Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6217s&link_type=abstract
American Astronomical Society, DPS meeting #38, #62.17; Bulletin of the American Astronomical Society, Vol. 38, p.608
Astronomy and Astrophysics
Astronomy
Scientific paper
An improved laboratory measurement system has been developed at Georgia Tech to measure ammonia opacity in a hydrogen and helium atmosphere at wavelengths from 1.1 to 20 cm, pressures from 0.5 to 16 bars and temperatures from 185 to 550 K. The goal of this system is to provide high-precision measurements over a range of temperature and pressure conditions which correspond to altitudes probed by spacecraft radio occultation experiments (e.g. Cassini), by entry probe signal extinction measurements (e.g. Galileo), and by passive radio emission measurements from both spacecraft (e.g. Juno) and earth-based radio telescopes.
One key problem with all previous centimeter-wavelength measurements of ammonia is the uncertainty in mixing ratio due to adsorption of ammonia gas by the test chambers. At the higher pressures of these measurements, it is necessary to use a metallic pressure vessel to contain the simulated deep atmospheres. Ammonia is easily adsorbed onto the metal surfaces during such measurements, which makes knowledge of the measured ammonia concentration difficult, especially at colder temperatures. Our new laboratory system employs an additional, glass-envelope Ka-Band (7-10 mm) Fabry-Perot resonator, kept at room temperature (minimizing adsorption), to enable differential measurements of the gas mixtures both before and after admission to the pressure vessel. With this method, we have been able to constrain the uncertainty in the ammonia mixing ratio of the gas mixture under test to less than 10% of the stated value under most measurement conditions. Additional monitoring of the precise quality factor (or Q) of the resonators in the system has also been used to characterize both the time variability and the spatial variability of adsorption within the system, further reducing uncertainties.
Hanley Thomas Ryan
Steffes Paul Gregory
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