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 ob tained on 26 March 1989 is presented. It is shown that atmospheric aer osol is responsible for almost all of the observed radiation beyond th e limb. This aerosol emission has two components: exponential with sca le height of about 10 km, and stratified (two layers with maxima at al titudes of about 23 and 33 km). These layers are a few kilometers thic k. The exponential component probably consists mainly of mineral dust, and the layered one of icy particles. The optical depth of the expone ntial component at 11.25 mum is 0.06(-0.015)+0.04, which corresponds t o a visible optical depth of 0.13(-0.03)+0.09. This value is in accord ance with the results of three other remote optical experiments on the Phobus mission. However, it is approximately 3 times smaller than tha t measured on Viking landers in the same season. One of the possible e xplanations can be that a global change of the dust content in the Mar s atmosphere occurred in the time interval of 10 years. But other type s of variabilities also cannot be excluded. The layers at 23-33 km hav e an optical depth of 0.005-0.02. The total column mass is about 5 x 1 0(-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 de nsity near the surface is about 2 cm-3.