Observation of DCl and upper limit to NH3 on Venus

Abstract

To search for DCl in the Venus atmosphere, a spectrum near the D35Cl (1–0) R4 line at 2141.54cm−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±6.8ppb. Comparing to the HCl abundance of 400±30ppb (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 70–110km. Photochemistry in the Venus mesosphere converts H from HCl to that in H2O with a rate of 1.9×109cm−2s−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 ∼3years 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 ∼100km 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 4481.11cm−1 was observed at NASA IRTF and resulted in a two-sigma upper limit of 6ppb 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.