IT IS the best of times and the worst of times for lunar scientists. “We’ve got a revolution going on in our understanding of the lunar surface,” says Rick Elphic of NASA Ames Research Center in Mountain View, California. Three recent missions have found an unexpectedly large supply of water on the moon that could both quench the thirst of future lunar dwellers and produce fuel for missions to other places in the solar system.
Yet the prospect of astronauts getting there any time soon is receding fast. In February, President Barack Obama announced his intention to cancel NASA’s Constellation programme, which included plans to get astronauts back to the moon by the early 2020s. His decision leaves the US without a reliable means of transport to low Earth orbit, let alone the moon.
Even so, the 2010s are shaping up to be a boom time for lunar science. The Obama administration’s plans give strong financial support to robotic exploration of the solar system, including the moon. NASA already has four robotic missions in the works, designed to explore the moon’s atmosphere, gravity and seismology. And one of the three finalists to be NASA’s next medium-sized mission is a robot called MoonRise, which would land in the vast South Pole-Aitken basin, dig up soil samples and return them to Earth. China and India, too, are planning follow-ups to their successful Chang’e and Chandrayaan orbiters, and Russia and Germany have missions in development.
Advertisement
What has reinvigorated lunar science most of all are the discoveries of water made last year by NASA’s (LRO) and (LCROSS), as well as India’s .
“Not only is there water on the moon,” says , the chief scientist for the , “but there are three different kinds of water.”
Pieters is referring to the discovery in 2008 of trace amounts of water in volcanic glasses from deep in the moon’s interior, surface water detected by Chandrayaan-1, and buried water at the poles dug up by LCROSS. “They are all different, and they all have different sources and implications,” she says.
“This is not your father’s moon,” says Greg Delory, a space scientist at the University of California, Berkeley. “Rather than a dead and unchanging world, it could be a very dynamic and interesting one.”
“The moon is far from a dead and unchanging world”
Support for a wet moon has ebbed and flowed throughout history. After Galileo’s telescope proved in the early 1600s that the moon was another world, it was widely assumed it had water. The moon’s dark blotches were named “maria”, Latin for “seas”. Yet by the end of the 17th century the evidence was already beginning to point towards a dry moon. More powerful telescopes discerned craters within the maria, which would not be visible if they were indeed oceans.
Fast forward to 1969, and the consensus had swung firmly in favour of an arid moon. The two Apollo 11 astronauts who landed on one of those so-called seas found it to be a dry lava plain. Back on Earth, the Apollo rock and soil samples were pronounced “dry as a bone”. In fact, they were a great deal drier, because living bone is never less than 10 per cent water by weight.
But even then, there were cracks in the orthodoxy. Lunar soil contains hydrogen ions carried from the sun by the solar wind. Constantly bombarding the moon, these ions can knock atoms loose from rocks, creating a plethora of oxygen atoms with dangling bonds. This could mean that hydrogen ions might attach to the oxygen to form hydrated minerals, hydroxyl ions (OH–) or water (H2O), one ion or molecule at a time.
of NASA’s Johnson Space Center in Houston, Texas, found evidence that this was more than a theory. In 1977, he showed that the Apollo rocks released water and hydroxyl when heated. Nevertheless, he failed to change the conventional wisdom that moon rocks were dry. There was a possibility that the samples had been contaminated by the supposedly dry nitrogen they were stored in on Earth. “The ‘dry’ nitrogen had about 20 parts per million of water,” says , who studies lunar rocks at the University of Tennessee in Knoxville. Alternatively, heating could have caused the hydrogen implanted by the solar wind to react with iron oxides in the rock, so that the water was actually formed in Gibson’s lab, not on the moon.
Around the same time, James Arnold of the University of California, San Diego, suggested looking for water near the lunar poles, which were not visited by Apollo. Because the moon’s poles receive only grazing sunlight throughout the year, Arnold pointed out that the base of a deep crater near a lunar pole would never see any light. Such craters could act as “cold traps” for water or any other volatile compounds in the tenuous lunar atmosphere.
Taylor compares the cold traps to a glass of ice tea on a summer day: “You can sit there and watch it gather all the moisture out of the air, and start dripping water. That’s what the poles are doing.”
It took 15 years before the “ice tea” theory was tested. In 1994, radio telescopes on Earth picked up radar waves bounced off the moon’s surface by a satellite called . The results were tantalisingly inconclusive. Clementine got only one good look at the south pole and while the radar did pick up what looked like the signature of water ice, the same type of signal could have been produced by the roughness of the lunar surface.
In 1998, orbited the moon carrying a neutron spectrometer, an instrument widely used in oil-prospecting to look for water and hydrocarbons. It measures the amount of hydrogen present, which is a plausible proxy for the amount of water. Lunar Prospector definitely saw hydrogen – enough that the soil could contain 1.5 per cent water by weight. Of course, that depends on whether you believe the assumption that water accounts for the hydrogen. The hydrogen atoms could instead be unattached, or be present thanks to other hydrogen-bearing compounds such as methane or ammonia. Also, Lunar Prospector could not resolve features smaller than 50 kilometres, so it could not tell whether the putative water was indeed concentrated in the permanently shadowed craters or spread out uniformly.
Lunar dew
The real test would be to land on the lunar surface and dig up a sample of the soil. Lunar Prospector did the next best thing: it crash-landed near the lunar south pole in 1999. Mission controllers had hoped to blast enough water from the moon to be able to detect it from Earth – but none was seen. A similar attempt with a European Space Agency probe called in 2006 also failed to detect any signs of ice. Either the water didn’t exist, or the two spacecraft were too small, hit the moon too glancing a blow, or simply hit the wrong places.
The stage was now set for two discoveries that would radically change our picture of the moon. In October 2008, India launched Chandrayaan-1, equipped with an imaging spectrometer, an instrument which had never gone to the moon before. It allows us to determine exactly which chemicals occur at which pixels in an image.
Soon after Chandrayaan-1 began mapping the moon, Pieters and her team realised that they were seeing large areas of hydroxyl and water. At first they couldn’t believe their results, but team member Roger Clark remembered seeing something similar when he worked on the mission to Saturn and its moons.
Cassini also carries an imaging spectrometer. Like an athlete going through practice workouts before the real competition, Cassini trained its spectrometer on the moon during a fly-by in 1999.
During a team discussion last year, Clark mentioned Cassini picking up hydroxyl and water on the moon. Then Jessica Sunshine, a Chandrayaan-1 team member who was also working on the Deep Impact mission to visit comets, spoke up. “Deep Impact is going by the moon in June, so I’ll be able to tell you what it sees.” It also carries an imaging spectrometer.
Sure enough, Deep Impact confirmed the Cassini and Chandrayaan-1 measurements. “Both of those instruments really helped us understand what was occurring there,” says Pieters. In fact, Deep Impact added a new piece of information: the water and hydroxyl seem to disappear during the lunar day, only to reappear at sundown and sunrise, like dew. This “dew” is, however, only a few molecules deep, so the lunar soil is still drier than the driest desert on Earth.
The discovery of mobile surface water was so unexpected that scientists are still debating where it comes from. One possibility is that individual molecules are created by the solar wind and hop around the lunar surface for a while, until they either get caught in a cold trap or escape out to space.
Cometary impact
Lunar scientists received another jolt when LCROSS crashed into a crater called Cabeus last October. LCROSS was designed to do what Lunar Prospector and SMART-1 could not. It walloped the moon head-on with a spent rocket engine the size of a large van, followed by a “shepherding satellite” that observed the impact from directly overhead before it, too, crashed into the moon 4 minutes later. It had the best possible vantage point for seeing whether the debris contained water.
In choosing to switch the impact site to Cabeus, LCROSS’s lead scientist Tony Colaprete had gone for a very deep crater with a very high ridge on its edge, which effectively blocked astronomers’ view of the impact from Earth. It was a calculated gamble. From earlier satellite observations, Cabeus appeared to have the highest concentrations of water, and Colaprete was betting that the shepherding satellite would be able to see it without any help from Earth-based telescopes.
“When Tony told us he was changing the [impact] site, we honestly thought we were screwed,” says Pete Schultz, a member of the LCROSS team. Indeed, from Earth the impact did at first seem to be a dud, but within days the team knew they had hit a water-rich site. The spectra showed clear signs of water, and the team voted to go public with their findings in November, a month ahead of schedule.
It is now certain that at least 1 per cent of the excavated material was water ice, perhaps as much as several per cent. To put it in simpler terms, the LCROSS impact site was damper than the driest desert on Earth.
But the biggest surprise was an abundance of other stuff in the debris plume, especially volatile compounds like carbon dioxide, ammonia, sulphur dioxide, methane and ethylene. “Maybe we can distil alcohol and make a little moonshine!” jokes at the University of Colorado, Boulder.
While water can be created on the moon by the solar wind, these other compounds cannot. If they are present in Cabeus, it is likely that they came from an outside source – a meteoroid or cometary impact. That could mean that localised deposits are far richer in water than the solar wind theory predicts.
Since November, LCROSS’s sister spacecraft, LRO, has continued to gather information. One of its instruments, Diviner, has measured the lunar temperature from orbit. Instead of guessing where cold traps are likely to be on the basis of solar illumination maps, we can now measure where they are. In many cases, they extend beyond the range of permanent shadow.
Diviner has also found that the shadowed regions themselves are colder than anyone expected. In fact, some of them are even colder than Pluto, where temperatures of 30 to 40 kelvin are typical.
Meanwhile, LRO’s neutron spectrometer, called LEND, is refining Lunar Prospector’s maps of hydrogen deposits. Already LEND has obtained results that are sharply at variance with the ice tea theory, identifying regions that are potentially as hydrogen-rich as Cabeus but are not located anywhere near a permanently shadowed crater. It will take time for researchers to assimilate these results, determine how reliable they are, and develop new hypotheses to account for them.
The discovery of water raises all sorts of scientific questions, which inevitably morph into questions about resource extraction. , chair of NASA’s Lunar Exploration Analysis Group, checks off a few of them: “Where does the water come from? How long has it been there? How much is there? What form is it in? Is it a sheet, or is it patchy? What are the complications of extracting it from an environment that is around 30 or 35 kelvin? How much is it going to cost?”
The best way to answer these questions, most lunar scientists agree, is to send robotic landers to the moon. Ideally, they would be rovers, like the ones that have been so successful on Mars. A rover could carry a neutron spectrometer, for instance, as a high-tech dowser for subsurface water. Here the interests of science dovetail nicely with the proposed NASA budget, which includes a $3.2 billion programme of “robotic precursor missions”. Though these will probably have various destinations, it is a good bet that one of the first will be the moon. Because the moon is only 1.3 light seconds away, scientists on Earth could operate a robot in near-real time – a distinct advantage when compared with the way the Mars rovers are controlled.
Given the scarcity of money and the uncertainties of politics, it is still unclear how to get from here to a sustainable human presence on the moon. For the next 10 or 20 years, the only Earth creatures on the moon will have silicon brains. However, the year 2009 may well go down in history as the year we struck water on the moon.
Gas station in the sky
Lunar water and other volatiles will be essential ingredients for any sustained human presence in space – if that presence ever materialises.
, Texas, divides space exploration into three stages: “Arrive, survive and thrive.” We have already arrived at the moon, thanks to Apollo. To survive, we don’t necessarily need lunar water. “You can take your water with you and recycle,” says Taylor. But the availability of water makes survival much easier, because it frees future astronauts from umbilical dependence on Earth. With locally produced water, they can drink, create a breathable atmosphere, make concrete, build a shelter against cosmic rays and grow plants.
For us to thrive on the moon, water and volatiles are even more crucial. As Taylor says: “The moon is a gas station in the sky.” The economic value of water – not to mention the frozen methane that was also detected by LCROSS – is as a propellant for interplanetary travel. The water can be converted for use as fuel by separating it into oxygen and hydrogen, while the methane is useful as it is. Both of these substances would be more expensive than gold if they had to be brought from Earth; far cheaper to mine them on the moon, if at all possible.
At the , Spudis estimated that there are 600 million tonnes of water in the moon’s north polar region, which would be enough to launch one space shuttle a day for 2000 years. While that’s a cute factoid, not all the water will be easy to extract. Spudis’s radar instrument on Chandrayaan found near the north pole, with the best examples on the floors of the Peary and Rozhdestvensky craters. All of them do have permanently shadowed regions, but the presumed water deposits extend outside of the shadow. Other nearby craters that look similar do not have water deposits for reasons that are still unclear.
