N. Gopal Raj
Interest in lunar exploration has flared up anew. What’s left to find out about the Moon?
It was nearly 50 years ago that the Soviet Union sent the world’s first spacecraft to the Moon. But the sphere-shaped Luna 1 did no more than fly past Earth’s natural satellite at a distance of several thousand kilometres in January 1959 before settling into an orbit around the Sun.
During the next 15 years or so, in a frenetic burst of technological one-upmanship, the Soviet Union and the United States despatched over 40 more spacecraft to photograph and study the Moon in great detail. In July 1969, Neil Armstrong became the first of a dozen men to set foot on it in the course of the Apollo programme. Those astronauts brought back close to 400 kg of lunar soil and rock samples. The Soviets relied on robotic craft and rovers to explore the Moon’s surface and return samples.
Once the space race ended, interest in sending spacecraft to the Moon rapidly waned. After Soviet Union’s Luna 24 brought back samples in August 1976, the small Japanese probe, Hiten, journeyed to our cosmic neighbour only 14 years later. Then the U.S. sent two spacecraft, the Clementine in 1994 and the Lunar Prospector in 1998.
After such an intense burst of space exploration and careful analysis of the lunar data and samples that were garnered, one would think that the Moon has become a well understood entity with much of the scientific juice already wrung out of it.
But interest in lunar exploration has flared up anew in recent years. In 2003, Europe sent the SMART-1 spacecraft. Last year, Japan’s Kaguya and China’s Chang’e-1 probes followed and are at present circling the Moon. India’s Chandrayaan-1 is currently scheduled to set off for the Moon on October 22. Early next year, the U.S. is planning to send the Lunar Reconnaissance Orbiter.
So what’s left to find out about the Moon?
“We know more about many aspects of the Moon than about any world beyond our own, and yet we have barely begun to solve its countless mysteries,” states a report from the U.S. National Research Council that was published last year. The report, titled ‘The Scientific Context for Exploration of the Moon’ and prepared at the request of the National Aeronautics and Space Administration (NASA), is eloquent about how much more the Moon has to offer science.
“The Moon is, above all, a witness to 4.5 billion years ... of solar system history, and it has recorded that history more completely and more clearly than any other planetary body. Nowhere else can we see back with such clarity to the time when Earth and the other terrestrial planets were formed and life emerged on Earth.” Besides, the Moon’s proximity makes it accessible to a degree that other planetary bodies are not.
The U.S. is interested in looking for resources that could support future human exploration of the Moon, but “that is not [our] primary goal,” said J. N. Goswami, director of the Physical Research Laboratory (PRL) at Ahmedabad. In the Indian Space Research Organisation, PRL will have a major role in the analysis of the scientific data sent back by Chandrayaan-1.
“We still feel that in spite of all these [earlier] missions, our understanding of many aspects of the Moon is very rudimentary,” he told this correspondent. Many hypotheses were based on samples brought back by the Apollo and Luna missions from a few places on the Moon’s Earth-facing side. But quite a few of these views were not supported by the comprehensive lunar surveys that the Clementine and Lunar Prospector spacecraft carried out, he said.
During the two years that Chandrayaan-1 is scheduled to spend orbiting the Moon, the stream of data from its suite of 11 instruments, several of which are supplied by the U.S. and Europe, will cast new light on many of these issues and perhaps help resolve some of the current controversies.
How Earth came to acquire so large a moon is still a big mystery. Currently the most favoured hypothesis is that a Mars-sized body, given the name Theia, slammed into Earth some 4.5 billion years ago. The vast cloud of debris and vapourised material thrown into space by the gargantuan collision is thought to have later coalesced to form the Moon.
It is believed that in its early days the Moon may have been covered with molten rock (or magma). Then a crust solidified, made up of lighter minerals that floated to the top.
“But we don’t know whether the magma ocean covered the whole Moon or how deep it was,” according to Narendra Bhandari, a leading planetary scientist who was closely involved in drawing up the scientific programme for the Chandrayaan-1 before he retired from the PRL.
“We need to have detailed information about the chemical and mineralogical composition” of the Moon and how the composition changes with depth, said Dr. Bhandari. Those who study the Moon would like to know how many layers make up its crust, the composition of the mantle (the part of the interior of Moon below the crust) and so on, he remarked.
It is this sort of data that Chandrayaan-1 has been configured to provide. Modellers will then be able to use the information to try and figure out the Moon’s hidden past.
Chandrayaan-1’s great advantage is that its instruments can survey the Moon in several different ways: using visible wavelengths of light, ultra-violet, infra-red, x-ray, low-energy gamma ray and even radar. Doing so should provide not only the detailed topography of the Moon but also an accurate, high-resolution map of the chemicals and minerals that make it up.
When, for instance, there is a solar flare and more energetic x-rays emanate from the sun, iron atoms in minerals on the Moon are prodded into giving off x-rays with a characteristic energy that can be readily picked up by an instrument on the spacecraft known as the Chandrayaan-1 Imaging X-ray Spectrometer. That information would help calibrate data from other instruments, such as the Hyper-spectral Imager and Moon Mineralogy Mapper, which can then be used to estimate more precisely the amount of iron in minerals all over the Moon.
The iron-to-magnesium ratio is a key number that scientists need to figure out the early stages of the Moon’s evolution, said Dr. Goswami. If all goes well, the Chandrayaan-1 should be able to provide that information with greater accuracy and a resolution that is an order of magnitude better than is currently available, he added.
The pockmarked surface of the Moon, the result of collisions with numerous bodies left over after the formation of the Solar System, offers an opportunity to study its sub-surface composition as well. The Chandrayaan-1’s high-resolution cameras will be able to pick out the “central hill” in craters where material from the interior of the Moon has rebounded and become exposed after such collisions.
Then there is the issue of whether water is present on the Moon. Both the Clementine and Lunar Prospector missions found strong indications that water in the form of ice could be present in permanently shadowed areas at the poles.
Just a few months ago, a group of U.S. researchers reported in the prestigious scientific journal Nature that they had used a new and more sensitive technique to analyse again some of the lunar volcanic soil samples brought back by the Apollo astronauts. They found that these samples still carried minute traces of water, suggesting that the water had come from deep within the Moon. “Thus, the presence of water must be considered in models constraining the Moon’s formation and its thermal and chemical evolution,” pointed out Alberto Saal of Brown University and the other scientists in their paper.
Water could also have been deposited on the Moon by comets and meteorites that crashed into it, and produced locally by interaction of the solar wind with oxygen-bearing minerals. As a result of heating by sunlight, much of this water would have evaporated and been lost to space. But some water might have been transported to places at the poles that never receive sunlight.
Finding water is important for sustaining a permanently manned lunar base. The U.S.-built Miniature Synthetic Aperture Radar on the Chandrayaan-1 is specifically intended to detect water ice up to a depth of a few metres at the poles.
The spacecraft will also look for signs of how volatile substances, such as water, move along the hot, sunlit surfaces of the Moon till they get trapped in shadowed places at the poles. To this end, its High-Energy X-ray Spectrometer will be used to try and pick up faint signals from gamma rays released during decay of a radioactive form of the element radon, which is volatile, said Dr. Goswami.
Other instruments on the spacecraft will make measurements to better model the lunar gravity field and study the radiation environment there. The spacecraft will also drop off an impactor that will crash land on the Moon.
For scientists, the excitement from a mission like the Chandrayaan-1 lies not just in using its data to validate existing ideas about the Moon. A bigger thrill would be coming across new and unexplained phenomena that then open up fresh avenues of research.
“In natural science, you approach the truth but never probably [reach] the whole truth,” says Dr. Goswami.Chandrayaan to be launched on October 22Moon mission is not expensive, says ISROChandrayaan-1 taking final testsCabinet clears Chandrayaan-2Chandrayaan-1 on course for launch