The moon was formed by a violent, head-on collision between
the early Earth and a "planetary embryo" called Theia approximately
100 million years after the Earth formed, UCLA geochemists and colleagues
report.
Scientists had already known about this high-speed crash,
which occurred almost 4.5 billion years ago, but many thought the Earth
collided with Theia (pronounced THAY-eh) at an angle of 45 degrees or more -- a
powerful side-swipe (simulated in this 2012 YouTube video). New evidence
reported Jan. 29 in the journal Science substantially strengthens the case for
a head-on assault.
The researchers analyzed seven rocks brought to the Earth
from the moon by the Apollo 12, 15 and 17 missions, as well as six volcanic
rocks from the Earth's mantle - five from Hawaii and one from Arizona.
 |
| By Gregory H. Revera (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons |
The key to reconstructing the giant impact was a chemical
signature revealed in the rocks' oxygen atoms. (Oxygen makes up 90 percent of
rocks' volume and 50 percent of their weight.) More than 99.9 percent of
Earth's oxygen is O-16, so called because each atom contains eight protons and
eight neutrons. But there also are small quantities of heavier oxygen isotopes:
O-17, which have one extra neutron, and O-18, which have two extra neutrons.
Earth, Mars and other planetary bodies in our solar system each has a unique
ratio of O-17 to O-16 - each one a distinctive "fingerprint."
In 2014, a team of German scientists reported in Science
that the moon also has its own unique ratio of oxygen isotopes, different from
Earth's. The new research finds that is not the case.
"We don't see any difference between the Earth's and
the moon's oxygen isotopes; they're indistinguishable," said Edward Young,
lead author of the new study and a UCLA professor of geochemistry and
cosmochemistry.
Young's research team used state-of-the-art technology and
techniques to make extraordinarily precise and careful measurements, and
verified them with UCLA's new mass spectrometer.
The fact that oxygen in rocks on the Earth and our moon
share chemical signatures was very telling, Young said. Had Earth and Theia
collided in a glancing side blow, the vast majority of the moon would have been
made mainly of Theia, and the Earth and moon should have different oxygen
isotopes. A head-on collision, however, likely would have resulted in similar
chemical composition of both Earth and the moon.
"Theia was thoroughly mixed into both the Earth and the
moon, and evenly dispersed between them," Young said. "This explains
why we don't see a different signature of Theia in the moon versus the
Earth."
Theia, which did not survive the collision (except that it
now makes up large parts of Earth and the moon) was growing and probably would
have become a planet if the crash had not occurred, Young said. Young and some
other scientists believe the planet was approximately the same size as the
Earth; others believe it was smaller, perhaps more similar in size to Mars.
Another interesting question is whether the collision with
Theia removed any water that the early Earth may have contained. After the
collision - perhaps tens of millions of year later - small asteroids likely
hit the Earth, including ones that may have been rich in water, Young said.
Collisions of growing bodies occurred very frequently back then, he said,
although Mars avoided large collisions.
A head-on collision was initially proposed in 2012 by Matija, now a research scientist with the SETI Institute, and Sarah Stewart, now a
professor at UC Davis; and, separately during the same year by Robin Canup of
the Southwest Research Institute.
Co-authors of the Science paper are Issaku Kohl, a
researcher in Young's laboratory; Paul Warren, a researcher in the UCLA
department of Earth, planetary, and space sciences; David Rubie, a research
professor at Germany's Bayerisches Geoinstitut, University of Bayreuth; and
Seth Jacobson and Alessandro Morbidelli, planetary scientists at France's
Laboratoire Lagrange, Université de Nice.
The research was funded by NASA, the Deep Carbon Observatory
and a European Research Council advanced grant (ACCRETE).
For more information visit:-
No comments:
Post a Comment