University of Arizona and Japanese scientists are convinced thatevidence at last settles decades-long arguments about what objectsbombarded the early inner solar system in a cataclysm 3.9 billion yearsago.
Ancient main belt asteroids identical in size to present-dayasteroids in the Mars-Jupiter belt -- not comets -- hammered the innerrocky planets in a unique catastrophe that lasted for a blink ofgeologic time, anywhere from 20 million to 150 million years, theyreport in the Sept. 16 issue of Science.
However, the objects that have been battering our inner solar systemafter the so-called Late Heavy Bombardment ended are a distinctlydifferent population, UA Professor Emeritus Robert Strom and colleaguesreport in the article, "The Origin of Planetary Impactors in the InnerSolar System."
After the Late Heavy Bombardment or Lunar Cataclysm period ended,mostly near-Earth asteroids (NEAs) have peppered the terrestrial region.
Strom has been studying the size and distribution of craters acrosssolar system surfaces for the past 35 years. He has long suspected thattwo different projectile populations have been responsible forcratering inner solar system surfaces. But there's been too little datato prove it.
Now asteroid surveys conducted by UA's Spacewatch, the Sloan DigitalSky Survey, Japan's Subaru telescope and the like have amassed fairlycomplete data on asteroids down to those with diameters of less than akilometer. Suddenly it has become possible to compare the sizes ofasteroids with the sizes of projectiles that blasted craters intosurfaces from Mars inward to Mercury.
"When we derived the projectile sizes from the cratering recordusing scaling laws, the ancient and more recent projectile sizesmatched the ancient and younger asteroid populations smack on," Stromsaid. "It's an astonishing fit."
"One thing this says is that the present-day size-distribution ofasteroids in the asteroid belt was established at least as far back as4 billion years ago," UA planetary scientist Renu Malhotra, a co-authorof the Science paper, said. "Another thing it says is that themechanism that caused the Late Heavy Bombardment was a gravitationalevent that swept objects out of the asteroid belt regardless of size."
Malhotra discovered in previous research what this mechanism musthave been. Near the end of their formation, Jupiter and the other outergas giant planets swept up planetary debris farther out in the solarsystem, the Kuiper Belt region. In clearing up dust and pieces leftoverfrom outer solar system planet formation, Jupiter, especially, lostorbital energy and moved inward, closer to the sun. That migrationgreatly enhanced Jupiter's gravitational influence on the asteroidbelt, flinging asteroids irrespective of size toward the inner solarsystem.
Evidence that main belt asteroids pummeled the early inner solarsystem confirms a previously published cosmochemical analysis by UAplanetary scientist David A. Kring and colleagues.
"The size distribution of impact craters in the ancient highlands ofthe moon and Mars is a completely independent test of the inner solarsystem cataclysm and confirms our cosmochemical evidence of an asteroidsource," Kring, a co-author of the Science paper, said.
Kring was part of a team that earlier used an argon-argon datingtechnique in analyzing impact melt ages of lunar meteorites -- rocksejected at random from the moon's surface and that landed on Earthafter a million or so years in space. They found from the ages of the"clasts," or melted rock fragments, in the breccia meteorites that allof the moon was bombarded 3.9 billion years ago, a true global lunarcataclysm. The Apollo lunar sample analysis said that asteroids accountfor at least 80 percent of lunar impacts.
Comets have played a relatively minor role in inner solar systemimpacts, Strom, Malhotra and Kring also conclude from their work.Contrary to popular belief, probably no more than 10 percent of Earth'swater has come from comets, Strom said.
After the Late Heavy Bombardment, terrestrial surfaces were socompletely altered that no surface older than 3.9 billion years can bedated using the cratering record. Older rocks and minerals are found onthe moon and Earth, but they are fragments of older surfaces that werebroken up by impacts, the researchers said.
Strom said that if Earth had oceans between 4.4 billion and 4billion years ago, as other geological evidence suggests, those oceansmust have been vaporized by the asteroid impacts during the cataclysm.
Kring also has developed a hypothesis that suggests that the impactevents during Late Heavy Bombardment generated vast subsurfacehydrothermal systems that were critical to the early development oflife. He estimated that the inner solar system cataclysm produced morethan 20,000 craters between 10 kilometers to 1,000 kilometers indiameter on Earth.
Inner solar system cratering dynamics changed dramatically after theLate Heavy Bombardment. From then on, the impact cratering recordreflects that most objects hitting inner solar system surfaces havebeen near-Earth asteroids, smaller asteroids from the main belt thatare nudged into terrestrial-crossing orbits by a size-selectivephenomenon called the Yarkovsky Effect.
The effect has to do with the way asteroids unevenly absorb andre-radiate the sun's energy. Over tens of millions of years, the effectis large enough to push asteroids smaller than 20 kilometers acrossinto the jovian resonances, or gaps, that deliver them toterrestrial-crossing orbits. The smaller the asteroid, the more it isinfluenced by the Yarkovsky Effect.
Planetary geologists have tried counting craters and their sizedistribution to get absolute ages for surfaces on the planets and moons.
"But until we knew the origin of the projectiles, there has been somuch uncertainty that I thought it could lead to enormous error," Stromsaid. "And now I know I'm right. For example, people have based thegeologic history of Mars on the heavy bombardment cratering record, andit's wrong because they're using only one cratering curve, not two."
Attempts to date outer solar system bodies using the inner solarsystem cratering record is completely wrong, Strom said. But it shouldbe possible to more accurately date inner solar system surfaces onceresearchers determine the cratering rate from the near-Earth asteroidbombardment, he added.
The authors of the Science paper are Strom, Malhotra and Kring fromthe University of Arizona Lunar and Planetary Laboratory, and TakashiIto and Fumi Yoshida of National Astronomical Observatory, Tokyo, Japan.
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