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Extraordinary colors of asteroidal object (5145) 1992 AD
ICARUS 97, 150--154 (1992)
NOTE Extraordinary Colors of Asteroidal Object (5145) 1992 AD BI~ATRICE E . A . M U E L L E R Kitt Peak National Observatory. Tucson, Arizona 85719 DAVID J. THOLEN University q( Hawaii, Honolulu, Hawaii 96822 WILLIAM K . HARTMANN Planetao' Science Institute, Tucson, Arizona 85719 AND DALE P. CRUIKSHANK NASA Ames Research Center, Mq~fett Field, Cal(tbrnia 94035 Received March 9, 1992: revised April 28, 1992
discovery was made shortly after perihelion passage, which occurred on 1991 September 26 UT, and quite close to both opposition and the object's passage through the ascending node. The figure demonstrates that (5145) 1992 AD passes very close to the orbit of Saturn (although Saturn itself is presently on the opposite side of its orbit), which implies that the object's lifetime in its current orbit is rather limited. In fact, a simple two-body extrapolation of the orbit into the future shows a moderately close approach to Saturn just three orbital periods from now (less than 300 years). A more rigorous orbital integration to examine the possibility of evolution toward the inner Solar System is beyond the scope of this Note.
The recently discovered outer Solar System object, (5145) 1992 AD, in a s o m e w h a t Chiron-like orbit, has colors far redder t h a n any other k n o w n asteroids or comets, and represents a hithertou n k n o w n spectral class. The red color may be associated with exposure o f organics that are purer or more pristine t h a n those f o u n d on the surfaces o f C, P, a n d D asteroids, and comets, and such m a t e r i a l s a r e likely to show diagnostic spectral features in the infrared. ~ 1992 Academic Press, Inc.
2. Observations. Rabinowitz reported the discovery and unusual motion to one of us (BEAM), who succeeded in acquiring VR1 colorimetry on that same night. Similarly, Scotti reported the discovery to another one of us (DJT), who was able to acquire BVRI colorimetry from Mauna Kea on 1992 January 23 (Table I). Both sets of observations were reported along with the discovery announcement on this latter date (Scotti 1992, Mueller 1992a, Tholen 1992), and indicated that 15145) 1992 AD was uniquely red among the observed asteroids and comets. Additional confirmation of the extremely red color of the object was later provided by Hainaut and Smette 11992). The Kitt Peak observations by BEAM were made with the 2. l-m telescope equipped with a Tektronix 1024 × 1024 CCD (scale 0.Y'/pixel). The standard Harris filter set (Kron-Cousins system) was used. The sky was photometric with seeing of about 1.8". A digital aperture of 5. l" was used for the photometry. Landolt Stars (Landolt 1983) and the field of NGC 2419 (L. Davis 1991, private communication) were observed as standards. The Mauna Kea observations were made using the submicron photometer (SUMP) and BVRI filters on the NASA Infrared Telescope Facility (IRTF). A 3-ram aperture, corresponding to -6", was utilized for the observations. Standard stars from Landolt 11983) were used to reduce
1. Introduction. 15145) 1992 AD was discovered on 1992 January 9 UT by D. L. Rabinowitz with the Spacewatch telescope on Kitt Peak (Scotti 1992). The object was also independently identified by C. E. Shoemaker on films taken with the Palomar 18" Schmidt telescope on 1992 January 1 UT. The very slow opposition motion immediately suggested that the object was quite distant, and thus a rare find, so a special effort was made to obtain additional astrometric and physical data. Extrapolation of the preliminary orbit backward in time enabled prediscovery images of (5145) 1992 AD to be found on various archival films and plates dating back as far as 1977, which have permitted the computation of a definitive orbit. Although a similar object, 2060 Chiton, turned out to be a comet, deep CCD images of 15145) 1992 AD have failed to reveal any trace of coma, even though the object is at perihelion, and a spectrum shows no emission lines (Hainaut and Smette 1992), thus the object has been added to the numbered asteroid catalog. The orbit, as shown in Fig. 1, is highly eccentric (0.58), with perihelion and aphelion distances of 8.7 and 32.3 AU, respectively. The orbital inclination is 25 deg, and the semimajor axis of 20.5 AU implies an orbital period of 92.7 years, the longest of any known asteroid. The 15 fl
0019-1035/92 $5.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
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FIG. 1. The orbit for (5145) 1992 AD is shown along with those of the five outermost planets. The dots represent the positions of the asteroid and planets as of 1992 January 9 UT. The dashed portions of the orbits lie below the plane of the ecliptic, and the distances from the ecliptic plane are indicated by the vertical lines (shown for (5145) 1992 AD and Pluto only). The numbers indicate the ecliptic longitude in degrees.
the instrumental magnitudes and colors to the Johnson (BV) and KronCousins (RI) system. The geometry for both observations was nearly identical. The heliocentric distance in both cases was 8.70 AU, the geocentric distance decreased from 7.74 to 7.72 AU between the two dates, and the phase angle decreased from 1.4 to 0.4 deg. Almost all known interplanetary bodies in the outer Solar System, including Trojan asteroids, comets, and Chiron, have low albedos (-0.02-0.1) and fall in a series of spectral classes known as C, P, and D (Hartmann et al. 1982, 1985, A'Hearn 1988), which range in color from neutral (solar) for Chiron and C asteroids to red for D asteroids, which are concentrated among the Trojans and outermost belt. The
TABLE I Observed Magnitudes Date
B
V
R
I
Observer
92 Jan 9 92 Jan 23
-17.97
16.86 16.62
16.11 15.96
15.35 15.28
BEAM DJT
N o t e . DJT magnitudes are preliminary reductions. Differences between BEAM and DJT colors (i.e., V-R) are within the measured amplitude of the lightcurve (0.17 mag; Buie e t al. 1992). To derive the colors plotted in Fig. 2, we used solar colors of B - V = 0.67, V - R = 0.36, V - I = 0.69 (Hartmann et al. 1990).
outermost satellites of Jupiter and Saturn, believed to be captured, also appear to be members of these classes (Tholen and Zellner 1984, Luu 1991) suggesting that most dark materials of the outer Solar System fall within this scheme. Figure 2 plots the colors of (5145) 1992 AD against a background of known outer Solar System planetesimal colors from visible to infrared wavelengths. We utilize VJHK colors of seven C's, six P's, eight D's, and Chiron, reported by Hartmann et al. (1990), and 10 different comets observed by DJT, WKH, and DPC, plus Comet P/Tempel 2 (observed in VRI, Mueller 1992b). Detailed spectra of CPD asteroids are relatively featureless; thus the eight-color plus VJHK colorimetry gives a rough approximation to their spectra; e.g., the line marking the upper limit of the D field in Fig. 2 matches eight-color colorimetry. The comet data in Fig. 2 mostly refer to coma plus nucleus, but include some with very thin comae, such as P/Schwassmann-Wachmann 1 in a state of minimum activity; on the basis of reports of nuclear colors by other observers, we believe our observed colors are comparable to those of inactive nuclei. For clarity, P's are left out of the figure, but six additionally observed P's fall approximately in the gap between the C's and D's. Figure 2 also includes JHK observations of (5145) 1992 AD reported by Davies and Sykes (1992). It must be noted that no simultaneous V and J colors were obtained by these observers. Davies and Sykes extrapolated the previous V observations to the date of their own observation, and used that to compute a V - J color which ties their JHK data to our BVRI data. In addition to this uncertainty, the phase of the lightcurve was unknown, introducing up to 0.15 mag of additional uncertainty. In other words, the BVRI data set and the JHK data set are each internally consistent, but the link between
152
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FIG. 2. Comparison of our BVRI and Davies and Sykes' JHK colorimetry of (5145) 1992 AD (open circles and heavy solid line) to colors of other outer Solar System interplanetary bodies. The V-J color link is uncertain (see text). Colors of 21 asteroids of classes C, P, and D by Hartmann et al. (1982) are plotted and define shaded fields; P's concentrate in between. Colors of 11 different comets (with two sets on P/SchwassmannWachmann I in two different years--dashed) are also plotted (data of DJT, DPC, WKH, and BEAM). Neutral colors for 2060 Chiron are shown by open circles and heavy dashed line at bottom (Hartmann et al. 1990). BVRI colors of (5145) 1992 AD show that it is far redder than other objects. Also included are laboratory colorimetry of a number of organic substances, such as "tholin 3" (dotted line), residue from bombardment of methane ice (M), asphaltite (A, off figure at top in JHK) and other substances whose symbols and significance are described in the text.
them and the vertical position of the JHK curve on our diagram are less certain. The important result is that (5145) 1992 AD is far redder in BVR1 and higher in relative JHK reflectance (even allowing generous error bars) than previously observed outer Solar System interplanetary objects. (We do not include other objects on the figure, but we note that (5145) 1992 AD is also redder than the higher-albedo asteroids, which predominantly populate the asteroid belt inside 2.7 AU; we believe those asteroids are less relevant as comparison objects because they appear to be limited to the inner Solar System. To give the reader a sense of these colors, we also note that (5145) 1992 AD is not as red as Mars from V to R, but redder than the Moon and Io. Interestingly, (5145) 1992 AD has about the same V - R color as Titan, but Titan's brightness drops from R to I, while (5145) 1992 AD's increases.) In Fig. 2 we compare (5145) 1992 AD to outer Solar System asteroids, and because of the possibility that it might have a yet-undetected coma, we also compare with comets which do have comae. We do not mean to imply that coma-free asteroids can be directly compared with comets, because systematic color difference may arise due to scattering effects as comae develop. Our point is that (5145) 1992 AD is redder than known objects of either asteroid or comet character.
These data indicate that (5145) 1992 AD is a member of a hithertounknown spectral taxonomic class of interplanetary bodies. We assign the notation Z for this new taxonomic class, although it remains to define this class in terms of the eight-color photometric system. Data of Howell et al. (1992) indicate a low geometric albedo (preliminary result -<0.08), suggesting that (5145) 1992 AD belongs among the generally dark (carbonaceous'?) outer Solar System objects. The problem posed by (5145) 1992 AD is the meaning of the unusual colors compared to other outer Solar System bodies. 3. Significance q# observations. Observations of Chiron-class objects with different spectral classes may eventually clarify the population of the Oort cloud, which is believed to consist of planetesimals ejected from the regions of the giant planets in the early Solar System, especially Uranus and Neptune. For example, "comets thrown into the Oort cloud from different initial positions among the giant planets may have somewhat different compositions that could lead to detectably different colors" (Hartmann et aL 1982). What have we seen so far in this regard? 2060 Chiron, which is now displaying cometary activity, appears to be a recent arrival from the
MUELLER ET AL. Oort cloud, presently on an unstable orbit that will be changed by Saturn on a time scale of 103-104 years (Everhart 1977, Marsden 1977). It has a very neutral color from V to K wavelengths, as shown by the heavy dashed line at the bottom of Fig. 2. However, the rest of the more "traditional" comets show a range of colors, generally in the P to D fields, with some preference toward redder P's and D's (Hartmann et al. 1982, 1985, A'Hearn 1988). As long ago as 1980, Gradie and Veverka proposed that the D asteroids are colored by Iow-albedo, reddish organic compounds, and that comet nuclei would share this coloration. Vilas and Smith (1985) stressed that the incidence of red color among interplanetary bodies increases as one goes outward in the outer Solar System from the neutral, main-belt C's to the red, Trojan P's and D's. These papers led to an unconfirmed view of increasing formation efficiency of colored compounds (or decreasing destruction efficiency) as a function of solar distance. Chiron and Saturn's captured satellite Phoebe do not confirm this trend; however, their relevance is suspect because we do not know their points of origin. Searches for spectral signatures of organics in low-albedo objects have so far yielded only a few features, including a possible C - H stretching band and a band identified with C I N bearing material (Cruikshank and Brown 1987, Cruikshank et al. 1991). The literature thus reflects a view that objects in the Saturn/Uranus neighborhood would be redder than D's (Bell e t al. 1989, p. 926 and Fig. 7), and it has been suggested informally that (5145) 1992 AD thus confirms the canonical view. However, we point out that (5145) 1992 AD, like Chiron, is probably on its way in from the Oort cloud, having probably originated at some unknown place in the outer Solar System. Because the orbit is unstable, (5145) 1992 AD has almost certainly not been in its present location for much of Solar System history. Thus it is too early to say that (5145) 1992 AD confirms the trend. In any case, as Bell et al. predicted, we now know that the CPD scheme is not a complete description of outer Solar System materials. Carbonaceous and silicate rocks and minerals, and ices, generally fail to match the observed spectral characteristics of (5145) 1992 AD, whereas organic substances in low-albedo mixtures do. Thus, possible insight into the meaning of the colors of (5145) 1992 AD may come from spectroscopic studies of samples of organic materials prepared in various labs (Cruikshank et al., in preparation). These include an extract from the Murchison carbonaceous chondrite, bulk Murchison and Allende samples, kerogen, an HCN polymer (Matthews and Ludicky 1986), asphaltite (Moroz et al. 1991), a solid produced from energetic particle irradiation of frozen methane (Strazzulla et al. 1984), and four tholins made by UV irradiation and electrical discharges in atmospheres of primitive gases (Khare et al. 1981, McDonald et al. 1991). Some of these materials are neutral in color when seen in diffuse reflectance, but the tholins ( " T H " in Fig. 2), HCN polymer ("HCN"), and asphaltite ( " A " ) all have steeply upward sloping reflectances in the VRI region, redder than, or comparable to, (5145) 1992 AD. As shown in Fig. 2, all these materials are also redder than the D-class asteroids, formerly the reddest known small bodies in the outer Solar System. The sample called tholin 3 (the "Titan tholin" of Khare et al.--dotted line in Fig. 2) is redder at R and I than (5145) 1992 AD, and its reflectance at JHK levels off in the near infrared, a characteristic of many of the materials we have studied. The dashed lines in Fig. 2 show a similar leveling in the JHK region for two observations of Comet P/ Schwassmann-Wachmann 1, observed at different states of activity. In this context, it is especially interesting that Davies and Sykes' estimated VJHK colors (keeping the V-J uncertainty in mind) fit closely to the tholin 3 sample and mimic its leveling off in the JHK region. Also shown in Fig. 2 are colors of an organic residue produced by particle bombardment of methane ice ( " M " points). These may be especially relevant, since this synthesis process may occur on planetary surfaces (Strazzulla et al. 1984). Its colors are less red than (5145) 1992 AD but redder than D asteroids. Its colors also show some leveling off in the JHK region. The albedo and color of the residue depend on the
153
combination of total radiation dosage and initial carbon content. Some combination of radiation dosage and initial carbon might produce (5145) 1992 AD's color. Our other samples (labeled "other organics" in Fig. 2) show less red colors than (5145) 1992 AD, ranging from neutral to slightly redder than our reddest D asteroids. These comparisons show that organics, unlike other plausible materials, bracket the colors of (5145) 1992 AD. This suggests that (5145) 1992 AD's unique colors may arise from a concentration of solid organic substances of purity greater than that found on the surfaces of most outer Solar System bodies; contamination by very dark, neutral-colored carbon materials may be the rule on most bodies. The coloring agents may be most similar to the tholins of Khare et al., or the irradiated methane ice residue of Strazzulla et al. A similar conclusion was reached independently by Fink et al. (1992), who stressed the possible connection with tholins. (Their paper reached us after we completed our draft of this work.) The possible exposure of relatively pure organics makes it extremely important to acquire infrared spectra of (5145) 1992 AD; our laboratory spectra of the organic substances generally show strong features in the infrared, contrary to the spectra of most CPD asteroids. Therefore, the infrared spectra of (5145) 1992 AD may turn out to be more diagnostic than is the case for the relatively featureless CPD asteroids observed thus far. The properties of (5145) 1992 AD call for a vigorous search for more Chiron-like objects, and further efforts to clarify their spectral properties and relations to more "traditional" comets and asteroids. Infrared colors and spectra are especially needed.
ACKNOWLEDGMENTS We are grateful for the helpful comments from D. L. Rabinowitz, M. Sykes, J. F. Bell, PSI staff, and other colleagues. BEAM acknowledges support through NASA Planetary Astronomy Postdoctoral Award. DJT was supported by NASA Grant NAGW 3044, and WKH by NASA Planetary Astronomy Program Contract NASW-4573. The National Optical Astronomy Observatories are operated by AURA Inc. under cooperative agreement with the National Science Foundation. Planetary Science Institute is a nonprofit division of Science Applications International Corp. This is PSI Contribution 296.
REFERENCES A'HEARN, M. F. 1988. Observations of cometary nuclei. A n n . R e v . E a r t h P l a n e t . Sci. 16, 273-293.
BELL, J. F., D. R. DAVIS,W. K. HARTMANN, AND M. J. GAFFEY 1989. Asteroids: The big picture. In A s t e r o i d s H (R. P. Binzel, T. Gehrels, and M. S. Matthews, Eds.), pp. 921-945. Univ. of Arizona Press, Tucson. BUtE, M. W., S. J. Bus, AND B. A. SKIFF 1992. 1992 AD. 1 A U Circ. 5451.
CRUIKSHANK, D. P., L. J. ALLAMANDOLA, W. K. HARTMANN, D. J. THOLEN, R. H. BROWN, C. N. MATTHEWS, AND J. F. BELL 1991. Solid C ~ N bearing material on outer Solar System bodies, l c a r u s 94, 345-353. CRUIKSHANK, D. P., AND R. H. BROWN 1987. Organic matter on asteroid 130 Elektra. S c i e n c e 238, 183-185.
CRUIKSHANK, D. P., K. UCHICA, R. N. CLARK, J. W. SALISBURY, W. K. HARTMANN, B. N. KHARE, C. N. MATTHEWS. R. L. LUD1CKY, C. SAGAN, G. STRAZZULLA, M. J. BARTHOLOMEW, AND E. T. ARAKAWA. Infrared spectra of organic solids for comparison with dark Solar System bodies. In preparation. DAVIES, J. K., AND M. V. SYKES 1992. (5145) 1992 AD. I A U C i r c . 5480.
154
NOTE
EVERHART, E. 1977. The evolution of comet orbits as perturbed by
comets. In Comets, Asteroids, and Meteorites (A. H. Delsemme, Ed.), pp. 79-86. Univ. of Toledo Press, Toledo.
Uranus and Neptune. In Comets, Asteroids, and Meteorites (A. H. Delsemme, Ed.), pp. 99-104. Univ. of Toledo Press, Toledo.
MATTHEWS,C. N., AND R. LUDICKY 1986. The dark nucleus of Comet
FINK, U., M. HOFFMANN, W. GRUNDY, M. HICKS, AND W. SEARS 1992. The steep red spectrum of 1992 AD: An asteroid covered with organic material'? Submitted for publication.
Halley: Hydrogen cyanide polymers. In Proceedings o f the 20th ESLAB Symposium on the Exploration ~?f Halley's Comet, Heidelberg, ESA SP-250, pp. 273-277.
GRADIE, J., AND J. VEVERKA 1980. The composition of the Trojan asteroids. Nature 283, 840-842. HAINAUT, O., AND A. SMETTE 1992. 1992 AD. IAU Circ. 5449.
McDONALD, G. D., B. N. KHARE, W. R. THOMPSON, AND C. SAGAN 1991. CH4/NH3/H20 spark tholin: Chemical analysis and interaction with Jovian aqueous clouds. Icarus 94, 354-367.
HARTMANN, W. K., D. CRUIKSHANK, AND J. DEOEWlJ 1982. Remote comets and related bodies: VJHK colorimetry and surface materials. Iearus 52, 377-408.
MOROZ, L. V., C. M. PIETERS, AND M. V. AKHMANOVA1991. Spectroscopy of solid carbonaceous materials: Implications for dark surfaces of outer belt asteroids. Lunar Planet. Sei. XXII, 925-926.
HARTMANN, W. K., D. CRUIKSHANK, AND D. THOLEN 1985. Outer Solar System materials: Ices and color systematics. In h'es in the Solar System (J. Klinger et al., Eds.). Reidel, New York.
MUELLER, B. E. A. 1992a. 1992 AD. IAU Circ. 5434. MUELLER, B. E. A. 1992b. CCD-photometry of comets at large heliocentric distances. In Asteroids, Comets, Meteors 1991. (A. Harris and E. Bowell, Eds.). In press.
HARTMANN, W. K., D. THOLEN, K. MEECH, AND D. CRUIKSHANK1990. 2060 Chiron: Colorimetry and cometary behavior. Icarus 83, 1-15. HOWELL, E., R. MARClALIS, R. CUTRI, M. NOLAN, L. LEBOFSKV,AND M. SYKES 1992. 1992 AD. IAU Circ. 5449. KHARE, B. N., C. SAGAN, J, ZUMBERGE, D. SKLAREW. AND B. NAGY 1981. Organic solids produced by electrical discharge in reducing atmospheres: Tholin molecular analysis. Icarus 48, 290-297.
S¢OTTI, J. V. 1992. 1992 AD. 1AU Circ'. 5434. STRAZZULLA, G., R. S. CATALIOTT, L. CALCAGNO, AND G. FOTI 1984. The IR spectra of laboratory synthesized polymeric residue. Astron. Astrophys. 133, 77-79. THOLEN, D. J. 1992. 1992 AD. IAU Circ. 5434.
THOLEN, D. J., AND B. ZELLNER 1984. Multicolor photometry of outer
LANDOLT, A. U. 1983. UBVR1 photometric standards around the celestial equator. Astron. J. 88, 439-460. Luu, J. X. 1991. CCD photometry and spectroscopy of the outer Jovian satellites. Astron. J. 102, 1213-1225.
VILAS, F., AND B. A. SMITH 1985. Reflectance spectrophotometry (-0.5-1.0 p~m) of outer-belt asteroids: Implications for primitive, organic Solar System material. Icarus 64, 503-516.
Jovian satellites. Icarus 58, 246-253.
MARSDEN, B. G. 1977. Orbital data on the existence of Oort's cloud of
WILLIAMS, G. V. 1992. 1992 AD. IAU Circ'. 5462.
NOTE Extraordinary Colors of Asteroidal Object (5145) 1992 AD BI~ATRICE E . A . M U E L L E R Kitt Peak National Observatory. Tucson, Arizona 85719 DAVID J. THOLEN University q( Hawaii, Honolulu, Hawaii 96822 WILLIAM K . HARTMANN Planetao' Science Institute, Tucson, Arizona 85719 AND DALE P. CRUIKSHANK NASA Ames Research Center, Mq~fett Field, Cal(tbrnia 94035 Received March 9, 1992: revised April 28, 1992
discovery was made shortly after perihelion passage, which occurred on 1991 September 26 UT, and quite close to both opposition and the object's passage through the ascending node. The figure demonstrates that (5145) 1992 AD passes very close to the orbit of Saturn (although Saturn itself is presently on the opposite side of its orbit), which implies that the object's lifetime in its current orbit is rather limited. In fact, a simple two-body extrapolation of the orbit into the future shows a moderately close approach to Saturn just three orbital periods from now (less than 300 years). A more rigorous orbital integration to examine the possibility of evolution toward the inner Solar System is beyond the scope of this Note.
The recently discovered outer Solar System object, (5145) 1992 AD, in a s o m e w h a t Chiron-like orbit, has colors far redder t h a n any other k n o w n asteroids or comets, and represents a hithertou n k n o w n spectral class. The red color may be associated with exposure o f organics that are purer or more pristine t h a n those f o u n d on the surfaces o f C, P, a n d D asteroids, and comets, and such m a t e r i a l s a r e likely to show diagnostic spectral features in the infrared. ~ 1992 Academic Press, Inc.
2. Observations. Rabinowitz reported the discovery and unusual motion to one of us (BEAM), who succeeded in acquiring VR1 colorimetry on that same night. Similarly, Scotti reported the discovery to another one of us (DJT), who was able to acquire BVRI colorimetry from Mauna Kea on 1992 January 23 (Table I). Both sets of observations were reported along with the discovery announcement on this latter date (Scotti 1992, Mueller 1992a, Tholen 1992), and indicated that 15145) 1992 AD was uniquely red among the observed asteroids and comets. Additional confirmation of the extremely red color of the object was later provided by Hainaut and Smette 11992). The Kitt Peak observations by BEAM were made with the 2. l-m telescope equipped with a Tektronix 1024 × 1024 CCD (scale 0.Y'/pixel). The standard Harris filter set (Kron-Cousins system) was used. The sky was photometric with seeing of about 1.8". A digital aperture of 5. l" was used for the photometry. Landolt Stars (Landolt 1983) and the field of NGC 2419 (L. Davis 1991, private communication) were observed as standards. The Mauna Kea observations were made using the submicron photometer (SUMP) and BVRI filters on the NASA Infrared Telescope Facility (IRTF). A 3-ram aperture, corresponding to -6", was utilized for the observations. Standard stars from Landolt 11983) were used to reduce
1. Introduction. 15145) 1992 AD was discovered on 1992 January 9 UT by D. L. Rabinowitz with the Spacewatch telescope on Kitt Peak (Scotti 1992). The object was also independently identified by C. E. Shoemaker on films taken with the Palomar 18" Schmidt telescope on 1992 January 1 UT. The very slow opposition motion immediately suggested that the object was quite distant, and thus a rare find, so a special effort was made to obtain additional astrometric and physical data. Extrapolation of the preliminary orbit backward in time enabled prediscovery images of (5145) 1992 AD to be found on various archival films and plates dating back as far as 1977, which have permitted the computation of a definitive orbit. Although a similar object, 2060 Chiton, turned out to be a comet, deep CCD images of 15145) 1992 AD have failed to reveal any trace of coma, even though the object is at perihelion, and a spectrum shows no emission lines (Hainaut and Smette 1992), thus the object has been added to the numbered asteroid catalog. The orbit, as shown in Fig. 1, is highly eccentric (0.58), with perihelion and aphelion distances of 8.7 and 32.3 AU, respectively. The orbital inclination is 25 deg, and the semimajor axis of 20.5 AU implies an orbital period of 92.7 years, the longest of any known asteroid. The 15 fl
0019-1035/92 $5.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
MUELLER ET AL.
151 90
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FIG. 1. The orbit for (5145) 1992 AD is shown along with those of the five outermost planets. The dots represent the positions of the asteroid and planets as of 1992 January 9 UT. The dashed portions of the orbits lie below the plane of the ecliptic, and the distances from the ecliptic plane are indicated by the vertical lines (shown for (5145) 1992 AD and Pluto only). The numbers indicate the ecliptic longitude in degrees.
the instrumental magnitudes and colors to the Johnson (BV) and KronCousins (RI) system. The geometry for both observations was nearly identical. The heliocentric distance in both cases was 8.70 AU, the geocentric distance decreased from 7.74 to 7.72 AU between the two dates, and the phase angle decreased from 1.4 to 0.4 deg. Almost all known interplanetary bodies in the outer Solar System, including Trojan asteroids, comets, and Chiron, have low albedos (-0.02-0.1) and fall in a series of spectral classes known as C, P, and D (Hartmann et al. 1982, 1985, A'Hearn 1988), which range in color from neutral (solar) for Chiron and C asteroids to red for D asteroids, which are concentrated among the Trojans and outermost belt. The
TABLE I Observed Magnitudes Date
B
V
R
I
Observer
92 Jan 9 92 Jan 23
-17.97
16.86 16.62
16.11 15.96
15.35 15.28
BEAM DJT
N o t e . DJT magnitudes are preliminary reductions. Differences between BEAM and DJT colors (i.e., V-R) are within the measured amplitude of the lightcurve (0.17 mag; Buie e t al. 1992). To derive the colors plotted in Fig. 2, we used solar colors of B - V = 0.67, V - R = 0.36, V - I = 0.69 (Hartmann et al. 1990).
outermost satellites of Jupiter and Saturn, believed to be captured, also appear to be members of these classes (Tholen and Zellner 1984, Luu 1991) suggesting that most dark materials of the outer Solar System fall within this scheme. Figure 2 plots the colors of (5145) 1992 AD against a background of known outer Solar System planetesimal colors from visible to infrared wavelengths. We utilize VJHK colors of seven C's, six P's, eight D's, and Chiron, reported by Hartmann et al. (1990), and 10 different comets observed by DJT, WKH, and DPC, plus Comet P/Tempel 2 (observed in VRI, Mueller 1992b). Detailed spectra of CPD asteroids are relatively featureless; thus the eight-color plus VJHK colorimetry gives a rough approximation to their spectra; e.g., the line marking the upper limit of the D field in Fig. 2 matches eight-color colorimetry. The comet data in Fig. 2 mostly refer to coma plus nucleus, but include some with very thin comae, such as P/Schwassmann-Wachmann 1 in a state of minimum activity; on the basis of reports of nuclear colors by other observers, we believe our observed colors are comparable to those of inactive nuclei. For clarity, P's are left out of the figure, but six additionally observed P's fall approximately in the gap between the C's and D's. Figure 2 also includes JHK observations of (5145) 1992 AD reported by Davies and Sykes (1992). It must be noted that no simultaneous V and J colors were obtained by these observers. Davies and Sykes extrapolated the previous V observations to the date of their own observation, and used that to compute a V - J color which ties their JHK data to our BVRI data. In addition to this uncertainty, the phase of the lightcurve was unknown, introducing up to 0.15 mag of additional uncertainty. In other words, the BVRI data set and the JHK data set are each internally consistent, but the link between
152
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FIG. 2. Comparison of our BVRI and Davies and Sykes' JHK colorimetry of (5145) 1992 AD (open circles and heavy solid line) to colors of other outer Solar System interplanetary bodies. The V-J color link is uncertain (see text). Colors of 21 asteroids of classes C, P, and D by Hartmann et al. (1982) are plotted and define shaded fields; P's concentrate in between. Colors of 11 different comets (with two sets on P/SchwassmannWachmann I in two different years--dashed) are also plotted (data of DJT, DPC, WKH, and BEAM). Neutral colors for 2060 Chiron are shown by open circles and heavy dashed line at bottom (Hartmann et al. 1990). BVRI colors of (5145) 1992 AD show that it is far redder than other objects. Also included are laboratory colorimetry of a number of organic substances, such as "tholin 3" (dotted line), residue from bombardment of methane ice (M), asphaltite (A, off figure at top in JHK) and other substances whose symbols and significance are described in the text.
them and the vertical position of the JHK curve on our diagram are less certain. The important result is that (5145) 1992 AD is far redder in BVR1 and higher in relative JHK reflectance (even allowing generous error bars) than previously observed outer Solar System interplanetary objects. (We do not include other objects on the figure, but we note that (5145) 1992 AD is also redder than the higher-albedo asteroids, which predominantly populate the asteroid belt inside 2.7 AU; we believe those asteroids are less relevant as comparison objects because they appear to be limited to the inner Solar System. To give the reader a sense of these colors, we also note that (5145) 1992 AD is not as red as Mars from V to R, but redder than the Moon and Io. Interestingly, (5145) 1992 AD has about the same V - R color as Titan, but Titan's brightness drops from R to I, while (5145) 1992 AD's increases.) In Fig. 2 we compare (5145) 1992 AD to outer Solar System asteroids, and because of the possibility that it might have a yet-undetected coma, we also compare with comets which do have comae. We do not mean to imply that coma-free asteroids can be directly compared with comets, because systematic color difference may arise due to scattering effects as comae develop. Our point is that (5145) 1992 AD is redder than known objects of either asteroid or comet character.
These data indicate that (5145) 1992 AD is a member of a hithertounknown spectral taxonomic class of interplanetary bodies. We assign the notation Z for this new taxonomic class, although it remains to define this class in terms of the eight-color photometric system. Data of Howell et al. (1992) indicate a low geometric albedo (preliminary result -<0.08), suggesting that (5145) 1992 AD belongs among the generally dark (carbonaceous'?) outer Solar System objects. The problem posed by (5145) 1992 AD is the meaning of the unusual colors compared to other outer Solar System bodies. 3. Significance q# observations. Observations of Chiron-class objects with different spectral classes may eventually clarify the population of the Oort cloud, which is believed to consist of planetesimals ejected from the regions of the giant planets in the early Solar System, especially Uranus and Neptune. For example, "comets thrown into the Oort cloud from different initial positions among the giant planets may have somewhat different compositions that could lead to detectably different colors" (Hartmann et aL 1982). What have we seen so far in this regard? 2060 Chiron, which is now displaying cometary activity, appears to be a recent arrival from the
MUELLER ET AL. Oort cloud, presently on an unstable orbit that will be changed by Saturn on a time scale of 103-104 years (Everhart 1977, Marsden 1977). It has a very neutral color from V to K wavelengths, as shown by the heavy dashed line at the bottom of Fig. 2. However, the rest of the more "traditional" comets show a range of colors, generally in the P to D fields, with some preference toward redder P's and D's (Hartmann et al. 1982, 1985, A'Hearn 1988). As long ago as 1980, Gradie and Veverka proposed that the D asteroids are colored by Iow-albedo, reddish organic compounds, and that comet nuclei would share this coloration. Vilas and Smith (1985) stressed that the incidence of red color among interplanetary bodies increases as one goes outward in the outer Solar System from the neutral, main-belt C's to the red, Trojan P's and D's. These papers led to an unconfirmed view of increasing formation efficiency of colored compounds (or decreasing destruction efficiency) as a function of solar distance. Chiron and Saturn's captured satellite Phoebe do not confirm this trend; however, their relevance is suspect because we do not know their points of origin. Searches for spectral signatures of organics in low-albedo objects have so far yielded only a few features, including a possible C - H stretching band and a band identified with C I N bearing material (Cruikshank and Brown 1987, Cruikshank et al. 1991). The literature thus reflects a view that objects in the Saturn/Uranus neighborhood would be redder than D's (Bell e t al. 1989, p. 926 and Fig. 7), and it has been suggested informally that (5145) 1992 AD thus confirms the canonical view. However, we point out that (5145) 1992 AD, like Chiron, is probably on its way in from the Oort cloud, having probably originated at some unknown place in the outer Solar System. Because the orbit is unstable, (5145) 1992 AD has almost certainly not been in its present location for much of Solar System history. Thus it is too early to say that (5145) 1992 AD confirms the trend. In any case, as Bell et al. predicted, we now know that the CPD scheme is not a complete description of outer Solar System materials. Carbonaceous and silicate rocks and minerals, and ices, generally fail to match the observed spectral characteristics of (5145) 1992 AD, whereas organic substances in low-albedo mixtures do. Thus, possible insight into the meaning of the colors of (5145) 1992 AD may come from spectroscopic studies of samples of organic materials prepared in various labs (Cruikshank et al., in preparation). These include an extract from the Murchison carbonaceous chondrite, bulk Murchison and Allende samples, kerogen, an HCN polymer (Matthews and Ludicky 1986), asphaltite (Moroz et al. 1991), a solid produced from energetic particle irradiation of frozen methane (Strazzulla et al. 1984), and four tholins made by UV irradiation and electrical discharges in atmospheres of primitive gases (Khare et al. 1981, McDonald et al. 1991). Some of these materials are neutral in color when seen in diffuse reflectance, but the tholins ( " T H " in Fig. 2), HCN polymer ("HCN"), and asphaltite ( " A " ) all have steeply upward sloping reflectances in the VRI region, redder than, or comparable to, (5145) 1992 AD. As shown in Fig. 2, all these materials are also redder than the D-class asteroids, formerly the reddest known small bodies in the outer Solar System. The sample called tholin 3 (the "Titan tholin" of Khare et al.--dotted line in Fig. 2) is redder at R and I than (5145) 1992 AD, and its reflectance at JHK levels off in the near infrared, a characteristic of many of the materials we have studied. The dashed lines in Fig. 2 show a similar leveling in the JHK region for two observations of Comet P/ Schwassmann-Wachmann 1, observed at different states of activity. In this context, it is especially interesting that Davies and Sykes' estimated VJHK colors (keeping the V-J uncertainty in mind) fit closely to the tholin 3 sample and mimic its leveling off in the JHK region. Also shown in Fig. 2 are colors of an organic residue produced by particle bombardment of methane ice ( " M " points). These may be especially relevant, since this synthesis process may occur on planetary surfaces (Strazzulla et al. 1984). Its colors are less red than (5145) 1992 AD but redder than D asteroids. Its colors also show some leveling off in the JHK region. The albedo and color of the residue depend on the
153
combination of total radiation dosage and initial carbon content. Some combination of radiation dosage and initial carbon might produce (5145) 1992 AD's color. Our other samples (labeled "other organics" in Fig. 2) show less red colors than (5145) 1992 AD, ranging from neutral to slightly redder than our reddest D asteroids. These comparisons show that organics, unlike other plausible materials, bracket the colors of (5145) 1992 AD. This suggests that (5145) 1992 AD's unique colors may arise from a concentration of solid organic substances of purity greater than that found on the surfaces of most outer Solar System bodies; contamination by very dark, neutral-colored carbon materials may be the rule on most bodies. The coloring agents may be most similar to the tholins of Khare et al., or the irradiated methane ice residue of Strazzulla et al. A similar conclusion was reached independently by Fink et al. (1992), who stressed the possible connection with tholins. (Their paper reached us after we completed our draft of this work.) The possible exposure of relatively pure organics makes it extremely important to acquire infrared spectra of (5145) 1992 AD; our laboratory spectra of the organic substances generally show strong features in the infrared, contrary to the spectra of most CPD asteroids. Therefore, the infrared spectra of (5145) 1992 AD may turn out to be more diagnostic than is the case for the relatively featureless CPD asteroids observed thus far. The properties of (5145) 1992 AD call for a vigorous search for more Chiron-like objects, and further efforts to clarify their spectral properties and relations to more "traditional" comets and asteroids. Infrared colors and spectra are especially needed.
ACKNOWLEDGMENTS We are grateful for the helpful comments from D. L. Rabinowitz, M. Sykes, J. F. Bell, PSI staff, and other colleagues. BEAM acknowledges support through NASA Planetary Astronomy Postdoctoral Award. DJT was supported by NASA Grant NAGW 3044, and WKH by NASA Planetary Astronomy Program Contract NASW-4573. The National Optical Astronomy Observatories are operated by AURA Inc. under cooperative agreement with the National Science Foundation. Planetary Science Institute is a nonprofit division of Science Applications International Corp. This is PSI Contribution 296.
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