IRAS Survey-Mode Observations of Pluto–Charon

Icarus 142, 155–159 (1999) Article ID icar.1999.6199, available online at http://www.idealibrary.com on

IRAS Survey-Mode Observations of Pluto–Charon Mark V. Sykes Steward Observatory, University of Arizona, Tucson, Arizona 85721 E-mail: [email protected] Received May 16, 1997; revised July 21, 1999

Radiometric observations of Pluto and its moon, Charon, by the Infrared Astronomical Satellite (IRAS) in 1983 marked the first detection of the system at thermal wavelengths. Reexamination of the IRAS observations prove that Pluto–Charon was detected several times during the all-sky survey in addition to the pointed observations that targeted the system. Pluto–Charon exhibits thermal lightcurve variations that can be explained by the known variation in hemispherically averaged geometric albedo with sub-Earth longitude. Changes in albedo, however, are not sufficient to explain the lower than expected fluxes observed at microwave wavelengths. c 1999 Academic Press °

In 1983, sensitive detectors aboard the Infrared Astronomical Satellite (IRAS) measured thermal emission from the Pluto– Charon system for the first time (Sykes et al. 1987, Tedesco et al. 1987, Aumann and Walker 1987). Analysis of these observations indicated a surface that could not be isothermal as had been previously assumed (e.g., Trafton 1980, Trafton and Stern 1983). IRAS pointed observations made in “stare mode” were reproduced using a simple “isothermal latitude” model which assumes a surface in radiative equilibrium with the average daily insolation. The “warmest” region of Pluto and Charon was determined to be around 59 K at the equator, decreasing as one moves toward the poles. Isothermal polar caps also were included (Sykes et al. 1987). Between 1984 and 1989, Pluto and Charon were observed to perform a series of mutual eclipses, which allowed brightness variations across the surfaces of the two bodies to be mapped (Buie et al. 1992, Young and Binzel 1993, Reinsch et al. 1994). Bright polar regions and generally darker equatorial regions (with variations in longitude and latitude) were inferred for Pluto. The distribution of these areas was generally confirmed through direct imaging by the Hubble Space Telescope (Stern et al. 1997). All other parameters being the same, a change in the albedo of a surface results in a change in its radiative equilibrium temperature. High-albedo surfaces reflect more sunlight and are consequently colder than low-albedo areas and are less bright in the infrared. Thus, as Pluto rotates we would expect to see a disk that increased and decreased in thermal brightness. If high

albedo correlates with the presence of volatiles, sublimation in the bright areas would decrease the temperature further than the effect of albedo alone, enhancing the variation in thermal brightness. Of course, were volatiles to correlate with dark areas, this variation might be damped or even result in dark areas being colder than bright areas! The IRAS pointed observations were made too close together in time to measure such an effect on Pluto. However, Pluto was reported to have been detected on two dates while IRAS was making its all-sky survey, from which a single flux density at 60 µm was reported (Tedesco et al. 1987). This flux was significantly lower than the pointed observations. Unfortunately, no timing information was provided which would have allowed for a determination of the hemisphere or hemispheres that were so observed. Detections of microwave (submillimeter and millimeter) thermal emission from Pluto–Charon (Altenhoff et al. 1988, Stern et al. 1993, Jewitt 1994) and the identification of nitrogen ice in combination with a thin atmosphere (Owen et al. 1993) argued for an extremely cold (35–40 K) surface for the planet, much colder than the 59 K equatorial temperature reported by Sykes et al. If Pluto was uniformly 35–40 K, the system would have been too faint to have been detected by IRAS. Explanations offered to explain the difference between the IRAS and microwave results include a wavelength-dependent thermal emissivity (Stern et al. 1993, Sykes 1993, Jewitt 1994), a combination of “hot spots” and cold regions on the surface (Stern et al. 1993, Jewitt 1994), and, more serious, the possibility that the IRAS observations of Pluto–Charon might have been either mistakenly identified or contaminated by a distant background source (Stern et al. 1993). A reexamination of the IRAS observations shows that in addition to the two pointed observations previously reported, the satellite scanned Pluto–Charon’s position four times over 3 days while conducting its general survey of the sky (Table I). Including the pointed observations, IRAS detected Pluto–Charon at six well-separated times distributed among three well-separated locations (Fig. 1). There were no predicted scans in which Pluto– Charon was not detected by IRAS. There are no apparent underlying sources in thermal images. A search for catalog sources within 2 arcmin of those positions found nothing. Catalogs examined include the IRAS Point Source Catalog (Version 2), the

155 0019-1035/99 $30.00 c 1999 by Academic Press Copyright ° All rights of reproduction in any form reserved.

156

MARK V. SYKES

FIG. 1. IRAS detections of Pluto–Charon are shown in images constructed of scans made by the satellite at 60 µm at different times. Survey observations are shown for (a) July 13, 1983 and (b) July 23–24, 1983. (c) One of the pointed observations made on August 16 is also shown. The predicted positions of Pluto– Charon at all three times are marked by circles in (a) and (b). For scale, each circle has a diameter of 1.5 arcmin. The predicted position for the time of each image is indicated by a bar. In (a) and (b) the location of the pointed observation is indicated by the lowest left circle. No underlying source is apparent at the locations of any of Pluto–Charon’s scanned positions.

IRAS Faint Source Survey, their corresponding reject files (containing lower signal-to-noise sources, many spurious), the IRAS Small Scale Structure Catalog, and a number of star catalogs, catalogs of galaxies and quasars, and others available on-line through the XCATSCAN facility at the Infrared Processing and Analysis Center (IPAC, http://www.ipac.caltech.edu). The two scans made on January 13, 1983, were separated by less than 2 h, which is very short compared with Pluto’s 6.38726day period (Tholen and Tedesco 1994). Using ADDSCAN software at IPAC, individual 60 (and 100)-µm detector outputs from both scans were combined and median averaged. Then measured peak flux densities and uncertainties were measured. This same procedure was applied to combine the scans of January 23 and 24, which were separated by less than 11 h. IRAS catalog flux densities are given for the central wavelength of each of its broad bandpasses and assume a ν −1 source function. Since the Pluto–Charon system has a source function analogous to that of a cold blackbody, the flux densities must be adjusted accordingly for each band. Color-correction factors for blackbodies having temperatures ranging between 40 and 60 K have values between 0.91 and 0.93 at 60 µm (these values are not necessarily monotonic with temperature and are tabulated in Beichman et al. 1988). These factors are divided into the IRAS catalog flux density to give the “true” flux density at a given wavelength. The color temperature corresponding to the 60- and 100-µm fluxes reported by Sykes et al. is 57 K. Blackbody radiation corresponding to this temperature is assumed to be descriptive of the overall shape of the Pluto–Charon source function. The color correction factor for blackbodies having temperatures between 52.5 and 57.5 K is 0.92 at 60 µm. This factor has been applied to the ADDSCAN output flux densities which appear in Table I. An examination of individual 100-µm detector outputs and images constructed from them show that survey-mode detections of Pluto at this wavelength are not reliable (primarily shot noise) and hence are excluded from this work. Such putative detections were in the range between 500 and 600 mJy, falling well below the 1.5-Jy limiting flux density expected for survey-mode detections at 100 µm (Beichman et al. 1988). By comparison, the 60-µm flux densities reported here are near the limiting flux density of 0.5 Jy for that wavelength. An examination of individual detector outputs and images constructed from them, however, indicates that Pluto was seen in all detector outputs at 60 µm. It is noted that Pluto’s brightness during survey mode is somewhat higher than the value of 420 ± 40 mJy reported by Tedesco et al. for coadded survey data from two dates. At the time, the Tedesco et al. flux density was treated with some suspicion, because it was significantly lower than the number derived from the pointed observations. Sykes et al. speculated that the difference was due either to noise or to real lightcurve variations. This work confirms that during survey-mode observations, Pluto–Charon was indeed fainter at thermal wavelengths than during pointing mode. Figure 2 shows the mean opposition V magnitude of the Pluto– Charon system plotted against the sub-Earth longitude (Buie

157

IRAS RADIOMETRY OF PLUTO–CHARON

TABLE I IRAS Observations of Pluto Sub-Earth Date 1983 UT S1 S2

July

S3 S4 P5 P6

Aug.

60 µm

100 µm

Lat.

Long.

Fv

σ

Fv

σ

Mode

SOP/OBS

R

1

ρ

13 13

07:55 09:38

−10.8 −10.8

219 215

446

67

—

—

Survey Survey

337/18 337/25

29.9 29.9

29.8 29.8

1.9 1.9

23 24

15:19 01:37

−10.8 −10.8

358 334

467

71

—

—

Survey Survey

357/43 358/27

29.9 29.9

29.9 29.9

1.9 1.9

16 16

04:27 11:18

−10.6 −10.6

111 95

581

58

721

123

Pointed Pointed

405/3 405/43

29.9 29.9

30.3 30.3

1.7 1.7

et al. 1997). The IRAS pointed observations were made at minimum light while the January 13 survey-mode observations were made at maximum light and the January 23/24 survey mode observations were made at an intermediate brightness. Ignoring the potential effect of sublimation cooling by volatiles, a dark, lowalbedo surface will be warmer (hence brighter at thermal wavelengths), than a bright, high-albedo surface. Since the pointed observations were made of an overall darker hemisphere than those observed by IRAS in survey mode, the larger flux densities of the pointed observations are consistent with this general picture. Tryka et al. (1994) showed convincingly, using thermal parameters similar to those of icy satellites, that Pluto should have a subsolar thermal bulge more similar to the asteroid standard thermal model (STM) (Lebofsky and Spencer 1989) than the isothermal latitude model plus ice caps of Sykes et al. Tryka et al. also concluded that cold nitrogen ice regions must be restricted to the poles and that the equatorial region is devoid of nitrogen ice because it is too warm.

To illustrate what kind of longitudinal variations in thermal emission we might expect as a consequence of global variations in albedo alone, a model was created, based on the asteroid standard thermal model, assuming unit thermal emissivity and no infrared beaming. Bolometric Bond albedos are assumed to be uniform over an observed hemisphere and equal to the hemispherically averaged geometric albedo, analogous to what is observed for Saturn’s icy satellites Rhea and Tethys (e.g., Morrison et al. 1986). The geometric albedo of Charon is set constant at 0.375 (Buie et al. 1997). The hemispherically averaged geometric albedo for Pluto is derived from the minimum and maximum values of 0.49 and 0.66, respectively, and the Pluto lightcurve given by Buie et al. (1997). The heliocentric distance of 29.9 AU, geocentric distance of 30.0 AU, and phase angle of 1.9◦ are assumed (representing an average over these values for the IRAS observations). Radii of 1142 and 596 km for Pluto and Charon, respectively, were taken from Tholen and Buie (1988). Isothermal polar ice caps are not included. The resultant source function at each sub-Earth longitude is integrated

FIG. 2. The visual lightcurve of the Pluto–Charon system is shown as a function of sub-Earth longitude (Buie et al. 1997). The central longitude during each IRAS scan is indicated. P, pointing-mode observation; S, survey-mode observation (see Table I).

158

MARK V. SYKES

remains constant in this model, is due to its lower albedo. There is good agreement between the IRAS observations and the STM model which suggests that there are diurnal temperature variations on the planet (as suggested by Tryka et al.) and that variations in observed thermal emission are due primarily to variations in albedo. Cooling of the surface by other sublimating volatiles such as methane is not required to explain the IRAS observations. Methane ice patches would have temperatures that are a function of both albedo and (N2 ) atmospheric pressure. Depending primarily on the value of the latter, temperatures above 55 K would be possible (Stansberry et al. 1996). So, the presence of methane ice in the equatorial region is not constrained by the IRAS observations and cannot be ruled out. As mentioned above, observations of Pluto–Charon at millimeter and submillimeter wavelengths have indicated an effective surface temperature lower than that needed to explain the IRAS observations (Altenhoff et al. 1988, Stern et al. 1993, Jewitt 1994). Could this discrepancy be explained by thermal lightcurve variations? The STM model of Fig. 3 was applied to the existing microwave observations (inputting the heliocentric distance, geocentric distance, and observational phase of epoch). The results are listed in Table II. A surprising fraction of these observations were made at sub-Earth longitudes where the hemispherically averaged geometric albedo was much higher than the darkest area (where the IRAS pointed observations were made). Albedo variations alone, however, in the context of the simple STM model, are insufficient to explain the lower millimeter flux densities observed. Thus, another mechanism such as wavelengthdependent emissivity must be invoked. Observations of a Pluto–Charon thermal lightcurve using the Infrared Space Observatory have been reported (Lellouch et al. 1998), generally confirming the IRAS results. New opportunities for observing Pluto at IRAS wavelengths will exist with the future Space Infrared Telescope Facility (SIRTF). Since 1983 Pluto has passed its perihelion and the redistribution of surface volatiles is in progress as the subsolar point shifts away from TABLE II Millimeter-Wave Thermal Emission Comparison with STM FIG. 3. Predicted thermal lightcurves at 60 and 100 µm for the standard thermal model (STM) and rapid rotator model (RRM), described in the text, are plotted with the observed IRAS flux densities (filled circles).

over the IRAS 60- and 100-µm bandpasses and plotted in Fig. 3 along with the IRAS observations. For comparison, a rapid rotator model (RRM) assuming the same input parameters is also shown. A RRM assumes radiative equilibrium with the average daily insolation at each point on the surface. If the RRM assumed a global albedo of zero, the 60- and 100-µm flux densities would be 332 and 522 mJy, respectively. The resultant STM subsolar temperatures for Pluto range between 55 and 61 K. Charon’s higher subsolar temperature of 64 K, which

λ (µm)

Sub-Earth long.

Fv (obs)

Fv (pred)

Reference

450 800 800 1100 1100 1200 1200 1200 1200 1300 1300 1300

315 330 315 315 293 356 321 17 293 315 65 213

<151 39 ± 6 33 ± 7 <24 <36 16 ± 5 11 ± 6 12 ± 5 15 ± 2 12 ± 10 10 ± 6 13 ± 4

130 55 48 26 25 24 24 25 24 19 21 20

Stern et al. 1993 Jewitt 1994 Stern et al. 1993 Stern et al. 1993 Stern et al. 1993 Altenhoff et al. 1988 Altenhoff et al. 1988 Altenhoff et al. 1988 Altenhoff et al. 1988 Stern et al. 1993 Stern et al. 1993 Jewitt 1994

IRAS RADIOMETRY OF PLUTO–CHARON

its equator. The IRAS observations mark a point in the evolving thermal nature of the planet that will not be reproduced for another 248 years. So, just as an asteroid observation made decades ago on a glass plate extends the observational baseline necessary to refine the asteroid’s orbit, the first radiometric observations of Pluto by IRAS will continue to provide an important constraint in modeling the nature and temporal evolution of our (generally) most distant planet. ACKNOWLEDGMENTS The staff of the Infrared Processing and Analysis Center are thanked for generating special IRAS data products in support of this work. T. Chester helped with the identification of IRAS survey observations of Pluto. M. Buie generously provided the sub-Earth latitudes and longitudes on Pluto for many of the observation times reported herein. R. Cutri is thanked for useful discussions about IRAS photometric uncertainties. This paper benefitted from the reviews of J. R. Spencer and S. A. Stern. This work was supported in part by NASA Grants NAG 5-1370 and NAG 5-3359.

REFERENCES Altenhoff, W. J., R. Chini, H. Hein, E. Krupa, P. G. Mezger, C. Sulter, and J. B. Schramb 1988. First radio astronomical estimate of the temperature of Pluto. Astron. Astrophys. 190, L15–L17. Aumann, H. H., and R. G. Walker 1987. IRAS observations of the Pluto–Charon system. Astron. J. 94, 1088–1091. Beichman, C. A., G. Neugebauer, H. J. Habing, P. E. Clegg, and T. J. Chester (Eds.) 1988. Infrared Astronomical Satellite Catalog and Atlases, Vol. 1: Explanatory Supplement. NASA RP-1190. Buie, M. W., D. J. Tholen, and K. Horne 1992. Albedo maps of Pluto and Charon: Initial mutual event results. Icarus 97, 211–227. Buie, M. W., D. J. Tholen, and L. H. Wasserman 1997. Separate lightcurve of Pluto and Charon. Icarus 125, 233–244. Jewitt, D. 1994. Heat from Pluto. Astron. J. 107, 372–378. Lebofsky, L. A., and J. R. Spencer 1989. Radiometry and thermal modeling of asteroids. In Asteroids II (R. Binzel, T. Gehrels, and M. Matthews, Eds.), pp. 128–147. Univ. of Arizona Press, Tucson.

159

Lellouch, E., R. Laureijs, B. Schmitt, E. Quirco, C. de Bergh, J. Grovisier, and A. Coustenis 1998. ISOPHOT observations of the Pluto–Charon system: Pluto’s thermal lightcurve. Bull. Am. Astron. Soc. 30, 2707. Morrison, D., T. Owen, and L. A. Soderblom 1986. The satellites of Saturn. In Satellites (J. Burns and M. Matthews, Eds.), pp. 764–801. Univ. of Arizona Press, Tucson. Owen, T., T. L. Roush, D. P. Cruikshank, J. L. Elliot, L. A. Young, C. de Bergh, B. Schmitt, T. R. Geballe, R. H. Brown, and M. J. Bartholomew 1993. Surface ices and the atmospheric composition of Pluto. Science 261, 745–748. Reinsch, K., V. Burwitz, and M. C. Festou 1994. Albedo maps of Pluto and improved physical parameters of the Pluto–Charon system. Icarus 108, 209– 218. Stansberry, J. A., J. R. Spencer, B. Schmitt, A.-I. Benchkoura, R. V. Yelle, and J. I. Lunine 1996. A model for the overabundance of methane in the atmospheres of Pluto and Triton. Planet. Space Sci. 44, 1051–1063. Stern, S. A., M. W. Buie, and L. M. Trafton 1997. HST high-resolution images and maps of Pluto. Astron. J. 113, 827–843. Stern, S. A., D. A. Weintraub, and M. C. Festou 1993. Evidence for a low surface temperature on Pluto from millimeter-wave thermal emission measurements. Science 261, 1713–1716. Sykes, M. 1993. Implications of Pluto–Charon radiometry. Bull. Am. Astron. Soc. 25, 1138. Sykes, M. V., R. M. Cutri, L. A. Lebofsky, and R. P. Binzel 1987. IRAS serendipitous observations of Pluto and Charon. Science 237, 1336. Tedesco, E. F., G. J. Veeder, R. S. Dunbar, and L. A. Lebofsky 1987. IRAS constraints on the sizes of Pluto and Charon. Nature 327, 127–129. Tholen, D. J., and M. W. Buie 1988. Further analysis of Pluto–Charon mutual event observations—1988. Bull. Am. Astron. Soc. 20, 807. Tholen, D. J., and E. F. Tedesco 1994. Pluto’s lightcurve: Results from four oppositions. Icarus 108, 200–208. Trafton, L. 1980. Does Pluto have a substantial atmosphere? Icarus 44, 53–61. Trafton, L., and S. A. Stern 1983. On the global distribution of Pluto’s atmosphere. Astrophys. J. 267, 872–881. Tryka, K. A., R. H. Brown, D. P. Cruikshank, T. C. Owen, T. R. Geballe, and C. DeBergh 1994. The temperature of nitrogen ice on Pluto and its implication for flux measurements. Icarus 112, 513–527. Young, E. F., and R. P. Binzel 1993. Comparative mapping of Pluto’s sub-Charon hemisphere: Three least squares models based on mutual event lightcurve. Icarus 102, 134–149.