[meteorite-list] NASA Scientists Theorize Final Growth Spurt For Planets

[Ron Baalke]

Dec. 9, 2010

Dwayne Brown 
Headquarters, Washington                                
202-358-1726 
dwayne.c.brown at nasa.gov 

Cathy Weselby                                          
Ames Research Center, Moffett Field, Calif.                     
650-604-2791 
cathy.weselby at nasa.gov 

Maria Martinez 
Southwest Research Institute, San Antonio 
210-522-3305 
maria.martinez at swri.org 
RELEASE: 10-329

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 
	
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