Update The impact plumes of the Lunar CRater Observation and Sensing Satellite (LCROSS) and its Centaur rocket stage in Cabeus crater near the south pole of the Moon on 9 October 2009 showed the spectral signature of hydroxyl, a key indicator that water ice is present in the floor of the crater (1). Analysis of the results indicates concentrations of roughly 6% water in the impact area, including nearly pure ice crystals in some spots. The Indian Chandrayaan-1 Moon Mineralogy Mapper experiment showed low-concentration hydroxyl signatures over much of the lunar surface, not just in permanently shadowed craters (2), and the Mini-SAR experiment indicated possible large deposits of water-ice in the northern lunar craters (3).
The discussion below is historical, based primarily on results from the Clementine and Lunar Prospector missions, our view of water on the Moon is changing continuously with results from the Lunar Reconnaissance Orbiter and analyses of lunar samples on Earth.
On 5 March 1998 it was announced that data returned by the
spacecraft indicated that water ice might be present at both the north and south lunar poles,
in agreement with interpretations of
results for the south pole reported in November 1996.
The ice originally appeared to be mixed in with the lunar regolith
(surface rocks, soil, and dust) at low concentrations conservatively estimated at 0.3
to 1 percent. Subsequent data from Lunar Prospector taken over a longer period has
indicated the possible presence of discrete, confined, near-pure water ice deposits
buried beneath as much as 18 inches (40 centimeters) of dry regolith, with the water
signature being stronger at the Moon's north pole than at the south (4).
The ice was thought to be spread over 10,000 to 50,000 square km (3,600 to 18,000
square miles) of area near the north pole and 5,000 to 20,000 square km (1,800 to
7,200 square miles) around the south pole, but the latest results show the water
may be more concentrated in localized areas (roughly 1850 square km, or 650 square
miles, at each pole) rather than being spread out over these large regions. The
estimated total mass of ice is 6 trillion kg (6.6 billion tons).
Uncertainties in the models mean this estimate could be off considerably.
The Lunar Prospector, a NASA Discovery mission, was launched into lunar orbit in
January 1998. Included on Lunar Prospector is an experiment called the
This experiment is designed to detect minute amounts of water ice at a level of
less than 0.01%. The instrument concentrated on areas near the lunar poles
where it was thought these water ice deposits might be found. The Neutron
Spectrometer looks for so-called "slow" (or thermal) and "intermediate"
(or epithermal) neutrons which result from collisions of normal "fast" neutrons
with hydrogen atoms. A significant amount of hydrogen
would indicate the existence of water. The data
show a distinctive 4.6 percent signature over the north polar region and
a 3.0 percent signature over the south, a strong indication that water is
present in both these areas. The instrument can detect water to a depth of
about half a meter.
The Clementine imaging experiment showed that such permanently shadowed
areas do exist in the bottom of deep craters near the Moon's south pole.
In fact, it appears that approximately 6000 to 15,000 square kilometers
(2300 to 5800 square miles) of area around the south pole is permanently
shadowed. The permanently shadowed area near the north pole appears on
Clementine images to be considerably less, but the Lunar Prospector results
show a much larger water-bearing area at the north pole. Much of the area
around the south pole is within the South Pole-Aitken Basin (shown at left
in blue on a lunar topography image), a giant impact crater 2500 km
(1550 miles) in diameter and 12 km deep at its lowest point.
Many smaller craters exist on the floor of this basin. Since they are
down in this basin, the floors of many of these
craters are never exposed to sunlight. Within these craters the
temperatures would never rise above about 100 K
(280 degrees below zero F) (5). Any water ice at the bottom of the crater
could probably exist for billions of years at these temperatures.
In a Science magazine article (8) on 29 November 1996, it was announced that interpretation of data from a Clementine spacecraft experiment suggested the possibility of ice on the surface of the Moon. The ice was believed to be in the bottom of a permanently shadowed crater near the Moon's south pole (at the center of the Clementine mosaic shown at the top of the page). It was also thought likely that other frozen volatiles, such as methane, were in the deposit. The deposit was estimated to be approximately 60,000 to 120,000 cubic meters in volume. This would be comparable to a small lake in size, four football fields in surface area and 16 feet deep. This estimate was very uncertain, however, due to the nature of the data.
One of the problems in studying a permanently shadowed area is that no pictures
can be obtained. The Clementine spacecraft searched for the ice using an
investigation known as the
Bistatic Radar Experiment.
Basically, this experiment consisted of
having the Clementine spacecraft transmit an S-band radio signal through its high
gain antenna towards a lunar target. The signals reflected off the Moon and
were received by a 70 meter Deep Space Network (DSN) antenna on the Earth.
Frozen volatiles such as water ice are much more reflective to S-band radio waves
than lunar rocks. Radio waves also have different characteristics when reflected
off ice than off silicate rock. An analysis of the signals returned from orbit
234 showed reflection characteristics suggestive of water ice for the
permanently shadowed areas near the south pole. Reflections from regions
which are not permanently shadowed do not show these characteristics. It is
possible that other scattering mechanisms could be responsible for this result,
but the interpretation of the radio returns and the fact that they are associated
only with the permanently shadowed regions seem to indicate that water ice is the
most likely possibility. However, Arecibo radio telescope studies using the same
radio frequency as Clementine showed similar reflection patterns from areas
which are not permanently shadowed. These reflections have been interpreted as
being due to rough surfaces, and it was suggested that the Clementine results may
have been due to roughness, rather than water ice, as well.
Bistatic Radar Experiment Parameters
9-10 April 1994
Transmission: S-Band 2.273 GHz (13.19 cm wavelength)
Polarization: Right Circular (RCP)
Signal Power: 6 Watts
Axial Tilt: 4.5 to 5.5 degrees (Moon to Earth)
Orbits Used: 234 and 235
Ice at the South Pole of the Moon - Lunar and Planetary Institute
Ice on the Bone Dry Moon - Paul Spudis