The moon’s surface used to be covered in an ocean of magma before it cooled. Afterwards, it was constantly bombarded by meteorites, which dug up heavier minerals that had sunk to the layers below, according to a study published by a team of Indian researchers in the journal Nature on August 21.
It settles earlier debates about whether the magma ocean covered all of the moon’s surface.
The theory of a magma ocean
According to one theory, the top 1,000 kilometres of the moon’s surface was covered in molten magma immediately after its birth about 4.5 billion years ago.
In the next hundreds of millions of years, heavier minerals like pyroxene and olivine (which are rock-forming mineral groups that are also present in the earth’s mantle) settled below lighter elements like plagioclase (a mineral belonging to what is called the feldspar group, which contain both alumina and silica). As the moon cooled, these elements settled in layers.
Rock samples from the Apollo and Luna missions from near the equator and Chinese missions from the mid-latitudes supported this theory, and the present study using Chandrayaan-3 data from near its poles provides more evidence in its favour.
A team of Indian researchers studied the elements on the lunar soil at about 70 degrees latitude of the moon. They made 23 observations using the Alpha Particle X-ray Spectrometer (APXS) on the Pragyan rover, dispatched by the Vikram lander of Chandrayaan-3 last August.
The team was based in the Physical Research Laboratory (PRL) and Space Application Centre, Ahmedabad, the Hemvati Nandan Bahuguna University in Uttarakhand’s Srinagar, and the Indian Space Research Organisation (ISRO), Bengaluru.
Like the previous missions that studied rocks from the lower and mid-latitudes, the study found rocks of the type ferroan anorthosite, rich in plagioclase.
“The southern region being dominated by these rocks gives strong evidence that these rocks are spread all over the moon, which could only happen because of the magma ocean,” said Santosh V. Vadawale from the PRL, one of the authors of the study.
However, the study found that this type of rock was not the only kind in these latitudes: about one-fourth of rocks contained lighter magnesium minerals, pyroxene and olivine.
These lighter elements were later dug over billions of years by meteorites, which formed craters of various sizes inside bigger craters called basins. Each meteorite carried bits and pieces of these lighter rocks to the top layers, giving rise to a mixture of rock types at these latitudes.
“All the pieces are coming together,” said Vadawale.
Senthil Kumar Perumal, a scientist at the CSIR-National Geophysical Research Institute, Hyderabad, who is not associated with the study, supported the findings. He explained that the location of the Chandrayaan-3 landing site exposed it to a large amount of lighter minerals from the lower layers due to a larger possibility of meteorites’ impact.
“The South Pole Aitken basin is the largest impact basin on the Moon, and the Chandrayaan-3 landing site is located on top of the debris materials that were produced from the … basin,” he said.
But could there be more to the layers?
But not all experts agree with this interpretation of the measurements.
A planetary geologist who did not wish to be named for fear of retaliation said that the results could be explained without the mixing caused by meteorites. They said the mixing was advanced by measurements from low latitudes by the Apollo missions, but the reality could be a more complicated mixture of magnesium-rich rocks near the poles.
The mixing hypothesis rests on the fact that there is a lower fraction of magnesium-heavy minerals in the top layers. However, if there were more pyroxene and olivine-heavy rocks near the poles as the moon cooled, then mixing is not required at all.
There is some evidence for this claim. Lunar meteorites are pieces of moon rock launched from various parts of the moon by meteorites that eventually land on Earth. Since 1979, close to 400 lunar meteorites have been discovered. Scientists studying these rocks have proposed a complicated mixture of magnesium rocks at high latitudes, which could naturally explain the present observations, said the expert.
Vadawale agreed that the interpretation of their Nature study is debatable, but that their measurements of the fractions of the different minerals in the high-altitude rocks are robust. All experts The Wire spoke to agreed.
Debdutta Paul is a freelance science journalist.
