By Joshua Howgego
Millions of individual bits of space debris hit Earth every day. Most are little more than dust particles, yet occasionally we encounter something serious. A “superbolide” is the loosely defined term for a shooting star that creates a flash more than twice as bright as the full moon. But sometimes they get even bigger than that.
What exactly happened on 30 June 1908 over the Tunguska River in central Siberia is a century-old mystery. There was certainly an explosion: 200 square kilometres of trees were flattened, and the indigenous Evenki people who live in the area reported that their animals were thrown into the air by a shockwave. We also know that there was a bright flash of light visible in London. So the prevailing theory is that a massive space rock caused the blast. Yet we haven’t found convincing evidence of the meteorite. Neither is there an obvious crater – unless you accept the problematic theory that nearby Lake Cheko is it. An alternative idea is that the explosion came from a sudden huge belch of subterranean gas.
Read more: Rocks of ages: How meteorites reveal the solar system’s history
The lights that appeared in 2013 over Chelyabinsk, a city in southwest Russia, certainly were caused by a superbolide – it was caught on film by numerous people (see video below).
Particularly strange was that astronomers were expecting a large meteoroid called 2012 DA14 to pass close to Earth that very day. About 16 hours before it did, the Chelyabinsk rock came down – seemingly by pure coincidence. More than 1000 people were injured by flying glass blown out by the shockwave. And this time we did find fragments of the rock, revealing that it was a chondrite, a non-metallic meteorite, that was originally 19 metres across.
It’s the most famous cataclysm ever, the meteorite that probably snuffed out the dinosaurs (along with three-quarters of all life on Earth) when it smashed into our planet 65 million years ago. The crater it left behind is actually one of the youngest and most accessible in the solar system. Last year, a research team drilled down more than a kilometre to reach the rocks that bear witness to the impact. The Earth’s crust flowed like liquid in the wake of the event, lifting and then collapsing mountains 25 kilometres high in 3 minutes. But one surprise finding is that the rocks didn’t actually melt – they seem to have been pushed around like a fluid by the force of the impact alone. That suggests that visiting impact craters elsewhere in the solar system might allow us a sneak peek into the what lies below the surface without any need to drill.
In its early history, Earth was bombarded with large chunks of space debris. But unlike those on the moon, most of Earth’s craters have been eroded away. One of the oldest we know of is the Vredefort Dome in South Africa, which is some 2 billion years old. At 300 kilometres across, it’s also one of the largest, created by a rock that could have been 10 kilometres wide.
Read more: Impact sights: Six of Earth’s most impressive craters
Morokweng is another huge and ancient crater in South Africa, this time on the edge of the Kalahari Desert. It has been weathered away so much that it was only discovered by means of circular magnetic anomalies in the rock, which were found by mineral prospectors in the 1990s. But in 2006 it gained notoriety. Researchers were drilling deep into crater when, about 770 metres down, they found a 25-centimetre fragment of the meteorite. That was quite a shock: the crater is 70 kilometres wide, and it was previously assumed that any impact big enough to produce such a hole would melt the meteorite beyond recognition.
Allan Hills 84001
This lump of rock made an impact not because of its huge size – it only weighed about 176 grams – but because it contained signs of alien life. At least, that was the idea aired by NASA scientists in 1996 – a claim so bold that even then US President Bill Clinton gave a press conference about it on the day of the discovery. The rock was blown from the surface of Mars about 15 million years ago by a huge asteroid impact and eventually fell in Antarctica about 13,000 years ago. The researchers who analysed it spotted what they thought were the outlines of tiny cells, organic molecules and iron-containing deposits similar to those produced by some bacteria on Earth. Since then, however, natural mechanisms that can explain each of those things have been identified. These days, its only claim to fame is more modest: it is the oldest piece of Mars on Earth.
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