Over 4.5 billion years ago, a small planet smashed into molten young Earth. Magma careened into space, clumped in the cold void together with debris from the shattered alien planet, and thus the moon was born. Then shortly afterward (in cosmic terms), an even smaller planet thumped into the moon itself.
A second collision could explain a host of weirdness observed by Apollo astronauts and remote observation, suggests Meng Hua Zhu of Macau University of Science and Technology, the lead author of the study published Monday in the American Geological Union’s Journal of Geophysical Research: Planets.
This postulated second impact happened about 4.3 to 4.5 billion years ago, after the young moon’s crust had solidified, Zhu told Haaretz. Happily for posterity, it was apparently a dwarf planet less than 800 kilometers (about 500 miles) in diameter.
“That would be the equivalent of an object a bit smaller than the dwarf planet Ceres moving at a speed about one-quarter as fast as the meteor pebbles and sand grains that burn up as ‘shooting stars’ in Earth’s atmosphere,” the AGU’s Lauren Lipuma writes.
To put the postulated dwarf into proportion, the moon is 3,474 kilometers in diameter at this point in time, Earth is 12,742 kilometers and Mars is 6,779 kilometers.
The dwarf planet hit the nearside of the moon in a massive impact that, among other things, sent ejecta flying upward, which landed on the farside of the moon. So much material reached the farside that it created a whole new layer of rock between 5 to 10 kilometers thick, Zhu says, based on remote observations.
Also, the impact seems to have been relatively low velocity: The dwarf orb was traveling at a mere 22,500 kilometers per hour, the team estimates. Or it may have been a little smaller and a little faster, but the gist is clear, and cosmic trauma ensued.
The collision explains certain otherwise baffling asymmetries and anomalies observed by Apollo astronauts between the nearside and farside of the moon, postulate Zhu and the team. Moved in part by the mysterious asymmetries, in 2011 the Americans launched a double-probe mission called GRAIL (Gravity Recovery and Interior Laboratory).
GRAIL mapped the crust more thoroughly, and provided new information about the composition of the crust.
After 88 days of data collection, the GRAIL probes were allowed to crash, or crashed, on the lunar surface, and that was that for them. Scientists, on the other hand, are still analyzing the information they provided. The data so far suggest that one fond theory — that Earth used to have two moons that merged in the early days of the solar system — is wrong.
Meanwhile, on the farside
We don’t see the differences between the near and “dark” sides of the moon with the naked eye mainly because we only see the nearside. The American space missions never landed on the farside: the first time that ever happened was this year when the Chinese achieved that very thing.
But as they orbited the moon decades ago, the Apollo missions could see the farside and discovered that, for one thing, the farside is much more pocked with craters than the nearside, which is typified by low-lying “seas” — actually, basins in the dry crust, also called mares.
Even more weirdly, the farside crust is significantly thicker than the crust on the nearside. Data from GRAIL has shown the farside crust sports a metal-rich layer over the primordial crust that’s a whopping 10 kilometers thick.
For its part, the much thinner nearside has a large area of low-calcium pyroxene rock that is believed to have originated in the traumatic impact.
The answer to all that, says the new theory, is that when the dwarf planet crashed into the moon, it naturally caused massive ejection of material that wound up on the farside of the moon. That ejecta created the 5 to 10 kilometer extra layer on the farside crust, Zhu tells Haaretz.
Thing is, if all this happened within 200 million years of the moon’s creation, none of this would explain why the farside of the moon has much more impact crater marks than the nearside. All sides of the moon (if one can say an orb has sides) would have been slammed by meteors and cosmic whatnot over the ensuing 4.3 billion years.
“Actually, they should be the same,” Zhu tells Haaretz.
The answer, he suggests, is that the nearside is hotter than the dark side and not because of the sun. “The nearside of the moon is enriched in heat-producing (radioactive) elements, like thorium, potassium and uranium,” Zhu explains. These elements provide heat and make the crust and upper mantle of Moon’s nearside much warmer than that of the farside. True, the impact produced a massive scar on the nearside, but since it is relatively warm, with a high internal temperature, the impact structures could have easily “relaxed,” effectively erasing the impact scars, he says.
“In addition, the nearside of the moon was almost covered by the mare basalts. Most of early relaxed structures may possibly have been covered by these late-erupting mare basalts. On the other hand, the top layer of the farside is much colder and has a higher strength, so the impact scars keep until now.”
And that may explain why we find more impact scars on the farside than the nearside.
The theory presented by Zhu and colleagues could explain another anomaly. The moon is believed to be made chiefly of material from Earth and a planet named Theia that crashed into our planet 4.5 billion years ago. The new theory could explain differences in isotopes of potassium, phosphorus and rare-earth elements such as tungsten-182 between the surfaces of the Earth and moon, the AGU explains: These elements could have come from the second impact, which would have added that material to the moon after its formation.
But there's one mystery Zhu can't explain yet. The moon seems to still have something of a molten core from its beginnings in magma and catastrophe. How long did it take the thing to cool down?
"In general, we think the Moon was initially covered by a thick magma ocean 500 to 1,000 km thick," he tells Haaretz. "This thick magma ocean was theoretically thought to be solid within 10 million years. However, from the lunar samples, we find that the moon's magma ocean might have lasted 100 to 150 million years. Currently, we still do not know what the heating source is that kept the magma ocean for such a long time."
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