Mars’s oddest cloud may be even odder than we thought

Every spring and summer in Mars’s southern hemisphere, during the Martian dusty season, a spectacular cloud emerges: the Arsia Mons Elongated Cloud (AMEC), the most visually striking cloud on the Red Planet. This white wisp of water ice appears downwind of the 20-km-tall Arsia Mons volcano, forms, grows, and fades daily, stretching up to 1800 km—nearly twice the length of the UK—before quickly evaporating. The cycle repeats every morning for several months, and Mars Express first revealed the AMEC in 2018 and has viewed it repeatedly since.

Using ESA’s Mars Express in combination with a state-of-the-art meteorological model of Mars, scientists have now found there may be very exotic physics behind this cloud. Researchers have explored its evolution, vivid dynamics, and intriguing behavior, determining it is an orographic cloud: a type also seen on Earth that forms as wind flows past the craggy topography of a volcano or mountain. But when they tried to model exactly how the cloud springs to life, their simulations did not reproduce what the images show—until now. Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, lead author of the new study, said that creating the AMEC in their modeling required including some exotic physics, physics that is included in textbooks but treated as theoretical and usually thought not to happen in nature, and that certainly hasn’t been seen in action before. Once this physics was included, the AMEC emerged just as they hoped.

On Earth and elsewhere, clouds typically form when moist air cools and water vapor condenses into liquid droplets or ice crystals, usually via heterogeneous nucleation. That process requires specks of other “stuff” in the atmosphere—salt, pollen, soot, or dust, for instance—for vapor to cling to and condense onto; on Mars, that stuff is thought to be dust. For the AMEC, however, cloud formation seems to take place without needing any of this stuff: water vapor turns directly into icy cloud particles without any middle step. Hernández-Bernal compared it to droplets of condensation appearing in the middle of a room rather than on a window.

The team calls this process homogeneous nucleation, and says it has never before been seen in a planetary atmosphere—making it wholly unexpected. Scientists had suggested this process might take place in the upper atmospheres of Earth and Venus, but it hasn’t been spotted there. Its rarity is due to it requiring exceptional circumstances, including extreme relative humidity levels over 100,000 times those usually experienced in daily life on Earth.