Welcome to my blog "Mystery of Galaxy".
Today our topic is "origin of our Moon".
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 protoplanet. This is the
Giant Impact hypothesis and it's been the favored explanation for the Moon's
formation for the last 50 years.
 |
| Giant Impact hypothesis Source: NASA/JPL-Caltech/T. Pyle (SSC) / Public domain |
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. Planetesimals collided with each other, often obliterating
themselves in the process.
 |
| Planetesimals Source: The original uploader was 1981willy at English Wikipedia. Later versions were uploaded by WilyD at en.wikipedia. / Public domain |
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 cool 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.
 |
| Source: Citronade / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) |
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 planetesimals 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 impactor. 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 that5 degree tilt of the Moon's orbit is really significant.
 |
| Ecliptic plane Source: Tfr000 (talk) 01:59, 15 March 2012 (UTC) / CC BY-SA |
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 last50
years or so trying to make the model work somehow.
In 2012, Dr. Robin Canup of
the South western 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 45and 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’ll 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 plausible 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 Ćuk 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 Earth radii, 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 Ćuk 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.
We're going to learn what
that something is in a moment. In 2012 Ćuk 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 liquidly. 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 the oryon 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. That's all. Thank you.
Comments
Post a Comment