Why we should worry about comets, asteroids and mass extinctions
Every once in a while, the Earth presses the reset button on life. Virtually all plant and animal species that have ever existed on Earth are extinct today. The fossil record and geological data show that mass extinctions occur periodically when life on Earth undergoes short periods of intense stress. Yet mass extinctions play a vital role in the evolution of life on Earth. New science points to major comet or asteroid impacts as the cause of mass extinction events.
Comets and asteroids come in all shapes and sizes: large asteroids (over 1 km in diameter) are thought to impact earth approximately every 500,000 years and have severe global repercussions. Small asteroids (10-20 m / 33-66 feet) impact Earth frequently, and most disintegrate in the atmosphere, sometimes producing powerful airburst or "fireballs".
Although large impacts are rare, even a 60-meter (197-foot) asteroid can release the energy equivalent of about 185 Hiroshima A-bombs and destroy a large metropolitan area. NASA warns that we are woefully unprepared for comet and asteroid impacts and that the risks are underestimated.
Mass extinctions in Earth's history
There are five known major mass extinction events (“The Big Five”):
439 million years ago, 70% species life on Earth was wiped out
364 million years ago, 70% of species were lost
252 million years ago: 90-96% of all species became extinct.
201.3 million years ago. 70-75% of all species are wiped out.
65 million years ago: 76% of life on Earth disappears, including the dinosaurs.
But there have been many more extinction events, that have decimated a huge fraction of the Earth’s species diversity. The red line shows the rate of extinction of marine genera over time (the peaks in the graph show the highest extinction rates):
Extinction rate of marine invertebrate genera over time in millions of years (Alroy 2008)
What causes mass extinction events?
Mass extinctions are thought to be caused by a combination of massive climate change, including ice ages, drastic sea level rise and fall, earthquakes and increased volcanism, all of which may be caused by the impact and aftermath of comet or asteroid impacts. Such impacts could also have caused megatsunamis and global forest fires.
A comet fragment or asteroid only a few kilometers in diameter is estimated to release the energy of several million nuclear weapons detonating simultaneously (e.g. Comet Shoemaker–Levy 9).
But comets and asteroids are not all bad. Major impact events by comets or asteroids are probably responsible for the formation of the Earth–Moon system, the origin of water on Earth and the evolutionary history of life, which unfortunately includes periodic mass extinctions.
The link between mass extinctions and impact craters
This 2015 study shows a striking correlation between comet or asteroid impact craters and extinction events over the last 260 million years, suggesting a cause-and-effect relationship.
The arrows in this figure point to the major extinction events, which correlate to the increased impact craters over the last 260 million years.
Mass extinctions and crater formation rate over time in millions of years (from Rampino & Caldera 2015)
Chicxulub crater and the dinosaurs
One of the craters considered in this study is the famous Chicxulub crater located in the Gulf of Mexico, which dates back to about 65 million years ago—the time of a great mass extinction that included the dinosaurs. It is the third largest known impact crater at 180 km wide (112 mi); the asteroid or comet that caused the crater is estimated to be 10 km (6.2 mi) in diameter and delivered an estimated energy of over a billion times the energy of the atomic bombings of Hiroshima and Nagasaki.
On the bright side, the extinction of the dinosaurs provided an opportunity for mammals to diversify, and eventually, humans to flourish – but not for long before another major comet impact occurred.
The Younger Dryas comet impact
Another devastating comet impact occurred much more recently, just as human civilization was emerging.
Recent and mounting scientific evidence (also see this article) demonstrates that a massive impact occurred over North America, Europe and parts of South America around 12,800 years ago. The estimated explosive capacity of this comet impact is about one thousand times greater than the entire world's nuclear device stockpile.
The comet impact is thought to have been composed of a conglomerate of comet fragments that entered the Earth over Canada, which would have been covered by an ice sheet two miles thick at the time. Due to this ice sheet, any disturbances caused by the falling fragments, one of which could have been 4 km (2.5 mi) in diameter, would have left only transient marks or depressions under the Canadian Shield (e.g. Great Lakes, Hudson Bay). Other fragments would have exploded upon entry into the atmosphere and caused powerful air bursts (i.e. "fireballs").
The impact caused drastic and abrupt climate and environmental changes, major ecological reorganization, broad-scale species extinctions and rapid human behavioural shifts. This devastation occurred just as humans were beginning to organize cities and agriculture, and the Younger Dryas must have been a major setback for human civilization.
Rapid climate change during the Younger Dryas is clearly observable in paleoclimate records:
Abrupt climate change during the Younger Dryas. Source: Abrupt Climate Change; Inevitable Surprises
Recent evidence for the Younger Dryas comet impact includes nanodiamonds, melt-glass, carbon spherules and other high-temperature materials, which are found in abundance in the sediments from this time period, throughout the Younger Dryas Boundary field (seen below). Because these materials formed at temperatures in excess of 2200 degrees Celsius, the fact they are present together in a thin layer of sediments dated to 12,800 years ago suggests they were likely created by a major comet impact event.
Only two layers of sediments containing these nanodiamonds exist: the Younger Dryas boundary, dated to 12,800 years ago, and a deeper layer, dated to 65 million years ago, which is when the dinosaurs became extinct.
Map showing 24 sites containing Younger Dryas Boundary nanodiamonds distributed over 150 million km2 on four continents (Source: Kinzie et al. 2014)
Comets and asteroids today
As seen in this map, released by NASA's Near Earth Object (NEO) Program in 2014, small asteroids roughly 1 to 20 meters in diameter frequently impact Earth's atmosphere.
Of the millions of estimated near-Earth asteroids of 10–20 meters in diameter, only these 556 impacts have been cataloged!
Asteroids with a diameter of 7 meters enter Earth’s atmosphere about every 5 years and generate as much kinetic energy as the atomic bomb dropped over Hiroshima, typically exploding in the upper atmosphere (i.e. "fireballs"). They are usually harmless but there have been some exceptions, causing deaths, injuries, property damage, or other significant localized consequences.
The 2013 Chelyabinsk event
On February 15, 2013, a meteor exploded in the sky over Chelyabinsk, southern Russia. The fireball and was caught in numerous videos, mainly by dashboard cameras throughout the region.
Although no people or buildings were hit by the asteroid, the shockwave from the exploding object injured about 1500 people and caused damage to 7200 buildings in the region.
The asteroid was completely undetected before it impacted earth’s atmosphere. Models suggest that an object the size of the Chelyabinsk asteroid hits Earth once every 150 years on average.
Although the Chelyabinsk meteorite probably measured 17 to 20 m in diameter before it exploded, it was very small compared to other objects that could potentially hit the earth. To observers on the ground, it shone 30 times brighter than the Sun. The explosion released energy estimated at about 500 kilotons of TNT (about 20 to 30 times more energy than the Hiroshima atomic bomb).
The event brought to the world's attention the very real hazards associated with the impact of objects from outer space.
The 1908 Tunguska event
The Tunguska event was a large airburst caused by a comet or asteroid, which fell near the Stony Tunguska River in Russia’s Eastern Siberian Taiga on June 30th, 1908. The explosion occurred over a largely unpopulated area, and flattened 80 million trees over 2,150 square kilometers (830 sq. mi), devastating local plants and animals.
Trees flattened by the Tunguska event
The impactor was somewhere between 60 to 190 meters (200 to 620 feet) depending on whether it was a heavier asteroid or a lighter comet, yet the explosion released the energy equivalent to about 185 Hiroshima bombs. An explosion of this magnitude can destroy a large metropolitan area.
These two recent impact events helped spark the discussion of the very real hazards associated with comet or asteroid impacts.
We are woefully unprepared
Few programs exist that are working towards detecting Near Earth Objects (NEOs are objects that orbit close to the Earth) that may pose a risk of earth impact, and not nearly enough is invested in this research.
Number of Near Earth Objects detected by various projects per year.
NASA launched the Asteroid Grand Challenge in 2013 which aims to create a plan to find all asteroid threats to human populations:
The job is too big for NASA, and so we'll have to find them!
NASA launched the Planetary Defense Coordination Office in 2016. Its mission is to lead the coordination of interagency and intergovernmental efforts to plan responses to potential impact threats. Most of their current work involves leading the cataloging and tracking of potentially hazardous Near Earth Objects, such as asteroids and comets that are larger than 30-50 meters in diameter.
Unfortunately, US president Donald Trump recently cut NASA’s multi-billion dollar Asteroid Redirect Mission (ARM) from the 2018 budget, which hoped to identify, redirect and send astronauts to explore an asteroid. One of the mission goals was to test basic planetary defense techniques for asteroid deflection.
NASA warns that the earth is unprepared for a major impact and that there is little we could do about it right now. Although it’s unlikely a large impact will happen in the short-term, one will eventually happen.
Asteroids and comets are some of the greatest threats to humanity, and even relatively small impacts can create explosions hundreds of times stronger than nuclear bombs. Anthropogenic climate change and nuclear war are very real threats today. However, a large comet or asteroid strike would be like detonating the planet's nuclear arsenal simultaneously many times over, and would cause sudden and drastic climate change as seen in the Younger Dryas period.
Unfortunately, scientific research in this field has little support nor interest from governments. We are beginning to understand that extinction level events are far more probable than once thought, and that research for detection and possible deflection are indeed crucial to our survival as a species and not just for paranoid alarmists.