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| Titel |
ORbiter Terahertz-Infrared Sensor - ORTIS |
| VerfasserIn |
Patrick G. J. Irwin, Brian N. Ellison, Simon B. Calcutt, Neil E. Bowles, Jane Hurley, Ranah Irshad, Nicholas Teanby, Brian Alderman, Simon Rae, Remco de Kok |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250035370
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| Zusammenfassung |
| 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, 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 ~ 2 GHz from the main methane absorption line that could be measured
simultaneously.
Sub-millimetre wave spectroscopy is advancing rapidly in the terahertz frequency range
and we believe that observation of the required spectral features will be technically feasible
from a highly compact in situ payload within the timeframe of EJSM. The advantages over
lower frequency measurements include: 1) the methane absorption line is stronger, allowing
sounding to higher altitudes; 2) the field-of-view is smaller for the same antenna size
allowing easier near-limb-sounding; 3) the Doppler shifting of lines due to winds is larger;
and 4) the instrument payload is more compact.
A sub-millimetre wave instrument would provide valuable information about
Jupiter’s stratosphere, but the radiance away from the line centres is governed by
the temperature and also the ammonia abundance in the upper troposphere. Our
instrument will combine the sub-millimetre wavelength device with an infrared
spectrometer/radiometer, which will be able to measure the upper tropospheric
temperatures in the H2-H2, H2-He collision induced continuum (200 -– 650 cm-1), not
affected by ammonia, simply extending the vertical coverage of temperature of the
combined instrument right down to the cloud tops and simultaneously allowing the
determination of the upper tropospheric ammonia abundance and para-H2 fraction.
A feasibility study is currently under way to assess possible infrared component
designs, ranging from a simple channel radiometer to a more ambitious spectrometer
designs.
Since the ESA Jupiter Ganymede Orbiter will be in orbit about Ganymede our proposed
instrument would spend much of its time viewing the surface of Ganymede rather
than the atmosphere of Jupiter. Observing over a wide frequency range from far-IR
to mid-IR wavelengths offers significant advantages over purely THz devices in
determining surface temperature variations and provides complementary information on
composition to the measurements made in the visible and near-IR. In particular,
the inclusion of the thermal IR may allow measurements of ‘hot spots’ and help
characterise any sources of endogenic heat flow at unprecedented spatial and temporal
resolution. |
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