The Week in Space and Physics: An Antarctic Airburst
On airbursts over Antarctica, Mimas' ocean, Bennu and the hottest place in the solar system
I’m pleased to announce a new partnership with V101 Space, a Youtube channel covering space topics in a clear and engaging way. V101 Space will be making video versions of some of my articles. The first - based on last month’s article ‘The Year of the Sun’ - is already online!
In 1908 a big chunk of rock or ice smashed into the atmosphere, exploded with the force of a large atomic bomb, and devastated a thousand square miles of the Siberian wilderness. It was the largest impact event in recorded history, and yet it was at first little noticed by human civilization. So remote was the blast that few people directly saw it, and almost two decades passed before anyone made a serious effort to find its crater.
Despite extensive later searches, no such crater was ever found. Instead the evidence suggests the object exploded a few miles above the surface, sending out a blast wave that flattened the surrounding forest. Such events are known as airbursts, and though they can cause a lot of damage on the ground, they tend to leave little in the way of long lasting evidence.
In recent years, however, researchers have found traces of ancient airbursts buried in the Antarctic ice sheets. Small spheres of rock and metal have been found there, some of which have been linked to airbursts taking place more than four hundred thousand years ago. Now researchers claim to have found evidence of an even older airburst, one that exploded over two million years ago.
The claim comes from an analysis of metallic spheres found in 1994. More than one hundred of these small spheres were uncovered in the Allen Hills of Antarctica, and then dated to an age of about 2.8 million years. Chemical analysis showed the spheres probably came from outer space, and that they thus came from an ancient asteroid impact.
A fresh analysis of the spheres, reported in a recent paper, strengthened that idea. However, the mix of elements found within them suggest they came from an unusually large airburst, one that exploded just above the surface. That implies a truly devastating event, in which a plume of gas heated to thousands of degrees struck the Antarctic ice at incredible speed.
If something like that happened today, and took place over a densely inhabited area of our planet, the outcome would be catastrophic. Such a large airburst could destroy an entire city, leaving thousands, or even millions, of people dead. This is a frightening scenario, and one we may have been lucky to avoid so far.
Fortunately such events seem to be rare. The one found in Antarctica took place at least two million years ago, and was of unusual size. Smaller ones - such as that of 1908 - occur only once or twice per millennium. Astronomers are actively searching for asteroids that may present a threat - and none, so far at least, have been found.
Mimas’ Secret Ocean
Three years after Star Wars hit the movie screens, Voyager I flew past Mimas. The photos it sent back came as a surprise: the moon of Saturn, which had otherwise seemed wholly unremarkable, bore an uncanny resemblance to the Death Star. A vast circular crater dominated its surface, perfectly poised amidst a grey and desolate landscape.
Fortunately Mimas is probably not a hidden superweapon. It is instead a small moon, measuring a few hundred miles across and made mostly of ice. Its interior is not home to an army of stormtroopers, but instead, according to a recent discovery, to a vast ocean of liquid water.
Such oceans are, of course, common among the moons of Jupiter and Saturn. Oceans are believed to lie under the frozen surfaces of Enceladus and Europa, Ganymede and Callisto, and perhaps even Titan. All are drawing increasing attention from scientists, not least for the possibility that primitive life could be found within them.
Yet Mimas had seemed unlikely to host an ocean. Though the possibility was briefly considered in 2014, later research ruled the idea out. An ocean would mean tides, and those would create cracks in the icy surface of Mimas. No such cracks have ever been seen, which led researchers to conclude that no ocean existed.
Mimas, however, likes to surprise. A recent study of its orbit and spin, based on data from the Cassini probe, found patterns of motion that point strongly towards a liquid interior. Indeed, so strong is this evidence that researchers now think a deep ocean lies under a layer of ice twenty miles thick.
The lack of cracks on Mimas’ surface can, they say, be explained if the ocean only formed a few million years ago. That seems possible. The orbit of Mimas is slowly changing over time, thanks to constant tugs from Saturn’s other moons. Models suggest that those tugs could have heated the interior of Mimas, allowing the ocean to form.
Sadly, that ocean may not last long. The same models suggest Mimas will soon begin to cool. If it does, its ocean will eventually freeze solid once more.
Bennu and OSIRIS
In 2016, NASA launched OSIRIS-REx, a probe that headed towards the asteroid Bennu. After spending two years in orbit around that asteroid, OSIRIS touched down on its surface in 2020, grabbed a sample of rock and dirt, and headed back to Earth.
Late last year, OSIRIS fired a capsule containing that sample towards the deserts of Utah, where it was later picked up by NASA. OSIRIS itself flew onwards, aiming itself towards another asteroid, Apophis, which it should reach by the end of the decade.
Despite the safe return on the capsule, however, NASA scientists were dismayed to find they couldn't open it. Two of the lid’s fasteners refused to budge, leaving the capsule stuck shut and its contents inaccessible. After three months of effort, and with the help of some specially designed tools, NASA finally got it open in January.
That allowed scientists to finally begin studying the rocks inside. Though official results will not come until at least March, reports in Space.com and New Scientist have revealed some early details.
Asteroids like Bennu are thought to be fragments from the early days of the Solar System. Back then the system was likely filled with dozens of small worlds. A few of these may have survived and become moons or dwarf planets. Yet most were destroyed, either pulverised in violent collisions or ripped apart by the gas giants.
The rocks brought back by OSIRIS show that Bennu could be a piece of one of these shattered planets. Many of them have a phosphate crust, implying that they formed in the presence of water. If so, Bennu might once have belonged to a small water-filled world, somewhat akin to Saturn’s moon Enceladus.
The Hottest Place on Earth
Most of the time, the hottest place in the Solar System lies deep inside the Sun, where temperatures can reach as much as fifteen million degrees celsius. Sometimes, however, the honour has belonged to a building in the heart of Britain’s Oxfordshire; a building known as the Joint European Torus, or JET.
JET is an experimental nuclear fusion reactor, one of the largest on Earth. Sadly it is now shutting down. It carried out its last experiments at the end of 2023, and will soon be decommissioned. In a few years a new reactor - ITER, based in the south of France - will take over its mission to prove the viability of nuclear fusion as a power source.
In its final run, however, JET set a world record for energy production by nuclear fusion. The experiment extracted sixty-nine megajoules of energy from a speck of matter weighing just a fifth of a milligram. That’s impressive: you’d need to burn ten million times as much coal to produce the same amount of energy.
I first read about the Tunguska event when I was a young boy. Although I was too young to really understand it, I was fascinated. I enjoyed reading your article :-)