Tuesday, August 11, 2009

Baptistina's terrible daughters; Asteroid impacts.(The source of the dinosaur's demise).

IT WAS once suggested, to illustrate the chaotic and unpredictable way in which natural systems behave, that the beat of a butterfly's wing in China could eventually trigger a hurricane in the Atlantic. A bit of an exaggeration, perhaps, but the point was that even in the theoretically deterministic world of Newtonian mechanics, only a small amount of complexity is needed to make practical prediction well nigh impossible.

Thus it is perhaps not as far-fetched as it sounds to suggest that the collision 160m years ago of two space rocks, albeit quite large ones, resulted in the stormy death almost 100m years later of the dinosaurs and many other species on Earth. For although the orbits of the planets look to astronomers like a model of regular, Newtonian clockwork, on a scale of millions of years, the solar system is every bit as chaotic as the Earth's weather.

That such a collision was, indeed, responsible for the demise of the dinosaurs is a suggestion made in the September 6th issue of Nature by William Bottke and his colleagues at the Southwest Research Institute in Boulder, Colorado. Dr Bottke has been tracking the orbits of asteroids. These rocky minor planets, most of which are located between the orbits of Mars and Jupiter, often come in families. Members share both chemical composition (which can be deduced from the spectrum of the light they reflect) and orbital characteristics. The assumption is that such families are the daughters of single, larger bodies that have been hit by other asteroids and smashed into pieces. Dr Bottke thinks he has traced one such family back to its original collision.

A smashing time

The largest member of the family in question is called Baptistina, and is some 40km (25 miles) across. In addition to her, the team identified many more than 2,000 smaller objects as belonging to the same family. They were able to do this by looking at those asteroids' orbits in detail.

One of the chaotic things that happen to the paths taken by asteroids is caused by sunlight, and is known as the Yarkovsky effect. Just as most orbiting planets spin on their axes, so asteroids also rotate. When part of an asteroid's surface passes from night to day, it starts to warm up. Because of thermal inertia, the time at which it reaches its maximum temperature comes somewhat after local "noon", a phenomenon that any sunbather will recognise. Since the heat it radiates back into space depends on its temperature, the afternoon side of an asteroid radiates more heat than the morning side. The reaction to this radiation is greater on the afternoon side than on the morning side and there is thus a small force pushing on the afternoon side that gradually distorts the asteroid's orbit. The smaller the asteroid in question, the faster that distortion happens.

The result is that the daughters of Baptistina have become scattered farther and farther away from their mother. From the pattern of this scattering, and from the sizes of the asteroids (which tells you how fast they will scatter), it is possible to calculate the date of the original collision. That turns out to be 160m years ago, give or take about 20m years.

That the dinosaurs were killed off 65m years ago by an asteroid impact is now widely accepted. Dr Bottke's contention is that the deed was done by one of Baptistina's daughters. His evidence is twofold. First, the surviving debris from the devastating impact has the same composition as Baptistina and her offspring. That is a necessary but not a sufficient condition, as the type of rock involved, called carbonaceous chondrite, is found in other asteroids, too. The second line of evidence is the "case of the missing daughters".

The cluster of Baptistina's young has a gap in it. This corresponds to a place where the gravitational pulls of Mars and Jupiter have conspired to change the orbits of any asteroids much faster than the Yarkovsky effect could manage alone. Such asteroids would rapidly have adopted orbits that cut across those of the inner planets, including Earth. Many of the larger ones would have started arriving about 100m years ago. Sooner or later, collisions would have been inevitable.

Those collisions are why the asteroids that ought to be in the space are missing. They are probably also the explanation for the fact that, over the past 100m years or so, the number of asteroids that have hit the Earth is about twice that which would have been expected. Taking into account the circumstances that pertained 65m years ago, Dr Bottke reckons it more than 90% probable that the dinosaur killer was one of Baptistina's daughters.

The hole made by that collision, in what is now the Yucatan Peninsula in southern Mexico, has long since been buried in thick layers of sediment under the sea. But anyone with a pair of field glasses can see for himself what Baptistina's daughters are capable of.

The lunar crater named after Tycho Brahe, a 16th century Danish astronomer, near the moon's south pole, is 85km across. It is surrounded by rays of ejecta that stretch across much of the southern lunar hemisphere, indicating the violence of the impact that formed it. It is thought to be just over 100m years old (Apollo 17 brought back material from the site, which allowed it to be dated), and it is reckoned by Dr Bottke to be the result of the moon encountering one of the first of Baptistina's progeny to arrive in the inner solar system. It is a beautiful sight. But you would not have wanted to witness, at close quarters, its creation.

Source Citation:"Baptistina's terrible daughters; Asteroid impacts.(The source of the dinosaur's demise)." The Economist (US) 384.8545 (Sept 8, 2007): 82US. Academic OneFile. Gale. BROWARD COUNTY LIBRARY. 11 Aug. 2009

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