August 2005

What's new in exploration

Weird earth science
Vol. 226 No. 8 

It’s not your usual rock hammer. The tools that geologists and geophysicists use are getting incredibly sophisticated nowadays. For more than a century, sound waves have been the primary tool to look into Earth’s interior. But now we have a new tool for studying that.

The heat source within the Earth’s interior from gravity-induced friction and pressure, plus radioactive decay, is not a settled issue. And there remains long-shot hypothesis that there could be a fission reactor at the Earth’s center. Through detection of maddeningly tiny, virtually massless subatomic particles called neutrinos and their contrary cousins, the antineutrinos, questions like these can now be answered. Rough calculations show that about a hundred trillion neutrinos zip through your body every second. Not long ago, detecting them was an extremely difficult task.

Because they are produced by various radioactive processes and, depending on their energy, can go through nearly everything, detecting their source is very difficult. It recently became possible to detect the long-sought electron antineutrino, which can be produced by the decay of 238Uranium and 232Thorium within the Earth. A new detector, the Kamioka liquid scintillator antineutrino detector (KamLAND) has the sensitivity to detect these, and could yield important geophysical information.

The Earth emits only a modest number of antineutrinos, so scientists need a huge detector to be able to see them. The KamLAND was built with the size and sensitivity required to detect these Earth-made antineutrinos. It is housed in a cavern underneath a Japanese mountain, which shields it from the background noise of cosmic radiation. KamLAND consists of about 2,000 photomultiplier tubes, each 20 inches in diameter, and contained in a 59-foot vessel. The photomultiplier tubes are bathed in 1,000 tons of liquid scintillator, which is essentially a mix of baby oil, benzene and a little fluorescent material. When particles interact with this mixture, they make a little flash of light that is detected by light sensors.

A team of 87 researchers from Japan, the United States, China and France just published the results of their geoneutrino detections in Nature. There haven’t been enough of them detected to make an accurate measurement of the heat produced by radioactivity. But this should improve as they gather more data and combine them with those obtained from a similar detector, called Borexino, in Italy, which is scheduled to begin operations in 2006. It should be possible, with several of these detectors in operation, to conduct a sort of radioactive tomography of the Earth, measuring how the elements are distributed and, therefore, how uniform the mantle is.

Fig 1

Supernova SN 1993J: Blue is low energy, red is high. Image courtesy of NRAO/ AUI and N. Bartel, M. Bietenholz, M. Rupen, et al. 

So far, the concept has been proven, and the KamLAND result is consistent with geophysical models that put the amount of heat coming from radioactive decay at an upper limit (at the 99% confidence level) of 60 terawatts (1 TW=1012 watts) of radiogenic heat from Th and U, and a central value of 16 TW that is consistent with model predictions. This is about half of Earth’s total measured heat-dissipation rate.

A new way to date rocks. Scientists at the US National Science Foundation (NSF) and the European Commission of the European Union announced a radical new initiative – using cosmic rays from distant supernovae to measure the geochronology of the Earth’s surface. They call it CRONUS, for Cosmic-Ray Produced Nuclide Systematics.

Supernovae occur when a star explodes in a remarkably short amount of time – in a matter of hours or days. These explosions unleash torrents of incredibly energetic atomic particles, loosely called cosmic rays. Billions of cosmic rays impact Earth every year. The particles blast apart the atoms of Earth’s atmosphere and rocks, changing them into new elements. Now, NSF has awarded $5.8 million over five years for geologists to measure the accumulated results of these atomic transmutations in rocks at the Earth’s surface.

Cosmic-ray particles penetrate only a few feet below the Earth’s surface, so deeper rocks are shielded from the buildup of cosmic-ray transmutations. The number of new atoms produced by cosmic rays can therefore show the amount of time passed between geological events, such as earthquakes, landslides and glaciers. They can also reveal how fast Earth’s surface changes from erosional forces.

The US part of the project will include 13 US universities. “The CRONUS initiative will benefit all disciplines in the Earth sciences,” said Herman Zimmerman, director of NSF’s division of earth sciences. Whether geomorphology, tectonics, volcanology, hydrology, geologic hazards, or paleoclimatology, he said, “Each needs an improved understanding of geochronology at the Earth’s surface.”

The EU, through its Marie Curie Actions, awarded 3.4 million Euro ($4.4 million) over four years for the project and a research-training network that involves teams in France, Germany, the Netherlands, Slovakia, Switzerland and the UK.

“As scientists who use geochronology techniques in the course of their research,” US coordinator Fred Phillips said, “We need to know exactly how cosmic rays are distributed on our planet’s surface, taking into account variables like longitude, latitude, and elevation, as well as changes occurring over geologic time scales, such as periodic shifts in Earth’s magnetic field.”

Scientists from the United States and Europe will work together sampling rocks from key sites around the world, exposing elements to nuclear beams in high-energy accelerators, and counting cosmic-ray impacts with detectors aboard high-altitude aircraft. These results will all be synthesized in a broad-ranging effort to understand all aspects of the cosmic phenomenon. When perfected, the new cosmic-ray methods will shed light on Earth’s past climate cycles, changes in soil erosion, and the frequency of floods and landslides. WO

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