Scientists theorize final growth spurt for planets
Researchers contend that planets gained their final mass from comet or asteroid impacts.
December 13, 2010
A team of 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.
Image: NASA/Don Davis
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 impacts, and how they affect the evolution of the planets are unknown," said William Bottke from the Southwest Research Institute (SWRI) in Boulder, Colorado. "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 a process called late accretion during a planet’s final growth spurt added the elements.
"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 converters."
The results indicate the largest Earth impactor was between 1,500 to 2,000 miles (2,400 to 3,200 kilometers) 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 to 200 miles (240 to 320 km) 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°, 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."