NASA's LRO Exposes Moon's Complex, Turbulent Youth
The moon was bombarded by two distinct populations of asteroids or comets in its youth, and its surface is more complex than previously thought, according to new results from NASA's Lunar Reconnaissance Orbiter (LRO) spacecraft featured in three papers appearing in the Sept. 17 issue of Science. In the first paper, lead author James Head of Brown University in Providence, R.I., describes results obtained from a detailed global topographic map of the moon created using LRO's Lunar Orbiter Laser Altimeter (LOLA). "Our new LRO LOLA dataset shows that the older highland impactor population can be clearly distinguished from the younger population in the lunar 'maria' giant impact basins filled with solidified lava flows," says Head.

"The highlands have a greater density of large craters compared to smaller ones, implying that the earlier population of impactors had a proportionally greater number of large fragments than the population that characterized later lunar history." Meteorite impacts can radically alter the history of a planet. The moon, Mars, and Mercury all bear scars of ancient craters hundreds or even thousands of miles across. If Earth was subjected to this assault as well and there's no reason to assume our planet was spared these enormous impacts could have disrupted the initial origin of life. Large impacts that occurred later appear to have altered life's evolution. The approximately 110-mile-diameter, in the Yucatan Peninsula of Mexico, is from an impact about 65 million years ago that is now widely believed to have led or contributed to the demise of the dinosaurs and many other lifeforms.

Scientists trying to reconstruct the meteorite bombardment history of Earth face difficulty because impact craters are eroded by wind and water, or destroyed by the action of plate tectonics, the gradual movement and recycling of the Earth's crust. However, a rich record of craters is preserved on the moon, because it has only an extremely thin atmosphere – a vacuum better than those typically used for experiments in laboratories on Earth. The moon’s surface has no liquid water and no plate tectonics. The only source of significant erosion is other impacts. "The moon is thus analogous to a Rosetta stone for understanding the bombardment history of the Earth," said Head. "Like the Rosetta stone, the lunar record can be used to translate the 'hieroglyphics' of the poorly preserved impact record on Earth."

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