A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

Abstract

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν / δ ν = 76 , 000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50–112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO 2 lines. The derived NO mixing ratio is 5.5 ± 1.5 ppb below 60 km and its flux is ( 6 ± 2 ) × 10 7 cm −2 s −1 . Predissociation of NO at the (0–0) 191 nm and (1–0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of 0.19 ± 0.06 erg cm −2 s −1 . For a flash energy on Venus similar to that on the Earth (∼ 10 9 J ), the global flashing rate is ∼90 s −1 and ∼6 km −2 y −1 which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO 2 lines, and the line at 1900.076 cm −1 is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO ( v = 1 ) in the lowest 20 km on Mars. However, quenching of NO ( v = 1 ) by CO 2 is very effective below 40 km. Excitation of NO ( v = 1 ) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.