Yardangs on Mars

The atmosphere of Mars is about a hundred times thinner than that of Earth, but the planet still manifests distinct weather patterns – with highly variable temperatures, winds transporting air masses, dust storms and ice clouds, even dust devils playing across its rusty surface.

As well as gathering data on the ‘chemical landscape’ of the Martian surface and atmosphere, PFS returns detailed information on the components making up the planet’s weather system including vertical temperature and pressure profiles, surface temperatures and wind fields, the distribution and type of atmospheric aerosols and water vapour variations.

The atmosphere of Mars is 95% carbon dioxide, and strong CO2 bands in the thermal infrared region of the PFS spectrum can be employed to take temperature soundings all the way down to the planet’s surface.

Imagine the atmosphere as divided into concentric layers of carbon dioxide, with the surface emitting heat radiation. If this happens at a wavelength that carbon dioxide does not absorb at all, then the radiation from the surface passes through the whole of the atmosphere to reach the orbiting instrument.

However, if we proceed along  the spectrum to a wavelength absorbed by CO2, then each layer absorbs radiation from below, before emits a different radiation on the basis of its own temperature. The same process occurs in the next layer up, with the radiation again absorbed and then re-emitted according to its own lower temperature – temperature assumed to decrease with altitude.

At the lowest radiance point along the band all the radiation other than the top of the atmosphere has been completely absorbed, and the atmosphere is effectively opaque – anyone using those wavelengths to see the planet would be blind. However, at nearby wavelengths the absorption coefficient decreases, and the same process of absorption and emission is less efficient: a fraction more radiation comes through, mostly from the upper atmospheric layers.

Mars from above

And moving sideways along the spectrum from the maximum absorption coefficient sees radiation from progressively lower layers comes through, until finally all layers of the atmosphere are represented. The PFS team have developed dedicated software tools to derive the vertical temperature (and hence pressure) profile from the shape of the bands. The measurement is accurate to about three Kelvin with a vertical resolution of three to five kilometres.

In reality this band pattern is not always constant – the presence of dust and ice makes a difference. In addition it is common to find temperature inversions, with a cooler layer below a warmer one, just like on Earth, mostly at night and over the poles. The ground typically cools faster than the atmosphere, so low layers of air end up warmer than the underlying soil, this inversion altering the shape of the band.

Measurements of the surface temperature can be converted into thermal fields, and from those surface winds can be computed. Those winds in turn transfer air masses and local temperatures. Local heat is also influenced by the dust lifted by the wind, and the condensation of ice particles to form clouds. However the presence of dust and ice clouds can be inferred from the PFS spectrum – as can fields of water abundance also strongly linked to temperature.

Just like the Earth, Mars has summer and winter seasons – the global surface pressure actually drops by a quarter during an average winter, as atmospheric carbon dioxide freezes into a solid, deposited on the polar caps. The eventual aim of the PFS team is to acquire a sufficient amount of measurements to create a global circulation model of the atmosphere. That goal would require a minimum of one Martian year’s worth of data, but may take much more actual time to accomplish.