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Relative molecular abundances measure the chemical composition of an astronomical source or object. Such abundances can be either by mass or by number of molecules (see Abundances). For interstellar clouds, molecular abundances are typically presented as fractional abundances constructed from the measured molecular column density, divided by the estimated H2 column density of the source (Irvine et al. 1987). For planetary or satellite atmospheres, abundances are usually given in terms of the “volume mixing ratio”, the fractional number of molecules or atoms in a given volume relative to the total number of particles in the volume (de Pater and Lissauer 2001).
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References
de Pater I, Lissauer JJ (2001) Planetary sciences, Cambridge University Press, Cambridge, p 77
Irvine WM, Goldsmith PF, Hjalmarson Å (1987) Chemical abundances in molecular clouds. In: Hollenbach DJ, Thronson HA (eds) Interstellar processes. Reidel, Dordrecht, pp 561–610
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Charnley, S. (2011). Molecular Abundances. In: Gargaud, M., et al. Encyclopedia of Astrobiology. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-11274-4_1008
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