65 million years ago a massive asteroid, five to ten kilometers across, hit the Earth at a speed exceeding 30,000 kilometers per hour. As a result of this catastrophic collision, giant creatures, known to us as dinosaurs, who dominated the Earth for more than 100 million years, were destroyed. Remarkably, about 30% of all species now existing on Earth were destroyed at that time. That time was far from the first, when a catastrophic object enters the Earth, and certainly did not become the last. There is an opinion that such events happen on a periodic basis because of the movement of the Sun along the galaxy. If this is so, we should be able to predict when the next such event is coming and whether we should be worried about our own destiny.
The threat of mass extinction always exists, but it is not always possible to calculate it precisely. Threats in our solar system – related to space bombardment – usually come from two sources: the asteroid belt between Mars and Jupiter and the Kuiper belt and the Oort cloud beyond the orbit of Neptune. For the asteroid belt, which is suspected (but it’s not certain) in the destruction of dinosaurs, our chances of getting a large object in the face decrease with time. Because the material between Mars and Jupiter is gradually depleted, and there is nothing that could fill 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 satellites of Jupiter and Saturn.
The history of the 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 the heavy bombardment of the times of the young Solar System more than 4 billion years ago. There are many large craters with smaller craters inside, which indicates an extremely high level of activity in those times. However, if you look at the dark areas (the lunar seas), you will see not many craters inside. Radiometric dating shows that most of these zones are from 3 to 3.5 billion years old. The youngest areas found in the largest ocean of the Moon are Oceanus Procellarum, which is only 1.2 billion years old and relatively recently established.
Based on these data, we can conclude that the belt of asteroids is scaling with time and the rate of crater formation is falling. There is an opinion that we are still far from this, but in the next few billion years the Earth will receive the last serious impact of an asteroid, and if there is still life on it, mass extinction is inevitable. Today, the asteroid belt presents a lesser threat than in the past.
But the Oort cloud and the Kuiper belt are completely different stories.
Outside of Neptune, in the outer solar system, lies a deep threat. Hundreds of thousands – if not millions – of large blocks of ice and stone float in sparse orbits around the Sun, in anticipation of perturbations caused by the passage of large masses. The violation of the orbit can lead to different outcomes, among them the sending of an object to the internal solar system, where he will arrive with a brilliant comet and, possibly, with something will collide.
Interactions with Neptune or other objects of the Kuiper belt and the Oort cloud are random and independent of the processes of our galaxy, but there is a possibility that passing through a star-rich region – like a galactic disk or one of the spiral arms – can 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 mechanics, since the Sun and all the stars move along elliptical roads around the center of the galaxy. But some people argued that the periodic extinction occurred exactly the same frequency. That is, these extinctions could have been caused by cometary rain, which happens once in 31 million years.
Is it possible? The answer can be found in the data. We can consider the major events of extinction on Earth as markers in the fossil record. We can calculate the number of births (this is slightly higher than the “species” in our classification of living beings, the human race is homo in homo sapiens) that existed at a certain time. We can do this by returning 500 million years ago in time, thanks to the discoveries made in the sedimentary rocks.
We can look for patterns in these extinction events. The easiest way to do this is quantitatively – the Fourier transform with the subsequent search for regularities. If we see events of mass extinction every 100 million years, for example, with a large disappearance 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 do the data on extinction show?
There are several relatively weak evidence for the frequency of 140 million years and a little more feasible – for jumps every 62 million years. Where the orange arrow, you see a periodicity of 31 million years. These two jumps seem huge, but only relative to other jumps, which are completely insignificant. How strong, objectively, are these two leaps demonstrating periodicity?
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 was in the past, there is an increased chance that this will happen in 62 or 140 million years. However, the frequency of 26-30 million as such is not observed.
If we begin to study craters on Earth and the geological composition of sedimentary rocks, this idea crashes 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 show that only the extinction 65 million years ago has a layer of dust and ash that we could associate with A major blow.
The boundary layer of the Cretaceous and Paleogene periods is characteristic of 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 mass extinction.
The idea that mass extinctions occur on a periodic basis is interesting and convincing, but it simply does not have conclusive evidence. The idea that the passage of the Sun through the galactic plane leads to periodic extinctions is also interesting, but unproven. We know that every half a million years, within the reach of the Oort cloud, stars pass, but at the present time we are far from these events. In the foreseeable future, the Earth is not threatened by the natural cataclysm caused by the universe. On the contrary, we are the greatest threat to ourselves.