The research, led by Juan Alday of the Instituto de Astrofísica de Andalucía in Spain and The Open University in the United Kingdom, together with Francisco González-Galindo and an international team of collaborators, draws on simulations from the Mars Planetary Climate Model, a general circulation model capable of simulating the entire Martian atmosphere from the surface all the way to the exosphere. Below the homopause lies the homosphere, where turbulence keeps the mixing ratios of long-lived gases essentially constant with altitude, so all species share a common scale height set by the mean molecular weight of the air. Above it lies the heterosphere, where molecular diffusion takes over and each species settles according to its own molecular weight — heavier gases like argon fall off rapidly with altitude while lighter ones extend further. Because this diffusive separation governs how chemical species are distributed in the upper atmosphere, the altitude of the homopause directly influences how much gas is exposed to escape processes at the top of the atmosphere.

The team traced the homopause using the ratio of argon to molecular nitrogen, two chemically inert gases that are well mixed at low altitudes but separate above the homopause because argon, with a molecular weight of 40, is heavier than nitrogen at 28. They developed two methods to identify the boundary from the model’s density profiles: one based on the altitude where the argon-to-nitrogen ratio drops 5 percent below its lower-atmosphere value, and another based on finding the inflection point in the curvature of the ratio profile. Both methods gave similar results for most of the planet, but they diverged by tens of kilometres in the polar regions, where strong downward winds associated with the winter Hadley circulation push air depleted in nitrogen down from above, breaking the assumption of a well-mixed atmosphere below the homopause. The researchers adopted the curvature-based method as their preferred approach, noting that the argon-to-nitrogen ratio exhibits a sharper transition at the homopause than the carbon dioxide mixing ratio used in earlier studies, which tends to yield estimates 5 to 10 kilometres higher.

The simulations reveal a striking seasonal pattern. The homopause reaches its highest altitudes over the summer poles of each hemisphere, peaking at roughly 120 kilometres over latitudes above 60 degrees north during northern summer and around 130 kilometres over the corresponding southern latitudes during southern summer. At the same time, the homopause plunges to its lowest values in the winter hemisphere, dropping as far as 60 to 90 kilometres over the southern polar region during southern winter. These asymmetries reflect the organisation of the global circulation: during the solstices, the atmosphere is dominated by a single Hadley cell centred near 30 degrees latitude in the summer hemisphere, and during southern summer this circulation intensifies because Mars is near perihelion, its closest approach to the Sun, where increased solar heating, elevated dust loading and the planet’s topography all combine to enhance mixing. The atmosphere also physically expands near perihelion, further lifting the homopause. During the equinoxes, when two roughly symmetric Hadley cells drive a weaker circulation, the homopause settles at more modest altitudes of 90 to 110 kilometres, with the autumn equinox values running about 10 kilometres higher than those of the spring equinox owing to the difference in Mars–Sun distance.

Perhaps most surprising is the sheer range of pressures at which the homopause sits. Rather than tracking a fixed pressure or density level, as it would if it simply rose and fell with the thermal expansion and contraction of the atmosphere, the homopause wanders across four orders of magnitude in pressure, from about 10 to the minus 4 to 10 to the minus 1 pascals, with a modal value near 0.3 millipascals. This behaviour confirms a long-standing hint from NASA’s MAVEN mission: variations in the homopause altitude are not merely a breathing of the atmosphere with the seasons but reflect genuine changes in the strength of turbulent mixing driven by breaking waves and atmospheric tides. The temperatures at the homopause are comparatively well behaved, mostly falling between 100 and 150 kelvin, with the mean homopause temperature rising from about 115 kelvin at altitudes above 110 kilometres to 142 kelvin below 80 kilometres.

The model also predicts a pronounced diurnal cycle. In most latitudes and seasons, the homopause altitude peaks in the afternoon between 12:00 and 18:00 local time and falls to a minimum in the pre-dawn hours around 5:00 to 7:00, with typical day-night amplitudes of 5 to 15 kilometres. The largest swings, reaching 30 to 40 kilometres, occur in transition seasons at latitudes between 40 and 70 degrees, where the homopause can collapse dramatically in the early morning hours as the circulation reorganises between solstice and equinox configurations. Atmospheric tides imprint additional structure, producing double-peaked diurnal patterns at some mid-latitudes of the winter hemisphere that mirror the well-documented semidiurnal tide in Martian surface pressure and temperature. By contrast, the homopause over the summer pole during the season of continuous daylight, and over the winter pole during polar night, barely moves at all, pinned in place by the near-constant illumination conditions.

Dust storms add another layer of variability. When the team ran simulations tailored to individual Martian years from MY32 to MY35, incorporating the observed day-to-day dust loading and solar flux, they found that regional dust storms and the great global dust storm of MY34 lifted the homopause by 10 to 20 kilometres, with the magnitude depending on the intensity of the event. This finding dovetails with published analyses of MAVEN measurements showing that homopause altitudes rose during the growth phase of the 2018 planet-encircling dust storm and subsided as it decayed. Outside of dust events, interannual variability of 5 to 10 kilometres concentrates around the equinoxes and over the southern polar region during the first half of the year, while the summer hemispheres near the solstices are remarkably repeatable from year to year, suggesting that the vigorous, self-consistent solstitial circulation anchors the homopause, whereas the weaker equinoctial circulation leaves it exposed to planetary waves and other transient phenomena.

To validate the model, the researchers compared its predictions against two independent observational datasets. The first came from the Neutral Gas and Ion Mass Spectrometer on NASA’s MAVEN spacecraft, whose measurements of argon and nitrogen densities between 2015 and 2020 yield homopause altitudes averaging 110 kilometres with a standard deviation of about 12 kilometres — almost identical to the model’s 110 plus-or-minus 11 kilometres. Applying the same retrieval methodology to synthetic data extracted from the model at the exact times and locations of the MAVEN measurements allowed the team to probe the biases hidden in the observational technique itself. They found that in the winter polar regions, where downwelling air distorts the argon-to-nitrogen ratio, the standard method systematically underestimates the homopause altitude, and they caution that the sharp decrease of the homopause over the winter pole inferred from MAVEN data should be interpreted with care. They also conclude that the carbon dioxide densities at the homopause reported from MAVEN data are likely underestimated, a bias that appears even when the technique is applied to model data. Overall, the model captures the magnitude and seasonal-latitude structure of the homopause well, although it appears to underpredict the strength of the day-night cycle, with observed homopause altitudes running roughly 20 kilometres higher than simulated during the afternoon at certain seasons.

The second comparison used solar occultation profiles of pressure, temperature and carbon dioxide density from the Atmospheric Chemistry Suite on ESA’s ExoMars Trace Gas Orbiter, applying a published technique that locates the turbopause where the eddy and molecular diffusion coefficients are equal. Applied to model profiles, this method reproduces the homopause altitude with a mean difference of only 0.2 plus-or-minus 6.1 kilometres, although the assumption of a fixed reference eddy diffusion coefficient introduces errors of up to 20 kilometres at some times and places, and simply doubling or halving that coefficient shifts the inferred altitude by about 10 kilometres. Against the real TGO data, the differences average 1.2 plus-or-minus 7.9 kilometres, confirming that the model tracks the observed seasonal trends.

Finally, the team turned the model results into something immediately useful: a parameterisation of the eddy diffusion coefficient for one-dimensional photochemistry and escape models, which sacrifice the full three-dimensional dynamics of a general circulation model in exchange for finer detail on chemistry and escape processes. By running a one-dimensional diffusion model on argon and nitrogen profiles and finding the mixing strength that best reproduces the global model, they showed that a constant eddy diffusion coefficient fits best near the homopause, and they constructed a hybrid profile that follows the density-dependent form suggested by earlier analyses of carbon monoxide and carbon dioxide from TGO at lower altitudes before saturating to a constant value below the homopause. Applied across a full Martian year, the derived coefficient spans one to two orders of magnitude, and the team provides a simple linear relation linking the coefficient to homopause altitude. With this tool, future models of hydrogen and water escape on Mars — processes known to vary dramatically with season and dust storms — can finally incorporate a realistically variable homopause, bringing the study of atmospheric loss on Mars a significant step closer to reality.