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 is 21 kilometres wide and about as deep as the Earth's oceans.
"We decided we would study the living daylights out of this crater," said lead investigator Maria 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 pole taking millions of images 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 from spots inside the crater and how much infrared light was absorbed and reflected by the crater, a property known as the albedo 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 nearby craters, an indication that ice could be present. They estimated that if the relative brightness was being caused by ice, it was likely to be contained in a micron-thick layer on the crater floor that 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 less damaged by the micrometeorites that bombard the moon than have other parts of the lunar surface because the floor remains permanently in the sun's shadow.
Thanks to the specific tilt of the moon's axis, both its north and south poles 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 moon to possibly support human life in the future.
Scientists have long suspected that the deep craters located at the two poles of the moon might contain frozen water since parts of their interior are always in shadow, with some areas not having seen the sun in billions of years, and they are some of the coldest surfaces in the solar system, with temperatures of 40 Kelvin, or about –233 C.
These dark crater regions, scientists speculated, would make perfect "cold traps" for volatile water molecules deposited on the lunar surface during collisions with comets. The water molecules, scientists believe, evaporated from the surface 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 craters for signs of water, and deposits of ice and water vapour have been detected at 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.
One 2008 study published 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 an email. "The  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."
Topography a clue to crater'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 also provide 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 occasionally illuminated by sunlight, they figured the brightness was not likely to be caused by the presence of ice.
"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 to water 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 that the crater walls were brighter because they were younger than both the crater rim and the floor.
"This is because there have been landslides and other downslope movement on the walls that covered up wall craters," Zuber said. "When material falls downslope, it exposes fresher, brighter material from below."
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.
The findings are published in Wednesday's edition of the journal Nature.