Deep, frigid craters at the moon's poles appear to hold a wider variety of flash-frozen resources for future explorers than scientists thought even a year ago – from water to carbon dioxide, methane, mercury, and even small amounts of silver.
This is the implication scientists are drawing from data gathered from two NASA missions to explore the moon and take the measure of resources that astronauts might be able to use to sustain bases there.
The two missions were launched on the same rocket from the Kennedy Space Center in Florida in June 2009. The Lunar Reconnaissance Orbiter (LRO) arrived at the moon several days after launch and began mapping the lunar surface. It also gathered data on lunar temperatures and composition of the moon's surface. The second landing, four months later, made a bigger impression. LCROSS – the spent upper stage from the rocket that had launched the two craft, plus a package of sensors and instrumets that guided the upper stage and recorded its impact – deliberately collided with the bottom of Cabeus Crater near the moon's south pole.
The 5,500-mile-an-hour impact created what LCROSS lead scientist Anthony Colaprete calls "a hot, steaming crater," briefly warmed by energy released in the collision and humidified by the vaporized water ice carried up in the debris plume.
Researchers report that the spot LCROSS hit contained amounts of water comparable to some of Earth's driest deserts – each ton of lunar soil at the impact site could produce about 12 gallons of water. But that's still enough to be useful for splitting water into its constituents, oxygen and hydrogen, for rocket fuel, researchers say.
Even more water appears to be locked up below the lunar soil in broad, sunlit regions outside the crater, where it would be far more accessible to explorers.
The missions' results appear in six research papers appearing in Friday's issue of the journal Science. Taken together, the papers reveal a moon hosting "a more complex environment than we expected," says Richard Vondrak, project scientist for the LRO mission. "It has some of the coldest spots in the solar system with a treasure trove" within the surface layer of these areas.
Far from being the moon's dusty attic, a picture some researchers had painted of Cabeus prior to the LCROSS collision, the crater appears to be the storage freezer in the basement, team members say. Indeed, even during the lunar summer, temperatures at the bottom of Cabeus are among the coldest in the solar system -- falling lower than those at Pluto's poles or rivaling those on distant comets.
Given the amounts and variety of compounds found in the crater, it's clear that material is accumulating there faster than natural processes can get rid of it, notes Peter Schultz, a planetary scientist at Brown University in Providence, R.I., and a LCROSS team member.
Where the compounds come from and how they find their way into the "freezer" remain somewhat mysterious, although scientists have some ideas about the processes that might be involved.
Based on the crater LCROSS left, as well as evidence gathered while material was still rising from the collision point, "the surface is very fluffy," Dr. Schultz says. "I think that's because these things are added atom by atom over billions of years,"
Meteroids, comet debris, and cosmic dust rain down on the moon constantly. On average, 25 objects punch craters at least 60 feet wide into the lunar surface each year, he explains. These collisions can release atoms and molecules from material in the incoming object – including water – to circulate around the moon.
In addition, water can form as hydrogen ions from the sun smack the moon's surface and react with oxygen bound up in minerals there.
Crater-bottom deposits may also trace their origins to volcanic activity on a younger moon, or from gases that still appear to vent from deep fissures in the lunar surface. Interstellar clouds of gas between stars also may contribute material as the solar system sweeps through them on its way around the spinning Milky Way.
Whatever their source, atoms and molecules from around the lunar surface repeatedly rise and fall during the long lunar day, energized by the sun's warmth, researchers say. After bounding around the moon's sunlight half, they come to rest on the surface during the lunar night. Over time, they randomly encounter a polar crater, with temperatures so cold the atoms and molecules flash freeze to the soil there when they fall in.
It will take additional strike-the-moon missions to analyze the debris in ways that will help distinguish among the various sources for the compounds found in the moon's polar cold traps, Schultz says.