Mars is home to the highest clouds ever discovered above the surface of a planet, astronomers said today.
The European Space Agency's orbiting Mars Express spacecraft found clouds that are between 50 and 62 miles (80 to 100 kilometers) above the red planet.
The highest clouds on Earth top out at about 52 miles (84 kilometers).
The clouds encircling Mars are most likely made of carbon dioxide, researchers said. There were detected with a camera that senses ultraviolet and infrared light, so there is no conventional picture of them.
The clouds were spotted by observing distant stars just before they disappeared behind Mars. The stars would dim as they went behind clouds.
"If you wanted to see these clouds from the surface of Mars, you would probably have to wait until after sunset" says Franck Montmessin, a French researcher who works with the camera team.
This is because the clouds are very faint and can only be seen reflecting sunlight against the darkness of the night sky. In that respect, they look similar to the mesospheric clouds, also known as noctilucent clouds on Earth, which occur about 50 miles (80 kilometers) above our planet.
For clouds to form, vapor needs solid particles around which to condense. The researchers found a previously unknown population of minuscule dust grains above 38 miles (60 kilometers) in the Martian atmosphere. The dust, perhaps blown skyward from the surface, is likely the stuff around which the carbon dioxide forms. An alternative idea is that the dust if debris from meteors that have burned up in the thin air of Mars.
Mars is known to create dust storms that can shroud the entire planet.
The discovery suggests the upper layers of Mars' atmosphere can be denser than previously thought. That would be important to mission planners, who use the Martian atmosphere to "aerobrake" incoming spacecraft.