Observations of D/H ratios in H2O, HCl, and HF on Venus and new DCl and DF line strengths

Abstract

Intensities of the spectral lines in the fundamental bands of D35Cl and DF were calculated using the semi-empirical dipole moment functions derived from the most accurate and precise measurements of intensities of the ro-vibrational lines of H35Cl and HF. Values obtained in this way for the deuterated species are superior to any available measured or calculated data to date.Our study of the D/H ratios in H2O, HCl, and HF on Venus is based on spatially-resolved high-resolution spectroscopy using the CSHELL spectrograph at NASA IRTF. Search for DF on Venus using its R5 (1-0) line at 3024.054cm−1 results in a DF mixing ratio of 0.23±0.11ppb that corresponds to (D/H)HF=420±200 times that in the Standard Mean Ocean Water (SMOW). H2O abundances on Venus were retrieved using lines at 3022.366 and 3025.761cm−1 that were observed at an exceptionally low overhead telluric water abundance of 0.3pr.mm. The measured H2O mixing ratios at 74km vary insignificantly between 55°S and 55°N with a mean value of 3.2ppm. When compared with simultaneous observations of HDO near 2722cm−1, this results in (D/H)H2O=95±15 times SMOW. Reanalysis of the observation of the D35Cl R4 (1-0) line at 2141.540cm−1 (Krasnopolsky, V.A. [2012b]. Icarus 219, 244–249) using the improved line strength and more thorough averaging of the spectra gives (D/H)HCl=190±50 times SMOW.The similarity of the measured (D/H)H2O=95±15 at 74km with 120±40 observed by De Bergh et al. (De Bergh, C., Bezard, B., Owen, T., Crisp, D., Maillard, J.P., Lutz, B.L. [1991]. Science 251, 547–549) below the clouds favors the constant (D/H)H2O from the surface to the mesosphere, in accord with the prediction by theory. D/H≈100 removes a difference of a factor of 2 between H2O abundances in the observations by Krasnopolsky (Krasnopolsky, V.A. [2010b]. Icarus 209, 314–322) and the Venus Express nadir observations (Cottini, V., Ignatiev, N.I., Piccioni, G., Drossart, P., Grassi, D., Markiewicz, W.J. [2012]. Icarus 217, 561–569). Equivalent widths of the HDO and H2O lines are similar in our observations; therefore some errors cancel out in their ratios. Photochemistry of HCl in the mesosphere tends to enrich D in HCl and deplete it in H2O. This may be an explanation of the twofold difference between the observed D/H in HCl and H2O.An alternative explanation is based on (D/H)H2O≈200 observed in the mesosphere by Bjoraker et al. (Bjoraker, G.L., Larson, H.P., Mumma, M.J., Timmermann, R., Montani, J.L. [1992]. Bull. Am. Astron. Soc. 24, 995) and Fedorova et al. (Fedorova, A. et al. J. Geophys. Res. 113, E00B22). This means an effective exchange of D between H2O and HCl and almost equal D/H in both species. However, this requires a twofold increase in D/H from the lower atmosphere to the mesosphere. This increase is not supported by theory; furthermore, condensation processes usually deplete D/H above the clouds.Photochemistry of HF has not been studied; it proceeds mostly in the lower thermosphere, and D/H in HF may be very different from that in H2O. Overall, the observational data on D/H in all hydrogen-bearing species on Venus are helpful to solve the problem of deuterium fractionation on Venus.