Shown in this HRSC color image captured in April 2018 is a precursor of this year’s unusual intense dust storm season on Mars. This small-scale, local storm front is situated north of Utopia Planitia, in the vicinity of the north polar ice cap of Mars. Another, much larger storm emerged further southwest in Arabia Terra at the end of May, and developed into a global, planet-encircling dust storm event within several weeks. The latter is one of the most intense dust storms ever observed on Mars and is currently monitored by all five active ESA and NASA orbiters.

Local and regional dust storms often occur on Mars, but only few of them grow in size to become global phenomena that can last for months. This happens only every three to four Mars years, with one Mars year lasting about two Earth years. Although Martian dust storms are very impressive, due to Mars’ lower atmospheric pressure they are generally weaker compared to hurricanes on Earth, with less than half the typical hurricane wind speed.

Dust storms occur statistically more often in the northern autumn and winter season when Mars is around perihelion, which is the closest point to the sun. On its elliptical orbit around the sun Mars receives more sunlight at perihelion, so the atmosphere warms up which causes the air to move.

Previous massive dust storms were observed by NASA orbiters like Mariner 9 in 1971, Viking I in 1977, and by Mars Global Surveyor in 2001. The Mariner 9 dust storm experience was in particular remarkable, since upon arrival at Mars the probe transmitted only images that did not show any surface features – the planet was completely hulled in a dust storm. Only the summit of the 22 km high Olympus Mons peaked out of the clouds. When the storm calmed down, Mariner 9 mapped Mars globally for the first time.

Only very little sunlight reaches the Martian surface during the storm, such that the solar panels of NASA’s Opportunity rover cannot supply energy at the moment. Consequently, it went into sleep mode and stopped communicating in June 2018. At the same time, NASA’s Curiosity rover continues operating on the Martian surface and can observe the evolution of the storm, thanks to its nuclear-powered battery.