Mars’s strangest cloud forms through physics never seen on any planet

Every spring and summer in Mars’s southern hemisphere, as the dusty season grips the Red Planet, one of the most visually spectacular phenomena in the solar system unfolds above a colossal volcano. A long, brilliant white streak of water ice, known as the Arsia Mons Elongated Cloud, or AMEC, emerges downwind of Arsia Mons, a volcano whose summit towers roughly 20 kilometres above the surrounding plains. The cloud forms, grows and fades on a daily basis, stretching out for up to 1800 kilometres, nearly twice the length of the United Kingdom, before rapidly evaporating. This remarkable cycle then repeats every morning for several months. Now, a team of scientists working with data from the European Space Agency’s Mars Express spacecraft and a state-of-the-art meteorological model of Mars has discovered that behind this curious cloud may lie some genuinely exotic physics, a process long confined to textbooks and never before witnessed in action in any planetary atmosphere.

Mars Express first revealed the AMEC in 2018, and the spacecraft has viewed the recurrent cloud repeatedly in the years since. Researchers have used its observations to explore the cloud’s evolution, its vivid dynamics and its intriguing behaviour, eventually determining that it is an orographic cloud, a type of cloud also familiar on Earth that forms as wind flows past the craggy topography of a mountain or volcano. Yet despite this basic classification, something fundamental refused to fit. When scientists attempted to model exactly how the cloud springs to life each morning, their simulations simply could not reproduce what the spacecraft images showed. The modelled atmosphere of Mars, it seemed, was missing a crucial ingredient, and identifying that missing piece would push the researchers toward the very limits of known cloud physics.

The breakthrough came from Jorge Hernández-Bernal of LMD/CNRS/Sorbonne Université in Paris, France, lead author of the new study published in Nature Geoscience. To create the AMEC in their modelling, the team found they needed to include physics that, while described in textbooks, is treated as theoretical and is usually thought not to occur in nature. Once this physics was incorporated into the simulations, the elongated cloud emerged just as the observations demanded. The discovery suggests that Mars’s most striking cloud is not merely an unusually dramatic example of a familiar phenomenon, but a natural laboratory in which one of the rarest processes in atmospheric science plays out every single morning of the martian dusty season.

To appreciate why this finding is so unexpected, it helps to consider how clouds normally form. On Earth and elsewhere, clouds typically arise when moist air cools and water vapour condenses into liquid droplets or icy crystals. This usually happens through a process known as heterogeneous nucleation, which requires tiny specks of other material to be present in the atmosphere for the vapour to cling to and condense upon. Salt, pollen, soot or dust can all serve this purpose. On Mars, atmospheric dust has long been assumed to play this role, providing the surfaces onto which the planet’s scarce water vapour can freeze. Without such seeding particles, standard cloud physics says that cloud formation should be extremely difficult, if not impossible, under most natural conditions.

For the AMEC, however, it appears that cloud formation takes place without needing any of this material at all. According to Hernández-Bernal, the water vapour turns directly into icy cloud particles without any intermediate step. He offers a vivid analogy: it is akin to droplets of condensation appearing in the middle of a room, rather than on a window. This direct vapour-to-ice transformation is called homogeneous nucleation, and it has never been seen before in a planetary atmosphere. The researchers describe the result as wholly unexpected, a process that atmospheric scientists had theorised about for decades but had never caught in the act anywhere in the solar system beyond laboratory experiments.

Scientists had previously suggested that homogeneous nucleation might take place in the upper atmospheres of Earth and Venus, but the process has never actually been spotted there. Its elusiveness stems from the exceptional circumstances it requires. For water vapour to spontaneously freeze into ice without a seeding surface, relative humidity must reach extreme levels, more than 100,000 times those usually experienced in daily life on Earth. Such conditions are so far removed from ordinary experience that most atmospheric scientists considered homogeneous nucleation essentially a theoretical curiosity. The new modelling of the AMEC now strongly suggests that the humidity of the martian atmosphere can indeed reach these extraordinary levels, at least in the specific circumstances that prevail near Arsia Mons during the dusty season.

The key to the phenomenon lies in a unique combination of factors. The AMEC sits in a position where Mars’s thin atmosphere and the towering height of the nearby Arsia Mons volcano come together to create precisely the conditions needed for this rare process. As winds flow past Arsia Mons, the volcano’s immense bulk triggers a powerful atmospheric wave that lifts moist parcels of air several kilometres upward in just a few minutes. This rapid ascent cools the atmosphere dramatically, causing temperatures to drop by 30 degrees in only 10 minutes. Relative humidity levels then spike to the extreme values required, and the water vapour spontaneously freezes directly into cloud particles, giving birth to the elongated cloud that then stretches downwind for hundreds of kilometres before the warming day causes it to evaporate.

While some aspects of the modelled cloud do not exactly match the observations, Hernández-Bernal describes the result as remarkable. Because scientists do not have nearly as much information about Mars’s atmosphere as they do about Earth’s, reproducing the AMEC to this degree represents a significant success for the meteorological model. The research was built on data from the three cameras aboard Mars Express: the Visual Monitoring Camera, the High Resolution Stereo Camera and OMEGA. Mars Express can image large swathes of the martian surface at high resolution, and it is one of the few Mars-orbiting spacecraft, alongside ESA’s ExoMars Trace Gas Orbiter, able to observe during the morning hours when the AMEC is present. This latest release also marks the first time ESA has featured High Resolution Stereo Camera imagery of the cloud, revealing it in striking detail.

The ability to monitor the cloud on short timescales is central to what makes the discovery possible. As ESA Mars Express Project Scientist Colin Wilson notes, the spacecraft can track how the cloud changes on timescales of mere hours, providing an unrivalled view of short-lived phenomena on the planet. He characterises the finding as a true accomplishment for the mission and its scientists: Mars Express discovered the AMEC, has followed up and monitored it for years, and is now helping to reveal the secrets of its formation. The combination of long-term monitoring, high-resolution imaging and morning observation capability has allowed a fleeting, daily metamorphosis high above a martian volcano to be captured, characterised and finally explained through simulation.

Beyond deepening our understanding of atmospheric processes on Mars, the result carries a broader message for planetary science, including the study of exoplanets orbiting distant stars. It highlights that unlikely processes should not be discounted when exploring the planets of the universe. As Wilson observes, while clouds on Earth and Mars seem to be governed by the same basic rules, understanding this exotic martian cloud required exotic physics, and the same may prove true elsewhere in the cosmos. For now, the Arsia Mons Elongated Cloud stands as a reminder that even a world we have orbited for more than two decades can still harbour phenomena that challenge our assumptions, and that the most familiar-looking features of a planet’s sky can conceal some of the rarest physics known to science.

Subject of Research: Homogeneous ice nucleation in the formation of the Arsia Mons Elongated Cloud on Mars

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

Article References: Mars’s oddest cloud may be even odder than we thought. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Mars, Mars Express, Arsia Mons Elongated Cloud, homogeneous nucleation, orographic clouds, water ice, ESA, martian atmosphere, meteorological modelling, relative humidity, Nature Geoscience, planetary science