Weird Science | A hop for a lander, a leap for ISRO, a giant stride in knowledge
Moon samples brought back by American and Soviet missions taught us about the solar system. Now India wants to contribute
The Moon is 40 million years older than previously believed, with a new estimate putting its age at 4.46 billion years instead of 4.42 billion years. The finding is based on the analysis of lunar dust brought back by astronauts on NASA’s Apollo 17 mission in 1972.
What this goes to show is the importance of studying samples brought back by lunar expeditions. Apart from the new analysis, published in Geochemical Perspectives Letters on October 23, lunar samples are topical for another reason. The Indian Space Research Organisation (ISRO) wants to return samples to Earth in future expeditions.
ISRO’s optimism stems from the success of its Chandrayaan-3 mission, specifically a “hop” performed by the Vikram lander, which demonstrated that it could take off from one place and land on another part of the lunar surface. That could be a small step towards developing the technology to bring back future landers to Earth — with samples from the Moon.
If this does happen, the source of those samples may be key. Chandrayaan-3’s Vikram landed on the lunar south pole, the first lander to do so, and no sample from that region has ever been brought to Earth yet. ISRO has not announced the schedule or landing site for Chandrayaan-4, a collaboration with Japan, but should it choose the South Pole again, samples from there would open up new avenues for research.
The moon and its mysteries
In the composition of lunar samples lies the key to answering many questions about the moon, including how it came into being. In the process, we could also learn more about the early history of the solar system, including that of the Earth.
The study of moon rocks for such purposes, in fact, has been going on for decades already. Successive missions by the US (Apollo) and the former Soviet Union (Luna) have brought back several samples, which have been shared with research organisations elsewhere. These include the Physical Research Laboratory (PRL) in Ahmedabad.
PRL was among the first to receive rocks from the crewed Apollo 11 mission in 1969 and continued to receive more until Apollo 17 in 1972. In between, the USSR too presented PRL with samples from two Luna missions.
The six Apollo missions brought back 2,200 samples weighing a total of 382 kg, and nearly 400 samples are distributed each year for research and teaching projects, according to NASA, whose Johnson Space Center has a repository called the lunar sample building. Researchers study the composition of these rock and soil samples and try to figure out, among other things, how the moon was formed, and when.
There are various theories about the formation of the moon, but one has gained precedence over the others. According to this leading theory, when the Earth itself was young, a Mars-sized object, informally known as Theia, struck the evolving planet. This caused much of both objects to melt and merge into a new object, which is now Earth, while a small portion spun off to form the moon.
Once the crust of the moon was formed, it kept being bombarded by meteorites, resulting in an outpouring of lava. Meanwhile, radiation from the sun kept getting trapped in the lunar soil. The lava outpourings and the trapped radiation remain a permanent record of the activity that has taken place on the moon.
The reason this theory is widely accepted today is the knowledge gained from lunar samples; their composition and age are consistent with such a chain of events.
“We think that the Moon formed from a collision between an impactor and the proto-Earth. Models make predictions on what the moon should look like, and how old it should be. By comparing analyses and model predictions, we can better understand how the moon formed,” University of Chicago planetary scientist Nicolas Dauphas said over email.
Dauphas, who has previously analysed samples brought by Apollo, was not involved in the study giving a new estimate for the Moon’s age.
Dust tells moon's age
In 1972, Apollo 17 astronauts sampled a lunar valley called Taurus-Littrow. The dust samples they brought back contained tiny crystals of the mineral zircon, which had formed billions of years ago. Researchers from the Fields Museum, University of Glasgow and Northwestern University used a new analytical method, called atom probe tomography, to determine the age of the oldest known lunar crystal. Ultraviolet lasers evaporate atoms on the surface of the sample, and the atoms are then taken through radiometric dating. The sample was found to be 4.46 billion years old; the Moon must be at least that old.
All this is based on the understanding that the zircon crystals are no older than the moon itself. If the impact with Theia did happen, sending debris spinning off to form the moon, the crystals would have formed only after this molten mass cooled. This discounts the possibility that crystals from elsewhere, possibly older than the moon, were deposited on the lunar surface after the satellite had formed.
“A non-lunar source for these grains is highly unlikely, as the mechanism for depositing a zircon [crystal] in these rocks at this time period from a source that had zircons is a much more convoluted and complicated situation than in-situ formation,” the study’s lead author, Jennika Greer, an earth scientist at the University of Glasgow, said over email.
Taurus-Littrow is part of the lunar highlands, which preserve older material than plain regions called the lunar maria, Greer said. The shadowy south pole, on the other hand, is expected to provide clues to processes that took place subsequent to the moon's formation.
The promise of more lessons
Since the moon formed, asteroids have hit it several times and created craters, many of which are visible as pockmarks even from the Earth. The size of a crater gives a measure of its age, and age-dating a moon crater also allows scientists to estimate the age of craters on other celestial objects.
Dauphas of UChicago said studying moon samples can also inform scientists how the satellite cooled and differentiated. While many studies have gone into these aspects, there is still much to learn. That makes it important to look at samples from different lunar regions, if scientists get them.
“We have explored only a small part of the moon. It is the same as the earth. You would not be able to tell the whole story of Earth’s origin and evolution with just a handful of samples from one location,” Dauphas said.
At this stage, we don’t know if and when ISRO’s future missions will return lunar samples, or even which lunar regions the missions will go to. The fact remains, however, that Chandrayaan-3 has already landed on the south pole, which raises interesting possibilities.
“While the moon-forming event may have been the starting point for the moon, that doesn't mean that subsequent processes affected the moon uniformly. If you were interested in the ices that form in the shadowed regions of the moon, the south pole is a better place to go to than the [lunar maria],” Greer said.
Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but it isn't.