THERMAL STRUCTURE AND DUST LOADING OF THE MARTIAN ATMOSPHERE DURING LATE SOUTHERN SUMMER - MARINER-9 REVISITED

The temperature structure and dust loading of the Martian atmosphere are investigated using thermal emission spectra recorded by the Mariner 9 infrared interferometer spectrometer (IRIS). The analysis is restricted to a subset of the IRIS data consisting of approximately 2400 spectra in a 12-day period extending from L(S) = 343-degrees to L(S) = 348-degrees, corresponding to late southern summer on Mars. Simultaneous retrieval of the vertical distribution of both atmospheric temperature and dust optical depth is accomplished through an iterative procedure which is performed on each spectrum. The inclusion of dust opacity in the retrieval algorithm causes the retrieved temperatures to change by more than 20 K in some atmospheric layers. The largest column-integrated 9 mum dust optical depths (approximately 0.4) occur over the equatorial regions. The highest atmospheric temperatures (> 260 K) are found at low altitudes near the subsolar latitude (approximately 6-degrees-S), while the coldest temperatures (< 150 K) are found at levels near 1.0 mbar over the winter pole. A comparison of temperature maps for 1400 LT and 0200 LT indicates diurnal temperature variations as large as 80 K at low altitudes near the subsolar latitude, whereas diurnal temperature changes at pressures less than 1.0 mbar are typically about 10 K. Both dayside and nightside temperatures above about 0.1 mbar (approximately 40 km) are warmer over the winter (north) polar region than over the equator or the summer (south) polar region. This thermal structure suggests the existence of a net zonally averaged meridional circulation with rising motion at low latitudes, poleward flow at altitudes above 40 km, and subsidence over the poles. Because a meridional circulation transports atmospheric constituents as well as heat, it has significant implications for the net flux of dust and water into the polar regions.