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Planetary News: Asteroids and Comets (2007)Ancient Collision between Asteroids Likely Transformed Life on EarthBy Amir AlexanderSeptember 21, 2007
It happened 160 million years ago in the inner asteroid belt: a massive space rock, around 60 kilometers in diameter, slammed into a planetoid with a diameter three times as large and broke it into small pieces. It was an anonymous catastrophe, and it took place in the silent emptiness of space hundreds of millions of miles away, somewhere between the orbits of Jupiter and Mars. Nevertheless, according to a recent paper published in Nature, this ancient collision ultimately transformed life on Earth and changed the course of evolution. The reason for this surprising conclusion is that according to the paper’s authors, the collision of the two giant rocks created a new family of asteroids known today as the “Baptistina asteroid family.”Using both observations and computer simulations, the authors concluded that a significant segment of the Baptistina family asteroids escaped the asteroid belt and entered orbits that cross the paths of the four inner planets – Mercury, Venus, Earth, and Mars. A portion of these, they calculated, slammed into Earth, creating a two-fold increase in the number of impactors raining down on our planet. In particular, the authors argue, there is a 90% probability that the Chicxulub crater in the Yucatan peninsula was created by a massive rock over 10 kilometers in diameter that was initially part of the Baptistina family. It is widely accepted today that the impact that formed the Cicxulub crater is largely responsible for the mass extinction of species known as the "K/T extinction," which took place on Earth 65 million years ago. The impact sent volumes of dust, ash, and debris into the atmosphere, darkening the skies and initiating a global winter that lasted years. In the harsh logic of evolution, species that could not cope with the suddenly changed conditions died out, making room for other more adaptive creatures. The extinction most famously did away with the dinosaurs, who had roamed the Earth for hundreds of millions of years, making possible the emergence of mammals as the dominant class of large animals. We do not know what course evolution would have taken were it not for that cataclysmic event. There is no doubt, however, that the rich landscape of life on our planet would have looked vastly different than it actually does today. The article that carries such far-reaching implications about the history of our planet is entitled “An asteroid breakup 160 Myr Ago as the probable source of the K/T impactor” and appeared in the September 6, 2007, issue of Nature. It is the result of an international collaboration between William F. Bottke and David Nesvorny of the Southwest Research Institute in Boulder, Colorado, and David Vokrouhlicky of the Institute of Astronomy at Charles University in Prague. In their analysis the researchers traced the space rocks belonging to the asteroid family clustered around the present day 40 kilometer wide asteroid known as “Baptistina.” In order to determine the event that gave birth to this family of rocks the researchers considered how the orbits of asteroids evolve over time. Over millions of years, light from the Sun which illuminates one side of a rock but not the other will affect an asteroid’s orbit. Under the influence of these photonic effects, known as Yarkovsky and YORP effects, space rocks who have been moving in close orbits will move further apart and spread into a range of paths. The most affected are the lightest asteroids, which will stray the forthest from their original orbits, whereas the heavier rocks will remain relatively unmoved. Starting with the current orbit and size distribution of the Baptistina family, Bottke and his colleagues then worked backwards to determine the original event that spawned it. They concluded that the present day Baptistina asteroid is in fact a fragment of an ancient planetoid with a diameter of around 170 kilometers. By running computer simulations they concluded that the observable mass distribution among the asteroid family is best explained by a collision between the ancient Baptistina and a 60 kilometer wide asteroid. They also determined that this cataclysmic event took place around 160 million years ago, give or take 20 million years.
But when looking at the orbital distribution of the asteroid family, the researchers also noted a striking anomaly. Not far inside the orbit of the current Baptistina asteroid is a region with an unusual feature: the orbital periods of objects within this region are at the elegant ratio of 7:2 to the period of Jupiter, and of 5:9 to the period of Mars. This means that an asteroid in this range (known as the J7:2/M5:9 range) will complete seven revolutions around the Sun every two Jupiter years, and five revolutions every nine Mars years. In their current dispersion asteroids of the Baptistina family are well represented both inside and outside J7:2/M5:9, but within the range itself the asteroid population is noticeably sparse. The reason for this became clear once Bottke and his colleagues started running simulations of the Baptistina family’e evolution over time. The orbits of Space rocks caught in the J7:2/M5:9 region resonate strongly with the orbits of the planets, Jupiter and Mars. The result is that the asteroids’ orbit becomes more and more elongated, until their path takes into the inner solar system, crossing the paths of Mercury, Venus, Earth, and Mars. The J7:2/M5:9 range, in other words, acts as a “dynamical superhighway,” sending asteroids into the inner solar system and collision courses with the terrestrial planets. But how many of these actually slammed into Earth? To estimate this the researchers ran computer simulations tracking the fate of 9,024 hypothetical space rocks orbiting in the J7:2/M5:9 region. They found that within 200 million years about 1.7% of these asteroids impacted the Earth, with the shower peaking about 40 million years after the original Baptistina event and then tapering off. Overall, Bottke and his colleagues concluded, the Baptistina event doubled the rate of impacts of large space rocks on Earth over a period of around 100 million years. “The Baptistina bombardment produced a prolonged surge in the impact flux that peaked roughly 100 million years ago” Bottke said, and pointed out that this agrees well with what is known about the impact record on Earth. “We are now in the tail end of this shower” he added. But long before the Baptistina asteroid shower tapered off, it may had already left its mark on our planet. 65 million years ago a massive space rock, around 10 kilometers in diameter, slammed into the coast of the Yucatan peninsula, initiating the K/T extinction. When life revived the giant dinosaurs were gone, and warm-blooded mammals had taken their place in the Earth’s biosphere. Can this momentous transformation of life on Earth be traced to the break-up of the Baptistina asteroid 160 million years ago? Almost certainly yes, according to Bottke and his colleagues. Studies on Earth indicate that the Cicxulub impactor was a primitive asteroid, a variety of the type known as “carbonaceous chondryte.” This identification excludes from consideration the vast majority of the ordinary run of space rocks that pass by the Earth. But it is perfectly consistent with what is known of the composition of asteroids of the Baptistina family, which constituted about half of the population of near Earth objects (NEO’s) at the time of the Chicxulub impact.
Taking this into account, the authors then ran computer simulations to estimate the likelihood that a giant asteroid, 10 kilometers in diameter or more, would slam into the Earth. They found the chances of such an impact from an asteroid belonging the Baptistina family were as high as 60% in the past 160 million years ; in contrast, a similar impact from any other source is likely to occur on our planet only once every 2 billion years or more! The conclusion was inevitable: there is a greater than 90% chance that the Chicxulub crater was formed by a cataclysmic collision between the Earth and an asteroid of the Baptistina family. One can only imagine what life on our planet might look like today, were it not for this errant refugee from the inner asteroid belt slamming into central America. Would the dinosaurs still be with us? Would mammals have remained the small and opportunist creatures they had been, before the demise of the dinosaurs allowed them to spread and flourish? Would higher intelligence ever have evolved under these circumstances, and what form would it take? We will never know. All we know is that 160 million years ago two primitive rocks collided in the vast emptiness of space, somewhere between the orbits of Mars and Jupiter; and for life on Earth, for the dinosaurs, for mammals, and ultimately for us humans, that has made all the difference. |
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