Apr 9 2022
Cosmic Rays : Part 4 : Paleoclimatology and Muons
The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics©2021 The Pasayten Institute cc by-sa-4.0The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov.The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.The Particle Data Group's write up on cosmic rays. See Figure 29.8 for a representation of the "ankle" feature in the spectrum.https://pdg.lbl.gov/2019/reviews/rpp2019-rev-cosmic-rays.pdfAnother representation of the power laws can be found in Professor Peter Gorham's Coursework on Ultra High Energy Cosmic Rays: http://www2.hawaii.edu/~gorham/UHECR.htmlNatalie Wolchover has written two great articles in Quanta on Cosmic Rays, both which talk about what might accelerate these particles.The Particle That Broke a Cosmic Speed Limit and Cosmic Map of Ultrahigh-Energy Particles Points to Long-Hidden TreasuresColussi & HoffmannIn situ photolysis of deep ice core contaminants by Çerenkov radiation of cosmic originGephysical Research Letters: https://doi.org/10.1029/2002GL016112Guzmán, Colussi & HoffmannPhotolysis of pyruvic acid in ice: Possible relevance to CO and CO2 ice core record anomaliesAtmospheres: https://doi.org/10.1029/2006JD007886A quick primer on Cherenkov Radiation: https://www.radioactivity.eu.com/site/pages/Cherenkov_Effect.htmTheme music "Sneaking Up on You" by the New Fools, licensed by Epidemic Sound.Cosmic RaysPart 4 - Paleoclimatology and MuonsOur atmosphere is one giant filter for cosmic rays. The sparse molecules near the top of our atmosphere begin the process of catching the energy of those energetic particles from space and transferring it into heat or muons. These cosmogenic muons that typically make it all the way down to the surface.Near the surface, the atmosphere is a lot thicker, but it’s still just a collection of ballistic molecules bashing into each other at 1000 miles per hour. Some of those molecules hit us, and some hit the ground. We perceive these molecular impacts as air pressure. By contrast, cosmogenic muons are moving through this mess at over 600 million miles per hour. To those muons, the surface of the Earth is barely noticeable. They fly through a lot of things: hundreds of meters of rock, oceans, plants and animals before colliding or decaying. By contrast, those particles of atmospheric gas typically reflect off the surface of the Earth. Rocks just aren’t that permeable to most gas. As we explained in the ALPHA particle miniseries, helium gas generated from radioactive decay deep within the earth collects underground, trapped by rocks.One thing gas can permeate is surface water.Quite a bit of our atmospheric gases get dissolved into the ocean. Oxygen in the air allows the fish to breathe too, once dissolved into the water so it can be picked up by their gills. Increased carbon dioxide levels also imply more CO2 gets put under water. When the water on Earth’s surface freezes, as it might do near the polar ice caps, it traps some of that dissolved gas with it. This has been happening for millions of years, and until somewhat recently at least, that ice has been compounding. New ice forms above, pushing old ice down. This has resulted in a LOT of ice.In Antarctica there are areas where the ice is over four kilometers deep! That’s miles of ice! Greenland also carries massive glaciers, two to three kilometers deep, built up in same fashion.The gases trapped in that glacial ice is a frozen relic of an older atmosphere. The deeper the ice, the older the dissolved gases. As the mixture of molecules in our atmosphere changes over time, it sets down a record in the glacial ice. The deepest ice, millions of years old, can tell us what the atmosphere was like millions of years ago.Extracting that ice is quite the scientific adventure!This all easy to say in theory - but the practice of Science requires a lot of gory, technical detail. Different measurements from different samples of ice at different depths from different parts of the world need to be calibrated. Ice can form at different rates in different places under different conditions.But, at least averaged over a given year or decade or so, the atmosphere should be well mixed. Huge weather patterns around the world mix the air, ensuring should be about the same. And so the Scientific logic goes like this:Assuming older ice is usually below the younger ice and the atmosphere is well mixed, then given any two ice sheets on earth, there should be a way compare them. The concentrations of different gases dissolved at different times should sequentially be the same. Like multi-colored stripes on a pole. The stripes may be different sizes, but they should be in the same order. If we can find the same sequences in gas concentrations across different ice sheets then we can start to put together a history of the Earth’s atmosphere.Near the turn of the 21st century, geophysicists were working on exactly this problem. They were trying to calibrate the gas concentrations trapped in ancient ice samples by comparing ice from Antarctica with Greenland. And things just weren’t adding up. The sequences didn’t align. The gas concentrations were just too different. There was some kind of missing variable in the data.As it turned out, that variable involved cosmogenic muons.The Speed of Sound and LightTo understand how muons resolved this Paleoclimatology puzzle, we need to go back to the source. The source of cosmic rays.In episode two of this series we talked about Fermi Acceleration - the process by which electrically charged particles like protons get accelerated to outrageous velocities by SHOCKWAVES in astrophysical plasmas.And shockwaves occur in glacial ice too.To understand shockwaves, let’s think about sound waves.Sound usually travels in the atmosphere like a wave. A wave of air pressure. Those atmospheric particles slam against each other in an organized and oscillating way, spreading out away from source.The speed of those waves depends on the amount and types of molecules present, as well as the overall temperature of the atmospheric gas. The sound waves we experience travel at around 343 meters per second, which is about 767 miles per hour.Here’s the thing, humans routinely fly supersonic jets that travel faster than that.Supersonic jets - like fighter jets - travel faster than the speed of sound. They travel faster than noise they make. You can’t hear them coming until they’re already past you. And when you do finally hear them, it’s a tremendous noise.It’s a shockwave, actually, that you hear. The particles of air are being disturbed faster than speed of sound. In some sense, the sound waves that are produced all kind of pile up on each other, forming the shock front or wall of pressure that some folks call a sonic boom.It’s a wall of energy collected by atmospheric particles moving far from equilibrium. This wall is similar to those plasma shockwaves that accelerated the cosmic rays deep in outer space.The important point is that the shockwave was generated by something moving faster than normal waves could. The jet was moving faster than speed of sound.As we’ll now see, another kind of shockwave - one driven by cosmogenic muons - is responsible for disrupting the gases dissolved in that ancient ice.Quasi Particles of LightSo fighter jets move faster than the noise they make. That’s a nice trick to try to sneak up on folks, but we have radar. Radar works by using radio waves - electromagnetic or light waves with really long wavelengths - and reflecting it off of objects. Unless the fighter jet is moving faster than the speed of light, we can still see it coming.But this whole idea presents a fun riddle. Question: When does the speed of light not equal the speed of light?Answer: When it is SLOWER than the speed of light.Wait. What?!Question: When is the speed of light SLOWER than the speed of light?Answer: When light moves through water. Or glass. Or. You guessed it. Ice.Wait. What?!Glass, like water, reflects and refracts light. You can typically tell when there’s water in a glass, or when you’re looking through a window. The light coming through them behaves in a funny way. Things just look different. A straw inside your glass of water usually looks disconnected from the part of it that is outside.We usually say that water “bends” light. In physics class we say it refracts it. And this happens because light SLOWS DOWN A LOT when its inside water. Or glass. Or Ice. By a lot I mean like 30 percent.Microscopically, at the level of photons, of course that notion is silly. The speed of light is a constant. It’s not LIGHT that’s moving through the water, it’s not a pure collection of photons per say. It’s something else, something that connects with light, and it is light that comes out the other side.If that sounds a little wild, don’t panic. It has a very simple physical analogy.Imagine being inside your home when a supersonic jet flies by. The shockwave of that sonic boom slams into your walls, shaking the windows and rattling your doors. Did the sound you hear come from molecules in the air? Sure. But the air inside your house. The molecules from the sonic boom slammed into your walls and windows, which in turn shook themselves. They vibrated in place. They vibrated in such a way that it shook the air molecules in your room, and the sound made it to your ears.Inside or outside, the sonic boom sounded basically the same. A bit muffled sure, but otherwise the same. Those sound waves from the air outside where transferred to the air inside through the physical materials of your house.Inside that glass of water, the electromagnetic energy is still moving. It’s just tangled up now with all the electromagnetic fields of all the molecules moving around inside the fluid. The resulting excitations - the slower light waves if you like - aren’t really made up of photons, they’re collective excitations of an electromagnetic disturbance passing through. But once out the other side, they spit out photons again. The air of course also has an index of refraction so this is something of a simplification, but hopefully the point is clear. It’s not pure photons that are traveling through the water, the glass or the ice. It’s something else. And that something else - those quasiparticles - don’t quite move as fast as light. They move a lot slower. 30 percent slower.Cherenkov RadiationCosmogenic muons travel at 99.4% of the speed of light. But light - or the quasiparticles that appear as light anyway - moves 30% slower in water or ice.So in water you cannot see those cosmogenic muons coming. Effectively, they’re moving faster than the speed of light. And that’s trouble because they carry an electric charge.As you might recall from our earliest episodes, electrically charged particles transfer energy with each other by exchanging photons. Therefore, cosmogenic muons moving through an electromagnetically dense medium like glacial ice are creating distortions in that electromagnetic field faster than those distortions can propagate as waves.In short, cosmogenic muons create electromagnetic shockwaves in water, and glass and ice. Just like with the fighter jets whose sound waves all piled up into a sonic boom, the cosmogenic muons create electromagnetic disturbances in the ice that pile up to create a shockwave of light. Or you know, quasi-particle light inside the ice. Or water.Traditionally, those electromagnetic shockwaves are called CHERENKOV RADIATION.Cherenkov radiation is famous for the eerie blue glow it gives to the water inside of radioactive cooling ponds near nuclear reactors. It appears blue but the shockwaves are mostly in the ultraviolet or UV spectrum. UV photons - or their associated quasi particles - have a bit higher energy than visible light.And if there’s one thing we know about ultraviolet light, it’s powerful enough to burn our eyes and skin. That’s because it’s powerful enough to break down chemical bonds between organic molecules.Given that, can you guess what the different is between Greenland Ice and Antarctic Ice?The PaperOrganic Molecules. Frozen plant matter. Greenland’s got it. Antarctica doesn’t. Surrounded by water and much closer to life as we know it, Greenland ice has much more contaminants that the center of antactiac, which though covered in ice, is effectively a desert.In a 2003 paper published in Geophysics Research Letters, entitled “*In situ photolysis of deep ice core contaminants by Çerenkov radiation of cosmic origin*, the authors Augstin Colussi and Michael Hoffmann argued that an unexplained excess of carbon monoixde gas trapped was consistent with the disintegration of the tiny bits of plant matter present in the Greenland ice by Cherenkov radiation induced by the flux cosmic rays.Remember, that’s over a hundred cosmic rays per square meter per second!In 2007, those authors, together with Marcelo Guzman, now at university of Kentucky, published a follow on study describing concrete chemical mechanisms that could generate carbon monoxide and carbon dioxide from cosmic rays.While protected from the sun’s natural ultraviolet rays by layers upon layers of ice, atmospheric gases from over a 1000 years go are still exposed to the penetrating flux of muons from cosmic rays. And the electromagnetic shockwave of those ridiculously fast muons - their Cherenkov radiation - constantly exposes organic matter to tiny bits of ultraviolet radiation. Just enough, as it turns out, to rip a few carbon atoms off of some big, frozen organic molecules to mix with the otherwise trapped, historical atmospheric gas.Like adventure, elementary particles are everywhere, my friends. Go seek them out.