NASA Scientists Theorize Final Growth Spurt For Planets
MOFFETT FIELD, Calif -- A team of NASA-funded researchers has unveiled a new theory that contends planets gained the final portions of their mass from a limited number of large comet or asteroid impacts more than 4.5 billion years ago. These impacts added less than one percent of the planets' mass.Scientists hope the research not only will provide a better historical picture of the birth and evolution of Earth, the moon and Mars, but also allow researchers to better explore what happened in our solar system's beginning and middle stages of planet formation.
"No one has a model of precisely what happened at the end of planet
formation-we've had a broad idea-but variables such as impactor size,
the approximate timing of the impacts, and how they affect the
evolution of the planets are unknown," said William Bottke, principal
investigator from the Southwest Research Institute (SWRI) in Boulder,
Colo. "This research hopefully provides better insights into the
early stages of planet formation."
The team used numerical models, lunar samples returned by Apollo
astronauts and meteorites believed to be from Mars to develop its
findings. The scientists examined the abundances of elements such as
gold and platinum in the mantles, or layers beneath the crust, of
Earth, the moon and Mars. Consistent with previous studies, they
concluded the elements were added by a process called late accretion
during a planet's final growth spurt.
"These impactors probably represent the largest objects to hit Earth
since the giant impact that formed our moon," Bottke said. "They also
may be responsible for the accessible abundance of gold, platinum,
palladium, and other important metals used by our society today in
items ranging from jewelry to our cars' catalytic convertors."
The results indicate the largest Earth impactor was between 1,500 -
2,000 miles in diameter, roughly the size of Pluto. Because it is
smaller than Earth, the moon avoided such enormous projectiles and
was only hit by impactors 150 - 200 miles wide. These impacts may
have played important roles in the evolution of both worlds. For
example, the projectiles that struck Earth may have modified the
orientation of its spin axis by 10 degrees, while those that hit the
moon may have delivered water to its mantle.
"Keep in mind that while the idea the Earth-moon system owes its
existence to a single, random event was initially viewed as radical,
it is now believed that large impacts were commonplace during the
final stages of planet formation,' Bottke said. "Our new results
provide additional evidence that the effects of large impacts did not
end with the moon-forming event."
The paper, "Stochastic Late Accretion to the Earth, Moon, and Mars,"
was published in the Dec. 9 issue of Science. It was written by
Bottke and David Nesvorny of SWRI; Richard J. Walker of the
University of Maryland; James Day of the University of Maryland and
Scripps Institution of Oceanography, University of California, San
Diego; and Linda Elkins-Tanton of the Massachusetts Institute of
Technology. The research is funded by the NASA Lunar Science
Institute (NLSI) at the agency's Ames Research Center in Moffett Field, Calif.
The NLSI is a virtual organization that enables collaborative,
interdisciplinary research in support of NASA lunar science programs.
The institute uses technology to bring scientists together around the
world and comprises competitively selected U.S. teams and several
international partners. NASA's Science Mission Directorate and the
Exploration Systems Mission Directorate at the agency's Headquarters
in Washington, funds the institute, which is managed by a central office at Ames.
For more information on NLSI, visit:
http://lunarscience.nasa.gov
Source: NASA
