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Detection of a CH4 atmosphere on Pluto
ICARUS 44, 62--71 (1980)
Detection of a OH4 Atmosphere on Pluto U W E F I N K , B R A D F O R D A. S M I T H , D. C H R I S B E N N E R , 1 J A M E S R. J O H N S O N , H. J. R E I T S E M A Lunar and Planetary l, aboratory. University of Arizona, Tucson, Arizona 85721 AND
J A M E S A. W E S T P H A L Hale Observatories. ('alifornia Institute of Technology Pasadena, California 91109 Received August 11, 1980 Using a low-resolution spectrograph and a C C D array, a s p e c t r u m o f Pluto from 0.58 to 1.06 #,m was obtained. T h e s p e c t r u m had a resolution o f -25 A and a signal-to-noise ratio of - 3 0 0 . It s h o w e d CH4 absorption b a n d s at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 A. T h e strongest o f t h e s e b a n d s was at 8900 A with an absorption depth o f 0.23. This band was heavily saturated, c o m p a r e d to the weaker bands, providing proof for the g a s e o u s origin of the o b s e r v e d absorptions. By applying CH4 band model p a r a m e t e r s to our data, a total CH4 a b u n d a n c e o f 80 ± 20 m - a m was derived. This translates into a o n e - w a y a b u n d a n c e of 27 ± 7 m - a m and a CH4 surface pressure of 1.5 × 10-4 atm. An upper limit to the total p r e s s u r e of ,-0.05 atm could be set. First-order calculations on atmospheric escape showed that this m e t h a n e a t m o s p h e r e would be stable if the m a s s o f Pluto is increased 50% o v e r its current value and its radius is 1400 kin. Alternatively a heavier gas mixed with the CH4 a t m o s p h e r e would aid its stability. T h e relatively large a m o u n t o f g a s e o u s CH4 o b s e r v e d implies that the absorption b a n d s recently reported at 1.7 and 2.3 p,m are likely due to a t m o s p h e r i c CH~ absorptions rather than surface frost as interpreted earlier.
spectrum of Pluto which clearly shows the presence of a CH4 a t m o s p h e r e , and we describe our o b s e r v a t i o n s and their interpretation in this paper.
INTRODUCTION
A few years ago we obtained an imagetube s p e c t r u m o f Pluto (Benner et al., 1978) which gave indications of a m e t h a n e atmosphere on Pluto and, to a m u c h lesser degree of certainty, on Triton. Unfortunately the image tube response fell off sharply near 9000 A, preventing adequate c o v e r a g e of the strongest CH4 band below 1 tzm, at 8900 A. Since then, Si array detectors have been developed which have longer wavelength r e s p o n s e and provide good c o v e r a g e o f the 8900-A CH4 band. To confirm or refute our earlier conclusions, we have built a low-resolution spectrograph to which we attached a C C D (charge coupled device) array. We obtained a good
OBSERVATIONS, INSTRUMENTATION. AND DATA REDUCTION
The observations were conducted at the 155-cm Catalina O b s e r v a t o r y telescope. An instrument test run was carried out 1980 February 22 and immediately yielded good e x p o s u r e s of Pluto. T h e s e spectra clearly showed the presence of CH4 bands. After i m p r o v e m e n t s to the instrument and refinements to the data reduction procedure, Pluto was again o b s e r v e d 1980 May 4 from 7:30 to 8:30 UT. Three good exposures of 5 m and two of 10m were obtained. A n u m b e r of stars were o b s e r v e d and two e x p o s u r e s each of the G - t y p e stars SAO 120173 and 120940 were selected for corn-
I Present address: N A S A - A m e s Research Center, Astrophysical e x p e r i m e n t s b r a n c h N245-6, Moffett Field, Calif. 94035. 62
0019-1035/80/100062-10502.00/0 Copyright ~) 1980by Academic Press, Inc. All rights of reproduction in any fon'a reserved.
CH 4
ATMOSPHERE ON PLUTO
parison with Pluto. T h e average Pluto air m a s s was 1.21 and that of the c o m p a r i s o n stars 1.25. The instrumentation consisted of a C C D array attached to a spectrograph. The C C D was m a n u f a c t u r e d by T e x a s I n s t r u m e n t s and contained 500 x 500 elements, each being 15.2/zm square. During its operation it was cooled to - 1 2 0 ° C to minimize dark noise. The detector and associated electronics were part o f a ground-based system built at the California Institute o f Technology for use by the Space T e l e s c o p e Wide Field Planetary C a m e r a investigation definition team. 2 T h e spectrograph was built at the L u n a r and Planetary L a b o r a t o r y and was designed to m a k e o p t i m u m use o f the C C D array. It e m p l o y e d a transmission grating and had a spectral range 5700-11,000 A and a m a x i m u m first-order resolution o f 11 /~. The long-wavelength limit w a s determined by the C C D sensitivity cutoff and in practice was close to 1.06 tzm as can be seen in Fig. 2. The short-wavelength limit followed the sharp cutoff o f a Schott O G 570 filter which eliminated overlapping orders. The object to be o b s e r v e d was focused on the spectrograph entrance slit which was in turn imaged on the C C D array. F o r m o s t e x p o s u r e s the entrance slitwidth was 0.5 m m which c o r r e s p o n d s to 5" field o f view with the f / 1 3 . 5 secondary ot" the telescope. The slit length was 18 m m (18if'), thus giving good c o v e r a g e of the sky background. A slice across the s p e c t r u m at 8000 A, perpendicular to the direction of dispersion is shown for Pluto and a c o m p a r ison star in Fig. 1. For Pluto the sky background shows up distinctly a b o v e the DC offset count o f the CCD. It is, h o w e v e r , less than one-tenth of the Pluto signal. The star essentially exhibits only the dc bias o f the CCD. T h e halfwidth of the star signal is about 3", and its 2Team members are: W. B. Baum, A. D. Code, D. G. Currie, G. E. Danielson, J. E. Gunn, T. Kelsall, J. Kristian, C. R. Lynds, P. K. Seid¢lman, B. A. Smith, and J. A. Westphal.
SAO
63
120173
PLUTO
-60
-40
t
-20 0 20 row n u m b e r
40
60
-,~-~-~ -~ -~ ~ ~ ~ ~ ~ ,b i
, i J -80-60-40-20
m m olong shl t I i i i 0 20 40 60 orc-sec in sky
i 80
,
FIG. 1. Slice across the spectrum perpendicular to the direction of dispersion at 8000 A, for a comparison star and a Pluto exposure. Ordinate is linear in intensity and has been normalized to one. Abscissa can be expressed in terms of row number of the CCD array, millimeters along the slit, or arc-second field of view in the sky. apparent full-width at the base is 10". T h e s e widths result from the combination of seeing, guiding, and the finite resolution o f the c a m e r a lens. The width of Pluto is almost identical to that of the star, showing that tracking and guiding during the Pluto exposure were excellent. At the two ends o f the s p e c t r u m the image widths were slightly larger due to optical aberrations and imperfect alignment. F o r the data reduction, the Pluto counts were s u m m e d o v e r 11 pixels at each column (or wavelength) thus collecting all o f the Pluto signal. N e x t the sky and dc offset were subtracted by using the 5 pixels on both sides of Pluto. The five Pluto spectra w e r e then averaged with weights assigned according to their m a x i m u m signal level. The standard stars were reduced in the same m a n n e r and the Pluto average was then divided by the star average. The w a v e length scale was derived from helium and
64
FINK ET AL.
xenon comparison spectra exposed during 11 pixeis in common. The CCD sensitivity the run. The oxygen A band at 7620 ~k variations were therefore canceled to a high served as a fiducial mark for the wavelength degree. calibration. Sensitivity variations were not taken into The resulting Pluto and star averages and account when the sky signal was subtracted their ratio are shown in Fig. 2. Both the from the Pluto signal. However, as can be Pluto and standard-star spectra contain the seen in Fig. 1 RMS variations in the sky sensitivity variations which exist across the background are - 0 . 6 % which is reduced to CCD array. These variations are removed -0.2% after l l object and 10 sky pixels only in the ratio. Great care was therefore have been averaged. A sky-only exposure taken to place the Pluto and comparison- showed that no undue sensitivity fluctuastar spectra on identical locations on tion occurred at the position of the Pluto the CCD. The spectrum centerline of the spectrum. Pluto exposures and the comparison stars The expected maximum signal to rmsall fell within plus or minus one pixel of noise ratio from photon statistics is 400. each other. Three of the Pluto spectra were Additional noise is introduced by the sky at identical locations as the comparison subtraction, but as discussed above, this stars, while the other two still had l0 out of should be somewhat below the photon noise. Outright pixel defects, cosmic ray strikes, or larger than average sensitivity Pluto / .S"~""'~$//) 1 variations in any pixel used during the data reduction (object, comparison, or sky for each) can introduce additional noise. About , l m a dozen serious defects could easily be recognized and have been left blank in the spectra and their ratio in Fig. 2. Additional 6000 8000 tO00O smaller defects however are still present in coo. s,:7 % 4~ some places of the spectra and raise the I Av ,¥ 1 noise level somewhat above the statistical one. Ignoring the readily recognized larger i / of these, we measured a peak-to-peak signal-to-noise ratio of -100 (300 rms) which -- .-. _;::==,..I is quite close to the expected statistical 6000 8000 0000 photon noise value. It is this high signal-toRofio noise ratio which, we believe, is responsi- ~ "-*". . . . . x~. . . . ~m'~,,.\., v . " f " ~ l ~ ! ble for the detection of the weak CH4 bands i not observed until now. The spectral resol lution is given by the half-width of the Pluto seeing disk (cf. Fig. l) and is -25 ]k. This is quite sutticient for the relatively broad CH.8 ....... 6~oo . . . . . 8~obo . . . . . . . . . . -co'oo ~ Wovelength features investigated. The Pluto ratio spectrum in Fig. 2 is quite Fu(;. 2. The average spectrum of Pluto, compariflat showing a slight Pluto reddening as son stars and their ratio. Ordinate is linear in intensity and is normalized to the maximum signal for measured by an intensity ratio of 1.06 bePluto and the comparison star. The ratio spectrum tween 9000 and 6000 A. This was checked is one where there are no absorptions. The Pluto by comparing the two SAO stars used in the and comparison-star spectra contain the sensitivity ratio to the stars BS 5183 (dG2),72 Her variations across the CCD. Serious CCD defects (G212), and BS 5384 (dG3) which were h a v e b e e n d e l e t e d a n d a r e left a s b l a n k s in t h e s p e c observed during a run 1980 June. The latter tra. .
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i
CH~ ATMOSPHERE ON PLUTO
65
stars have accurate broadband UBVRI, or density was so high that the absorptions narrowband photometry (Johnson et al., exhibited a pseudocontinuum behavior, 1966; Wamsteker, 1975) and follow the and only the strongest bands at 8900 and color of the Sun very closely. It was found 10,000 ,~ displayed a weak pressure effect. This situation is expected to change for that the SAO stars used in our ratio were definitely G-type stars but were slightly less the much lower temperature on Pluto red than the Sun. Thus the slope in Pluto's (-60°K). The pressure coefficient is defined ratio spectrum of Fig. 2 should probably be as the ratio of the line pressure broadening increased to 1.25 _ 0.10 between 9000 and coefficient, (3'L, cm-~/atm) divided by the 6000/~. This agrees quite well with avail- mean line spacing (8, cm-t). Because of the able VIR photometry (Harris, 1961). With redistribution of the total band intensity this slight reddening correction, we feel into a few low J lines, at low temperature, that our spectrum accurately represents the average line spacing is expected to Pluto's relative albedo from 0.6to 1.0/.tm at increase considerably. Estimates made by Benner (1979) for Uranus at a temperature 25-/~ resolution. of -80°K show that 8 should increase by a factor of 2-10 depending on the model ANALYSIS assumptions. For Pluto at 60°K higher In addition to the slight slope, Pluto's values can be expected. The band-model absorption coefficients, ratio spectrum clearly shows methane bands at 6200, 7200, 7900, 8400, 8600, 8900, on the other hand, are expected to display and 10,000 A. The strongest of these bands only minor changes for the lower Pluto is at 8900 A, as expected. The absorption temperatures. The integrated absorption depth for this band is 0.23, which is very strength of a CH4 band, arising from the close to that of our earlier image tube data ground state, does not change with temper(Benner et al., 1978). However, the high ature, and the absorptions below 1 p.rn are quality of our present data set also brings known to consist of hundreds of more or out all the other methane absorptions. Par- less randomly spaced and overlapping ticularly, the presence of the weak 6200-A bands. Experimental proof is provided by band was a pleasant surprise. These weak the measurement of the absorption bands allow us to determine a good meth- strengths of liquid CH4 by Ramaprasad et ane abundance, independent of the model al. (1978), who found excellent agreement employed. The relatively large 0.3-m ampli- between their data and the room-temperatude of Pluto's light curve (Neffet al., 1974; ture gas results. We thus have all the parameters for a Tedesco and Tholen, 1980) gives indications that we are seeing through a clear Voigt profile band-model calculation which atmosphere and a reflecting-layer model automatically includes the Lorentz and will be used in the following calculations. Doppler profile limits. The absorption The analysis made use of the CH4 band- coefficient is that determined by Fink et al. model coefficients from 4500 to 10,500 A (1978). The average CH4 Lorentz pressure measured by us a few years ago (Dick and broadening coefficient (3'L) is --0.085 Fink, 1977; Fink et al., 1977) and subse- cm-l/atm at room temperature (Rank et al. 1960) and was converted to 0.25 cm-I/atm quently by Giver (1978). In the band-model theory (cf. Goody, 1964), the methane ab- at 80°K (Varanasi et al., 1973). The Doppler sorption in a small wavelength interval is half-width is a function of wavelength and characterized by an absorption coefficient at 60°K and 8900 A is 0.0094 cm-L The k~, (km-am) -1 and a pressure coefficient y~ mean line spacing (8) for the 8900-/~ band atm-'. For the complex CH4 absorptions was found to be -0.0061 cm -~ at room below 1 p.m, Fink et al. found that the line temperature (Fink et al., 1978), but needs
66
FINK ET AL.
to be increased by a factor m, which was left as a free parameter. A number of model calculations varying the input parameters were carried out, and the best fit is compared to our Pluto ratio spectrum in Fig. 3. The calculations were made with an input parameter spacing of 10 A and thus have slightly higher resolution than the Pluto observations. The small discrepancy in the middle of the 8900-,~ band can probably be ascribed to a slight change in the shape of the absorption coefficient curve at lower temperatures, which can result in sharper peaks and a change in their relative intensity. Otherwise the synthetic CH4 spectrum fits the Pluto ratio very well. The abundance of CH4 was determined by the weak bands at 6200, 7900, and 8400/~ and yielded a value of 80 _ 20 m-am of CH4 for the two-way transmission. These bands will remain on the linear portion of the curve of growth regardless of the model used and independent of any reasonable value of the line spacing. They thus yield a good measure of the CH4 abundance on Pluto. Assuming a Lambert sphere with an air mass factor for the whole disk of 3, a one-way CH4 abundance of 27 _+ 7 m-am results. While the weak bands used for the total CH~ determination are linear with abundance, the strong bands definitely exhibit effects of saturation. A simple calculation shows that the 8900-A band, if it were linear, should have a transmission of only 8% with 80 m-am of CH4, rather than the 77% actually observed. The room-temperature value of the line spacing only increases the transmission to 16%. In order to match the Pluto data the line spacing had to be increased to 0.085, 0.10, 0.17, and 0.17 cm -1 for the 10,000, 8900, 8600, and 7200/~ regions, respectively. These results imply an increase in the line spacing by a factor of 10, 16.5, and 12 over the room-temperature value for the first three bands, which compares quite favorably to the expectation from theory. For the 7200-A region the mean line spacing came out somewhat
larger than might have been expected from the greater complexity of this band (Dick and Fink, 1977) but since no room-temperature values have been measured a direct comparison was not possible. Under the assumption of a clear atmosphere and a reflecting-layer model, the Pluto data thus allowed us to obtain average line spacings for CH4 at 60°K which would be a difficult feat in the laboratory. The saturation of the strong bands provides persuasive evidence that we are observing gaseous CH4 absorptions rather than liquid or solid features. In both the liquid and solid state, CH~ possesses a true continuum absorption coefficient for which the absorptions are linear with abundance and exhibit no saturation effect. An additional argument is provided by the amount of solid or liquid needed to match the observed Pluto data. For the 6200-/~ band it would take 10-cm, for the 7200-A band, 3.2cm, and for the 8900-/~ band, 1. l-cm of clear transmission path length using absorption coefficients derived from Ramaprasad et al. (1978). Even a thick scattering surface frost does not provide more than about 1 mm of effective path length. An unpublished spectrum by H. Kieffer shows an absorption at 8900 A of - 5 % for a completely covered surface of a thick methane frost. For a planetary surface with only partial coverage, at most a few percent contribution could therefore be expected from a frost alone. The p a r t i a l p r e s s u r e , p, of the CH4 atmosphere can readily be calculated from the relationship p = toga,
where m is the mass of a CH4 molecule, g is Pluto's surface gravity, and a is the number column abundance of CH4 molecules. If we use a value of 80 cm/sec 2 for g (see discussion below), the surface pressure of CH4 is related to its one-way abundance, a oH,, by p~ (atm) = 5.6 x 10-6acH4 (m-am). An abundance of 27 m-am of CH4 thus
CH4 ATMOSPHERE ON PLUTO
67
0 0 -0 0
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.=o.- -~
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~
~
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-I
iO
~F-,
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u 'x~
o
O
-3 t,.O
cD 0 I
o
n
68
FINK ET AL.
translates into a surface pressure of -- 1.5 x 10-4 atm or, via the Curtis-Godson approximation, into an average atmospheric pressure of 0.8 × 10-4 atm. At this pressure the Lorentz half-width is about 1000 times smaller than the Doppler half-width. Although of great interest, it was not possible to place a stringent limit on the t o t a l a t m o s p h e r i c pressure on Pluto. For, as discussed above, the Doppler width dominates the line profile to fairly high pressures, and the average line spacing at low temperatures for CH4 has not been measured and had to be left as a free parameter. In order for the effects of pressure to become noticeable in our synthetic specta, the Lorentz half-width must become close to the Doppler half width which requires a total pressure --0.05 atm. From our Voigt profile calculations we therefore deduce an upper limit to the total pressure of that order. DISCUSSION A number of investigations in the region below 8000 .~, have been carried out in the past to seek a CH4 atmosphere on Pluto: Kuiper, 1944, 1952; Martin, 1975; Benner et al., 1978; and Barker et al., 1980. All except Benner et al. obtained negative results. Kuiper stated no upper limits. Martin gave an upper limit of 6-8 m-am, while Barker et al. derived an upper limit of !-3 m-am. These upper limits are considerably below our detection and for the earlier observations may be explained by the lack of laboratory data and incorrect analysis which did not take saturation into account. The recent observations of Barker et al. require further examination. We find that, although their noise level is considerably higher than ours, their data shows evidence for the 7200-/~, CH4 absorption and moreover, within their noise level, its absorption depth and equivalent width is virtually the same as in our data. However, Barker et al. report an upper limit which is a factor of 8 smaller and neglect saturation in their anal-
ysis. This explains their very small upper limit. We do not consider the above negative results as evidence for a time-varying Pluto atmosphere, although, as was first suggested by Benner et al.. it is quite possible that Pluto has an atmosphere only near perihelion (coming up in 1989). Because of the much lower signal-to-noise ratio of the spectra resulting in a negative detection, their actual upper limits to the CH4 equivalent widths are considerably above our detected values. We have evidence that, at least in the last few years, Pluto's CH4 atmosphere has not altered substantially. The absorption depth for the 8900-,~ band in this paper is very close to that found earlier by Benner et td. (1978) for their observations in 1976 June 18 and 19. Recent observations in the infrared beyond 1/zm using J H K L photometry (Cruikshank et al., 1976) and narrow band filter photometry (Lebofsky et al., 1979; Soifer et al., 1980, Cruikshank and Silvaggio, 1980) have shown absorptions in this spectral region. Cruikshank et al., and Cruikshank and Silvaggio have interpreted these as CH.~ frost on the surface of Pluto. Lebofsky et al. concluded that "t he detailed match between the spectrum of Pluto and the laboratory spectrum of CH4 frost is poor" while Soifer et al. reported that their resolution was not high enough to distinguish between surface frost or a gaseous atmosphere. Cruikshank and Silvaggio stated that "'the methane ice spectrum matches the Pluto data better than do the gas spectra." However, for their gaseous atmosphere models they used a CH.j abundance which was more than a factor of 5 smaller than our detection. The relatively large amount of gaseous methane now found by us requires reexamination of the origin of the infrared absorption features past 1 p.m. A rough calculation with recently acquired band-model coefficients (Benner and Fink, to be published), shows that the absorption depths seen by the above authors can be repro-
CH4 A T M O S P H E R E
duced by atmospheric CH4. The gaseous absorption features coincide with and mask the solid absorptions. We do not, of course, advocate the absence of CH4 frost on Pluto's surface. We point out, however, that at the presently available resolution, the infrared absorptions cannot be construed as direct observational evidence for surface frost, until the gaseous absorptions are first properly accounted for. The stability of a substantial CH4 atmosphere on Pluto can be a problem depending on its mass and radius. This has recently been investigated by Trafton (1979, 1980). Trafton suggested that a heavier gas mixed with the CH4 would prevent its escape. We have therefore made some firstorder calculations of the mean life, ~-, of a CH4 atmosphere on Pluto using the Jeans' escape formula:
ON PLUTO
69
i 0 '~
i012
, 0
~,o
%
°~
I
@ ~.
g
I0 '°
E 9 "6 10 a) c
g
10 9
=E 10 7
i0 s
I 600
I 800
I I000
~ I 1200
'i 1400
1 1600
Radius (kin]
FIG. 4. Mean life of a CH4 atmosphere, using the Jeans' escape formula with Pluto's m a s s and radius as parameters.
r-' = (U/20r)'l=)(l + ~,)e-~ Here U is the mean thermal velocity and ~, is the square of the ratio of the escape velocity to the mean thermal velocity. (See Spizter, 1952 and Hunten and Donahue, 1976 for a more detailed discussion of thermal escape). The calculations were made with the mass and radius of Pluto as parameters and are plotted in Fig. 4. For the present nominal value of Pluto's mass, 1.53 x 102s g (Harrington and Christy, 1980, and neglecting the mass of the much smaller satellite), the radius would have to be of the order of 950 km or less, for the atmosphere to be stable -109 years. This small radius would lead to a geometric albedo greater than 1 and a density that is unacceptably high. This set of parameters would therefore cause serious problems for the stability of a CH4 atmosphere on Pluto. However the upper limit to Harrington and Christy's mass determination (1.81 x 1025g) yields a radius of -1100 kin, a geometrical albedo of 0.67 and a density of 3.25. These values are within the range of possibility for Pluto and would provide a stable atmosphere. If
the mass is increased by 50% over Harrington and Christy's nominal value to 2.3 x 10zs g a radius of 1400 km, a geometric albedo of 0.45, a density of 2.0, and a surface gravity of 80 cm/sec 2 result. We consider the latter as very plausible parameters for Pluto and have adopted them in our calculation of surface pressure in this paper. Through Kepler's third law, the mass of Pluto depends on the third power of the separation of the satellite, which is very difficult to determine and according to Harrington and Christy, " a fully resolved observation of the satellite has not yet been reported." Thus we feel that an increase in mass by 50% is entirely within the present errors3 The radius of 1400 km also falls within the errors of a recent speckle interferometry determination (1500 _+ 200 km, Arnold et al., 1979). We therefore feel that the stability of a CH4 atmosphere on Pluto poses at present no a A recent additional m a s s determination is 1.99 x 10~ g by Bonneau et al. (IAU Circular No. 3509); from the same data however, Harrington and Christy derived a m a s s of 1.43 × 10~ g (IAU Circular No. 3515).
70
F I N K ET AL.
u n d u e p r o b l e m . In t h e c o n t e x t o f t h e simp l e r h y p o t h e s i s t h a t t h e r e is o n l y o n e a t m o spheric constituent of Pluto, and considering t h a t its m a s s a n d r a d i u s a r e still q u i t e u n c e r t a i n , t h e s t r o n g d e p e n d e n c e o f the atmospheric stability on these two paramet e r s h a s a l l o w e d us to n a r r o w t h e i r p e r m i s sible r a n g e . If t h e m a s s o f P l u t o t e n d s to the smaller value then Trafton's suggestion of t h e a d d i t i o n o f a h e a v i e r g a s is r e q u i r e d to k e e p t h e a t m o s p h e r e s t a b l e o v e r the lifetime of the solar system. Unfortunately, o u r s e n s i t i v i t y to the t o t a l p r e s s u r e is at p r e s e n t not sufficient to d e t e c t o r r e f u t e the p r e s e n c e o f an a d d i t i o n a l b r o a d e n i n g gas. Higher resolution observations beyond 1 txm c o m b i n e d w i t h l o w - t e m p e r a t u r e l a b o r a t o r y d a t a will b e m u c h m o r e s e n s i t i v e to this p r o b l e m . In o r d e r to s u s t a i n a CH4 a t m o s p h e r e , t h e s u r f a c e t e m p e r a t u r e o f P l u t o h a s to b e in a fine b a l a n c e . I f t h e t e m p e r a t u r e is t o o c o l d , the a t m o s p h e r e will f r e e z e out. If it is t o o hot, the v a p o r p r e s s u r e o f f r o z e n CH4 will e x c e e d t h e s u r f a c e p r e s s u r e o f the CH~ a t m o s p h e r e , w i t h the r e s u l t t h a t a net upw a r d f o r c e e x i s t s in t h e a t m o s p h e r e , and it will be r a p i d l y l o s t ( T r a f t o n , 1980). T h e t e m p e r a t u r e at w h i c h t h e solid v a p o r p r e s s u r e b e c o m e s e q u a l to o u r s u r f a c e p r e s s u r e o f 1.5 × 10 -4 a t m is 6 & K . S e t t i n g t h e B o n d a l b e d o e q u a l t o t h e g e o m e t r i c a l b e d o (0.45 for o u r p r e f e r r e d set o f P l u t o p a r a m e t e r s a b o v e ) , r e s u l t s in a m a x i m u m s u r f a c e t e m p e r a t u r e o f 62°K a n d an a v e r a g e t e m p e r a t u r e f o r a s l o w r o t a t o r o f 52°K. T h u s it a p p e a r s t h a t t h e s u r f a c e c o n d i t i o n s on P l u t o a r e p r e s e n t l y a p p r o p r i a t e for t h e a t m o sphere that we observe. B e c a u s e o f P l u t o ' s s m a l l size, l a r g e ecc e n t r i c i t y , high i n c l i n a t i o n , and large dist a n c e f r o m the S u n , P l u t o h a s so far b e e n l o o k e d u p o n as a r a t h e r i r r e g u l a r t y p e o f p l a n e t o f t h e s o l a r s y s t e m . W e feel, t h a t the r e c e n t d i s c o v e r y o f its s a t e l l i t e , and the p r e s e n t d e t e c t i o n o f an a t m o s p h e r e , will d o m u c h to e n h a n c e its i m a g e a n d e s t a b l i s h P l u t o as a m o r e r e g u l a r a n d r e s p e c t a b l e member of the planetary community.
ACKNOWLEDGMENTS We would like to thank Dr. R. V. Shack of the University of Arizona, Optical Science Center for his helpful discussions regarding lens aberration, R. F. Poppen for the computer reduction programs, and R. James for completing the instrument in such an expeditious and efficient manner. This research has been supported by NASA grants NSG 7070, NGL 05-002003 and NASA contract NAS5-25451. REFERENCES ARNOLD,S. J., BOKSENBERG,A., ANDSARGENT,W. L. W. (1979). Measurement of the diameter of Pluto by speckle interferometry. Astrophys. J. 234, L159L163. BARKER,E. S., COCHRAN,W. D., AND COCHRAN, A. L. (1980). Spectrophotometry of Pluto from 3500 to 7350 A. Icarus, 44, 43-52. BENNER, D. C. (1979). The visual and near infrared spectrum of methane and its application to Uranus, Neptune, Triton and Pluto. PH.D. dissertation, University of Arizona. BENNER, D. C., FINK. O., AND CROMWELL, R. H. 11978). Image tube spectra of Pluto and Triton from 6800 to 9000 ,~,. Icarus 36, 82-91. CRUIKSHANK, D. P., AND SII.VAGGIO,P. M., (1980), The surface and atmosphere of Pluto. Icarus 41, 96102. CRUIKSHANK,D. P., PILCHER, C. B.. AND MORRISON, D. (1976). Pluto: Evidence for methane frost. Science 194, 835-837. DtCK, K. A., AND FINK, U. (1977). Photoelectric absorption spectra of methane, methane and hydrogen mixtures and ethane. J. Quant. Spectrosc. Radial. Tranffer 18, 433-446. FINK, U., BENNER, D. C., AND DICK, K. A. (1977). Band model analysis of laboratory methane absorption spectra from 4500-10,500 ~. J. Quant. Spectrosc. Radial. Transfer 18, 447-457. GIVER, L. P. (1978). Intensity measurements of the CH4 bands in the region 4350 to 10,600 ,~. J. Quant. Spectrosc. Radial. Tranffer 19, 311-322. GOODY, R. M. (1964). Atmospheric Radiation, Oxford Univ. Press (Clarendon), London. HARRINGFON, R. S., AND CHRISTY,J. W. (1980). The satellite of Pluto. II. Astron. J. 85, 168-170. HARRIS, D. L. (1961). Photometry and colorimetry of planets and satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 272342. Univ. of Chicago Press, Chicago. HUNTEN, D. M., AND DONAHUET. M. (1976). Hydrogen loss from the terrestrial planets. Ann. Rev. Earth Planet. Sci. 4, 265-292. JOHNSON, H. L., MITCHELl., R. I., IRIANTE, B., AND WISNIEWSKI,W. Z. (1966). UBVRIJKL photometry of the bright stars. Comm. Lunar and Planetary Lab 4, 99-110.
CH4 A T M O S P H E R E KUIPER, G. P. (1944). Titan: A satellite with an atmosphere. Astrophys. J. 100, 378-383. KUIPER, G. P. (1952). In The Atmospheres o f the Earth and Planets. pp. 306-405. Univ. of Chicago Press, Chicago. LEBOFSKY, L. A., RIEgE, G. H., AND LEBOFSKY, M. J. (1979). Surface composition of Pluto. Icarus 37, 554-558 MARTIN, T. Z. (1975). Saturn and Jupiter: A study of atmospheric constituents. Ph.D. thesis, University of Hawaii. NEFF, J. S., LANE, W. A., AND FIX, J. D. (1974). An investigation of the rotational period of the planet Pluto. Publ. Astron. Soc. Pacific 86, 225-230. RAMAPRASAD, K. R., CALDWELL, J., AND McCLURE, D. S. (1978). The vibrational overtone spectrum of liquid methane in the visual and near infrared: Applications to planetary studies. Icarus 35, 4(K)409. RANK, D. H., FINK, U., AND WIGGINS, T. A. (1966). Measurements on spectra of gases of planetary interest. II. Hz, SO~, NH:~, and CH~. Astrophys. J. 143, 980-988.
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SOIFER, B. T., NEUGEBAUER, G.. AND MATTHEWS, K. (1980). The 1.5-2.5 p.m spectrum of Pluto. Astron. J. 85, 166-167. SPIrZER, L., JR. (1952). The terrestrial atmosphere above 300 km. In The Atmospheres o f the Earth and Planets (G. P. Kuiper, Ed.), pp. 211-247. Univ. of Chicago Press, Chicago. TEDESCO, E. F., AND THOLEN, D. S. (1980). Photometric observations of Pluto in 1980. Bull Amer. Astron. Sac. 12, 729. TRAFTON, L. M. (1979). Pluto's atmosphere--Reconciliation of the detection gaseous methane with Pluto's small mass. Bull. Amer. Astron. Soc. !1, 570. TRAFTON, L. M. (1980). Does Pluto have a substantial atmosphere? Icarus, 44, 53-61. VARANASI, P., SARANGI, S., AND PUGH. L. (1973). Measurements on the infrared lines of planetary gases at low temperatures. I. ~,:~ fundamental of methane. Astrophys. J. 179, 977-982. WAMSTEKER0 W. (1975). Ph.D. dissertation. University of Leiden. Holland.
Detection of a OH4 Atmosphere on Pluto U W E F I N K , B R A D F O R D A. S M I T H , D. C H R I S B E N N E R , 1 J A M E S R. J O H N S O N , H. J. R E I T S E M A Lunar and Planetary l, aboratory. University of Arizona, Tucson, Arizona 85721 AND
J A M E S A. W E S T P H A L Hale Observatories. ('alifornia Institute of Technology Pasadena, California 91109 Received August 11, 1980 Using a low-resolution spectrograph and a C C D array, a s p e c t r u m o f Pluto from 0.58 to 1.06 #,m was obtained. T h e s p e c t r u m had a resolution o f -25 A and a signal-to-noise ratio of - 3 0 0 . It s h o w e d CH4 absorption b a n d s at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 A. T h e strongest o f t h e s e b a n d s was at 8900 A with an absorption depth o f 0.23. This band was heavily saturated, c o m p a r e d to the weaker bands, providing proof for the g a s e o u s origin of the o b s e r v e d absorptions. By applying CH4 band model p a r a m e t e r s to our data, a total CH4 a b u n d a n c e o f 80 ± 20 m - a m was derived. This translates into a o n e - w a y a b u n d a n c e of 27 ± 7 m - a m and a CH4 surface pressure of 1.5 × 10-4 atm. An upper limit to the total p r e s s u r e of ,-0.05 atm could be set. First-order calculations on atmospheric escape showed that this m e t h a n e a t m o s p h e r e would be stable if the m a s s o f Pluto is increased 50% o v e r its current value and its radius is 1400 kin. Alternatively a heavier gas mixed with the CH4 a t m o s p h e r e would aid its stability. T h e relatively large a m o u n t o f g a s e o u s CH4 o b s e r v e d implies that the absorption b a n d s recently reported at 1.7 and 2.3 p,m are likely due to a t m o s p h e r i c CH~ absorptions rather than surface frost as interpreted earlier.
spectrum of Pluto which clearly shows the presence of a CH4 a t m o s p h e r e , and we describe our o b s e r v a t i o n s and their interpretation in this paper.
INTRODUCTION
A few years ago we obtained an imagetube s p e c t r u m o f Pluto (Benner et al., 1978) which gave indications of a m e t h a n e atmosphere on Pluto and, to a m u c h lesser degree of certainty, on Triton. Unfortunately the image tube response fell off sharply near 9000 A, preventing adequate c o v e r a g e of the strongest CH4 band below 1 tzm, at 8900 A. Since then, Si array detectors have been developed which have longer wavelength r e s p o n s e and provide good c o v e r a g e o f the 8900-A CH4 band. To confirm or refute our earlier conclusions, we have built a low-resolution spectrograph to which we attached a C C D (charge coupled device) array. We obtained a good
OBSERVATIONS, INSTRUMENTATION. AND DATA REDUCTION
The observations were conducted at the 155-cm Catalina O b s e r v a t o r y telescope. An instrument test run was carried out 1980 February 22 and immediately yielded good e x p o s u r e s of Pluto. T h e s e spectra clearly showed the presence of CH4 bands. After i m p r o v e m e n t s to the instrument and refinements to the data reduction procedure, Pluto was again o b s e r v e d 1980 May 4 from 7:30 to 8:30 UT. Three good exposures of 5 m and two of 10m were obtained. A n u m b e r of stars were o b s e r v e d and two e x p o s u r e s each of the G - t y p e stars SAO 120173 and 120940 were selected for corn-
I Present address: N A S A - A m e s Research Center, Astrophysical e x p e r i m e n t s b r a n c h N245-6, Moffett Field, Calif. 94035. 62
0019-1035/80/100062-10502.00/0 Copyright ~) 1980by Academic Press, Inc. All rights of reproduction in any fon'a reserved.
CH 4
ATMOSPHERE ON PLUTO
parison with Pluto. T h e average Pluto air m a s s was 1.21 and that of the c o m p a r i s o n stars 1.25. The instrumentation consisted of a C C D array attached to a spectrograph. The C C D was m a n u f a c t u r e d by T e x a s I n s t r u m e n t s and contained 500 x 500 elements, each being 15.2/zm square. During its operation it was cooled to - 1 2 0 ° C to minimize dark noise. The detector and associated electronics were part o f a ground-based system built at the California Institute o f Technology for use by the Space T e l e s c o p e Wide Field Planetary C a m e r a investigation definition team. 2 T h e spectrograph was built at the L u n a r and Planetary L a b o r a t o r y and was designed to m a k e o p t i m u m use o f the C C D array. It e m p l o y e d a transmission grating and had a spectral range 5700-11,000 A and a m a x i m u m first-order resolution o f 11 /~. The long-wavelength limit w a s determined by the C C D sensitivity cutoff and in practice was close to 1.06 tzm as can be seen in Fig. 2. The short-wavelength limit followed the sharp cutoff o f a Schott O G 570 filter which eliminated overlapping orders. The object to be o b s e r v e d was focused on the spectrograph entrance slit which was in turn imaged on the C C D array. F o r m o s t e x p o s u r e s the entrance slitwidth was 0.5 m m which c o r r e s p o n d s to 5" field o f view with the f / 1 3 . 5 secondary ot" the telescope. The slit length was 18 m m (18if'), thus giving good c o v e r a g e of the sky background. A slice across the s p e c t r u m at 8000 A, perpendicular to the direction of dispersion is shown for Pluto and a c o m p a r ison star in Fig. 1. For Pluto the sky background shows up distinctly a b o v e the DC offset count o f the CCD. It is, h o w e v e r , less than one-tenth of the Pluto signal. The star essentially exhibits only the dc bias o f the CCD. T h e halfwidth of the star signal is about 3", and its 2Team members are: W. B. Baum, A. D. Code, D. G. Currie, G. E. Danielson, J. E. Gunn, T. Kelsall, J. Kristian, C. R. Lynds, P. K. Seid¢lman, B. A. Smith, and J. A. Westphal.
SAO
63
120173
PLUTO
-60
-40
t
-20 0 20 row n u m b e r
40
60
-,~-~-~ -~ -~ ~ ~ ~ ~ ~ ,b i
, i J -80-60-40-20
m m olong shl t I i i i 0 20 40 60 orc-sec in sky
i 80
,
FIG. 1. Slice across the spectrum perpendicular to the direction of dispersion at 8000 A, for a comparison star and a Pluto exposure. Ordinate is linear in intensity and has been normalized to one. Abscissa can be expressed in terms of row number of the CCD array, millimeters along the slit, or arc-second field of view in the sky. apparent full-width at the base is 10". T h e s e widths result from the combination of seeing, guiding, and the finite resolution o f the c a m e r a lens. The width of Pluto is almost identical to that of the star, showing that tracking and guiding during the Pluto exposure were excellent. At the two ends o f the s p e c t r u m the image widths were slightly larger due to optical aberrations and imperfect alignment. F o r the data reduction, the Pluto counts were s u m m e d o v e r 11 pixels at each column (or wavelength) thus collecting all o f the Pluto signal. N e x t the sky and dc offset were subtracted by using the 5 pixels on both sides of Pluto. The five Pluto spectra w e r e then averaged with weights assigned according to their m a x i m u m signal level. The standard stars were reduced in the same m a n n e r and the Pluto average was then divided by the star average. The w a v e length scale was derived from helium and
64
FINK ET AL.
xenon comparison spectra exposed during 11 pixeis in common. The CCD sensitivity the run. The oxygen A band at 7620 ~k variations were therefore canceled to a high served as a fiducial mark for the wavelength degree. calibration. Sensitivity variations were not taken into The resulting Pluto and star averages and account when the sky signal was subtracted their ratio are shown in Fig. 2. Both the from the Pluto signal. However, as can be Pluto and standard-star spectra contain the seen in Fig. 1 RMS variations in the sky sensitivity variations which exist across the background are - 0 . 6 % which is reduced to CCD array. These variations are removed -0.2% after l l object and 10 sky pixels only in the ratio. Great care was therefore have been averaged. A sky-only exposure taken to place the Pluto and comparison- showed that no undue sensitivity fluctuastar spectra on identical locations on tion occurred at the position of the Pluto the CCD. The spectrum centerline of the spectrum. Pluto exposures and the comparison stars The expected maximum signal to rmsall fell within plus or minus one pixel of noise ratio from photon statistics is 400. each other. Three of the Pluto spectra were Additional noise is introduced by the sky at identical locations as the comparison subtraction, but as discussed above, this stars, while the other two still had l0 out of should be somewhat below the photon noise. Outright pixel defects, cosmic ray strikes, or larger than average sensitivity Pluto / .S"~""'~$//) 1 variations in any pixel used during the data reduction (object, comparison, or sky for each) can introduce additional noise. About , l m a dozen serious defects could easily be recognized and have been left blank in the spectra and their ratio in Fig. 2. Additional 6000 8000 tO00O smaller defects however are still present in coo. s,:7 % 4~ some places of the spectra and raise the I Av ,¥ 1 noise level somewhat above the statistical one. Ignoring the readily recognized larger i / of these, we measured a peak-to-peak signal-to-noise ratio of -100 (300 rms) which -- .-. _;::==,..I is quite close to the expected statistical 6000 8000 0000 photon noise value. It is this high signal-toRofio noise ratio which, we believe, is responsi- ~ "-*". . . . . x~. . . . ~m'~,,.\., v . " f " ~ l ~ ! ble for the detection of the weak CH4 bands i not observed until now. The spectral resol lution is given by the half-width of the Pluto seeing disk (cf. Fig. l) and is -25 ]k. This is quite sutticient for the relatively broad CH.8 ....... 6~oo . . . . . 8~obo . . . . . . . . . . -co'oo ~ Wovelength features investigated. The Pluto ratio spectrum in Fig. 2 is quite Fu(;. 2. The average spectrum of Pluto, compariflat showing a slight Pluto reddening as son stars and their ratio. Ordinate is linear in intensity and is normalized to the maximum signal for measured by an intensity ratio of 1.06 bePluto and the comparison star. The ratio spectrum tween 9000 and 6000 A. This was checked is one where there are no absorptions. The Pluto by comparing the two SAO stars used in the and comparison-star spectra contain the sensitivity ratio to the stars BS 5183 (dG2),72 Her variations across the CCD. Serious CCD defects (G212), and BS 5384 (dG3) which were h a v e b e e n d e l e t e d a n d a r e left a s b l a n k s in t h e s p e c observed during a run 1980 June. The latter tra. .
.
.
.
.
.
.
.
.
i
CH~ ATMOSPHERE ON PLUTO
65
stars have accurate broadband UBVRI, or density was so high that the absorptions narrowband photometry (Johnson et al., exhibited a pseudocontinuum behavior, 1966; Wamsteker, 1975) and follow the and only the strongest bands at 8900 and color of the Sun very closely. It was found 10,000 ,~ displayed a weak pressure effect. This situation is expected to change for that the SAO stars used in our ratio were definitely G-type stars but were slightly less the much lower temperature on Pluto red than the Sun. Thus the slope in Pluto's (-60°K). The pressure coefficient is defined ratio spectrum of Fig. 2 should probably be as the ratio of the line pressure broadening increased to 1.25 _ 0.10 between 9000 and coefficient, (3'L, cm-~/atm) divided by the 6000/~. This agrees quite well with avail- mean line spacing (8, cm-t). Because of the able VIR photometry (Harris, 1961). With redistribution of the total band intensity this slight reddening correction, we feel into a few low J lines, at low temperature, that our spectrum accurately represents the average line spacing is expected to Pluto's relative albedo from 0.6to 1.0/.tm at increase considerably. Estimates made by Benner (1979) for Uranus at a temperature 25-/~ resolution. of -80°K show that 8 should increase by a factor of 2-10 depending on the model ANALYSIS assumptions. For Pluto at 60°K higher In addition to the slight slope, Pluto's values can be expected. The band-model absorption coefficients, ratio spectrum clearly shows methane bands at 6200, 7200, 7900, 8400, 8600, 8900, on the other hand, are expected to display and 10,000 A. The strongest of these bands only minor changes for the lower Pluto is at 8900 A, as expected. The absorption temperatures. The integrated absorption depth for this band is 0.23, which is very strength of a CH4 band, arising from the close to that of our earlier image tube data ground state, does not change with temper(Benner et al., 1978). However, the high ature, and the absorptions below 1 p.rn are quality of our present data set also brings known to consist of hundreds of more or out all the other methane absorptions. Par- less randomly spaced and overlapping ticularly, the presence of the weak 6200-A bands. Experimental proof is provided by band was a pleasant surprise. These weak the measurement of the absorption bands allow us to determine a good meth- strengths of liquid CH4 by Ramaprasad et ane abundance, independent of the model al. (1978), who found excellent agreement employed. The relatively large 0.3-m ampli- between their data and the room-temperatude of Pluto's light curve (Neffet al., 1974; ture gas results. We thus have all the parameters for a Tedesco and Tholen, 1980) gives indications that we are seeing through a clear Voigt profile band-model calculation which atmosphere and a reflecting-layer model automatically includes the Lorentz and will be used in the following calculations. Doppler profile limits. The absorption The analysis made use of the CH4 band- coefficient is that determined by Fink et al. model coefficients from 4500 to 10,500 A (1978). The average CH4 Lorentz pressure measured by us a few years ago (Dick and broadening coefficient (3'L) is --0.085 Fink, 1977; Fink et al., 1977) and subse- cm-l/atm at room temperature (Rank et al. 1960) and was converted to 0.25 cm-I/atm quently by Giver (1978). In the band-model theory (cf. Goody, 1964), the methane ab- at 80°K (Varanasi et al., 1973). The Doppler sorption in a small wavelength interval is half-width is a function of wavelength and characterized by an absorption coefficient at 60°K and 8900 A is 0.0094 cm-L The k~, (km-am) -1 and a pressure coefficient y~ mean line spacing (8) for the 8900-/~ band atm-'. For the complex CH4 absorptions was found to be -0.0061 cm -~ at room below 1 p.m, Fink et al. found that the line temperature (Fink et al., 1978), but needs
66
FINK ET AL.
to be increased by a factor m, which was left as a free parameter. A number of model calculations varying the input parameters were carried out, and the best fit is compared to our Pluto ratio spectrum in Fig. 3. The calculations were made with an input parameter spacing of 10 A and thus have slightly higher resolution than the Pluto observations. The small discrepancy in the middle of the 8900-,~ band can probably be ascribed to a slight change in the shape of the absorption coefficient curve at lower temperatures, which can result in sharper peaks and a change in their relative intensity. Otherwise the synthetic CH4 spectrum fits the Pluto ratio very well. The abundance of CH4 was determined by the weak bands at 6200, 7900, and 8400/~ and yielded a value of 80 _ 20 m-am of CH4 for the two-way transmission. These bands will remain on the linear portion of the curve of growth regardless of the model used and independent of any reasonable value of the line spacing. They thus yield a good measure of the CH4 abundance on Pluto. Assuming a Lambert sphere with an air mass factor for the whole disk of 3, a one-way CH4 abundance of 27 _+ 7 m-am results. While the weak bands used for the total CH~ determination are linear with abundance, the strong bands definitely exhibit effects of saturation. A simple calculation shows that the 8900-A band, if it were linear, should have a transmission of only 8% with 80 m-am of CH4, rather than the 77% actually observed. The room-temperature value of the line spacing only increases the transmission to 16%. In order to match the Pluto data the line spacing had to be increased to 0.085, 0.10, 0.17, and 0.17 cm -1 for the 10,000, 8900, 8600, and 7200/~ regions, respectively. These results imply an increase in the line spacing by a factor of 10, 16.5, and 12 over the room-temperature value for the first three bands, which compares quite favorably to the expectation from theory. For the 7200-A region the mean line spacing came out somewhat
larger than might have been expected from the greater complexity of this band (Dick and Fink, 1977) but since no room-temperature values have been measured a direct comparison was not possible. Under the assumption of a clear atmosphere and a reflecting-layer model, the Pluto data thus allowed us to obtain average line spacings for CH4 at 60°K which would be a difficult feat in the laboratory. The saturation of the strong bands provides persuasive evidence that we are observing gaseous CH4 absorptions rather than liquid or solid features. In both the liquid and solid state, CH~ possesses a true continuum absorption coefficient for which the absorptions are linear with abundance and exhibit no saturation effect. An additional argument is provided by the amount of solid or liquid needed to match the observed Pluto data. For the 6200-/~ band it would take 10-cm, for the 7200-A band, 3.2cm, and for the 8900-/~ band, 1. l-cm of clear transmission path length using absorption coefficients derived from Ramaprasad et al. (1978). Even a thick scattering surface frost does not provide more than about 1 mm of effective path length. An unpublished spectrum by H. Kieffer shows an absorption at 8900 A of - 5 % for a completely covered surface of a thick methane frost. For a planetary surface with only partial coverage, at most a few percent contribution could therefore be expected from a frost alone. The p a r t i a l p r e s s u r e , p, of the CH4 atmosphere can readily be calculated from the relationship p = toga,
where m is the mass of a CH4 molecule, g is Pluto's surface gravity, and a is the number column abundance of CH4 molecules. If we use a value of 80 cm/sec 2 for g (see discussion below), the surface pressure of CH4 is related to its one-way abundance, a oH,, by p~ (atm) = 5.6 x 10-6acH4 (m-am). An abundance of 27 m-am of CH4 thus
CH4 ATMOSPHERE ON PLUTO
67
0 0 -0 0
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68
FINK ET AL.
translates into a surface pressure of -- 1.5 x 10-4 atm or, via the Curtis-Godson approximation, into an average atmospheric pressure of 0.8 × 10-4 atm. At this pressure the Lorentz half-width is about 1000 times smaller than the Doppler half-width. Although of great interest, it was not possible to place a stringent limit on the t o t a l a t m o s p h e r i c pressure on Pluto. For, as discussed above, the Doppler width dominates the line profile to fairly high pressures, and the average line spacing at low temperatures for CH4 has not been measured and had to be left as a free parameter. In order for the effects of pressure to become noticeable in our synthetic specta, the Lorentz half-width must become close to the Doppler half width which requires a total pressure --0.05 atm. From our Voigt profile calculations we therefore deduce an upper limit to the total pressure of that order. DISCUSSION A number of investigations in the region below 8000 .~, have been carried out in the past to seek a CH4 atmosphere on Pluto: Kuiper, 1944, 1952; Martin, 1975; Benner et al., 1978; and Barker et al., 1980. All except Benner et al. obtained negative results. Kuiper stated no upper limits. Martin gave an upper limit of 6-8 m-am, while Barker et al. derived an upper limit of !-3 m-am. These upper limits are considerably below our detection and for the earlier observations may be explained by the lack of laboratory data and incorrect analysis which did not take saturation into account. The recent observations of Barker et al. require further examination. We find that, although their noise level is considerably higher than ours, their data shows evidence for the 7200-/~, CH4 absorption and moreover, within their noise level, its absorption depth and equivalent width is virtually the same as in our data. However, Barker et al. report an upper limit which is a factor of 8 smaller and neglect saturation in their anal-
ysis. This explains their very small upper limit. We do not consider the above negative results as evidence for a time-varying Pluto atmosphere, although, as was first suggested by Benner et al.. it is quite possible that Pluto has an atmosphere only near perihelion (coming up in 1989). Because of the much lower signal-to-noise ratio of the spectra resulting in a negative detection, their actual upper limits to the CH4 equivalent widths are considerably above our detected values. We have evidence that, at least in the last few years, Pluto's CH4 atmosphere has not altered substantially. The absorption depth for the 8900-,~ band in this paper is very close to that found earlier by Benner et td. (1978) for their observations in 1976 June 18 and 19. Recent observations in the infrared beyond 1/zm using J H K L photometry (Cruikshank et al., 1976) and narrow band filter photometry (Lebofsky et al., 1979; Soifer et al., 1980, Cruikshank and Silvaggio, 1980) have shown absorptions in this spectral region. Cruikshank et al., and Cruikshank and Silvaggio have interpreted these as CH.~ frost on the surface of Pluto. Lebofsky et al. concluded that "t he detailed match between the spectrum of Pluto and the laboratory spectrum of CH4 frost is poor" while Soifer et al. reported that their resolution was not high enough to distinguish between surface frost or a gaseous atmosphere. Cruikshank and Silvaggio stated that "'the methane ice spectrum matches the Pluto data better than do the gas spectra." However, for their gaseous atmosphere models they used a CH.j abundance which was more than a factor of 5 smaller than our detection. The relatively large amount of gaseous methane now found by us requires reexamination of the origin of the infrared absorption features past 1 p.m. A rough calculation with recently acquired band-model coefficients (Benner and Fink, to be published), shows that the absorption depths seen by the above authors can be repro-
CH4 A T M O S P H E R E
duced by atmospheric CH4. The gaseous absorption features coincide with and mask the solid absorptions. We do not, of course, advocate the absence of CH4 frost on Pluto's surface. We point out, however, that at the presently available resolution, the infrared absorptions cannot be construed as direct observational evidence for surface frost, until the gaseous absorptions are first properly accounted for. The stability of a substantial CH4 atmosphere on Pluto can be a problem depending on its mass and radius. This has recently been investigated by Trafton (1979, 1980). Trafton suggested that a heavier gas mixed with the CH4 would prevent its escape. We have therefore made some firstorder calculations of the mean life, ~-, of a CH4 atmosphere on Pluto using the Jeans' escape formula:
ON PLUTO
69
i 0 '~
i012
, 0
~,o
%
°~
I
@ ~.
g
I0 '°
E 9 "6 10 a) c
g
10 9
=E 10 7
i0 s
I 600
I 800
I I000
~ I 1200
'i 1400
1 1600
Radius (kin]
FIG. 4. Mean life of a CH4 atmosphere, using the Jeans' escape formula with Pluto's m a s s and radius as parameters.
r-' = (U/20r)'l=)(l + ~,)e-~ Here U is the mean thermal velocity and ~, is the square of the ratio of the escape velocity to the mean thermal velocity. (See Spizter, 1952 and Hunten and Donahue, 1976 for a more detailed discussion of thermal escape). The calculations were made with the mass and radius of Pluto as parameters and are plotted in Fig. 4. For the present nominal value of Pluto's mass, 1.53 x 102s g (Harrington and Christy, 1980, and neglecting the mass of the much smaller satellite), the radius would have to be of the order of 950 km or less, for the atmosphere to be stable -109 years. This small radius would lead to a geometric albedo greater than 1 and a density that is unacceptably high. This set of parameters would therefore cause serious problems for the stability of a CH4 atmosphere on Pluto. However the upper limit to Harrington and Christy's mass determination (1.81 x 1025g) yields a radius of -1100 kin, a geometrical albedo of 0.67 and a density of 3.25. These values are within the range of possibility for Pluto and would provide a stable atmosphere. If
the mass is increased by 50% over Harrington and Christy's nominal value to 2.3 x 10zs g a radius of 1400 km, a geometric albedo of 0.45, a density of 2.0, and a surface gravity of 80 cm/sec 2 result. We consider the latter as very plausible parameters for Pluto and have adopted them in our calculation of surface pressure in this paper. Through Kepler's third law, the mass of Pluto depends on the third power of the separation of the satellite, which is very difficult to determine and according to Harrington and Christy, " a fully resolved observation of the satellite has not yet been reported." Thus we feel that an increase in mass by 50% is entirely within the present errors3 The radius of 1400 km also falls within the errors of a recent speckle interferometry determination (1500 _+ 200 km, Arnold et al., 1979). We therefore feel that the stability of a CH4 atmosphere on Pluto poses at present no a A recent additional m a s s determination is 1.99 x 10~ g by Bonneau et al. (IAU Circular No. 3509); from the same data however, Harrington and Christy derived a m a s s of 1.43 × 10~ g (IAU Circular No. 3515).
70
F I N K ET AL.
u n d u e p r o b l e m . In t h e c o n t e x t o f t h e simp l e r h y p o t h e s i s t h a t t h e r e is o n l y o n e a t m o spheric constituent of Pluto, and considering t h a t its m a s s a n d r a d i u s a r e still q u i t e u n c e r t a i n , t h e s t r o n g d e p e n d e n c e o f the atmospheric stability on these two paramet e r s h a s a l l o w e d us to n a r r o w t h e i r p e r m i s sible r a n g e . If t h e m a s s o f P l u t o t e n d s to the smaller value then Trafton's suggestion of t h e a d d i t i o n o f a h e a v i e r g a s is r e q u i r e d to k e e p t h e a t m o s p h e r e s t a b l e o v e r the lifetime of the solar system. Unfortunately, o u r s e n s i t i v i t y to the t o t a l p r e s s u r e is at p r e s e n t not sufficient to d e t e c t o r r e f u t e the p r e s e n c e o f an a d d i t i o n a l b r o a d e n i n g gas. Higher resolution observations beyond 1 txm c o m b i n e d w i t h l o w - t e m p e r a t u r e l a b o r a t o r y d a t a will b e m u c h m o r e s e n s i t i v e to this p r o b l e m . In o r d e r to s u s t a i n a CH4 a t m o s p h e r e , t h e s u r f a c e t e m p e r a t u r e o f P l u t o h a s to b e in a fine b a l a n c e . I f t h e t e m p e r a t u r e is t o o c o l d , the a t m o s p h e r e will f r e e z e out. If it is t o o hot, the v a p o r p r e s s u r e o f f r o z e n CH4 will e x c e e d t h e s u r f a c e p r e s s u r e o f the CH~ a t m o s p h e r e , w i t h the r e s u l t t h a t a net upw a r d f o r c e e x i s t s in t h e a t m o s p h e r e , and it will be r a p i d l y l o s t ( T r a f t o n , 1980). T h e t e m p e r a t u r e at w h i c h t h e solid v a p o r p r e s s u r e b e c o m e s e q u a l to o u r s u r f a c e p r e s s u r e o f 1.5 × 10 -4 a t m is 6 & K . S e t t i n g t h e B o n d a l b e d o e q u a l t o t h e g e o m e t r i c a l b e d o (0.45 for o u r p r e f e r r e d set o f P l u t o p a r a m e t e r s a b o v e ) , r e s u l t s in a m a x i m u m s u r f a c e t e m p e r a t u r e o f 62°K a n d an a v e r a g e t e m p e r a t u r e f o r a s l o w r o t a t o r o f 52°K. T h u s it a p p e a r s t h a t t h e s u r f a c e c o n d i t i o n s on P l u t o a r e p r e s e n t l y a p p r o p r i a t e for t h e a t m o sphere that we observe. B e c a u s e o f P l u t o ' s s m a l l size, l a r g e ecc e n t r i c i t y , high i n c l i n a t i o n , and large dist a n c e f r o m the S u n , P l u t o h a s so far b e e n l o o k e d u p o n as a r a t h e r i r r e g u l a r t y p e o f p l a n e t o f t h e s o l a r s y s t e m . W e feel, t h a t the r e c e n t d i s c o v e r y o f its s a t e l l i t e , and the p r e s e n t d e t e c t i o n o f an a t m o s p h e r e , will d o m u c h to e n h a n c e its i m a g e a n d e s t a b l i s h P l u t o as a m o r e r e g u l a r a n d r e s p e c t a b l e member of the planetary community.
ACKNOWLEDGMENTS We would like to thank Dr. R. V. Shack of the University of Arizona, Optical Science Center for his helpful discussions regarding lens aberration, R. F. Poppen for the computer reduction programs, and R. James for completing the instrument in such an expeditious and efficient manner. This research has been supported by NASA grants NSG 7070, NGL 05-002003 and NASA contract NAS5-25451. REFERENCES ARNOLD,S. J., BOKSENBERG,A., ANDSARGENT,W. L. W. (1979). Measurement of the diameter of Pluto by speckle interferometry. Astrophys. J. 234, L159L163. BARKER,E. S., COCHRAN,W. D., AND COCHRAN, A. L. (1980). Spectrophotometry of Pluto from 3500 to 7350 A. Icarus, 44, 43-52. BENNER, D. C. (1979). The visual and near infrared spectrum of methane and its application to Uranus, Neptune, Triton and Pluto. PH.D. dissertation, University of Arizona. BENNER, D. C., FINK. O., AND CROMWELL, R. H. 11978). Image tube spectra of Pluto and Triton from 6800 to 9000 ,~,. Icarus 36, 82-91. CRUIKSHANK, D. P., AND SII.VAGGIO,P. M., (1980), The surface and atmosphere of Pluto. Icarus 41, 96102. CRUIKSHANK,D. P., PILCHER, C. B.. AND MORRISON, D. (1976). Pluto: Evidence for methane frost. Science 194, 835-837. DtCK, K. A., AND FINK, U. (1977). Photoelectric absorption spectra of methane, methane and hydrogen mixtures and ethane. J. Quant. Spectrosc. Radial. Tranffer 18, 433-446. FINK, U., BENNER, D. C., AND DICK, K. A. (1977). Band model analysis of laboratory methane absorption spectra from 4500-10,500 ~. J. Quant. Spectrosc. Radial. Transfer 18, 447-457. GIVER, L. P. (1978). Intensity measurements of the CH4 bands in the region 4350 to 10,600 ,~. J. Quant. Spectrosc. Radial. Tranffer 19, 311-322. GOODY, R. M. (1964). Atmospheric Radiation, Oxford Univ. Press (Clarendon), London. HARRINGFON, R. S., AND CHRISTY,J. W. (1980). The satellite of Pluto. II. Astron. J. 85, 168-170. HARRIS, D. L. (1961). Photometry and colorimetry of planets and satellites. In Planets and Satellites (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 272342. Univ. of Chicago Press, Chicago. HUNTEN, D. M., AND DONAHUET. M. (1976). Hydrogen loss from the terrestrial planets. Ann. Rev. Earth Planet. Sci. 4, 265-292. JOHNSON, H. L., MITCHELl., R. I., IRIANTE, B., AND WISNIEWSKI,W. Z. (1966). UBVRIJKL photometry of the bright stars. Comm. Lunar and Planetary Lab 4, 99-110.
CH4 A T M O S P H E R E KUIPER, G. P. (1944). Titan: A satellite with an atmosphere. Astrophys. J. 100, 378-383. KUIPER, G. P. (1952). In The Atmospheres o f the Earth and Planets. pp. 306-405. Univ. of Chicago Press, Chicago. LEBOFSKY, L. A., RIEgE, G. H., AND LEBOFSKY, M. J. (1979). Surface composition of Pluto. Icarus 37, 554-558 MARTIN, T. Z. (1975). Saturn and Jupiter: A study of atmospheric constituents. Ph.D. thesis, University of Hawaii. NEFF, J. S., LANE, W. A., AND FIX, J. D. (1974). An investigation of the rotational period of the planet Pluto. Publ. Astron. Soc. Pacific 86, 225-230. RAMAPRASAD, K. R., CALDWELL, J., AND McCLURE, D. S. (1978). The vibrational overtone spectrum of liquid methane in the visual and near infrared: Applications to planetary studies. Icarus 35, 4(K)409. RANK, D. H., FINK, U., AND WIGGINS, T. A. (1966). Measurements on spectra of gases of planetary interest. II. Hz, SO~, NH:~, and CH~. Astrophys. J. 143, 980-988.
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SOIFER, B. T., NEUGEBAUER, G.. AND MATTHEWS, K. (1980). The 1.5-2.5 p.m spectrum of Pluto. Astron. J. 85, 166-167. SPIrZER, L., JR. (1952). The terrestrial atmosphere above 300 km. In The Atmospheres o f the Earth and Planets (G. P. Kuiper, Ed.), pp. 211-247. Univ. of Chicago Press, Chicago. TEDESCO, E. F., AND THOLEN, D. S. (1980). Photometric observations of Pluto in 1980. Bull Amer. Astron. Sac. 12, 729. TRAFTON, L. M. (1979). Pluto's atmosphere--Reconciliation of the detection gaseous methane with Pluto's small mass. Bull. Amer. Astron. Soc. !1, 570. TRAFTON, L. M. (1980). Does Pluto have a substantial atmosphere? Icarus, 44, 53-61. VARANASI, P., SARANGI, S., AND PUGH. L. (1973). Measurements on the infrared lines of planetary gases at low temperatures. I. ~,:~ fundamental of methane. Astrophys. J. 179, 977-982. WAMSTEKER0 W. (1975). Ph.D. dissertation. University of Leiden. Holland.