Watery Eyes

A new telescope on the bottom of the ocean may be able to map out the inner depths of Earth’s core

By Carl Zimmer|Sunday, March 01, 1992
For ten years University of Hawaii physicist John Learned has been leading a team of researchers who are designing a telescope to be stationed at the bottom of the ocean. That in itself is enough to raise eyebrows, but there’s an even stranger wrinkle: new research shows that the device may someday be able to map not only the outer reaches of the firmament but also the inner depths of Earth’s core.

Learned calls the telescope Dumand, an acronym for deep underwater muon and neutrino detector. As its name implies, Dumand will be tracking neutrinos, elusive subatomic particles that have no charge and little or no mass, and muons, short-lived, highly energetic particles that can be created by a number of different particle collisions, including neutrino collisions. Celestial objects like the sun produce copious numbers of neutrinos, but because the particles interact with matter so rarely, they are extremely difficult to detect.

Physicists generally try to filter out solar neutrinos from the other particles emanating from space by setting up shop a mile or two underground, where a detector--which typically consists of a tank of chlorine--won’t be bombarded by stray signals. Most neutrinos pass through the ground and the tank unimpeded. But when an occasional neutrino does hit a chlorine nucleus, it turns one of the neutrons in the nucleus into a proton. In other words, the chlorine atom is transformed into argon. The amount of argon in the tank corresponds to the number of neutrino collisions.

Such underground detectors are adequate tools for catching neutrinos from the sun. But there’s another class of the cagey particle that those detectors can’t see. Physicists believe that quasars and certain other galaxies contain supermassive black holes at their cores; from these galactic engines come neutrinos a million times more energetic than the ones from the sun. Since these neutrinos emanate from what are some of the farthest objects known in the universe, they could help astronomers chart the distant cosmos. But the neutrinos are produced so far away that very few cross the orbit of Earth, and only a vast tank would catch a significant quantity of them. Learned wants to use the ocean as a tank instead.

Dumand will look something like a giant upside-down chandelier. The apparatus will consist of nine 1,000-foot-long cables anchored to the ocean floor at one end and kept taut by buoys tied to the other end. Some drifting will inevitably take place, but researchers will be able to track it precisely with sonar. The cables will form a circle 300 feet in diameter off the coast of Hawaii. Strung along the cables every 30 feet will be 16- inch-wide glass spheres that house the detectors.

When a neutrino strikes an atom in seawater, it can create a muon, which in turn generates a conical streak of blue light. The detectors will catch the light and relay a signal to the base of the array. From there information will travel 20 miles of fiber-optic cable to land, where computers will piece together the paths of neutrinos. Learned will have to discount muons coming in from above Dumand--muons are created not only by neutrino collisions but also by the interactions of cosmic rays with atoms in the atmosphere. However, muons that travel up from the ocean floor can only have been the product of neutrino collisions because only neutrinos can penetrate through the entire Earth. With enough of these collisions, astrophysicists will be able to paint a neutrino portrait of the sky.

Learned’s team is already rolling. Our plan is to lay cable this summer and fall, he says, and we’ll have the whole thing done in 1993. We’ll be seeing neutrinos the year after that.

As it turns out, they may be seeing a lot more than they bargained for. Recently Floyd Stecker and his colleagues at NASA’s Goddard Space Flight Center and the University of Utah used satellite data to estimate how many neutrinos galactic engines churn out. They claim that Dumand will catch as many as 10,000 every year. That’s many powers of ten higher than we had ever dreamed of, says Learned.

With this embarrassment of riches, Learned may be able to learn more about the Earth as well as the heavens. Although the great majority of neutrinos will pass through the planet unhindered, a few will be stopped; how many depends on the density of the material they encounter. As Earth rotates, the neutrinos Dumand detects will pass through different cross sections of the planet. Different neutrino counts from those sections will indicate different densities.

That’s exactly what people do with CT scans, says Learned. They rotate the beam around your head and create a cross section. Until now, geologists could learn about the core only by tracking seismic waves. Although Dumand itself may not be powerful enough for physicists to construct a detailed, CT-scan-like image, Learned believes the next generation of ocean-based detectors will be. We hadn’t been counting on any of this, Learned says. But he and his colleagues will have plenty of counting to do now.
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