The moon is pockmarked with cold, wet oases that could contain enough water ice to be useful to manned missions.
A year after NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) smashed into the surface of the moon, astronomers have confirmed that lunar craters can be rich reservoirs of water ice, plus a pharmacopoeia of other surprising substances.
The debris plume about 20 seconds after LCROSS impact.
On Oct. 9, 2009, the LCROSS mission sent a spent Centaur rocket crashing into Cabeus crater near the moon’s south pole, a spot previous observations had shown to be loaded with hydrogen. A second spacecraft flew through the cloud of debris kicked up by the explosion to search for signs of water and other ingredients of lunar soil.
And water appeared in buckets. The first LCROSS results reported that about 200 pounds of water appeared in the plume. A new paper in the Oct. 22 Science ups the total amount of water vapor and water ice to 341 pounds, plus or minus 26 pounds.
Given the total amount of soil blown out of the crater, astronomers estimate that 5.6 percent of the soil in the LCROSS impact site is water ice. Earlier studies suggested that soils containing just 1 percent water would be useful for any future space explorers trying to build a permanent lunar base.
“The number of 1 percent was generally agreed to as what was needed to be a net profit, a net return on the effort to extract it out of the dark shadows,” said NASA planetary scientist Anthony Colaprete in a press conference Oct. 21. “We saw 5 percent, which means that indeed where we impacted would be a net benefit to somebody looking for that resource.”
Water could lurk not just in the moon’s deep dark craters, but also as permafrost beneath the sunlit surface. Based on the impact data, water is probably mixed in to the soil as loose ice grains, rather than spread out in a concentrated skating rink. This distribution could make the water easier to harvest.
“The water ice is in this rather malleable, dig-able kind of substrate, which is good,” Colaprete said. “At least some of the water ice, you could go in and literally just scoop it up if you needed to.”
But the plume wasn’t just wet. A series of papers in Science report observations from both LCROSS and LRO that show a laundry list of other compounds were also blown off the face of the moon, including hydroxyl, carbon monoxide, carbon dioxide, ammonia, free sodium, hydrogen, methane, sulfur dioxide and, surprisingly, silver.
Temperature map of the lunar south pole from the LRO Diviner Lunar Radiometer Experiment, showing several intensely cold impact craters. UCLA/NASA/Jet Propulsion Laboratory, Pasadena, Calif./Goddard
The impact carved out a crater 80 to 100 feet wide, and kicked between 8,818 pounds and 13,228 pounds of debris more than 6 miles out of the dark crater and into the sunlight where LCROSS could see it. Astronomers, as well as space enthusiasts watching online, expected to see a bright flash the instant the rocket hit, but none appeared.
The wimpy explosion indicates that the soil the rocket plowed into was “fluffy, snow-covered dirt,” said NASA chief lunar scientist Michael Wargo.
The soil is also full of volatile compounds that evaporate easily at room temperature, suggests planetary scientist Peter Schultz of Brown University, lead author of one of the new papers. The loose soil shielded the view of the impact from above.
Data from an instrument called LAMP (Lyman Alpha Mapping Project) on LRO shows that the vapor cloud contained about 1256 pounds of carbon monoxide, 300 pounds of molecular hydrogen, 350 pounds of calcium, 265 pounds of mercury and 88 pounds of magnesium. Some of these compounds, called “super-volatile” for their low boiling points, are known to be important building blocks of planetary atmospheres and the precursors of life on Earth, says astronomer David Paige of the University of California, Los Angeles.
Compared to the amount of water in the crater, the amounts of these materials found were much greater than what is usually found in comets, the interstellar medium, or what is predicted from reactions in the protoplanetary disk.
“It’s like a little treasure trove of stuff,” said planetary scientist Greg Delory of the University of California, Berkeley, who was not involved in the new studies.
Astronomers picked Cabeus crater partly because its floor has been in constant shadow for billions of years. Without direct sunlight, temperatures in polar craters on the moon can drop as low as -400 degrees Fahrenheit, cold enough for compounds to stick to grains of soil the way your tongue sticks to an ice cube.
Other factors, like micrometeorite impacts and ultraviolet photons that carry little heat but significant amounts of energy, can release these molecules from the moon’s cold traps. The composition of the lunar surface represents a balancing act between what sticks and what is released.
The fact that so many different materials, most of which are usually gaseous at room temperature and react easily with other chemicals, remain stuck to the moon gives astronomers clues as to how they got there.
“Perhaps the moon is presently active and there’s all kinds of chemistry going on and stuff being produced, continually collecting in these polar regions,” Delory said. “Maybe it’ll tell us the moon is in fact a much more active and dynamic system than we thought, and there’s water being concentrated at the poles by present-day ongoing processes.”
Another possibility is that these materials hitched a ride on comets or asteroids, Schultz suggests. Compounds deposited all over the moon could have migrated to the poles over the course of billions of years, where they were trapped by the cold or buried under the soil.
There’s only one sure way to find out.
“We need to go there,” Delory said. Whether the water will be a useful resource for future astronauts or not, the ice itself is a rich stockpile of potential scientific information, he said. “That’s as much a reason to go there, for the story that this water tells.”