A barrier above Mars limits mixing in the atmosphere

The Martian atmosphere is a natural laboratory for planetary atmospheres. Dust and water vapour interact there with circulation, climate and the surface. Dust absorbs and scatters radiation, which changes temperature and the movement of air. Water vapour ties the atmosphere to the polar caps and to the long-term water balance. Observations show a clearly uneven distribution of these components across the planet. No consistent mechanism explained it.

A team from the Southern University of Science and Technology (SUSTech) in southern China tracked particles in the Lagrangian framework, following them as the atmospheric flow carried them along. The results appeared in "Nature Geoscience".

A barrier at the edge of the circulation

The first finding concerns the edge of the planetary Hadley circulation, in which air rises near the equator and sinks at higher latitudes. That boundary forms a dynamic barrier to the transport of material. Over 30 Earth days, only about 15 per cent of the particles released inside the circulation cross it. About 20 per cent of those released outside get through. This state holds for about 80 per cent of the Martian year. For the remaining 20 per cent or so, the structure weakens into two weaker cells with a different pattern of isolation.

A teleconnection between the poles

The second result is more surprising still. Above the Hadley cell, the authors found a teleconnection between the polar regions of both hemispheres. Within 10 Earth days, a particle can travel from high latitudes in the summer hemisphere to high latitudes in the winter hemisphere. Dimensional analysis indicates that the fast rotation of Mars would let vortices form much as they do on Earth. The thin atmosphere reaches radiative equilibrium so quickly, though, that those vortices are effectively suppressed. Transport on Mars therefore runs mainly along the mean flow, unlike on Earth, where vortices dominate. Venus has a mean flow too, but its strong superrotation confines transport to narrow bands of latitude.

This has concrete consequences. When dust and water vapour separate, the formation of water ice can be suppressed, which in turn alters the radiative balance and the water cycle. Components processed photochemically in the upper layers are delivered straight over the winter pole. Over the longer term, such a pattern would favour the deposition of water and its isotopes, semi-heavy water among them, only at high latitudes. Atmospheric escape would further modify the planet's water resources. The questions are not closed, but this is the first consistent picture of how a planet with a thin atmosphere filters its own transport.