Mars cloud mystery points to exotic physics
A recurring cloud above Mars may form through a rare ice-making process that has never been observed in another planetary atmosphere.
A strange cloud that appears above Mars every morning may be the first observed example of a planetary atmosphere creating ice through a process scientists usually treat as theoretical.
The Arsia Mons Elongated Cloud (AMEC) forms downwind of Arsia Mons, a volcano that rises about 20 kilometres above the Martian surface. During spring and summer in Mars’s southern hemisphere, the white water-ice cloud can stretch for up to 1,800 kilometres before fading. It returns the next morning, repeating the cycle for several months.1
The cloud was first revealed by the European Space Agency’s Mars Express mission in 2018. Researchers have watched it repeatedly since then, but previous atmospheric models could not reproduce the way it forms. A new study, supported by Mars Express observations and a meteorological model of Mars, offers a possible explanation.2
The unusual physics behind the cloud
Clouds on Earth usually form through heterogeneous nucleation. Water vapour condenses into droplets or ice crystals around tiny particles such as dust, salt, pollen or soot. Scientists had generally expected Martian clouds to depend on the same kind of airborne material.
The new modelling instead points to homogeneous nucleation. In that process, water vapour changes directly into ice particles without first attaching itself to dust or another piece of material. The research team describes it as being like condensation appearing in the middle of a room rather than on a window.2
That distinction matters because homogeneous nucleation has not previously been observed in a planetary atmosphere. The process needs exceptionally high humidity and a rapid temperature drop. The model suggests that air moving over Arsia Mons is lifted several kilometres in only a few minutes. Temperatures then fall by roughly 30 degrees Celsius in about 10 minutes, while relative humidity rises to more than 100,000 times the levels commonly experienced on Earth.
Those conditions allow water vapour to freeze directly into cloud particles. The volcano’s height, Mars’s thin atmosphere and the direction of the prevailing winds appear to combine into a setting where the unusual process can occur. The result is the long, delicate cloud seen trailing away from Arsia Mons.
What Mars Express found
Mars Express was important not just because it captured the cloud, but because it can observe Mars during the morning hours when the AMEC is present. The mission’s cameras have tracked the feature as it forms, lengthens and disappears over timescales of only a few hours.
The new model does not match every detail of the observations, and the researchers do not present it as the final word on the cloud. It does, however, reproduce the AMEC closely enough to show that the proposed mechanism is physically plausible. The work also suggests that extreme humidity levels previously not known to exist on Mars may occur in the planet’s upper atmosphere.
The finding could have implications beyond this single Martian cloud. Scientists have considered whether similar processes might occur in the upper atmospheres of Earth and Venus, although the required conditions are rare. It may also help researchers think about cloud formation on exoplanets, where the atmosphere can operate under conditions far removed from those on Earth.
Our opinion
This is a particularly satisfying kind of planetary discovery because the headline phenomenon is easy to picture, while the explanation is genuinely counterintuitive. Mars is not simply producing a familiar Earth-style cloud on a larger scale: the data suggest it may be making ice in a way that has never been directly seen in any planetary atmosphere.
The sensible caveat is that the model is not a complete explanation yet. Some details still do not line up perfectly with the observations, and the result depends on limited knowledge of Mars’s atmosphere. That makes the story more interesting, not less. Mars Express has turned a recurring visual curiosity into a test of atmospheric physics, and future observations will show whether this exotic mechanism survives closer scrutiny.
The research was published in Nature Geoscience. ESA says Mars Express will remain valuable for this work because its instruments can follow short-lived atmospheric features during the part of the Martian day when the cloud is visible.