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Dr. Phil Metzger Profile picture
@DrPhiltill
Aug 19, 2019 • 3 tweets • 1 min read • Read on X
Granular physics is awesome because non-intuitive macroscopic behaviors appear as the result of sand-grain-sized (mesoscopic) interactions. The existence of grain-scale physics makes granular matter different than ordinary continuum matter (solid, liquid, or gas). HT @IanMolony
2/ When I did my doctoral work in the physics of granular materials, this sort of non-intuitive sand experiment was all the rage. This particular experiment is new, demonstrating that there are still surprises to discover in physics!
3/ For another example of freakishly amazing granular physics, check out the three short clips of water drops landing in sand, here: pnas.org/content/112/2/…

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More from @DrPhiltill

Dr. Phil Metzger Profile picture
Apr 28
Untrue. This does touch on something related that actually happened, which people have apparently distorted and used to prop up the dumb conspiracy theory. I will explain… 1/N
2/ First I’ll tell you what I know about the videos, then the telemetry.

When I analyzed the plume effects of the lunar landings, starting in the late 1990s and early 2000s, I tracked down the original data. One of the guys on my team worked with Houston to get the videos.
3/ The originals had been converted to digital and this was more convenient for us to use, since we wouldn’t need reel-to-reel NTSC video equipment, so this is what we got. I had high resolution copies of all the landing videos. There was no lost video. It all exists.
Read 15 tweets
Dr. Phil Metzger Profile picture
Apr 18
NASA now building a flight-ready lunar excavator for a resource utilization pilot plant (not a demonstration — the actual pilot plant) on the Moon.
Discussing the challenges of reoeatably setting up the correct lunar soil conditions (compaction, rocks) for testing the lunar excavator on Earth. Image
3/ The robot will not be joystick-operated from Earth due to time delay and bandwidth limits. It will have software for autonomous mining & roving.
Read 8 tweets
Dr. Phil Metzger Profile picture
Apr 10
I'm tired of reading in the news people proclaim that starting a city on another planet is economically ridiculous when clearly they are just guessing. So I'm finally starting to write a paper on the analysis I did a few years ago that found (to my surprise) it is quite feasible.
The main thing ppl don't seem to grasp is that the cost of the extra stuff for Mars, like building a dome, recycling air, using mass for radiation shielding, washing perchlorates from dirt, etc., are utterly trivial compared to the cost of frivolous things we do in our economy.
The 2nd main point that ppl don't seem to grasp is that you don't need any particular advantage from being on Mars to make it economically viable. Mars doesn't need special minerals or anything. Any location becomes economically viable simply by there being enough humans there.
Read 4 tweets
Dr. Phil Metzger Profile picture
Mar 31
Part 2. Another thing I think is cool in the papers I linked a few days ago. The quoted thread was about the granular physics of gas digging craters in small experiments, which I thought was cool. This new thread is about lunar geology. 🧵 1/N
2/ The papers described how those small experiments give physics insight that leads to a new equation predicting erosion rate when there is no saltation. I took that new equation and applied it to the Apollo Lunar Modules to predict how much soil was blowing.
Image
Image
3/ We can compare the theory’s predictions to the images looking out the Lunar Module (LM) window. As the dust blows, the image gets brighter. A histogram of pixel brightnesses gets narrower (less contrast) and shifts to the right (more bright pixels). Image
Read 42 tweets
Dr. Phil Metzger Profile picture
Mar 29
Here’s something I think is cool in the new papers that I linked yesterday.

My research group over the years has run many, many small scale experiments where a jet digs a crater against a window so we can see into it.
2/ Something weird we see in these experiments is that the depth of the crater is perfectly described by the logarithm function. Like I mean, perfectly. There are two parameters: a and b, the length scale and (inverse) time scale.
Image
Image
3/ You can use different gas speed, molecular weight, diameter jets, grain sizes, mineral density, gravity, etc. The crater depth is always a perfect logarithm of time. In fact, if you plot it versus the logarithm of time, the depth turns out to be a perfectly straight line. WHY? Image
Read 15 tweets
Dr. Phil Metzger Profile picture
Mar 18
I’m not so sure. The link to the prior estimate is a paper that measures the “blast zone”, which is the region around a lander where the reflectivity of the surface has changed. We have never known exactly what causes this change. Is it from gas blowing the dust texture flat? /1
2/ Or is it from blowing dust plowing across the surface? Or from engine shutoff when the last sputter of the engine cause a low velocity blanket of dust to fly out to a much smaller distance than normal? The problem has always been that this blast zone is *too small* to be…
3/ …the area where the dust finally lands, because the dust in rocket exhaust is going far to fast in low gravity to travel only that far. (We think we might know the cause now, but I don’t want to tell here since we will probably write a paper on it.)

So the prior estimate…
Read 10 tweets

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