GUESTWORDS: Cosmic Messengers

By Stephen Rosen

    Yes, Virginia, Santa’s North Pole is covered with snow and ice. But this just in: Trillions of tons of ice really exist at the North Pole of Mercury, our Sun’s hottest companion.
    NASA’s Messenger spacecraft recently detected the nuclear signatures of water ice using galactic cosmic rays as probes. On other NASA missions, cosmic rays helped discover water on the Moon and Mars. Cosmic rays probe and “X-ray” volcanoes to predict when they will erupt. Very high-energy cosmic rays also generate recurring avalanches of nuclear particles, even Higgs bosons, everywhere in our atmosphere. These “cosmic showers” rain down over square miles of the Earth’s surface, penetrating our bodies — for the most part harmlessly.
    Cosmic rays are the only samples of matter that arrive on Earth from outside our solar system, and we now know that they are very energetic extraterrestrial protons (nuclei of hydrogen) and nuclei of heavier elements. Some low-energy cosmic particles originate in the Sun, high-energy cosmic rays come from stars in our own Milky Way galaxy, and very high-energy cosmic rays come from beyond our galaxy.
    The highest energy cosmic rays observed, if somehow converted into useful work, could probably lift a locomotive — but there is no simple way to harness this immense deluge of energy constantly raining down on us. (Very sad, really, when you think about how much oil and gas we consume . . . and how expensive electricity is on the East End.) Astronauts sent into space are exposed to the primary or extraterrestrial cosmic ray beam and can experience serious health consequences if they are unshielded on long voyages. At the bottom of the atmosphere, where we reside, we are mostly protected by the blanket of air above us, except when a rare energetic particle strikes a chromosome. Genetic changes may ensue; fit mutations will survive.
    Some cosmic rays come from the core or nucleus of our galaxy, where stars are born — and a giant black hole lives. NASA’s Spitzer Space Telescope has recently detected huge bubbles there that emit gamma radiation and X-rays. Cosmic rays are messengers carrying scientific information as gifts from beyond.
    The information content of cosmic rays tells us their energy, their origins, their age, the directions from which they come, and what they passed through on their way here. Evidence comes from very sensitive, sophisticated scientific instrumentation on mountaintops or sent aloft in special balloons and satellites. Data also come from the International Space Station, the Mars rover Curiosity, and Mercury’s recent visitor, the Messenger spacecraft.
    The most energetic cosmic particles striking nitrogen and oxygen air molecules at the top of our atmosphere generate a cascade of nuclear reactions, fanning out over thousands of acres by the time they reach the Earth’s surface. These “cosmic showers” are an avalanche of nuclear air-molecule fragments sharing the high energy of the initial incoming cosmic ray. Vast arrays of nuclear particle detectors placed at ground level are set to count only those nuclear events that occur simultaneously. This indicates that the original triggering cosmic ray came from interstellar or intergalactic space. Cosmic particles come from all directions in the universe uniformly. The cosmic ray beam is roughly constant in time. The lower-energy particles are more abundant than the higher-energy particles.
    Another source of information about the constancy of the primary cosmic ray beam over time is the observation of the composition of meteorites that have been traveling around our solar system for millions of years or more. Modified by cosmic ray bombardments, they act as if they were cosmic ray dosimeters, showing that the primary galactic cosmic rays have been constant in intensity within a factor of two over a million years, and constant within a factor of three or four over a billion years.
    Where do cosmic rays come from? The most likely sources are supernovae — stars that have reached a stage in their evolution when they become unstable and erupt in a spectacular outburst of energy in the form of light, X-rays, gamma rays, and nuclear particles.
    Supernovae may be visible to the naked eye for months: the Crab Nebula (seen in 1054 A.D.), the Tycho Brahe supernova (seen in 1572), and the Kepler supernova (seen in 1604). Similar spectacular events have been documented in ancient annals and astronomical observations from China and Japan. A supernova flares up in our galaxy every 50 years or so — but there are 100 billion distant galaxies in the universe that contribute their share to the primary beam of ultra-high-energy cosmic rays finding their way to us.
    The relative constancy of cosmic rays in time and space says that these sources of cosmic rays are also distributed uniformly in space and time. Magnetic fields bend the paths of positively charged cosmic rays (mostly protons) so much so that there is no obvious correlation between the directions they arrive from here and the direction they had at their source. Light rays move in a (mostly) straight line-of-sight to us from distant stars. Unlike light rays, cosmic rays move in a sort of random walk, drunkard’s stagger, or pinball’s path before they reach us, disguising their source’s origin and suppressing any variations in space or time. Imagine the Earth bathed in an immense reservoir or rippling sea of cosmic rays.
    Our galaxy looks like a flat pancake (the “disk”), with a scoop of ice cream at its center (the “core”); our solar system is located about halfway out within one of the many spiral arms in the disk. Cosmic rays may be “stored” by bouncing around in our galaxy in the sense that a pinball bouncing from obstacle to obstacle is “stored” in the pinball machine. Some cosmic rays bounce off light nuclear targets and are deflected slightly. Others strike heavy nuclear targets and are deflected a great deal so the original particle source-direction is not preserved.
    How could the cosmic ray particles from exploding stars be accelerated to such incredibly high energies — energies not even remotely available in terrestrial nuclear particle accelerators? An unusual acceleration mechanism was famously suggested by the great physicist Enrico Fermi.
    Imagine a tennis ball bouncing up and down on a flat horizontal surface. Visualize a tennis racket placed above the ball parallel to the horizon moving slowly downward. The vertical bounces of the ball between the two approaching flat surfaces will increase rapidly. Rather, slow motion downward of the tennis racket results in a rather surprising rapid increase in the speed of the bouncing ball, as the frequency rise of the audible bounces would indicate. Imagine the ball represents the cosmic ray particle, and the two approaching flat surfaces represent reflecting “mirrors,” or reflecting walls. A high magnetic field can act as a reflector of charged particles.
    Of course, these are simple analogies, and conditions in interstellar or intergalactic space are what they are.
    The physicist Karl Darrow said that cosmic radiation “is unique in modern physics for the minuteness of the phenomena, the delicacy of the observations, the adventurous excursions of the observers, the subtlety of the analysis . . . and the grandeur of the inferences.”
    The cosmos is extremely generous, showering us with cosmic messages and gifts of grandeur. Happy holidays!

    Stephen Rosen, the author of “Cosmic Ray Origin Theories,” is a former research physicist at the Institut d’Astrophysique in Paris and at the Centre d’Etude Nucleaire de Saclay. He lives in East Hampton and New York.