Lunar ice

LUNAR ICE

The Moon is a completely different place from Earth. Despite being very close, it is a dead world, lifeless, sterile. From space it is easy to tell the difference: the Moon is gray, but the Earth is blue. And that is one of the main reasons why our world is full of life: water. The discovery of ice at the lunar poles by the Lunar Prospector probe could help change the future of our natural satellite as an inhabited world.

LUNAR ENVIRONMENT

We remember the people who traveled to the Moon in their typical astronaut outfits: thick suit, helmet, backpack. All this should not only give them oxygen to breathe, because there is no air, but by the same token, should provide adequate pressure, temperature control (which varies from +120 Celsius degrees in sunlight to -150 degrees C in the shade) and shielding from harmful radiation and micro-meteorites.

LUNAR SOIL

The chemical composition of the lunar soil is very similar to that of Earth: oxides of silicon, magnesium, calcium, aluminum. There is less iron and more titanium than here. But rocks are terribly dry, and there is no sign that they had ever been in water. In fact, the astronauts' footprints, that here on Earth would be erased in a little while, will last a million years on the lunar surface, by the total absence of wind and rain.

Yet there is some erosion though, caused precisely by the bombardment of meteorites and cosmic dust, to the point that the lunar surface is very disjointed, as on Earth. There is a layer of loose material, ranging from a few meters to 100 m deep, known as regolith.

IN SEARCH OF WATER ON THE MOON

While the Moon has been explored by astronauts between 1969 and 1972, the scientific research conducted during the Cold War era were necessarily inadequate. The Apollo missions 11, 12, 14, 15, 16 and 17 brought nearly 400 kg of lunar samples. The Soviets, in the same period, with the robotic spacecraft Luna 16, 20 and 24 also brought a few but important grams. But despite the different missions having collected samples from varied terrain like plains and mountains, for communication and propulsion reasons they could not land very far from the Moon equator. So, we don't have samples of the poles.

With the end of the space race, the Moon was almost abandoned. Only in 1994 the U. S. launched another probe to the Moon: Clementine, a military spacecraft, whose objective was to test new sensors. It made high-quality photographic maps, even of the poles. The Moon has day and night, as on Earth (except that last longer: 29 days in total) but in these pictures polar craters whose bottoms are remarkably always in the dark were discovered: as the lunar axis has almost no tilt the light from the Sun can never reach those points. As the Moon was bombarded by comets countless of times, which are basically ice CO₂ and H₂O, it was speculated that they vaporized on impact and many of the water molecules could migrate to these dark craters where the cold would freeze them again.

The Clementine transmitting antenna was used to launch a microwave beam to these craters; this beam bounced off the surface and was picked up by antennas on Earth. The manner in which the signal was altered on the rebound gave reason to believe that ice crystals could be there.

Thus, shortly after NASA funded the first civilian mission to the Moon in three decades: Lunar Prospector. The spacecraft, of 296 kg total mass, was built by Lockheed Martin at a cost of US$ 34 million, plus 4 million to operate it. A three-stage, solid fuel rocket, the Athena II, of 25 million dollars, would thrust it fast enough to reach the Moon. With a total budget of 63 million dollars, it was the cheapest planetary mission in NASA's history.

Launched on 7 January 1998, it reached the Moon after 105 hours. There, 138 kg of hydrazine in its tanks allowed it to maneuver into a circular orbit of 100 km altitude, passing over the poles. It went around every 118 minutes.

THE EXPERIMENT

The spacecraft was a graphite-epoxy drum of 1,37 m in diameter by 1,28 m high, covered with solar cells, spinning like a wheel to maintain stability. As commands reached the Moon in just 1,3 s, there was no need to install a computer; a solid-state recorder retained the data when it passed behind the Moon, for later broadcast. Three masts protruded from the spacecraft body, where scientific instruments were installed: a magnetometer, responsibility of Mario Acuña, of the University of Arizona, which together with an electron reflectometer, responsibility of Robert Lin of the University of California at Berkeley, was used for mapping magnetic fields and anomalies, coming both from the surface as from a possible lunar core; an alpha-particle detector, responsibility of Alan Binder, from Lockheed, in charge of measuring certain gases that may emanate from the interior of the Moon; a gamma-ray spectrometer, responsibility of Scott Hubbard, from NASA Ames Research Center, was in charge of mapping the surface chemical elements; Alex Konopliv of NASA Jet Propulsion Laboratory was responsible for analyzing variations in the radio signal coming from the spacecraft to measure changes in its trajectory and thus map the Moon's gravitational field.

But the most striking experiment was the neutron spectrometer, responsibility of William Feldman of Los Alamos National Laboratory. As there is no atmosphere, the Moon's surface is constantly bombarded by atoms torn from the stars, the so-called cosmic rays. When they hit the ground they generate neutrons flying into space. These neutrons can be of three types: fast neutrons are those who escaped instantly at being hit; those left on the ground lose velocity in contact with hydrogen becoming medium-speed neutrons; and later slow neutrons, whose minimum speed depends on the temperature of the soil. A small cylinder with a special type of helium captured slow and medium-speed neutrons. A second cylinder, identical, had a shielding that discarded the slow neutrons. A special device in the gamma-ray spectrometer detected only the rapid ones. The readings of the three devices were used to map the lunar surface and see from where each type of neutron "sprouts" or "stop sprouting".

THE DATA

Data made available by researchers for consideration by the American Association for the Advancement of Science and published in its journal "Science" on 4 September 1998, shows a lack of up to 4,6% of medium neutrons (i.e., fast neutrons became slow neutrons in a very short period of time, making it rare to see medium-speed neutrons) precisely in the regions where photos of Clementine show that sunlight never reaches the ground. This is a signal that much hydrogen, that according to calculations is in the form of frozen H₂O, is mixed with the soil in a proportion of 0,3 to 1 kg per ton. And there may be even pure-ice deposits.

That there is water on the Moon is very significant for the future human exploration of our natural satellite, especially when we consider that to bring one liter of water from Earth to space costs about US$ 20,000. Water is not only a source of life for future lunar colonists, but also oxygen for breathing and hydrogen for fuel can be extracted from it. So these colonies can begin to make the Moon no longer being the sterile world that it has always been.

Aldo Loup.

Based on a lecture given at the USP, on 20 March 1999. Originally published in ABC Color, on 25 June 2006. Illustration: This artist's concept shows the Lunar Prospector spacecraft into orbit around the Moon. The masts of instruments are fully deployed. Lunar Prospector spent about a year and a half studying and mapping the Moon. After the mission was completed, it crashed into a crater at the lunar South Pole. On board the ship were ashes of Eugene Shoemaker, a pioneer of Astrogeology, who thus became the first human being buried on another celestial body. Credit: NASA Ames Research Center.