We
have understood for a long time that the earth moon system is the
result of a massive collision that an accretion disk and two major
bodies in orbit around each other. So far so good. What is critical
is that center of mass was so far from the main body it allowed ample
accretion around the moon without been drained off into the parent
through collision. There should be ample solutions available with
this constraint.
The
key point is that the bulk of the mass was captured in the resultant
system and that the primary orbit kept the center of mass outside the
parent from the beginning.
After
all than, Newton went to work and with the help of Jupiter and the
Sun we have the present configuration.
This is presently much better than what was first presented decades ago when I first encountered such efforts.
Huge Moon-Forming
Collision Theory Gets New Spin
by Mike Wall,
SPACE.com Senior Writer
Date: 17 October 2012
Time: 03:01 PM ET
This artist's
conception of a planetary smashup whose debris was spotted by NASA's
Spitzer Space Telescope three years ago gives an impression of the
carnage that would have been wrecked when a similar impact created
Earth's Moon. A team at Washington University in St. Louis has
uncovered evidence of this impact that scientists have been trying to
find for more than 30 years. Image released Oct. 17, 2012.
CREDIT: NASA/JPL-Caltech
The moon did indeed
coalesce out of tiny bits of pulverized planet blasted into space by
a catastrophic collision 4.5 billion years ago, two new studies
suggest.
The new research
potentially plugs a big hole in the giant impact theory, long
the leading explanation for the moon's formation. Previous versions
of the theory held that the moon formed primarily from pieces of a
mysterious Mars-size body that slammed into a proto-Earth — but
that presented a problem, because scientists know that the moon and
Earth are made of the same stuff.
The two studies both
explain how Earth and the moon came to be geochemical
twins. However, they offer differing versions of the enormous smashup
that apparently created Earth's natural satellite, giving scientists
plenty to chew on going forward.
One of the studies —
by Matija Cuk of the SETI (Search for Extraterrestrial Intelligence)
Institute in Mountain View, Calif., and Sarah Stewart of Harvard —
suggests the answer lies in Earth's rotation rate. [Video: New Ideas
About the Moon-Forming Impact]
If Earth's day had
been just two to three hours long at the time of the impact, Cuk and
Stewart calculate, the planet could well have thrown off enough
material to form the moon (which is 1.2 percent as massive
as Earth).
This rotational speed
might sound incredible, and indeed it's close to the threshold beyond
which the planet would begin to fly apart. But researchers say the
early solar system was a "shooting gallery"
marked by many large impacts, which could have spun planets up to
enormous speeds.
Cuk and Stewart's
study, which appears online today (Oct. 17) in the journal Science,
also provides a mechanism by which Earth's rotation rate could have
slowed over time.
After the collision, a
gravitational interaction between Earth's orbit around the sun and
the moon's orbit around Earth could have put the brakes on the
planet's super-spin, eventually producing a 24-hour day, the
scientists determined.
###
Simulation
of an off-center, low-velocity collision between two protoplanets
containing 45 percent and 55 percent of Earth's mass. Color scales
with particle temperature in kelvin, with blue-to-red indicating
temperatures from 2,000 K to in excess of 6,440 K. After the initial
impact, the protoplanets re-collide, merge and form a rapidly
spinning Earth-mass planet surrounded by an iron-poor protolunar disk
containing about 3 lunar masses. The composition of the disk and the
final planet's mantle differ by less than 1 percent.
A bigger impactor
Cuk and Stewart's
version of the cosmic smashup posits a roughly Mars-size impactor —
a body with 5 percent to 10 percent the mass of Earth. However, the
other new study — being published in the same issue of Science
today — envisions a collision between two planets in the same
weight class.
"In this impact,
the impactor and the target each contain about 50 percent of the
[present] Earth's mass," Robin Canup, of the Southwest Research
Institute in Boulder, Colo., told SPACE.com via email.
"This type of
impact has not been advocated for the Earth-moon before (although a
similar type of collision has been invoked for the origin of the
Pluto-Charon pair)," Canup added, referring to the largest moon
of Pluto.
In her computer
models, the symmetry of this collision caused the resulting
moon-forming debris disk to be nearly identical in composition to the
mantle of the newly enlarged Earth.
Canup's models further
predict that such an impact would significantly increase Earth's
rotational speed. But that may not be a big issue, since Cuk and
Stewart's work explains how Earth's spin could have slowed over time.
A third study,
published today in the journal Nature, determined that huge amounts
of water boiled away during the moon's birth. The finding, made
by examining moon rocks brought back to Earth by Apollo astronauts,
further bolsters the broad outlines of the giant impact theory.
Though the gigantic
smashup occurred 4.5 billion years ago, scientists may one day be
able to piece together in detail how it all went down, Canup said.
"Models of
terrestrial planet assembly should be able to evaluate the relative
probability of, e.g., the collision I advocate vs. the one proposed
by Cuk and Stewart," she said.
No comments:
Post a Comment