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A Sensitive Search for Biomarker Gases on Mars: Vertical Distribution of Polar Ozone
The search for life on Mars has previously focused on the presence of water on the planet. However, other biomarkers, such as O2, CH4, and O3, can also be indicative of life. A recent multi-instrument campaign has sampled various atmospheric gases on Mars including CH4, H2O, O2, and others. Data of Mars were collected using the Cryogenic Echelle Spectrograph (CSHELL) at the NASA Infrared Telescope Facility (IRTF) atop Mauna Kea in Hawaii. The data span ten months from August 2009 to June 2010 and include 28 different settings. The data were processed using a standard cleaning procedure, and the frames were spatially and spectrally aligned. Processing also involved utilizing a general circulation model (GCM) and accessing the Mars Climate Database (MCD) in order to create detailed weather forecasts specific to the observation date and time.
Further processing was conducted solely on the O2 data from December 12, 2009. The wavenumber range of the observation (7898.5 cm-1 to 7916.7 cm-1) includes eight different rotational transitions of O2(a1∆g) → O2(Χ3Σg). By modeling an O2 spectrum at a specific temperature and comparing it to the data, we were able to determine the temperature of the transitions. We found the mean weighted temperature of the O2(a1∆g)→O2(Χ3Σg) rotational transitions to be 149 ± 1.4K. For further comparison with previous studies, we also determined the retrieved temperature for O2(a1∆g) for each hemisphere. The retrieved temperature for just the northern hemisphere was found to be 149 ± 1.6K, whereas the temperature for just the southern hemisphere was found to be 151 ± 2.7K.
Above 20 km in the Martian atmosphere, the formation of O2(a1∆g) results from the destruction of ozone. By comparing retrieved temperatures of the O2(a1∆g)→O2(Χ3Σg) rotational transitions derived from the data, we were able to map the relative abundance of ozone in the Martian atmosphere. We found the highest relative abundance of ozone to occur in the northern polar region. A second maximum in abundance occurs at southern high latitudes, with the abundance of ozone substantially decreased in the mid-latitudes of both hemispheres and near the equator. Our results are consistent with previous studies that have found maximum zone abundances at the poles, and decreased abundance near the equator and mid-latitudes.
This research represents the first retrieval of the vertical structure of ozone in the Martian atmosphere from retrieval of O2(a1∆g) temperatures using ground-based infrared telescopes.