The fall of a meteorite from space may seem exceptional, but it’s extremely common: a 2020 study calculated that an average of 17,600 meteorites heavier than 50 grams fall to Earth every year, with an estimated total mass of 16,600 kilos. The vast majority of these space rocks are lost at sea, but those that are recovered help scientists understand the history of the Solar System, acting as fossils of our cosmic past. But this precious legacy is now in danger of disappearing.
Countless meteoroids—pieces of rock or metal smaller than asteroids and comets, from microscopic to several metres across—are circulating in space. Ninety-nine percent come from the asteroid belt between Mars and Jupiter, but there are also fragments of comets, material ejected by Mars or the Moon after an impact, remnants from before the birth of the Solar System, and it has even been suggested that some may have come from other solar systems.
A trail of valuable information
When one of these meteoroids enters the Earth’s atmosphere, it forms a meteor, which appears as a streak of light. Millions of space fragments reach Earth, but the vast majority are too small to survive being burned up by atmospheric friction, as in shooting star showers. Only a small fraction reach the surface; these are meteorites and they number in the thousands each year (estimates vary). And although they can fall anywhere, they are most abundant at latitudes near the equator. Only very rarely do they become dangerous, such as the one that exploded over Chelyabinsk (Russia) in 2013, the one that wiped out a large area of forest in Tunguska (Siberia) in 1908 or, on a much larger scale, in the asteroid category, the one that caused the extinction of the dinosaurs 66 million years ago.
Scores of meteorites are recovered each year, from which scientists extract valuable information, providing geological and chemical snapshots of times and places in the history of the nearby cosmos. Those originating on Mars can travel through space for millions of years before arriving here by chance, but those from asteroids are as old as the Solar System itself, 4.5 billion years. In some cases, they have even been thought to contain the remains of extraterrestrial life, such as the one that fell at Orgueil (France) in 1864, possibly from a comet, or the one called Allan Hills 84001 (ALH84001), of Martian origin, found in Antarctica in 1984; neither of these claims has been substantiated.
But many more meteorites remain undiscovered. As 71% of the Earth’s surface is covered by water, most are lost in the oceans. They are easier to find in deserts. Some are only found thousands of years after they fall: ALH84001, which was ejected by an impact on Mars 17 million years ago, landed in Antarctica about 13,000 years ago. In fact, the frozen continent is the place where most meteorites have been recovered, not only because they are easy to spot in the ice, but also because glacier movements tend to concentrate them in certain areas, and the ice protects them from erosion. Some 50,000 have been found in Antarctica, almost 63% of the total.
The secrets of the universe
If a meteorite lands in Antarctica and sinks deep into the ice, it is probably irretrievable. This is not the case for those that land on so-called blue ice, which has a visible colour due to compression that removes air bubbles and causes crystals to grow, resulting in hard, bare ice where it is easy to spot the rocks that are exposed as the upper layers melt. Using artificial intelligence (AI) to calculate the conveyor belt effect of glaciers, scientists have been able to map some 600 areas of blue ice, 1% of the Antarctic surface, where meteorites are most likely to concentrate. Through this analysis, the researchers have estimated that only 15% of existing meteorites have been recovered so far, or much less: there could be between 300,000 and 850,000 meteorites near the surface of the Antarctic ice sheet that have not yet been located.
But the same team of researchers who produced this scientific treasure map warns that almost all of them are at risk of disappearing forever because of the great environmental challenge of our time: climate change. According to study co-leader Veronica Tollenaar of the Université Libre de Bruxelles (ULB), “even when temperatures of the ice are well below zero, the dark meteorites warm so much in the sun that they can melt the ice directly under the meteorite.” This creates a depression that causes the rock to gradually sink. “As atmospheric temperatures increase, the surface temperature of the ice increases, intensifying this process, since less heat from meteorites is required to locally melt the ice,” Tollenaar adds.
Using their AI system, the researchers estimate that about 5,000 meteorites are being lost each year due to this process—compared to about 1,000 that are discovered—and that by 2050, 24% of the total will have inexorably disappeared, which could soar to 76% by 2100 in a worst-case scenario if greenhouse gas emissions are not reduced. For study co-leader Harry Zekollari, also from ULB, this is an unexpected impact of climate change that needs to be addressed: “To secure this valuable extraterrestrial material, we must intensify and coordinate the recovery of Antarctic meteorites before we lose them to climate change.” According to Zekollari, when these rocks disappear into the ice, “so do some of the secrets of the universe.”
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