Published online (and openly accessible) in the Early Edition of the Proceedings of the National Academy of Sciences:

H. C. Tung, N. E. Bramall, and P. B. Price
Microbial origin of excess methane in glacial ice and implications for life on Mars
PNAS published December 7, 2005, 10.1073/pnas.0507601102 (Environmental Sciences-Physical Sciences , Geophysics)
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