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5 December 2013 11:26 am ,
Vol. 342 ,
Dyslexia, a learning disability that hinders reading, hasn't been associated with deficits in vision, hearing, or...
Exotic, elusive, and dangerous, snakes have fascinated humankind for millennia. They can be hard to find, yet their...
Researchers have sequenced and analyzed the first two snake genomes, which represent two evolutionary extremes. The...
Snake venoms are remarkably complex mixtures that can stun or kill prey within minutes. But more and more researchers...
At age 30, Dutch biologist Freek Vonk has built up a respectable career as a snake scientist. But in his home country,...
Since arriving on the island of Guam in the 1940s, the brown tree snake ( Boiga irregularis ) has extirpated native...
An animal rights group known as the Nonhuman Rights Project filed lawsuits in three New York courts this week in an...
Researchers have been hot on the trail of the elusive Denisovans, a type of ancient human known only by their DNA and...
- 5 December 2013 11:26 am , Vol. 342 , #6163
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Where Did Those Craters Come From?
15 September 2005 (All day)
As scientists accumulate evidence that something battered the inner planets 3.9 billion years ago, some say they are homing in on what did the pummeling. New findings indicate that the massive cratering seen on Earth and its neighbors originated in the asteroid belt. That in turn would point to a reshuffling of the outer planets as the ultimate cause of a cataclysmic bombardment that may have short-circuited the rise of life on Earth.
The most obvious clues to the source of the so-called late heavy bombardment are the many craters left behind and the hulking impactors responsible, as derived from crater size. So Robert Strom, a professor emeritus at the University of Arizona, Tucson, and co-workers compiled Strom's published and unpublished crater counts from the most pockmarked planetary surfaces, such as the highlands of the moon. They did the same for younger, more lightly cratered areas, such as certain volcanic plains on Mars.
An unusual preponderance of the objects that hit the younger surfaces were small, they found, a size distribution that matches that of the near-Earth asteroids that have drifted in from the main belt more recently. By contrast, a greater proportion of large impactors had cratered older terrains. That indicated that a different mechanism must have driven the ancient bombardment--one that did not discriminate between large and small asteroids.
The group argues that the asteroids must have pummeled the inner solar system after a rearrangement of the outer planets. Perhaps Jupiter and Saturn teamed up to scatter asteroids gravitationally (Science, 3 December 2004, p. 1676), or Neptune and Uranus formed long after the rest of the planets, the team proposes in the 16 September Science. Cratering specialist William Bottke of the Southwest Research Institute in Boulder, Colorado, suspects that Strom and his colleagues are on to something, but says the case remains open.
More on the late heavy bombardment