A Scarcity of Gas

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Science  22 Jun 2001:
Vol. 292, Issue 5525, pp. 2219-2221
DOI: 10.1126/science.292.5525.2219e

Temperatures within Earth increase with depth. Much of Earth's heat is generated internally by radioactive decay of U and Th; this decay also produces 4He (alpha particles). A measurement of the flux of 4He at Earth's surface should agree with estimates based on Earth's content of U and Th and the observed heat flow, but the measured flux of 4He is much less than what is predicted.

Helium isotopes are used as an important tracer of mantle processes, and, as a gas, it might be thought that helium should simply escape upward. Using a series of models, van Keken et al. examine the history of work on this fundamental problem and pose possible solutions. One possibility is that there were massive short-lived releases of helium or heat from Earth's mantle in the geologic past, although this seems unlikely. Alternatively, helium may be efficiently trapped or stored in some part of Earth's mantle, whereas the heat escapes more quickly. — BH

Earth Planet. Sci. Lett.188, 421 (2001).

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