Mars Express, PFS

Mars Express, PFS

zum Bild PFS

The PFS is one of the instruments of priority on-board of the spacecraft Mars Express that was launched on June 2nd from the Russian-Kasakhian launch site Baikonur, Central Asia. The main effort in research and development of PFS was done by a team in Italy (principal investigator V. Formisano, Istituto di Fisica dello Spazio Interplanetario CNR, Rome), completed by teams in Poland (Space Research Center, Warsaw), Russia (Space Research Institute, Moscow), Germany (DLR, Optical Information Systems, Berlin), France (Observatoire de Paris-Meudon), and Spain (Instituto de Astrofisica de Andalucia, Granada).

The PFS is a two-channel Michelson interferometer operating in the IR from 1.25 to 45 µm wavelength. It is optimized for the study of the Martian atmosphere. The spectra will also be used for investigations of the Martian soil.

The field of view (FOV) of the two channels is about 9 km squared in the short-wave channel (SWC) ranging from 1.25 to 5 µm, and 18 km in diameter in the long-wave channel (LWC) ranging from 6 to 45 µm, respectively, when the spacecraft is at minimum altitude of 250 km (pericenter). The scanning fore optics allow the field of view (FOV) to be pointed perpendicular to the flight direction within ± 45° besides  the nadir direction perpendicular to the Martian surface. This enables the PFS to map the major part of the Martian surface during the different seasons in one Martian year (about two terrestrial years).


Main Scientific Objectives of PFS

1.   Study of the Martian atmosphere

  • Mapping of the three-dimensional temperature field by analyzing the CO 2 absorption bands at 4.3 and 15 µm. The CO 2 molecules of the lower and warmer atmospheric layers contribute to the center and wings of the absorption profile whereas the upper and cooler layers contribute mainly to the center. Deconvolution of these contributions retrieves the temperature-height profile.
  • Study of the atmospheric composition, variation of the content of H 2 O, CO, HDO, etc., search for unknown minor constituents.
  • Comparison of the distribution of temperature and composition in time and space allows to calculate the three-dimensional wind field.
  • Investigation of the aerosols like hoar frost, dust, ice clouds, hazes, etc.

2.   Investigation of the Martian soil

  • Determination of surface temperature and thermal inertia.
  • Investigation of the mineralogical composition including the estimation of the water content of minerals. This is an important investigation for the selection of promising areas where to search for traces of simple life forms providing they existed (or exist).


    The Instrument

    The Principle of a Fourier Transform Spectrometer

    In contrast to resolving spectrometers, the incident radiation in a Fourier transform spectrometer is not split into different wavelengths by a prism or grating. The beam is divided by a beamsplitter in two partial beams. Two mirrors reflect the partial beams back to the beamsplitter where they interfere depending on the difference of twice the path length beamsplitter - mirror 1 or beamsplitter - mirror 2, respectively. The intensity of the interfering beams is recorded by the detector. At least one of the mirrors moves in a precisely controlled way, thereby changing the path difference. The signal as a function of the path difference is the interferogram, which is the Fourier transform of the spectrum. Re-transforming the interferogram provides the spectrum. Fourier transform spectrometers have advantages over other types of spectrometers, at least in the IR, concerning the field of view and the noise.

    The Performance of the PFS

    zum Bild Messprinzip

    The PFS is a two-channel Fourier transform spectrometer. Two channels indicate two spectrometers, one on top of the other. Both are equipped with a pair of retroreflectors, i.e. three flat mirrors assembled to a corner of a cube. They are attached by brackets to an axle moved by a torque motor. This angular movement changes the path difference.


    Main Parameters of the PFS

    Spectral range [µm] 1.25 - 5 6 - 45

    Spectral resolution [cm -1 ]

    2 (0,3-1,25 nm) 2 (1,5-11,3 nm)
    FOV [mrad] 35 x 35 70 ø
    Detector PbSe LiTaO 3
    NESR [W/(cm 2 sr cm -1 )] 3 x 10 -10 @ 195 K 2 x 10 -8 @ 290 K
    NEP [W Hz -1/2 ] 1 x 10 -11 2 x 10 -10
    Beamsplitterr CaF 2 CsI
    Interferogram, samples 16.384 4.096

    Optischer Gangunterschied [cm] ± 0.5
    Messzeit je Interferogramm [s] 4
    Leistungsaufnahme [W] 29
    Masse, Spektrometer [kg] 25
    Maße, Spektrometer [mm 3 ] 250 x 210 x 320

    SWC - short-wave channel; LWC - long-wave channel; FOV - field of view; NESR - noise equivalent spectral radiance; NEP - noise equivalent power


    First Scientific Results of PFS  (J. Helbert, Institute of Planetary Resarch)

    PFS covers simultaneously two spectral regions which are highly interesting for planetary scientist. In the short wavelength channel (SWC) PFS measures solar light reflect from the surface of the planet. In the long wavelength channel (LWC) the solar radiation has a very low intensity and instead PFS measures directly the "glow" of the planetary surface due to its temperature.

    zum Bild Spektrogram

    Solar light reflected from the surface of Mars passes twice through its atmosphere. The constitutents of the atmosphere adsorb the solar light differently. Furthermore the reflection on the Martian surface depends on der properties of the surface material. Therefore using data from the SWC of PFS we can learn something about the composition of the atmosphere and the surface at the same time.
    The image on the left (Copyright: ESA - V. Formisano 2004) shows a so called spectrogram derived using data from the shortwave length channel of PFS. Mars Express has crossed the equator flying from south to north. Clearly visible are the spectral signatures of CO and CO2, two of the most important gases in the Martian atmosphere. About 97% of the atmosphere are CO2, a gas which is very efficient in absorbing infrared radiation. Therefore it is one of the major green house gases and measuring its abundance and distribution with height gives important inputs to climate modells.

    zum Bild Oberflächentemperatur

    Each planetary body emits radiation due to its own temperature. The intensity and wavelength distribution of this radiation depends on the temperature of the surface. Since PFS can measure this intensity distribution in the long wavelength channel with high accuracy, we are able to derive the surface temperature from orbit. As for the reflection, the emission depends on the surface material. Therefore we can not only derive the temperature, but at the same time we get information about the composition and the physical properties of the surface.
    A number of minerals, most important the silicates, show spectral features in the wavelength range above 55µm. By combining PFS measurements with laboratory measurements performed in our group, it is possible to study the mineralogy in great details. For this task PFS is complementary to OMEGA which has a higher spatial resolution, but can measure only up to 55µm.
    The image on the right shows surface temperatures derived directly from the long wavelength spectra for the first orbit PFS has obtained. The data are plotted over a martian globe with the topography color coded. Clearly visible is the Hellas Basin in the south. As expected temperature peak when Mars Express crosses the equator. The orbit end at the edge of the North polarcap and temperatures drop here down to 175K or nearly -100°C.

    The surface temperature is an important parameter for models of the upper layer of the Martian surface. By comparing modeled and observed temperature for different times of the day the thermal inertia can be derived. Thermal inertia values are a good indication for the thermal conductivity of the surface material. With this and some more physical parameters of the soil, one can use models to determine for example the minimal burial depth of ice or search for hotspots as a tell-tale sign of current vulcanic activity. For the scientific interpretation of the PFS data the Berlin Mars near Surface Thermal model (BMST) is used.


    PD Dr. rer. nat. habil. Gabriele Arnold
    German Aerospace Center

    Institute of Planetary Research
    , Management and Infrastructure
    Tel: +49 30 67055-370

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