65 million years ago, a massive asteroid, five to ten kilometers across, struck Earth at speeds exceeding 30,000 kilometers per hour. This catastrophic collision destroyed the giant creatures we know as dinosaurs, which have ruled the Earth for over 100 million years. Remarkably, about 30% of all species currently existing on Earth were destroyed at that time. That time was far from the first when a catastrophic object falls into the Earth, and it certainly did not become the last. It is believed that such events occur on a periodic basis due to the movement of the Sun through the galaxy. If so, we should be able to predict when the next such event is coming and whether we should worry about our own destiny.
The threat of mass extinction always exists, but it is not always possible to calculate it accurately. Threats in our solar system - associated with space bombing - typically come from two sources: the asteroid belt between Mars and Jupiter and the Kuiper belt and the Oort cloud outside Neptune's orbit. For the asteroid belt, which is suspected (but not certain) of killing dinosaurs, our chances of getting in the face of a large object diminish over time. Because the material between Mars and Jupiter is gradually depleting and there is nothing to replace it. We understand this when we look at two things: the young solar system, the early models of our solar system, and most airless worlds without active geology: the Moon, Mercury, most of the moons of Jupiter and Saturn.
The history of falls in our solar system is literally written on the faces of worlds like the moon. The lunar highlands - bright spots - show us the history of heavy bombing from the early solar system more than 4 billion years ago. There are many large craters with smaller craters inside, indicating an extremely high level of activity at the time. However, if you look at the dark areas (lunar seas), you will not see many craters inside. Radiometric dating shows that most of these zones are between 3 and 3.5 billion years old. The youngest regions to be found in the largest sea of the Moon, Oceanus Procellarum, are only 1.2 billion years old and relatively recently created.
Based on these data, we can conclude that the asteroid belt is getting thinner over time and the rate of crater formation is falling. It is believed that we are still far from this, but in the next few billion years, the Earth will receive the last serious impact of the asteroid, and if there is still life on it, a mass extinction is inevitable. The asteroid belt poses less of a threat today than in the past.
But the Oort cloud and the Kuiper belt are completely different stories.
Promotional video:
Beyond Neptune, in the outer solar system, a deep threat lurks. Hundreds of thousands - if not millions - of large chunks of ice and rock float in tenuous orbits around the Sun, waiting for the perturbations caused by the passage of large masses. A violation of the orbit can lead to different outcomes, among them the sending of an object into the inner solar system, where it will arrive as a brilliant comet and, possibly, collide with something.
Interactions with Neptune or other Kuiper Belt and Oort Cloud objects are random and independent of our galaxy's processes, but there is a possibility that passing through a stellar-rich region - like the galactic disk or one of the spiral arms - could increase the chances of comet rain and comet impact on Earth. As the Sun moves through the Milky Way, every 31 million years it passes through the galactic plane. This is a purely orbital mechanic, as the Sun and all stars move along elliptical paths around the center of the galaxy. But some people have argued that periodic extinctions occurred at exactly the same frequency. That is, these extinctions could be caused by comet rain, which occurs once every 31 million years.
Is it possible? The answer can be found in the data. We can view major extinction events on Earth as landmarks in the fossil record. We can count the number of genera (this is just above the "species" in our classification of living things; the human race is homo in homo sapiens) that existed at a certain time. We can do this by going back 500 million years in time thanks to discoveries made in sedimentary rocks.
We can look for patterns in these extinction events. The easiest way to do this quantitatively is the Fourier transform followed by a search for patterns. If we see events of mass extinction every 100 million years, for example, with a large extinction of the number of species after a certain period of time, the Fourier transform will show a large burst with a frequency of 1 / (100 million years). What does the extinction data show?
Measurement of biodiversity, as well as changes in the number of genera, at a point in time, revealing most of the major extinction events in the past 500 million years
There is some relatively weak evidence for a frequency of 140 million years and even stronger evidence for jumps every 62 million years. Where the orange arrow is, you see a periodicity of 31 million years. These two jumps seem huge, but only in relation to other jumps, which are completely insignificant. How strong are these two leaps, objectively, demonstrating periodicity?
This figure shows the Fourier transform for extinction events over the past 500 million years. The orange arrow shows where the 31 million year periodicity would fit.
In just 500 million years, you can place three possible mass extinctions with a period of 140 million years and eight with a period of 62 million years. What we see does not fit into such periods with such events; rather, if such an event happened in the past, there is an increased chance that it will happen in 62 or 140 million years. However, the frequency of 26-30 million is not observed as such.
If we start studying craters on Earth and the geological composition of sedimentary rocks, this idea collapses completely. Of all the craters that formed on Earth due to falls, less than a quarter are formed by objects from the Oort cloud. Moreover, the boundaries between geological periods (Triassic / Jurassic, Jurassic / Cretaceous, Cretaceous / Paleogene) and geological records that correspond to extinction events indicate that only the extinction 65 million years ago has a layer of dust and ash that we might associate with a big blow.
The boundary layer of the Cretaceous and Paleogene periods characteristically stands out in the sedimentary rock, but is represented by a thin layer of ash, and its composition tells us about the extraterrestrial origin of the body, which led to the mass extinction
The idea that mass extinctions occur on a periodic basis is interesting and compelling, but there is simply no conclusive evidence for it. The idea that the passage of the Sun through the galactic plane leads to periodic extinctions is also interesting, but unsubstantiated. We know that stars pass within the reach of the Oort cloud every half a million years, but we are far from these events at present. In the near foreseeable future, the Earth is not threatened by a natural cataclysm caused by the Universe. On the contrary, we ourselves pose the greatest threat to us.
ILYA KHEL