The abundance of carbon monoxide in Neptune's atmosphere
Carbon Monoxide (CO) was discovered in the stratosphere of Neptune from the detection of the J=3-2 and J=2-1 rotational transitions in emission at 345.8 and 230.5 GHz respectively. It was conventionally thought that all of the atmospheric carbon should be in its reduced form of methane (CH4). Two sources of stratospheric CO have been postulated: CO transported from the interior by convection due to Neptune's strong internal heat source (internal source); or, CO produced through photochemical reactions from an external supply of water (external source). In this research project the J=3-2 transition of CO was observed to find the CO profile in Neptune's atmosphere and determine the mechanism producing CO. Three instruments were used at the James Clerk Maxwell Telescope (JCMT) to measure the CO line: the heterodyne receiver B3; the University of Lethbridge Fourier Transform Spectrometer (FTS); and, the Submillimeter Common User Bolometer Array (SCUBA). The high resolution (1.25 MHz) of the heterodyne observations over a large frequency range (~20 GHz) produced a very powerful result because the narrow emission core from the stratosphere and the broad absorption feature arising in the lower atmosphere were measured simultaneously. The CO abundance profile was determined using a model of the J=3-2 CO transition in Neptune's atmosphere developed for this project. Calculations indicate a CO abundance of 1.9+0.5-0.3x10-6 in the upper stratosphere and (0.8±0.2)x10-6 in the lower stratosphere and troposphere. The moderate resolution of the FTS data allowed the broad absorption feature to be measured. Uranus was originally chosen as the calibration source, but the discovery of CO in Uranus by Encrenaz et al. (2004), while this project was in progress, prompted both Neptune and Uranus to be examined for CO absorption. Two data sets (1993 and 2002) were analyzed and it was found that the 1993 spectra produced superior results, giving a CO mole ratio in the lower atmosphere between 0.8x10-6 and 2x10-5; this agrees, within the uncertainty limit, with the lower atmosphere heterodyne result. A tentative detection of CO in Uranus was also obtained from the 1993 data, with a CO abundance profile constrained to pressures greater than 0.5 bar with an abundance between 5x10-7 and 1x10-5. The 2002 data were found to be inferior to the 1993 data because of imperfect cancellation of thermal emission from the terrestrial atmosphere. The 850ìm SCUBA filter profile is well matched to the width of the CO feature. Photometric observations of Neptune and Uranus were used to determine if the reduction in integrated flux due to CO absorption could be detected using SCUBA. A CO mole ratio in the range (1.2-1.7) x10-6 was found for Neptune, calibrated against Uranus and assuming no CO in Uranus. Calibration of the Neptune and Uranus SCUBA data against Mars to produce an independent estimate of the CO abundance in both planets did not produce a useful result because of large calibration errors. Comparison of the results from the three techniques determined that the heterodyne measurement was superior and the derived CO profile was used to determine the source of neptunian CO. It was concluded that the source of CO in Neptune is both internal and external. The lower atmosphere result indicates an interior dominated by water ice. The most likely mechanism for the upper atmosphere CO involves meteoritic ablation, photolysis of H2O, and chemical reaction with by-products of methane photochemistry. The required H2O influx for this mechanism is at least two orders of magnitude higher than previously observed, indicating either that the observed H2O abundance is too small or that CO is produced by a different mechanism.
submillimetre, Neptune, atmosphere, carbon monoxide, fourier transform spectroscopy, radiative transfer
Doctor of Philosophy (Ph.D.)
Physics and Engineering Physics
Physics and Engineering Physics