PFS: Planetary Fourier Spectrometer
Studying the composition of the atmosphere
From space and Earth-based observations so far, we know that the Martian atmosphere is about one hundred times less dense than the Earth's at ground level, that it's composed almost entirely of carbon dioxide
But there are many gaps in our knowledge. We still don't know, for example, how temperature and pressure vary with altitude, what the global circulation patterns are, how the composition of the atmosphere varies with time and place, and what all the trace constituents are. We also don't know for sure how much dust there is in the atmosphere, how it's transported there, what it's made of and how it affects the Martian weather. The Planetary Fourier Spectrometer (PFS) on board Mars Express will help find the answers to some of these outstanding questions.
The principle behind PFS is similar to that behind OMEGA: both instruments will identify complex molecules from the wavelengths the molecules absorb from sunlight and from the infrared radiation (heat) they emit. PFS, however, will take measurements over a wider wavelength range
PFS's bread and butter studies, though, will centre on the 15 µm waveband, which is strongly absorbed by carbon dioxide. The instrument will perform a complex mathematical procedure on spectra at this wavelength to obtain the vertical temperature-pressure profile of the gas. As carbon dioxide makes up more than 98% of the Martian atmosphere, this measurement will form the basis of global atmospheric temperature and pressure studies.
"If we have a million profiles of pressure and temperature, we will be able to compute the wind in three dimensions," says Formisano. "We will be able to produce our own map of global circulation. I hope to have a 3D map of the atmosphere for each of the eight Martian seasons during which Mars Express will be operating." Such information should be of use to teams that are building up global circulation models of the Martian atmosphere.
PFS will also record spectra of the dust in the atmosphere. These will reveal the dust composition and also allow the obscuring effects of dust to be extracted from spectra of minor constituents, the most important of which are likely to be water, carbon monoxide (CO), methane
Formaldehyde, in particular, is expected to show up at the poles rather than elsewhere. "When you've got water and carbon dioxide ice and you bombard them with energetic particles (which stream into the poles from the solar wind), you can easily generate complex organic molecules such as formaldehyde," says Formisano. Formaldehyde has already been observed at the Martian poles from Earth-based telescopes. One of the most interesting molecules to search for, though, will be methane. "Methane can exist within certain low limits without any organic chemistry. But if you have a 'lot' of methane, there must be a source for it because methane is easily oxidised and destroyed. That source could be life," says Formisano.
Principal Investigator: Dr. Vittorio Formisano, Instituto Fisica Spazio Interplanetario, Rome, Italy.
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