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Recent infrared spectra of Mars and Venus
d. Quant. Spectrosc. Radiat. Transfer. Vol. 3, pp. 551-558. Pergamon Press Ltd., 1963. Printed in Great Britain
RECENT INFRARED SPECTRA OF MARS AND VENUS WILLIAM M. SINTON Lowell Observatory, Flagstaff, Arizona Abstract--Infrared spectra of Mars and Venus are discussed with regard to the composition of these two planets' atmospheres. The evidence for carbon monoxide in the Venus atmosphere is presented. The transparency of the Venus cloud level is found to be less than 0.1 per cent between 3 and 4 t~. Spectra of Mars are analyzed for the COs content of that planet's atmosphere. A new instrument, a birefringent interferometer that has just been put into operation, is described briefly. THE STUDY of the infrared spectra of the planets can yield m u c h information which is not otherwise obtainable. The presence of m a n y atmospheric molecules m a y be revealed b y their vibration-rotation bands. The extension of observation farther into the infrared (i.e., into the PbS region and beyond) permits the detection of fairly minor constituents in atmospheres. Such has been the aim at Lowell Observatory and we have particularly investigated that portion of the spectrum between 2 and 4 t~. This paper will be in the nature of a progress report on our studies of Mars and Venus. Figures 1 and 2 present spectra of Venus taken with the 42 in. Lowell reflector and a prism spectrometer equipped with a liquid-nitrogen-cooled lead-sulfide detector. In Fig. 1 we see several CO2 bands in Venus at 1.96, 2.01, and 2.06/~. These bands are also visible in the solar and lunar spectrum but are much weaker. We also see in Venus spectrum a band at 2.16/~ which was earlier found by KUIPER. C1~This band has now been identified as the 2v3 band of C1201~O1S by F. A. DIAZ. ~z~ The same transition in C120216 is strictly forbidden. The twin dips at 2.35/~, which are labeled CO, appear to be real but m a y not indeed be the P and R branches of the C O harmonic b a n d as we shall see later. The spectra of Fig. 2 show the region f r o m 2.9 to 3.8/~. N o new bands in the Venus spectrum appear in this region as c o m p a r e d to the sun. However, the Venus spectrum, when c o m p a r e d to the sun, falls rather rapidly with wavelength. The spectrum of Mercury, which is also included in this illustration, shows the effect of emission of that o b j e c t - - m o s t of the light recorded here is thermal emission and not reflected sunlight. Figure 3 shows the average of 6 spectra of Venus between 2.25 and 2.42 /~ after division by the spectrum of the sun to remove terrestrial atmospheric components. The twin depression centered on 2.345/z is shown, but a comparison with the two l a b o r a t o r y spectra of 79 c m - a t m of CO in an 8-5 m cell shows that either the Venus CO is at an extremely low temperature, or that the absorption is due primarily to something else. We will not know the answer to this interesting question until spectra with resolution sufficient to resolve the rotational structure of CO are obtained. 551
552
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Recent infrared spectra of Mars and Venus I
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FIG. 3. The mean of 6 spectra of Venus after division by the solar spectrum is shown in the uppermost curve. The lower two curves are laboratory spectra of 79 cm-atm of carbon •monoxide in a 8.5 m cell. In view of the very high temperatures known to exist at the surface of Venus it would be interesting to learn whether any of this radiation escapes through windows in the near infrared. We know that it does not escape in the 8-13/~ region, but can we say anything about windows in the Venus atmosphere in the 3-4/z region where absorption by CO2 and H 2 0 would be small? On the sunlit hemisphere such emission may be hidden by overriding reflected sunlight. But one can observe the dark hemisphere at inferior conjunction. A series of measurements were made at inferior conjunction in April 1961 with the spectrometer fixed at 3.75/z. F r o m a comparison to the emission from blackbodies, temperatures of 227, 242, and 238°K were derived for the dates April 12, 15, and 17, respectively. At the succeedinginferior conjunction in November, 1962 the experiment was repeated with another wavelength setting at 3.5/~ in addition to the one at 3.75/z. The new wavelength was chosen because at 3.75 t~ there may be absorption by the wings of the 4.3 /~ CO2 band. In these measurements no definite emission from Venus was observed and the sensitivity was such that a deflexion equal to the standard deviation would have been produced by emission at 230°K at either wavelength. The temperature found in the 8-13/~ region is about 235°K and it is now believed to belong to the top of the cloud deck. F r o m the above observations we may say that this cloud deck is also opaque at 3.5 and 3.75/~. Gaps in the deck must also be less than 0.1 per cent or otherwise radiation from a 600°K surface beneath would produce significant emission. We note that Fig. 2 showed a decrease in reflectivity of the clouds toward longer wavelengths in agreement with the thermal-emission findings. Figures 4, 5, and 6 present spectra o f Mars. The first spectrum of this group shows CO2 absorptions in the 1-2.7/z region. These bands are considerably stronger than in
554
WILLIAM M.
SINTON
2.06
z.,
/
./
j
v~ LeS[
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FIG. 4. A spectrum of Mars between 1 and 2'5 t~. spectra of the sun and we shall return to this point. In this spectrum no significant indication of the CO at 2.34/~ appears. In Fig. 5 the region f r o m 2.9 to 3.8/z is shown. Here we find the absorptions by C - H linkages in the d a r k region of Syrtis Major which have been discussed earlier.Ca)The methane band at 3.3 /z appears no stronger in the Martian spectra than in the solar spectrum. In the region beyond 3.8/z (Fig. 6) we see the NzO bands of the terrestrial atmosphere. These bands are also not enhanced in the Mars spectrum. At the opposition in 1960 a special study of the 2/z CO2 bands in the spectrum of
CH4 H20
Hz0
FIG. 5. Spectra of Mars and the Sun between 2.9 and 3"8/~. The inset circles show sketches of the areas of Mars observed for the two spectra.
Recent infrared spectra of Mars and Venus
555
/
i,oo// o/r
FIG. 6. Spectra of Mars and of the Sun between 3.8 and 4.2/~.
Mars was made. At nearly the same time a number of spectra were made of the bands in the solar spectrum at varying air mass. In Fig. 7 the percentage absorption at the center of the 2.06/z band is shown. If the horizontal differences between the observed Mars absorptions and the line through the solar data are taken it is found that the absorption on Mars is equivalent to that occurring in 1.86 air masses above Palomar. 60
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6
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FIG. 7. Percentage of absorption at the center of the 2.06 ~ CO2 band versus the terrestrial air mass penetrated. Observations of Mars are designated by ~; observations of the Sun are designated b y Q
556
WILLIAMM. SINTON
The dashed line in Fig. 7 is displaced by 1.86 air-masses f r o m the line through the solar data. We now assume that the band absorption on Mars varies with the same relationship on total gas pressure as it does with the total a m o u n t of CO2. F o r example, if the absorption varies as the square root of the a m o u n t of CO2, it is assumed that it also varies as the square root of the total pressure. The total pressure on Mars is a b o u t 0.1 of that at Palomar. F r o m the equivalence assumption the a m o u n t of CO2 in the total observed p a t h through the Martian atmosphere is 18.6 times the a m o u n t above Palomar. Since the Martian atmosphere is traversed twice this becomes 9.3 times the a m o u n t of CO2 above Palomar, which yields 17.4 m - a t m of CO2 on Mars. This a m o u n t m a y be compared with the result (29 m-atm) obtained by GRANDJEAN and GOODY(4) f r o m KUIPER'S (5) earlier observations of the 1.6/z COz bands.
Fic. 8. The principle of the birefringent interferometer. The upper graphs show the type of fringe patterns recorded by each of the detectors and also by the difference of the signals from the two detectors. Another experiment that was tried at the same time was an attempt to determine the way in which the 2/z bands varied with the distance f r o m the limb on Mars, i.e., with the Martian air mass. The spectrometer was fixed at 2.0/z and scans were made across the disc with an aperture whose size was about one-fifth of the disc. These scans were repeated with the spectrometer at 2-2/z. N o detectable difference between the two sets of curves was found, but this is not surprising. The penetrated air mass does not reach large values except at the extreme limb. But here the large-size aperture included too m a n y values of low air-mass nearer to the center of the disc. At Lowell Observatory we are now attempting to perform two-beam infrared interferometry of the planets and of cool stars between 2 and 4/z. Our approach to interferometry has been rather different f r o m most other approaches. The light is polarized parallel to the fast and slow axes of a birefringent material and the planes of polarization are recombined on emergence to produce interference as has been done by L. MERTZ.t6~ The principle of operation is shown in Fig. 8. The light is polarized at 45 ° to the optical
Recent infrared spectra of Mars and Venus
557
axis of a rutile Soliel prism compensator. The p a t h difference that is introduced by the Soliel prism is partially reduced by a rutile bias plate, which has its axis at 90 ° to the axis of the compensator. The emerging light is analyzed by a rutile Wollaston prism. Both beams emerging f r o m the Wollaston prism are detected. One b e a m yields fringes which have a bright zero-path-difference fringe, and the other b e a m has a dark fringe at zero p a t h difference. The difference of the two detected signals is recorded. F o r a nearly continuous source the difference signal is near zero for most of the interferogram. In this way most og the harmful effects of inadequate guiding or of atmospheric transmission fluctuations are removed. Fig. 9 shows the scheme of the constructed instrument. TWO PARALLEL
MOVABLE PRISM
LIGHT B E A M S
ROT,LE
I
PLATE
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FIG. 9. The birefdngent intefferometer as constructed.
2.16
VENUS RUN 009 OCT. 2 4, 1962 FIG. 10. One of the first computed spectra which was obtained from an interferogram of Venus that was recorded with the interferometer attached to the Lowell 42 in. telescope. The automatic data plotter which produced this curve did not reproduce the calculated points with sufficient vertical resolution. Wollaston prisms are used as polarizers in order to utilize all of the available light. The polarized beams f r o m the initial Wollaston prism on the left traverse identical sets of Soliel prisms and bias plates. The final Wollaston prism at the right serve to recombine the diverging beams of opposite polarization so that an optimumly designed condenser can be used with detectors of the smallest p o s s ~ l e size. Figure 10 shows a spectrum of Venus (one of the first) which was obtained with the instrument when it was attached to the Lowell 42 in. telescope. This is an unapodized transform of the interferogram. M a n y improvements in the operation of the instrument have been made since the taking of the early interferogram, but these later observations have not yet been computed. A recent interferogram which was obtained of Mars on M a r c h 1, 1963 with the
558
WILLIAM M. SINTON
i n t e r f e r o m e t e r a t t a c h e d to the 200 in. H a l e t e l e s c o p e is s h o w n in Fig. 11. T h e l o w e r c u r v e s h o w s t h e r e c o r d i n g o f the difference signal o f the t w o d e t e c t o r s while the u p p e r c u r v e s h o w s the s u m o f the t w o signals. T h e s u m signal shows the v a r i a t i o n o f t h e t o t a l i n t e r f e r o m e t e r energy. T h i s v a r i a t i o n r e s u l t e d f r o m t r y i n g to guide w i t h a s m a l l a p e r t u r e o n t h e l i m b o f t h e p l a n e t a n d b e c a u s e the o b s e r v a t i o n was m a d e t h r o u g h light c i r r u s c l o u d s . T h e curves were r e p r o d u c e d f r o m the t r a c i n g o n a t w o - c h a n n e l r e c o r d i n g p o t e n t i o m e t e r . T h e difference: signal was also d i g i t i z e d a n d r e c o r d e d o n m a g n e t i c t a p e f o r s u b s e q u e n t processing.
\ A-B
k
MARS (SOUTH LIMB)
FIG. 11. A recent interferogram of Mars that was obtained with the interferometer attached to the 200 in. Hale telescope. The upper curve shows the sum-signal from the two detectors and illustrates the variation of the total energy through the interferometer. The lower curve shows the difference signal and shows how the fringes are relatively unaffected by the fluctuations of the total energy. I n m y o p i n i o n t h e i n t e r f e r o m e t e r offers the g r e a t e s t p r o m i s e f o r m o d e r a t e - r e s o l u t i o n s p e c t r a o f s m a l l a r e a s o f b r i g h t p l a n e t s a n d also f o r m o d e r a t e - r e s o l u t i o n s p e c t r a o f v i s u a l l y - f a i n t l a t e - t y p e stars. I n t e r e s t i n g i n t e r f e r o m e t r i c tracings were o b t a i n e d at P a l o m a r o f l a t e - t y p e stars o f 9th visual m a g n i t u d e . I say interesting b e c a u s e in o r d e r f o r t h e stars to h a v e p r o d u c e d t h e interf.erograms t h e y m u s t have h a d c o n s i d e r a b l e b a n d a b s o r p t i o n in their spectra. I r e g r e t t h a t t h e d i s c u s s i o n o f the i n t e r f e r o m e t e r c o u l d n o t h a v e given m o r e c o m p u t e d results b u t t i m e has n o t b e e n a v a i l a b l e to get m o r e o f the o b s e r v a t i o n s c o m p u t e d . Acknowledgements--I am indebted to Miss ANN GEOFI~RIONwho assisted with the interferometric observations and who worked out the program for the reductions. I am also indebted to the Air Force Cambridge Research Laboratories who supported the observational program through their contract A F 19(604)-5874.
REFERENCES 1, G. P. KtJIPER, The Atmospheres of the Earth and Planets, revised, p. 355, University Press, Chicago (1952). 2. F. A. DIAZ, J. Opt. Soc. hmer. 53, 203 (1963). 3. W. M. SINTON, Science, 130, 1234 (1959). 4. J. GRANDJEANand R. M. GOODY,Astrophys. J. 121,548 (1955). 5. G. P. KUIPER, The Atmospheres of the Earth and Planets, Revised ed. p. 358, University Press, Chicago (1952). 6. L. MERTZ, J. Phys. Radium 19, 233 (1958).
RECENT INFRARED SPECTRA OF MARS AND VENUS WILLIAM M. SINTON Lowell Observatory, Flagstaff, Arizona Abstract--Infrared spectra of Mars and Venus are discussed with regard to the composition of these two planets' atmospheres. The evidence for carbon monoxide in the Venus atmosphere is presented. The transparency of the Venus cloud level is found to be less than 0.1 per cent between 3 and 4 t~. Spectra of Mars are analyzed for the COs content of that planet's atmosphere. A new instrument, a birefringent interferometer that has just been put into operation, is described briefly. THE STUDY of the infrared spectra of the planets can yield m u c h information which is not otherwise obtainable. The presence of m a n y atmospheric molecules m a y be revealed b y their vibration-rotation bands. The extension of observation farther into the infrared (i.e., into the PbS region and beyond) permits the detection of fairly minor constituents in atmospheres. Such has been the aim at Lowell Observatory and we have particularly investigated that portion of the spectrum between 2 and 4 t~. This paper will be in the nature of a progress report on our studies of Mars and Venus. Figures 1 and 2 present spectra of Venus taken with the 42 in. Lowell reflector and a prism spectrometer equipped with a liquid-nitrogen-cooled lead-sulfide detector. In Fig. 1 we see several CO2 bands in Venus at 1.96, 2.01, and 2.06/~. These bands are also visible in the solar and lunar spectrum but are much weaker. We also see in Venus spectrum a band at 2.16/~ which was earlier found by KUIPER. C1~This band has now been identified as the 2v3 band of C1201~O1S by F. A. DIAZ. ~z~ The same transition in C120216 is strictly forbidden. The twin dips at 2.35/~, which are labeled CO, appear to be real but m a y not indeed be the P and R branches of the C O harmonic b a n d as we shall see later. The spectra of Fig. 2 show the region f r o m 2.9 to 3.8/~. N o new bands in the Venus spectrum appear in this region as c o m p a r e d to the sun. However, the Venus spectrum, when c o m p a r e d to the sun, falls rather rapidly with wavelength. The spectrum of Mercury, which is also included in this illustration, shows the effect of emission of that o b j e c t - - m o s t of the light recorded here is thermal emission and not reflected sunlight. Figure 3 shows the average of 6 spectra of Venus between 2.25 and 2.42 /~ after division by the spectrum of the sun to remove terrestrial atmospheric components. The twin depression centered on 2.345/z is shown, but a comparison with the two l a b o r a t o r y spectra of 79 c m - a t m of CO in an 8-5 m cell shows that either the Venus CO is at an extremely low temperature, or that the absorption is due primarily to something else. We will not know the answer to this interesting question until spectra with resolution sufficient to resolve the rotational structure of CO are obtained. 551
552
WILLIAM M. SINTO~
f~ o
.
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1 f~t,,I
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~.
Recent infrared spectra of Mars and Venus I
553
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I
4
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2.3p
FIG. 3. The mean of 6 spectra of Venus after division by the solar spectrum is shown in the uppermost curve. The lower two curves are laboratory spectra of 79 cm-atm of carbon •monoxide in a 8.5 m cell. In view of the very high temperatures known to exist at the surface of Venus it would be interesting to learn whether any of this radiation escapes through windows in the near infrared. We know that it does not escape in the 8-13/~ region, but can we say anything about windows in the Venus atmosphere in the 3-4/z region where absorption by CO2 and H 2 0 would be small? On the sunlit hemisphere such emission may be hidden by overriding reflected sunlight. But one can observe the dark hemisphere at inferior conjunction. A series of measurements were made at inferior conjunction in April 1961 with the spectrometer fixed at 3.75/z. F r o m a comparison to the emission from blackbodies, temperatures of 227, 242, and 238°K were derived for the dates April 12, 15, and 17, respectively. At the succeedinginferior conjunction in November, 1962 the experiment was repeated with another wavelength setting at 3.5/~ in addition to the one at 3.75/z. The new wavelength was chosen because at 3.75 t~ there may be absorption by the wings of the 4.3 /~ CO2 band. In these measurements no definite emission from Venus was observed and the sensitivity was such that a deflexion equal to the standard deviation would have been produced by emission at 230°K at either wavelength. The temperature found in the 8-13/~ region is about 235°K and it is now believed to belong to the top of the cloud deck. F r o m the above observations we may say that this cloud deck is also opaque at 3.5 and 3.75/~. Gaps in the deck must also be less than 0.1 per cent or otherwise radiation from a 600°K surface beneath would produce significant emission. We note that Fig. 2 showed a decrease in reflectivity of the clouds toward longer wavelengths in agreement with the thermal-emission findings. Figures 4, 5, and 6 present spectra o f Mars. The first spectrum of this group shows CO2 absorptions in the 1-2.7/z region. These bands are considerably stronger than in
554
WILLIAM M.
SINTON
2.06
z.,
/
./
j
v~ LeS[
\ H,(~
v
J GAIN X2
FIG. 4. A spectrum of Mars between 1 and 2'5 t~. spectra of the sun and we shall return to this point. In this spectrum no significant indication of the CO at 2.34/~ appears. In Fig. 5 the region f r o m 2.9 to 3.8/z is shown. Here we find the absorptions by C - H linkages in the d a r k region of Syrtis Major which have been discussed earlier.Ca)The methane band at 3.3 /z appears no stronger in the Martian spectra than in the solar spectrum. In the region beyond 3.8/z (Fig. 6) we see the NzO bands of the terrestrial atmosphere. These bands are also not enhanced in the Mars spectrum. At the opposition in 1960 a special study of the 2/z CO2 bands in the spectrum of
CH4 H20
Hz0
FIG. 5. Spectra of Mars and the Sun between 2.9 and 3"8/~. The inset circles show sketches of the areas of Mars observed for the two spectra.
Recent infrared spectra of Mars and Venus
555
/
i,oo// o/r
FIG. 6. Spectra of Mars and of the Sun between 3.8 and 4.2/~.
Mars was made. At nearly the same time a number of spectra were made of the bands in the solar spectrum at varying air mass. In Fig. 7 the percentage absorption at the center of the 2.06/z band is shown. If the horizontal differences between the observed Mars absorptions and the line through the solar data are taken it is found that the absorption on Mars is equivalent to that occurring in 1.86 air masses above Palomar. 60
i
i
,
/
5C o
j ~z
f
40
G~
~ 30
g n-
O
/°
20
2.06p. I0
O
I
i
i
L
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r
I
2
3
4
5
6
AIR
7
MASS
FIG. 7. Percentage of absorption at the center of the 2.06 ~ CO2 band versus the terrestrial air mass penetrated. Observations of Mars are designated by ~; observations of the Sun are designated b y Q
556
WILLIAMM. SINTON
The dashed line in Fig. 7 is displaced by 1.86 air-masses f r o m the line through the solar data. We now assume that the band absorption on Mars varies with the same relationship on total gas pressure as it does with the total a m o u n t of CO2. F o r example, if the absorption varies as the square root of the a m o u n t of CO2, it is assumed that it also varies as the square root of the total pressure. The total pressure on Mars is a b o u t 0.1 of that at Palomar. F r o m the equivalence assumption the a m o u n t of CO2 in the total observed p a t h through the Martian atmosphere is 18.6 times the a m o u n t above Palomar. Since the Martian atmosphere is traversed twice this becomes 9.3 times the a m o u n t of CO2 above Palomar, which yields 17.4 m - a t m of CO2 on Mars. This a m o u n t m a y be compared with the result (29 m-atm) obtained by GRANDJEAN and GOODY(4) f r o m KUIPER'S (5) earlier observations of the 1.6/z COz bands.
Fic. 8. The principle of the birefringent interferometer. The upper graphs show the type of fringe patterns recorded by each of the detectors and also by the difference of the signals from the two detectors. Another experiment that was tried at the same time was an attempt to determine the way in which the 2/z bands varied with the distance f r o m the limb on Mars, i.e., with the Martian air mass. The spectrometer was fixed at 2.0/z and scans were made across the disc with an aperture whose size was about one-fifth of the disc. These scans were repeated with the spectrometer at 2-2/z. N o detectable difference between the two sets of curves was found, but this is not surprising. The penetrated air mass does not reach large values except at the extreme limb. But here the large-size aperture included too m a n y values of low air-mass nearer to the center of the disc. At Lowell Observatory we are now attempting to perform two-beam infrared interferometry of the planets and of cool stars between 2 and 4/z. Our approach to interferometry has been rather different f r o m most other approaches. The light is polarized parallel to the fast and slow axes of a birefringent material and the planes of polarization are recombined on emergence to produce interference as has been done by L. MERTZ.t6~ The principle of operation is shown in Fig. 8. The light is polarized at 45 ° to the optical
Recent infrared spectra of Mars and Venus
557
axis of a rutile Soliel prism compensator. The p a t h difference that is introduced by the Soliel prism is partially reduced by a rutile bias plate, which has its axis at 90 ° to the axis of the compensator. The emerging light is analyzed by a rutile Wollaston prism. Both beams emerging f r o m the Wollaston prism are detected. One b e a m yields fringes which have a bright zero-path-difference fringe, and the other b e a m has a dark fringe at zero p a t h difference. The difference of the two detected signals is recorded. F o r a nearly continuous source the difference signal is near zero for most of the interferogram. In this way most og the harmful effects of inadequate guiding or of atmospheric transmission fluctuations are removed. Fig. 9 shows the scheme of the constructed instrument. TWO PARALLEL
MOVABLE PRISM
LIGHT B E A M S
ROT,LE
I
PLATE
/,~'~--",. /
I
I/I
~ " ~ ,~RRoMA'
.
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J,~---t-'71
ACHROMATS
F;XEO PR,SM
FIG. 9. The birefdngent intefferometer as constructed.
2.16
VENUS RUN 009 OCT. 2 4, 1962 FIG. 10. One of the first computed spectra which was obtained from an interferogram of Venus that was recorded with the interferometer attached to the Lowell 42 in. telescope. The automatic data plotter which produced this curve did not reproduce the calculated points with sufficient vertical resolution. Wollaston prisms are used as polarizers in order to utilize all of the available light. The polarized beams f r o m the initial Wollaston prism on the left traverse identical sets of Soliel prisms and bias plates. The final Wollaston prism at the right serve to recombine the diverging beams of opposite polarization so that an optimumly designed condenser can be used with detectors of the smallest p o s s ~ l e size. Figure 10 shows a spectrum of Venus (one of the first) which was obtained with the instrument when it was attached to the Lowell 42 in. telescope. This is an unapodized transform of the interferogram. M a n y improvements in the operation of the instrument have been made since the taking of the early interferogram, but these later observations have not yet been computed. A recent interferogram which was obtained of Mars on M a r c h 1, 1963 with the
558
WILLIAM M. SINTON
i n t e r f e r o m e t e r a t t a c h e d to the 200 in. H a l e t e l e s c o p e is s h o w n in Fig. 11. T h e l o w e r c u r v e s h o w s t h e r e c o r d i n g o f the difference signal o f the t w o d e t e c t o r s while the u p p e r c u r v e s h o w s the s u m o f the t w o signals. T h e s u m signal shows the v a r i a t i o n o f t h e t o t a l i n t e r f e r o m e t e r energy. T h i s v a r i a t i o n r e s u l t e d f r o m t r y i n g to guide w i t h a s m a l l a p e r t u r e o n t h e l i m b o f t h e p l a n e t a n d b e c a u s e the o b s e r v a t i o n was m a d e t h r o u g h light c i r r u s c l o u d s . T h e curves were r e p r o d u c e d f r o m the t r a c i n g o n a t w o - c h a n n e l r e c o r d i n g p o t e n t i o m e t e r . T h e difference: signal was also d i g i t i z e d a n d r e c o r d e d o n m a g n e t i c t a p e f o r s u b s e q u e n t processing.
\ A-B
k
MARS (SOUTH LIMB)
FIG. 11. A recent interferogram of Mars that was obtained with the interferometer attached to the 200 in. Hale telescope. The upper curve shows the sum-signal from the two detectors and illustrates the variation of the total energy through the interferometer. The lower curve shows the difference signal and shows how the fringes are relatively unaffected by the fluctuations of the total energy. I n m y o p i n i o n t h e i n t e r f e r o m e t e r offers the g r e a t e s t p r o m i s e f o r m o d e r a t e - r e s o l u t i o n s p e c t r a o f s m a l l a r e a s o f b r i g h t p l a n e t s a n d also f o r m o d e r a t e - r e s o l u t i o n s p e c t r a o f v i s u a l l y - f a i n t l a t e - t y p e stars. I n t e r e s t i n g i n t e r f e r o m e t r i c tracings were o b t a i n e d at P a l o m a r o f l a t e - t y p e stars o f 9th visual m a g n i t u d e . I say interesting b e c a u s e in o r d e r f o r t h e stars to h a v e p r o d u c e d t h e interf.erograms t h e y m u s t have h a d c o n s i d e r a b l e b a n d a b s o r p t i o n in their spectra. I r e g r e t t h a t t h e d i s c u s s i o n o f the i n t e r f e r o m e t e r c o u l d n o t h a v e given m o r e c o m p u t e d results b u t t i m e has n o t b e e n a v a i l a b l e to get m o r e o f the o b s e r v a t i o n s c o m p u t e d . Acknowledgements--I am indebted to Miss ANN GEOFI~RIONwho assisted with the interferometric observations and who worked out the program for the reductions. I am also indebted to the Air Force Cambridge Research Laboratories who supported the observational program through their contract A F 19(604)-5874.
REFERENCES 1, G. P. KtJIPER, The Atmospheres of the Earth and Planets, revised, p. 355, University Press, Chicago (1952). 2. F. A. DIAZ, J. Opt. Soc. hmer. 53, 203 (1963). 3. W. M. SINTON, Science, 130, 1234 (1959). 4. J. GRANDJEANand R. M. GOODY,Astrophys. J. 121,548 (1955). 5. G. P. KUIPER, The Atmospheres of the Earth and Planets, Revised ed. p. 358, University Press, Chicago (1952). 6. L. MERTZ, J. Phys. Radium 19, 233 (1958).