Direct observations of nighttime clouds in the Mars atmosphere are difficult to obtain. (Cloud observations obtained by Mars Global Surveyor's Thermal Emission Spectrometer in the years 2000-2006 were limited to the dayside of the planet). The Mars Climate Sounder instrument on board the Mars Reconnaissance Orbiter spacecraft now provides routine nightside observations of atmospheric temperature and opacity that document the presence of rapidly evolving water ice cloud layers in the Martian tropics during the northern summer season. The MCS observations confirm global circulation model predictions made in 2004 by David Hinson and John Wilson.
A daily cycle of cloud formation in the Martian tropics during northern summer is driven largely by solar radiation and the associated solar thermal tide. Models predict a striking pattern of cloud formation above the high altitude volcanoes of the Tharsis region. At mid-day, the atmosphere is typically free of clouds due to elevated temperatures. Beginning in the early evening hours, atmospheric cooling at altitudes above 20 kilometers begins to foster water ice cloud formation. During the night, the altitude of cloud formation descends closer to the surface. The atmospheric models predict that the cloud layer eventually merges with ground fog in the early morning hours, thereafter dissipating as the solar radiation increases on the dayside.
Clouds form in association with a strong temperature inversion within the atmosphere, where temperatures higher up are significantly warmer than those below. This inversion is seen in the accompanying figure at an altitude of about 10 kilometers above the surface, where the black line (denoting temperature) swings strongly toward colder temperatures. The figure also shows a strong maximum of water ice opacity (blue curve) at the same altitude, indicating the active formation of water ice clouds in association with the temperature inversion. This MCS snapshot in time shows both the inversion and the cloud layer at the precise attitude predicted by the global circulation model for this time of day.
The unique capability of MCS to provide linked atmospheric temperature and aerosol opacity data represents an important advance for studies of the atmospheric circulation of Mars.