Voyager Infrared Spectrometer by Stuart A. Borman
Flgure 1. IRIS spectrometer. Interferometer is to right of primary mirror Each of the two Voyager spacecraft currently exploring the outer reaches of our solar system carries sophisticated infrared and ultraviolet spectrometers. In this month’s INSTRUMENTATION, the Voyager Fourier transform infrared (FTIR) interferometer spectrometer will be described. The spectrometer was developed in a joint effort spearheaded by the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center, with the participation of research and development groups from the University of Michigan and Texas Instruments, Inc. The instrument, named IMS for infrared interferometer spectrometer, flew on the Nimbus 3 and Nimbus 4 meteorological satellites in earth orbit and on the Mariner 9 spacecraft that orbited Mars, in addition to the Voyager spacecraft. One individual who has participated in the development of IRIS from the 1544A
ANALYTICAL CHEMISTRY. VOL. 53, NO. 13, NOVEMBER 1981
very beginning is Rudolf A. Hanel of the Goddard Space Flight Center’s Laboratory for Extraterrestrial Physics. Hanel is presently in charge of the infrared spectrometer investigation on the two Voyager spacecraft.
Interferometry An FTIR spectrometer like IRIS acquires a signal called an interferogram in the time domain, instead of direct acquisition in the frequency domain, as with grating instruments, for example. An interferogram is a plot of the autocorrelation function of electromagnetic radiation as a function of time. If monochromatic light were involved, for example, the time domain signal would be a sine wave. A much more complicated interferogram results when polychromatic light is analyzed. Unfortunatelv. there is no transduc. er with a s u f f i c i h y rapid response time to follow the oscillations of elec0003-2700181~A351-1544$010010 1981 A m I a n Chemacal Soclew
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Instrumentation
I Flgute 2. IRIS spectrometer. Interferometer at center. radiometer at left
tromagnetic radiation in the infrared region. What is needed is a black box able to “slow down” the electromagnetic radiation so it can be detected, without distorting the information carried in the original signal. The Michelson interferometer operates on electromagnetic radiation to “slow it down” in just this way-a process known as modulation. As mentioned above, if a single frequency is incident on an interferometer, the time domain signal generated, the interferogram, will consist of one pure sine wave whose frequency is proportional to but much lower than the frequency of the incident radiation. But most radiation sources in the real world (or real universe) contain a mixture of many frequencies. These different frequencies added together form a complex interferogram that often looks something like a decaying transient disturbance. One might think that such a pattern could not
possibly contain any useful information. But this jumble of frequencies can be deconvoluted by the mathematical process of Fourier transformation, converting the information contained in the interferogram from the time domain to the frequency domain. A power vs. time signal is thus converted to a sDectrum of radiance vs. frequency. An excellent descriDtion of Fourier transform spectromeiry and the Michelson interferometer can be found in Reference 1. Other books containing more detailed information include References 2,s. and 4. IRIS
The IHISinstrument on the Voyager spacecraft does not operate like a conventional infrared laboratory spectrometer, with a source, a sample, and a detector, in that order. With IRIS, the sample is also the source, because the instrument measures thermal (in-
frared) emission. Since infrared emission from molecules at low temperature is very weak, it is evident why a Michelson interferometer, with its high signal-to-noise ratio, was chosen for the Voyager mission. This higher S/N ratio, the “Fellgett advantage” Michelson interferometers possess over prism or grating instruments, arises from the fact that the deteetor in the Michelson interferometer sees all spectral intervals simultaneously instead of sequentially. The instrument and its optical layout are shown in Figures 1,2, and 3. The most prominent feature in Figure 1is the spectrometer’s telescope. Radiation collected by the 50-cm-diameter primary and the secondary mirror is directed through a field stop (Figure 3) to a spherical dichroic mirror (5). The interferometer itself can he seen behind the primary mirror in Figure 1. It is also clearly visible in Figure
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
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Flgure 3. Optical layout of the Voyager IRIS. Reprinted from Reference 5 with permission of the Optical Society of America
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2. The device behind the dichroic mirror (on the left in Figure 2) is a radiometer for measurements in the visible and near-infrared regions. Radiation reflected by the spherical dichroic mirror is directed to a beam splitter (Figure 3), and thence to the fixed and stationary mirrors of the interferometer. The interferometer modulates the incident radiation by splitting it into two beams. One beam is directed to a fixed mirror and the other to a movable mirror called the Michelson mirror. The two beams are then recombined at the beam splitter, and an interference pattern generated by the interaction of the two beams is focused on a thermopile detector. For clarity, in Figure 3 both the main and reference interferometers are rotated 90’ about an axis between the main interferometer beam splitter and the dichroic mirror; also, the reference interferometer is not to scale. For the interferometer to operate properly, the instrument’s temperature must be held constant, in this case at 200 0.5 K. Therefore, the optical components of the Voyager IRIS (except the active surface of the primary) are wrapped in multilayer thermal blankets for the flight. A thermal radiator mounted on the interferometer cools the instrument by radiating to deep space. Three sets of heaters provide fine thermal control for the instrument. In addition to tberwal protection, considerable effort was invested in radiation-hardening the instrument, to ensure proper operation in the high-energy particle environment of Jupiter’s magnetosphere. The reference interferometer (Figure 3) contains a neon source, from which the 5852 8, line is isolated by a narrow-band interference filter. The moving mirror of the reference inter-
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ANALYTICAL CHEMISTRY, VOC. 53. NO. 13, NOVEMBER 1981
ferometer is operated by the same linear motor that operates the moving mirror of the main interferometer. The interferogram generated by the reference interferometer is a pure sine wave. The reference interferometer serves two functions: It provides feedback control of the Michelson mirror motion, and, by sensing the zero crosings of the neon sine wave, it initiates the sample and bold commands needed for the acquisition of discrete data points in the interferogram. Each interferogram is acquired in 45.6 s with a rate of 80 words s-l. Each interferogram thus contains 3648 digital words. Hundreds of individual spectra are averaged to produce an emission spectrum of high S/N ratio for scenes under study. The output of the detector is quantized and temporarily stored in the spacecraft for transmission to earth, where the data are processed in a digital computer. The interferogram is Fourier-transformed to yield an amplitude and a phase spectrum. The amplitude in each channel is proportional to the net spectral radiance between the instrument and the scene within the field of view. This raw amplitude vs. frequency spectrum must then be corrected for phase and calibrated against an absolute radiance source. The phase of the signal in each spectral interval of the raw amplitude spectrum depends on whether the scene is warmer or Folder than the instrument. The phase of a colder source is 180’ from the phase of a warmer source. After the amplitude spectrum has been phase-corrected, it is calibrated. Two points are used in the calibration: deep space, assumed to be a perfect beat sink, is one point, and the accurately known temperature of the instrument (200 K) is the other. With (continued on page 1550 A)
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these two calibration points, relative amplitudes can be converted to absolute radiance units. Ice Clouds on Mars The infrared emission spectrum of the atmosphere of a planet is a difficult thing to interpret, since spectral radiance reaching the interferometer is a complicated composite of emission and absorption. For instance, emitted radiation by CO2 in one atmospheric layer may be reabsorbed by C02 molecules higher up in the atmosphere. As Hanel put it, “The primary goal of remote sensing in the infrared is to extract the chemical composition and physical parameters . from the measurement of spectral radiances. In the early days of evolution of this technique, the problem was often compared to the task of unscrambling an omelet and reconstructing the eggs” ( 6 ) . One spectrum Hanel and his associates were able to unscramble is shown in Figures 4a and 4b (7). Figure 4a shows Mariner 9 IRIS measurements of the lower Arcadia region of Mars under clear weather conditions and of the Tharsis Ridge region under conditions of partial cloudiness. Although it had been suspected that clouds of the type that appeared in the Tharsis Ridge region that day were composed of water ice, no direct spectral evidence had been found. c The spectrum measured over lower I Arcadia showed an approximately constant brightness temperature, except for the C02 absorption band centered at 667 cm-1 and the rotational water vapor absorption lines below 400 cm-’. In contrast, the Tharsis Ridge spectrum exhibited a strikingly broad absorption feature extending from 550 to 950 cm-l, with a second RXC the unique, new Whatman Rapid Analysis Chromatogbroad absorption feature between 225 raphy column saves you time . and money. How? and 350 cm-’. Superimposed on the by being fast. Optimized efficiency and selectivity, but a t speeds up latter was a sharp spike near 227 to four times faster than standard LC analytical columns. This cm-l. The theoretical water ice cloud means greater throughput a t a fraction of the cost per analysis. spectrum (Figure 4b) had similar features. These spectroscopic data by being durable. Low back pressure, even at flow rates of 5.0 strongly indicated that the Tharsis ml/min, minimizes internal friction. Results: a longer-lasting column Ridge clouds were composed of water with precise quantitation, analysis after analysis. ice. by being convenient. No need for expensive, space-consuming auxVoyager iliary equipment. RAC is immediately compat@le with all standard Voyager 1 IRIS spectra of the planet LC instrumentation. And translation is fast. RAC chemistry is idenJupiter showed clear evidence of a tical to that of the columns you’re probably now using. number of compounds in the Jovian Faster speeds . , , lower costs. Compare R k with the column atmosphere, including hydrogen, you’re using. Available in highly stable Partisil 5, Partisil 5 CCS/C8 methane, acetylene, ethane, ammonia, and Partisil 5 ODs-3. phosphine, water, CH3D, and GeH4. Pearl et al. (8) painstakingly identiCall Whatman 800-631-7290 or send today for complete technical fied the presence of SO2 on Jupiter’s information. volcanic moon, Io, by comparing tiny blips on spectra from Io’s atmosphere -to peaks in a synthesized spectrum of
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During the passage of Voyager I through the Saturn system, IRIS ac-
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Wavenumber (crn-l) Figure 4. (a) Marlner 9 IRIS spectra from Mars. Arcadia spectrum is offset for clarity. (b) Theoretical water Ice clod spectrum. Brightness temperature (y axis) is a function of spectral radiance. Reprinted from Reference 7, copyright 1973 by the American Association for the Advancement of Science
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quired data on the planet, its rings, Titan, and other satellites. Infrared spectra of Saturn indicated the presence of hydrogen, methane, ammonia, phosphine, acetylene, ethane, and possibly methyl acetylene (C3H4)and propane (CsH8). On Titan, positive identifications were made for methane, acetylene, ethylene, ethane, and HCN, but again only tentative identifications could at first be made for methyl acetylene and propane. Positive identification of these last two components had to wait for a more detailed analysis of Titan spectra, as shown in Figure 5 (9). At the top of Figure 5 are spectra acquired from the center of Titan’s disk and from its north pole. At the bottom of the fiiure are spectra of propane and methyl acetylene taken in the laboratory. The average disk spectrum shows weak but definite spectral features of the propane fundamentals at 748,922,1054,and 1158 cm-I. The disk spectrum and, to a greater extent, the polar spectrum show the fundamentals of methyl acetylene at 328 cm-I and at 633.2 cm-1. These data provided convincing evidence for the existence of these gases in the atmosphere of Titan. Voyager 2 made its closest approach to Saturn on Aug. 25,1981. Due to a rare planetary alignment occurring only once every 175 years, Voyager 2 should he able to continue on to a rendezvous with Uranus in January 1986,and perhaps even Neptune in August 1989.Eventually the Voyager spacecraft and their remarkable spectrometers will leave the solar system. But instruments like IRIS will no douht fly again when new opportunities for the spectrometric exploration of space present themselves. ThanLe to Rudolf H a e l for reviewing this material and to Ron Joho of Hewlett-Packard far suggesting the topic.
References (1) Skoog, Dou las A.; West, Donald Mi, “Principles of Instrumental ~na~ysla, 2nd ed.; Saunders CollegetHolt, Rinehart and Winston: Philadelphia, 1980;
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241-54. (2) Bell, Robert John: “Introductory Fou-
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rier TransformSpectroscopy”; Academia Press: New York, 1972. (3) Griffiths, Peter R.“Chemical Infrared Fourier Transform S ectroscopy”;John Wiley & Sons: New Ark, 1975. (4) Griffiths, Peter R.,Ed. “Transform Techniques in Chemistry”;Plenum Press: New York, 1978. (5) Hanel, R. A,, et al. Appl. Opt. 1980,19,
8.0 0.0
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1391-1400. (6) Hanel, R. A., “Cpmical Eyolution 09
the Giant Planets ,Academic Press: New York, 1976; Chapter 13. (7) Curran, Robert J.,et al. Science 1973,
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Flgure 5. Comparison of ObSeNed Titan spectrum with laboratoly spectra of propane and methyl abtylene. Reprinted frcin Reference9 by permission of Macmllian Journals Ltd.
(8)Pearl, J., et al. Nature 1979,280,7558. (9) Maguire, W. C., et al. Nature 1981. 292,6834.
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