N. Gopal Raj
A paper by a team of Japanese scientists is bound to fuel the controversy further.
Whether or not water in the form of ice exists on the Moon is a hotly debated issue. In fact, India’s Chandrayaan-1 spacecraft, which was launched on Wednesday, is carrying a U.S.-built radar that is specifically intended to look for water-ice at the lunar poles.
Meanwhile, a paper from a team of Japanese scientists that is coming out this week is bound to fuel the controversy further. Based on images from a camera on Japan’s Kaguya lunar probe that was launched last year, their findings question whether such ice does indeed exist.
On the Moon’s surface, any water would be rapidly vapourised by the heat of the Sun’s rays and lost to space. But since the early 1960s, it has been argued that some water might have been transported to the polar regions. There, over millions of years, the water could have accumulated as ice at the bottom of craters that are permanently in shadow. Indeed, two U.S. spacecraft that went to the Moon in the 1990s found indications of water-ice in the permanently shadowed areas at the poles.
Now, Junichi Haruyama of the Japan Aerospace Exploration Agency and other scientists have studied images taken by Kaguya’s Terrain Camera of the Shackleton crater at the lunar south pole, which is one of the prime locations on the Moon where it is believed water-ice could be found.
Imaging the interior of the crater, which never receives any direct sunlight, is not easy. “Our observation was based on the idea that the [permanently shadowed] area is weakly lit by sunlight scattered from nearby higher terrain,” said Dr. Haruyama and the others in a paper being published this week on Science Express, the advance electronic publication wing of the journal Science. For their analysis, the scientists examined images captured at the end of 2007 when illumination of the area around the Shackleton crater was maximum.
The team found that the temperature of the crater floor was low enough to hold ice, said Dr. Haruyama in an email to this correspondent. “However, the images show lack of water-ice deposits.”
In their paper, the scientists wrote, “there is no extensively exposed pure water-ice deposit occupying a larger area than seen in the [Terrain Camera’s] spatial resolution [of 10 metres].” It could be that the ice was “dirty” by being mixed with lunar soil.
Also, the water-ice may be buried under a thin layer of lunar soil, said Dr. Haruyama in his email.
The Japanese are not the only ones to cast doubt on there being water-ice on the Moon. Three years ago, a team of U.S. astronomers led by Donald Campbell of Cornell University used two radio telescopes in the United States like a radar to bounce radio signals off the Moon.
In a paper published in the journal Nature, Prof. Cambell and others reported that the distinctive polarisation properties of the returning radio signals, which are normally associated with icy surfaces in the Solar system, had been found not just in the Shackleton crater but also in sunlit areas where ice would not exist. They argued that the polarisation properties they observed were “strongly correlated with the rock-strewn walls and ejecta of young craters.”
But Paul Spudis, a proponent of the view that water-ice is likely to be found on the Moon, is unmoved. Dr. Spudis, who is currently with the Lunar and Planetary Institute at Houston in the United States, is the principal investigator for the Mini Synthetic Aperture Radar (MiniSAR) that the India’s Chandrayaan-1 spacecraft is carrying to the Moon.
The Japanese team was claiming that it did not see any evidence of ice inside the Shackleton crater, remarked Dr. Spudis who was at Sriharikota for the launch of Chandrayaan-1. “But the simple fact is no one that I know of ever predicted that there would be pools of ice visible to imaging from a camera if you were able to illuminate the [crater] floor. So effectively they have knocked down a straw man of their own creation,” he told this correspondent.
Dr. Spudis was deputy leader of the science team for the Clementine mission that the U.S. sent to the Moon in 1994. The team improvised an experiment in which the satellite beamed radio waves at the lunar south pole and picked up the return signals back on Earth. The Clementine team argued that water-ice in the permanently shadowed region at the south pole was the probable explanation for the polarisation response seen in the radar echo.
“Now, if you go back and look at the paper that we wrote in 1996 when we talked about the Shackleton crater, we basically said that the ice, if there was ice in the crater, it was patchy and dirty,” said Dr. Spudis. “And effectively that is exactly what you see in this [Japanese] image. ... There is no reason to suppose that ice inside a lunar polar crater would be smooth and mirror-like or even white, because it is all going to be coated and mixed in with dirt.”
As for the findings of Prof. Campbell and his team, Dr. Spudis thinks they were incorrectly interpreting their Earth-based radar data. In a 2007 article in The Space Review, an online publication, he said that the same factor need not have been responsible for the distinctive radar polarisation seen from the dark Shackleton crater and the sunlit Schomberger G. crater. Ice as well as surface roughness could both produce that same radar signature. If the radar signature came from a spot in sunlight, it must be due to surface roughness. “But, if it’s in permanent darkness, ice cannot be ruled out,” he noted.
In the wake of all this controversy, the data from Chandrayaan-1 MiniSAR will be even more eagerly awaited. After the satellite enters lunar orbit and begins to circle the Moon from pole to pole, the radar will be able to look down into the permanently shadowed polar areas up to a depth of a few metres.
With the MiniSAR, “for the first time, we will map the dark areas of the poles using an appropriate radar, but also at the optimum viewing geometry,” said Dr. Spudis. The MiniSAR “will map both poles at a uniform resolution, at uniform illumination conditions and look for the signature of the possible presence of water-ice.”
But even the MiniSAR data may not prove that water-ice exists. “But what we hope to do is to collect enough data such that we will create candidate sites so that we will know where to look in the future when we actually land on the Moon and sample the material,” he added.