1

. Vittorio Formisano et al., “Detection of Methane in the Atmosphere of Mars,” Science, Vol. 306, 3 December 2004, pp. 1758–1761.

Sushil K. Atreya, “The Mystery of Methane on Mars and Titan,” Scientific American, Vol. 296, May 2007, pp. 42–51.

2

. If considerable oxygen and few anaerobic bacteria are present, water and carbon dioxide will be produced, instead of methane.

3

. Microbial cells, such as bacteria, are extremely small. Our bodies contain 10 times more microbes than human cells.

4

. “For a 600-year lifetime, a little over 100 metric tons of methane would have to be produced [on Mars] each year to maintain a constant global average of 10 ppbv [parts per billion by volume].” Atreya, p. 46.

About 45% of organic matter and 75% of methane is carbon by weight. Anaerobic bacteria convert about 76% of the available carbon to methane. Assume that eleven comets (or asteroids) weighing 10¹⁶ grams each struck Mars and only one hundred thousandth of each impactor consisted of organic matter. This would allow 100 metric tons of methane to enter Mars’ atmosphere for each of 5,000 years. (1 metric ton = 10⁶ grams.)

Other reasonable combinations of numbers produce similar results. Certainly, more carbon is still trapped in Mars’ soil.