Measurement of Mars Atmosphere Argon Density with the APXS on the Opportunity site

Using the APXS on board the Opportunity rover on MER mission, we were able to measure the argon density variation in the martian atmosphere as a function of seasons. The freezing and melting of the CO2 in the martian atmosphere at the poles creates local atmospheric lows/highs that moves the air mass. The argon, however, never freezes and stays in the air. An enhancement of Ar/CO2 mixing ratio by a factor of six at the martian South pole during the winter has been observed by the GRS onboard the Odyssey orbiter. In order to study this effect from the ground at the Opportunity site on MER mission, we have been making dedicated APXS measurements of the Ar in the atmosphere. In this case, the only real peak in the X-ray spectrum is the Ar line. To a good approximation, the APXS count rate is proportional to the number of argon atoms in the sensing volume, and hence measures the atmospheric density of argon, ρAr. If the atmosphere were perfectly mixed, the argon partial pressure would be constant. If we define the Local Mix- ing Ratio (MRlocal) as equal to the ratio of the local Ar partial pressure to the global average, then the measurement PAr/T is proportional to the local mixing ratio. The APXS Ar experiment thus gives a direct measurement of the local mixing ratio at the MER landing sites and it is a direct probe of the global circulation between the polar CO2 resource/sink and the equatorial regions. We have found that the Ar amount in the martian atmosphere at the Opportunity landing site is not constant and it is changing with the changing seasons. The change of the Ar in the atmos-phere follows the overall change in the atmospheric pressure but it is not in phase with it. There is a delay of many months between the maximum in Ar/CO2 mixing ratio and the pressure maximum. In addition, we have even seen the asymmetry between the south and north martian poles due to a different degree of CO2 contribution from both poles during the martian year. The actual Ar abundance that is realized at the near equatorial location of the rovers is controlled principally by the efficiency with which the atmosphere can mix away the Ar abundance gradients that occur from the localized condensation of CO2 at the poles. While we understand the overall condensation/sublimation cycles of CO2 in Mars" atmosphere, we do not have a good understanding of the meridional mixing that controls the equatorial abundance of Ar that we are measuring on the rovers with their APXS instruments. We expect that (at least) two factors are important in controlling this mixing that smooth the Ar gradients. The first is the Hadley-cell type circulation of the Martian atmosphere that moves the bulk of the atmosphere between the regions it is heated (near the equator) and more poleward latitudes where it is cooled. However, the Hadley cell meridional circulation only extends up to ~60 degrees latitude near the winter pole. At this point, a polar vortex of fast zonal winds exists, and the Hadley cell circulation is closed off from the winter pole.Transport across Mars' polar vortex is not well understood, and indeed not only important for Ar abundance on Mars, but also the transport of H2O and dust to the polar regions of Mars. Thus, by studying the Ar abundance at the equator on Mars, we will have some insight into the meridional circulation and mixing present on Mars, not only in the organized Hadley cell, but also across the polar vortex.