University of Manchester/Katie Joy

Antarctic meteorites yield global bombardment rate

UK scientists provide a new estimate for the amount of space rock falling to Earth each year.

By Jonathan Amos • bbcnews
April 30, 2020 5 minSource

A team of UK scientists has provided a new estimate for the amount of space rock falling to Earth each year.

It's in excess of 16,000kg. This is for meteorite material above 50g in mass.

It doesn't take account of the dust that's continuously settling on the planet, and of course just occasionally we'll be hit by a real whopper of an asteroid that will skew the numbers.

But the estimate is said to give a good sense of the general quantity of rocky debris raining down from space.

"The vast, vast majority of objects to hit the Earth are really small," explained Dr Geoff Evatt.

"We're talking about objects for which, when they strike the ground, the fragments sum together to over 50g. So, typically, 50g-10kg in total. Objects bigger than this are very, very infrequent," the University of Manchester mathematician told BBC News.

One of the other outcomes of the study - produced in conjunction with colleagues from Cambridge University, Imperial College London, and the British Antarctic Survey - is that it enables a risk assessment to be made for the entire planet.

This reveals that the number of falls at the poles is about 60% of what you would expect at the equator.

It explains why you would absolutely want to put any long-term contingency facilities at higher latitudes.

The Global Seed Vault, for example, which aims to retain copies of Earth's plants in case of crisis, is sited at 78 degrees North on the Svalbard archipelago.

The new estimate, published in the journal Geology , grew out of the project to undertake the first UK-dedicated meteorite hunt in the Antarctic.

Researchers involved in that effort wanted to be sure they would visit the most productive areas to perform such a quest.

The White Continent is the place on Earth where most meteorites have been recovered - with good reason: the "black on white" contrast of fallen space rocks on snow and ice makes searching a lot easier.

And hunters will typically go to places where the movement of the ice sheet concentrates the meteoritic material - so called stranding zones.

Dr Evatt and colleagues worked out how many objects ought to be in their chosen area - a place called the Outer Recovery Ice Fields, close to the Shackleton range of mountains in East Antarctica.

And they were virtually bang on with their expectation, finding close to 120 meteorites in two systematic searches over 2019 and 2020.

But having worked out a reliable flux for the number of falls at their chosen terrain, the scientists realised they could then use this knowledge to anchor a global assessment.

This incorporated orbital mechanics - how Earth's gravity will pull in nearby passing material - to work out how rates might vary by latitude. The model outputs a grand total of about 17,000 falls a year.

And this can be tested by looking at the data from fireball events. Satellites in orbit tracking the lightning in storms will also catch the blazing trail of a space rock plunging into the atmosphere.

"Satellites monitor these explosions in the sky, working out the energy of the events and also the longitude and latitude of where they happen. And from this you can see how they vary across the globe with latitude, and very nicely the curve you get from these fireballs fits with what we independently modelled using purely an applied mathematical approach," said Dr Evatt.

Prof Sara Russell leads the planetary materials group at the Natural History Museum in London. She wasn't involved in the research, but commented: "I think this is an amazing study, and this estimate sounds like it is in the right sort of ballpark.

"We think a total of about 40,000 tonnes (so 40,000,000 kg) of extra-terrestrial material falls to Earth each year, but the vast majority of this is in the form of tiny dust grains.

"This is a very difficult measurement to make with any accuracy and only about half a dozen meteorites are actually seen to fall each year, but of course almost all meteorite falls are not observed because they fall in the sea, in unpopulated areas or just no-one is looking!" she told BBC News.

Fireballs reported by US Government sensors (Apr 1988 to Mar 2020)

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