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Reliable due to PTB calibration: Mars Express discovers water and methane

The European Spacecraft Mars Express discovered considerable, hitherto unknown water quantities as “permafrost” ice at the south pole of the red planet. Recently, one of the measuring instruments, the Planet Fourier Spectrometer (PFS), also succeeded in providing evidence of methane in the Martian atmosphere. PTB contributed to these discoveries: the radiometric calibration of the short-wave PFS channel is traced back to PTB standards – not only prior to start, but also in the Mars orbit.

Details of two PFS spectra taken at the south pole of Mars and the equator. The pole spectrum clearly shows the absorption band of water ice between 3750 cm-1 and 3950 cm-1 which is lacking in the equator spectrum. Both spectra show absorption lines of water vapour. In addition, the pole spectrum shows an absorption band of solid carbon dioxide at 4370 cm-1. Inserted is a view of the Mars surface at the south pole in visible light. The grid of the PFS measurements which provide proof of water ice is superposed.

At the end of last year, Mars Express as the first European Mars mission reached its Martian orbit. Only a short time afterwards, infrared measurements of OMEGA (combined camera and spectrometer), SPICAM (UV and IR spectrometer) and PFS (the Planet Fourier Spectrometer) enabled the sensational discovery of water at the south pole. Shortly after that, methane was detected as trace gas in the carbon dioxide atmosphere of Mars by PFS measurements.

The PFS, developed, constructed and operated in a cooperation project between Italy, Poland and Germany, is a two-channel spectrometer for the wavelength range from 1.2 µm to 45 µm. PFS measures the solar radiation reflected by Mars. This also allows ice mixed with sand and stones, which is “hidden” in the visible spectral range, to be detected on the Mars surface.

The short-wave channel of the spectrometer (1.2 µm to 4.8 µm) was radiometrically calibrated with traceability to PTB standards. For this purpose, the spectral radiant intensities of five infrared miniature radiators in the PFS calibration device were calibrated over the channel’s whole spectral range by direct comparison with the PTB primary standard for spectral radiance, a high-temperature cavity radiator. The radiators calibrated by PTB first provided “preflight” calibration of the PFS. One of the calibrated infrared radiators was then integrated into the PFS and now allows regular “inflight“ calibration of the short-wave channel in the Mars orbit. This ensures that no structures will be pretended in the measured spectra as a result of unrecognized modifications of the PFS sensitivity.

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