Massive moon crater might contain ice

Scientists probing the three-billion-year-old Shackleton crater at the south pole of the moon believe their observations could support the suspicion that there is ice within the four-kilometre-deep pockmark on the lunar surface.

Researchers at the Massachusetts Institute of Technology, Brown University and NASA's Goddard Space Flight Centre used millions of measurements made by an instrument called a laser altimeter to map the changes in elevation and brightness within the crater, which is21kilometres wide and about as deep as the Earth's oceans.

"We decided we would study the living daylights out of this crater," said lead investigatorMaria Zuber, a professor of geophysics at MIT's department of earth, atmospheric and planetary sciences, in a news release. "From the incredible density of observations, we were able to make an extremely detailed topographic map."

The altimeter used to make the measurements is mounted on a spacecraft called the Lunar Reconnaissance Orbiter, which orbits the moon from pole to poletakingmillions ofimages and measurements of the moon's topography, temperature, chemical composition, ability to reflect light and other characteristics.

In the case of the MIT study, scientists were interested in how long it took for a laser light to bounce back fromspots insidethe crater and how much infrared light was absorbedand reflected by the crater, a property known as thealbedo or natural reflectance.

They analyzed more than five million measurements of Shackleton made by the altimeter and compared them with those for other parts of the lunar surface.

Tilt of moon's axis leaves crater floor in the dark

They found that the floor of the Shackleton crater was brighter than that of other nearbycraters, an indication that ice could be present.They estimated that if the relative brightness was being caused by ice, it waslikely to be contained ina micron-thick layer on the crater floorthat would be made up of about 22 per cent ice mixed with rock.

But the researchers acknowledged that the simpler explanation for the relative brightness could be that the crater floor has been lessdamaged by the micrometeorites that bombard the moon than haveother parts of the lunar surface becausethe floorremains permanently in the sun's shadow.

Thanks to the specific tilt of the moon's axis, bothits north and southpoles contain areas that are in near-permanent sunlight and others that remain dark unique qualities that scientists say make the poles the most likely places on the moonto possibly support human life in the future.

Scientists have long suspected that the deep craters located at the twopoles of the moonmight contain frozen water since parts of their interiorarealwaysin shadow, with some areas not havingseen the sunin billions of years, and they are some of the coldest surfaces in the solar system, with temperatures of 40 Kelvin, orabout 233 C.

Thesedark craterregions, scientists speculated,would makeperfect "cold traps" for volatile water molecules deposited on thelunar surface during collisions with comets. The water molecules, scientists believe,evaporated from thesurface when the sun shone on the moon but were not energetic enough to escape the moon's gravitational field.

"When the sun set, they would re-deposit on the surface," Zuber wrote in an email. "When the sun came back up, they would re-mobilize, and when darkness returned, they would re-deposit somewhere else on the surface.

"This 'random walk' of volatiles would continue until the molecules found their way to a 'cold trap,' that is, a permanently shadowed region."

NASA spacecraft have explored lunar cratersfor signs of water, and deposits of ice and water vapour have beendetectedat both poles, but the mapping of the Shackleton crater has thus far,in the words of the study authors,"yielded conflicting interpretations" with regard to the presence of ice.

One2008 studypublished in the journal Science found that the crater's albedo indicated there were no ice deposits on the crater floor and that "water-ice may be disseminated and mixed with soil over a small percentage of the area or may not exist at all."

"We obtained a different result than the previous measurement, but the two studies are not totally comparable," Zuber said when asked about the earlier study in anemail. "The[2008] paper imaged the crater interior at visible wavelengths and did not detect brightening. In contrast, we detected brightening within the crater, albeit at infrared wavelength."

Topographya clue tocrater's history

The authors of the recent study say that although the new measurements are not definitive proof of the presence of ice, they do offer an "unprecedented topographic characterization" of the Shackleton crater that past radar mapping and optical imaging has not.

They alsoprovide insights into how lunar craters are formed and how their geometry and composition has changed over time.

While measuring the light absorption inside the crater, the study authors found that the crater walls were even brighter than the floor. Since those parts of the crater are occasionallyilluminated by sunlight, they figured the brightness was not likely to be caused by the presence ofice.

"We suspected that there was more than one process involved in the brightening when we noticed that the walls were brighter than the floor," Zuber said. "If the brightening were due only towater ice, we would expect the floor to be at least as bright and probably brighter than the walls.

"This is because water ice would be expected to migrate to the coldest, darkest place in the crater, which is the floor."

Instead, the researchers determined thatthe crater walls were brighter because they were younger than boththe crater rim andthe floor.

The older soil and indentations on the walls were likely sloughed off and covered up during the various seismic shake-ups the moon experiences when it collides with other celestial bodies or is affected by the Earth's gravitational tides, the authors said.

Thefindings are publishedin Wednesday's edition of the journal Nature.