Mars’s oddest cloud may be even odder than we thought
Researchers using ESA’s Mars Express and a Martian weather model have found that Mars’s Arsia Mons Elongated Cloud forms through an unprecedented atmospheric process called homogeneous nucleation, where water vapour freezes directly into ice without dust particles.
Every spring and summer in Mars’s southern hemisphere, a striking white cloud emerges near the 20-kilometre-tall Arsia Mons volcano, stretching up to 1,800 kilometres before vanishing within hours. Known as the Arsia Mons Elongated Cloud (AMEC), this feature was first spotted by ESA’s Mars Express in 2018 and has since been observed repeatedly during dusty seasons. The cloud forms daily as winds interact with the volcano’s towering slopes, creating a distinctive orographic feature unlike typical Earth-bound clouds.
Scientists initially modelled the AMEC as an orographic cloud, but simulations failed to replicate its behaviour until exotic physics were introduced. The breakthrough came when researchers incorporated homogeneous nucleation—a process where water vapour freezes directly into ice crystals without dust or other particles acting as nuclei. This phenomenon had never been observed in a planetary atmosphere before.
The rare conditions required for homogeneous nucleation were met near Arsia Mons, where winds lift moist air rapidly, cooling it by 30 degrees in just 10 minutes and spiking relative humidity to extreme levels. This rapid cooling triggers spontaneous freezing, forming the AMEC in a process akin to condensation appearing mid-air rather than on a surface.
The discovery relied on data from Mars Express’s three cameras, including high-resolution imagery that captured the cloud’s formation in unprecedented detail. The mission’s ability to observe the AMEC during morning hours provided critical insights, reinforcing Mars Express’s role in unravelling the planet’s atmospheric mysteries and highlighting the need to consider unconventional processes in planetary science.