On December 13, 1972, Apollo 17 astronaut Garisson Schmitt approached a boulder in the Sea of Tranquility on the Moon. “This boulder has its own little path leading straight to the hill,” he informed his commander, Eugene Cernan, noting where the boulder was before rolling down the hill. Cernan took some samples.
"Imagine what it would have been like if you had stood there before this boulder rolled," Cernan said thoughtfully. "I probably wouldn't do better," Schmitt replied.
The astronauts carved pieces of the moon from the boulder. Then, using a rake, Schmitt scraped off the dusty surface and lifted up a pebble that would later be called troctolite 76536.
That rock and its boulder brothers were supposed to tell the story of how our moon came to be. In this creation story, recorded in countless textbooks and science museum exhibits over the past forty years, the Moon was melted out of a catastrophic collision between a germ Earth and a solid world the size of Mars. The other world was called Teia, after the Greek goddess who gave birth to Selene, the moon. Theia crashed into the Earth so hard that both worlds melted. The streams of molten material thrown out by Theia then cooled and solidified, forming the silvery companion we all know well.
But modern measurements of troctolite 76536 and other rocks from the Moon and Mars have questioned this theory. Over the past five years, multiple studies have uncovered a problem: the canonical giant collision hypothesis is based on assumptions that do not match the evidence. If Theia hit the Earth and later formed the Moon, the Moon must be made of Theia's material. But the Moon is not like Theia - or Mars, for that matter. To the very atoms, it looks almost the same as the Earth.
Faced with this inconsistency, lunar explorers sought new ideas to understand how the moon came to be. The most obvious solution may be the simplest, but it gives rise to other problems with understanding the young solar system: perhaps Theia formed the Moon, but Theia also consisted of a substance that is almost identical to the earth. Alternatively, the collision process mixed everything, homogenizing the individual pieces and liquids in the cake, which was then cut into portions. In this case, the collision had to be extremely powerful, or there had to be several of them. The third explanation questions our understanding of the planets. It may be that the Earth and Moon we have today have gone through strange metamorphoses and wild orbital dances that have radically changed their rotation and future.
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Bad news for Teia
To understand what could have happened on the most important day for the Earth, you need to start by understanding the youth of the solar system. Four and a half billion years ago, the Sun was surrounded by a hot cloud of donut-shaped debris. The stellar elements revolved around our newborn sun, cooling and - over the years - merging together in a process we do not fully understand. First into clumps, then into planetesimals, then into planets. These solids were rigid and often collided, evaporated and reappeared. It was in this incredibly hard stellar billiards that the Earth and the Moon were forged.
To get the moon we have today, with its size, rotation and the speed at which it is moving away from the Earth, our best computer models say that whatever the Earth collides with, it must be something the size of Mars. Anything more or less would already produce a system with much greater angular momentum than we observe. A larger projectile would also throw too much iron into Earth's orbit and produce a moon much richer in iron than we observe.
The first geochemical studies of troctolite 76536 and other rocks supported this story. They showed that the lunar rocks must have been born in the lunar ocean of magma, which could in turn have emerged from a giant collision. Troctolite floated in the molten sea like an iceberg in Antarctica. Based on these physical limitations, scientists decided that the Moon was made from Theia's remains. But there is a problem.
Let's go back to the young solar system. As the solid worlds collided and evaporated, their contents mixed, eventually settling in separate regions. Closer to the Sun, where it was hotter, the lighter elements were more likely to heat up and escape, leaving an excess of heavy isotopes (variations of elements with extra neutrons). Farther from the Sun, rocks were able to hold more water and lighter isotopes remained. Therefore, a scientist can examine a mixture of isotopes to determine in which part of the solar system it appeared, just as an accent betrays a person's homeland.
These differences are so pronounced that they are used to classify planets and types of meteorites. Mars is so different from Earth, for example, that its meteorites can be identified by simply measuring the ratio of three different oxygen isotopes.
In 2001, using advanced mass spectrometry techniques, Swiss scientists re-examined troctolite 76536 and other lunar samples. It turned out that their oxygen isotopes are indistinguishable from those on Earth. Geochemists have since studied titanium, tungsten, chromium, rubidium, potassium, and other not-so-ordinary metals on Earth - and they all looked pretty much the same.
This is bad news for Teia. If Mars is so different from Earth, Theia - and therefore the Moon - must be different too. If they are the same, it means that the moon should have formed from molten pieces of the Earth. The rocks collected by Apollo, it turns out, will directly contradict what physics insists.
"The Canonical Model is in serious crisis," says Sarah Stewart, a planetary scientist at the University of California, Davis. "She has not been completely killed yet, but her current status is that she is not working."
Moon of steam
Stewart has tried to rethink the physical limitations of this problem - the need for a specific size of impact body that moves at a specific speed - against the backdrop of new geochemical evidence. In 2012, she and Matiya Zhuk, now at the SETI Institute, proposed a new physical model for the formation of the moon. They stated that the young Earth was a spinning dervish whose day lasted for two to three hours when Theia struck her. The collision produced a disk around the Earth - like the ring of Saturn - but that only lasted 24 hours. Ultimately, the disk cooled and solidified to form the moon.
Supercomputers are not powerful enough to fully simulate this process, but they have shown that a projectile crashing into such a rapidly spinning world can shear off enough Earth, completely destroy Theia, and scrape off enough skin from both to create a Moon and Earth with the same isotopic ratios. Like a potter on a potter's wheel.
For the rapidly rotating Earth explanation to be correct, however, there must be something else slowing the planet's rotation rate down to its present state. In their 2012 paper, Stewart and Chuck argued that for certain orbital-resonance interactions, the Earth should have transferred angular momentum to the sun. Later, Jack Wisdom of the Massachusetts Institute of Technology proposed several alternative scenarios for extracting angular momentum from the Earth-Moon system.
However, none of the explanations were satisfactory. The 2012 models have never been able to explain the Moon's orbit or its chemistry, Stewart says. And so, last year, Simon Locke, a Harvard graduate and Stuart student at the time, presented an updated model that suggested a previously unseen planetary structure.
In his opinion, every piece of Earth and Teia evaporated and formed a swollen, swollen cloud in the form of a thick donut. The cloud rotated so fast that it reached a point called the co-rotation limit. At this outer edge of the cloud, the evaporated rock circled so quickly that the cloud took on a new structure, with a thick disk circling the inner region. Importantly, the disc was not separated from the central region in the same way that Saturn's rings are.
The conditions in this structure are indescribably hellish; there is no surface, instead clouds of molten rock, with each area of the cloud forming raindrops of molten rock. The moons grew inside this vapor, Locke says, before the vapor finally cooled down and left behind the Earth-Moon system.
Given the unusual characteristics of the structure, Locke and Stewart felt that it deserved a new name. They tried several versions before arriving at "synestia," which uses the Greek prefix "sin," meaning "together," and the goddess Hestia, which represents home, hearth and architecture. This word means "linked structure," Stewart says.
“These bodies are not what you think. And they don't look the way you thought they would look."
In May, Locke and Stewart published a paper on the physics of synesthesia; their work on lunar synesthesia is still pending. They presented it at a planetary conference and said that their colleagues were interested, but hardly agree with the idea. Perhaps because synesty remains just an idea; unlike ringed planets, which are many in the solar system, and protoplanetary disks, which are many in the universe, no one has ever seen a single one.
But it's a fun way to explain the peculiarities of our Moon when our models don't seem to work.
Ten moons
Among the natural satellites of the solar system, the earth's moon may be the most amazing because of its loneliness. Mercury and Venus do not have natural satellites, in part because of their proximity to the sun, the gravitational effect of which makes the orbits of the satellites unstable. Mars has tiny Phobos and Deimos, which some believe are captured by asteroids; others speak in favor of large bodies falling to Mars. The gas giants have many satellites, both hard and soft.
Unlike these satellites, the Earth satellite also stands out for its size and the physical stress it carries. The moon makes up less than 1% of the Earth by mass, and the total mass of the satellites of the outer planets is less than 1/10 percent of their parents. More importantly, the Moon accounts for 80% of the angular momentum of the Earth system -
Moon. In other words, the Moon is responsible for 80% of the movement of the system as a whole. For the outer planets, this value is less than 1%.
Perhaps Luna did not always carry all this burden. The satellite's face shows evidence of heavy bombing; why, then, should we assume that only one blow molded the moon out of the earth? The moon may have formed in the course of many collisions, according to Raluka Rufu, a planetary scientist at the Weizman Research Institute in Israel.
In a paper published last winter, she argued that Earth's satellite might not be original. Instead, it became a collection of thousands of pieces - at least ten, based on her calculations. The projectiles flew at different angles and at different speeds to the Earth and formed discs that merged into “moon debris”, eventually blinding the moon we know today.
Planetary scientists noted her work. Robin Canup, a lunar scientist at the Southwest Research Institute and an expert on theories of lunar formation, says the theory is worth considering. However, more research is needed. Rufu isn't sure if the debris was moving in the same direction, just as the moon is constantly looking in the same direction. If so, how could they have merged at all? This remains to be seen.
Meanwhile, others have turned to another explanation for the similarities between the Earth and the Moon, which could have a very simple answer. From synesties to lunar belts, new physics models - and new physics - can be controversial. Perhaps the Moon is similar to the Earth only because Theia was similar.
Same
The moon is not the only "earthly" thing in the solar system. Rocks like troctolite 76536 have the same oxygen isotope ratio as terrestrial rocks, as well as groups of asteroids - enstatite chondrites. The oxygen isotopes of these asteroids are similar to those on Earth, says Miriam Telus, a cosmochemist who studies meteorites at the Carnegie Institution in Washington. “One of the arguments is that they formed in hot areas of the disk that could be closer to the sun,” she says. They may have formed near Earth.
Some of these rocks came together to form the Earth; others formed Theia. Enstatite chondrites are residual rocks that have never been collected or grown large enough to form mantles, cores, and fully formed planets.
In January, Nicholas Daufas, a geophysicist at the University of Chicago, stated that most of the rocks that became Earth were enstatite-type meteorites. He argued that everything that formed in one region would be collected from them. Planetary construction took place using the same mixed materials that we now find on Earth and the Moon; they look the same because they are the same. The giant body that formed the Moon likely had an isotopic composition similar to that of Earth.
David Stevenson, a California Institute of Technology planetary scientist who has studied the moon's origins since Theia's hypothesis was first presented in 1974, says he considers this work to be the most important contribution to the controversy over the past year. Because it focuses on a problem that geochemists have been trying to solve for decades.
“This is a clever story about how the various elements that make it to Earth should be viewed,” Stevenson says.
But not everyone agrees. Questions remain about the isotopic ratios of elements like tungsten, Stewart notes. Tungsten-182 is derived from hafnium-182, so the ratio of tungsten to hafnium works like a clock to determine the age of a particular rock. If one rock has more tungsten-182 than another, you can safely say that the tungsten-rich rock formed earlier. But the most accurate measurements show that the ratios of tungsten to hafnium are the same for the Earth and the Moon. Two bodies had to be in special conditions for this to happen.
Based on materials from Quanta
Ilya Khel