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Post-eclipse brightening and non-brightening of Io
ZC~RUS 25, 439-446 (1975)
Post-Eclipse Brightening and Non-Brightening of Io HERBERT FREY Astronomy Program, University of Maryland, College Park, Maryland 20740 and Geophysica Branch, GoddardSpace Flig~ Center, Greenbelt, Maryland 20771 1Received J u n e 17, 1974; r e v i s e d F e b r u a r y 3, 1975 I t m a y b e possible t o u n d e r s t a n d t h e a p p a r e n t i n t e r m i t t e n t n a t u r e of t h e post-eclipse b r i g h t e n i n g s a n d n o n b r i g h t e n i n g s o f I o in t e r m s of a n o n u n i f o r m d i s t r i b u t i o n of blue reflectors g r o u p e d in t h e h e m i s p h e r e c e n t e r e d a t 0 ° l o n g i t u d e . T h e d i m e n s i o n s r e q u i r e d for s u c h blue m i r r o r s a r e c o n s i s t e n t w i t h v e r y large c r a t e r s . T h e h i g h blue a l b e d o of w a t e r f r o s t a n d o t h e r ices m a k e s t h e s e m a t e r i a l s likely c a n d i d a t e s for t h e reflectors. I . OBSERVATIONAL HISTORY
The problem of an anomalous, posteclipse brightening of Io remains unsolved. Binder and Crnickshank (1964) first reported an apparent brightening in the blue ( 2 ~ 0.45pm) of approximately 0.1 magnitudes which decayed in about 15 min to the pre-eclipse level. O'Leary and Veverka (1971) reported uncertain results during the reappearance of March 31, 1969, while observing at 0.70pm; the uncertainty was attributed to corrections for scattered light from Jupiter. These same authors did, however, observe a 0.1-0.2 magnitude brightening at 0.5pro wavelength on the night of May 2, 1969, an event also observed by Johnson (1971 ). Johnson confirmed the brightening, b u t reported an increase of some 0.7 magnitudes at 0.44pm. O'Leary and Veverka (1971) found no enhancement at 0.85pro on June 3, 1969, but suggested a possible positive result at 0.35pm on the same night, depending on the sky subtraction used. ' Franz and Millis (1971) searched unsuccessfully for the brightening on the nights of June 10 and J u l y 3, 1969 (A 0.55t~m) and May 21 and June 13, 1970 (A~0.45~m). Because their technique provided better correction for scattered light, these authors concluded that the previously reported anomalous brightening did not exist. Fallen and Murphy (1971) confirmed the negative result on June 13, 1970 ( ) ~ 0 . 4 4 p m ) , and also Copright © 1975by AcademicPre~. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
found no brightening on June 4, June 11, and June 20, 1970. O'Leary and Miner (1973) obtained uncertain results ("probable") for the June 25, 1971, event. Cruikshank and Murphy (1973) found no brightening on J u l y 25, 1971, uncertain results for the nights of July 9 and August, 17, 197l, and positive brightening on August 28 and September 20, 1972. In reviewing the observations, these authors concluded that the brightening is real b u t intermittent. Cruikshank and Murphy (1973) attributed the intermittent nature of the eclipse brightening to a marginally stable frost condition, and suggested that the changing heliocentric radius of Jupiter correlates with the observed brightenings and nonbrightenings. With Jupiter at perihelion, Io's tenuous atmosphere would condense during eclipse and sublime upon emergence, b u t at aphelion the atmosphere is always frozen out. Sinton (1973) suggested that a condensable ammonia atmosphere, responding to the changing insolation between solstice and equinox, not only could account for the post-eclipse brightening, but also would explain the discordant 10 and 20/~m brightness temperatures observed for Io (Morrison et al., 1972; Hansen, 1973), as well as the departure of Io's post-eclipse heating curve from the two-layer model inferred from the cooling curve (Morrison and Cruikshank, 1973). The atmosphere he requires for this has a surface pressure of only 5.5 × 10 -8 bars, which is within the
439
O J O O O F F
69 69 69 69 69 69 69
70 70 70 70 70 70
31 Mar. 2May 2 May 3 June 3 June 10 J u n e 3 July
21 M a y 4 June 11 J u n e 13 J u n e 13 J u n e 20 J u n e
25 J u n e 71 9 J u l y 71
B &C B &C B &C
11 Oct. 63 19 Dec. 63 26 Dec. 63
O &M C &M
F &• Fa & Mu Fa & Mu F & 1~[ Fa & Mu Fa & Mu
&V &V &V &M &M
&V
B &C
Reference
15 o c t . 62
Date
®t ®2
o o o o o o
@ • • l) o o o
• • •
@
Code
0.35, 0.40 0.47
0.45 0.65 0.44 0.45 0.44 0.41, 0.47
0.70 0.44/0.56 0.50 0.35 0.85 0.55 0.55
0.45 0.45 0.45
0.45
Wavelength
8 5 5 1 1 3 3
27 03 03 41 41 36 50
4 12 4 56 6 51
25 9
5 31 9 21
21 6 05 4 9 54 11 11 49 13 6 18 13 6 18 20 8 13
31 2 2 3 3 10 3
11 19 26
15 d 2h41 m
Time
6.27 8.31
5.68 7.82 8.65 8.88 8.88 9.56
1.93 7.50 7.50 10.41 10.41 10.64 10.49
0.63 11.15 11.39
8.°57
As - A~
POsT-EcLiPSE OBSERVA~ONS OF I o
TABLE I
15.95 17.99
15.36 17.50 18.33 18.56 18.56 19.24
11.67 17.18 17.18 20.09 20.09 20.32 20.17
10.31 20.83 21.07
18725
.Le~
De)~2
--2.99 -2.95
--3.04 --3.01 --3.00 -2.99 --2.99 --2.98
--2.30 --2.26 --2.26 --2.23 --2.23 --2.23 --2.24
+2.77 +2.66 +2.68
+1.°26
(Ds +
+0.11 +0.08
+0.10 +0.01 -0.03 -0.04 -0.04 -0.09
+0.21 -0.05 -0.05 -0.27 -0.27 -0.30 -0.38
--0.31 +0.24 +0.28
+0.°13
(Ds -- D£)
tv
m
C &M C &M
Met Met Met Met f &m Met Met f &m Met f &m f &m Met
28 Aug. 72 20 Sept. 72
26 Aug. 73 2 Sept. 73 11 Sept. 73 18 Sept. 73 18 Sept. 73 25 Sept. 73 30 Sept. 73 4 Oct. 73 7 Oct. 73 11 Oct. 73 27 Oct. 73 15 Nov. 73
References: B & C ' -O'&V J F~M Fa & Mu O&M C&M Met f&m
C &M C &M
25 J u l y 71 17 Aug. 71 0.47 0.47 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45
• • o o
0.45 0.45
o
Binder and Cruikshank (1964). O'Leary and Veverka (1971.). Johnson (1971). Franz and Millis (1971). Fallen and Murphy (1971 ). O'Leary and Miner (1973). Cruiksbank and Murphy (1973). Millis eta/. (1974). Franz and Millis (1974).
o
0.45
o
o
o o
o o
o
o
0.47 0.47
0 ®3 7 17 7 31
7 39 7 53
19.62 20.60 19.92 20.80 15.17 16.43 17.81 18.73 18.73 19.51 19.99 20.25 20.48 20.66 21.00
20.45
9.94 10.92 10.24 11.12 5.47 6.73 8.11 9.03 9.03 9.81 10.29 10.55 10.78 10.96 11.30
10.75
--0.39 --0.39 --0.37 --0.36 --0.36 -0.35 --0.34 --0.33 --0.33 --0.31 --0.26 -0.20
--1.97 --1.97
--2.91 --2.85
+0.04 +0.09 +0.14 +0.20 +0.20 +0.23 +0.26 +0.28 +0.29 +0.30 +0.36 +0.39
+0.18 +0.19
+0.03 --0.01
Code : • o
Brightening observed. No brightening observed. Brightening w/N-sky subtraction (none w/S-sky subtraction). qD Brightening w/S-sky subtraction (none w/N-sky subtraction). @ Uncertain results: 1 "yes?", 2 "probably", a "uncertain".
26 3 04 2 4 58 11 1 22 18 3 15 18 3 15 25 5 12 30 12 38 4 1 34 7 14 33 11 3 29 27 1 49 15 13 07
28 20
25 17
Q
o
Q
t~
o
442
HERBERT FREY
upper limit set by the occultation of fl Scorpii C (Smith and Smith, 1972), and is approximately the amount inferred from the Pioneer 10 radio occultation experiment (Kliore et al., 1974). Both of the above models suggest that post-eclipse brightenings should have occurred in 1973, which is spring equinox for Jupiter (and for Io), and which is also a time when Jupiter is approaching a 1975 perihelion. Millis and co-workers searched for an anomalous brightening during 11 reappearances in 1973 (Millis et al., 1974; Franz and Millis, 1974). No brightening greater than a few hundreths of a magnitude was observed. The above models cannot, by themselves, explain the apparent intermittent nature of the events. In this paper we assume t h a t both positive and negative observations (summarized in Table I) are real : the brightening is assumed to exist at some times, but is not present at other observations. We suggest a possible cause of t h e variable nature of the phenomenon. I I . GEOMETRY OF THE ECLIPSE REAPPEARANCE
I t is possible to determine the Io-centric longitude observed from Earth (the subEarth longitude) at the moment of eclipse emergence from the size of the shadow cone on Io's orbit due to Jupiter and the known positions of the Earth and Sun. The orbital inclination of Io from the Jovian equator is negligible; we further assume a spin-lock or synchronous rotation for Io, as indicated by its light curve (Harris, 1961; Johnson, 1971). We take as the Io central meridian (0 ° longitude)
a.
the great circle through the sub-Jupiter point and the two poles. Figure 1 represents the geometry of the reappearance. The angular size of the Jovian disk as seen from Io is 19.°4. Ignoring refraction due to Jupiter's atmosphere, this is also the size of the shadow cone cast by Jupiter on Io's orbit. The longitude seen on Io from the Sun at the moment of emergence is then 9?7, assuming there are no librations. This is also the longitude seen from the Earth at emergence during opposition. For emergence after opposition, the possible Io-centric longitudes seen from Earth at time of reappearance are 9.7 ~ -~fe ~ 21.7, because the maximum phase angle (¢ in Fig. l) is 12° for Jupiter. I f ~e is the angle Sun-Jupiter-Io at time of emergence, the Io-centric longitude seen from the Earth is -~'e = 180° A- ~b+ ~E = ~b+ 9.07. The angle ¢ may be determined from the known Jovicentric right ascension of the Sun and Earth, ¢ = A s - AE. These quantities are tabulated in the American Ephemeris for 0 ~ UT; in Table I, ¢ and ~ r are given at the moment of reappearance for each of the observed events. Also listed are the quantities (D s + DE)~2 and (D s - De). D s and Dr are the declination of the Sun and Earth measured, as are A s and An, in the Jovicentric coordinate system. The quantity (D s - D r ) is hereafter called the illumination angle; it is the north-south component of the combined angles of incidence and reflection at Io. (D s + DE)~2 is the effective latitude on Io for a specular reflection on a surface assumed to be spherical.
jl
\,~
~\
,
/ 1
FIG, 1. Geometry of eclipse reappearance for Io. ~b-- angle Sun-Jupiter-Earth. ~ -~ angle SunJupiter-Io.
POST-ECLIPSE
BRIGHTENING
I I I . RESULTS Figure 2 is a plot of the effective latitude (Ds + DE)[ 2 against the Io-centric longitude at time of eclipse reappearance. The code used to represent the observations is the same as found in Column 3 of Table I. All northern hemisphere (defined by (Ds + De)/2 > 0) events are positive: these are the observations of Binder and Cruikshank (1964). Because of the relative orbital inclinations of the Earth and Jupiter, all later observations fall in the southern hemisphere of the satellite. Of these later observations, positive brightenings are reported in a band - 2 . 0 < (D s - DE)/2 <--2.25, but this band also includes two uncertain and three negative observations. Below this all observations are negative or uncertain. The fall 1973 observations lie in the region -0.2 to -0.4, and these are all negative. Observations over the next several years will again yield points in the northern hemisphere of Io. This same clustering is obvious on plots of ( D s - De) against the effective latitude and against the longitude on Io, but the correlation is not so striking. In summary, positive observations consist of only 25% (8/32) of the total, and in the southern hemisphere only 14% (4/28). Nonetheless there remains the important
L i I I ~ '•I w [t
+~f+2 ~ I
0
e~
®
® °o~
I I i I 26
24
2';'
0
• ®o
, ( , I i I ~ 20 ~B 16 ~4 10 - C e n t r i c
I j ~2
i
i
I
Ion~ilude
FIG. 2. Location of sub-Earth point during eclipse reappearance of Io. Io-centric longitude is measured from the meridian passing through the poles and the sub-Jupiter point on Io. Code used is same as in Table 1.
OF Io
443
fact t h a t there has to date been no simultaneous positive and negative observations by independent workers, while one positive and two negative events have been independently confirmed by different observers. It m a y be that the intermittent nature of the apparent brightenings is in part due to the different sub-Earth points on Io during the different events. IV. Dlscusslo~ The cause of the brightening, if real, remains unexplained. It is likely t h a t some nonuniform distribution of reflective surfaces is involved. The possible nature of these surfaces is discussed below. The photometry of Harris (1961), Johnson (1971), Owen and Lazor {1973), and Morrison et al. (1974) have demonstrated t h a t the hemisphere centered on 0 ° Io-centric longitude is red compared to the bluer disk seen at 180% Morrison et al. (1974) find that the u--y color (on the Str6mgren system) ranges from +2.60 at 0 ° to +2.15 at 180 °. Johnson (1971) reports a change in the intensity ratio at 0.4~m/ 0.56~m from 0.330 at 0 ° to a peak at 160 ° of 0.390. Detailed examination of the magnitudes and color curves as a function of rotational phase reveals much structure, consistent with the mottled surface reported by visual observers of Io (e.g., Dollfus, 1961). Table I indicates t h a t positive results are nearly always made at short wavelengths, but on a hemisphere t h a t is "red." No observations at wavelengths longer than 0.6 have confirmed a brightening. Io is very red (B-V ~ 1.17) compared to the other Galilean satellites (B-V ~ 0.85) (Harris, 1961). The spectral reflectivity of this satelhte shows a sharp drop in the blue (Johnson, 1971 ; Johnson and McCord, 1970; Wamsteker et al., 1974), far steeper than t h a t of the other three large satellites. The near-infrared reflectivity is higher for Io than for the others, and the albedo at 3.5~m and 5.0~m is very high compared to J I I , J I I I , and J I V (Gillett et ed., 1970). I t is possible to account for the observed brightening by an increase in the blue
44 4
HERBERT F R E y
albedo at specific Io-graphic locations on a nominally red hemisphere. A perfect blue mirror (albedo = 1.0 at 0.45pro) located on the hemisphere centered at 0 ° longitude would have to be only 200-500km in radius to produce a 10% increase in the refieetivity of the observed hemisphere at this wavelength. We have assumed that the mirror obeys the same scattering law as does the rest of the surface. The uncertainty in the size is due to the uncertainty in Io's blue albedo. This area is not inconsistent with that of large impact craters. I t is not likely that any one brightening can be ascribed to a single mirror with the above dimensions: the proximity of positive and negative sub-Earth points in the longitude range 18°-21 ° argues against such a situation. The effective area of the mirror m a y be composed of several smaller mirrors which, if properly aligned, may be widely spaced. The points shown in Fig. 2 are only the centers of the effective single mirror. Furthermore, we have chosen an unfortunate case by permitting the mirrors to obey only the scattering law applicable to the general surface of Io. A function with a higher backscattering component would allow the reflectors to be smaller. For example, ice has a stronger specular component than does fresh snow (see Veverka, 1973). It is of interest to compare the dimensions of such a blue reflector with that described by the ground trace covered for the reported duration of the brightening. The difference between the beginning and ending longitudes of the enhancement is / I L = L e n d - - Lbcsln = (Llt/Plo) × 360°, where/It is the duration of the brightening and Plo is the period of revolution of Io, both in minutes of time. For a brightening lasting 15 minutes, the range in longitude is 2.°2. Using a radius of 1.82× 103km (O'Leary and Van Flandern, 1972), this corresponds to some 70kin at the equator, which is a reasonable fraction of the effective mirror for a specular reflection. I f the brightening is due to a high albedo, short wavelength reflector, it is not surprising that the phenomenon is generally not seen on the other satellites, whose
reflectivity in the blue is not nearly so low, and whose color is not nearly so red as that oflo. The nature of such a blue reflector is difficult to determine, b u t again some data are available. Pilcher et al. (1972) and Fink e t a / . (1973) have identified water frost in the infrared reflection spectra of / i I , J I I I , and p o s s i b l y / I V . The observations of J I suggest a small amount of ice, b u t probably less than 10% of the surface refiectivity in the 1-2.5pro region is due to water frost. Io's high reflectivity in the near infrared is indicative of frosts, b u t the sharp decrease in the blue for J I is not suggestive of the naturally occurring frosts of H20, CH4, or N i l 3 (Morrison and Cruikshank, 1973; Johnson and McCord, 1970). Polarization measurements are consistent with a thin, multiple-scattering surface layer on J I , J I I , and J I I I (Veverka, 1971). Eclipse curves at 10 and 20/~m can be best represented by a twolayer model in which a thin layer of low conductivity is underlain by a much thicker layer of high conductivity (Morrison and Cruikshank, 1973). Because of its high blue reflectivity, frost nonuniformly distributed on the leading side of Io is a candidate for the reflector that may explain the sometimes observed post-eclipse brightenings. Several naturally occurring ices may fit the observations (cf. Wamsteker et al., 1974). Several questions are raised by this model. Are these permanent reflectors, or are they transistory in the sense suggested by Cruikshank and Murphy (1973) and Sinton (1973). This question cannot be answered at this time. Repeated observations at sub-Earth longitudes already observed are required to solve the problem. Clearly the models suggested previously cannot account for the lack of positive results in 1973. It may be that a timevariable phenomenon combined with the Io-graphic variable suggested here will be required to satisfactorily account for the observations. I f the reflectors are permanent features on the surface of Io, are they, by chance, located only in the narrow longitude range suggested by the eclipse reappearances.
POST-ECIXPSE BRIGHTENING OF Io
Do they exist elsewhere on the surface, and if so, why have they not been observed. The hemisphere centered on 160°-180 ° is the "blue" side of Io; if blue reflectors such as suggested here were located on this hemisphere, it is unlikely they could be observed against the intrinsically blue background. Indeed, it might be argued t h a t the increasing blue refleetivity of the satellite from 90°-180 ° is due to an increase in the number of blue reflectors, which eventually come to dominate the surface at these longitudes. The post-eclipse brightening is then due to a relatively small number of blue mirrors on the redder hemisphere seen during eclipse reappearances. Blue reflectors must be nearly absent from the hemisphere centered at 270 ° . Morrison et al. (1974) and Johnson (1971) both show a sharp drop in the blue reflectivity of the satellite at these longitudes. Mirrors may exist at other phases, but it would require careful and extended patrol work to reveal their presence if the Io-graphic locations of the specular component are the important variable. The fine structure of the color curves with orbital phase suggests a nonuniform distribution of reflective surfaces whose size is smaller than the observed hemisphere. I t is not at all unlikely t h a t blue reflectors exist elsewhere on the surface, and may be observable against a relatively "red" background. V'. CONCLUSIONS I t may be possible to explain the intermittent history of the post-eclipse brightening and nonbrightening of Io in terms of a nonuniform distribution of high-blue albedo reflectors, grouped on the "red" hemisphere centered at 0 ° longitude of the satellite. The required dimensions of such reflectors are consistent with large impact craters observed on the Moon and on Mars. Naturally occurring ices in such bowlshaped depressions is a likely candidate for the blue reflector. The brightening is then essentially a contrast effect for this very red satellite. Admittedly oversimplified, this explan-
445
ation has assumed t h a t all published observations are correct as stated. Because of the serious and difficult problem of correcting for scattered light from Jupiter (see Franz and Millls, 1974), it is not clear t h a t the reported anomalous brightenings are indeed real. I f they are real, then the fact t h a t some observers could not find such an enhancement has a reasonable explanation in terms of the geographic distribution of blue reflectors. Many more observations are needed, both to confirm the brightenings and the locations of the above suggested mirrors. ACKNOWLEDGMENTS Special thanks to Bob Millis of the Lowell Observatory for several instructive discussions, especially on the scattered light problem. Thanks also to M. F. A ' H e a r n for a critical reading of a rough draft of this manuscript, and for suggestions along relevant lines of study. This work was supported in part by NASA Grant NGL-21002-033., REFERENCES BINDER, A. B., A~N'DCRUIKSHAI~K,D. P. (1964). Evidence for an Atmosphere on Io. Icarus 3, 299-305.
CRUIZSHA~rK,D. P., ANDMt~RP~y,R. E. (1973). The post-eclipse brightening of Io. Icarus 20, 7-17. DOLLF~S, A. (1961). Visual and photographic studies of planets at Pic du Midi. In P/anet~ and Sate/l/te~ (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 534-571. University of Chicago Press, Chicago. FAE,LOlq', F. W., AND mURPHY, R. E. (1971). Absence of post-eclipse brightening of Io and Europa in 1970. Icarus 15, 492-496. FIN~:, U., DEKKERS, N. H., A~rD L~aso~r, H. P. (1973). Infrared spectra of the Galilean satellites of Jupiter. Astrophys. J. 179, L155. FRA_~z, O. G., AND M~LIS, R. L. (1971). A search for an anomalous brightening of Io after eclipse. Icarus 14, 13--15. FRAI~Z, O. Cv., AND MILLIS, R. L. (1974). A search for post-eclipse brightening of Io in 1973. II. Icaru~ 23, 431-436. GILLETT, F. C., x-~ERRTLL, K. M., AZ~D STERN, W . A. (1970). Albedo and thermal omission of Jovian satellites I-IV. Astrophys. LetS. 6, 247. HA~SEN, O. L. (1973). Ten micron eclipse observations of Io, Europa and Ganymede. Icaru~ 18, 237-246.
446
HERBERT
FSEY
HARRIS, D. L. (1961}. Photometry and coloriof the Galilcan satellites. Icarua 23, 399metry of planets and satellites. I n P/aneta and 416. Satellites (G. P. Kuiper and B. M. Middlehurst, O'LEARY, B., AND MINER, E. (1973). Another possible post eclipse brightening of Io. Icarus Eds.), pp. 272-342. University of Chicago. Press, Chicago. 20, 18-20. JOHNSON, T. V. (1971). Galilean satellites: 0'LEA~tY, B., AND V ~ FLAZrD~RN, T. C. (1972). Io's triaxial figure. Icarus 17, 209-215. narrow-band photometry 0.30 to 1.10 microns. O'LEARY, B., AND VEV~aKA, J. (1971). On the Icarus 14, 94--111. anomalous brightening of Io after eclipse. JOHNSON, T. V., ~ I~[cCoRD, T. B. (1970). Icarus 14, 265-268. Galilean satellites: The spectral reflectivity OwE~, F. :N., AND LAZOR, F. J. (1973). Surface 0.30-0110 microns. Icarus 13, 37-42 color variations of the Galilean satellites. KLZORE, A., CAr~', D., FJELDBO, G., AI~D Icarus 19, 30-33. SErD~_~, B. L, (1974). Preliminary results on the atmospheres of Io and Jupiter from PILCKER, C. ]3., RIDGWAY, S. T., AN~ McCORD, T. B. (1972). Galilean satellites: Identification Pioneer 10S-band occultation experiment. of water frost. Science 178, 1087. Sc/ence 183, 323. SzNro~-, W. M. (1973). Does Io have an ammonia MILLIS, :R. L., THOMPSON, D. T., ~I.%~RIS, B. J., atmosphere? Icarus $0, 284-296. BLaCH, P., ~.~-DSEXTON,R. (1974). A search for S~zrH, B. A., AND SMITH, S. A. (1972). Upper post-eclipse brightening o~ Io in 1973. I. I c a r ~ limits for an atmosphere on Io. Icarus 17, 23, 425-430. 218-222. I~ORRISON, D., AND CRUIKSHANK, D. P. (1973). VEV~KA, J. (1971). Polarization measurements Thermal properties of the Galilean satellites. of the Galilean satellites of Jupiter. Icarus 14, Icarus 18, 224-236. 355-359. MORRISON,D., CRUIKSHA17K,D. P., ANDMURPHY, V~VF~RKA,J. (1973). The photometric properties R. E. (1972). Temperature of Titan and the of natural snow and of snow-covered planets. Galilean satellites at 20 microns. As~rophys. J'. Icar~q 20, 304-310. 173, L143. WAMSTEKER,W., K_ROES,R. L., AND FOUl,'rAIN, J. A. (1974). On the surface composition of Io. MORRISON, D., I~/~ORRISON,1%T.D., AND LAZAREWZCZ, A. R. (1974). Four-color photometry Icarus 23, 417-424.
Post-Eclipse Brightening and Non-Brightening of Io HERBERT FREY Astronomy Program, University of Maryland, College Park, Maryland 20740 and Geophysica Branch, GoddardSpace Flig~ Center, Greenbelt, Maryland 20771 1Received J u n e 17, 1974; r e v i s e d F e b r u a r y 3, 1975 I t m a y b e possible t o u n d e r s t a n d t h e a p p a r e n t i n t e r m i t t e n t n a t u r e of t h e post-eclipse b r i g h t e n i n g s a n d n o n b r i g h t e n i n g s o f I o in t e r m s of a n o n u n i f o r m d i s t r i b u t i o n of blue reflectors g r o u p e d in t h e h e m i s p h e r e c e n t e r e d a t 0 ° l o n g i t u d e . T h e d i m e n s i o n s r e q u i r e d for s u c h blue m i r r o r s a r e c o n s i s t e n t w i t h v e r y large c r a t e r s . T h e h i g h blue a l b e d o of w a t e r f r o s t a n d o t h e r ices m a k e s t h e s e m a t e r i a l s likely c a n d i d a t e s for t h e reflectors. I . OBSERVATIONAL HISTORY
The problem of an anomalous, posteclipse brightening of Io remains unsolved. Binder and Crnickshank (1964) first reported an apparent brightening in the blue ( 2 ~ 0.45pm) of approximately 0.1 magnitudes which decayed in about 15 min to the pre-eclipse level. O'Leary and Veverka (1971) reported uncertain results during the reappearance of March 31, 1969, while observing at 0.70pm; the uncertainty was attributed to corrections for scattered light from Jupiter. These same authors did, however, observe a 0.1-0.2 magnitude brightening at 0.5pro wavelength on the night of May 2, 1969, an event also observed by Johnson (1971 ). Johnson confirmed the brightening, b u t reported an increase of some 0.7 magnitudes at 0.44pm. O'Leary and Veverka (1971) found no enhancement at 0.85pro on June 3, 1969, but suggested a possible positive result at 0.35pm on the same night, depending on the sky subtraction used. ' Franz and Millis (1971) searched unsuccessfully for the brightening on the nights of June 10 and J u l y 3, 1969 (A 0.55t~m) and May 21 and June 13, 1970 (A~0.45~m). Because their technique provided better correction for scattered light, these authors concluded that the previously reported anomalous brightening did not exist. Fallen and Murphy (1971) confirmed the negative result on June 13, 1970 ( ) ~ 0 . 4 4 p m ) , and also Copright © 1975by AcademicPre~. Inc. All rights of reproduction in any form reserved. Printed in Great Britain
found no brightening on June 4, June 11, and June 20, 1970. O'Leary and Miner (1973) obtained uncertain results ("probable") for the June 25, 1971, event. Cruikshank and Murphy (1973) found no brightening on J u l y 25, 1971, uncertain results for the nights of July 9 and August, 17, 197l, and positive brightening on August 28 and September 20, 1972. In reviewing the observations, these authors concluded that the brightening is real b u t intermittent. Cruikshank and Murphy (1973) attributed the intermittent nature of the eclipse brightening to a marginally stable frost condition, and suggested that the changing heliocentric radius of Jupiter correlates with the observed brightenings and nonbrightenings. With Jupiter at perihelion, Io's tenuous atmosphere would condense during eclipse and sublime upon emergence, b u t at aphelion the atmosphere is always frozen out. Sinton (1973) suggested that a condensable ammonia atmosphere, responding to the changing insolation between solstice and equinox, not only could account for the post-eclipse brightening, but also would explain the discordant 10 and 20/~m brightness temperatures observed for Io (Morrison et al., 1972; Hansen, 1973), as well as the departure of Io's post-eclipse heating curve from the two-layer model inferred from the cooling curve (Morrison and Cruikshank, 1973). The atmosphere he requires for this has a surface pressure of only 5.5 × 10 -8 bars, which is within the
439
O J O O O F F
69 69 69 69 69 69 69
70 70 70 70 70 70
31 Mar. 2May 2 May 3 June 3 June 10 J u n e 3 July
21 M a y 4 June 11 J u n e 13 J u n e 13 J u n e 20 J u n e
25 J u n e 71 9 J u l y 71
B &C B &C B &C
11 Oct. 63 19 Dec. 63 26 Dec. 63
O &M C &M
F &• Fa & Mu Fa & Mu F & 1~[ Fa & Mu Fa & Mu
&V &V &V &M &M
&V
B &C
Reference
15 o c t . 62
Date
®t ®2
o o o o o o
@ • • l) o o o
• • •
@
Code
0.35, 0.40 0.47
0.45 0.65 0.44 0.45 0.44 0.41, 0.47
0.70 0.44/0.56 0.50 0.35 0.85 0.55 0.55
0.45 0.45 0.45
0.45
Wavelength
8 5 5 1 1 3 3
27 03 03 41 41 36 50
4 12 4 56 6 51
25 9
5 31 9 21
21 6 05 4 9 54 11 11 49 13 6 18 13 6 18 20 8 13
31 2 2 3 3 10 3
11 19 26
15 d 2h41 m
Time
6.27 8.31
5.68 7.82 8.65 8.88 8.88 9.56
1.93 7.50 7.50 10.41 10.41 10.64 10.49
0.63 11.15 11.39
8.°57
As - A~
POsT-EcLiPSE OBSERVA~ONS OF I o
TABLE I
15.95 17.99
15.36 17.50 18.33 18.56 18.56 19.24
11.67 17.18 17.18 20.09 20.09 20.32 20.17
10.31 20.83 21.07
18725
.Le~
De)~2
--2.99 -2.95
--3.04 --3.01 --3.00 -2.99 --2.99 --2.98
--2.30 --2.26 --2.26 --2.23 --2.23 --2.23 --2.24
+2.77 +2.66 +2.68
+1.°26
(Ds +
+0.11 +0.08
+0.10 +0.01 -0.03 -0.04 -0.04 -0.09
+0.21 -0.05 -0.05 -0.27 -0.27 -0.30 -0.38
--0.31 +0.24 +0.28
+0.°13
(Ds -- D£)
tv
m
C &M C &M
Met Met Met Met f &m Met Met f &m Met f &m f &m Met
28 Aug. 72 20 Sept. 72
26 Aug. 73 2 Sept. 73 11 Sept. 73 18 Sept. 73 18 Sept. 73 25 Sept. 73 30 Sept. 73 4 Oct. 73 7 Oct. 73 11 Oct. 73 27 Oct. 73 15 Nov. 73
References: B & C ' -O'&V J F~M Fa & Mu O&M C&M Met f&m
C &M C &M
25 J u l y 71 17 Aug. 71 0.47 0.47 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45 0.45
• • o o
0.45 0.45
o
Binder and Cruikshank (1964). O'Leary and Veverka (1971.). Johnson (1971). Franz and Millis (1971). Fallen and Murphy (1971 ). O'Leary and Miner (1973). Cruiksbank and Murphy (1973). Millis eta/. (1974). Franz and Millis (1974).
o
0.45
o
o
o o
o o
o
o
0.47 0.47
0 ®3 7 17 7 31
7 39 7 53
19.62 20.60 19.92 20.80 15.17 16.43 17.81 18.73 18.73 19.51 19.99 20.25 20.48 20.66 21.00
20.45
9.94 10.92 10.24 11.12 5.47 6.73 8.11 9.03 9.03 9.81 10.29 10.55 10.78 10.96 11.30
10.75
--0.39 --0.39 --0.37 --0.36 --0.36 -0.35 --0.34 --0.33 --0.33 --0.31 --0.26 -0.20
--1.97 --1.97
--2.91 --2.85
+0.04 +0.09 +0.14 +0.20 +0.20 +0.23 +0.26 +0.28 +0.29 +0.30 +0.36 +0.39
+0.18 +0.19
+0.03 --0.01
Code : • o
Brightening observed. No brightening observed. Brightening w/N-sky subtraction (none w/S-sky subtraction). qD Brightening w/S-sky subtraction (none w/N-sky subtraction). @ Uncertain results: 1 "yes?", 2 "probably", a "uncertain".
26 3 04 2 4 58 11 1 22 18 3 15 18 3 15 25 5 12 30 12 38 4 1 34 7 14 33 11 3 29 27 1 49 15 13 07
28 20
25 17
Q
o
Q
t~
o
442
HERBERT FREY
upper limit set by the occultation of fl Scorpii C (Smith and Smith, 1972), and is approximately the amount inferred from the Pioneer 10 radio occultation experiment (Kliore et al., 1974). Both of the above models suggest that post-eclipse brightenings should have occurred in 1973, which is spring equinox for Jupiter (and for Io), and which is also a time when Jupiter is approaching a 1975 perihelion. Millis and co-workers searched for an anomalous brightening during 11 reappearances in 1973 (Millis et al., 1974; Franz and Millis, 1974). No brightening greater than a few hundreths of a magnitude was observed. The above models cannot, by themselves, explain the apparent intermittent nature of the events. In this paper we assume t h a t both positive and negative observations (summarized in Table I) are real : the brightening is assumed to exist at some times, but is not present at other observations. We suggest a possible cause of t h e variable nature of the phenomenon. I I . GEOMETRY OF THE ECLIPSE REAPPEARANCE
I t is possible to determine the Io-centric longitude observed from Earth (the subEarth longitude) at the moment of eclipse emergence from the size of the shadow cone on Io's orbit due to Jupiter and the known positions of the Earth and Sun. The orbital inclination of Io from the Jovian equator is negligible; we further assume a spin-lock or synchronous rotation for Io, as indicated by its light curve (Harris, 1961; Johnson, 1971). We take as the Io central meridian (0 ° longitude)
a.
the great circle through the sub-Jupiter point and the two poles. Figure 1 represents the geometry of the reappearance. The angular size of the Jovian disk as seen from Io is 19.°4. Ignoring refraction due to Jupiter's atmosphere, this is also the size of the shadow cone cast by Jupiter on Io's orbit. The longitude seen on Io from the Sun at the moment of emergence is then 9?7, assuming there are no librations. This is also the longitude seen from the Earth at emergence during opposition. For emergence after opposition, the possible Io-centric longitudes seen from Earth at time of reappearance are 9.7 ~ -~fe ~ 21.7, because the maximum phase angle (¢ in Fig. l) is 12° for Jupiter. I f ~e is the angle Sun-Jupiter-Io at time of emergence, the Io-centric longitude seen from the Earth is -~'e = 180° A- ~b+ ~E = ~b+ 9.07. The angle ¢ may be determined from the known Jovicentric right ascension of the Sun and Earth, ¢ = A s - AE. These quantities are tabulated in the American Ephemeris for 0 ~ UT; in Table I, ¢ and ~ r are given at the moment of reappearance for each of the observed events. Also listed are the quantities (D s + DE)~2 and (D s - De). D s and Dr are the declination of the Sun and Earth measured, as are A s and An, in the Jovicentric coordinate system. The quantity (D s - D r ) is hereafter called the illumination angle; it is the north-south component of the combined angles of incidence and reflection at Io. (D s + DE)~2 is the effective latitude on Io for a specular reflection on a surface assumed to be spherical.
jl
\,~
~\
,
/ 1
FIG, 1. Geometry of eclipse reappearance for Io. ~b-- angle Sun-Jupiter-Earth. ~ -~ angle SunJupiter-Io.
POST-ECLIPSE
BRIGHTENING
I I I . RESULTS Figure 2 is a plot of the effective latitude (Ds + DE)[ 2 against the Io-centric longitude at time of eclipse reappearance. The code used to represent the observations is the same as found in Column 3 of Table I. All northern hemisphere (defined by (Ds + De)/2 > 0) events are positive: these are the observations of Binder and Cruikshank (1964). Because of the relative orbital inclinations of the Earth and Jupiter, all later observations fall in the southern hemisphere of the satellite. Of these later observations, positive brightenings are reported in a band - 2 . 0 < (D s - DE)/2 <--2.25, but this band also includes two uncertain and three negative observations. Below this all observations are negative or uncertain. The fall 1973 observations lie in the region -0.2 to -0.4, and these are all negative. Observations over the next several years will again yield points in the northern hemisphere of Io. This same clustering is obvious on plots of ( D s - De) against the effective latitude and against the longitude on Io, but the correlation is not so striking. In summary, positive observations consist of only 25% (8/32) of the total, and in the southern hemisphere only 14% (4/28). Nonetheless there remains the important
L i I I ~ '•I w [t
+~f+2 ~ I
0
e~
®
® °o~
I I i I 26
24
2';'
0
• ®o
, ( , I i I ~ 20 ~B 16 ~4 10 - C e n t r i c
I j ~2
i
i
I
Ion~ilude
FIG. 2. Location of sub-Earth point during eclipse reappearance of Io. Io-centric longitude is measured from the meridian passing through the poles and the sub-Jupiter point on Io. Code used is same as in Table 1.
OF Io
443
fact t h a t there has to date been no simultaneous positive and negative observations by independent workers, while one positive and two negative events have been independently confirmed by different observers. It m a y be that the intermittent nature of the apparent brightenings is in part due to the different sub-Earth points on Io during the different events. IV. Dlscusslo~ The cause of the brightening, if real, remains unexplained. It is likely t h a t some nonuniform distribution of reflective surfaces is involved. The possible nature of these surfaces is discussed below. The photometry of Harris (1961), Johnson (1971), Owen and Lazor {1973), and Morrison et al. (1974) have demonstrated t h a t the hemisphere centered on 0 ° Io-centric longitude is red compared to the bluer disk seen at 180% Morrison et al. (1974) find that the u--y color (on the Str6mgren system) ranges from +2.60 at 0 ° to +2.15 at 180 °. Johnson (1971) reports a change in the intensity ratio at 0.4~m/ 0.56~m from 0.330 at 0 ° to a peak at 160 ° of 0.390. Detailed examination of the magnitudes and color curves as a function of rotational phase reveals much structure, consistent with the mottled surface reported by visual observers of Io (e.g., Dollfus, 1961). Table I indicates t h a t positive results are nearly always made at short wavelengths, but on a hemisphere t h a t is "red." No observations at wavelengths longer than 0.6 have confirmed a brightening. Io is very red (B-V ~ 1.17) compared to the other Galilean satellites (B-V ~ 0.85) (Harris, 1961). The spectral reflectivity of this satelhte shows a sharp drop in the blue (Johnson, 1971 ; Johnson and McCord, 1970; Wamsteker et al., 1974), far steeper than t h a t of the other three large satellites. The near-infrared reflectivity is higher for Io than for the others, and the albedo at 3.5~m and 5.0~m is very high compared to J I I , J I I I , and J I V (Gillett et ed., 1970). I t is possible to account for the observed brightening by an increase in the blue
44 4
HERBERT F R E y
albedo at specific Io-graphic locations on a nominally red hemisphere. A perfect blue mirror (albedo = 1.0 at 0.45pro) located on the hemisphere centered at 0 ° longitude would have to be only 200-500km in radius to produce a 10% increase in the refieetivity of the observed hemisphere at this wavelength. We have assumed that the mirror obeys the same scattering law as does the rest of the surface. The uncertainty in the size is due to the uncertainty in Io's blue albedo. This area is not inconsistent with that of large impact craters. I t is not likely that any one brightening can be ascribed to a single mirror with the above dimensions: the proximity of positive and negative sub-Earth points in the longitude range 18°-21 ° argues against such a situation. The effective area of the mirror m a y be composed of several smaller mirrors which, if properly aligned, may be widely spaced. The points shown in Fig. 2 are only the centers of the effective single mirror. Furthermore, we have chosen an unfortunate case by permitting the mirrors to obey only the scattering law applicable to the general surface of Io. A function with a higher backscattering component would allow the reflectors to be smaller. For example, ice has a stronger specular component than does fresh snow (see Veverka, 1973). It is of interest to compare the dimensions of such a blue reflector with that described by the ground trace covered for the reported duration of the brightening. The difference between the beginning and ending longitudes of the enhancement is / I L = L e n d - - Lbcsln = (Llt/Plo) × 360°, where/It is the duration of the brightening and Plo is the period of revolution of Io, both in minutes of time. For a brightening lasting 15 minutes, the range in longitude is 2.°2. Using a radius of 1.82× 103km (O'Leary and Van Flandern, 1972), this corresponds to some 70kin at the equator, which is a reasonable fraction of the effective mirror for a specular reflection. I f the brightening is due to a high albedo, short wavelength reflector, it is not surprising that the phenomenon is generally not seen on the other satellites, whose
reflectivity in the blue is not nearly so low, and whose color is not nearly so red as that oflo. The nature of such a blue reflector is difficult to determine, b u t again some data are available. Pilcher et al. (1972) and Fink e t a / . (1973) have identified water frost in the infrared reflection spectra of / i I , J I I I , and p o s s i b l y / I V . The observations of J I suggest a small amount of ice, b u t probably less than 10% of the surface refiectivity in the 1-2.5pro region is due to water frost. Io's high reflectivity in the near infrared is indicative of frosts, b u t the sharp decrease in the blue for J I is not suggestive of the naturally occurring frosts of H20, CH4, or N i l 3 (Morrison and Cruikshank, 1973; Johnson and McCord, 1970). Polarization measurements are consistent with a thin, multiple-scattering surface layer on J I , J I I , and J I I I (Veverka, 1971). Eclipse curves at 10 and 20/~m can be best represented by a twolayer model in which a thin layer of low conductivity is underlain by a much thicker layer of high conductivity (Morrison and Cruikshank, 1973). Because of its high blue reflectivity, frost nonuniformly distributed on the leading side of Io is a candidate for the reflector that may explain the sometimes observed post-eclipse brightenings. Several naturally occurring ices may fit the observations (cf. Wamsteker et al., 1974). Several questions are raised by this model. Are these permanent reflectors, or are they transistory in the sense suggested by Cruikshank and Murphy (1973) and Sinton (1973). This question cannot be answered at this time. Repeated observations at sub-Earth longitudes already observed are required to solve the problem. Clearly the models suggested previously cannot account for the lack of positive results in 1973. It may be that a timevariable phenomenon combined with the Io-graphic variable suggested here will be required to satisfactorily account for the observations. I f the reflectors are permanent features on the surface of Io, are they, by chance, located only in the narrow longitude range suggested by the eclipse reappearances.
POST-ECIXPSE BRIGHTENING OF Io
Do they exist elsewhere on the surface, and if so, why have they not been observed. The hemisphere centered on 160°-180 ° is the "blue" side of Io; if blue reflectors such as suggested here were located on this hemisphere, it is unlikely they could be observed against the intrinsically blue background. Indeed, it might be argued t h a t the increasing blue refleetivity of the satellite from 90°-180 ° is due to an increase in the number of blue reflectors, which eventually come to dominate the surface at these longitudes. The post-eclipse brightening is then due to a relatively small number of blue mirrors on the redder hemisphere seen during eclipse reappearances. Blue reflectors must be nearly absent from the hemisphere centered at 270 ° . Morrison et al. (1974) and Johnson (1971) both show a sharp drop in the blue reflectivity of the satellite at these longitudes. Mirrors may exist at other phases, but it would require careful and extended patrol work to reveal their presence if the Io-graphic locations of the specular component are the important variable. The fine structure of the color curves with orbital phase suggests a nonuniform distribution of reflective surfaces whose size is smaller than the observed hemisphere. I t is not at all unlikely t h a t blue reflectors exist elsewhere on the surface, and may be observable against a relatively "red" background. V'. CONCLUSIONS I t may be possible to explain the intermittent history of the post-eclipse brightening and nonbrightening of Io in terms of a nonuniform distribution of high-blue albedo reflectors, grouped on the "red" hemisphere centered at 0 ° longitude of the satellite. The required dimensions of such reflectors are consistent with large impact craters observed on the Moon and on Mars. Naturally occurring ices in such bowlshaped depressions is a likely candidate for the blue reflector. The brightening is then essentially a contrast effect for this very red satellite. Admittedly oversimplified, this explan-
445
ation has assumed t h a t all published observations are correct as stated. Because of the serious and difficult problem of correcting for scattered light from Jupiter (see Franz and Millls, 1974), it is not clear t h a t the reported anomalous brightenings are indeed real. I f they are real, then the fact t h a t some observers could not find such an enhancement has a reasonable explanation in terms of the geographic distribution of blue reflectors. Many more observations are needed, both to confirm the brightenings and the locations of the above suggested mirrors. ACKNOWLEDGMENTS Special thanks to Bob Millis of the Lowell Observatory for several instructive discussions, especially on the scattered light problem. Thanks also to M. F. A ' H e a r n for a critical reading of a rough draft of this manuscript, and for suggestions along relevant lines of study. This work was supported in part by NASA Grant NGL-21002-033., REFERENCES BINDER, A. B., A~N'DCRUIKSHAI~K,D. P. (1964). Evidence for an Atmosphere on Io. Icarus 3, 299-305.
CRUIZSHA~rK,D. P., ANDMt~RP~y,R. E. (1973). The post-eclipse brightening of Io. Icarus 20, 7-17. DOLLF~S, A. (1961). Visual and photographic studies of planets at Pic du Midi. In P/anet~ and Sate/l/te~ (G. P. Kuiper and B. M. Middlehurst, Eds.), pp. 534-571. University of Chicago Press, Chicago. FAE,LOlq', F. W., AND mURPHY, R. E. (1971). Absence of post-eclipse brightening of Io and Europa in 1970. Icarus 15, 492-496. FIN~:, U., DEKKERS, N. H., A~rD L~aso~r, H. P. (1973). Infrared spectra of the Galilean satellites of Jupiter. Astrophys. J. 179, L155. FRA_~z, O. G., AND M~LIS, R. L. (1971). A search for an anomalous brightening of Io after eclipse. Icarus 14, 13--15. FRAI~Z, O. Cv., AND MILLIS, R. L. (1974). A search for post-eclipse brightening of Io in 1973. II. Icaru~ 23, 431-436. GILLETT, F. C., x-~ERRTLL, K. M., AZ~D STERN, W . A. (1970). Albedo and thermal omission of Jovian satellites I-IV. Astrophys. LetS. 6, 247. HA~SEN, O. L. (1973). Ten micron eclipse observations of Io, Europa and Ganymede. Icaru~ 18, 237-246.
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HERBERT
FSEY
HARRIS, D. L. (1961}. Photometry and coloriof the Galilcan satellites. Icarua 23, 399metry of planets and satellites. I n P/aneta and 416. Satellites (G. P. Kuiper and B. M. Middlehurst, O'LEARY, B., AND MINER, E. (1973). Another possible post eclipse brightening of Io. Icarus Eds.), pp. 272-342. University of Chicago. Press, Chicago. 20, 18-20. JOHNSON, T. V. (1971). Galilean satellites: 0'LEA~tY, B., AND V ~ FLAZrD~RN, T. C. (1972). Io's triaxial figure. Icarus 17, 209-215. narrow-band photometry 0.30 to 1.10 microns. O'LEARY, B., AND VEV~aKA, J. (1971). On the Icarus 14, 94--111. anomalous brightening of Io after eclipse. JOHNSON, T. V., ~ I~[cCoRD, T. B. (1970). Icarus 14, 265-268. Galilean satellites: The spectral reflectivity OwE~, F. :N., AND LAZOR, F. J. (1973). Surface 0.30-0110 microns. Icarus 13, 37-42 color variations of the Galilean satellites. KLZORE, A., CAr~', D., FJELDBO, G., AI~D Icarus 19, 30-33. SErD~_~, B. L, (1974). Preliminary results on the atmospheres of Io and Jupiter from PILCKER, C. ]3., RIDGWAY, S. T., AN~ McCORD, T. B. (1972). Galilean satellites: Identification Pioneer 10S-band occultation experiment. of water frost. Science 178, 1087. Sc/ence 183, 323. SzNro~-, W. M. (1973). Does Io have an ammonia MILLIS, :R. L., THOMPSON, D. T., ~I.%~RIS, B. J., atmosphere? Icarus $0, 284-296. BLaCH, P., ~.~-DSEXTON,R. (1974). A search for S~zrH, B. A., AND SMITH, S. A. (1972). Upper post-eclipse brightening o~ Io in 1973. I. I c a r ~ limits for an atmosphere on Io. Icarus 17, 23, 425-430. 218-222. I~ORRISON, D., AND CRUIKSHANK, D. P. (1973). VEV~KA, J. (1971). Polarization measurements Thermal properties of the Galilean satellites. of the Galilean satellites of Jupiter. Icarus 14, Icarus 18, 224-236. 355-359. MORRISON,D., CRUIKSHA17K,D. P., ANDMURPHY, V~VF~RKA,J. (1973). The photometric properties R. E. (1972). Temperature of Titan and the of natural snow and of snow-covered planets. Galilean satellites at 20 microns. As~rophys. J'. Icar~q 20, 304-310. 173, L143. WAMSTEKER,W., K_ROES,R. L., AND FOUl,'rAIN, J. A. (1974). On the surface composition of Io. MORRISON, D., I~/~ORRISON,1%T.D., AND LAZAREWZCZ, A. R. (1974). Four-color photometry Icarus 23, 417-424.