An Atmosphere on Ganymede from Its Occultation of SAO 186800 on 7 June 1972 Author(s): R. W. Carlson, J. C. Bhattacharyya, B. A. Smith, T. V. Johnson, B. Hidayat, S. A. Smith, G. E. Taylor, B. O'Leary and R. T. Brinkmann Source: Science, New Series, Vol. 182, No. 4107 (Oct. 5, 1973), pp. 53-55 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/1736235 . Accessed: 14/11/2014 12:17 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org This content downloaded from 131.215.225.130 on Fri, 14 Nov 2014 12:17:50 PM All use subject to JSTOR Terms and Conditions asteroid belt at 8 X 108 This 3. J. R. Arnold, Astrophys. J. 141, 1536, 1548 relative position of the two bodies was g/year. (1965). result is similar ,to the observational 4. E. J. Opik, Advan. Astron. Astrophys. 4, then estimated as 0.3". The adopted of 109 obtained 302 (1966). diameter for was estimate g/year by 5. G. W. Wetherill, in Physical Studies of the prediction purposes Hawkins (16) as well as that obtained Minor Planets, T. Gehrels, Ed. (NASA SP- 5550 ? 130 km (2). 267, National Aeronautics and Space Admin- Successful by Latham et al. (17) from lunar seis- istration, Washington, D.C., 1971), p. 447. observations of the oc- mic data, although the flux obtained by 6. R. E. McCrosky, Smithson. Astrophys. Obs. cultation were made by Hidayat, Carl- Spec. Rep. 252 (1967); Eos Trans. Amer. the Prairie Network (5 X 1010 g/year) Geophys. Union 53, 724 (1972). son, and Johnson at the Bosscha Ob- from all sources, including fireballs 7. D. E. Gault, Meteoritics 4, 177 (1969); J. S. servatory near Lembang, Java, and by Dohnanyi, J. Geophys. Res. 75, 3468 (1970); (18), is higher. Including uncertainties G. W. Wetherill, ibid. 72, 2429 (1967). Bhattacharyya at the Kavalur field in there may be an uncer- S. G. W. Wetherill, Science 159, 79 (1968). station of the Kodaikanal Observatory (dMe/dt), 9. I. S. Astopovich, Astron. Zh. 16, 15 (1939); tainty of an order of magnitude in our F. L. Whipple and R. F. Hughes, in Meteors, in India. B. A. Smith and S. A. Smith, T. R. Kaiser, Ed. (Pergamon, London, 1955), with a calculations; however, the resonant ex- p. 149. observing portable photometer traction mechanism we propose should 10. Y. Kozai, Astron. J. 67, 591 (1962); A. J. and 20-cm telescope from Darwin, Smith, Jr., NASA Rep. TR R-194 (1964). result in a significant meteorite flux at 11. J. G. Williams, thesis, University of Cali- Australia, were just slightly (50 km; Earth. fornia, Los Angeles (1969). see Fig. 1) south of the ,actualocculta- 12. C. J. van Houten, I. van Houten-Groeneveld, Spectrophotometric study (19) of P. Herget, T. Gehrels, Astron. Astrophys. tion zone and obtained negative results. asteroids such as (511) Davida, (814) Suppl. 2, 339 (1970). The sky was of excellent photometric 13. B. G. Marsden, Astron. J. 75, 206 (1970). Tauris, (31) Euphrosyne, (175) 14. G. W. Wetherill and J. G. Williams, J. quality with scintillation noise below Andromache, and Hecuba Geophys. Res. 73, 635 (1968). average and all functioned (108) lying 15. D. E. Gault, E. M. Shoemaker, H. J. Moore, equipment near the boundary of the Kirkwood NASA Tech. Note D-1767 (1963). properly. An attempt was also made by 16. G. S. Hawkins, Astron. J. 65, 318 (1960). Gap could permit their identification 17. G. V. Latham, M. Ewing, F. Press, G. S. D. Sinvhal at the Uttar Pradesh Ob- with known classes of meteorites. Sutton, J. Dorman, Y. Nakamura, N. Toksoz, servatory in Naini Tal, India, with D. Lammlein, F. Duennebier, in Apollo 16 Another asteroidalresonance extraction Preliminary Science Report (NASA SP-315, photoelectric equipment but the fluctu- mechanism, utilizing "secular" reson- National Aeronautics and Space Administra- ations in sky transparency were too tion, Washington, D.C., 1972), pp. 9-19. ances rather than commensurabilities, 18. R. E. McCrosky, Smithson. Astrophys, Obs. large for any events to be detected. has recently been proposed by Williams Spec. Rep. 280 (1968). The photoelectric observations of 19. C. R. Chapman, thesis, Massachusetts Institute (20) and may be of comparable im- of Technology (1972); T. B. McCord, J. B. the event from Lembang (107.6?E, Adams, T. V. Johnson, Science 168, 1445 6.8?S, 1300 m above sea were portance. (1970). level) PETER D. ZIMMERMAN* 20. J. G. Williams, Eos Trans. Amer. Geophys. made with the Bosscha twin 60-cm Union 54, 233 (1973), Paper No. G46. G. W. WETHERILL 21. We thank Dr. J. G. Williams for his con- refracting telescopes mounted in ithe Department of Planetary and Space tinuing assistance and especially for providing same tube. The sky was clear and of several computer subroutines and his un- Science, University of California, published data. We also thank J. G. Higdon excellent photometric quality. Measure- Los Angeles 90024 for writing some computer programs used in ments were obtainedwith a two-channel this work. Research supported by NASA grant NGL-05-007-005. photometer, one channel for the red References and Notes * Present address: National Accelerator Lab- [wavelength (X) ; 6000 A] and the Post Office Box 1. .E. Advan. Astron. oratory, 500, Batavia, Illinois pik, Astrophys. 2, 219 60510. other for the blue (Xk 4500 A), with (1963). 2. E. Anders, Space Sci. Rev. 3, 583 (1964). 18 June 1973 m cooled photomultipliersand pulse-count- ing electronics. Only the data from the red channel, which were of higher quality, are reportedhere. The accumu- An Atmosphereon Ganymedefrom Its Occultation lated counts (at a samplingrate of 22.25 sec- ) were displayed on a visual read- of SAO 186800 on 7 June 1972 out and recorded with a 16-mm cine camera operated at a framing rate of Abstract. On 7 June 1972 the third Jovian satellite Ganymede occulted the about 24 sec-1. Also photographedwas eighth-magnitude star SAO 186800. -Successful photoelectric observations ob- the display from an accurate quartz tained at Lembang, Java (Indonesia), and Kavalur, India, show nonabrupt im- oscillator referenced before and after mersions and emersions, indicating the presence of an atmosphere whose surface the event to radio station WWV by pressure is greater than about 10-3 millibar. By fitting the two occultation dura- using an observatorychronometer. The tions as chords to a model disk, the diameter is found to be 5270 (+ 30, - 200) absolute accuracy of the timing is esti- kilometers, the major error contribution arising from the uncertain atmospheric mated to be ? 1 second; the relative thickness below the occultation layer. The derived mean density is 2.0 (- 0.03, + accuracy during the event was + 0.1 0.2) grams per cubic centimeter. second. Dead-time corrections of ap- proximately 25 percent have been ap- A search for occultation'sof stars by predictions, photographs taken in plied to the data. Direct photographsof planets carried out by Taylor at the March 1972 at the observatoriesat the the event were also taken and have been Royal Greenwich Observatory indi- Cape of Good Hope, South Africa, published elsewhere (3). The photoelec- cated that the star SAO 186800 (mag- and Perth, Australia, were analyzed to tric tracing obtained at Lembang is nitude 8.0, type KO) would be occulted yield more accurate relative positions shown in Fig. 2. The immersion and by Ganymede (JIII) on 7 June 1972 of Ganymede and the star. The pre- emersion midpoint times are 18h47m- (1). The predicted intensity drop was dictions were accordingly refined. The 40.9+ ? 1.2s and 18h50m22.4s 1.2s in about 5 percent at visible wavelengths. predicted area of visibility included corrected universal time (U.T.C.). Since the stellar position was the southern Asia, northern Australia, and The observationsat Kavalur (78.7?E, largest single source of error in the eastern Africa. The uncertainty in the 12.6?N, 800 m above sea level) were 5 OCTOBER 1973 53 This content downloaded from 131.215.225.130 on Fri, 14 Nov 2014 12:17:50 PM All use subject to JSTOR Terms and Conditions made at the Cassegrainfocus of a 1-m N Scale to guiding errors occurred primarily in telescope with an 8-arc-second aper- - the postemersion portion of the trace E-- North Pole?, ture. The photometer signal, recorded and have been suppressed. The Lem- through a Wratten 89B filter by a bang data also show an irregularmodu- cooled RCA 7102 photomultiplier,was lation of several seconds which is due amplified by a GR 1230A electrometer ' / to scattered light from nearby Jupiter. and displayedon one of the four traces f - Kavalur, India This did not interfere with identifica- of a Tektronix 533A oscilloscope with P + tion of the event. In addition there is a a four trace plug-in module in the gradual decrease in overall intensity chopped display mode and a sweep rate during the period of observationdue to Lembang,Java of 2 cm/sec. The other three traces Ganymede's motion away from Jupiter displayed 0.1-second pips from a quartz during the course of the observations.