Surface of Miranda: Identification of water ice

ICARUS 58, 288--292 (1984)

Surface of Miranda: Identification of Water Ice* ROBERT HAMILTON BROWN 1 AND ROGER N. CLARK 1 Planetary Geosciences Division, Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honohdu, Hawaii 96822 Received August 22, 1983; revised November 21, 1983 Near-infrared spectrophotometry at 5% resolution shows Miranda to have a water-ice surface. Estimates of Miranda's albedo made from the depth of its 2.0-/~m absorption band suggest that its visual geometric albedo is likely to be between 10 and 70%, which when combined with the satellite's visual magnitude, yields a diameter of 500 + 225 km. There is some evidence that suggests the visual geometric albedo of Miranda may be ->0.3, which implies that its diameter may lie near the lower end of the estimated range. With these results all the Uranian satellites are now known to have water-ice surfaces.

ditional work by Brown (1982, 1984) and Brown and Cruikshank (1983) showed that the near-infrared reflectance spectra of the four bright satellites of Uranus are consistent with the presence of a dark component with the i c e - - a component, they concluded, that is spectrally bland and has reflectance characteristics similar to those of carbon black, carbonaceous chondritic material, and other neutrally colored, low-reflectance materials. In contrast to the rapidly increasing data base for the bright satellites of Uranus, before 1983 little was known about Miranda beyond the visual brightness and orbital parameters; a summary of what was known about Miranda before 1982 was published by Cruikshank (1982), and a compilation of all the astrometric observations of Miranda since its discovery has been analyzed by Veillet (1983). The work of Veillet has resulted in the most accurate characterization of the orbital properties of Miranda to date, and for the first time has resulted in reasonably accurate masses for Ariel, Umbriel, Titania, and Oberon. This paper reports the first near-infrared reflectance spectrum of Miranda--the faintest of the five known satellites of Uranus--and demonstrates the presence of water-ice on its surface.

INTRODUCTION

It has been known for some time that water ice is present on the surfaces of the Uranian satellites Ariel, Umbriel, Titania, and Oberon. The discovery of water-ice absorptions in the spectra of Titania and Oberon was made by Cruikshank (1980), and in a follow-up study, Cruikshank and Brown (1981) found water-ice absorptions in the spectra of Ariel and Umbriel. A contemporaneous study by Soifer et al. (1981) confirmed the results of Cruikshank, and Cruikshank and Brown, and suggested that the albedos of Titania, Oberon, and Umbriel were lower than those of relatively pure water-ice surfaces. Shortly after the discovery of water ice on the surfaces of the four bright Uranian satellites, Brown et al. (1982) succeeded in measuring the 20-/zm thermal flux from Ariel, Umbriel, Titania, and Oberon, and found that all four satellites were of relatively low albedo, indicating the presence of a dark component on their surfaces. Ad* Paper presented at the "Natural Satellites Conference," Ithaca, N.Y., July 5-9, 1983. Visiting astronomer at the Infrared Telescope Facility which is operated by the University of Hawaii under contract to the National Aeronautics and Space Administration. 288 0019-1035/84 $3.00 Copyright © 1984 by Academic Press, Inc. All fights of reproduction in any form reserved.

SURFACE OF MIRANDA

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OBSERVATIONS

Reflectance spectra of Miranda in the region 1.62-2.47 /xm were obtained at the NASA Infrared Telescope Facility (IRTF) at Mauna Kea Observatory on UT June 7, 1983, using a 5% bandpass circular variable filter and the standard IRTF indium-antimonide photometer cooled with liquid helium. Observing conditions were generally excellent, with precipitable atmospheric water vapor being - 1 mm per air mass, enabling excellent extinction correction of the spectra in the region of the strong 1.9-/~m telluric absorption. Two standard stars were used to calibrate the spectra to reflectance and to provide extinction correction: 49 Librae, the local standard (being only a few degrees from Uranus) used for extinction correction, and the reflectance standard, 16 Cygni B, a solar-type star known for its spectral similarity to the Sun (Hardorp, 1981). Since Miranda is close to Uranus and could not be seen in the IRTF television or visual guiding system, special observing procedures were used. Normally, sky subtraction is accurate in the chopping mode when the scattered-light field has no strong gradients; however, the scattered light close to Uranus has a strong radial gradient, and accurate measurement of the reflected light from Miranda requires special care. Therefore, to ensure that Uranus' scattered light contribution to the measurements of Miranda was well compensated, a measurement of the sky diametrically opposite Miranda relative to Uranus was made for each Miranda measurement (see Fig. 1), and subtracted from the corresponding observations of Miranda. In all observations the direction of oscillation of the IRTF secondary mirror was oriented to be perpendicular to the line that joined Miranda, Uranus, and a spot on the sky diametrically opposite Miranda. The use of a 6-arcsec aperture and a 30-arcsec chopper throw, combined with Miranda's position angle on the sky, resulted in the reference

) FIG. 1. Shown is the observing geometry used to obtain the reflectance spectrum of Miranda plotted in Fig. 2. The aperture size, chop length, and satellite orbits are shown to scale. The outermost four circles are the orbits of Oberon, Titania, Umbriel, and Ariel in order of decreasing distance from Uranus. The central dot is Uranus, immediately concentric to the rings.

beams being in a region of the sky far from possible contamination owing to diffraction around the supports of the telescope secondary mirror. Fortunately, in the spectral region 1.6-2.5/zm, strong methane absorptions in the reflected light spectrum of Uranus result in much less scattered light than in the visual region of the spectrum, somewhat simplifying the problem of scattered light correction. An average of all our measurements of the sky opposite Miranda showed no signals different from zero by more than one standard deviation of the mean, supporting our belief that the Miranda observations are adequately corrected for scattered light. Because Miranda could not be seen in the field viewing system, offsets from the center of Uranus to the predicted position of Miranda were used to obtain the observations. Offsets from Uranus were calculated assuming a circular orbit for Miranda and adopting the nearest predicted time of greatest northern elongation found in the The Astronomical Almanac. Positional checks on the four other satellites of Uranus demonstrated offset accuracies of 1 arcsec. A log of observations is shown in Table I and a tabulation of the Miranda/ 16 Cyg B flux ratio is shown in Table II.

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TABLE I LOG OF OBSERVATIONS 1983 Date June June June June

6 6 7 7

Universal time

Object

09:51-12:50 13:07-13:30 09:14-12:14 09:23-12:29

49 Lib 16 Cyg B 49 Lib Miranda

DISCUSSION

After correction for scattered light and extinction, 11 spectra totaling 110 sec of integration time per spectral channel were coadded and converted to reflectance, resulting in the spectrum of Miranda in Fig. 2. Also displayed in Fig. 2 is a laboratory spectrum of water frost (convolved to the same resolution as the Miranda spectrum) drawn from Clark (1981). Both spectra are normalized to 1 at 1.79/zm. A broad, deep absorption at 2.0/xm can be seen in the Miranda spectrum that is well matched

T A B L E II REFLECTANCE OF MIRANDA

Wavelength

Reflectance

o-

1.615 1.675 1.735 1.795 1.853 1.913 1.970 2.028 2.084 2.140 2.196 2.252 2.306 2.361 2.414 2.466

0.951 0.984 0.699 1.000 0.830 0.566 0.519 0.329 0.399 0.729 0.315 0.674 1.116 0.574 0.201 0.073

0.197 0.122 0.187 0.163 0.248 0.269 0.218 0.173 0.179 0.219 0.288 0.244 0.156 0.270 0.188 0.617

Note. T h e relative reflectance of Miranda as ratioed to the solar-type star 16 Cygni B. T h e flux ratio of Miranda to 16 C y g B is 1.010 x 10 -4 at 1.795 /xm (channel 4). Errors given are one standard deviation of the mean.

in both shape and wavelength of minimum reflectance by the water-frost spectrum. There are large one-channel fluctuations at 2.2 and 2.3/xm that do not fit well a waterice spectrum, but they are likely the result of both statistical fluctuations and residual errors in scattered light correction. This, we believe, constitutes evidence for the presence of water frost or ice on the surface of Miranda. Some cosmochemically abundant ices, such as CH4 and NH3, have strong absorptions near 2.3/zm, but absorptions such as these are not seen in our spectrum of Miranda. Though both of the aforementioned ices may be present in small amounts on Miranda's surface, water ice seems to be spectrally dominant and is likely to be compositionally dominant as well. This is further demonstrated in Fig. 3 where we have plotted the spectra of NH3 frost, NH4OH ice, and CH4 ice along with our spectrum of Miranda (all spectra are normalized to 1.0 at 1.79/zm). The closest match to Miranda's spectrum of the lab spectra in Fig. 3 is that of a sample of NH4OH that is over 70% H20 by weight, but the error bars in Miranda spectrum are too large to permit conclusions regarding the presence or absence of absorption feaI.S

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FIG. 2. The spectra of M i r a n d a and of a sample o f medium-fine-grained water frost from Clark (1981). Both spectra are normalized to 1.0 at 1.79 t~m and the lab s p e c t r u m h a s been c o n v o l v e d to the resolution of the Miranda spectrum.

SURFACE OF MIRANDA

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FIG. 3. The spectrum of Miranda plotted with lab spectra of frozen NH4OH, NH3 frost, and CH4 ice. The NH4OH spectrum is of a sample containing 29% by weight NH3, which was frozen and then ground to a fine powder at 77°K under a dry nitrogen atmosphere. The NH3 frost spectrum is drawn from Slobodkin et al. (1978) and the CH4 ice spectrum is from Brown et al. (1984). All laboratory spectra have been convolved to the resolution of the Miranda spectrum.

tures other than the strong 2.0-/zm waterice absorption. Although the data are noisy, it is instructive to estimate what albedo might be inferred for Miranda on the assumption that the water-ice band depth at 2.0/xm is related to Miranda's albedo. Brown (1982) and Brown and Cruikshank (1983) have shown that there is a rough correlation of the 2.0-/zm water-ice band depth in the Uranian satellite spectra with the satellites' visual geometric albedos (pv). If we assume that Miranda's albedo is similarly related to its 2.0-/~m band depth, we may estimate a range of possible albedos based on the observed range of band depths in Miranda's spectrum. Within the lo- uncertainties, Miranda's 2.0-/xm absorption can be seen to have a depth between 40 and 85%. Furthermore, the 2.0-/zm band depths in the spectra of Ariel, Umbriel, Titania, and Oberon

291

(Brown, 1982; Brown and Cruikshank, 1983) lie between 30 and 50%, while the satellites have visual geometric albedos between 0.2 and 0.3. If albedo is similarly related to 2.0-/zm absorption band depth for Miranda, then Miranda's visual geometric albedo probably lies between 0.2 and 0.7 (the upper limit owing to Enceladus and Tethys being the only objects in the solar system known to have visual geometric albedos significantly greater than 0.7). Reitsema et al. (1978) have reported that V(1,0) = 3.8 for Miranda, which when used in a standard reduction of visual magnitude and geometric albedo to diameter results in the implied diameter of Miranda being between 275 and 500 km. If Miranda has a surface composed of water ice intimately mixed with dark, spectrally neutral particulates, then the study of Clark and Lucey (1984) requires that we include a pv of 0.1 in our estimate. This results because one of their samples of a mixture of charcoal grains and water ice shows a 2.0-/zm band depth roughly equal to the minimum depth of the 2.0-/zm band in our Miranda spectrum while having a normal reflectance in the visual of about 10%. Thus, we have the more conservative range of 275-730 km for the diameter of Miranda. Casting this in a more conventional form gives 500 --- 225 km for the diameter of Miranda with the error bars resulting from our estimate of a plausible range of uncertainty in pv. Despite our conservative estimate of a range of albedos for Miranda, there are reasons to believe that the albedo of Miranda can be further constrained to lie near the upper end of the interval 10-70%. Statistically, the most probable 2.0-~m band depth seen in our spectrum is about 65% of the continuum (approximated by a straight line through the reflectance peaks near 1.8 and 2.2/xm). If it is remembered that Miranda's 2.0-/zm band depth was reasonably matched by that of a laboratory spectrum of pure water frost, it can be concluded that the albedo of Miranda could be well above the lower end of our estimated range. Fur-

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thermore, if one is permitted to use the albedo and 2.0-t~m band depths of the other Uranian satellites as an analogy, one can conclude that the albedo of Miranda may be greater than 30% because the minimum 2.0/~m band depth in Miranda's spectrum is roughly equal to that of Ariel (pv = 0.3). Finally, if we use our measurement of Miranda's broadband K (2.2/~m) brightness derived from the spectrophotometric observations (K - 15.4) and combine it with the V mag from Reitsema e t al*. (1978), we find that the V - K magnitude of Miranda is about 1.1. Noting that V - K ~ 0.8 for Saturn's Rhea and V - K ~ 1.4 for Titania, Oberon, and Umbriel (representing an albedo range of 0.2 to 0.7), we see that Miranda's V - K mag is intermediate. Therefore, we believe that the diameter of Miranda may lie near the lower end of our range of 275 to 730 km and its albedo may lie near the center of the range 0.2 to 0.7. It is tempting to speculate on why Miranda might have a higher albedo than the other Uranian satellites, but there is still enough uncertainty in the results described here to render such speculation inappropriate. We would point out, however, that recent theoretical work by Squyres and Reynolds (1983) may provide an explanation by the mechanism of tidal heating if further work confirms that Miranda indeed has a high albedo. ACKNOWLEDGMENTS This work was supported by NASA Grants NSG 7323 and NSG 7312. We thank Pam Owensby for generating offsets for Miranda, Marci Nelson for help at the telescope, and Priscilla Piano for the fine illustrations. This paper has benefited from critical reviews by Dale Cruikshank, Michael Feierberg, and one anonymous referee. REFERENCES BROWN, R. H. (1982). The Satellites of Uranus: Spectrophotometric and Radiometric Studies of Their Surface Properties and Diameters. Ph.D. dissertation, University of Hawaii, Honolulu.

BROWN, R. H. (1983). The Uranian satellites and Hyperion: N e w spectrophotometry and compositional implications. Icarus 56, 414-425. BROWN, R. H., R. N. CLARK,J. HAYASHI, AND D. P. CRUIKSHANK (1984). Near-infrared reflectance spectra of solid methane, ethane, ethylene and ammonium hydroxide. In preparation. BROWN, R. H., AND D. P. CRU1KSHANK (1983). The Uranian satellites: Surface compositions and' opposition brightness surges. Icarus 55, 83-89. BROWN, R. H., D. P. CRUIKSHANK,AND D. MORRISON (1982). Diameters and albedos of satellites of Uranus. Nature 300, 423-425. CLARK, R. N. (1981). Water frost and ice: The nearinfrared spectral reflectance 0.65-2.5 microns. J. Geophys. Res. 86, 3087-3096. CLARK, R. N., AND P. G. LUCEY (1984). Spectral properties of ice-particulate mixtures and implications for remote sensing, I: Intimate mixtures. J. Geophys. Res., in press. CRUIKSHANK, D. P. (1980). Near-infrared studies of the satellites of Saturn and Uranus. Icarus 41, 246258. CRUIKSHANK,D. P. (1982). The satellites of Uranus. In Uranus and the Outer Planets (G. E. Hunt, Ed.), pp. 193-216. Cambridge Univ. Press, London/New York. CRUIKSHANK, D. P., AND R. H. BROWN (1981). The Uranian satellites: Water ice on Ariel and Umbriel. Icarus 45, 605-611. HARDORP, J. (1981). The Sun among the stars. V. A second search for solar spectral analogs. The Hyades distance. Astron. Astrophys. 105, 120-132. REITSEMA, H. J., B. A. SMITH, AND D. E. WEISTROP (1978). Visual and near-infrared photometry of the Uranian satellites. Bull. Amer. Astron. Soc. 10, 585 (Abstract). SLOBODKIN, L. S., I. F. BUYAKOV, R. D. CESS, AND J. CALDWELL 0978). Near-infrared reflection spectra of ammonia frost: Interpretation of the upper clouds of Saturn. J. Quant. Spectrosc. Radiat. Transfer 20, 481-490. SOIFER, B. T., G. NEUGEBAUER, AND K. MATTHEWS (1981). Near-infrared spectrophotometry of the satellites and rings of Uranus. Icarus 45, 612-617. SQUYRES, S. W., AND R. T. REYNOLDS (1983). Tidal evolution of the Uranian satellites. Abstract submitted to fifteenth annum meeting of the Division of Planetary Sciences of the American Astronomical Society, 16-20 October 1983, Ithaca, N.Y. VEILLET, CH. (1983). De l'observation et du mouvement des satellites d'Uranus. Ph.D. dissertation, University of Paris, Paris.