Mars is a world defined by extremes: it boasts the largest volcano in the solar system, canyons that dwarf Earth’s Grand Canyon, and dust storms that can swallow the planet for weeks at a time. Yet alongside these dramatic features lies a far subtler enigma, a ribbonlike cloud that mysteriously appears each year, stretching for thousands of kilometres across the Martian sky. Scientists have known of this recurring formation for decades, but only now are they uncovering the mechanics behind it.
According to the Daily Galaxy, this ghostly feature, formally known as the Arsia Mons Elongated Cloud (AMEC), emerges on the western side of Arsia Mons, a dormant giant of a volcano nearly 20 kilometres high. Each Martian morning during the southern summer solstice season, water vapour condenses into ice crystals, forming a bright streak that can extend over 1,800 kilometres. By late morning, sunlight and changing winds cause the cloud to dissipate, only for the cycle to repeat daily for nearly three months. Despite its plume-like look, the cloud has nothing to do with volcanic activity, the volcano itself has been silent for millions of years.
Cracking the Mystery
For years, computer models struggled to explain how such a massive and consistent structure could arise in Mars’ thin, dusty atmosphere, an atmosphere just 1 per cent the density of Earth’s. The key turned out to be a process called homogeneous nucleation, in which ice particles form without the usual dust grains serving as seeds. This requires extreme supersaturation of water vapour, something researchers once thought impossible on Mars. But when scientists simulated conditions with unusually high water content interacting with the slopes of Arsia Mons, the AMEC appeared in their models just as it does in the sky.
Much of this progress stems from long-term monitoring by the European Space Agency’s Mars Express orbiter, which has been circling the planet since 2003. Its High Resolution Stereo Camera (HRSC) has captured a vast array of Martian clouds, from fluffy, Earthlike formations to structures shaped by atmospheric waves. Using this treasure trove of images, researchers at the German Aerospace Centre have built the HRSC Cloud Atlas, a detailed catalogue that is allowing scientists to trace how topography, winds, and dust interact to create Mars’ atmospheric patterns.
Why It Matters
The AMEC is more than a spectacular curiosity. Its regular appearance shows how water vapour, terrain, and air currents combine to create surprisingly stable weather systems in Mars’ fragile atmosphere. Unlocking these processes not only sharpens our understanding of Martian meteorology but also sheds light on where and how water moves around the Red Planet, vital knowledge for future exploration and the search for habitability.