The process in question is called homogeneous ice nucleation. In most planetary atmospheres, water vapour does not condense into ice on its own. Instead, it freezes onto airborne motes of dust, salt or other tiny solids, a process known as heterogeneous nucleation. These foreign particles lower the energetic barrier to ice formation, allowing ice to grow at modest levels of supersaturation, the condition in which the air holds more water vapour than it can stably support. Homogeneous nucleation, by contrast, requires water molecules to organise themselves spontaneously into a stable ice embryo purely by chance collisions. The energy cost of creating a new ice surface from nothing is so high that the vapour must be supersaturated by many hundreds of percent before this becomes statistically likely. On Earth, such extreme supersaturation is essentially never reached in the lower atmosphere, which is why the process is considered a curiosity rather than a working mechanism.

There is, however, one place on Earth where homogeneous nucleation is thought to operate: the polar summer mesosphere, at altitudes near eighty kilometres, where temperatures plunge below minus one hundred and fifty degrees Celsius and noctilucent clouds of tiny ice crystals form. Theoretical studies have also suggested that ephemeral ice clouds could nucleate homogeneously in the upper mesosphere of Venus. Yet homogeneous nucleation from water vapour has never been directly observed in nature on any planet, and atmospheric scientists have generally regarded it as too demanding to matter under realistic conditions. The new work challenges that assumption by pointing to a planet where the conditions are, paradoxically, far more favourable than on Earth, despite Mars having much less water overall.

Mars’s atmosphere is thin, cold and nearly dust-dominated, and it sits well below the triple point of water, meaning liquid water cannot persist and water cycles almost entirely between vapour and ice. Crucially, the Martian atmosphere contains very few condensation nuclei at high altitude. Airborne dust exists, but its abundance drops sharply with height, and the number of available particles per cubic centimetre in the upper troposphere can be minuscule compared with Earth’s aerosol-rich air. When an air parcel rich in water vapour is rapidly cooled in such a particle-poor environment, supersaturation can climb to extreme values before any heterogeneous freezing gets a chance to consume the excess vapour. In that regime, homogeneous nucleation becomes not just possible but competitive, and the model developed by Hernández-Bernal and colleagues shows that it can take over entirely.

The team embedded a detailed scheme of homogeneous ice nucleation from water vapour into meteorological models of Mars, including the Mars Planetary Climate Model developed at the Laboratoire de Météorologie Dynamique and a mesoscale model capable of resolving the winds around individual volcanoes. The mesoscale simulations capture a key dynamical ingredient of the AMEC: a cold pocket of air that forms on the western, leeward side of Arsia Mons in the early morning hours, where orographic flow, gravity waves and the volcano’s enormous height combine to chill the air dramatically. As air parcels traverse this cold pocket, temperatures fall low enough, and supersaturation rises high enough, for water vapour to freeze spontaneously into vast numbers of minuscule ice crystals, each only a fraction of a micrometre across.

The results were striking. With homogeneous nucleation switched on, the model reproduced the distinctive characteristics of the AMEC that had eluded conventional microphysics: the cloud’s extreme elongation, its recurrent diurnal cycle of growth and dissipation, its high altitude near the top of the tropospheric boundary, and its optical brightness. The homogeneously nucleated particles, being numerous and tiny, scatter sunlight efficiently and give the cloud its characteristic appearance, while the dynamics of the cold pocket stretch the freshly formed ice into the long tail that streams away from the volcano. Simulations using only heterogeneous nucleation on dust, the standard assumption for Martian water ice clouds, failed to match the observed cloud, regardless of how the properties of the dust nuclei were tuned. Sensitivity tests showed that the outcome depends delicately on thermodynamic parameters such as the surface energy of ice and the vapour pressure over ice at very low temperature, with small adjustments shifting the cloud’s brightness and extent in ways consistent with the observational constraints.

Those observational constraints come from an unusually rich set of spacecraft data, all openly available. The Visual Monitoring Camera on the European Space Agency’s Mars Express orbiter, originally a humble engineering webcam, has provided a long record of the cloud’s daily evolution, complemented by the high-resolution imagery of the HRSC instrument on the same spacecraft. Images from the Mars Colour Camera aboard the Indian Space Research Organisation’s Mars Orbiter Mission and from the Emirates eXploration Imager on the United Arab Emirates’ Hope probe added further coverage. This multi-mission archive allowed the team to pin down the cloud’s morphology, timing and radiative properties with enough precision to make the model comparison meaningful, and the modelling code and datasets have been released openly for other researchers to scrutinise and extend.

The implications extend well beyond one photogenic cloud. If homogeneous nucleation from water vapour is operating on Mars, then the standard picture of cloud formation on that planet, in which dust grains almost always serve as the scaffolding for ice, is incomplete. High-altitude water ice clouds influence the Martian climate in important ways, affecting the vertical distribution of water vapour, the radiative budget of the atmosphere and even the formation of snowfall. Clouds that nucleate homogeneously consist of far more, and far smaller, particles than heterogeneously nucleated ones at the same water content, which changes how they scatter light and how they sediment. Climate models that omit this pathway may therefore misrepresent the water cycle and its climatic feedbacks, particularly in the cold, clean air above the great volcanoes and in the polar regions.

For Earth, the finding is a provocative nudge rather than a direct overturning. Terrestrial clouds below the mesosphere almost certainly form heterogeneously, thanks to the planet’s abundant aerosols, and the extreme supersaturations required for homogeneous freezing of water vapour are not reached in the troposphere. But the Mars result demonstrates that the process is viable under real planetary conditions, not merely in laboratory chambers, and it encourages a re-examination of assumptions about where and when homogeneous nucleation might matter, including in the mesospheres of Earth and Venus and potentially in the atmospheres of exoplanets with cold, clean, water-bearing air. The study also highlights how laboratory measurements of the vapour pressure and surface tension of supercooled water and ice, performed over the past decade, now feed directly into planetary-scale conclusions.

There is also a lesson in how the discovery was made. The AMEC was long known to observers and had been modelled before, but only by combining a decade of serendipitous spacecraft imagery with high-resolution mesoscale dynamics and a microphysical scheme that most modellers had set aside as irrelevant did the pieces fall into place. A cloud that looked like a mere curiosity of Martian meteorology has turned out to carry a fundamental message about how matter changes phase in extreme environments. As missions continue to monitor Mars’s atmosphere with ever finer instruments, the elongated cloud of Arsia Mons may prove to be the first confirmed natural laboratory for a process that textbooks had filed away as theoretically possible but practically impossible, a reminder that the universe often finds ways to cross thresholds we assumed were out of reach.

Subject of Research: Homogeneous ice nucleation from water vapour in the Martian atmosphere

Article Title: Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars

Article References: Hernández-Bernal, J., Määttänen, A., Spiga, A., & Forget, F. (2026). Homogeneous ice nucleation from water vapour suggested by elongated clouds on Mars. Nature Geoscience, 19(10), 1213-1217. https://doi.org/10.1038/s41561-026-02089-9

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02089-9

Keywords: Mars, Arsia Mons Elongated Cloud, homogeneous ice nucleation, cloud microphysics, water ice clouds, planetary atmospheres, supersaturation, Nature Geoscience, Martian climate, mesoscale modelling, heterogeneous nucleation, atmospheric science

The puzzle the researchers set out to solve has been staring planetary scientists in the face for decades. On Earth, planetary-scale circulation is generally assumed to homogenize atmospheric composition, smoothing out differences in the mixing ratios of trace gases and aerosols over synoptic timescales. Mars refuses to conform. Observations from orbiting instruments reveal enormous planetary-scale inhomogeneities in the distributions of dust, water vapor, carbon monoxide, and argon. Water vapor climbs steeply in the northern summer while the southern winter hemisphere stays comparatively dry. Carbon monoxide accumulates toward the winter poles as the gas freezes out at the cold polar surface. Argon, an inert tracer, collapses dramatically over the winter pole as carbon dioxide condenses out of the air and concentrates the remaining constituents. If global circulation is supposed to mix everything together, Mars did not get the memo.

To find out why, the team turned to a Lagrangian particle-tracking approach built on the ensemble Mars atmosphere reanalysis system, known as EMARS. Rather than examining the atmosphere from a fixed grid of points, Lagrangian analysis follows individual parcels of air as they are carried by the winds, hour by hour, through the reanalysis data. The researchers released vast numbers of virtual tracers, some seeded inside the Hadley cell and some seeded outside it, and tracked their displacements over 30 Earth days. They also performed sensitivity experiments in which the strength of the mean circulation and the strength of transient eddies—the chaotic, wave-like disturbances superimposed on the mean flow—were independently amplified, allowing the team to disentangle which component of the flow controlled the transport regime.

The result was unambiguous. Tracers launched within the Hadley cell tended to stay there, riding the cell’s coherent conveyor belt, while tracers outside it were largely excluded, unable to penetrate the circulation’s boundaries. The edges of the Hadley cell, in other words, act as transport barriers: surfaces across which material exchange is strongly suppressed, much like the walls of the stratospheric polar vortex on Earth that help preserve ozone-depleted air over Antarctica. At the same time, the interior of the cell functions as a pole-to-pole teleconnection pathway. Air rising in the rising branch over the warm hemisphere is carried aloft, crosses the equator in the upper branch, and descends over the opposite pole, delivering material—dust, water, chemistry—from one hemisphere directly to the other in a single coherent loop. The two poles of Mars, though separated by half a planet, are dynamically coupled in a way that mid-latitudes are not.

Determining why Mars behaves this way required a dimensional argument. The team compared the relative importance of the mean overturning circulation against eddy-driven mixing, and found that on Mars the mean circulation dominates overwhelmingly. This dominance, they showed, arises from the combined effect of two planetary properties: Mars rotates nearly as fast as Earth, which suppresses large-scale turbulent mixing by strengthening rotational constraints on the flow, while its atmosphere is vanishingly thin—roughly one hundred times less massive per unit area than Earth’s. The thin atmosphere means the circulation responds rapidly to thermal forcing, and the single-cell Hadley circulation that emerges in each solstice season is both stronger and more coherent relative to the eddies than its terrestrial counterpart. The outcome is a transport regime in which advection by the mean flow overwhelms diffusive eddy transport, the opposite of the balance that prevails on our own planet.

To test whether this regime is truly unique, the researchers repeated the same tracer experiments on Earth and Venus, using the ERA5 reanalysis for our planet and the Venus Climate Database for our inner neighbor. Earth, with its two-cell Hadley circulation, vigorous baroclinic eddies, and comparatively thick atmosphere, mixed tracers broadly across latitudes, erasing sharp compositional gradients on short timescales. Venus, whose atmosphere superrotates and whose eddy field dominates the meridional overturning, showed an entirely different pattern again. Only Mars, sandwiched between these two extremes, produces the distinctive combination of isolation inside a coherent cell and pole-to-pole teleconnection along its spine. The three terrestrial planets, made of similar materials orbiting the same star, thus host three qualitatively different atmospheric transport regimes.

The implications reach well beyond atmospheric dynamics as a curiosity. The isolation of material within the Hadley cell helps explain a suite of long-standing Martian observations: the striking latitudinal gradients in dust, water vapor, and carbon monoxide; the formation of a polar ozone layer driven by transport rather than local photochemistry, as earlier work by Montmessin and Lefèvre proposed; and the delivery of water and dust to the poles that ultimately becomes recorded in the polar layered deposits, the ice-rich archives of Martian climate history. If tracers cannot readily escape the Hadley circulation, then the compositional signal deposited at each pole reflects material that traveled along a well-defined dynamical pathway, not a well-mixed global average. That insight could sharpen interpretations of isotope ratios in polar ice, including the deuterium-to-hydrogen histories used to reconstruct how much water Mars has lost to space.

Perhaps the deepest consequence of the study is conceptual. The conventional view of planetary atmospheres holds that global-scale circulation is, above all, a mixing machine: it redistributes heat, momentum, and composition, and iron out inhomogeneities. The new results demonstrate that this is not a universal law but a regime-specific outcome of planetary parameters. Change the rotation rate, the atmospheric mass, or the balance between mean flow and eddies, and a circulation that mixes on one world can confine and channel on another. Planetary atmospheric dynamics, the authors conclude, can actively restrict redistribution, creating compositional reservoirs that persist in plain sight of one another. For Mars, that means the air above the tropics and the air above the mid-latitudes live in partial isolation, while the poles trade material as though connected by a private telegraph line.

For scientists preparing the next generation of Mars missions, the findings offer practical guidance as well. Interpreting measurements of water vapor, argon, carbon monoxide, or photochemical species requires knowing where a parcel of air has been and which dynamical basin it belongs to. Lagrangian tools of the kind released by the team—publicly available alongside the EMARS and ERA5 datasets—can now provide that context, converting snapshots of composition into narratives of transport. And as exoplanet astronomers characterize atmospheres of worlds with rotation rates and atmospheric masses far from those of the solar system’s terrestrial trio, the Martian lesson looms large: the same circulation pattern that on Earth homogenizes the air can, on a thin-atmosphere world, fence it off—and link its poles in one seamless loop.

Subject of Research: Atmospheric material transport by the Martian Hadley circulation

Article Title: Hadley circulation drives material isolation and pole-to-pole teleconnection on Mars

Article References: Fan, C.-S., Sun, C., Xie, Z., Luo, Y., Gu, L., & Fan, S. (2026). Hadley circulation drives material isolation and pole-to-pole teleconnection on Mars. Nature Geoscience. https://doi.org/10.1038/s41561-026-02090-2

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02090-2

Keywords: Mars, Hadley circulation, atmospheric dynamics, Lagrangian particle tracking, polar vortex, planetary atmospheres, dust transport, water vapor, Nature Geoscience, teleconnection, Venus, EMARS reanalysis