Title: The Moon rediscovered.
Subject(s): MOON -- Exploration; LUNAR Prospector (Artificial
satellite)
Source: Sky & Telescope, Dec98, Vol. 96 Issue 6, p32, 3p
Author(s): Foust, Jeffrey A.
Abstract: Reports on lunar data returned by the Lunar Prospector
spacecraft. Water ice at the lunar poles; Distinctly magnetic areas on
the lunar far side, directly opposite the impact basins that underlie
Mare Imbrium and Mare Serenitatis; Higher-resolution mapping of the
surface.
AN: 1252859
ISSN: 0037-6604
Note: Tucson-Pima Public Library subscribes to this magazine.
Database: MasterFILE Elite

Section: MISSION UPDATE

THE MOON REDISCOVERED

Lunar Prospector, the first craft in NASA's low-budget Discovery
series, has proved itself a first-rate science million.

One might think that the Moon, the most intensely studied body beyond
Earth, would have been stripped of any mysteries long ago. However,
recent date returned by the Lunar Prospector spacecraft show how much
we have yet to learn about our nearest neighbor. A bounty of findings,
published in the September 4th issue of Science, have provided new
insights -- and raised new questions -- about the nature and origin of
the Moon.

Once again, attention has focused on water ice at the lunar poles.
Prospector scientists have revised upward their estimate of the ice's
total mass to as much as 6 billion metric tons, 20 times more than
what was announced in March. The frozen water cannot be seen directly,
however, because it lies areas that remain permanently hidden from
sunlight. Instead prospector counts the neutrons that emanate from the
lunar surface after cosmic-ray strikes. Decreases in the counts of
medium-energy ("epithermal") neutrons over both poles, relative to
those for low-energy ("thermal") neutrons, mean that the former have
interacted with hydrogen atoms. The additional three months of data
collected since the March announcement have allowed project scientists
to map the neutron flux more carefully. They find that the
concentrations of hydrogen -- and thus presumably of ice -- are
confined to a limited number of areas near the poles, rather than
spread out evenly over relatively large polar regions as first
thought. The strongest indications come from poleward-facing rims of
the craters Peary, Hermite, Rozhdestvenskiy, and Plaskett in the north
and within the giant South Pole-Aitken basin in the south. Notably,
the polar regions do not exhibit corresponding dips in the counts of
highest-energy, "fast" neutrons. Prospector scientists interpret this
to mean that the water ice is not finely mixed in the surface but may
exist as near-pure deposits buried under as much as 40 centimeters of
regolith.

William C. Feldman (Los Alamos National Laboratory), who heads the
neutron-spectrometer team, cautions that the estimated mass and depth
of the water ice hinge on several assumptions. For example, the ice is
thought to extend up to 2 meters below the surface because that's the
depth estimated to have been "gardened" by small-scale impacts in the
course of the last two billion years. Also, if the ice is pure, then
it is confined to less than 2,000 square kilometers of permanently
shadowed terrain at each pole, an estimate whose exact value Feldman
says "at this point no one really knows." One study has shown that ice
could theoretically exist inside craters as much as 13 1/2degrees in
latitude from the pole.

Nor does all the hydrogen have to be coming from ice. Faith Vilas
(NASA/Johnson Space Center) and her colleagues suggest that the polar
regions also contain claylike minerals called phyllosilicates, whose
crystal structure traps water as hydroxyl (OH) radicals. Vilas
suspects that the energy to alter rock-ice mixtures into
phyllosilicates comes from micrometeoritic impacts. Multispectral data
collected from Earth and during the Galileo spacecraft's two flybys of
the Moon hint weakly that phyllosilicates might lie in -- and beyond
-- the polar regions. Feldman notes that the neutron spectrometer
cannot distinguish between these hydrated silicates and ice.

Another issue is how ice got to the lunar poles in the first place.
One logical source is comets and carbonaceous asteroids, some of the
water from which might be expected to freeze in the poles' dark
recesses after an impact. However, comets strike the lunar surface at
such high speeds that most of their water (along with a chunk of the
Moon) escapes to space. Some hydrogen must come from the solar wind,
which adheres to the outermost veneer of lunar rocks. Over time some
of this hydrogen reacts with iron oxides to form water. Finally, the
Moon must have released some water from its interior over time.
Whether any of that water could remain today -- or whether our
satellite continues to exude any gas at all -- remains unanswered.
Alan B. Binder, Lunar Prospector's project scientist, points out that
the spacecraft can detect the alpha particles given off by radon and
other gases that might be escaping from the interior. However, no
results have yet been announced, as the instrument readings have been
complicated by strong solar-wind activity.

Mysterious Magnetism 

Even lacking results from the alpha-particle spectrometer,
investigators have plenty of other unexpected findings to mull over.
Prospector's magnetometer and electron reflectometer turned up two
distinctly magnetic areas on the lunar far side, directly opposite the
impact basins that underlie Mare Imbrium and Mare Serenitatis. First
found during the Apollo missions more than 25 years ago, these
magnetic fields remain coherent over hundreds of kilometers. The field
strengths there peak at only 40 nanoteslas, nearly a thousand times
weaker than the field at Earth's equator. Even so, report Robert P.
Lin (University of California, Berkeley) and his team, the
concentration of field lines at the Imbrium antipode is creating a
"mini-magnetosphere" strong enough to hold off the ionized solar wind.

The location of these magnetic bundles directly opposite impact basins
is strong evidence that they were created by the impacts themselves.
According to one explanation, a large impact would send an expanding
fireball of ionized plasma racing around the Moon in just a few
minutes, piling up the magnetic field lines in front of it. The plasma
cloud soon reached the impact's antipode, where the surface rocks were
simultaneously being heated by converging seismic shock waves. The
"squeezed" magnetic field became frozen into the rocks as they cooled,
creating localized magnetizations that survived long after the Moon
lost its global magnetic field.

Mini-magnetospheres may explain the existence of light-colored patches
on the lunar surface like the Reiner Gamma swirls in western Oceanus
Procellarum. This irregularly shaped area was first linked to residual
magnetic fields in Apollo data 20 years ago, though it has yet to be
studied in detail by Lunar Prospector. A magnetosphere there would
keep the solar wind from implanting hydrogen, a key component of the
iron-reduction process believed to darken lunar soil.

If the Moon once had a global magnetic field, then it most probably
had a molten core to sustain a dynamo to generate the field. The
existence of any kind of dense lunar core has been uncertain, but new
data from Lunar Prospector are making a strong case for one. Careful
tracking of the spacecraft reveals variations in the frequency of its
radio beacon. These result from irregularities in the Moon's gravity
field from place to place and within its interior. Prospector
scientists have combined this gravity signature with models of lunar
composition to show that the Moon really does have a core, whose
diameter lies somewhere between 600 km across (if mostly iron) and
1,000 km (if mostly iron sulfide). The existence of a metallic core
has implications for the leading theory of lunar formation, which
holds that the Moon coalesced out of material blasted away from Earth
during a giant impact. If this event occurred after Earth's iron had
sunk inward into its core, then the Moon should contain little or no
iron in its interior. Thus a metallic core containing at least 1
percent of the Moon's total mass argues that the giant impact took
place before Earth had fully differentiated into a crust, mantle, and
core.

Thanks to Lunar Prospector's gravity data, mission scientists have
also discovered several new mass concentrations, or mascons, beneath
the lunar surface -- four of them on the far side. All previously
known mascons are near-side impact basins topped with mare basalts
several kilometers thick (presumably representing the excess mass). By
contrast, only some of the new finds are covered with basalt. This
means that some mascons may also owe their existence to subterranean
"plumes" of dense mantle material that rose into the thinned,
fractured crust left in the aftermath of basin-forming impacts.

Other clues to what fills the lunar interior come from Lunar
Prospector's gammaray spectrometer, which has found concentrations of
thorium and potassium around the rim of Mare Imbrium. The implication
is that these elements were dredged up from deep within the Moon
during an impact. However, the huge South Pole-Aitken basin, though
larger than Imbrium, has less thorium and potassium exposed on its
margins.

"Something special happened around Imbrium; you don't see this sort of
chemistry anywhere else on the Moon," says Richard Elphic (Los Alamos
National Laboratory). "It also confirms that the Moon is very
inhomogeneous, at least for these elements." Elphic's conclusions are
underscored by neutron-spectrometer data that show wide variation in
the proportions of iron and titanium across the lunar landscape.

Next Steps 

Lunar Prospector's mission enters a new stage in January, when the
spacecraft moves out of its circular, 100-km orbit into one only 25 to
30 km above the highlands and maria. This lower orbit will allow much
higher-resolution mapping of the surface -- but it also demands
greater vigilance on the part of flight engineers to keep the altitude
from inadvertently dropping to zero. Throughout the first half of
1999, the neutron spectrometer will refine the locations of polar ice
deposits, while the Doppler gravity experiment will attempt to resolve
the mascons well enough to let geophysicists deduce their true origin.
The lower orbit will also be a boon to Binder's search for evidence of
radon outgassing.

But the data in hand have already given a big boost to lunar science.
"The Moon was previously interpreted as just a dead body with nothing
interesting going on," observes investigator Mario Acuna (NASA/Goddard
Space Flight Center). Prospector's findings have convinced him that
the Moon is anything but dead. "This is typical of science," Acuna
says. "When you look closer, you see a lot more complexity."

PHOTO (COLOR): The Galileo orbiter looked own on the Moon during ...

The Galileo orbiter looked own on the Moon during a close flyby on
December 7, 1992. Some crater floors near the lunar north pole lie in
permanent shadow and likely contain deposits of water ice. Courtesy
NASA and the U.S. Geological Survey.

PHOTO (COLOR): Top right: A Galileo near-infrared map reveals
concentrations of ...

Top right: A Galileo near-infrared map reveals concentrations of
iron-rich silicates (purple) across the Moon's northern hemisphere.

PHOTO (COLOR): Right: Colored squares, each about 60 kilometers on a
...

Right: Colored squares, each about 60 kilometers on a side, map out
the counts of medium-energy neutrons emanating from the Moon's north
polar region. The lowest values, in dark pink and purple, correspond
to the greatest concentration of hydrogen and thus, Prospector
scientists claim, water ice.

PHOTOS (COLOR): Most geophysicists now believe the Moon had a small,
...

Most geophysicists now believe the Moon had a small, molten core and
perhaps a global magnetic field early in its history (left). Giant
impacts, like the one that created the Imbrium basin (middle), created
an ionized fireball and powerful seismic waves that raced toward the
point opposite the impact. Magnetic-field lines became concentrated at
that point and became "frozen" in the antipode's shocked rocks. Lunar
Prospector has confirmed that magnetized regions remain today (right)
at the antipodes of Imbrium and other large impact basins.

PHOTO (COLOR): The near-vertical orientation of the Moon's spin axis
to ...

The near-vertical orientation of the Moon's spin axis to the ecliptic
plane creates pockets of permanent night at the lunar poles.

~~~~~~~~

By Jeffrey A. Foust

JEFFREY A. FOUST researches planetary astronomy at the Massachusetts
Institute of Technology. His article "NASA's New Moon" appears in the
September issue.
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Source: Sky & Telescope, Dec98, Vol. 96 Issue 6, p32, 3p.
Item Number: 1252859