There’s Water On the Moon’s Surface, But Interior Could Be Dry

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With all the recent news of water on the Moon, a new paper published today in the journal Science may offer a surprise – or it may bring us back to previous assumptions about the Moon. A new analysis of eleven lunar samples from the Apollo missions by Zachary Sharp from the University of New Mexico and his colleagues indicates that when the Moon formed, its interior was essentially dry. While the recent findings of ubiquitous water and hydroxyl on the surface as well as water ice in the lunar poles are not challenged by this new finding, it does dispute — somewhat — two other recent papers that proposed a wetter lunar interior than previously thought. “The recent LCROSS findings were of water on the lunar surface due to cometary impacts, and the ice is from the comets themselves,” Sharp told Universe Today. “We are talking about water that was present in the molten early Moon 4.5 billion years ago.”

The accepted theory of how the Moon formed is that a Mars-sized body slammed into our early Earth, creating a big disk of debris that would ultimately form into the Moon.

Although planetary scientists are still refining models of the Moon’s formation, there is much to suggest a dry Moon. Any water would have been vaporized by the high temperatures generated by the impact and cataclysm that followed, and vapor would have escaped into space. The assumption is that the only way there could be water in the Moon’s interior if is the impactor was especially water-rich, and also if the Moon solidified quickly, which is considered unlikely.

But earlier this year, Francis McCubbin and his team from the Carnegie Institution for Science released their findings of a surprisingly high abundance of water molecules — as high as several thousand parts per million — bound to phosphate minerals within volcanic lunar rocks, which would have formed well beneath the lunar surface and date back several billion years.

Additionally, in 2008, Alberto Saal of Brown University and colleagues found a slightly lower abundance of water in the lunar mantle, but it was significantly higher than the previous estimate of 1 part per billion.
These two findings have been pushing lunar scientists to find possible alternative explanations for the Moon’s formation to account for all the water.

But now, Sharp and his team studied a wide range of lunar basalts and measured the composition of chlorine isotopes. Using gas source mass spectrometry they found a wide range of chlorine isotopes contained in the samples which are 25 times greater than what is found in rocks and minerals from Earth and from meteorites.
Chlorine is very hydrophilic, or attracted to water, and is an extremely sensitive indicator of hydrogen levels. Sharp and his team say that, if lunar rocks had initial hydrogen contents anywhere close to those of terrestrial rocks, then the fractionation of chlorine into so many different isotopes would never have happened on the Moon. Because of this Sharp and his colleagues say their results suggest a very dry interior of the Moon.

Sharp proposes that Saal and McCubbin’s calculations of high hydrogen contents in some lunar samples are not typical, and perhaps those samples are the product of certain igneous processes that resulted in their “extremely volatile enrichment.” They do not, however, represent the high and variable isotopic chlorine values reported in the majority of lunar rocks, Sharp said.

Still, there could be a compromise between the varied findings. “There are uncertainties that one has to take into account when doing this type of study, ” Sharp told Universe Today, “and if we take the low estimates of Saal and McCubbin’s papers, they are not so different from our findings.”

But the discrepancies, however small, show that perhaps we can’t make generalizations about the entire Moon from limited samples.

“We have not yet looked for water in a wide range of lunar samples,” said Jeff Taylor from the University of Hawaii, who was not involved in any of the aforementioned studies. “It is quite possible that the initial differentiation of the Moon and subsequent processes such as mantle overturn concentrated whatever water the Moon had into certain areas. Until we measure more samples, including samples from the farside (represented by many of the lunar meteorites and eventually by sample-return missions), we will not know for sure how much water is in the bulk Moon.”

In combination, all the recent studies of the lunar surface show there is likely a complex chemistry on the Moon that we have yet to understand.

“In other words,” said Taylor, “we need more work!”

Source: Science News

Earlier Papers:

Nominally hydrous magmatism on the Moon by Francis McCubbin et al., 2010.

Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior, Alberto Saal et al. Nature.

Water Cycle on the Moon Remains a Mystery

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“Water cycle on the Moon” is a phrase that many people – including lunar scientists – were never expecting to hear. This surprising new finding of ubiquitous water on the surface of the Moon, revealed and confirmed by three different spacecraft last year, has been one of the main topics of recent discussion and study by lunar researchers. But figuring out the cycle of how water appears and disappears over the lunar day remains elusive. As of now, scientists suspect a few different processes that could be delivering water and hydroxyl (OH) to the lunar surface: meteorites or comets hitting the Moon, outgassing from the Moon’s interior, or the solar wind interacting with the lunar regolith. But so far, none of the details of any of these processes are adding up.

Dana Hurley from The Johns Hopkins University Applied Physics Laboratory is part of team of scientists attempting to model the lunar water cycle, and she discussed the work at the NASA Lunar Science Institute’s third annual Lunar Forum at Ames Research Center, July 20-22, 2010.

“When we do the model, we assume the way that the water is lost is through photodissociation, and so that sets the timescale,” Hurley told Universe Today. “And using that timescale the amount that is coming in through the solar wind or micrometeorites can’t add up to the amount observed if it is in steady state, so something is not jiving.”

Photodissociation involves the breaking up of a substance into simpler components by the radiant energy of sunlight.

It appears the amount of water varies over the course of the lunar day. Two observations a week apart by a spectrometer on the repurposed Deep Impact spacecraft (now called EPOXI) showed the region that was near the Moon’s terminator at dawn had a detectable amount of water and hydroxyl, and a week later when it was near noon, those substances were gone. But the new region at dawn then had H2O and OH.

One theory holds that the water and hydroxyl are, in part, formed from hydrogen ions in the solar wind. By local noon, when the moon is at its warmest, some water and hydroxyl are lost. By evening, the surface cools again, and the water and hydroxyl return.

But, Hurley said, the solar wind in steady state does not reproduce the observed surface density of water and hydroxyl.

Additionally, looking at the other possible sources — the known source rate of micrometeoroids and comets — doesn’t provide the amount of observed H20 and OH either.

“We’d really like to have a lot more observations to understand how it evolves over the course of the day,” Hurley said.

Water in Polar Regions on the Moon Credit: ISRO/NASA/JPL-Caltech/Brown Univ./USGS

In her talk, Hurley said her team has been trying to look at all possible angles and ideas, including recent larger comet hits on the Moon, or potentially a seasonal event where water deposited at winter poles could be released when it warms up in summer. But so far none of these ideas have been tested or modeled, and as of now do not provide a solution to the daily cycle of water that was observed.

She also noted that since there are obviously some unique processes going on, the interaction between the surface and atmosphere needs more study.

“The surface and atmosphere are coupled,” Hurley said in an interview with Universe Today. “The atmosphere is produced from the surface; there is no atmosphere that lasts for a long time on the Moon and it is constantly being produced and lost. And so it is coming from the surface, either from something that is coming from the lunar regolith grains or something that is interacting with those grains, whether it is solar wind or something that is impacting. So, the surface is the source of the atmosphere and that atmosphere comes back and interacts with the surface again. And you really have to understand that whole system.”

So, what is her best guess as to the source of the water?

Hurley said there has to be some sort of recycling going on within the regolith, and perhaps a complex surface chemistry that allows the H20 and OH to exist for longer periods of time, which would better explain the surface density.

“What I’ve looked at is what could be happening in the atmosphere and how things hop around from the surface up and then back down to the surface,” she said. “The lunar regolith is rather loose, and these small particles and gases can go down within the regolith and be within the top several centimeters and work their way down and back out. So there is an exchange going on in that top layer that is kind of acting as a reservoir. That is my best guess of what is going on.”

Water Could Be Widespread in Moon’s Interior

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A new look at Moon rocks from the Apollo missions, along with a lunar meteorite show a much higher water content in the Moon’s interior than previously thought. Using secondary ion mass spectrometry (SIMS) which can detect elements in the parts per million range, scientists at the Carnegie Institution’s Geophysical Laboratory found the minimum water content ranged from 64 parts per billion to 5 parts per million—at least two orders of magnitude greater than previous results. The science team says their research suggests that the water was preserved from the hot magma that was present when the Moon began to form some 4.5 billion years ago. “The concentrations are very low and, accordingly, they have been until recently nearly impossible to detect,” said team member Bradley Jolliff of Washington University in St. Louis. “We can now finally begin to consider the implications—and the origin—of water in the interior of the Moon.”

The prevailing belief is that the Moon came from a giant-impact event, when a Mars-sized object hit the Earth and the ejected material coalesced into the Moon. In this new study of lunar samples, scientists determined that water was likely present very early in the formation history as the hot magma started to cool and crystallize. This result means that water is native to the Moon.

The SIMS technique measures hydroxyl by bombarding the grains of a type of phosphorous, water-bearing mineral called apatite with high-energy particles and counting the ions that are ejected. Based on the SIMS measurements, the scientists authors place the lower limit for the total lunar water at 100 times greater than previous estimates, and speculate that water may be “ubiquitous” in the moon’s interior.

The study could alter current theories about lunar magmatism (how igneous rock formed from magma), and how the moon formed and evolved.

Water is showing up in all sorts of unexpected places on the Moon. In September of 2009, a trio of spacecraft detected a ubiquitous layer of a combination of water (H2O) and hydroxyl (OH) that resides in upper millimeter of the lunar surface. It doesn’t actually amount to much; only about two tablespoons of water is believed to be present in every 1,000 pounds (450 kg). Then in October of 2009, the LCROSS impactor and spacecraft detected “buckets” of water in the permanently shadowed region of Cabeus crater near the moon’s south pole.

In 2008 water was found inside volcanic glass beads in Apollo Moon rocks, which represent solidified magma from the early moon’s interior. That finding led to this new study, using the SIMS. The scientists combined the measurements taken with the spectrometer with models that characterize how the lunar magma crystallized as the Moon cooled. They then inferred the amount of water in the apatite’s source magma, which allowed them to extrapolate the result to estimate the total amount of water that is present on the moon.

“For over 40 years we thought the Moon was dry,” said lead author of the new study, Francis McCubbin.

The research is published in the on-line early edition of the Proceedings of the National Academy of Sciences the week of June 14.

Water Ice Found on Moon’s North Pole

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It’s no longer a question of if there is water on the Moon; now it is how much. Scientists using the Mini-SAR instrument on India’s Chandrayaan-1 spacecraft have detected water ice deposits near the moon’s north pole. Mini-SAR, a lightweight, synthetic aperture radar, found more than 40 small craters with water ice. The craters range in size from 2 to15 km (1 to 9 miles) in diameter. Although the total amount of ice depends on its thickness in each crater, it is estimated there could be at least 600 million metric tons of water ice.

“The emerging picture from the multiple measurements and resulting data of the instruments on lunar missions indicates that water creation, migration, deposition and retention are occurring on the moon,” said Paul Spudis, principal investigator of the Mini-SAR experiment at the Lunar and Planetary Institute in Houston. “The new discoveries show the moon is an even more interesting and attractive scientific, exploration and operational destination than people had previously thought.”

During the past year, the Mini-SAR mapped the moon’s permanently-shadowed polar craters that aren’t visible from Earth. The radar uses the polarization properties of reflected radio waves to characterize surface properties. Results from the mapping showed deposits having radar characteristics similar to ice.

Fresh crater, Main L, 14 km diameter, 81.4° N, 22° E. Credit: NASA

“After analyzing the data, our science team determined a strong indication of water ice, a finding which will give future missions a new target to further explore and exploit,” said Jason Crusan, program executive for the Mini-RF Program for NASA’s Space Operations Mission Directorate in Washington.

The results are consistent with recent findings of other NASA instruments and add to the growing scientific understanding of the multiple forms of water found on the moon. Previously, the Moon Mineralogy Mapper discovered water molecules in the moon’s polar regions, while water vapor was detected by NASA’s Lunar Crater Observation and Sensing Satellite, or LCROSS.

Mini-SAR and Moon Mineralogy Mapper are two of 11 instruments on Chandrayaan-1. The Mini-SAR’s findings are being published in the journal Geophysical Research Letters.

Source: NASA

Look for “Flood” of News This Week About Water on the Moon

LCROSS Mission

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Almost five months ago, the LCROSS spacecraft had an abrupt end to its flight when it impacted a crater on the Moon’s south pole. But that was only the beginning of the work of principal investigator Tony Colaprete and the rest of the science teams, who have since been working non-stop to get their initial results out to the public. Look for a flood of ‘water on the Moon’ news to be announced at the Lunar and Planetary Science Conference this week.

“The data set from LCROSS is a lot more interesting that we thought it would be,” said Colaprete, speaking on a “My Moon” webcast, sponsored by the Lunar and Planetary Institute. “A big part of our time has been making sure the data is properly calibrated. That takes a lot of time and effort, but the other side of the equation is understanding all the stuff you don’t understand in the data, and there was a lot we didn’t initially understand.”

The LCROSS team will present six papers, 11 posters and several oral sessions at the LPSC.
While the results are still under embargo, Colaprete was able to discuss the basics of what the science teams have found.

LCROSS impact site. Credit: NASA

One surprise for the teams was the low “flash” produced by the impact of the spacecraft. “We didn’t see a visible flash, even with sensitive instruments,” Colaprete said. “There was a delayed and muted flash and the impactor was essentially buried, with all the energy apparently deposited at a depth. So it is very likely that there were volatiles in the vicinity.”

The second surprise was the morphology of the impact plume. “We had reason to believe there would be high angle plume,” said Colaprete. “But we had a lower angle plume. We had a signal of a debris curtain in the spectrometers in LCROSS all the way down in the four minutes following the impact of the Centaur stage. That was corroborated with DIVINER measurements with LRO (a radiometer on the Lunar Reconnaissance Orbiter.) They were able to make some great observations of the ejecta cloud with DIVINER, and we had good signals with our instruments all the way down to impact.”

Most surprising, Colaprete said, was all the “stuff” that came up from the impact. “Everyone was really excited and surprised about all the stuff that we threw up with the impact.”

The LRO spacecraft was able to be tilted in orbit so the LAMP (Lyman-Alpha Mapping Project) instrument could observe impact plume. It observed a plume about 20 km tall, and observed a “footprint” of a plume up to 40 km above the Moon’s surface.
“They saw vapor cloud fill the ‘slit’ of the spectrometer’s observations at about 23 seconds after impact and it remained there through the entire flyby,” Colaprete said. “What that corresponds to is a hot vapor cloud of about 1000 degrees that was observed.”

A closer view of the moon as the LCROSS spacecraft approaches impact. Credit: NASA

Two exciting species found in the cloud were molecular hydrogen and mercury. “What is fantastic about that, is that there was an article written a couple of decades ago, regarding the possibility of mercury and water at the poles, and they said don’t drink the water!”

Colaprete said observing molecular hydrogen is spectacular because normally it doesn’t stay stable even at 40 Kelvin. The teams are still speculating how it was trapped and what form it was in. They found about 150 kg of molecular hydrogen in the plume.

All the elements found in the plume must be coming from cometary and asteroidal sources, Colaprete said. They also found water ice, sulfur dioxide, methane, ammonia, methanol, carbon dioxide, sodium and potassium. “We haven’t identified everything yet, but what we’re seeing is similar to what you would see in an impact of a comet, like what happened with the Deep Impact probe, which is exciting and surprising. The mineralogy in the dust itself that we kicked up corresponds to what was seen by M Cubed instrument, and also what we see in chondrite asteroids.”

One of the most pleasing aspects of this scientific process, Colaprete said, was the different teams being able to verify what other teams were finding.

“The concentration of hydrogen we saw in the regolith was higher than expected,” Colaprete said. “We ran the numbers again, and we said, ‘Oh, we can’t wiggle out of this answer.’ Then the PI for the LEND (Lunar Exploration Neutron Detector on LRO, which can acquire high-resolution neutron datasets) instrument confirmed that their numbers were entirely consistent with what we got. It was surprising because it wasn’t what we expected. But that is why you make measurements.”

“This should be a fun year as we pull this all together, and get it released to the public so we can get a lot more neurons looking at this,” Colaprete said. “I think this will really change our understanding of the Moon and how we think about it.”

Water on the Moon

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Water has long been suspected to exist in the permanently shadowed polar craters on the Moon, and now the LCROSS impact has allowed scientists to make a direct and definitive finding of this precious resource in a place NASA and other space agencies are considering exploring with human expeditions. Many say this could be a game-changing discovery for the future of lunar science and exploration. Unlike the previous announcement in September of water on the Moon, where water exists diffusely across the moon as hydroxyl or water molecules adhering to the surface in low concentrations, this new discovery could mean underground reservoirs of water ice. “There is too much water to be just absorbed in the soil,” said Anthony Colaprete of the LCROSS mission at Friday’s press conference. “There has to be real solid ice there. You could melt it and drink it.”

But could you really drink it? “Well, not if it has methanol in it. We need to sort out the flavor of the water,” said Colaprete, “meaning we need to find out if it is water, ice, or vapor. We still need to do that math.”

Colaprete said from the amount of water the spectrometers on the LCROSS spacecraft detected, initial indications are it is ice. However, Colaprete added that the impacting Centaur upper stage didn’t hit appear to hit something hard and frozen, from the images of the crater.

If someone was walking on the Moon and was able to walk in Cabeus crater where the impact took place, would the regolith there look different compared to other places on the Moon? “That’s a good question – and we’ve been talking about that,” Colaprete said. “It would be an interesting place to walk around. With our near infrared camera we can relate the the data to what the human eye can see, and try to understand what the terrain looks like. We never saw the crater floor before impact, but now we can see what the floor looks like.”

Did they find anything else in the plume created by the impact? “We’re seeing a lot of stuff,” Colaprete said. “I think there’s a little bit of everything. We’re seeing other emission lines in the spectroscopic data we haven’t completely identified. We’re still working on those — I don’t know what all else is in there just yet. We’ve been focusing on the water quest so far.”

As to whether they’re seeing any organics, the team couldn’t yet say definitively. Colaprete said they are seeing compounds similar to those seen previously in asteroids and comets.

“This is only another snapshot in time of our understanding of the moon,” said Mike Wargo, NASA’s chief lunar scientist, ” and we’ll be continuing to work to get more details on the water and everything else. We’re not done yet.”

More Water on the Moon: Second Instrument Confirms Findings

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In late September, a team of scientists announced finding water molecule signatures across much of the Moon’s surface. Now, a second instrument on board India’s Chandrayaan-1’s lunar orbiter confirms how the water is being produced. The Sub keV Atom reflecting Analyzer (SARA) corroborates that electrically charged particles from the Sun interact with the oxygen present in some dust grains on the lunar surface to produce water. But the results bring out a new mystery of why some protons get reflected and not absorbed.

Scientists likened the Moon’s surface to a big sponge that absorbs the electrically charged particles. The lunar surface is a loose collection of irregular dust grains, or regolith, and the incoming charged particles should be trapped in the spaces between the grains and absorbed. When this happens to protons they are expected to interact with the oxygen in the lunar regolith to produce hydroxyl and water.

The SARA results confirm findings from Chandrayaan-1’s Moon Mineralogy Mapper (M3) that solar hydrogen nuclei are indeed being absorbed by the lunar regolith; however SARA data show that not every proton is absorbed. One out of every five rebounds into space. In the process, the proton joins with an electron to become an atom of hydrogen.

“We didn’t expect to see this at all,” says Stas Barabash, Swedish Institute of Space Physics, who is the European Principal Investigator for SARA.

The Sub Kev Atom reflecting Analyser (SARA)on board the lunar mission Chandrayaan-1.  SARA is the first-ever lunar experiment dedicated to direct studies of plasma-surface interactions in space.  Credits: ISRO/ESA/Swedish Institute Of Space Physics
The Sub Kev Atom reflecting Analyser (SARA)on board the lunar mission Chandrayaan-1. SARA is the first-ever lunar experiment dedicated to direct studies of plasma-surface interactions in space. Credits: ISRO/ESA/Swedish Institute Of Space Physics

Although Barabash and his colleagues do not know what is causing the reflections, the discovery paves the way for a new type of image to be made. Unfortunately, since the Chandrayaan-1 orbiter is no longer functioning, new data can’t be taken. However, the team can work with data already collected to further study the process.

The hydrogen shoots off with speeds of around 200 km/s and escapes without being deflected by the Moon’s weak gravity. Hydrogen is also electrically neutral, and is not diverted by the magnetic fields in space. So the atoms fly in straight lines, just like photons of light. In principle, each atom can be traced back to its origin and an image of the surface can be made. The areas that emit most hydrogen will show up the brightest.

While the Moon does not generate a global magnetic field, some lunar rocks are magnetized. Barabash and his team are currently creating images from collected data, to look for such ‘magnetic anomalies’ in lunar rocks. These generate magnetic bubbles that deflect incoming protons away into surrounding regions making magnetic rocks appear dark in a hydrogen image.

The incoming protons are part of the solar wind, a constant stream of particles given off by the Sun. They collide with every celestial object in the Solar System but are usually stopped by the body’s atmosphere. On bodies without such a natural shield, for example asteroids or the planet Mercury, the solar wind reaches the ground. The SARA team expects that these objects too will reflect many of the incoming protons back into space as hydrogen atoms.

Scientists with the ESA’s BepiColombo mission to Mercury are hoping to study the interaction between charged particles and the surface of Mercury. The spacecraft will be carrying two similar instruments to SARA and may find that the inner-most planet is reflecting more hydrogen than the Moon because the solar wind is more concentrated closer to the Sun.

Source: ESA