How Earth's Moon was Born

 


Our Moon is weird. At 1% of Earth's mass is the largest satellite relative to its planet. It orbits at a large distance from the Earth and its orbit is tilted from both the Earth's equator and from the plane of Earth's orbit around the Sun. Samples returned from the Apollo missions show that despite its appearance the Moon is chemically and identical twin of Earth. These findings sparked a new debate over how the Moon was formed. Planetary scientists quickly converged on the idea that the Moon formed from a collision between the still forming proto-Earth and a Mars-sized proto planet. This is the Giant Impact hypothesis and it's been the favored explanation for the Moon's formation for the last 50 years. But the Giant Impact doesn't really hold up too well under close scrutiny. Models and simulations show that it doesn't quite explain the Moon’s composition and orbit as well as previously thought. Scientists have been trying to rescue the Giant Impact model because it otherwise explains so much about the Earth-Moon system. In the last few years some new ideas have emerged that take the Giant Impact hypothesis to the next level, and in some cases turns the idea on his head. And a long way scientists discovered a new type of astronomical phenomena that may have turned the Earth inside out. Four and a half billion years ago the solar system was a demolition derby. Planets collided with each other, often obliterating themselves in the process. Those that could survive would go on to form protoplanets which in turn would go on to form the planets that we know today. The proto-Earth was in a semi-molten state when a Mars-sized protoplanet named Theia collided with it. The impact sheared off the Earth's mantle forming a disk around a planet. A large object formed in the disk and accreted material. Both bodies pool to become the Earth and the Moon. This is the Giant Impact hypothesis in a nutshell. It's a great explanation for how the Earth and Moon got here, except that it's wrong. Well, it's not completely wrong but there are some real problems with it. First, the Moon is about 1% of Earth's mass. Now that may not seem like a lot, but believe it or not that’s actually the largest moon in the solar system relative to its home planet. This is why you gotta have a Mars-sized impact or to create the Moon in the first place. But that impact has to take place at just the right angle and at just the right speed or else you can't generate enough mass around the Earth to form the Moon. This makes the Giant Impact not impossible but very improbable. Another problem is that the Moon is lacking volatiles. Volatiles are elements that turn into a gas when heated such as water. The Moon does have some water and indeed some traces of other volatiles but it's missing several other volatiles that are rather common on Earth such as oxygen and carbon dioxide. But the real problem with the Giant Impact hypothesis is that the Earth and Moon have the same isotopic profile. This makes them effectively like genetic twins. Now to remember, isotopes are a variant of an element that include more or fewer neutrons in its nucleus than the standard version of that element. For example, a hydrogen atom normally has one proton in its nucleus. If you add a neutron to the nucleus you get the isotope of hydrogen called deuterium. If you had two neutrons you get a second isotope called tritium. Isotopes are formed in chemical reactions such as those that take place during planet formation. But planet formation is chaotic and so different planets end up with their own unique set of isotopic profiles. And this is a problem for the Giant Impact hypothesis because it predicts that the Moon should for mostly from the impact. That would leave a little with a different isotopic profile from Earth, but in reality they're isotopic twins. At first glance it seems that the Giant Impact hypothesis must be wrong and that the Moon had to have formed by some other mechanism. But other hypotheses have even bigger problems. One of the earliest ideas is that the Moon was flowing out from a rapidly spinning proto-Earth. The idea was first suggested by George Darwin in 1879. He calculated the Earth would need to have reached at rotation period of about two and a half hours to fling out the Moon. However, this turns out to be dynamically impossible. The emerging bulge would destabilize the Earth’s spin, causing it to wobble. This wobbling would slow it down before it could even reach fission speed. Even if the Earth could still somehow achieve fission, the Moon would be flung out around the Earth's equator. But the Moon doesn't orbit around Earth's equator. Its orbit is tilted by about 18 degrees with respect to the equator and by about 5 degrees with respect to the Earth's orbital plane around the Sun. This plane is called the ecliptic and that 5 degree tilt of the Moon's orbit is really significant. A second idea is that the Moon formed else where and it was later captured in Earth orbit. This allows for the Moon to have it's rather large mass as well as have its own unique orbital inclination. However, a freely moving Moon would carry a lot of kinetic energy and so it would have to slow down by a lot in order for the Earth to capture it. One way to do that was to imagine that the Earth might have been surrounded by a really thick cloud of gas at the time. That would have dragged on the Moon it passed through the cloud and the Moon would have slowed down a little bit. But it would still require a really lucky shot by the Earth to somehow capture into a stable orbit. And the idea that the Moon could form someplace else in the solar system with the exact same isotopic profile is Earth just seems like a bridge too far. This leaves the Giant Impact as the best explanation despite its many flaws. So astronomers have been spending the last 50 years or so trying to make the model work somehow. In 2012, Dr. Robin Can up of the Southwestern Research Institute came up with a really interesting idea. She suggested that rather than have an Earth and Mars-sized impactor, the impactors themselves would be basically the same size about 45 and 55% of Earth's present day mass. The two objects strike each other off center at a low-velocity blend to form a rapidly spinning proto-Earth surrounded by a proto-lunar disk. In this model, the mantles of the original impactors are well-mixed, creating identical isotopic profiles in the planet and in the disk. The proto-Moon begins accreting material in the outer disk where temperatures are lower and rock begins to condense out of the vapor. As the proto-Moon grows it drags on the disk material and spirals inward to the inner disk. At this point the Moon is the closest it will ever be the Earth. But over time, tidal interactions between the Earth, Moon, and other disk material caused its orbit to gradually expand outward. Eventually it emerges from the other end of the disk. Meanwhile, the disk rapidly cools and shrinks,leaving the newly-formed Moon behind. This Moon would be loaded with basalts and silicates, would have an isotopic profile that is identical to the Earth, and at the same time be devoid of any volatiles. The key feature of this model is that it allows a disk with the right composition and mass to form over a fairly broad range of impact angles. This makes Canup’s model a lot more probable than the canonical Giant Impact. But this model creates an Earth-Moon system with about twice the angular momentum than it presently has. This means either the Earth has to be spinning twice as fast, or the Moon needs to be orbiting twice as far away. However Dr. Matija Cuk at the SETI Institute and Dr. Sarah Stewart who was then at Harvard University came up with a pretty clever way to get rid of some of that angular momentum. Shortly after the Earth and Moon form, tidal forces cause the Moon's orbit to expand away from the Earth, and the Earth slows down in response to conserve angular momentum.

This is just like a spinning figure skater slowing down her spin by extending her arms outward. As the Moon gets farther away from Earth,it begins to feel tidal forces from the Sun. This elongates the Moon’s orbit and locks its long axis perpendicular to the Earth and Sun line. This kind of orbital lock up is called an  evection resonance. Earth is still slowing down and transferring angular momentum to the Moon. But because the Moon cannot change its orbit,it transfers that angular momentum from the Earth to the Sun. This causes the Earth's orbit to expand slightly in response. Today in the Moon orbits at about 60 Earthradii, but when was caught the infection residence it would appear 20 times larger in the sky as it was making its closest approach to Earth. But one problem with the infection residence is that it may not last long enough to transfer enough angular momentum, That doesn't mean that Canup’s model is wrong, it just means that the evection resonance model may need a little help from some other mechanisms. But an impact between a larger and smaller body would be a lot more likely. And that got Cuk and Stewart wondering if there was still a way to rescue the canonical Giant Impact hypothesis. What they ended up coming up with was an origin story of the Moon that is so strange it turns the Earth into something that’s completely unrecognizable. In 2012 Cuk and Stewart wondered what would happen if the Earth were already spinning rapidly at the time of impact. The idea is not so far-fetched because there were a number of giant collisions taking place at this time. They found that under these conditions, Earth would be rotating so fast it could barely hold itself together. Earth would be deformed from a sphere into a rugby ball-shaped ellipsoid. Now even a smaller impactor moving at a really high velocity could break the Earth apart into a disk, allowing the Moon to form. It was an intriguing idea, but this turned out to be just the beginning. When Stewart and her graduate student Simon Lock looked further into this idea, they discovered something that we've never seen in the solar system before. After enough Giant Impacts, Earth no longer has a surface, there's just a layer of gas that gets denser with depth. As the Earth spins faster and gets hotter, the equator widens until it can no longer remain coupled to the planet. Chunks of the equator break off and vaporize and settle into an orbit. But here's the really weird part: depending on its rotation and temperature, the disk rapidly flares out into a giant donut-shaped structure of molten lava and vaporized rock. Stewart and Lock call this object a “Synestia”. The name comes from Hestia, the Greek goddess of Home and Hearth, and “syn” meaning “together”. So “synestia” roughly translates into“connected home”. Depending on the angular momentum and the energy of a collision, the synestia could have been a hundred times the size of Earth. Inside the synestia, gas pressures reach tens of bars and the temperature climbs from 4000 to 6000 degrees Fahrenheit. Somewhere in the surrounding vapor, fragments of molten rock combined together to form the “lunar seed”. The seed becomes a tiny satellite orbiting in the vapor. The surface of the synestia cools rapidly,allowing tiny droplets of rock to condense out of the vapor. The droplets fall tour the center in a torrential rock rain, coming in ten times faster than rainfall in a hurricane. Most of this rock rain re-vaporizes in the searing heat of the synestia. But some of it lands on the lunar seed, growing the proto-Moon. As the synestia cooled, more of the vapor condensed and the body contracted rapidly. After just a few years, the synestia shrank to inside the Moon's orbit and the newly-formed Moon emerged. Not only does this explain the Moon's large mass, but it also explains why the Moon lacks all those volatiles. Inside the synestia it was much too hot for volatiles to condense on the lunar seed. But because both the Moon and the Earth were formed from the same synestia, both objects share the same isotopic profile. Stewart and Lock modelled the dynamics of synestia and discovered that don't really last for very long. And then they condense very rapidly to form the Earth. The molten Earth would then be spinning very rapidly, so once again there would be a lot of angular momentum to get rid of. The Moon could have helped the Earth out by getting into an evection resonance and transferring angular momentum out of the system. But evection resonances are a little bit tricky to set up and may not last long enough to get rid of enough angular momentum. In 2016, Stewart and her team came up with another out-of-the-box model that not only explains how to get rid of the excess angular momentum, but also explains why the Moon's orbit is inclined with respect to the ecliptic. They found that there's about a 30% chance that a giant impact could increase the Earth's axial tilt - or obliquity - from 23 1/2 degrees to as high as 80 degrees. Now 30% may not seem like particularly good odds, but after enough giant impacts, one of them is gonna knock the planet over. After the Moon forms and the synestia contracts,the Moon is initially orbiting around the Earth's equator. Earth spinning so rapidly at this point that its equator is twice the length of its pole and a day is about two to three hours long. Tidal interactions between the Earth and the Moon caused the Moon's orbit to expand as before, but because this time the Moon's orbit is so highly inclined with respect to the ecliptic, it abruptly breaks away from the Earth's equator and begins oscillating up and down with respect to the ecliptic. The Moon's increasing distance allows it to act as a kind of gravitational lever arm against the Earth's rotation axis - gradually pulling it back from 70 degrees to 23 1/2 degrees - very close to was present day value. Meanwhile the Moon undergoes changes to its own or liquidy. Initially it started out with the same obliquity as Earth but then the Moon soon finds himself lopped over on its side. As its orbit expands, its inclination and obliquity both lower to their present-day values. As the Moon's orbit expands its oscillations above and below the ecliptic gradually dampen out and settle down to its present-day inclination of about 5 degrees. This new model puts the Giant Impact theory on steroids. The initial energy is 10 times greater, the angular momentum twice as much, and the initial axial tilt is about 40 to 50 degrees greater than it is now. But it allows a single event to create the unique chemical relationship between the Earth and Moon, set the rotation period and even the axial tilt of the Earth, as well as the orbital inclination, and even the axle of the Moon itself. No other theory can explain the formation of the Moon so completely as this. But any theory has to be tested. the Apollo missions brought so much information from the Moon literally back to Earth, just imagine what we could learn if we ever go back. 

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