What scientists are most certain of is that the two planets became a mass of molten material that gradually cooled to form the Earth and the Moon.
“This molten mass spun and formed a disk, which existed for a few days. The temperature, which was very high, cooled slowly and everything heavy merged to form the Earth today, “says Razvan Caracas, a physicist who studies the interior of planets at the French National Center for Scientific Research in Lyon, France.
Caracas is running computer simulations of what happened to this mass of atoms and materials immediately after the collision as part of a project called IMPACT. It uses a computer power equivalent to 200 desktop computers running for two weeks to calculate what happens under a set of conditions after the collision. Three supercomputing centers in France will provide the 120 million hours of computing time required by the project in just five years.
The initial shock wave after the proto-Earth and Tea collision generated crushing pressures and temperatures perhaps as high as 10,000 ° C at the center.
“It is difficult to imagine what the conditions were like,” Caracas clarifies. Almost the entire periodic table of elements would have been in a “supercritical condition”: a haze of disarmed atoms in neither gaseous nor liquid form.
During the following days, heavy materials such as iron began to form the center of a new planet: Earth.
“The metals would slowly separate into droplets as they cooled, and then the silicates (minerals) would liquefy. The heaviest inner part of the Earth was formed, then the material would have fallen on the planet, “continues the expert.” The outer part of the disk would have formed a ring and eventually would have accumulated to form the moon.
Clock
While the Earth came together in a few days, the moon likely took weeks or months to take shape, according to Caracas. There may even be two moons circling the early Earth, one colliding with the other to create the moon we see today. Both newly minted bodies had an ocean of molten rock.
Joshua Snape, a lunar geologist at VU Amsterdam, The Netherlands, is interested in the early moon. It hopes to determine its age within a range of tens or hundreds of millions of years. The traditional view is that it was formed about 4.5 billion years ago.
“Something profound was happening on the Moon between 4.35 and 4.4 billion years ago. The simplest explanation is that the lunar magma ocean (which covers the moon) cooled, “says Snape.
Thanks to the moon’s lack of tectonic activity, all its rocks can tell us about this magma period , an important stage in the moon’s formation.
How long it took for the magma to cool is a crucial question, says Snape. “It is important to understand how long this takes, because we extend what we know about the moon to other planets.”
As the moon is the only substantial body in the solar system to which we have traveled and extracted rocks, its samples are valuable to scientists. Snape has studied the proportions of lead and uranium isotopes in rocks returned by the Apollo missions and from lunar meteorites. This relationship acts like a deep time clock that you have used to calculate when a rock formed.
“The moon has a record and acts as a beautiful laboratory for understanding early planetary processes. This will be applicable to Mars, Mercury or Venus, places that are difficult for us to access, and it can even inform us about our own planet, ”says Snape.
Earth is not that useful because plate tectonics buries and recycles rocks.
“This is why we love the moon so much,” he said. “It is a treasure, geologically speaking.”
Their studies can reveal, for example, how long a planetary body with volcanic eruptions remains active when there are no tectonic plates to drive them. This could be important when it comes to studying planets around other stars.
Ocean of magma
Snape is currently working on a project called MoonDiff that involves trying to recreate rock compositions that existed in the lunar magma ocean. The minerals would not have crystallized right away, but they would have done so in a sequence that Snape is trying to reconstruct.
It crushes and heats the recreated rocks in conditions similar to those of the Moon when its surface was a mass of molten rock. “This week I’m doing experiments at one gigapascal (one billion pascals, one unit of pressure) and 1,200 ° C,” he said.
Knowing the sequence in which the minerals separated from the magma ocean would help explain the history of the moon and its current geology.
“The oldest rocks on the moon (the clearest visible parts) are formed mainly by a mineral called feldspar, which would have floated to the top of the liquid magma ocean,” argues the expert. “On the other hand, that Apollo 12 basalt sample and other similar rocks (which make up the darker gray parts) are mainly made up of minerals that would have been denser and sunk to the bottom.”
Maud Boyet, a geochemist at the University of Clermont Auvergne, France, studies the Earth’s early melting period and hopes to pinpoint when it first cooled and became habitable. To do this, it is examining the Earth and lunar rocks, as well as meteorites, using new mass spectroscopy techniques, among others, for a project called ISOREE. He says the moon rocks could tell us when the big collision took place, but to do that we still need to understand the early history of the moon. The rocks on the other side of the moon could help.
Moon rocks collected during the Apollo missions come from the side of the moon that faces Earth. The side facing out has a different surface composition. This could be because the moon underwent a second melt, perhaps caused by a second massive collision.
“We have no samples (collected) from the other side of the moon,” Boyet says. “But we have some meteorites (that landed on Earth) that we think came from there.”
Original article
This article was originally published in Horizon, the EU Research and Innovation Magazine
