Height ices Mars on top

Martian atmosphere churns harder in south making north wetter.

Scientists have figured out why it’s wet up north – on Mars. A new computer simulation of the martian atmosphere suggests that the planet’s geography causes differences in atmospheric circulation within the northern and southern hemispheres. These differences dump more water on the martian north pole, where it adds to the seasonal ice-cap.

Mark Richardson of the California Institute of Technology in Pasadena and John Wilson of the Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, find that the thin martian air, which is mostly carbon dioxide, rises and falls more vigorously in the southern than in the northern hemisphere¹.

This difference all but disappears when the duo remove from their simulations the height variations of the martian plains. Mars’ south pole is about six kilometres higher than its north pole. By comparison, Earth’s Tibetan plateau is on average just four kilometres above sea level.

It is this height difference that makes the atmospheric circulation dissimilar in the two hemispheres, the researchers conclude. The discrepancy creates an overall south-to-north transport of water vapour as the water ice in the polar ice-caps melts during their respective summers.

Richardson and Wilson reckon that the other potential cause of the asymmetry in atmospheric circulation – the eccentricity of Mars’ orbit around the Sun -doesn’t have a major role. The shape of the orbit determines how close Mars is to the Sun around the time of the southern summer solstice. But changing this distance in the simulations doesn’t alter the asymmetry of the circulation pattern.

Hadley cells

Atmospheric circulation on Mars happens much as it does on Earth. Gases warmed at the equator rise by convection, before passing towards the poles, where they cool, sink and flow back to the equator. This creates two great lobes of circulating gas, called Hadley cells, one in each hemisphere.

The Hadley cells carry water vapour and dust picked up from the planet’s surface. Richardson and Wilson’s simulations show that Mars’ southern Hadley cell spins more vigorously. Leaking across the equator, dust and water are then borne northwards.

The Mars Global Surveyor spacecraft showed very clearly in the late 1990s that the north and south polar ice-caps are not the same. The seasonal northern ice is rough and pitted, and looks much the same all over. The southern ice, on the other hand, is sculpted by natural erosion into strange shapes, more like a permanent ice sheet.

Some of these differences may be caused by the asymmetry in circulation – although it is not yet clear how much of either ice-cap is water ice. Most of it is frozen carbon dioxide: dry ice. The Mars Odyssey spacecraft began sending data back to Earth last month, and this information is beginning to clarify how water ice is distributed over Mars.

One clear prediction of the new results concerns the formation of polar ice features called layered deposits, which are thought to consist of alternating layers of dust and ice. They have been seen at both poles, but Richardson and Wilson calculate that their growth is likely to be more rapid in the north. We haven’t watched Mars close up for long enough yet to know if this is true.

References

1. Richardson, M. I. Wilson, R. J. A topographically forced asymmetry in the martian circulation and climate. Nature, 416, 298 – 301, (2002).