O2 abundances in the Martian atmosphere determined using Mars Express SPICAM UV stellar occultation data

Details O2 abundances in the Martian atmosphere determined using Mars Express SPICAM UV stellar occultation data O2 abundances in the Martian atmosphere determined using Mars Express SPICAM UV stellar occultation data

Sep 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..185l&link_type=abstract

European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a

Computer Science

Scientific paper

The distribution of O2 with altitude, latitude, and season is an important factor in the evolution and current stability of Mars' CO2 rich atmosphere. CO2 is photolyzed in the Martian atmosphere to form CO and O according to the following process: CO2 + hn→ CO + O. The atomic oxygen then preferentially recombines to form O2. If this simple reaction is indeed the dominant process in the Martian atmosphere then O2 should be more abundant than the currently accepted value of 0.12 percent [1]. Nair et al. (1994) present a detailed photochemical model of the Martian atmosphere, which shows that the abundance of O2 is largely controlled by reactions with odd hydrogen radicals from photolyzed water in the lower atmosphere. While the Nair et al. (1994) model certainly helps to explain the major photochemical processes at work in theMartian atmosphere, it assumes the abundance of O2 does not vary with latitude and season and is roughly constant with altitude. Our study probes the abundance of O2 in theMartian atmosphere during winter in the southern hemisphere (Ls=90-180) when CO2 condenses out of the atmosphere to form a polar cap. This enrichment of O2 with respect to CO2 during southern Martian winter allows for a more robust detection of O2 in addition to probing the effect of seasonal variations on the photochemistry of the Martian atmosphere. The European Space Agency's Mars Express spacecraft was placed in orbit around Mars on 25 December 2003. The SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) instrument aboard Mars Express measures stellar occultations in the 118-320 nm wavelength region [2]. The stellar occultation technique determines the abundance of chemical species by comparing a reference stellar spectrum (I0) to the same stellar spectrum attenuated by the planetary atmosphere (I). The slant densities, Ni(z), are related to the transmission, Tz(l), through (1) where z is the minimum altitude along the line of sight to the star, l is the wavelength, and si is the absorption cross section for the ith species [3]. The occultation technique is powerful because many of the systematics of the instrument are inherently corrected for when dividing the occulted spectrum by the reference spectrum, I/I0. In this study, we use the SPICAMUV stellar occultation data to determine the abundance of O2 and CO2 in the Martian atmosphere in the 50-120 km altitude range. Measuring O2 Abundances Figure 1 shows SPICAM UV transmission curves at three altitudes. The main absorbing species at all three altitudes is CO2 although absorption due to O3 can be clearly seen in the 30 km transmission curve in the 225-275 nm wavelength range. Absorption of stellar light by O2 can occur in 130-205 nm wavelength range due to the Schumann-Runge continuum and bands. Since CO2 is a strong absorber in the 110-210 nm wavelength range it is difficult to distinguish the portion of the absorbed stellar light due to CO2 vs. O2. Often the contribution of O2 to the absorption of stellar light is ignored, and the absorption is entirely attributed to CO2. Ignoring O2 absorption can lead to errors in the estimate of the abundance of CO2 in the Martian atmosphere, which can be quite significant (˜10%) during southern polar winter. In order to reduce noise and scaling errors, the transmission curves are fit using the function: Tl(z) = exp(-exp(-å n an(z-z0)n)) (2) where Tl(z) is the transmission curve at a given wavelength as a function of altitude, an is the nth coefficient, and z0 is the altitude of the halflight point, Tl(z0) = 0.5 [4]. This exponential fit to the data is then used to construct transmission curves at a given altitude as a function of wavelength, Tz(l), which are fit using a Levenberg- Marquardt least squares fit to Equation 1. The fitting procedures include O2, CO2, and O3 as absorbers. Absorption cross sections in the 110-300 nm wavelength range for each species were selected from those available in the literature with high wavelength resolution and close to the expected atmospheric temperature in the 50-120 km range of ˜130K [1]. The O2 cross-sections used in this study are from Yoshino et al. (2005) for the Schumann-Runge continuum (130-175 nm) and Minschwaner et al. (1992) for the Schumann- Runge bands (175-205 nm). The fit to the transmission curves is first performed assuming no O2 then repeated allowing for O2 absorption. The c2 n from the fits with and without O2 are compared using an F Test to determine the confidence level of the O2 detection at each altitude. Figure 2 shows the CO2 and O2 slant density profiles for a single line of sight around 75°S latitude at Ls=93. The open squares in Figure 2 highlight those points on the density profiles where the fit to the transmission curves including O2 passed the F Test at the 90%confidence level. Using these points where we are confident of the presence of O2 we find the ratio of O2 to CO2 to be six times the commonly accepted value of 1.2x10-3. Additionally, we measure for both the O2 and CO2 populations a scale height and temperature of 6.49 km and 127 K respectively. Ls and Latitude Variations With a reliable method of determining O2 abundances in the Martian atmosphere from SPICAM UV data, seasonal and latitudinal variations in the abundance of O2 can be measured from the more than 1200 stellar occultations currently available. Our current work focuses on the variations in the O2 abundances during southern winter (Ls=90- 180) in the southern hemisphere (0-90°S). Preliminary work shows that the altitude vs. O2 density profiles can vary strongly with latitude. Also, the O2 abundance in the Martian atmosphere varies within the southern winter with a peak abundance around Ls=115. These seasonal and latitudinal trends will be further explored and presented during the poster session of the EPSC2008 meeting. References [1] Nair, H. et al.(1994) Icarus, 111, 124-150 [2] Bertaux, J.-L. et al. (2006) JGR, 111, E10S90 [3] Qúemerais, E. et al. (2006) JGR, 111, E09S04 [4] Yelle, R. V. et al. (1993) Icarus, 104, 38-59 [5] Yoshino et al. (2005) J. Mol. Spectrosc., 229, 238-243 [6] Minschwaner et al. (1992) JGR, 97, 10,103- 10,108

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