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8 February, 2013  ▪  Leonid Zalizniak

The Earth’s “Star Scars”

Impact events have greatly changed our planet, including the territory of contemporary Ukraine

“For in much wisdom is much grief, and he that increaseth knowledge increaseth sorrow,” Ecclesiastes says. This biblical truth best captures the woeful essence of some of the scientific discoveries of recent decades. Among these, we have become aware of numerous dangers originating from space that threaten our planet and its living organisms. Many scientists now believe that life on Earth and human civilization emerged and have persisted only due to a miraculous coincidence of fortunate circumstances.

“INVISIBLE SCARS”

Jest decades ago, debates raged about the origins of hundreds of craters on the surface of the Moon that are visible to the naked eye. Today there is no doubt that most of them were caused by meteorites, not volcanoes. Scientists call these craters impact structures or astroblemes, meaning “star wounds”. If the many craters on the Moon’s surface are “scars” caused by meteorites, asteroids and comets, the Earth must have experienced even more impact events due to its larger size. However, erosion and weathering have largely “healed” them, and many of these “star scars” were only recently identified.

The Earth’s most famous astrobleme is the relatively “young” Barringer crater in Arizona (USA) which has the form of a bowl 1,200 metres in diameter and 175 metres deep. It was created about 40,000-50,000 years ago under the impact of a nickel-iron meteorite 50 metres across. Fragments of the meteorite have been found in and around the crater. The meteorite struck the surface of the Earth at a speed of 12-15 kilometres per second, releasing energy 1,000 times greater than the power of the atomic bomb dropped on Hiroshima. Researchers studying the crater in 1902 were the first to suggest that objects of this kind were caused by meteorite impact.

American scientists first launched scientific research into astroblemes in the 1960s. Similar research began in the USSR in 1969 with the examination of the “Popigai” meteorite crater in northwestern Yakutia. Formed 36 million years ago under the impact of an asteroid 5-8 kilometres across, it is around 100 kilometres in diameter and 2 kilometres deep. The colossal force from its impact transformed graphite in the ground into valuable materials. Unique industrial black diamond deposits were harvested there by GULAG prisoners under Stalin in the postwar years.

When Soviet scientists established that Popigai was a meteor impact crater and analysed the effects of the impact, it triggered a search for diamond-rich craters across the USSR, including Ukraine. The majority of the eight astroblemes identified in Ukraine were discovered as a result of this search activity. In particular, the Illyinets impact crater measuring 7,000 metres in diameter and 700 metres deep was discovered near Vinnytsia. The crater was created by an asteroid measuring around 250 metres across. Small impact diamonds were discovered at the bottom of the crater but not enough for industrial extraction.

The Boltysh crater, which is 24 kilometres across and up to 900 metres deep, is located near Oleksandriia (Kirovohrad Oblast) and is believed to be Ukraine’s biggest crater. Scientists claim that the impact was caused by a cosmic object 1,000 metres in diameter which struck the Earth around 65 million years ago.

There are about 200 known “star scars” on the surface of the Earth. Large craters have been found in Germany, Finland, Australia and Africa. In Russia, 15 “scars” measuring 10-110 kilometres in diameter have been discovered. Scientists also studied a depression 300 kilometres in diameter in South Africa and a smaller one, 250 kilometres across, in Canada. The biggest astrobleme on Earth, around 500 kilometres across, was discovered near Wilkes Land in Antarctica under a kilometre-thick ice sheet. This immense crater spans an area greater than the distance from Kyiv to Odesa. According to one hypothesis, it was excavated 250 million years ago by a gigantic asteroid measuring 48 kilometres across. According to some researchers, this cataclysm was the cause of the greatest annihilation of living organisms in the history of our planet – the Permian–Triassic extinction event that killed nearly 80 per cent of all biological species. A colossal astrobleme (600 by 400 kilometres) named Shiva was recently discovered at the bottom of the Indian Ocean.

However, the greatest impact event occurred in the early days of the Earth when it came in contact with a somewhat smaller plant roughly the mass of Mars. The collision released a tremendous amount of energy, the Earth melted, and a great mass of materials was released into its orbit where it stayed for a long time. Later, gravitation forces formed the Moon out of this debris.

UNINVITED GUESTS FROM SPACE

The catastrophic consequences of asteroids crashing into a planet are due to their high velocity (11-76 kilometres per second), which triggers the release of colossal amounts of destructive energy. It is believed that only cosmic objects over 100 metres across pose significant threat to mankind. Their sheer size enables them to penetrate the Earth’s atmosphere without significant loss of velocity, while smaller objects lose speed, disintegrate or explode in the air. Such an air burst was the likely cause of the Tunguska event of 1908 when a cosmic body (probably a comet) measuring 50-100 metres in diameter exploded over the Tunguska River in Siberia. The impact gave rise to numerous legends. The explosion was so powerful that it knocked down trees within dozens of kilometres. Scientists estimate the power of the explosion at 10-50 megatonnes, which is close to that of a hydrogen bomb. If this meteorite had exploded over London or Moscow, it would have destroyed the entire megalopolis.

Impact craters on the Earth’s surface are proof of numerous encounters with uninvited guests from space. The main source of these is the asteroid belt between the orbits of Mars and Jupiter. It contains more than 400,000 asteroids constituting 98 per cent of all small bodies orbiting the Sun. These sometimes leave their orbits when pulled by larger bodies (such as Jupiter) or as a result of collisions and can then cross the trajectory of the Earth with a possibility of contact.

The problem of catastrophic asteroid impacts rose to prominence in the early 1980s when scientists established that a gigantic meteorite crashed into the Earth’s surface 65 million years ago, putting an end to the predominance of reptiles and destroying 60-70 per cent of living organisms on our planet.

THE KILLER OF DINOSAURS

In 1977, American geologist Walter Alvarez noticed that one particular geological layer of clay 1-2 centimetres thick, located to the northwest of Rome was blue in colour. It occurred at the boundary between Mesozoic deposits containing the remains of dinosaurs and Cenozoic-era deposits. The strange colour was explained by high levels of the rare-earth element iridium. It is found on the Earth in very small quantities but abounds in space objects. In 1980, a hypothesis was put forward that this geological layer, 65 million years old, received its iridium content due to an asteroid impact event. Melted quartz and glass (tektites) and even microscopic diamonds that could only be a result of a powerful explosion were also found at the site. Such iridium-rich deposits with tektites were discovered in various countries, suggesting the planetary scale of the disaster. A tremendously powerful explosion (equal to 10 million hydrogen bombs) that occurred 65 million years ago when an asteroid 10 kilometres in diameter struck the Earth destroyed every living organism within 1,000 kilometres. The impact triggered an unprecedented earthquake (13.0 magnitude), which caused volcanoes to erupt in different parts of the world. A 300-metre-high tsunami swept across the oceans, killing all living creatures many kilometres inland. The explosion ejected a huge mass of melted and evaporated earth materials that fell as a fiery meteorite rain on the surface causing global fires. The velocity of fragments was so high that they exploded like atomic bombs on contact with the atmosphere. Within the first couple of hours after the collision, the temperature of the earth’s atmosphere shot up to 200°C, according to some estimates. All creatures on land that did not hide under the ground or in the water died. The bigger the animal was, the smaller its chances of survival were. According to palaeontologists, all land creatures weighing over 10 kilogrammes died in the event.

The release of tremendous amounts of sulphur compounds into the atmosphere caused ubiquitous acid rains. The abundance of dust in the stratosphere and atmosphere brought out a planetary night that lasted an entire year. The dust from the explosion eclipsed the Sun for several years, preventing its rays from reaching the planet’s surface and causing it to cool. A planetary winter set in and lasted for several years. Temperatures plummeted, causing many living organisms to freeze to death. Even worse, much lower levels of solar radiation slowed down photosynthesis. Plants were scorched, covered with dust and deprived of sunlight. They eventually died, resulting in the death of herbivorous animals, which decimated the population of predators. Within a short period, nearly 75 per cent of all living organisms vanished from the face of the Earth. Ocean creatures suffered less because of a slower temperature decline. That is the reason why there are so many relic species in the ocean.

Reptiles, especially the largest ones—dinosaurs—dominated the Earth prior to this event. The heat shock and subsequent temperature drop proved to be fatal to cold-blooded reptiles as they were unable to control their body temperature. The only reptiles that survived were the ones that buried themselves in the ground (lizards and snakes) or inhabited bodies of water (turtles and crocodiles).

The asteroid impact put an end to the Mesozoic era of reptiles and began a new, Cenozoic era ruled by mammals. The latter had emerged even in the Mesozoic era but had been nocturnal creatures similar to rats. Unlike reptiles, mammals can regulate their body temperature. This helped them survive the asteroid winter, and they came to rule the world after the extinction of dinosaurs. The Cenozoic era was characterized by an incredible diversity of mammal species and the arrival of primates.

Thus, asteroid impacts and their consequences were powerful factors in the development of life on Earth. Although most contemporary scholars link the great extinction 65 million years ago precisely to an impact event that left behind the Chicxulub crater in Mexico, there is now an alternative conception under which the disaster was caused by multiple impact events.

One such event could be the cause of the Manson impact crater 35 kilometres in diameter that was found near Iowa City (USA), 2,500 kilometres north of the Yucatán Peninsula. The crater was created by an asteroid about the same age as the one in Mexico. A joint Ukrainian-British mission that studied the Boltysh crater in Ukraine suggests that it was formed around the same time.

There is some evidence that the Kara crater (110 kilometres in diameter) in Siberia also belongs to this group. The Shiva asteroid struck the Indian Ocean not long afterwards, triggering mass expulsion of magma into the ocean. All this suggests that the “star scars” were caused by fragments of one mammoth space object that disintegrated in the Earth’s atmosphere.

Close to the end of the dinosaur era, our planet may have found itself under an asteroid rain that once again killed a large number of living creatures. Scientists suggest that new craters dating back to the same time of impact will be discovered in the future.

WHEN’S THE NEXT ONE?

Scientists have calculated that an average of four asteroids one kilometre across strike the Earth every million years. Their impact poses a threat to individual countries or parts of continents. A cosmic object at least ten times larger would threaten all of humankind. It is believed that over the past 570 million years, four or five such monstrous objects struck the Earth, approximately one per 100-150 million years. These include the asteroids that excavated the Chicxulub, Shiva and Land Wilkes craters. Humanity, which has existed for around three million years, has never experienced a cataclysm of this scale.

Asteroids and comets are not the only dangers to the Earth that originate in outer space. In particular, scientists believe that the great extinction of the late Ordovician period 450 million years ago was caused by the explosion of a supernova that exposed the Earth to powerful gamma rays. The ozone layer was destroyed, and ultraviolet rays killed nearly 60 per cent of all living creatures. Space has always been a source of potential threat to life on Earth. Humans have yet to reach a technological level that would permit us to protect our planet against new impacts by large cosmic objects. At the same time, not only did the bombardment of Earth by space objects destroy all living things, it was also a great factor in its formation and the emergence of life. For example, one hypothesis states that water was brought to the Earth by numerous asteroids that bombarded its surface in its earliest days. Other scientists have suggested that life on Earth is also of cosmic origin, allegedly delivered here by an asteroid.

An asteroid impact 65 million years ago killed billions of living creatures and put an end to the era of reptiles. However, the global extinction of dinosaurs paved the way for new life and the domination of mammals, eventually laying the foundation for humanity.


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