Visible Dayglows of the CO2 Planets

Abstract

Spacecraft instrumentation typically favors the UV and IR portions of the optical spectrum over the visible. As a result, there are questions that have not been answered about excited state production in various environments, including the atmospheres of the CO2 planets. For example, there are almost no oxygen green and red line measurements of Mars and Venus, and in particular, the possibility of CO emission in the Martian dayglow has not been investigated. The same holds for Venus, but dayglow Venus measurements are very difficult because of solar scattering from the clouds. CO2 photodissociation below 108 nm leads to production of the CO(a-X) Cameron bands, a well-known feature of the Mars dayglow, seen by the Mariner spacecraft [Barth et al., 1971] and by SPICAM on Mars Express [Simon et al., 2009]. However, there are three nearby CO triplet states that lie somewhat higher than the CO(a) state, the upper level of the Cameron bands, and these have fully allowed visible and IR transitions that can populate CO(a). Thus, the observed Cameron bands are the sum of the emission from the direct photodissociation process and the cascading emission [Judge and Lee, 1973]. In order to evaluate the relative contributions of each, we have performed laboratory experiments on CO2 photodissociation in the appropriate spectral region, utilizing synchrotron radiation from the Berkeley Advanced Light Source (ALS) in the 11-14 eV range. We measured the Cameron band emission at 180 260 nm, and the visible/IR emission from the triplet transitions. We could only make measurements down to 850 nm in the IR, but fortunately were able to avail ourselves of other data [Burke et al., 1996] to extend the useful range to 1.4 microns. This is very important, because the strongest bands lie in the IR. Spectra of the UV dayglow of Mars were first obtained by the Mariner probes, and it was determined [Conway, 1981] that the Cameron bands have an extremely high rotational temperature, several thousand degrees K. Our analysis of the current Mars Express spectra leads to the same conclusion, and we do not yet know whether this unusual rotational distribution originates with the cascading or the direct emission. This research was supported by the NASA Outer Planets program. Barth, C.A., et al., J. Geophys. Res. 76, 2213-2227, 1971. Burke, M.L. et al. J. Phys. Chem. 100, 138, 1996. Conway, R.R., J. Geophys. Res. 86, 4767, 1981. Judge, D.L. and L.C. Lee, J. Chem. Phys. 58, 104, 1973. Simon, C., et al., Planet. Space Sci. 57, 1008-1021, 2009.