AEROSOL VERTICAL PROFILE ON MARS FROM THE MEASUREMENTS OF THERMAL-RADIATION ON THE LIMB

The measurements of the thermal infrared emission near and even beyond the limb of Mars, where the atmospheric contribution is substantial, were carried out by means of the scanning radiometer TERMOSKAN onboard the Phobos 2 spacecraft. The interpretation of brightness profiles obtained on 26 March 1989 is presented. It is shown that atmospheric aerosol is responsible for almost all of the observed radiation beyond the limb. This aerosol emission has two components: exponential with scale height of about 10 km, and stratified (two layers with maxima at altitudes of about 23 and 33 km). These layers are a few kilometers thick. The exponential component probably consists mainly of mineral dust, and the layered one of icy particles. The optical depth of the exponential component at 11.25 mum is 0.06(-0.015)+0.04, which corresponds to a visible optical depth of 0.13(-0.03)+0.09. This value is in accordance with the results of three other remote optical experiments on the Phobus mission. However, it is approximately 3 times smaller than that measured on Viking landers in the same season. One of the possible explanations can be that a global change of the dust content in the Mars atmosphere occurred in the time interval of 10 years. But other types of variabilities also cannot be excluded. The layers at 23-33 km have an optical depth of 0.005-0.02. The total column mass is about 5 x 10(-5) g cm-2 for the mineral dust and about 1 x 10(-5) g cm-2 for the icy particles in high altitude layers. The mineral particles number density near the surface is about 2 cm-3.