Remnants of Alien Planets Found on the Moon
The Moon is a crater-pocked world. It has been continuously bombarded by meteorites throughout its existence. Because it has no atmosphere, it has no shield to protect it against incoming objects of all shapes and sizes.
By studying the materials found in craters, geologists and geophysicists can learn much about the Moon’s history, and the external and internal forces that have helped shape its rugged surface. The materials in a crater can be a mixture of substances excavated from below the surface by the force of the impact, and possibly of substances carried in by the meteorite itself.
One interesting material found inside impact craters on the far side of the Moon are a type of crystal mineral known as spinels. These types of rocks are common components of asteroids and meteorites, and could conceivably have been delivered to the Moon as a result of collisions with incoming bodies.
After re-examining evidence collected during previous studies of lunar craters in 2013, an international team of scientists working in China and the United States published an article in the journal Nature Geoscience that put forward this very theory. Using their own models for calculating the effects of asteroid or meteor impacts, they concluded that this was most likely how spinels ended up inside Moon craters.
Spinels in asteroids and meteorites are remnants of more ancient and chaotic times. These bodies are pieces of broken planets or planetoids that were around in the earliest days of the solar system, while it was still in the process of forming. They would in a sense be “alien” visitors to the Moon, since they would have arrived from a far distance before crashing into the Earth’s satellite.
But is that actually how these magnesium-rich minerals ended up in Moon craters? That is difficult to determine with certainty. These types of minerals exist in a sort of gray area, where different theories for their origins are possible and it can be difficult to tell which scenario best fits the evidence.
Anatomy of a Scientific Debate
Even though the Moon is 384,400 kilometres away from the Earth, scientists have several methods for investigating its geological characteristics. Naturally, these include the study of lunar rocks and other materials collected by the Apollo astronauts. But the list also includes observations made using Earth-based telescopes, and the study of data obtained by an orbiting device known as the Moon Minerology Mapper. The latter is an imaging spectrometer developed by NASA and carried into lunar orbit in 2008 by Chandrayaan-1, the first-ever mission to the Moon launched by India.
With an image spectrometer, it has been possible for planetary geologists to create a detailed outline of the Moon’s mineral structure, all across its surface. One of the more notable finds were magnesium-rich spinels on the Moon’s far side, in an area known as Moscoviense Basin. This basin is located inside a massive impact crater, and inside of the basin itself is a large lunar mare. Mares on the Moon were created by ancient volcanic eruptions, from a time when the Moon was far more geologically active than it is today.
With evidence of volcanic activity found in the vicinity of the Moscoviense Basin, many scientists concluded that magma instrusions must have been responsible for the spinel deposits. This magnesium-rich rock would have come originally from the crust-mantle border area, and would have been buried well below the surface of the Moon before the impact that created the crater. When the crater-forming object hit, it would have penetrated through several layers of rock, excavating the spinels from their former location and laying them out across the lunar landscape.
This was the original interpretation of the discovery of spinels on the Moon. But the Chinese-American team challenged some of the assumptions behind the volcanic thesis, and were able to make a strong case for the possibility of an off-moon source.
In the past, it was assumed that high-impact meteorite or asteroid collisions would obliterate much of the striking object, with any surviving remnants thrown far clear of the crater. This is why most scientists were quick to conclude that the spinels found on the Moon were excavated from far below the surface when the catastrophic event occurred.
But this is not necessarily so, the scientists from Purdue University in the U.S. and the Institute of Remote Sensing in Beijing argued. If the speed of a meteor impact is below a certain threshold, they calculated, a different outcome could be expected.
At lower-than-average velocities, harder materials inside the meteor or asteroid could survive the initial impact, and then slide back down the rim of the crater instead of being ejected over its edge. This would explain how magnesium-rich spinels would show up in an analysis of the contents of the Moscoviense Basin, when they’d never been detected anywhere else on the Moon before.
More Wonders Await
It is notable that the discovery of spinels on the Moon has only happened within the past few years, four decades after the original Apollo missions that officially launched the era of intensive study of our sole satellite. And it is just as notable that scientists aren’t sure how to interpret their discoveries. They’ve developed two plausible theories to explain the facts, each of which has varying ramifications for understanding the Moon’s history and geology.
Currently, there seems to be renewed interest in traveling to the Moon, and resurrecting manned space exploration. Nation-states and private entrepreneurs are both looking to get in on the action.
Given these developments, we can only wonder what surprises might be in store for planetary scientists after the next generation of bold explorers return with new and exciting treasures. Will astronauts in the 2020s and 2030s bring back evidence that settles the question of how the Moon was actually created, once and for all? Or might they discover something even more profound and world-changing, such as evidence that the Moon was visited in the past by a spacefaring race that didn’t begin their voyage from only a few hundred kilometres away?
This is of course all speculation. But if the Moon is still surprising us after all this time, there’s no telling what kind of shockers it might have in store for us once we land on its surface again.
By Nathan Falde