
Pole-Aitken Basin. Scientists had thought the crater was 7
kilometers deep, but it actually extends 12 kilometers, or about
7 and a half miles, below the moon's surface and is now believed
to be the deepest impact crater in the solar system. The basin,
located on the side of the moon that perpetually faces away from
Earth, would swallow Mount Everest and is comparable to the
deepest Earth features, such as ocean-floor trenches. It dwarfs
the Grand Canyon, which is about 1.6 kilometers deep.
Dr. Zuber presented preliminary findings May 26 during a
meeting of the American Geophysical Union in Baltimore.
Clementine, a low-cost spacecraft launched Jan. 25, orbited the
moon for two months, gathering unprecedented volumes of
information about Earth's companion.
The scientists have used Clementine data to draw the first
precise and comprehensive topographic map of the moon, measuring
the depths and peaks to an accuracy of 100 meters, or about 330
feet. That's 10 times better than the previous knowledge of the
global topography of the moon.
The data allows scientists to clear up any doubt that many
structures suspected of being impact basins were created from
bombardment with debris left over after the creation of the solar
system, about 4.6 billion years ago. By taking a precise
inventory of the quantity and sizes of impact basins,
scientists will be able to learn more about how much heat was
produced by large impacts during the moon's early evolution.
"Impacts are the geologic process that were most important
in shaping the moon's early history," Dr. Zuber said. Analysis of
Clementine tracking data also support the possibility that the
moon may contain a small amount of partially molten material deep
in its interior. But another mission would be needed to fully
answer that question.
Clementine promises to aid scientists who are trying to
solve several mysteries about the moon's structure and evolution.
In its early history most, if not all, of the lunar surface and
near surface was molten, and the melted material ultimately
formed the lunar crust. Scientists believe that the side of the
moon that perpetually faces Earth has a thinner crust than the
back side. This effect is so large that the moon's center of mass
is about 2 kilometers closer to Earth than is its geometric
center, Dr. Zuber said.
But before scientists can understand why the moon has such
feature, and ultimately, how the moon formed, they must measure
globally the thickness of the lunar crust and understand details
about the moon's mass and shape, said Dr. Zuber, an associate
professor who holds the Second Decade Society Chair in the
Hopkins Department of Earth and Planetary Sciences.
Researchers are using Clementine data from laser
measurements to determine the height of the spacecraft above the
lunar surface, and radio signals to calculate changes in
Clementine's orbital velocity, which provide information on the
moon's gravitational field.
One of the means of understanding the lunar interior is to
make a detailed record of how gravitational strength fluctuates
from one part of the moon to another. Areas of greater density
exert a stronger gravitational pull on the space probe than areas
of less density. As the space craft orbited the moon, it dipped
down closer to the surface in places where the gravitational
field was stronger. Scientists recorded the spacecraft's ups and
downs as it circled the moon by measuring small changes in its
velocity. Using a phenomenon known as the Doppler effect, they
calculated minute differences in velocity by analyzing changes in
radio wavelengths. As an object moves toward you, its radio waves
are compressed. As an object moves away from you, its radio waves
are stretched into longer frequencies. The faster an object
moves, the more pronounced its Doppler shift.
The radio wave and laser data can be dovetailed to create a
picture of lunar topography and mass distribution in the moon.
Knowledge of the sub-surface mass will contribute to an
understanding of what the moon is made of, since different
substances have different densities. This information may help to
shed some light on how the moon was formed. One theory holds that
it was created in a collision shortly after the birth of the
solar system, when a Mars-size object possibly collided with
Earth, sending debris from the mantles of both the Earth and the
object into space. The debris possibly coalesced into the
Johns Hopkins University news releases can be found on the
World Wide Web at
Information on automatic e-mail delivery
of science and medical news releases is available at the