MARS OZONE ABUNDANCES FROM INFRARED HETERODYNE SPECTRA AND THEIR APPLICATION TO THE STUDY OF THE STABILITY OF THE MARTIAN ATMOSPHERE

Abstract

The stability of the carbon dioxide atmosphere of Mars is thought to be maintained through catalytic chemistry involving "odd hydrogen" products of water vapor photolysis. Ozone is destroyed by odd hydrogen and therefore its abundance is an excellent tracer of those species that play a vital role in reforming carbon dioxide. Photochemical models of the atmosphere of Mars rely on observables such as ozone to test their predictions.

Infrared heterodyne spectroscopy with a spectral resolution >=106 is the only technique that can directly measure ozone in the Martian atmosphere from the surface of the Earth. Observations were made using the Goddard Infrared Heterodyne Spectrometer and Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility. Ozone abundances from seven data sets taken between 1988 and 2003 are presented, along with observation and analysis techniques. Measured spatial, seasonal, and orbital variability of total ozone column abundance is compared to that predicted by the first three-dimensional gas phase photochemical model of Mars. Overall agreement in the behavior of ozone across aphelion and perihelion periods supports the theory that odd hydrogen chemistry is responsible for maintaining the stability of the carbon dioxide atmosphere. Underestimation of modeled low latitude ozone around aphelion may indicate the suppression of odd hydrogen abundance through heterogeneous processes involving water ice clouds. The weak but not strict anticorrelation of the observed total column densities of ozone and water supports the role that the altitude distribution of water vapor is thought to play in regulating ozone abundance. Ozone abundances from this work are compared with those retrieved using ultraviolet techniques, showing generally good agreement. Techniques for extracting ozone altitude distribution are investigated by incorporating O2(1D) dayglow observations which indirectly probe ozone above ~20 km altitude.

The abundance and altitude distribution of ozone in the Earth's atmosphere retrieved from calibration spectra are compared to nearby contemporaneous measurements using Dobson, lidar, and ozonesonde techniques. Excellent agreement with altitude distribution measured by lidar and ozonesonde is achieved when total ozone column densities from Dobson spectrophotometry are used as a constraint in the radiative transfer analysis of the spectra.