August 2004

Methane

Making a definitive identification of a previously unknown astromolecule can be very challenging. Methane (CH4) is a case in point. In 1978 Fox & Jennings, reported the detection of methane in Orion A via rotational lines. However, later observations and analysis by Elldér et al., Wilson & Snyder, and Knacke et al. found this evidence to be less than compelling. A definite identification of interstellar methane did not occur until 1991, when Lacy et al. reported the detection of characteristic infrared spectral features toward a number of young stars.

Methane has been observed in the solar system in the atmospheres of Jupiter, Saturn, Uranus, Neptune, Pluto, and in Saturn's moon Titan, where it is the most abundant species after N2, and in Neptune's moon Triton. The tentative detection of methane in the tenuous atmosphere of Mars in 2004 may have profound astrobiological implications.

Methane has also been identified in comets and in a class of brown dwarfs that includes ε Indi B and Gliese 229B.

March 2008 - Methane has been observed for the first time in the atmosphere of an extrasolar planet, HD 189733b, by Swain et al.

Methane is a highly symmetrical molecule. As a consequence, it has neither a permanent dipole or quadrupole electrostatic moment. Its lowest permanent moment is the octupole moment.


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