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Astronomers comb through Moon smash haul

Full analysis of plume from lunar crater impact throws up water ice, mercury, silver, hydrogen, and hydrocarbons.

Debris ejected from the Cabeus crater into the sunlight, about 20 seconds after the LCROSS impact. (Inset shows close up with direction of Sun and Earth). Credit: Science/AAAS

Loose rubble in permanent shadow at the Moon's south pole contains a hotchpotch of volatile materials, including hydrogen, mercury, silver, hydrocarbons - and substantial quantities of water ice.

The findings come from the first published results of NASA's LCROSS (Lunar Crater Observation and Sensing Satellite) experiment, which last October hurled a spent rocket into a portion of a crater where temperatures are so cold — about –230ºC — that water vapour from ancient comet or asteroid impacts would freeze permanently.

Although it did not produce as spectacular a blast as anticipated (see 'Moon smash gives off flash'), the impact created a crater 25–30 metres wide, and lofted a few tonnes of dust and vapour about a kilometre above the surface. That was analysed by LCROSS's trailing observation satellite, which watched the site for 4 minutes before it too crashed into the Moon, and by NASA's Lunar Reconnaissance Orbiter (LRO),which passed within 80 kilometres of the impact.

Anthony Colaprete, a space scientist at NASA's Ames Research Center in Moffett Field, California, led the LCROSS team, which announced its discovery of water in the crater last November (see 'Lunar impact tosses up water and stranger stuff'). The first of a suite of studies in Science reveals that about 5.6% of the thrown-up material was water1.

"Five percent is a lot of water," says Colaprete. "It is about twice as wet as the Sahara desert." That may not sound all that wet by earthly standards, he says, "but if one considers that there are places on the moon wetter than places on Earth, that is a shock!" And he notes, it means that 1000 kilograms of lunar dirt contains more than 12 gallons (45 litres) of water - so that there is a potential reservoir of a billion gallons (3.8 billion litres) in the top metre of soil in the 10 kilometre area surrounding the impact site.

The study published today also reveals that the water is present as relatively clean ice grains, making it fairly easily mined by future residents of a lunar base.

Fluffy target

The impact briefly heated the powdery rubble, or 'regolith', of the Moon's surface to more than 700°C, says planetary scientist Paul Hayne of the University of California, Los Angeles (UCLA). "Water would have been instantly vapourized." Hayne was lead author of a second study, which used an infrared temperature sensor called Diviner, aboard the LRO2.

The glowing dot in the centre of this swathe of colour shows that the impact generated temperatures above 700°C. The data was recorded by the Lunar Reconnaissance Orbiter. Credit: NASA/UCLA

When the LRO's orbit took it past the same area, two hours later, the crater was still relatively warm (about –173ºC). That explains why LCROSS saw a sustained source of water vapour during the 4 minutes before it crashed, says Hayne. "The crater would have been steaming during that entire time."

Because some part of the surface was holding onto the heat, LCROSS must have hit a mixture of ice and surface soil, rather than sheet ice atop a solid surface, says Hayne.

Peter Shultz, a planetary scientist at Brown University, Providence, Rhode Island, who is lead author of a third study3, adds that the explosion happened 0.4 seconds after the impact. "That's characteristic of a very fluffy target," he says.

Cold trap

The region targeted by the impact was fiercely cold — with temperatures estimated for those of Pluto, or for comets in the outer regions of the Solar System, adds Hayne's colleague, David Paige, another UCLA planetary scientist.

The extreme chill in this area is caused by the Moon's tilt to the Sun, which effectively creates patches of ground in permanent shadow next to crater rims at the poles. The low temperature means that ice persists indefinitely. Other regions nearby, Paige's team discovered, are also very cold, even though they sometimes see sunlight4. "The cold trap is much larger than we'd thought," he says. At the LCROSS impact site, he adds, the temperature was about –233ºC just before the time of impact.

That is cold enough to keep a lot more than water in situ. Using an ultraviolet spectrograph known as LAMP, also aboard the LRO, a team led by Randy Gladstone of the Southwest Research Institute in San Antonio, Texas, found trace gases including hydrogen, calcium, magnesium, carbon monoxide and mercury5. Schultz adds that the LRO instruments also found light hydrocarbons, sulphur-bearing compounds, carbon dioxide and silver.

"What's interesting is that there's a lot there, and it's not just water," says Humberto Campins, a planetary scientist at the University of Central Florida, Orlando, who was not involved with the LCROSS mission.

The hydrogen was good news in the context of a possible Moon base. Its presence in the plume means that if you heat up the soil, "hydrogen comes pouring out", Gladstone says. "You don't have to break up the water to make rocket fuel."

Both mercury and silver — whose gases are too heavy to escape easily into space — are likely to have made their way to the cold traps at the lunar poles after being released from lunar soil elsewhere, by comet or asteroid impacts. "This isn't simply the solar wind implanting [elements] into the soil. This comes from things getting delivered to the Moon, migrating, and finding this place," explains Schultz.

The lunar cold traps might preserve materials brought into the inner solar system by ancient bombardments, agrees Campins - such as from asteroids; on two of which water ice has been spotted.

Like Earth's polar ice caps, Hayne adds, the Moon's polar cold traps contain much information about its geological history. But the deepest ice cores dug from Earth's poles are only a few hundred thousand years old. The Moon's poles contain material that could have been frozen there for billions of years. "We're turning a brand new page in the study of the Moon," says Schulz.

References

  1. Colaprete, A. et al. Science 330, 463-468 (2010).

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  2. Hayne, P. O. et al. Science 330, 477-479 (2010).

  3. Schultz, P. H. et al. Science 330, 468-472 (2010).

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  4. Paige, D. A . et al. Science 330, 479-482 (2010).

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  5. Gladstone, G. R. et al. Science 330, 472-476 (2010).

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Lovett, R. Astronomers comb through Moon smash haul. Nature (2010). https://doi.org/10.1038/news.2010.556

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