The European Space Agency's Mars Express spacecraft has observed an elongated cloud structure superficially resembling a volcanic plume over the Arsia Mons volcano on the Tharsis Plateau of Mars. The cloud has been seen in several images taken since 15 September 2018, and extends about 1500 km westwards of the volcano, which itself rises 9 km above the surrounding plateau, but is not thought to be volcanic in origin, but rather to have been caused by ice crystals condensing in the atmosphere around the summit of the volcano, in the same way as the condensation trails that form behind aircraft high in the atmosphere of Earth.
Cloud trail over Arsia Mons volcano (bottom) on 17 September 2018. Other volcanoes are Pavonis Mons (middle) and Ascraeus Mons (top). Mars Express/European Space Agency.
This phenomenon has been observed on three previous occasions, in 2009, 2012, and 2015, each time around Mars' Northern Summer Solstice (the day on which the Sun rises highest in the sky in Mars' Northern Hemisphere), which falls on 16 October this year (because a Mars year is 687 Earth days long, Mars has a Northern Summer Solstice roughly once every two Earth years). The skies around Arsia Mons are typically cloudy during Mars' northern winter, and clear during the northern summer, but the elongate cloud structure around the volcano appears for a few weeks either side of the solstice, suggesting a regular atmospheric current.
Annotated image showing the position of the cloud and volcanoes on the surface of Mars. European Space Agency/Global Carbon Project/University of the Basque Country.
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