If quasars and supernovae do not seem to be likely methods for ending all life on Earth, are there other astronomical events that may be more likely to destroy the biosphere? Around 1980, interest in extraterrestrial explanations for extinctions switched from supernovae to the effects of collisions between the Earth and comets or asteroids.18 For the past 25 years, this idea has been the front-runner for the explanation of the end Cretaceous ('K-T') extinction mentioned earlier, coinciding with the disappearance of the dinosaurs among other notable groups. There is now a wide range of evidence consistent with an impact at approximately this time, and even a possible impact site located in the Yucatan in Mexico.19,20 Clearly, this event did not destroy the whole biosphere, but could more catastrophic impacts have happened in the remote geological past—or indeed happen in the future?
The surface of the moon is a good place to start when thinking about these questions, as it is covered in impact craters in a way the Earth is not. Indeed the maria (dark 'seas' on the moon's surface) are enormous craters filled with solidified lava created by massive past impact events.21 Our planet should have suffered even more impacts than the moon, because it is larger (and so a bigger target) and has a stronger gravity22— indeed it has been estimated that for every object that hit the Moon around 20 objects should have hit the Earth.23 However, the more active surface of the Earth, with both more erosion and rock recycling by plate tectonics, has removed much of the evidence for impacts in the distant past.
Early in the history of our solar system, impacts from extraterrestrial objects were much more common due to the greater abundance of asteroids and comets—the population sizes of these objects have been reduced over time as they crash into things. Indeed, the period around 4.4 to 3.8 billion years ago (here we are defining billion as 1,000 million, as is now standard in science) is often referred to as the 'heavy bombardment', because of the high frequency of such collisions.24 Some of these early collisions seem highly likely to have been capable of destroying any life that might have started on the early Earth. For example, about 100 million years after the formation of the solar system (thought to have formed just over 4.5 billion years ago) there is widely accepted evidence that a Mars-sized object collided with the Earth, and that debris from this collision eventually formed our Moon. This collision is predicted to have caused temperatures that would have melted the rocks forming the proto-Earth25—a situation presumably incompatible with life.
Lesser collisions could still have the potential to end the biosphere, either in the past or in the future. For example, Norman Sleep and colleagues22 calculated that a collision with a 440 km diameter rock (the size of one of the larger asteroids) would release enough energy to evaporate all of the Earth's oceans. Such events could certainly have happened during the heavy bombardment—but could similar collisions happen in the future, so ending our biosphere? Eros, a slightly smaller asteroid than those modelled by Sleep and colleagues, currently has an orbit that on occasion crosses the Earth's trajectory. Calculations suggest that there is approximately a 50% chance of a collision between Eros and Earth sometime in the next 100 million years.26 Although this would not produce enough energy to vaporize the oceans in the way described earlier, it is estimated that the collision would release an order of magnitude more energy than the impact that is suspected of ending the Cretaceous.26 The effect on the biosphere would certainly be severe.
Clearly, there is an element of chance in the Earth avoiding a potentially biosphere-destroying event, such as a collision with a large asteroid. Such events could certainly lead to the extinction of humans, elephants, oak trees, and the like—however, it is much more difficult to kill all microbial life on Earth.27 Even for the very largest collisions there is the possibility of microbes surviving inside rock material ejected from the surface of the Earth during the collision. Such microbe-rich rocks could return to Earth, attracted by its gravity, after conditions had improved and the microbes could potentially restart life on our planet24—the biosphere equivalent of rebooting a crashed computer. We will return to the difficulty in destroying the Earth's microbial biosphere later in this chapter.
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