  Coupled one-dimensional photochemical-climate calculations have been
performed for hypothetical Earth-like planets around M dwarfs. Visible,
near-infrared and thermal-infrared synthetic spectra of these planets were
generated to determine which biosignature gases might be observed by a future,
space-based telescope. Our star sample included two observed active M dwarfs,
AD Leo and GJ 643, and three quiescent model stars. The spectral distribution
of these stars in the ultraviolet generates a different photochemistry on these
planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially
longer lifetimes and higher mixing ratios than on Earth, making them
potentially observable by space-based telescopes. On the active M-star planets,
an ozone layer similar to Earth's was developed that resulted in a
spectroscopic signature comparable to the terrestrial one. The simultaneous
detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been
suggested as strong evidence for life. Planets circling M stars may be good
locations to search for such evidence.
