Since 2004, ESA’s Mars Express orbiter has observed the surface and atmosphere of Mars using a variety of visual, radar, and radio-science instruments. In 2018, it imaged a spectacular, elongated, water-ice cloud seeming to originate from the Arsia Mons volcano in the southern hemisphere of Mars. The cloud is notable for its length (up to 1,800 kilometers) and its appearance near the time of martian perihelion and the southern solstice, a season not known for cloud formation in the southern hemisphere. However, studying it has proved challenging as the cloud forms during the martian morning and disappears after only a few hours. Unfortunately, some of the probes currently in orbit of Mars, such as NASA’s Mars Reconnaissance Orbiter and Mars Global Surveyor, are in Sun-synchronous orbits, meaning their cameras are not able to image the area until the afternoon when the cloud has already evaporated.
To better understand this atmospheric phenomenon, an international team including members from ESA, Universidad País Vasco (Spain), and other institutions primarily utilized the Visual Monitoring Camera aboard Mars Express, an instrument originally designed to verify the separation of the Beagle 2 lander. Although the resolution of this camera is coarse at 12 kilometers per pixel, it has a wide field of view and can collect images in rapid succession, enabling the team to observe the cloud as it forms and evolves. They also studied images of the Arsia Mons cloud formation taken by other Mars orbiters, going as far back as Viking 2 in the late 1970s, to look for previous appearances of the cloud and to determine how it has evolved over multiple martian years.
The team concluded that the cloud is likely to be orographic in origin, meaning that it forms due to topography. Moist air near the surface encounters the slope of the Arsia Mons volcano, which forces it to ascend. As the air reaches altitudes of 40 to 50 kilometers, it condenses to form the head of the cloud. Then, high-altitude winds drag the cloud westward, rapidly elongating it. As the Sun rises and temperatures increase, the cloud evaporates. They also noted that the cloud was diminished during periods of dust storm activity, indicating that dust storms can interfere with cloud formation. Further detailed study of the daily, seasonal, and annual evolution of this cloud, and other clouds like it, can provide constraints on the time variability of moisture and wind patterns in the lower and upper atmosphere of Mars. READ MORE