No green cheese and maybe
no water
Eshleman,
Parks question existence of water on the moon
BY DAVID F. SALISBURY
On July 31, NASA intends
to crash the Lunar Prospector spacecraft into a small
crater near the moon's south pole. The 354-pound
spacecraft will be traveling at about 3,800 miles per
hour when it plows into the lunar surface. The purpose
for such a violent end to the $63-million mission is to
find out whether there is any water ice hidden in the
bottom of the crater, which is shrouded in perpetual
shadow.
But two Stanford
researchers predict that, rather than ice, the spacecraft
is more likely to hit deposits of "hydrous"
lunar minerals that contain the constituents of water
chemically bound into their crystal structure. Von R.
Eshleman, professor emeritus of electrical engineering,
and George A. Parks, professor emeritus of geological and
environmental sciences, argue that water ice left on the
moon from past cometary impacts has most likely combined
chemically with the water-free powdered rock that covers
most of the lunar surface. They outlined their reasoning
in a letter that was published in the July 23 issue of
the journal Science.
The small
white circle superimposed on the radar map of the moon's
south polar region outlines the crater where NASA
controllers intend to crash the Lunar Prospector
spacecraft on July 31.
Courtesy
U. of Texas, Austin
NASA scientists and other
advocates for establishing a permanent base on the moon
are eager to find a source of accessible water because it
could provide water, oxygen and rocket fuel for human
colonists. Water ice would be ideal for this purpose.
But, if Eshleman and Parks are right, "the
constituents of water are there, but obtaining water
would require large amounts of energy and so would be
much harder and more expensive," says Eshleman, who
has been involved with radioscience investigations of the
moon and planets since the 1950s.
When Prospector crashes on
the moon, the Hubble space telescope, along with a number
of its space and Earth-based cousins, will be focused on
the site. They will be equipped with sensitive
instruments that can determine the composition of the
plume of lunar material that the Prospector kicks up.
Mission scientists predict that this plume could contain
as much as 40 pounds of water, which sunlight should
break down into hydrogen and hydroxyl ions. If the cloud
contains significant amounts of hydroxyl ions, mission
scientists argue that it will verify their assertion that
large amounts of water ice exist in the region, something
that they have predicted based on indirect evidence
collected by the spacecraft's instruments.
The two Stanford
scientists argue that this test, however dramatic, will
not be definitive. If their theory is correct, they
consider it unlikely that the spacecraft will kick up
significant signs of water. But it is not impossible that
the energy from the impact might cause some of the
particles of hydrous rock to chemically dissociate,
producing hydrogen and hydroxyl ions and reconstituting
some of the water that it has held in chemical bondage,
perhaps for billions of years.
The idea that the polar
craters on the moon and the planet Mercury may contain
water ice dates back to the early 1960s. Scientists
recognized that the bottoms of some of these craters must
be shrouded in perpetual shadow, shielded from sunlight
by their uplifted rims. These shaded areas should have a
constant temperature of around 170 degrees Celsius below
zero, making them the coldest places on their respective
worlds. As a result, each time Mercury or the moon was
hit by a water-bearing comet, the water molecules it
deposited would either end up in one of these "cold
sinks" or escape into space.
In fact, radar
observations of Mercury appear to have found water ice in
some of its polar craters. At least, the radar
reflections closely resemble those produced by
icecovered objects, like several outer planet
satellites. On the moon itself, however, the evidence has
been more ambiguous.
The strongest evidence in
favor of the existence of ice on the moon is measurements
taken with an instrument called a neutron spectrometer
flown aboard the Prospector. This instrument cannot
detect water ice directly, but has measured excess levels
of hydrogen in the lunar polar regions.
Radar images taken of the
lunar poles, however, have not found the characteristic
signature of ice found on Mercury or other ice-covered
objects in the solar system. So proponents have explained
the radar returns as coming from "stealthy
ice," ice that is either buried below a layer of
lunar dust or mixed with lunar material to some depth.
Eshleman and Parks
consider this highly unlikely, pointing out that the
hydrogen detected by Prospector is to be expected if all
the ice has combined with surface minerals. In the past,
scientists thinking about this situation have simply
assumed that water molecules freezing out would clump
together to form ice. They haven't given any thought to
the chemical nature of the material the ice would be
forming on or the nature of the chemical interactions
that might occur.
The Stanford researchers
point out that the moon rocks brought back to Earth
during the Apollo program indicate that the material
covering the lunar surface is anhydrous, that is, it
doesn't contain any water at all. Due to continual
micrometeorite bombardment, this material has been ground
up into a fine powder. "The surface of these
particles is covered with broken bonds that will grab
hold of any water molecules that come along," says
Eshleman. Although no samples were returned from the
lunar poles, there is no reason for the material there to
be basically different from that which covers the rest of
the moon, he argues.
According to Eshleman,
this anhydrous lunar material contains two of the same
ingredients as Portland cement and should have similar
properties. While the lunar material is naturally
anhydrous, Portland cement is made from common earth
materials that are heated to drive out all the water.
"What happens when
you put an ice cube on a layer of Portland cement in a
vacuum and at a very low temperature?" Eshleman
asks. "We predict that the ice will eventually
disappear as it is absorbed by the cement."
Eshleman and Parks predict
that the lunar material should react in a similar
fashion. What is more, they calculate that this is a
oneway process: Once water is incorporated into the
lunar minerals, the resulting material would be extremely
stable.
Furthermore, the radar
reflection of the resulting hydrous material should be
nearly identical to that of the anhydrous material, and
so can explain the radar data from the poles without
having to bury or mix the ice as proposed by the
Prospector team, the Stanford scientists say.
The response of the
Prospector mission scientists -- W. C. Feldman and
colleagues from Los Alamos National Laboratory, S.
Maurice from the Observatoire Midi-Pyrenees in Toulouse,
France, and Alan Binder from the Lunar Research Institute
in Gilroy, Calif. -- is that Eshleman and Parks'
suggestions "are worthy of more extended
quantitative analysis." The Prospector researchers
defend their position by asserting that the hydroxyl ions
that they measure must be coming from something. The
levels that they have detected are extremely high and are
concentrated in two southern polar craters that are in
continuous shadow, making water ice a likely source. They
also say that the amount of water ice that they expect to
find is too small to detect by radar unless it takes the
form of pure ice.
So, regardless of what
happens when Prospector crashes, it is unlikely to
resolve the question of whether there are potentially
valuable caches of water ice hidden in perpetual shadow
near the moon's pole. SR
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