by Whit Gibbons

May 1, 2005

And now for something completely different. A recent scientific paper heralds potential ecological benefits that can result from enormous comets or asteroids striking Earth. Understanding, of course, that whatever or whoever happens to be at the point of contact when a sizable rock knocks a half-mile-diameter crater in the earth’s surface will find little positive to report. In fact, they will not be around to report at all. Nonetheless, according to the authors, the impacts of asteroids and comets “can also have beneficial influence on processes from the molecular to the evolutionary scale.”

Written for the journal “Trends in Ecology and Evolution,” the paper by Charles S. Cockell of the Open University in England and Philip A. Bland of Imperial College London gives an intriguing perspective and positive spin to what many would consider the world’s worst nightmare. But what happens to life on a global scale must be viewed differently from what happens to a particular species or region at ground zero. Hence, these “extraterrestrial agents of biological change” can be credited with contributing to important steps in the process of life on Earth.

First, making a distinction between the world colliding with a meteor that is a comet, which is made up of frozen gasses and rocks, and one that is an asteroid, which is almost all rock, is unnecessary for making the point that a sizable chunk of either would get our attention if it made it through Earth’s atmosphere to the ground. By one calculation, collision with a rock six miles in diameter would release an unimaginable explosion--“the equivalent energy of 10 million megatons of TNT”; 1,000 times more powerful than the atomic bombs dropped on Japan to end World War II. Whether the impact was made by rocks from a comet or an asteroid wouldn’t much matter.

According to the article, past collisions could have resulted in at least three notable outcomes for life on Earth. First, and most important if true, is that extraterrestrial impacts could have prestaged the beginning of life. The prelife scenario could have been one in which a meteor actually brought the organic materials necessary for the chemical reactions that eventually led to living matter. Another hypothesis is that the impact provided a dramatic jolt of energy that led to the development of complex molecular structures necessary for life. Even a significant portion of the life-essential water present on Earth today could have arrived in the form of ice-laden comets that melted upon impact. All of this would have happened about 4 billion years ago, give or take a decade or two, so it all is obviously theory, not fact. But providing alternative hypotheses, when no one knows for sure, is what science is all about.

A second beneficial feature of celestial bombardment would have occurred after life on Earth was already well established. Most meteors hitting Earth today land in the oceans, which cover three-fourths of the planet. One paleontological record in the Arctic Ocean suggests that an enormous tsunami was created from an impact several million years ago. Sediments today indicate that the inland wave was of such proportion that high productivity was created by the distribution of water inland that brought nutrients back to the sea. Impacts on land itself create craters that can become freshwater lakes that support life forms incompatible with marine environments. Such reorganization of hydrological and nutrient cycles, as well as the creation of new habitats, can be viewed as beneficial to life on Earth in the broadest sense.

Finally, the paper presents the positive side of the extinction of entire groups of organisms as a consequence of massive meteor collisions. For example, 200 million years ago a collision of such proportion occurred that much of life on Earth was diminished due to darkened skies, acid rain, flooding, and fires. Following this event, the dinosaurs evolved and became the reigning animals until 65 million years ago when it happened again. Following that event came the mammals and us.

We need not stay up nights worrying about the phenomenon reoccurring, as impacts of such magnitude occur only about once every 100 million years or so. With 35 million years to go, it's a bit early to start counting.

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