Nair H, Allen M, Yung YL, Clancy RT; Workshop on the Evolution of the Martian Atmosphere (1992 Jun 29-Jul 1 : Kona, HI).

Pap Present Workshop Evol Martian Atmos. 1992; 21-2.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125, USA.

The importance of odd hydrogen (or HOx) radicals in the catalytic recombination of carbon monoxide and oxygen in the martian atmosphere has been known for many years. The inclusion of recent chemical kinetics data, specifically temperature-dependent CO2 absorption cross sections, into our one dimensional photochemical model shows that HOx is too efficient in this regard. The absorption cross sections of CO2 are smaller than previously assumed; this leads to a reduction in the photolysis rate of CO2 while the photolysis rate of H2O has increased. As a consequence the predicted mixing ratio of CO in our models is substantially less than the observed value of 6.5 x 10(-4). Simultaneous measurements of water, ozone, and carbon monoxide have been recently obtained in the martian atmosphere in early December 1990 (Lg for Mars was 344 degrees). The global average water abundance was approximately 3 micrometers, with a mixing ratio ranging from 70 ppm at low latitudes to below 20 ppm at high latitudes. If such low water abundances are typical for the martian atmosphere over long timescales, the efficiency of the HOx catalytic cycle may be severely curbed. Indeed, Shimazaki pointed out that a "typical" water abundance of approximately 10 micrometers produced a model CO abundance much lower than observed, while a lower H2O abundance of approximately 1.4 micrometers produced a CO abundance consistent with observations. The seasonally varying concentration of ozone is a valuable indicator of the abundance of HOx, as it is typically destroyed by reactions such as H + O3 --> OH + O2. Its lifetime is on the order of tens of minutes, so that it adjusts virtually instantaneously to variations in the water abundance. Ozone amounts at the equator were measured to be <1 micrometer, which is smaller than what our photochemical model indicates. Our aim is to examine the ability of gas-phase chemistry to control the composition of the martian atmosphere. The expected results of lowering the abundance of HOx are higher mixing ratios for both CO and O3. Differences between models and observations may require the inclusion of additional gas-phase mechanisms, and possibly heterogeneous chemistry, for example, to curtail the HOx cycle as suggested by Anber et al. (1992).

Publication Types:

• Meeting Abstracts

Keywords:

• Atmosphere
• Carbon Dioxide
• Carbon Monoxide
• Hydrogen
• Mars
• Oxygen
• Ozone
• Temperature
• Time
• Water
• chemistry
• NASA Discipline Exobiology
• Non-NASA Center

Other ID:

• 99604017

UI: 102237325

From Meeting Abstracts