Computer Science – Sound
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..198i&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Computer Science
Sound
Scientific paper
The sub-millimetre wave spectra of the outer planets are rich in absorption line features that can be measured with extremely high spectral resolution (~106) with sub-millimetre wave heterodyne technology to determine temperature, winds and composition in the stratosphere. To measure stratospheric temperature requires the observation of the absorption of a well-mixed gas such as methane and while there is a methane absorption feature near 1.2 THz that is measurable with current solid-state receiver technology, this feature is relatively weak. Methane absorption lines become stronger at increasing frequency and the feature at 2.2 THz is particularly attractive as there is a strong water line lying ~2GHz from the line centre that could be measured simultaneously with the methane absorption line. The technology for sub-millimetre wave spectroscopy at this frequency has advanced very considerably in recent years and we believe that observation of this feature will be technically feasible from a highly efficient and integrated payload. The advantages of this observation scenario over lower frequency measurements are numerous and include, for example: 1) the methane absorption line is stronger, allowing sounding to higher altitudes; 2) the field-ofview is smaller for the same antenna size allowing the instrument to observe smaller features and also making limb-sounding easier; 3) it is easier to determine the Doppler shifting of lines due to winds at these higher frequencies; and 4) the instrument payload is more compact. The eventual target of the Outer Planet Flagship Mission (OPFM) has yet to be decided, but a submillimetre wave device would provide valuable information on stratospheric properties for Jupiter, Saturn and Titan. For Jupiter and Saturn, the radiance away from the line centres at THz frequencies is governed by the temperature in the upper troposphere and also by the abundance of variable gases such as ammonia and phosphine. We propose to combine the sub-millimetre wavelength device with a very simple far-IR channel radiometer, common bore-sighted with the sub-millimetre wave spectrometer and using the same antenna, with channels spread over the 200 - 650 cm-1 range. The far-IR radiometer would be able to measure the upper tropospheric temperature profile in the H2-H2, H2-He collision induced continuum, which is not affected by ammonia or phosphine and such an addition would very simply extend the vertical coverage of temperature of the combined instrument from the tropopause right down to the cloud tops. This addition would also mean that the sub-millimetre wave continuum measurements could be used to determine the variable abundance of ammonia or phosphine allowing the combined device to probe temperature, dynamics and abundances in the upper troposphere as well as in the stratosphere.
Alderman Byron
Calcutt Simon B.
de Kok Remco
Ellison Brian N.
Irwin Patrick G. J.
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