Ann Hodges was chilling on her couch in Alabama, US, one afternoon in 1954 when a meteorite burst through the roof and slammed into her side, leaving an enormous bruise.
It’s exceedingly rare to be hit by a big space rock – the one that hit Hodges weighed about four kilograms.
But scientists have found the Earth is under constant bombardment by thousands of tonnes of micrometeorites. The “cosmic dust”, though, is so fine someone might never realise it had rained down upon them.
A new study, “The micrometeorite flux at Dome C (Antarctica), monitoring the accretion of extraterrestrial dust on Earth”, has been published in Earth and Planetary Science Letters.
Researchers have melted snow from Antarctica (where it’s exceedingly pure) and studied the hundreds of micrometeorites found in various layers, dating back to the 1990s. They concluded about 5200 tonnes of space dust hit the Earth every year, from comets and asteroids.
Lead author Julien Rojas told Scientific American they can tell which is which because the “dust from comets is fluffier than from asteroids”.
Australian micrometeorite guru Andrew Tomkins – an associate professor at Monash University’s School of Earth, Atmosphere and Environment – says they’re so tiny you probably wouldn’t feel them shower down on you.
“The most abundant ones are the smallest ones, so they get exponentially more abundant the smaller they get,” he says.
“The really small ones are about 50 microns. Human hair is around 30 microns. Some are up to about 2 millimetres but they’re quite rare at that size.
“If you imagine a grain of sand on a beach and then imagine that coming through the atmosphere of Earth really really fast, most of them melt. The surface tension makes them form a little sphere.
“Some survive without melting, they slow down faster than they heat up.”
The late, great astronomer Carl Sagan once said we are all made of “star stuff”, which astrophysicist Neil deGrasse Tyson echoed when he said we are all “stardust”. Tomkins says that’s also true of micrometeorites – but because it’s true of everything in the universe.
“It’s better to think about them as little particles of asteroid dust,” he says.
“If you imagine the early Solar System with more asteroids than today, there were lots of collisions going on all the time so there. Comets were quite frequent as well.”
The smaller the dust, the less it heats up on atmospheric entry, so it survives to sprinkle over cities, land, water and us.
Tomkins’ own work, on micrometeorites in the Pilbara in Western Australia, found they contain iron oxide minerals, which showed the Earth’s atmosphere 2.7 billion years ago contained a distinct layer of oxygen.