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Terrestrial Atmosphere Studies: Iron Oxide in the Mesosphere
Spectral emission studies reveal meteorites’ contribution to airglow.
Planetary airglow is a faint, high-altitude emission that is the direct or indirect result of energetic solar photons or electrons interacting with atoms and molecules in the atmosphere. These emissions include spectral lines from atomic and molecular oxygen and nitrogen, the hydroxyl molecule (OH), and sodium.
Much of the recent work carried out in SRI’s Molecular Physics Program has been linked to the optical spectra that are routinely acquired at the giant telescopes, such as the Keck telescopes on Mauna Kea in Hawaii. These spectra demonstrate the complexity and richness of terrestrial nightglow. A recent discovery is a previously unidentified glow in the 500 to700-nm spectral region that is associated with the excited ferrous oxide molecule, FeO [Evans et al., 2010; Saran et al., 2011]. Comparison of the spectrum with emissions seen in meteor trains and laboratory studies confirms that the emitter is indeed FeO.
Iron, a component of the atmosphere, comes from the same source—meteor ablation—as sodium and other metals. It is the subsequent chemistry of iron atoms that leads to the FeO glow. Free iron atoms react with atmospheric ozone at an altitude of about 95 km, and the product is FeO in an electronically excited state, which emits photons in the observed spectral region. Other emissions depend on ozone, including sodium and vibrationally excited OH. Thus FeO observations provide an additional constraint on atmospheric ozone distribution.
SRI’s studies of FeO emission will lead to enhanced detection of free atmospheric iron. Because iron and iron oxide are tracers for the meteoric input into the atmosphere, SRI’s observations can effectively allow sampling of meteors in space to enhance information obtained from meteorites on the ground. It is possible that when advancing technology reveals the airglow of extra-solar planets, FeO emission may indicate both the presence of oxygen and ozone and the extent to which the meteor environment is similar to that of Earth.
In addition to the support of NSF project P18142, Dr. Deepali Saran has a post-doctoral fellowship from the NSF CEDAR program (project P19146), and work on the FeO emission is a major part of her research. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This material is based upon work supported by the National Science Foundation under Grant No. ATM-0637433.
Evans, W.F.J., R. L. Gattinger, T.G. Slanger, D.A., Degenstein, and E. J. Llewellyn, (2010), Discovery of the FeO orange bands in the terrestrial night airglow spectrum obtained with OSIRIS on the Odin spacecraft, Geophys. Res. Lett., 37, L22105, doi:10.1029/2010GL045310.
Saran, D.V., T.G. Slanger, W. Feng, and J.M.C. Plane (2011), FeO Emission in the Mesosphere: Detectability, Diurnal Behavior and Modeling, J. Geophys. Res. 116, D12303, doi:10.1029/2011JD015662