The Threat of Large Impacts
by Gabriel Chong
On February 15, 2013, a 17 meter-long meteor exploded in the air above Chelyabinsk, Russia, releasing energy equivalent to the detonation of approximately 30 Hiroshima bombs, and injured over a thousand people. According to NASA, it was the largest meteor impact in about 100 years. The following day, the asteroid DA14, measuring three times as large, zipped past the earth at a hair’s breadth of 17,100 miles. Had it collided with the Earth, it would have no doubt triggered an even more cataclysmic aftermath. The unexpectedness of the Russian meteor and the temporal proximity of both events have captured the imagination of the public regarding apocalyptic large impacts. But just how often do large impacts occur and how greatly do they pose as existential threats to life on earth?
Large impacts, as it turns out, are actually very common in the Earth’s history. A useful comparative phenomenon would be earthquakes. Large impacts, like earthquakes, are not singular, discrete events that only occur periodically. In fact, thousands of space rocks land on earth every day, though they are usually so tiny that their impact is negligible, just as thousands of tremors occur on a daily basis, though they are mostly so mild as to escape our perception. Large impacts occur because the earth does not orbit in a clear vacuum, but through a swarm of debris, asteroids, comets, meteoroids, and all kinds of hazardous near-earth objects (NEOs) – remnants of the formation of the solar system, just as earthquakes occur because our planet is not static, but tectonically active and heated by natural convection.
The relation between the magnitude and frequency of both large impacts and earthquakes is logarithmic. In other words, the frequency of an event decreases exponentially with regards to increase in magnitude. For example, it is estimated that earthquakes with a magnitude of 2.9 or lower on the Richter scale take place more than a million times annually, whereas catastrophic earthquakes such as the one that triggered the 2011 Japanese tsunami (measuring 8.0 or greater on the Richter scale) occur only about once a year. Likewise, asteroids measuring less than one meter in diameter collide with the Earth on an hourly basis (the majority of which take place in remote areas or the oceans that cover 70% of the earth’s surface). Meanwhile, very large impacts (objects measuring 1 km in diameter or more), such as the Chicxulub asteroid that wiped out the dinosaurs during the Cretaceous period – occur only once every few hundred millions of years.
“…asteroids measuring less than one meter in diameter collide with the Earth on an hourly basis…”
Curiously, as much as large impacts pose an existential threat to us, they might have been also crucial in shaping the Earth’s habitability. In fact, our very own moon was likely the product of a large impact. Although various theories abound regarding the origin of the moon,the most probable hypothesis was that a Mars-sized planetesimal collided with the young earth as early as within its first 100 million years. It was then gravitationally captured by the Earth and has remained as our only satellite ever since. The large impact hypothesis has been corroborated by various features of the Moon (that it lacks an iron core characteristic of the rocky planets, that it shares a similar density and isotopic content with the Earth’s mantle, etc.) and thus, conforms to existing theories of planet formation.
“…as much as large impacts pose an existential threat to us, they might have been also crucial in shaping the Earth’s habitability”
Without the Moon, life on the Earth may not have existed at all, or may have evolved very differently. The gravitational pull of the Moon creates the tides, which distribute heat across the globe and deposit chemicals from the continents into the oceans. The resulting mineral richness of the oceans may have been crucial for the emergence of the first complex organic compounds. Additionally, tidal heating contributes to the Earth’s average global temperature being higher than it would have been otherwise. Furthermore, the same gravitational pull has also significantly reduced the rotation speed of the earth since the early epochs of its formation, as well as kept it tilting within a small range of angles, both of which were vital to stabilizing the earth’s magnetic field and giving it a more even, temperate climate necessary for the development of complex life.
At present, the threat of an extremely large impact is unlikely to arrive as a surprise. The state of modern asteroid detection technology is such that any impending large NEO hurling towards the Earth will likely have been predicted at least months prior to the impact and we will have much warning in advance. However, three caveats abound regarding the threat of impact events. Firstly, we have not yet devised an efficient or economically sound method of deflecting a potentially devastating large impact. Secondly, while extremely large NEOs are reasonably easy to detect, there are many small ones which fall within the ‘blind spot’ of asteroid detection – asteroids that are small enough to escape our radar, yet massive enough to potentially result in moderate destruction (such as the recent Chelyabinsk meteor). In the short run (from a human perspective), these moderatesized impacts are likely to cause more aggregate damage than extremely large impacts. Last but not least, extremely large impacts are a type of ‘black swan’ event – events that are potentially extremely catastrophic, but their high improbability renders it almost impossible to predict their next occurrence.
ABOUT THE AUTHOR:
Yong Wei Chong Gabriel is a philosophy student at Wellesley College. Her column aims to break down popular topics in science into digestible bits for the lay reader. Gabriel can be contacted at [email protected]. Find out more by visiting her Scientific Malaysian profile at http://www.scientificmalaysian.com/members/gabrielle/