Finding Micrometeorites: Part 1
Micrometeorites are tiny little rocks that fell from the sky, and they're actually pretty common: on average, about one falls on the earth's surface per square meter, per year. Finding them is quite easy, too, if you live in Antarctica --- just melt some drinking water and save the particles at the bottom of the hole. After all, the entire continent is covered in ice, so said particles must have come from up above.
Everywhere else in the world, though, finding micrometeorites is not easy. In fact, it was only until 2016 that a Norwegian Jazz guitarist (really) proved to the world that you could find these particles amidst the noise of urban contamination. He has since found thousands, and leads an active Facebook group where many do the same. The secret is first physically reducing the noise as much as possible, and then being patient enough to sift through large amounts of material. I've thought for a while that a project like LadyBug could be used to automate both processes to some level, with the eventual goal of being able to take a scoop of dirt from anywhere, chuck it into a machine, and have it spit out tiny meteorites at the other end.
The reason I'm updating the project now is because my dad and I finally got around to emptying the rain barrel, which has been sitting under the spout for about five years now. Theoretically, any tiny rocks that landed on the roof would get flushed into the barrel, sink below the point of the spigot, and concentrate there over time.
I took this nice bucket of at-the-bottom rainbarrel sludge and set to work extracting the small mineral particles, as described by "on the trail of stardust". Basically, I added water, waited a little bit of time for heavy particles to settle, then poured off the liquid. Over and over, like gold panning. Once I had done this a few times in the bucket, I transferred it over to a smaller bowl and basically just directed a stream of running water into it, so that the container was constantly overflowing. This worked much better for removing all the tiny leaves and dirt, and, when dried, I was rewarded with a couple of spoonfuls of non-dirt that could be separated into different size fractions.
Finally, I took these size fractions (>0.5mm, 0.25-0.5, and 0.15-0.25, because these are the meshes I had) and separated out the magnetic particles by using a magnet covered by a plastic bag. Not all micrometeorites are magnetic, but most are, and the extra separation is necessary to have a chance at finding one, at least for an amateur like me.
From there, I immediately noticed my first spherule with the naked eye in the larger size fraction. Woohoo!
Unfortunately, the magnification/quality I can get with my USB microscopes turns not good enough to make any kind of identification other than being able to tell that it's round. I'll have to take this and any others I find in to a proper stereoscope and then possibly an SEM to be sure. Most spherules like this are of human formation, anyway.
Now, I probably have about half a gram of magnetic mid-sized particles, or a few thousand to tens of thousands. Really, not that much --- certainly not too many that I can't search by eye, and I plan to. But again, let's imagine that we are truly ambitious and want to come up with something that would scale to checking through millions and millions of particles a day.
Anyway, let's first start with the USB microscope sees in the magnetic 250-500 micron fraction:
(excuse the background --- it is sharpie'd cardboard. I am still trying to figure out the best thing for imaging rocks of varying but mostly blackish colors).
Note that we have quite a range of particle sizes and colors, and some things are shiny and some are not and some are blurry and some are not. The first thing we could easily do is separate to a very consistent size, which would make things easier. But let's brute force the issue with a stack+stitch:
(click for full size). Is that gold?
So now we've got a much larger field of view, and more rocks to see at once. We could feed this into some kind of image segmentatorator, but really, stitching doesn't actually help here, since the object of interest fits into a single image! And segmentor doesn't care whether it gets one big image or many small ones. Since we would presumably be keeping track of position information, stitching only adds an extra step where we now have to figure out where the rocks individually were.
Okay, but maybe your system does a scan at low magnification and then zooms in to a higher magnification to take a closer look at anything of interest.
Here's one rock:
And here's the whole stitched view (non-stacked --- there was a bump).
Stitching is a bit more useful at this magnification, because you don't have to worry about your particles fitting into each frame. But this is even less of an issue for the smaller rocks, which we haven't looked at yet.
And in any case, steady-state would probably make more sense than stitching over a wide field of view. Rather than having an operator spread some particles over a plate and replacing them after the scan is done and any particles have been picked it, a serious system should use a hopper and conveyor belt to spread them out thin past a stationary (or moving in 1 axis) microscope. If you see anything worth keeping, direct the rocks into a new container. Expert mode: Use a tiny electromagnetic needle to separate each grain of sand individually.
I'll leave you with an example of edge and roundness detection (on my smaller grains) using a script I found from someone who isn't me, Definitely some potential, but I think that it would also be worth trying to physically separate out by roundness rather than immediately jumping into computer vision. That is, if our goal is to solve the problem of finding micrometeorites, rather than forcing the problem to be solved using the hardware that we have.
Micrometeorites (part 2)
I just spent the last couple of hours on a dissecting microscope looking through grains of sand for things that were more round than usual. And I did find things. 7 or 8 candidate rough or shiny stones of size 150-500 microns, which I transferred to a tiny plastic bag using a moistened sliver of matchstick. It's not exactly the same, but right now I'm glad for my training in handling nematodes, which can be of similar size and which are also fragile and try to squirm away. Tiny rocks, in comparison, are relatively easy to pick up --- though I'm not convinced that every single one of the specks actually made it into the bag afterwards. (EDIT: A much better way is onto a sticky note (they stick) , or if going to be used for SEM, onto conductive tape. )
Unfortunately, while clearer when viewed directly with my eyeballs, the best pictures I could get with my phone on the eyepiece don't vastly exceed that of the USB microscope. Here's the large spherule I showed in the last post --- if it were extraterrestrial it would probably be "I type", but again, most that look like this are extraterrestrial.
So when I get a chance they'll go right into the SEM to look in proper detail. In the meantime, let's play: Spot the blurry spherule!
There's actually quite a bit of interesting "other" that I didn't transfer over, too:
is that an insect mandible? a thorn?
After spending this time looking through with my eyeballs, I am still convinced that some form of automation, or further enrichment of the sample, is possible. After all, we already did the latter with our washing, sieving, and magnetic sorting.
First we can be even stricter: Wash more thoroughly, in order to remove all the little tiny dirt flecks which complicate the search in the small size fraction, and allow non-magnetic particles to stick to the magnetic ones. And I would sieve to an even narrower particle size --- perhaps 50 microns in variation.
With that size fraction I would like to then try a method to physically separate out jagged samples from rounded ones. My ideas are to merely shake them dry in a container until the smaller, round objects end at the bottom, to put them on a shaker with a slight slant so that rounded particles tend to roll to the bottom, or to (most complicated) use a dense liquid like saturated sucrose solution to separate them out based on drag.
I still think the automated machine vision part is possible, too, again, trying to put the particles in single file so we can use good optics and not have to make them move. But the idea of a magnetic needle picking out individual particles is pretty much absurd. I mean yes, it's possible, but it would be a pain in the butt to build.
Something like what they use to separate out seeds would be ideal:
or, actually, it turns out they have this optimized for sand-sized particles --- for sand!
Apparently, the "Wesort Intelligent Full-Color CCD Quartz sand Color Sorter is specialized for the Quartz sand processing industry". What a world.
Micrometeorites (part 3)
Looked under the SEM and saw some promising stuff, but the veteran hunters think terrestrial. Dang!