The first image of our grand tour focuses on the toxic carbon-dioxide clouds of Venus. The feature may represent atmospheric waves, analogous to high and low pressure cells on Earth. Hubble has provided the most detailed complete global coverage of the red planet ever seen - resolving images as small as 30 km across. The image was snapped in February 1995 when Mars lay 103 million km from the Earth during opposition, the most favourable time to observe Mars when the Earth lies directly between it and the Sun. A crescent shaped orographic cloud formation is seen just right of the centre.
Orographic clouds are common on Earth.
They form when moist air is forced to rise over a land barrier such as a mountain. As the moist air rises it cools and precipitates into clouds that cling to the downwind face of the obstacle. On Mars, warm afternoon air is being pushed over Olympus Mons, the largest volcano in the solar system, which towers some 27 km high - three times the cruising height a commercial airliner. The moist air form orographic clouds of ice-crystals which linger downwind of the volcano. Ascraeus Mons to the north, Pavonis Mons and Arsia Mons to the south.