For a big view of inner Earth, catch a few … Geoneutrinos

Were the Earth a crystal ball, you might gaze 2,900 kilometers down to its outer core with a telescope. The Earth, though, is frustratingly opaque — to light. Most knowledge of the planet’s internal structure comes from studying seismic waves, which give a kind of ultrasound image. Inferences about Earth’s internal chemistry rely on the elements found in near-surface rocks, meteorites and the sun.

Recently, geoscientists have developed a new tool for probing the Earth’s innards. Borrowing a page from astrophysics, they are using the curious subatomic particles known as neutrinos. Astrophysicists have used neutrino telescopes for decades to study neutrinos originating in the sun and elsewhere in the cosmos. Now earth scientists are taking a neutrino telescope and looking down, to illuminate the Earth’s interior by detecting “geoneutrinos” — neutrinos produced within the planet itself.

Originally used to detect elusive particles from space called neutrinos, the four-story detector at the Sudbury Neutrino Observatory could be retrofitted to detect antineutrinos produced by natural radioactivity inside Earth. Courtesy of Lawrence Berkeley National Laboratory

A physicist boats in water surrounding Canada’s SNO detector, which is filled with heavy water to detect neutrinos. A new fluid will be used to detect geoneutrinos. SNO+ Collaboration

THEY CAME FROM EARTH | Like its cousin the neutrino, a geoneutrino (an anti-neutrino produced in Earth) can pass through Earth unimpeded and, researchers hope, into detectors built to catch it.

“Now, for the first time, we have the possibility of measuring the composition of the Earth in real time,” says William McDonough, a geochemist at the University of Maryland in College Park.

Geoneutrinos are actually antineutrinos, which are neutrinos’ antimatter counterpart, just as positrons are the antimatter partner to electrons. “ ‘Geoneutrinos’ is just an easier word to say than ‘antineutrinos coming from inside the Earth,’ ” McDonough says. Electrons and positrons have opposite electrical charges, but neutrinos and antineutrinos have no charge. So neutrinos and antineutrinos, confusingly, may or may not be the same particle.

Geoneutrinos were first observed in a detector deep inside a mine in Japan in 2005. Now an array of proposed new experiments are poised to get an even better glimpse of the Earth’s inner chemistry. These range from deep-mine detectors in Canada, the United States and Europe, to a mobile, submersible deep-ocean detector.

McDonough and two colleagues from the University of Hawaii gave an overview of the experiments in October in Eos, the weekly newspaper of the American Geophysical Union, and earth scientists discussed new developments with particle physicists at a conference in September in Sudbury, Canada — the site of one of the proposed experiments.

Geoneutrinos originate from the radioactive decay of uranium, thorium and potassium in the Earth’s crust and mantle. Earth scientists are keen to learn more about the crucial role the decay of these elements may play in heating up the Earth and, in turn, driving convection in the Earth’s mantle.

Powering Earth

“The convection in the mantle is responsible for essentially all of the dynamics of geology that we see — moving continents and seafloor spreading,” says John Learned, a particle physicist at the University of Hawaii at Manoa. But whether radioactive decay dominates the heating action or is one of a number of players isn’t known. There’s even controversy over how much heat, in terms of power, the Earth puts out; estimates range from 30 billion to 44 billion kilowatts.

Energy drives the movement of the geologic plates upon which the continents ride, says McDonough, “and the fuel for that is either entirely radioactive fuel or a subset of energy sources.” It’s like the energy mix in homes, he says. “We don’t get all of our electricity from coal, but some portion from nuclear and some portion from other sources. The question today is, ‘What are the energy sources driving the Earth’s engine?’ ”

Among the other possible energy sources is heat left over from the planet’s formation by colliding meteorites. These planetary building blocks eventually accreted enough mass to become Earth. As the meteorites slammed into each other, their kinetic energy became thermal energy. Over time, the Earth has radiated this heat into space.

“We could have started out with a large amount of kinetic energy, and we’ve slowly dissipated it,” says McDonough, “or we could have started with a large amount of kinetic energy and rapidly dissipated it, depending on the atmospheric conditions.”

It’s difficult to measure how much heat might have come from this or other sources. But the new suite of geoneutrino detectors could pin down better numbers for the radioactive contribution.

Estimates are that radioactivity, mainly from uranium and thorium but also from potassium, accounts for at least 40 to 60 percent of Earth’s interior heat. These elements are probably most abundant in the crust, the top 30 kilometers or so of rock. But key to understanding Earth’s dynamics is knowing the amount of these elements in the mantle — the vast, viscous, slowly churning layer that stretches 2,900 kilometers from crust to the molten outer core.

Like the better-known solar neutrinos, geoneutrinos can pass through thousands of miles of solid rock without being stopped or even deflected. That makes them ideal for studying deep Earth — but also makes them very difficult to catch.

Catching geoneutrinos

One surefire way to catch some is to build a detector near a concentrated source of antineutrinos. Conveniently, the uranium and other radioactive elements used in a nuclear reactor provide a flood of these ghostly particles.

That’s why the first geoneutrino detector was built near a cluster of reactors in Japan, with an aim to further characterize antineutrinos. Consequently, particles from the reactors swamped those produced by naturally occurring uranium in the crust and mantle.

Nonetheless, in 2005, this experiment, called KamLAND (short for Kamioka Liquid Scintillator Anti-Neutrino Detector), provided the first glimpse of geoneutrinos and a first approximation of uranium and thorium’s contribution to the Earth’s heat. Unfortunately, because the number of geoneutrinos was so small, the estimated heat contribution, in terms of power, had a range of 19 billion to 60 billion kilowatts; consistent with but not more precise than previous estimates.

Since then, the geoneutrino detector Borexino, located in an Italian mine, has come online. And new experiments are on the horizon, though it may be at least two years before the first of the next generation of detectors starts up.

The SNO+, or Sudbury Neutrino Observatory Plus, would sit two kilometers underground in the Creighton nickel mine near Sudbury, Ontario. SNO+ would piggyback on and use the same detector as a highly successful solar neutrino project called the Sudbury Neutrino Observatory, which played an important role in solving a long-standing conundrum. Researchers had detected fewer neutrinos coming from the sun than expected. SNO showed that neutrinos have a tiny bit of mass and are shape-shifters, turning from a detectable form into another, undetectable one.

Converting SNO into SNO+ — which would detect the lower-energy geoneutrinos — means changing out the fluid that filled the detector. SNO operated from 1999 to 2006 using heavy water — water with atoms of deuterium, heavy hydrogen — to snag solar neutrinos. Pending final approval of funding, the detector will be filled with a hydrocarbon-based scintillation fluid, which, when a geoneutrino is caught, will luminesce and trigger the detector.

The fluid is a common, mass-produced petrochemical called linear alkylbenzene, or LAB, used to make clear detergents, like liquid hand soap. It’s less toxic than most chemical liquid scintillators.

“It produces a lot of light, and it’s very transparent, but it’s a safer scintillator,” says SNO+ director Mark Chen. “It’s much easier to use it, especially in a setting where we are taking a thousand tons of it into an active mine.”

The detector is a four-story acrylic sphere surrounded by electronic eyes that scan the fluid for flashes of light characteristic of geoneutrinos’ presence.

Chen, a particle astrophysicist at QueensUniversity in Kingston, Canada, hopes SNO+ will start up in late 2010. It should catch about 50 geoneutrinos a year, considerably more than either KamLAND or Borexino. The longer
 SNO+ runs, the better the picture it will get of the inner Earth.

Ontario’s nuclear power plants are far enough away to not overwhelm the geoneutrino signal. “Certain problematic backgrounds from cosmic rays are even further reduced because we just happen to be deeper underground than other, similar detectors,” Chen says. With SNO+, he says, it will be possible to do some interesting things “with less background and improved precision.”

SNO+ has an ambitious scientific agenda that includes better understanding the fundamental nature of neutrinos. One goal is to pin down the mass of the neutrino — a quantity even more elusive than the neutrino itself. Another is to determine whether neutrinos and antineutrinos are the same particle — the lack of charge makes it difficult to tell.

That question “connects to our understanding of the early universe and might inform us about why … we see matter in the universe but much less antimatter,” Chen says.

Mantle signature

SNO+ is the furthest along of the up-and-coming geoneutrino experiments. Other detectors under discussion include one in the Homestake mine in South Dakota and a large detector to be built in Europe, possibly in Finland. Startup and operating costs are estimated to be on the order of hundreds of millions of dollars, and construction wouldn’t begin for at least several years.

These detectors would be, for the most part, counting geoneutrinos that originate in the Earth’s crust, where thorium and uranium are concentrated. Looking close to the crust is like having your eye close to a bright flashlight.

To get a better idea of what’s going on in the mantle — a dim and more distant flashlight — requires a geoneutrino detector situated in a place where the crust is only a few kilometers thick, like at the bottom of the ocean.

McDonough and his colleagues are proposing a 10,000-ton submersible detector they have named “Hanohano” (Hawaiian for “magnificent”), for the Hawaii Anti-Neutrino Observatory.

Hanohano would be about 10 times the size of the SNO+ detector and filled with the same scintillator fluid. It would be towed out to sea on a barge and sunk, anchored about 4,000 meters deep and about 90 m from the bottom. After catching neutrinos for a year or two, it could be serviced and redeployed elsewhere. Like SNO+, Hanohano will be a multi-faceted experiment, with research agendas in astrophysics and particle physics as well as earth science.

With funding, Hanohano could be built within about two years, McDonough estimates. Preliminary design studies for Hanohano are underway, and McDonough is hoping for another $5 million of seed money to continue design. But to construct and deploy Hanohano and to keep it running for 10 years could cost around $200 million, he estimates.

That’s expensive, but it’s about a factor of 10 less expensive than sending a spacecraft to another planet, points out David Stevenson, a planetary physicist at the California Institute of Technology in Pasadena, who is not directly involved in the geoneutrino experiments. And “it’s inconceivable to me that we could get the same information with the accuracy we desire by any other method,” he says.

He also hopes for the unexpected. “I’ve learned as a planetary scientist, that when you go to a planet, you actually discover things, you are surprised,” he says. “And in the case of the neutrinos, you may be surprised. You may be surprised, for example, to discover that there’s a major source of radioactivity in a layer just above the core” — an idea proposed early last year by Dutch and South African scientists writing in the South African Journal of Science.

Another scenario, which Stevenson thinks is extremely unlikely but Learned acknowledges “would be quite cool,” is that enough uranium exists in the core that there is essentially a nuclear reactor humming away down there.

San Diego–based independent scientist J. Marvin Herndon first proposed such a core reactor in 1993. Although not widely believed, his hypothesis would explain some puzzling observations, such as an excess of an isotope of helium emitted from volcanoes, Learned points out.

Hanohano would be able to tell fairly quickly whether such a reactor exists at all, Learned reported last May at a neutrino symposium in New Zealand.

For all their promise, these geoneutrino detectors won’t be able to unearth the whole picture of Earth’s interior.

Stevenson points out that all the geoneutrino detectors proposed share a shortcoming: They can’t detect the geoneutrinos coming from radioactive potassium-40. These geoneutrinos have too little energy to trigger the detector.

“So there is one part of the expected heat production that we cannot measure, and haven’t figured out how to measure,” he says. And although only a tiny fraction of potassium-40 is actually radioactive, and most of it has already decayed over Earth’s 4.6-billion-year history, potassium still contribute up to 20 percent of the radioactive heat, Stevenson says.

“But if you go back in time, potassium-40 becomes increasingly important,” he adds. “And that’s why, if you want to reconstruct the history of the Earth, you would like to know how much potassium you had.”

Diana Steele is a freelance science writer based in Ohio.

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