New paper: The spatial flux of Earth’s meteorite falls found via Antarctic data

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?

Schematic of meteorite stranding zone ice flow and loss used to help build our model of Antarctic meteorite stranding zone data. Image Andrew Smedley (as published in the supplementary materials of Evatt et al., 2020)

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

New paper

Andy Smedley | 23 March 2020

A little over two weeks ago the latest paper from the project was published. Given what’s happening in the world right now (it’s late March 2020), that seems a long time ago, but perhaps this short blogpost will provide some useful distraction whilst we all practise social distancing.

Occasionally meteorites will be found partly encased in the translucent ice, the ice underneath having melted and then refrozen [Credit: Katie Joy]

So our new study looks at how sunlight interacts with the blue ice where the vast majority of Antarctica’s meteorites are found. In the original paper that kicked off the project we used a simple mathematical model of how sunlight penetrates into the ice and how much is absorbed by the meteorite. How much sunlight is absorbed by a meteorite trapped in the ice determines how much it will heat up, and if it reaches the melting point of ice, how much the meteorite will then sink dow. In the original paper we used a fairly simple model of how sunlight is attenuated by blue ice and treated the system as one dimensional. In reality though it’s a 3D system and things are more complicated. One of the major ways in that it’s more complicated is that sunlight is made up of many different wavelengths (think the colours you see when it passes through a prism, or when a rainbow is visible, but extending beyond the range your eyes can see). Each of these wavelengths is affected by the ice properties slightly differently which means that the total attenuation is more subtle that we first assumed. When sunlight at infrared wavelengths hits the surface of the ice and passes into it, it’s rapidly absorbed by the ice. In contrast the part of sunlight corresponding to blue wavelengths is absorbed much less readily, and so is repeatedly scattered by the tiny bubbles within the ice. (These bubbles are actually tiny pockets of air trapped when snowflakes fell many thousands of years ago and their chemistry can help us understand how climate has changed on geological timescales.) As it so difficult for light at these blue wavelengths to be absorbed by the ice they continue to be scattered around inside the ice enhancing the amount of energy available to be absorbed by any dark meteorites present. As a result a meteorite sitting within the ice can act as a sink for nearby solar radiation, but, as well as absorbing more, because we now treat them 3-dimensionally, more energy is dissipated. To figure out how these different contributions balance out, we took the results of our sunlight modelling and added in the other things that might cause heating or cooling of the meteorite: the temperature of the air above the ice, the wind blowing over the surface, the motion of the ice, whether the meteorite gets warm enough to melt the ice and how far it then sinks and how these factors might vary over several years, plus the 3D nature of the problem.

It’s hard enough to find meteorites in Antarctica without them sinking into the ice. [Credit: Katie Joy]

All in, rather than iron meteorites being predicted to lie ~30 cm below the surface of the ice whilst their stony counterparts rise to the surface, this study suggests that the difference is much less, with iron meteorites being only 5-10 cm deeper than the stony ones. This isn’t a huge distance of course, and the blue ice is slightly translucent, but when you’re scanning from your skidoo for a speck of dark rock surrounded by the immensity of Antarctica, it’s enough to make spotting them virtually impossible. Interestingly though this new modelling shows the meteorite sinking mechanism is more nuanced than we first thought, with iron meteorites reaching the surface over the winter (when it is dark) before sinking into the ice early in the summer period after the sun rises. Though some questions remain, this seems more in line with what has been found in Antarctica as it gives the potential to find some iron meteorites if the conditions are right, and if the field expedition is during the early part of the summer.

If you fancy reading some of the technicalities of the paper, it can be found here.

Sledge Victor Over and Out

Katie Joy | 21 Jan 2020

The last few days have been somewhat manic to say the least. A final search on the 17th Jan didn’t yield any more meteorites, though we finished off the southern ice field and could put the doos (skidoos) to bed.

End of season thumbs up selfie from the two remaining Sledge Victor field party members (Katie in the stormtrooper mask at left, Taff in his duck beak mask at right). [Credit: Katie Joy]

A weather window then opened up across the Ronne Ice Shelf meaning that a plane was quickly dispatched from Rothera to come out to our field site at Outer Recovery to collect fieldguide Taff and I. We had a busy day of packing up camp, building and finalising the depots to be left over the Antarctic winter, sorting out all the kit to be brought back to Rothera.

Evening sun at our field site just before we broke camp. [Credit: Katie Joy]
Kelvin-Helmholtz clouds forming in the area, indicative shearing winds. [Credit: Katie Joy]

The weather was quite changeable in the day, with at one point snow blowing across the ice surface under 25 knot winds (not ideal for landing a plane when you need to see the skiway contrast). However, the Otter landed at about 10 pm on the 18th Jan with pilot Ian and co-pilot Callam, uplifting us back west, where we had a late night camping out in a mountain tent on Berkner Island. The air was still, but cold with our hair freezing up turning us grey, and the surface was snowy and looking like a million sugar lumps. The next morning on the 19th we departed for Rothera via Fossil Bluff, landing around 7 pm.

Flying back over the Peninsula – mountain ranges emerge out of the ice. [Credit: Katie Joy]
Melt ponds forming as the sea ice melts near Fossil Bluff. [Credit: Katie Joy]

Everything was rapidly unloaded from the aircraft and shifted to various parts of the base to be organised — the meteorites to the freezer, the rubbish to the rubbish centre, our science cargo to its storage area be sorted, all the field gear to the Fuchs building (sorry guys for making it a mess). Utterly overwhelming to see so many people after the relative quietness and tranquillity of the field. Finding our rooms, quick shower, making it to the dining hall (just) for the end of service (lamb roast – amazing). I was scheduled on the Dash 7 flight north to Chile for 9 am the next morning on the 20th…  so had to run around to organise all of my personal kit separating it from the BAS borrowed field kit and making sure things were washed (sorry to my roommate Sarah who’s bedroom was suddenly inundated with stinking clothes and four bags of stuff tipped out on the floor – I hope the smell of kerosene and field grime is not lingering) and the science cargo to be shipped back north later this month, to be ready to leave. Ahhhhh… finally got a beer at the end of a long day. Then the next morning saying goodbye to people, heading north on the Dash, dinner in Punta, an early night and collection at 4 am today for the next flights back to the UK via Santiago and Sau Paulo.

Flying over the Andes on route out of Santiago airport. [Credit: Katie Joy]

So yes, its been a bit mad for the last few days and I am missing life of being at Outer Recovery whizzing around the ice spotting meteorites. Antarctica grabs you (well it has certainly has got to me) and doesn’t let you go — hopefully I will get a chance to come back someday to continue the search for more space rocks on the ice.

It has been amazing that in just two field seasons with such small teams we have collected over 100 surface stones for future scientific study by the cosmochemistry community and I am very proud of what we have achieved and look forward very much to finding out what types of meteorites we have collected. Over to the laboratory and curation team now for the next phase of the science story, and hopefully we can continue to source more funding and the support of BAS to get back out to the ice in the next few years to continue our scientific success.

Saying goodbye to our fieldsite. [Credit: Katie Joy]

More blogs posts to come when we have results in from the last season’s meteorite haul, and to update everyone on our science research paper outcomes.

Some thanks and shout outs from me at this stage:

  • The rest of the Sledge Victor Manchester fieldteam — Geoff, Wouter, and Romain who fought a determined fight with the metal detector panels, and found some great meteorites during surface search days. To the story of 118-218-119-119alpha.
  • Andy Smedley, our Man back in Manchester, who has been receiving our emails from the field to post the blog. [BTW – we named the 3rd blue detector panel sledge Sledge Smedley (mentioned in this previous blog) in his honour at not having him with us in Antarctica]. Whilst Sledge Smedley only had a few days out and about, he lived his life to the full bouncing around and at least still has an intact bottom.
  • The Twin Otter flight teams (Mark, Ian, Dutch, and Dave) and co-pilots from Rothera and Halley who have come out to visit us this season and help get us and plane fuel across the enormity of the continent. The Rothera field operations managers who work 3-d chess to try and get everyone in the right place on the right day working around the ever changeable weather. The Rothera science coordinator Maz who has been so brilliant in helping out with requests for boxes, getting our cargo together for shipment, and just being completely fab. Everyone at Rothera and Halley who works hard to just get stuff done.
  • And a special thanks to our wonderful team of field guides Julie, Taff and Rob who have kept us safe, organised camp, provided great chats and moral, and have helped us to find the meteorites we have collected. Thanks guys for putting up with us all for the different parts of the project you have worked on this year and last, its been a privilege to spend time with you.

A productive few days out and about

Katie Joy | 16 Jan 2020

Although most of the team are now heading north back to the UK, Taff and I are still working hard at our field site and it’s been a productive and busy few days out in the field as we wrap up the season with some surface meteorite searching on two of the ice fields that we visited last year. Revisiting sites is interesting as some of our old tracks are still preserved as imprints in the snow patches like fossilised tracks, indicating past exploration. Both areas are much clearer of fresh snow now than last year (though they still have some thicker patches of older snow), and as there has been less wind it has been pleasant getting out and about and systematically covering the ground to try and find as many samples as possible.

Meteorite sample encased within ice. Just the top portion was poking out and we had to dig the rest of the sample out. [Credit: Katie Joy]

We managed to collect seven samples on the 14th (including a nice big one spotted from about 100 m away), two samples yesterday (rewards for a lot of driving around getting frustrated that we weren’t finding much), and four more today on the 16th (including a nearly completely ice submerged sample), bringing the total number of meteorite stones collected to 82 from this area in total from this year. Several meteorites found over the last few days have been stunning — really nice flight shaped stones preserving evidence of the orientation they travelled through Earth’s atmosphere. A couple others have very fresh fusion crusts suggesting they might be recent falls, and some have hints of pale coloured interiors which look different from the normal chondrite type (primitive asteroid) of samples that we most commonly collect.

We are not sure how many more days we will have in the field as we now await a break in the weather for a plane to travel over from Rothera to collect us — but tea supplies and moral levels are high, and we will keep getting out searching until before our skidoo petrol runs down. Then we will drink some more tea and reflect on a great end to the season.

Cracking ice – as the ice field extends and the ice speeds up it cracks and twists into small faults a few cms wide. We don’t typically find meteorites in areas like this, but sometimes have to drive past and it is like a structural geology lecture in action. [Credit: Katie Joy]
Taff pointing the way to the stunning meteorite (a whole stone) we found today with a rollover lip. [Credit: Katie Joy]

PS Thanks to our Sledge Victor teammates Romain, Geoff and Wouter, and quizmaster K for the amazing sausage roll song rendition over skeds. We didn’t think you would deliver, but you didn’t fail us. Quite magnificent. Who knew there were so many verses to get through. We hope that your travel back to the UK goes well, and see you back Manchester way. 119 is a lot cleaner without you. 🙂

Rothera, under and over

Geoff Evatt | 15 Jan 2020

Good morning from a rainy (yep) Rothera. Romain, Wouter and I have been here since Friday, occupying our time as best we can before we head home. To keep myself busy I’ve been out helping on the boats, which has involved holding on to the leashes of the BAS divers (doing wildlife survey work on the sea bed in the general vicinity of Rothera), should they need ever to be hauled back to the surface. Clearly this need is very unlikely, which affords the opportunity to do some wildlife spotting and take pictures of the glorious scenes. The highlight of which were a pair of feeding Minke whales circling around us, and at one point came within 10 m of the boat. Alas my lack of photographic skills (and general excitement) got the better of me, but at least the picture below captures the jist of things.

Fishing
Fishing for divers [Credit: G. Evatt]

In addition, I undertook the herculean task of waking up before 730am, to help launch a weather balloon. Balloons are launched all around Antarctica at the same time, with the aim of collecting comparable atmospheric data from similar elevations. I am pleased to announce that I currently sit in second place of highest elevations reached (28,939 ft), behind only the Radio 4 team…

Balloon
Weather balloon launch [Credit: G. Evatt]

And other than these we have been packing our bags and running around the airstrip (in so doing I have had the additional joy of seeing two humpback whales and a pair of chinstrap penguins) to burn off the vast quantities of food we’ve consumed.

It will be very strange to leave Antarctica after all this time, and after all the fantastic support we have received from the British Antarctic Survey. It is hard to understate just how complex the project’s logistics have been, and BAS have led the way brilliantly in ensuring we achieved as many of our aims as humanly possible. So thank you everyone for your help and support, it is very much appreciated and has been a privilege experiencing it first-hand. Next stop… the Falkland Islands!

Minke
Minke whale [Credit: G. Evatt]
Crabeaters
Crabeater seals [Credit: G. Evatt]
Adele
Adélie penguins [Credit: G. Evatt]
Shag
Antarctic shag [Credit: G. Evatt]

(Still) Chasing Meteorites

Katie Joy | 13 Jan 2020

The remaining team of Taff and Katie are still in the field at Outer Recovery and after two tent days (one because the winds were blowing at 30 knots all day, and yesterday due to bad contrast as it was cloudy) we made it out and about for some more meteorite searching. The high winds of a couple of days ago has blown away all the pretty hoar frost which was covering the area, but also more importantly has shifted the snow off our local ice field meaning that we could get back there to search the final southerly section which had previously been hidden from us under a couple of centimetres of covering.

We headed out in strong cold winds, today was the first day I have had to put on the big yellow down jacket over all the other layers I have been wearing. Pretty much as soon as we started out we were off the mark with a large 15 cm stone, and despite the cold, the day continued to prove fruitful with seven more meteorites collected and bagged*. These included a very nice hand sized complete fusion crusted stone which are pretty uncommon and a tiny pea sized perfect small rounded stone. By the time we headed back to camp the winds had dropped off, making for a pleasant returning home commute.

Katie_Meteorite
A lovely fusion crusted stone sitting on a small crack on the blue ice field. [Credit: Katie Joy].

We are not sure yet of our field retrieval date – but it feels really good to get a few more meteorites collected before we have to return to Rothera.


* Well a small one nearly wasn’t bagged when a gust of wind whipped the meteorite encased in its plastic collection bag away across the ice field — I jumped on the skidoo and whizzed off chasing after it, and upon catching up jumped off the skidoo on a snow patch to make a dive… Well, I missed and fell on my butt (you try running on ice when dressed like the Michelin man), and the bag and stone flew on past me at what I can only assume must be the quickest pace it has experienced since landing on Earth. Back on the skidoo I shot off again, overtook the flying bag again and this time aligned both me and the skidoo to make the interception. Bagged meteorite retrieved and safely stored despite its best attempts to get away. I was feeling pretty cold before this chase episode, but the rapid retrieval and adrenaline shot seemed to have done the trick to warm me up.

Meanwhile in Manchester…

13 Jan 2020

Readers will have seen and read about a lot of goings on “down south” in Rothera and at the Outer Recovery ice fields, and the results of the team’s searches at the field site near the Shackleton Mountains. This is only part of the story (though a key one)!

Back in Manchester the rest of the Lost Meteorites of Antarctica team have been busy, so we thought it only right that we give a brief overview of the work going on behind the scenes. Recently mentioned, Liam and John provided support to Wouter with the technical glitches and have of course been instrumental throughout the project from its initial design, build, lab testing and field testing.

Patches of blue ice at the base of cliffs in the Theron Mountains. Selecting the right spot is key to finding meteorites. [Credit: Romain Tartese]

In parallel with the detector system build, Andy has been working with lots of data analysis (using satellite datasets and climate model outputs) to figure out whereabouts the team was best searching for meteorites. Antarctica is a big place and meteorites are only found in a few spots. Sometimes people head out there to come back empty handed, so we wanted to do our best for last season to make sure we found a “blue ice area” that harboured meteorites. First of all, a selection of candidate sites were tracked down by Katie (before the current project was funded) and then reduced to a long-list of those accessible on a logistics basis with the help of BAS. Then, using a combination of estimates of snowfall (that tells us something about the rate at which meteorites accumulate in a given area), and the local surface ice flow and wind scouring (that tells about the rate of loss of meteorites), we came up with a prediction of what density of meteorites we expected across these candidate sites. That prediction enabled us to refine and rank our preferred areas for Katie to visit last year. Thankfully she and Julie Baum confirmed our estimates and found some meteorites! Once we had decided on particular areas, Andy was involved in making custom maps for the team’s GPSs from hi-res satellite imagery, more detailed estimates of which individual ice fields to return to (from the data and samples Katie collected last year), and the logistics involved in shipping and planning. At the moment he’s the main contact back in Manchester and has been responsible for posting updates sent through by satellite phone while Geoff, Katie, Wouter and Romain have been at the remote field site.

There’s lots of posts about trying to find meteorites on the blog, but once we find them — what happens to them? That job is being undertaken by Jane and Tom working with members of the isotope group.

Well, we’ve made sure the potential meteorites have all been collected following defined procedures to keep them as free from any contamination as possible, for example, they only come into contact with stainless steel equipment used to get them into polythene bags, and every sample is double-bagged. They are even kept at sub-zero temperatures throughout their journey back to the UK, giving us the best chance of keeping them in pristine condition for future science. Jane, working with Katie, Rhian Jones and with folks at the meteorite group at the NHM, has been working out the necessary steps for the preliminary examination plan for classifying the meteorites, to ensure the samples do not get contaminated, and that every stage of examination is thoroughly documented. In line with this, the first ten samples from last season have now been thawed and she is using “CT-scanning” to look inside the rock and get an initial idea of what it is made of, before deciding how to break or cut the sample. Small pieces will then be mounted on glass slides in order to examine them with microscopes so that they can be formally classified into their different classes.

The “light box” set up used to acquire the images for 3D photogrammetry scans. [Credit: Tom Harvey]

Now the first samples from last year’s reconnaissance trip have been defrosted, Tom has been working to scan the fresh sample exteriors with a technique called photogrammetry. Photogrammetry uses information in pictures of a sample (in this case a meteorite) which show overlapping surface features to position that bit of the sample in 3D space — meaning that we can generate an electronic 3D model of the sample! These models are really useful because they preserve a record of the sample exterior prior to analysis (or, if needs be, cutting), and mean that we can zoom in on parts of the surface that are particularly interesting, which is great for curation and initial characterisation purposes and gives a permanent record of what the meteorite looked like when it was found.

And as this post goes online, it sounds like this year’s samples might just be starting to make the long journey back to the UK… holding the promise of lots more interesting science.