A main source of the 44 trillion watts of heat that flows from the interior of the Earth is the decay of radioactive isotopes in the mantle and crust. Scientists using the KamLAND neutrino detector in Japan have measured how much heat is generated this way by capturing geoneutrinos released during radioactive decay. |
What
spreads the sea floors and moves the continents? What melts iron in the
outer core and enables the Earth’s magnetic field? Heat. Geologists
have used temperature measurements from more than 20,000 boreholes
around the world to estimate that some 44 terawatts (44 trillion watts)
of heat continually flow from Earth’s interior into space. Where does it
come from?
Radioactive
decay of uranium, thorium, and potassium in Earth’s crust and mantle is
a principal source, and in 2005 scientists in the KamLAND
collaboration, based in Japan, first showed that there was a way to
measure the contribution directly. The trick was to catch what KamLAND
dubbed geoneutrinos – more precisely, geo-antineutrinos – emitted when
radioactive isotopes decay. (KamLAND stands for Kamioka
Liquid-scintillator Antineutrino Detector.)
“As
a detector of geoneutrinos, KamLAND has distinct advantages,” says
Stuart Freedman of the U.S. Department of Energy’s Lawrence Berkeley
National Laboratory (Berkeley Lab), which is a major contributor to
KamLAND. Freedman, a member of Berkeley Lab’s Nuclear Science Division
and a professor in the Department of Physics at the University of
California at Berkeley, leads U.S. participation. “KamLAND was
specifically designed to study antineutrinos. We are able to
discriminate them from background noise and detect them with very high
sensitivity.”
KamLAND
scientists have now published new figures for heat energy from
radioactive decay in the journal Nature Geoscience. Based on the
improved sensitivity of the KamLAND detector, plus several years’ worth
of additional data, the new estimate is not merely “consistent” with the
predictions of accepted geophysical models but is precise enough to aid
in refining those models.
One
thing that’s at least 97-percent certain is that radioactive decay
supplies only about half the Earth’s heat. Other sources – primordial
heat left over from the planet’s formation, and possibly others as well –
must account for the rest.
Hunting for neutrinos from deep in the Earth
Antineutrinos
are produced not only in the decay of uranium, thorium, and potassium
isotopes but in a variety of others, including fission products in
nuclear power reactors. In fact, reactor-produced antineutrinos were the
first neutrinos to be directly detected (neutrinos and antineutrinos
are distinguished from each other by the interactions in which they
appear).
Because
neutrinos interact only by way of the weak force – and gravity,
insignificant except on the scale of the cosmos – they stream through
the Earth as if it were transparent. This makes them hard to spot, but
on the very rare occasions when an antineutrino collides with a proton
inside the KamLAND detector – a sphere filled with a thousand metric
tons of scintillating mineral oil – it produces an unmistakable double
signal.
The
first signal comes when the antineutrino converts the proton to a
neutron plus a positron (an anti-electron), which quickly annihilates
when it hits an ordinary electron – a process called inverse beta decay.
The faint flash of light from the ionizing positron and the
annihilation process is picked up by the more than 1,800 photomultiplier
tubes within the KamLAND vessel. A couple of hundred millionths of a
second later the neutron from the decay is captured by a proton in the
hydrogen-rich fluid and emits a gamma ray, the second signal. This
“delayed coincidence” allows antineutrino interactions to be
distinguished from background events such as hits from cosmic rays
penetrating the kilometer of rock that overlies the detector.
Says
Freedman, “It’s like looking for a spy in a crowd of people on the
street. You can’t pick out one spy, but if there’s a second spy
following the first one around, the signal is still small but it’s easy
to spot.”
KamLAND
was originally designed to detect antineutrinos from more than 50
reactors in Japan, some close and some far away, in order to study the
phenomenon of neutrino oscillation. Reactors produce electron neutrinos,
but as they travel they oscillate into muon neutrinos and tau
neutrinos; the three “flavors” are associated with the electron and its
heavier cousins.
The KamLAND antineutrino detector is a vessel filled with scintillating mineral oil and lined with photomultiplier tubes (inset), the largest scintillation detector ever constructed, buried deep underground near Toyama, Japan. |
Being
surrounded by nuclear reactors means KamLAND’s background events from
reactor antineutrinos must also be accounted for in identifying
geoneutrino events. This is done by identifying the nuclear-plant
antineutrinos by their characteristic energies and other factors, such
as their varying rates of production versus the steady arrival of
geoneutrinos. Reactor antineutrinos are calculated and subtracted from
the total. What’s left are the geoneutrinos.
Tracking the heat
All
models of the inner Earth depend on indirect evidence. Leading models
of the kind known as bulk silicate Earth (BSE) assume that the mantle
and crust contain only lithophiles (”rock-loving” elements) and the core
contains only siderophiles (elements that “like to be with iron”). Thus
all the heat from radioactive decay comes from the crust and mantle –
about eight terawatts from uranium 238 (238U), another eight terawatts
from thorium 232 (232Th), and four terawatts from potassium 40 (40K).
KamLAND’s
double-coincidence detection method is insensitive to the low-energy
part of the geoneutrino signal from 238U and 232Th and completely
insensitive to 40K antineutrinos. Other kinds of radioactive decay are
also missed by the detector, but compared to uranium, thorium, and
potassium are negligible contributors to Earth’s heat.
Additional
factors that have to be taken into account include how the radioactive
elements are distributed (whether uniformly or concentrated in a “sunken
layer” at the core-mantle boundary), variations due to radioactive
elements in the local geology (in KamLAND’s case, less than 10 percent
of the expected flux), antineutrinos from fission products, and how
neutrinos oscillate as they travel through the crust and mantle.
Alternate theories were also considered, including the speculative idea
that there may be a natural nuclear reactor somewhere deep inside the
Earth, where fissile elements have accumulated and initiated a sustained
fission reaction.
KamLAND
detected 841 candidate antineutrino events between March of 2002 and
November of 2009, of which about 730 were reactor events or other
background. The rest, about 111, were from radioactive decays of uranium
and thorium in the Earth. These results were combined with data from
the Borexino experiment at Gran Sasso in Italy to calculate the
contribution of uranium and thorium to Earth’s heat production. The
answer was about 20 terawatts; based on models, another three terawatts
were estimated to come from other isotope decays.
This
is more heat energy than the most popular BSE model suggests, but still
far less than Earth’s total. Says Freedman, “One thing we can say with
near certainty is that radioactive decay alone is not enough to account
for Earth’s heat energy. Whether the rest is primordial heat or comes
from some other source is an unanswered question.”
Better
models are likely to result when many more geoneutrino detectors are
located in different places around the globe, including midocean islands
where the crust is thin and local concentrations of radioactivity (not
to mention nuclear reactors) are at a minimum.
Says
Freedman, “This is what’s called an inverse problem, where you have a
lot of information but also a lot of complicated inputs and variables.
Sorting those out to arrive at the best explanation among many requires
multiple sources of data.”
Partial radiogenic heat model for Earth revealed by geoneutrino measurements
