ESA's Mars Express spacecraft has been observing local and regional dust storms forming at the north pole of the Red Planet over the last month, and watching as they disperse towards the equator. Local and regional storms lasting a few days or weeks and confined to a small area are commonplace on Mars, but at their most severe they can cover the entire planet, as happened last year during a global storm that encircled the planet for many months.
It is currently spring in the northern hemisphere, and water-ice clouds and small dust-lifting events are frequently observed along the edge of the seasonally retreating ice cap. Many of the spacecraft at Mars return daily weather reports from orbit or from the surface, providing global and local impressions of the changing atmospheric conditions.
Between 22 May and 10 June, ESA's Mars Express spacecraft observed at least eight different storms at the edge of the ice cap. These formed and dissipated very quickly, lasting between one and three days. The two cameras on board the spacecraft – the High Resolution Stereo Camera (HRSC) and the Visual Monitoring Camera (VMC) – have been monitoring the storms over the last few weeks.
The image at the top of this page, acquired by HRSC on 26 May, shows a spiral-shaped dust storm, its brown colour contrasting with the white ice of the north polar ice cap below. Meanwhile, the animated sequence (right) was compiled from images of a different storm, captured by the VMC over a period of 70 minutes on 29 May. This particular storm started on 28 May and continued until around 1 June, moving towards the equator during that time. The montage of images (below) shows three different storms developing on 22 May and 26 May, and between 6 and 10 June. In the latter case, the cameras watched the storm evolve for several days as it moved towards the equator.
At the same time, wispy patches of light-coloured clouds can be seen at the outer margin of the polar cap and also several thousand kilometres away, close to the volcanoes Elysium Mons and Olympus Mons. Together with the MARCI camera on board NASA’s Mars Reconnaissance Orbiter, Mars Express observed that when the dust storms reached the large volcanoes, orographic clouds – water ice clouds that form due to the influence of the volcano’s leeward slope on the airflow – that had previously been developing started to evaporate as a result of the air mass being heated by the influx of dust.
These regional dust storms last only a few days; the elevated dust is transported and spread out by global circulation into a thin haze in the lower atmosphere, at altitudes of between approximately 20 and 40 kilometres. Some traces of dust and clouds remained in the volcanic province until mid-June.