Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p41a0185j&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P41A-0185
Physics
6225 Mars, 6297 Instruments And Techniques
Scientific paper
The European Space Agency's cornerstone project HERSCHEL is a space based 3.5m-telescope for exploration of the Universe in the spectral range from far-infrared to sub-millimetre wavelength (57-670 μm). The science payload consists of three instruments: 1. PACS, a bolometer detector array camera and a low- to medium-resolution spectrometer to perform imaging photometry and imaging line spectroscopy in the 60--210μm wavelength band, 2. SPIRE, an imaging photometer covering the 250--500 μm range, and an imaging Fourier Transform Spectrometer covering 200--670 μm, and 3. HIFI, a high resolution heterodyne receiver covering the frequency range 480--1910 GHz with a resolving power of up to 107. In order to achieve an absolute flux calibration accuracy of better than 5 percent for HIFI, it is intended to use Uranus and Mars as primary flux standards for continuum radiation. But especially Mars seems to be a challenging calibration source, because numerous rotational absorption lines -- caused by CO and H2O in Mars tenuous atmosphere -- disturb the continuum radiation emitted by the surface. So usually these wavelength regions are omitted for an accurate instrument calibration. However, we have studied the emitted radiation especially in the deep core of the CO absorption lines, because the spectral lines are optically thick at these frequencies and Mars can be treated as a pure gas planet without any need to model the surface emission. Using a detailed radiation transfer model and a general circulation model to predict the seasonal variation of Mars' atmosphere we found for several CO lines a variability of Mars' brightness temperature of less than ±5 percent. So we suggest not to avoid the rotational CO absorption lines of Mars, but to observe especially the line center of all observable CO lines in order to increase the accuracy of the absolute flux calibration of HIFI.
Hartogh Paul
Jarchow Ch.
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