Infrared reflectance spectra of Hyperion, Titania, and Triton

ICARUS 46, 169-174 (1981)

Infrared Reflectance Spectra of Hyperion, Titania, and Triton L A R R Y A. L E B O F S K Y , * G E O R G E H. RIEKE,*'~ "~ AND MARCIA J. LEBOFSKYt *Lunar and Planetary Laboratory, and "~Steward Observatory, University of Arizona, Tucson, Arizona 85721 Received March 3, 1980; revised March 2, 1981 Medium-resolution infrared (1-2.5 /zm; AX/X - 0.05) photometry of Triton, Titania, and Hyperion and medium-resolution (1.5-2.4/.tm; Ah/h ~ 0.01) spectroscopy of Triton are presented. Hyperion and Titania have spectra roughly similar to the laboratory spectrum of water frost, while the spectrum of Triton is inconsistent with the spectra of frosts likely to be major surface constituents.

INTRODUCTION

Infrared reflectance studies of the surfaces of outer solar system bodies have shown the presence of frosts on their surfaces and the probable presence of gaseous atmospheres. There appears to be a trend from the most stable of the frost condensates, water frost, on the surfaces of the satellites of Jupiter and Saturn (Pilcher et al., 1972; Fink et al., 1973, 1976; Morrison et al., 1976; Lebofsky, 1977; Pollack et al., 1978; Clark and McCord, 1980) to the much less stable (more volatile) methane on the surface and in the atmosphere of Pluto (Cruikshank et al., 1976; Benner et al., 1975; Lebofsky et al., 1979; Cruikshank and Silvaggio, 1980; Soifer et al., 1980; Fink et al., 1980). This is in agreement with the cosmochemical models of solar system formation and volatile stability (Lewis, 1971, 1972; Lebofsky, 1975). It would obviously be of interest to determine the surface compositions of small bodies between the orbits of Saturn and Pluto; however, the relative faintness of these objects makes it difficult to obtain full spectral formation. Infrared spectral observations of the smaller satellites of Saturn and the satellites of Uranus and Neptune have been made recently (Cruikshank and Silvaggio, 1979; Cruikshank, 1980). For the smaller satel1 Alfred P. Sloan Fellow.

lites of Saturn and the satellites of Uranus, the spectra appear to be consistent with the presence of water fi'ost. The match is poor in certain parts of the spectra (a shift of the band positions on the satellites relative to laboratory frost spectra) which may imply the presence of some additional component (Cruikshank, 1980). For Triton, the spectrum appears to be consistent with the presence o f a methane atmosphere and perhaps some methane frost (Cruikshank and Silvaggio, 1979). The spectra obtained by Cruikshank and Silvaggio were undersampled to obtain adequate signal to noise at the wavelengths observed. It is therefore important to compare them with more fully sampled observations to test the identifications of surface compositions. OBSERVATIONS

Broadband (J, H , and K; A h / h ~ 0.3) and narrowband (1.5 to 2.4 /xm 2~h/h 0.05) infrared observations of three satellites of Saturn (Hyperion, $7), Uranus (Titania, U3), and Neptune (Triton, N1) were made during 1978 and 1979 with the Steward Observatory 90 in. (2.29-m) telescope, using a liquid-helium-cooled InSb detector. The solar calibration, 0-magnitude calibration, and filter characteristics may be found in Lebofsky et al. (1979). In May, 1980, the Steward Observatory Fourier Transform Spectrometer (FTS) was used on the 90 in. 169 0019-1035/81/050169-06502.00/0 Copyright© 1981by AcademicPress,Inc. All rightsof reproductionin any formreserved.

170

LEBOFSKY, RIEKE, AND LEBOFSKY I.O

TABLE I OBSERVATIONS Arid RESULTS FOR HYPERION AND TITANIA ( 4 / 1 9 / 7 9 U T ) O

hen

Hyperion Magnitude

1.220) 1.50 1.63(H) 1.70 2.00 2.11 2.20(K) 2.20 2.35

12.92 13.00 12.79 12.71 13.36 13.37 13.16 13.23 13.00

Titania

R~"

1.00 0.69 0.74 0.79 0.46 0.45 0.55 0.51 0.62

± + ± + -+ ± ± ± ±

Magnitude .04 .08 .03 .06 .05 .05 .02 .08 .06

12.41 12.41 12.32 12.24 12.56 13.14 12.55 12.47 12.54

¢K

Rx"

1.00 ± 0.74 ± 0.71 ± 0.77 ± 0.60 ± 0.35 ± 0.60 ± 0.64 ± 0.59 ±

.04 .08 .01 .03 .06 .04 .02 .05 .05

"Errors shown are 1~ statistical errors.

telescope to make higher spectral resolution (35 cm -x ---, A h / h ~ 0.007) observations of Triton in the region 1.5-2.4 /zm. The observations and results are presented in Tables I and II, and Fig. 1. DISCUSSION

The spectra of Cruikshank and Silvaggio (1979) and Cruikshank (1980) are sufficiently undersampled, (Ah = 0.07/zm; Ah/h ~ 0.03-0.05) that the underlying spectral structure may not be fully represented and could in fact distort the observed spectra. This can be seen clearly in

1.0

1.5

2.0

2.5

WAVELENGTH (/~m) FIG. 1. F T S s p e c t r u m o f T r i t o n .

Fig. 2, where we show the 1.0-2.5-/zm spectra of several frosts (Larson and Fink 1977). For example, in the case of CH4 frost, it may be possible to have spectral points centered in the middle of the three absorption minima in the range 1.6-1.8/zm and " m i s s " the fact that the spectrum is high between these spectral points, giving the impression of a broad feature. Our photometry should therefore provide a test of the accuracy of the spectra deduced by Cruikshank and Silvaggio (1979) and Cruikshank (1980), since we sample more fully and should detect any spectral components between their sample points. Indeed, at the current state of technology, the only feasible means to study such faint objects in the infrared may be to combine

T A B L E II O B S E R V A T I O N S A N D R E S U L T S FOR T R I T O N

~eff

1.220) 1.50 1.63(H) 1.70 2.00 2.11 2.20(K) 2.20 2.35

Magnitude

Rx"

6 / 1 3 / 7 8 LIT

4/17/79 UT

4/20/79 UT

-12.89 12.02 12.43 12.29 13.83 12.59 12.70 12.86

----12.15 12.47 12.39 12.22 12.79

12.26 12.40 12.52 12.35 12.25 12.66 12.52 12.79 12.87

6/13/78 UT

0.47 0.53 0.64 0.73 0.18 0.54 0.49 0.41

---+ -+ -+ -+ -+ -+

.06 .05 .05 .06 .07 .02 .04 .07

Mean~

4/17/79 UT ----0.67 -+ 0.50 0.54 -+ 0.63 -+ 0.36 -

.11 .08 .02 .09 .08

4/20/79 UT 1.00 0.65 0.52 0.61 0.69 0.47 0.54 0.42 0.38

_+ _+ -+ -+ +-+ -+ -+ -+

.02 .05 .01 .03 .06 .04 .01 .03 .04

1.00 0.58 0.52 0.62 0.70 0.41 0.54 0.46 0.38

_ -+ -+ -~ -+ +-+ _+ -

.02 .04 .01 .03 .04 .03 .01 .02 .03

a D a t a f o r 6 / 1 3 / 7 8 a n d 4 / 1 7 / 7 9 n o r m a l i z e d to K = 0.54 ( J = 1.0 f o r 4 / 2 0 / 7 9 ) . E r r o r s s h o w n are Itr statistical eITOFS,

b E r r o r s s h o w n a r e s t a n d a r d d e v i a t i o n s o f t h e m e a n (ltr).

IR SPECTRA OF HYPERION, TITANIA, AND TRITON

171

microns 4.0

3.0 I

2.4 I

2.0 l

18 i

1.6 ' t''

'

1.4 t . . . . . .

12 I '

'

-

~////~

~

~.,,._,

tI I 4000

I

I

6000

8000

C m =1 F I G . 2. S e l e c t e d s p e c t r a o f f r o s t s . F r o m

fully sampled low-resolution data with partiaUy sampled data at higher resolution, with more thorough investigation only of those cases where the two approaches do not yield a unique interpretation. The diag-

Larson

and Fink (1977).

nostic power of our filter set is clear from a comparison of the Pluto spectrum with those of the frosts, which indicates a match only with methane frost (Lebofsky et al. 1979).

172

LEBOFSKY, RIEKE, AND LEBOFSKY

In Fig. 3 we c o m p a r e the p h o t o m e t r y o f H y p e r i o n and Titania. The spectra are fairly similar, although Titania m a y h a v e a slightly lower reflectance at 2.1/xm. In b o t h cases, there a p p e a r to be broad absorption features n e a r 1.5 /~m and 2.0 or 2.1 /~m. L a b o r a t o r y spectra o f various frosts (Kieffer and Smythe, 1973; Smythe, 1975), convolved with our filter transmissions can be found in L e b o f s k y et al. (1979). The spectra o f H y p e r i o n and Titania do not agree perfectly with any o f the frosts. H 2 0 frost is one of the closer matches, but the e x p e c t e d p e a k at 2.2/~m is not apparent. Our m e a s u r e m e n t s of H y p e r i o n are in close a g r e e m e n t with C r u i k s h a n k ' s and those o f Titania are in r e a s o n a b l y good agreement. Cruikshank (1980) identified H 2 0 frost as a m a j o r surface constituent o f these two satellites, but also notes some discrepancies, including a possible anomalous absorption at 2.2/~m for H y p e r i o n and generally less spectral contrast for Titania than would be provided b y a surface c o m pletely c o v e r e d by H 2 0 frost. Additional observations are therefore required before a completely secure identification can be made. 1.0

(a)

0.5

HYPERION

Z

o

i

f

(b)

0.5

TITANIA

1.0

ll.5

2'.0

2.5

WAVELENGTH (p..m)

FIG. 3. (a) Normalized (J = 1.0) spectral reflectance of Hyperion. Error bars are 1~ statistical errors, x, Broadband filters; 0, Narrowband filters. (b) Normalized (J = 1.0) reflectance of Titania. Error bars and symbols as in Part (a).

TABLE III CO-ADDED FTS DATA, A~./~. 0.02 ~eff

Rk a

1.51 1.56 1.59 1.63 1.67 1.73 1.76 2.02 2.08

0.11 -+ 0.035 0.23 --4-0.03 0.11 -+ 0.03 0.25 -+ 0.02 0.17 -+ 0.02 0.30 -+ 0.03 0.19 +- 0.03 0.13 -+ 0.03 -0.04 - 0.03 0.08 - 0.03 0.04 - 0.03 0.09 -+ 0.03

2.14

2.22 2.28

Arbitrary normalization. Because o f the high resolution of the s p e c t r u m and the e x t r e m e faintness o f Triton (ink - 12.5), the F T S data w e r e noisy (see Fig. 1). F o r this reason, we have c o m b i n e d individual spectral points to give a s p e c t r u m with resultant resolution o f A~,/~, - 0.02, as s h o w n in Table III. This s p e c t r u m is c o m p a r e d with our p h o t o m e t r y and with the data of Cruikshank and Silvaggio (1979) in Fig. 4. The a g r e e m e n t b e t w e e n our n a r r o w b a n d p h o t o m e t r y and the F T S s p e c t r u m can be seen to be fairly good. The c o m p a r i s o n o f our data with the Cruikshank and Silvaggio (1979) s p e c t r u m is not as good. The Cruikshank and Silvaggio s p e c t r u m shows no absorption feature in the region 1.5-1.7 # m ; in the region 2 . 0 2.3 ~ m , it decreases toward longer w a v e lengths and has a m a x i m u m at 2.1 # m , in disagreement with b o t h the n a r r o w b a n d and F T S spectra. Cruikshank (private communication) has recently obtained data in closer a g r e e m e n t with ours. Our spectra of Triton are m a r k e d l y different f r o m the spectra o f the other two satellites, f r o m that o f Pluto, and f r o m the laboratory frost spectra. The structure of the spectrum of Triton is m o s t similar to that o f the methane spectrum, but the wavelengths o f the features differ b y several tenths o f a micron:

IR SPECTRA OF HYPERION, TITANIA, AND TRITON 1.0

TRITON

(o)

0.8 0.6 0.4 'z~ 0 . 2 ~ 0.4 b.i I1:

(b) A

0.2

0.0

1=.5 21o WAVELENGTH (/J.m)

I.O

21.5

FIG. 4. (a) Spectral data of Triton from Cruikshank and Silvaggio (1979). (b) FTS spectral data co-added to give a resolution of Ah/h - 0.01 for comparison with the data of Cruikshank and Silvaggio (1979). The large triangles represent our narrowband filter data. The typical 1~ error is approximately the height of the

triangles. the spectrum of Triton is not consistent with a methane-frost-covered surface. Also, Triton's spectrum is not consistent with a methane atmosphere, since there is no major shift in the absorption band centers between the gas and frost.

173

Significant structure in the spectrum of Triton, especially in the region 1.5-1.8/~m is apparent in Fig. 4. We wish to test whether the difference in the ratios of two adjacent spectral points is significant (an absorption feature), or whether the points may be regarded as belonging to the same population. Since we are considering the differences of means of small samples (< 10 samples per spectral point), the Student t test is a valid method for determining the significance of differences between adjacent points (Fisher, 1934). In Table IV we have tabulated the values of t and p, the probability that the difference between adjacent points is fortuitous. It would appear that the structure is in fact significant; i.e., that the true spectrum of Triton has absorption features that show high-resolution (-0.01/~m) structure. It should also be noted that the highresolution structure in the Triton spectrum could explain the discrepancies in the various narrowband spectra. Since the filters tend to overlap the absorption band edges, very slight changes in the depths and widths of the absorptions in the Triton spectrumcould significantly affect the narrowband filter measurements.

TABLE IV S I G N I F I C A N C E OF T R I T O N ' S S P E C T R A L F E A T U R E S

h

n a

Ratio

1 -

1.51 1.56 1.59 1.63 1.67 1.73 1.76 2.29 2.98 2.14 2.22 2.28

Student's t Test

o "b

(~m)

t 11 9 10 10 10 8 7 11 9 9 9 9

0.11 0.23 0.11 0.25 0.17 0.30 0.19 0.13 -0.04 0.08 0.04 0.09

0.035 0.03 0.03 0.02 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.03

2.54 2.82 3.89 2.82 3.51 2.58

0.02 0.01 0.001 0.01 <0.002 0.02

3.91 2.63 0.94 1.18

0.001 0.02 0.35 0.25

a n, Number of interferogram points averaged. b 1 - or, Standard deviation of the mean. c p , Probability of the difference between the pair of points being fortuitous.

Possibly significant Possibly significant Significant Possibly significant

Significant Possibly significant Significant Possibly significant Not significant Not significant

174

LEBOFSKY, RIEKE, AND LEBOFSKY SUMMARY

The results presented here seem to raise more questions rather than produce answers. Hyperion and Titania may be water f r o s t c o v e r e d , b u t as p o i n t e d o u t b y C r u i k s h a n k (1980) a n d m o r e s t r o n g l y in t h i s paper, the discrepancy between the satellite s p e c t r a a n d t h e l a b o r a t o r y s p e c t r a o f v a r i o u s f r o s t s r a i s e s q u e s t i o n s as t o t h e identification. Our results clearly show that the metha n e , as e i t h e r a s u r f a c e f r o s t o r in t h e a t m o s p h e r e , is n o t a m a j o r c o n s t i t u e n t o f T r i t o n . T h i s is in d i s a g r e e m e n t w i t h t h e r e s u l t s o f C r u i k s h a n k a n d S i l v a g g i o (1979) b u t is c o n s i s t e n t w i t h t h e n o n d e t e c t i o n o f a methane atmosphere b y J o h n s o n e t al. (1980). A m o n g p l a u s i b l e s u r f a c e c o n s t i t u e n t s , t h e r e is as y e t n o c l e a r i d e n t i f i c a t i o n for Triton.

ACKNOWLEDGMENT This work was supported by NASA Grant NSG7114.

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