High Clouds at the Sunrise Terminator of Mars - Revisited

On March 19, 2012, at areocentric Ls 85, a cloud that was very high in Mars atmosphere and visible beyond the sunrise terminator was discovered by Wayne Jaeschke. It was imaged by him and subsequently by others on many nights. The cloud at the time of discovery was centered at about 45° south latitude and about 197° west longitude, and though most images of it found it to be near this location, a few documented it or similar high terminator clouds at quite different locations. Images of the cloud are presented here. Comparable clouds have been detected in the past, and they were at areocentric longitudes of the Sun and areographic locations different from this one. Certain images of the cloud are measurable, and the measurements show that the cloud was in the Martian mesosphere. Mesospheric clouds are routinely detected by spacecraft orbiting Mars, but in recent years have seldom been detected by Earth-based observers. Review is made of Minami's proposal that the cloud was an aurora. The present authors suggest that the composition and cause of the clouds are uncertain, but prominent possibilities include condensation of CO2 in the Martian night, and water ice clouds related to atmospheric gravity waves.