We know a lot about the universe. Roughly how much black holes weigh, the atmosphere of planets light-years away and how leaky Uranus is.
WWhat we don’t know too much about is a lot closer than you think – the moon.
Scientists have long believed that our nearest cosmic neighbour holds clues to the history of the entire solar system.
Among them is Professor Yang Li, a geoscientist at Beijing’s Peking University and an honorary professor at the University College London.
His team have discovered that the inside of the mysterious ‘far side’ of the moon may be colder than the side always facing Earth.
The study, published today in the journal Nature Geoscience, looked at soil scooped up by China’s Chang’e-6 lunar lander last June.
The spacecraft dug through the South Pole-Aitken basin, a 1,600-mile-wide impact crater that is among the largest in the history of the solar system.
One of the samples was examined by Professor Yang’s team, who found that the 2.8 billion-year-old soil was formed from underground lava.
What took Professor Yang back was the temperature the 300g sample was formed at – 1,100°C, about 100°C cooler than samples from the near side.
‘Studying the far side isn’t just about curiosity,’ Professor Yang tells Metro.
‘It may hold clues to the early history of the Earth-moon system and planetary crust formation.’
Why is this a big deal?
If you stared up at the moon for one month, you’d only ever see one side of it – the ‘near side’ – because our natural satellite is tidally locked with the Earth, so it keeps the same hemisphere pointed towards us.
Astronomers call the side we can’t see the ‘far side’, sometimes called the ‘dark side’, even though it does see sunlight.
The two sides look very different from one another, Professor Yang says: ‘In plain language, the near side is covered by large dark plains – the lunar maria – created by ancient volcanic eruption.
‘In contrast, the far side is dominated by bright, rugged highlands with far fewer volcanic plains.’
How the moon became two-faced is one of the ‘most important questions that remains to be solved’ about our natural satellite.
Yet seeing the far side, let alone getting to it, is difficult for a simple reason.
‘We cannot see it directly from the Earth,’ says Professor Yang, ‘and we can not send signals directly unless you have a satellite to bridge this.’
This is why estimating how hot very old rocks were when they formed is such a big deal to scientists. They now know that the mantle – the layer between the crust and core – on the far side is cooler.
‘Because the far side has much less basaltic volcanism, it is generally believed that the far side’s thicker crust has prevented magma from reaching the surface. If we assume magmas generated at both sides are the same at depth,’ Professor Yang says.
‘Our study demonstrates that the far side actually is colder inside, hence there have to be fewer heat-producing elements, such as Uranium, Thorium and Potassium, whose decay can generate heat.’
Studies have suggested that this is because an asteroid slammed into the moon, jiggling its insides and pushing hotter elements to the near side.
What the findings also change is the understanding of ‘KREEP’, the chemical residue left behind when the moon’s magma ocean cooled.
Scientists assumed that KREEP only dusted the surface of the moon, but the study suggests it’s in the depths, too, and isn’t evenly spread.
‘Such a new finding also helps us to refine the origin of the moon, and hence the Earth,’ Professor Yang says.
Why we have a giant white-ish rock doing laps around us depends on who you ask.
The most popular theory – often called the ‘big whack’ – says that about 4.5billion years ago, an early, Mars-size planet named Theia slammed into Earth. Some tossed-out debris then squished together to form the moon.
Or, in the early days of our solar system, we might have had two, thermally different mini-moons, or ‘moonlets’, that whacked into one another.
‘So our results make it closer to distinguishing these scenarios,’ Professor Yang says.
‘If it is homogeneous at the beginning, then this redistribution must happen precisely at a time when this KREEP layer is still in a molten state.’
These are just some of the thoughts that Professor Yang has when he cranes his neck up at night to see a little white circle.
But he hopes that, one day, there might be people looking right back at him.
‘The far side’s “radio silence”, shielded from Earth’s radio noise, also makes it an ideal site for astronomy,’ he says, ‘such as low-frequency radio telescopes to study the early universe.’
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