By Geoff Evatt:
So, how many new meteorites are landing from space each? How much total mass of them are we gaining? And where about’s on Earth are we most likely to be hit? (especially timely this week given the news story about a 1 km asteroid passing within 4 million km of the Earth! ) This are just some of the questions answered in our latest publication, as published by the journal Geology this week.
In this study, we combined glaciology, mathematics and physics, and sprinkled it all with meteorite collection data, to produce an accturate estimate of these quantities. The headline figure being we estimate over 17,000 falls each year weighing over 50 gr (that is to say, some 17,000 objects fall from space and hit the earth every year, where each component fragment is known as a meteorite, and the summed mass of these meteorites are over 50 gr), and this equates to over 16,000 kg per year landing on the Earth. As for the regions most likely to be impacted, then this, it turns out, appears to be at the equator, where the poles receive about 60% of the equatorial flux.
The first part of the study was to work out the flux of extraterrestrial material in Antarctica. With it having the most documented meteorites on earth, and collected in a very systematic fashion, this meant we were able to harness the data from thousands of samples. However the nature of meteorites in Antarctica means that working out the area they originally landed on is not simple (because the ice is flowing). Combining mathematics with glaciology, we were able to invert for the effective surface area of ice which feeds into Meteorite Stranding Zones (the areas from which they are collected). And since we know flow speeds of the ice, and the number of meteorites collected from them, we were then able to solve for the flux of meteorites falling on a typical square kilometre of ice. Such a figure is useful, but beggars the question: how does that relates to elsewhere on Earth?
Solving for the places most likely to be impacted (the latitudinal variation) was a lovely problem, as the answer was not obvious because competing effects pulled the result to either the poles or the equator. Why the poles? Well, because material orbiting the sun might do so above/below the Earth, yet when in the vicinity of the Earth gravitational attraction, the objects would be deviated towards the polar regions. Conversely, the equatorial regions face head-on into the asteroid belt, and thus more surface area is available for receiving the material. As it turns out (after much old-school orbital mechanics) the equator still dominate for earth, but with the polar region receiving a decent whack – about 60% of the equatorial flux. This computed variation ties in very neatly with observations of the spatial distribution of fireballs across the globe – which was extremely reassuring. With us knowing a good estimate for the flux at the poles, is was then straight forward to use the derived latitudinal variation curve to estimate it for everywhere else.
Now, despite the equator being more likely to be hit, in regards being hit by anything dangerously big, this is not anything to worry about for many many years. This is because such events are extremely rare. And since the whole planet is receiving so many non-dangerous falls each year (17,000+), and each event creating a glorious fireball (much brighter than the shooting stars we see which are formed from dust-sized grains) it really tells us to head outside and look up: there is a good chance of seeing such a fireball event if you give it just a few nights.
Stay safe and look up!
Read the article (open access): G.W. Evatt, A.R.D. Smedley, K.H. Joy, L. Hunter, W.H. Tey,I.D. Abrahams, and L. Gerrish (2020) The spatial flux of Earth’s meteorite falls found via Antarctic data Geology https://doi.org/10.1130/G46733.1
Read a BBC Science online news story about the study