HR: 1330h
AN: A12A-0065 INVITED     [PDF]
TI: Photochemistry of the Atmospheres of Mars and Venus
AU: * Krasnopolsky, V A
EM: vkrasn@verizon.net
AF: Catholic University of America, Department of Physics, 200 Hannan Hall, Washington, DC 20064 United States
AB: Mars photochemistry is mainly the CO$_2$-H$_2$O chemistry, which was studied by McElroy and Donahue (1972) and Parkinson and Hunten (1972) thirty years ago. However, there is an increasing disagreement between the recent experimental data and the latest models of Mars photochemistry: (1) the standard gas-phase chemistry predicts too low CO and O$_3$, (2) all modifications in gas-phase chemistry suggested ten years ago to fit CO and O$_3$ have not been confirmed, (3) recent data on CO are even higher than that in the modified models, (4) the observed H$_2$ is far below the model values, (5) the MGS/TES mean H$_2$O exceeds that in the models, (6) the measured upper limit to H$_2$O$_2$ is much smaller than the model predictions, and (7) the low latitudinal variations of the O$_2$ dayglow at 1.27 $\mu$m and O$_3$ at late northern spring and summer disagree with the very strong variations of H$_2$O and question the basic concept of Mars photochemistry. However, the inclusion of even one heterogeneous reaction (sink of peroxide on ice and ice-covered dust) removes most of these disagreements. Heterogeneous chemistry should exist on Mars, and Mars photochemical models are inadequate without heterogeneous chemistry. A significant progress in Mars photochemistry and dynamics is expected from photochemical GCMs. These models do not account for small-scale mixing and long living species (H$_2$, O$_2$, and CO) and need too much computational time to try various version. Therefore, a combination of a photochemical GCM with a 1D global-mean model may be the best tool to study Mars photochemistry and dynamics. Venus photochemistry is a challenging problem, which was studied mostly in early 1980's after the success of the Pioneer Venus and Venera missions. Later, the important data on the composition of the lower (subcloud) atmosphere were obtained from ground-based spectroscopy of Venus nightside and analyzed using a chemical model. Venus photochemistry involves three basic tasks: the atmospheric composition above and below 60 km and the formation and structure of the cloud layer. H$_2$O is strongly depleted by sulfuric acid above the clouds, and photolysis of HCl is the main source of odd chlorine Cl$^*$ and odd hydrogen H$^*$. H$^*$ is much less abundant than Cl$^*$ because of the reaction OH + HCl $\rightarrow$ H$_2$O + Cl. SO$_2$ is a source of sulfur chemistry, which results in the formation of the H$_2$SO$_4$ clouds and is important in the balance of CO, O, and O$_2$. NO may be formed by lightning and significantly affect Venus chemistry. The major difficulty for the models above 60 km is to fit the upper limit O$_2\leq$0.3 ppm. The lower atmosphere is hot and dense, and slow reactions with high activation energy may proceed there. However, the simple assumption of thermochemical equilibrium is typically misleading. Analysis of the CO$_2$-CO-SO$_x$-H$_2$SO$_4$-OCS system agrees with the observational data on the OCS and CO vertical profiles. The clouds consist of H$_2$SO$_4$ and probably S$_n$ and FeCl$_3$. Venus photochemistry is complex, some observational data are insufficient and missing, and a new spacecraft mission with the advanced instruments for analysis of the chemical composition of the atmosphere and clouds is highly desirable.
DE: 5405 Atmospheres--composition and chemistry
DE: 5407 Atmospheres--evolution
DE: 5435 Ionospheres (2459)
DE: 6225 Mars
DE: 6295 Venus
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting