The submillimetre and near-millimetre spectral region, from 300μm to 1.2mm, offers great potential for planetary spectroscopy because most of the molecules which constitute the atmospheres of the planets exhibit spectral transitions in this region. Ground-based observations are possible at these wavelengths because the Earth's atmosphere transmits electromagnetic radiation in spectral bands known as "windows" which occur between the strong absorption lines of water vapour.
In collaboration with a team of colleagues, I am engaged in a campaign of observations to measure the emission spectra of the planets in the submillimetre and near-millimetre windows. We use a Fourier Transform Spectrometer (FTS) at the James Clerk Maxwell Telescope (JCMT). The team comprises the following people:
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Lethbridge, Canada |
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Hilo, Hawaii |
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Cardiff, UK |
The JCMT is situated at the summit of Mauna Kea, a dormant volcano on the island of Hawaii, at an altitude of 4092m above sea level. This is one of the best astronomical observing sites in the world, partly because the atmosphere above Hawaii is particularly stable and unpolluted. The JCMT is just one of a dozen or so observatories on the mountain.
At submillimetre wavelengths, the transmittance of the terrestrial atmosphere is determined primarily by the amount of water vapour in the line of sight. A high-altitude observing site is therefore an absolute requirement. Unfortunately, even though the summit is normally well above most of the weather, the water vapour content of the atmosphere above the telescope can be quite variable.
The telescope has a segmented primary mirror of diameter 15m and is the largest facility in the world devoted to submillimetre astronomy. For further information, follow these links:
The JCMT is operated by the Joint Astronomy Centre in Hilo, Hawaii on behalf of the parent organizations: the Particle Physics and Astronomy Research Council in the United Kingdom, the National Research Council of Canada and The Netherlands Organization for Scientific Research.
Our group is responsible for developing the imaging spectrometer for the upcoming SCUBA-2 instrument at the JCMT.
Please see the SCUBA-2 project page for more information.
The FTS for this project was designed and built at the University of Lethbridge. The instrument papers for the original design, as well as the new Mach-Zehnder design can be found in the papers section.
The FTS couples to the Nasmyth focus of the telescope and receives the f/35 beam. It is fundamentally similar to the Michelson interferometer in that it uses 2-beam interference to generate an interferogram, from which the spectrum can be recovered by a Fourier transform. The maximum optical path difference between the two beams is 1.2m, which provides a spectral resolution of 125 MHz (0.004 cm-1). The FTS also makes use of polarisation encoding, which confers a number of technical advantages.
Early measurements with this spectrometer used the JCMT's facility bolometric detector, UKT14. Analysis of the data obtained with this detector, however, revealed the presence of pervasive channel fringes in the spectra due to parasitic Fabry-Perot effects arising from parallel surfaces in the detector's internal optical system, a feature which is irrelevant for conventional photometry but which greatly complicates the spectral calibration. These fringes can be seen in some of the science results below. In part to circumvent this problem, a dual-polarisation 3He detector system has been purchased and was commissioned on the telescope in August 1996. The detector is photon-noise limited in the 350µm and 450µm bands, and nearly so at 850µm and 1400µm; the noise equivalent power, excluding "sky noise" caused by atmospheric turbulence, is <5×10-16 W.Hz-1/2, which places this detector amongst the best 0.3-K bolometers in the world.
The objective of this campaign of observations is to measure the emission spectra of the planets in the submillimetre and near-millimetre windows. For the giant planets, these spectra consist of two components:
- A continuum spectrum is produced in the deep atmosphere, mainly by molecular hydrogen (the dominant atmospheric constituent). Although the hydrogen molecule is symmetric and therefore has no permanent dipole moment, it does emit electromagnetic radiation under the conditions of high pressure and high temperature which exist deep in the giant planet atmospheres.
- An absorption line spectrum is produced against this background by other atmospheric constituents at higher (colder) altitudes. Most of the spectral lines at submillimetre wavelengths are due to rotational molecular transitions. Detailed measurements of the shapes of these spectral features, for which high spectral resolution is required, can be analysed to determine the abundance and distribution of the constituents which cause them.
Each constituent has a different story to tell about the physical and chemical conditions in the atmosphere in question. Here are some of our results so far from this ongoing programme:
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Naylor et al. (1994) reported the first detection of the carbon monoxide J=3-2 absorption feature in Neptune, centred at 354.5 GHz. CO was unexpectedly discovered in the stratosphere of Neptune in 1992 using heterodyne instrumentation, and this is the first detection of a tropospheric absorption component. The measured spectrum is indicated in red and a synthetic spectrum, which was derived for a uniform vertical distribution of carbon monoxide in the neptunian atmosphere, is in yellow. Despite the presence of channel fringes in the data (see above), the agreement between the two is encouraging. Further measurements of this line and of other CO lines are planned with the new detector system, with which we hope to eliminate the channel fringes and determine the tropospheric CO abundance more precisely. |
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A 1981 paper claimed to have detected hydrogen cyanide in Jupiter's atmosphere, with a mole fraction in the upper troposphere of 1.8×10-9. This was a surprising discovery since HCN should not be stable at observable levels, and over the next several years various theories were advanced to explain its formation. After several attempts to detect the strong submillimetre absorption lines of HCN, Davis et al. (1997) conclusively demonstrated that HCN does not exist in Jupiter at the abundance level originally claimed. The figure indicates our upper limits for five different assumed vertical distributions. |
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In the paper by Davis et al. (1997), we reported the first-ever measurement of the Jupiter continuum spectrum in the 850µm window. Calibration of the continuum is extremely difficult because of various effects, including for example fluctuations in atmospheric transmittance, which must be taken into account. In the present case, the continuum was obtained by ratioing against the Sun to remove the extended-source channel fringes. The red curves in the figure represent the measured spectrum and the error envelope; the error here is almost entirely due to the uncertainty in the solar temperature at these wavelengths. The blue and orange curves represent two synthetic spectra, one (blue) for a nominal ammonia abundance and one (orange) for an enhanced ammonia abundance; the latter is a better fit to our measurements. The hatched region represents a photometric measurement of the jovian brightness temperature by Griffin et al. (1986), with which our own measurement is in excellent agreement. |
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