Publication


V. Krasnopolsky
Observation of DCl and upper limit to NH3 on Venus
Icarus, 219(1), 244-249, 2012; doi: http://dx.doi.org/10.1016/j.icarus.2012.02.036,
Spectroscopy
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Abstract


To search for DCl in the Venus atmosphere, a spectrum near the D35Cl (1–0) R4 line at 2141.54 cm−1 was observed using the CSHELL spectrograph at NASA IRTF. Least square fitting to the spectrum by a synthetic spectrum results in a DCl mixing ratio of 17.8 ± 6.8 ppb. Comparing to the HCl abundance of 400 ± 30 ppb (Krasnopolsky [2010a] Icarus, 208, 314–322), the DCl/HCl ratio is equal to 280 ± 110 times the terrestrial D/H = 1.56 × 10−4. This ratio is similar to that of HDO/H2O = 240 ± 25 times the terrestrial HDO/H2O from the VEX/SOIR occultations at 70–110 km. Photochemistry in the Venus mesosphere converts H from HCl to that in H2O with a rate of 1.9 × 109 cm−2 s−1 (Krasnopolsky [2012] Icarus, 218, 230–246). The conversion involves photolysis of HCl; therefore, the photochemistry tends to enrich D/H in HCl and deplete in H2O. Formation of the sulfuric acid clouds may affect HDO/H2O as well. The enriched HCl moves down by mixing to the lower atmosphere where thermodynamic equilibriums for H2 and HCl near the surface correspond to D/H = 0.71 and 0.74 times that in H2O, respectively. Time to establish these equilibriums is estimated at ∼3 years and comparable to the mixing time in the lower atmosphere. Therefore, the enriched HCl from the mesosphere gives D back to H2O near the surface. Comparison of chemical and mixing times favors a constant HDO/H2O up to ∼100 km and DCl/HCl equal to D/H in H2O times 0.74. Ammonia is an abundant form of nitrogen in the reducing environments. Thermodynamic equilibriums with N2 and NO near the surface of Venus give its mixing ratio of 10−14 and 6 × 10−7, respectively. A spectrum of Venus near the NH3 line at 4481.11 cm−1 was observed at NASA IRTF and resulted in a two-sigma upper limit of 6 ppb for NH3 above the Venus clouds. This is an improvement of the previous upper limit by a factor of 5. If ammonia exists at the ppb level or less in the lower atmosphere, it quickly dissociates in the mesosphere and weakly affects its photochemistry.

Reference


@article{KRASNOPOLSKY2012244,
  author = "V. Krasnopolsky",
  title = "Observation of DCl and upper limit to NH3 on Venus",
  year = 2012,
  journal = "Icarus",
  volume = 219,
  number = 1,
  pages = "244-249",
  month = "May",
  keywords = "Spectroscopy",
  doi = "http://dx.doi.org/10.1016/j.icarus.2012.02.036",
  url = "http://www.sciencedirect.com/science/article/pii/S001910351200084X"
}
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