Mars Express watches a curious cloud

Since September 13, Mars Express, from ESA, observes the evolution of an elongated cloud formation hovering near the Arsia Mons volcano, 20 km high, close to the equator of the planet.

Despite its location, this atmospheric feature is not related to volcanic activity but rather is a cloud of water ice driven by the influence of the volcano's leeward slope on the airflow – what scientists call an orographic cloud or leeward – and a regular phenomenon. in this region.

The cloud is visible in this view taken on October 10 by Mars Express's visual surveillance camera (VMC) – which has reproduced it hundreds of times in recent weeks – as an elongated white line spanning 1500 km west of Arsia Mons. . For comparison, the cone-shaped volcano has a diameter of about 250 km; a view of the area with labels is provided here.

Mars has just experienced its winter solstice in the northern hemisphere on October 16th. In the months leading up to the solstice, most cloud activities disappeared on large volcanoes such as Arsia Mons; its summit is covered with clouds during the rest of the year Martian.

However, it is known that a recurrent water ice cloud, like the one shown in this image, forms along the southwestern flank of this volcano – it had already been observed by Mars Express and d & # 39; Other missions in 2009, 2012 and 2015.

The appearance of the cloud varies throughout the Martian day, its length increasing during the leeward morning of the volcano, almost parallel to the equator, and reaching a size as impressive as it is. could be seen even by telescopes on Earth.

The formation of water ice clouds is sensitive to the amount of dust present in the atmosphere. These images, obtained after the great dust storm that swept the entire planet in June and July, will provide important information on the effect of dust on cloud development and its variability throughout the year. .

The elongated cloud hovering near Arsia Mons this year has also been observed with the Visible and Near Infrared Mapping Spectrometer, OMEGA, and the High Resolution Stereoscopic Camera (HRSC) on Mars Express, providing scientists with a variety of different data to study. this phenomenon.

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