Some diurnal properties of clouds over the martian volcanoes

Abstract

Discrete white cloud systems have long been recognized as a martian atmospheric phenomenon. However, it was not until the Mariner 9 mission in 1971–72 that the cloud systems were found to be closely related to topographic features, in particular, the giant volcanoes on the Tharsis Ridge. Using the available Earth based observations, Smith and Smith1 investigated the seasonal and diurnal behaviour of the clouds the Olympus Mons and Elysium regions. They found that the cloud activity was generally confined to late spring and early summer seasons in the northern hemisphere. This corresponds to Ls=60–150°, where Ls is the aerocentric solar longitude and Ls = 0 for spring equinox in the northern hemisphere. The Viking Orbiters have observed martian atmospheric phenomena for eight seasons. During this period, the volcanoes have been frequently, but often irregularly, monitored. Unfortunately, the changes in the spacecraft orbit have meant corresponding variations in the surface resolution, viewing geometry and local times of the observations. However, the available observations do provide important new insights into the diurnal properties of the volcanic cloud systems, which we describe here.