When I was a kid living in Scotland, I remember my dad getting us out of bed in the middle of the night to show us a comet in the sky out of our bedroom window. This occurred in the 1950s. The comet was easily visible with the naked eye and had a very distinctive tail. I don’t know for sure which comet it was but a little research suggests it was either Arend-Roland or Mrkos. My money is on Mrkos.
Both these comets were visible from Earth months apart in 1957 and both were non-periodic comets; so they won’t be back. I have never seen another comet so clearly since then.
There’s a lot of fuss today about comets and asteroids and their possibility of impacting our planet. It turns out that we are struck by at least 6000 asteroids per year, most of which are small and burn up in the atmosphere. It’s estimated that about 100 tons of dust/sand-sized debris impact our atmosphere every day.
It seems that asteroid impacts are quite common so I think the fuss is really about those that are large enough to cause a threat to life and property. Asteroid impacts of this size are rare. Impacts that are large enough to cause damage and some loss of life happen about every 2000 years on average with those large enough to threaten our civilization occurring every few million years.
Most asteroids orbit the sun in a belt, the Asteroid Belt, between Mars and Jupiter. This belt lies in the plane of the ecliptic, the disk around the sun within which all the planets orbit. Now and again one of these asteroids is disturbed in its orbit, usually by a collision with another asteroid, and fall inward toward the sun in the plane of the ecliptic.
As it falls toward the sun it gains speed, swapping its potential energy for kinetic energy. When it gets close to the Earth it can be moving at up to 150,000 miles per hour. The law of gravity means that the speed of the falling asteroid does not depend on its mass, but its kinetic energy does. A large mile wide asteroid weighing millions of tons and traveling at 150,000 miles an hour will be an extinction-level event if it hits our planet; the planet will recover, we will not.
Comets are thought to come from either the Kuiper Belt, a disk-shaped cloud of objects and lies beyond the orbit of Neptune, or the Oort Cloud that is thought to be a spherical cloud far beyond Pluto that contains up to a trillion comet-like objects. Unlike asteroids, comets typically travel toward the sun on a path that is not in the plane of the ecliptic.
These more random trajectories and the relative infrequency of comets compared to asteroids makes them less of a threat. Although it might have been a comet that caused the impact 65 million years ago that is attributed to the extinction of the dinosaurs, it was more likely to have been an asteroid, if for no other reason than the abundance of Iridium that it seems to have left behind.
I do hear people worrying about the destruction of the planet; they should not fret about the Earth being destroyed. Our planet has survived many extinction events and come through it, changed to be sure, but still a vibrant home to new life forms to thrive. The human species may be exterminated by a comet or asteroid impact, but the planet will continue to spin on its axis and orbit the sun as before, it will just do so less a number of the species currently living upon it. When folks claim to be trying to ‘save the planet’ they aren’t, they are trying to save themselves in the manner that they find comfortable. I don’t hear them consulting cockroaches and ants for their opinion on planet-saving.
Our planet is about 4.5 billion years old. During that time it has gone through many changes. The late heavy bombardment is thought to have happened about 4 billion years ago and is thought to have been a time when Earth was hammered by a deluge of asteroids leaving the surface molten lava. Life would not have been possible during this time. As the Earth cooled the early atmosphere consisted mostly of nitrogen and carbon dioxide, 200 times what is in the atmosphere today, with an abundance of methane, ammonia, water vapor, hydrogen sulfide, and neon, but no oxygen. The oxygen-rich atmosphere occurred about 3 billion years ago and was caused by the emissions of oxygen by cyanobacteria, using photosynthesis, that thrived in the CO2 rich atmosphere. This was chemical warfare on the global scale as the planet was changed forever. The arrival of free oxygen kick-started the evolution of life on Earth. But it would not last as life on Earth endured a sequence of mass-extinctions beginning with the Ordovician-Silurian Extinction some 440 million years ago.
Life has thrived then been extinguished at least five times in the history of our planet. It seems that more than 99% of all species that ever lived on Earth are now extinct. The first mass extinction, the Ordovician-Silurian Extinction, occurred about 440 million years ago and killed off about 85% of all species, mostly small marine organisms. It is thought that the cause was the rapid swing of the global temperature from very much colder to a very much warmer bounce, with changes in sea levels and atmospheric CO2 concentration. It doesn’t seem that comet or asteroid impacts were responsible.
Life thrived again on Earth until the next mass extinction killed 75% of all species, falling hardest of marine organisms including corals and trilobites. This Late Devonian Extinction happened over a 20 million year span starting about 380 million years ago. It is thought that this extinction was caused by mass volcanism, probably in what is now Siberia, that resulted in lower oxygen levels in the atmosphere.
The Permian-Triassic Extinction, 252 million years ago, is also called the Great Dying. It killed off 96% of all marine species and 75% of all land species. The world’s forests were wiped out as well as large numbers of insect species. The largest contributor to this global calamity was the Siberian Traps, a complex of super volcanos that spewed 14.5 trillion tons of carbon into the atmosphere. The global temperature rose about 30F with the result that the oceans lost up to three-quarters of their oxygen.
Life on Earth got busy recovering from the Great Dying but only 50 million years later, about 201 million years ago, some 80% of all species were wiped out in the Triassic-Jurassic Extinction. The causes of this extinction event are the subject of much debate. Some contend that the cause was global warming caused by the release of CO2 into the atmosphere by volcanism associated with the break up of the super-continent Pangea. Others blame the release of methane into the atmosphere while a third group of scientists blames a combination of volcanism and the impact of a large comet or asteroid. What is not debated is that this extinction event cleared the way for the dinosaurs to rule the Earth.
The dinosaurs prospered for 150 million years until they were wiped out along with 76% of all species by the Cretaceous-Tertiary Extinction 65 million years ago. The causes of this extinction have been long debated but the majority view nowadays is that it was caused by a large asteroid impact off the coast of the Yucatan Peninsula. The jury is still out on the verdict on the cause with a vocal minority blaming global warming caused by volcanism and other tectonic effects. What is not in doubt is that land dinosaurs were profligate before this event and non-existent afterward, leaving the planet free for the rise of the Mammals, including us.
What can we conclude from this history of the evolution of life on our planet and the mass extinctions that eliminated most of the life at five times in our history? I think that the first obvious conclusion is that our planet is very robust and can shake off these calamitous events as can be seen by the average global temperature over the last 500 million years.
Life too seems to be very robust in the abstract if not the specific.After each annihilation of species, new life evolves and thrives on our planet. It seems therefore that while the life of humanity may be fragile and in danger of extinction from one calamity or another, life on planet Earth is not subject to the same danger. When we are gone, our planet will keep spinning and orbiting the sun and new life will keep evolving until our sun goes red giant and consumes it. I will not be here to see that event but I wish I could.
