انگارهی برخورد ِ غولآسا
engâre-ye barxord-e qulâsâ
Fr.: hypothèse de l'impact géant
A model for → Moon formation (initially put forward by
Hartmann and Davis, 1975,
Icarus 24, 504), according to which the → proto-Earth
suffered a collision with another → protoplanet
near the end of the → accretion process
that ejected material into a → circumterrestrial
disk, out of which the Moon formed. Also called
→ canonical model.
The giant impact hypothesis is the leading theory for lunar formation.
There are, however, some key observations that cannot be explained using this
model. First, the Moon is a large fraction of the mass of Earth (~ 1%) and it is
difficult to get enough mass into orbit to form such a massive Moon.
Second, the Moon has a similar bulk composition to the Earth, but
it is missing large amounts of more
→ volatile elements. The model does not properly
explain Moon's distinctive composition.
Finally, Earth and the Moon share virtually the same
→ isotopic ratios.
It is therefore expected that the body that hit the
Earth, often called → Theia,
would have had a different isotopic
ratio than the proto-Earth. In the canonical model, most of
the mass of the Moon comes from Theia and so the Moon should have a
different isotopic fingerprint than Earth, but it does not.
The type of impact that formed the Moon in the canonical model is
dictated by a very strong constraint, the → angular momentum
of the Earth-Moon system. It is
assumed that the angular momentum of the Earth-Moon system
immediately after the Moon formed was the same as it is today. This
assumption limits the velocity of the impact, the mass of the
impacting bodies, and the angle at which the two bodies
collided. It was found that only a grazing impact with a Mars-mass
impactor at near the escape velocity can put enough mass into orbit
to potentially form a lunar-mass Moon. This is why the canonical
model is such a specific type of impact.
However, the angular momentum of the Earth-Moon system could
have been reduced over time by competition between the
gravitational pull of Earth, the Moon and the Sun. Therefore,
the Moon-forming collision could have been much more energetic than
the canonical impact.
Simon Lock and Sarah Stewart (2017, J. Geophys. Res. Planets, 122, 950-982)
have shown that such high-energy, high-angular momentum impacts can produce
a different type of planetary object, → synestias.
High-energy impacts vaporize a substantial fraction (~ 10%) of the rock
of the impacting bodies and the resulting synestias can be huge, with equatorial
radii of more than ten times that of the present-day Earth.
Because the impact-produced synestia was so big, the Moon formed
inside the vapor of the synestia surrounded by gas at pressures of
tens of bars and temperatures of 3000-4000 K.
Fragments of molten rock from the impact collided together and
formed a lunar seed orbiting within the vapor of the synestia. The
surface of the synestia was hot (2300 K) and the body cooled
rapidly. The loss of energy led to the condensation of rock
droplets at the surface of the synestia, and a torrential rock
rain fell towards
the center of the synestia. Some of this rain was revaporized in
the hot vapor of the synestia, but some encountered the lunar seed,
and the Moon grew.
As the synestia cooled, more of the vapor condensed and the body
contracted rapidly. After ten years or so, the synestia shrank
inside the orbit of the Moon and the nearly fully-formed Moon
emerged from the vapor of the synestia. The synestia continued to
cool and became a planet within a thousand years or so of the Moon
emerging from the structure.
Without the tight constraint of the angular momentum, impacts that
form synestias can put a lot more mass into the outer regions of
the synestia than can be put into the disk in the canonical
impact. This makes forming a large, lunar-mass Moon much easier.
Moreover, because the Moon formed within the synestia, surrounded by
hot vapor, it inherited its composition from Earth but only
retained the elements that are more difficult to vaporize. The more
volatile elements remained in the vapor of the synestia. When the
synestia cooled and contracted inside the Moon's orbit, it took all
the more volatile elements with it.
This model can also help explain the isotopic similarity between
Earth and the Moon. The Moon inherited its isotopic fingerprint
from the vapor that surrounded it in the outer regions of the
impacts that form synestias tend to efficiently mix material from
the two colliding bodies, and the outer portions of the synestia in
which the Moon formed would have had an isotopic composition that
was similar to the rest of the synestia. Earth and the Moon
therefore share a similar isotopic fingerprint which is made by a
mixture of the isotopic compositions of both the bodies that
→ giant; → impact;