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"X-Raying" the Earth with Nutrinos

August 1, 1997

Imagine astronomers gazing up one end of a telescope, trying to create pictures of the sky, while geologists peer down the other end, looking through a kind of microscope that can penetrate the earth’s innermost sanctums. Sound improbable? Well, welcome to the wacky world of neutrino astronomy, where down is up and up is down and occasionally the twain do meet.

Astronomers have been laying traps for high-energy neutrinos in some of the more remote spots on the planet: far beneath the Mediterranean Sea, in Siberia’s Lake Baikal, and deep in the ice caps of the South Pole (see “Hunting the Wild Neutrino,” TR April 1997). They hope that these elusive particles-with little or no mass and no electric charge-will reveal secrets about the violent places in deep space from which they came: black holes, quasars, and pulsars.

Now, however, geologists are hoping to use the neutrinos snared by these detectors to see if they can learn something about the earth’s constitution. Despite their infinitesimal size and fleetness (bounding at or near the speed of light), some of these neutrinos will be stopped in their tracks as they crash into atoms inside the earth. The denser the region, the greater the likelihood that it will block a neutrino. By keeping track of how many neutrinos reach the detectors as they travel through the earth, scientists can calculate where they were absorbed and in what quantities to obtain a picture of the planet’s “internal density structure.”

Detecting Dense Regions

Medical computer tomography (CT) employs a similar approach. Machines record the transmission and absorption of x-rays as they criss-cross through the human body, allowing observers to detect tumors or other masses. “We want to do the same thing with the earth, using neutrinos instead of x-rays,” explains Raymond Jeanloz, a geologist at the University of California at Berkeley.

Neutrino tomography was first proposed in the late 1970s by two physicists, John Learned at the University of Hawaii and Hugh Bradner at the Scripps Institution in San Diego. The duo realized that neutrinos-produced as a byproduct of the reactions occurring at the heart of every star-abound in the universe. But they set the idea aside because there were no available means of capturing the high-energy particles as they reached-and passed through-the earth.

Now, new observatories under development-including AMANDA (the Antarctic Muon and Neutrino Detector Array), NESTOR (named after the famous Greek king) off the coast of Greece, the Neutrino Telescope in Lake Baikal, and RICE (the Radio Ice Cerenkov Experiment)-may soon have the capability to detect the particles. With some prodding from Learned, Chaincy Kuo, a geology graduate student at Berkeley, thus revived the concept in 1994, assembling a team of geologists and astrophysicists to develop a strategy for gleaning information about the earth from neutrino data.

To understand how the technique is expected to work, suppose there’s one cosmic source of high-energy neutrinos and one detector on earth. As the earth rotates, neutrinos, which travel in straight lines, would cut different swaths through the planet en route to the detector. Observers could note the number of neutrinos detected for each separate route and determine where the most were being absorbed. That information would indicate where the densest regions of the earth were.

In reality, there would be many sources and many detectors. In time, therefore, neutrino absorption could be measured along a web of lines that slice through the entire planet. A computer could then combine these measurements to produce a composite image of density variations.
Density variations are significant, according to Jeanloz, because “they drive geologic processes on a global scale.” Denser regions in the mantle tend to sink, whereas less dense materials tend to rise. This continual subterranean churn gives rise to the movement of tectonic plates as well as to earthquakes and volcanoes.

Estimates of the earth’s density now rely primarily on seismological techniques. After an earthquake, scientists can measure the velocity of seismic waves that travel through the ground to a network of sensors-the denser the material, the faster the waves move. Additional information comes from studying the vibrations (or ringing) of the planet after a large quake. Unlike neutrino tomography, however, seismology cannot map the distribution of the earth’s density with high resolution.

Buried Treasures

Neutrino tomography might eventually yield clues as to what exactly the earth’s interior is made of. This knowledge, in turn, might help us find various resources-water, oil, gas, metals, and other minerals-buried beneath the surface. George Frichter, a physicist at the University of Delaware’s Bartol Research Institute, suggests that the technique might even tell us something about the moon’s interior if we observe how neutrino measurements change as the moon passes in front of the earth detector.

But the viability of neutrino tomography still hinges on one question: Are there enough detectable high-energy neutrinos for this to work? Hawaii’s Learned has no doubt that high-energy neutrinos are abundant and are waiting to be nabbed. “But how many are out there? And are the detectors we’re building big enough?”

To capture as many neutrinos as possible, Learned is part of an international team that is planning to build a giant “kilometer-cubed” neutrino telescope that would be about 50 times larger than the latest generation of instruments. Construction could begin within 5 to 10 years, possibly at the NESTOR site in the Mediterranean. “Given its size,” Learned says, “this device should have a real ability to do earth tomography-not just gross density measurements, but high-resolution scans.”

Such a massive detector would not come cheap, costing $100-200 million. On the plus side, Learned says, the neutrino beam itself is free, produced by “cosmic accelerators that are not subject to the whims of political agencies.”

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