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Observations of the Saturn E ring and a new satellite
ICARUS 47, 288-290 (1981)
NOTES Observations of the Saturn E Ring and a New Satellite S. M. LARSON,
J. W.
FOUNTAIN,
B. A. SMITH,
A N D H . J. R E I T S E M A
Lunar and Planetary Laboratory, University o f Arizona, Tacson, Arizona 85721 Received May 22. 1981; revised June 29, 1981 The faint E ring o f Saturn a p p e a r s as a narrow ring 246,000 ± 4,000 km from the center of Saturn on p h o t o g r a p h s taken w h e n the ring-plane inclination w a s 50.4. The apparent brightness o f the ring was uniform at all o b s e r v e d orbital longitudes and permits an estimate of the normal optical thickness. A faint satellite (1981S 1) was observed near the L4 triangular libration point o f T e t h y s and is probably the same object as 1980S13. Until recently, g r o u n d - b a s e d observations o f the faint E ring o f Saturn have been made only w h e n the Earth was located close to the ring plane, in 1966 and 1979-1980. U n d e r s u c h conditions, the E-ring visibility is e n h a n c e d b e c a u s e , in their edgewise presentation, the bright rings contribute relatively little scattered light, and the E-ring scatterers are concentrated into a line. The first evidence for the existence of the E ring in 1966 was a single, marginal image by Feibelman (1967). In 1979-1980, efforts were made at a n u m b e r o f observatories to confirm the E ring by applying improved observing techniques and new detectors; the result o f these efforts was a large n u m b e r o f positive observations. A major reduction in instrumental diffraction and scattering w a s accomplished by the use of a focal plane c o r o n a g r a p h with occulting and apodizing m a s k s (Larson and Reitsema, 1979). The 1980 data show the E ring extending at least to 480,000 km (Brahic et al., 1980; Baum et al., 1980; Dollfus and Brunier, 1980; L a m y and Mauron, 1980; Reitsema et al.. 1980) with an apparent intensity m a x i m u m near the orbit of E n c e l a d u s ( B a u m et al., 1980; L a m y and Mauron, 1980; R e i t s e m a et al., 1980). A unique determination o f the position of m a x i m u m density of ring particles from the ground-based 1980 data is difficult b e c a u s e of the viewing geometry. The region of the intensity m a x i m u m is seen in a Voyager 1 image as a broad diffuse band b e t w e e n 210,000 and 300,000 k m from the c e n t e r o f Saturn (Smith et al.. 1981). Photographic images o f the E ring were obtained on the nights o f April 1 (Fig. i) and April 2, 1981, with a focal-plane c o r o n a g r a p h operating at f / 6 on the 154-cm Catalina O b s e r v a t o r y reflector. At that time, the Earth was 5°.4 above the ring plane and the phase angle was ~ 7 . Further o b s e r v a t i o n s were obtained on May 2 and May 3, 1981 w h e n the ring plane was inclined 40.5 and the phase angle was 30.9. The E ring was recorded on all images. T h e detector was Kodak 2415 Technical
Pan film, hypersensitized in 8% hydrogen-forming gas and processed to g a m m a of 1.7 in Kodak HC-110 Developer (dilution D) for 12 min at 20°C. Ten-minute e x p o s u r e s without filter brought the scattered-light background density to 0.6 or more above base plus fog. Background stars as faint as m = 19 were recorded on images guided for S a t u r n ' s apparent motion. The 2415 film has a panchromatic response with relatively low green sensitivity, making conversion o f the unfiltered image to a standard p a s s b a n d uncertain. The April images were digitized on the KNPO PDS microdensitometer and converted to intensity using calibration steps e x p o s e d on the film. Positions were m e a s u r e d relative to the bright Saturn satellites. All of the images show a narrow E ring as indistinguishable from an unresolved line source: the observed full width at half m a x i m u m o f 8000 km is the same as that o f faint field stars trailed by the motion of Saturn during the e x p o s u r e . M e a s u r e m e n t o f both ansae o f the E ring gives a distance o f 246,000 _+ 4000 km from the center of Saturn. T h u s , the brightest region o f the E ring is found to lie 8000 ± 4000 km outside o f the orbit of Enceladus. It is probable that this feature is associated with the brightness m a x i m u m seen in the 1980 data. There appears to be some indication o f light inside the bright core of the ring in these images, but photographic effects cannot be ruled out. This narrow feature is the only E-ring structure visible in the 1981 data, although there is an indication in our 1980 data o f a local intensity m i n i m u m at 253,000 km, outside the bright core and near the 3:1 R h e a resonance. No other discrete features are seen in the 1980 data. The ring brightness appears to be uniform at all o b s e r v e d orbital longitudes, but the film graininess affects its visibility. More observations will be needed to search for nonuniformities that might have been responsible for the temporal variations reported by
288 0019-1035/81/080288- 03502.00/0 Copyright © 1981by AcademicPress, Inc. All rights of reproduction in any form reserved.
NOTES
289
6:37UT
O 7:26 UT
star O ~ ~'E
E RING
~ , -
~ "" ~ . . . . D I O N E B
O,____c:7=_ O~"o----.~---- "~ . . . .
TETH~O .o
1981Sl ~ ' - . ~ / ~
T~...~.aJ _j~..j-
star
RHEA
0
0
W
ENCELADUS DIONE
FIG. 1. Coronographic photographs of the E ring and 1981S1 on April ], 1981. The objects are identified in the schematic. 1980S6 (popularly k n o w n as Dione B) w a s found to lead Dione by 48°, in good a g r e e m e n t with predictions (Reitsema, 1981a).
Dollfus and Brunier (1980). Using the brighter satellites as photometric standards a n d subtracting the light scattered by Saturn and its bright rings, we obtain a brightness o f 19.1 m a g n i t u d e s per linear arcsec. This is equivalent to a uniform ring o f conservative isotropically scattering particles having a width o f 8000 k m and normal optical thickness r = 1.5 × 10-6. Further observations are needed to s t u d y possible temporal variations and to determine additional scattering properties o f the ring particles. With the application o f coronagraphic observing techniques it should be possible to a c c o m p l i s h this goal. We are currently modeling the o b s e r v e d brightness profile in an effort to determine the radial density distribution. In addition to the E ring, a probable satellite o f
Saturn, 1981S1 (Smith, 1981) was recorded on five images taken b e t w e e n 06:30 and 07:30 U T on April 1. This object appeared to be either in or j u s t outside the E ring and w a s o b s e r v e d to m o v e toward the planet until it was lost in the glare o f the rings. Its motion over 60 min w a s indistinguishable from that o f either E n c e l a d u s or T e t h y s . The short arc over which 1981S 1 was o b s e r v e d w a s not sufficient to derive a period. P h o t o g r a p h s obtained on May 2 and 3 s h o w e d the s a m e object near w e s t e r n and e a s t e r n elongation, respectively. The m e a s u r e d elongation distance placed it near the orbit o f T e t h y s . The interval b e t w e e n the calculated greatest e a s t e r n elongation on April 1 and m e a s u r e d e a s t e r n elongation on May 3 was 32.093 days; an a s s u m e d total o f 17 revolutions gave a period
290
L A R S O N ET AL.
indistinguishable from that of Tethys. The average longitude of the satellite derived from the April 1 data was 5~5 (+_~8) preceding Tethys. It is probable that this satellite is the same as 1980S13 (Reitsema, 1980), which preceded Tethys by 6~4 on April 8, 1980. Harrington et al. (1981) first suggested that some observations in 1980 could be explained by satellites or clouds near the Tethys L4 and L5 triangular libration points. With the observations by Veillet (1981), Tethys is the first satellite known to have both L~ and L~ regions occupied by sizable objects (Reitsema, 1981b). After subtracting the light of the E ring, we obtain a magnitude of 18.5 (+_0.5) for 1981S1; this corresponds to a diameter of 30 km for a geometric albedo of 0.6. similar to that of Mimas, Dione, and Rhea (Smith et al., 1981).
AC KNOWLE DGMENTS We are indebted to E. Carder for assistance with the KPNO PDS microdensitometer, T. Duxbury and R. Doyle of JPL for providing accurate satellite positions and J. Gotobed for programming support. This work is supported by NASA Grant NGL 03-002-002.
RE FERENCES BAUM, W. A., KREIDL, T., WESTPHAL, J. A., DANIELSON, G. E., SEIDELMANN, P. K., PASCU, D., AND CURRIE, D. G. (1980). Profile of Saturn's E ring. Bull. A mer. Astron. Soe. 12, 702. (Abstract) BRAHIC, A., LECACHEAUX, J., AND SICARDY, B. (1980). Observations of Saturn's edgewise rings in France. Bull. A mer. Astron. Soc. 12, 700.
DOLLFUS, A., AND BRUNIER, S. (1980). Le coronagraphic focal et l'observation d'un nouvel anneau de Saturne. C. R. A c a d . Sei Paris Se t . B 290, 261-263. FEIBELMAN, W. A. (1967). Concerning the " D " ring of Saturn. N at ur e 214, 793-794. HARRINGTON, R. S., PASCU, O., AND SEIDELMANN, P. K. (1981). 1980SI3 - 1980S24 = 1980S25. IAU Circular No. 3583. LAMY, P. L., AND MAURON, N. (1980). The new satellite Dione B and outer ring of Saturn. Bull. Amer. Astron. Soc. 12, 728. LARSON, S. M., AND REITSEMA, H. J. (1979). A planetary coronagrph. Bull. Am e r . Astron. Soc. 11, 558. (Abstract) REITSEMA, H. J. (1980). Satellites ~f" Saturn. IAU Circular No. 3466. REITSEMA, H. J., SMITH, B. A., AND LARSON, S. M. (1980). The E ring of Saturn. Bull. Am e r . Astron. Soc. 12, 701. REITSEMA, H. J. (1981a). 1980S6. IAU Circular No. 3572. REITSEMA, H. J. (1981b). Orbits of the Tethys Lagrangian bodies, in preparation. SMITH, B. A. (1981). Satellites o f S a t a r n . 1AU Circular No. 3593. SMITH, B. A., SODERBLOM, L., BEEBE, R., BOYCE, J., BRIGGS, G., BUNKER,A., COLLINS, S. A., HANSEN, C. J., JOHNSON, T. V., MITCHELL, J. L., TERRILE, R. J., CARR, M., COOK, A. F., CUZZl, J., POLLACK, J. B., DANIELSON, G. E., INGERSOLL, A., DAVIES, M. E., HUNT, G. E., MASURSKY, H., SHOEMAKER, E., MORRISON, D., OWEN, T., SAGAN, C.. VEVERKA,J., STROM, R., AND SUOMI, V. E. (1981). Encounter with Saturn: Voyager I imaging science results. Science 212, 182-184. VEILLET, C. (1981). Satellites o f Saturn. IAU Circular No. 3593.
NOTES Observations of the Saturn E Ring and a New Satellite S. M. LARSON,
J. W.
FOUNTAIN,
B. A. SMITH,
A N D H . J. R E I T S E M A
Lunar and Planetary Laboratory, University o f Arizona, Tacson, Arizona 85721 Received May 22. 1981; revised June 29, 1981 The faint E ring o f Saturn a p p e a r s as a narrow ring 246,000 ± 4,000 km from the center of Saturn on p h o t o g r a p h s taken w h e n the ring-plane inclination w a s 50.4. The apparent brightness o f the ring was uniform at all o b s e r v e d orbital longitudes and permits an estimate of the normal optical thickness. A faint satellite (1981S 1) was observed near the L4 triangular libration point o f T e t h y s and is probably the same object as 1980S13. Until recently, g r o u n d - b a s e d observations o f the faint E ring o f Saturn have been made only w h e n the Earth was located close to the ring plane, in 1966 and 1979-1980. U n d e r s u c h conditions, the E-ring visibility is e n h a n c e d b e c a u s e , in their edgewise presentation, the bright rings contribute relatively little scattered light, and the E-ring scatterers are concentrated into a line. The first evidence for the existence of the E ring in 1966 was a single, marginal image by Feibelman (1967). In 1979-1980, efforts were made at a n u m b e r o f observatories to confirm the E ring by applying improved observing techniques and new detectors; the result o f these efforts was a large n u m b e r o f positive observations. A major reduction in instrumental diffraction and scattering w a s accomplished by the use of a focal plane c o r o n a g r a p h with occulting and apodizing m a s k s (Larson and Reitsema, 1979). The 1980 data show the E ring extending at least to 480,000 km (Brahic et al., 1980; Baum et al., 1980; Dollfus and Brunier, 1980; L a m y and Mauron, 1980; Reitsema et al.. 1980) with an apparent intensity m a x i m u m near the orbit of E n c e l a d u s ( B a u m et al., 1980; L a m y and Mauron, 1980; R e i t s e m a et al., 1980). A unique determination o f the position of m a x i m u m density of ring particles from the ground-based 1980 data is difficult b e c a u s e of the viewing geometry. The region of the intensity m a x i m u m is seen in a Voyager 1 image as a broad diffuse band b e t w e e n 210,000 and 300,000 k m from the c e n t e r o f Saturn (Smith et al.. 1981). Photographic images o f the E ring were obtained on the nights o f April 1 (Fig. i) and April 2, 1981, with a focal-plane c o r o n a g r a p h operating at f / 6 on the 154-cm Catalina O b s e r v a t o r y reflector. At that time, the Earth was 5°.4 above the ring plane and the phase angle was ~ 7 . Further o b s e r v a t i o n s were obtained on May 2 and May 3, 1981 w h e n the ring plane was inclined 40.5 and the phase angle was 30.9. The E ring was recorded on all images. T h e detector was Kodak 2415 Technical
Pan film, hypersensitized in 8% hydrogen-forming gas and processed to g a m m a of 1.7 in Kodak HC-110 Developer (dilution D) for 12 min at 20°C. Ten-minute e x p o s u r e s without filter brought the scattered-light background density to 0.6 or more above base plus fog. Background stars as faint as m = 19 were recorded on images guided for S a t u r n ' s apparent motion. The 2415 film has a panchromatic response with relatively low green sensitivity, making conversion o f the unfiltered image to a standard p a s s b a n d uncertain. The April images were digitized on the KNPO PDS microdensitometer and converted to intensity using calibration steps e x p o s e d on the film. Positions were m e a s u r e d relative to the bright Saturn satellites. All of the images show a narrow E ring as indistinguishable from an unresolved line source: the observed full width at half m a x i m u m o f 8000 km is the same as that o f faint field stars trailed by the motion of Saturn during the e x p o s u r e . M e a s u r e m e n t o f both ansae o f the E ring gives a distance o f 246,000 _+ 4000 km from the center of Saturn. T h u s , the brightest region o f the E ring is found to lie 8000 ± 4000 km outside o f the orbit of Enceladus. It is probable that this feature is associated with the brightness m a x i m u m seen in the 1980 data. There appears to be some indication o f light inside the bright core of the ring in these images, but photographic effects cannot be ruled out. This narrow feature is the only E-ring structure visible in the 1981 data, although there is an indication in our 1980 data o f a local intensity m i n i m u m at 253,000 km, outside the bright core and near the 3:1 R h e a resonance. No other discrete features are seen in the 1980 data. The ring brightness appears to be uniform at all o b s e r v e d orbital longitudes, but the film graininess affects its visibility. More observations will be needed to search for nonuniformities that might have been responsible for the temporal variations reported by
288 0019-1035/81/080288- 03502.00/0 Copyright © 1981by AcademicPress, Inc. All rights of reproduction in any form reserved.
NOTES
289
6:37UT
O 7:26 UT
star O ~ ~'E
E RING
~ , -
~ "" ~ . . . . D I O N E B
O,____c:7=_ O~"o----.~---- "~ . . . .
TETH~O .o
1981Sl ~ ' - . ~ / ~
T~...~.aJ _j~..j-
star
RHEA
0
0
W
ENCELADUS DIONE
FIG. 1. Coronographic photographs of the E ring and 1981S1 on April ], 1981. The objects are identified in the schematic. 1980S6 (popularly k n o w n as Dione B) w a s found to lead Dione by 48°, in good a g r e e m e n t with predictions (Reitsema, 1981a).
Dollfus and Brunier (1980). Using the brighter satellites as photometric standards a n d subtracting the light scattered by Saturn and its bright rings, we obtain a brightness o f 19.1 m a g n i t u d e s per linear arcsec. This is equivalent to a uniform ring o f conservative isotropically scattering particles having a width o f 8000 k m and normal optical thickness r = 1.5 × 10-6. Further observations are needed to s t u d y possible temporal variations and to determine additional scattering properties o f the ring particles. With the application o f coronagraphic observing techniques it should be possible to a c c o m p l i s h this goal. We are currently modeling the o b s e r v e d brightness profile in an effort to determine the radial density distribution. In addition to the E ring, a probable satellite o f
Saturn, 1981S1 (Smith, 1981) was recorded on five images taken b e t w e e n 06:30 and 07:30 U T on April 1. This object appeared to be either in or j u s t outside the E ring and w a s o b s e r v e d to m o v e toward the planet until it was lost in the glare o f the rings. Its motion over 60 min w a s indistinguishable from that o f either E n c e l a d u s or T e t h y s . The short arc over which 1981S 1 was o b s e r v e d w a s not sufficient to derive a period. P h o t o g r a p h s obtained on May 2 and 3 s h o w e d the s a m e object near w e s t e r n and e a s t e r n elongation, respectively. The m e a s u r e d elongation distance placed it near the orbit o f T e t h y s . The interval b e t w e e n the calculated greatest e a s t e r n elongation on April 1 and m e a s u r e d e a s t e r n elongation on May 3 was 32.093 days; an a s s u m e d total o f 17 revolutions gave a period
290
L A R S O N ET AL.
indistinguishable from that of Tethys. The average longitude of the satellite derived from the April 1 data was 5~5 (+_~8) preceding Tethys. It is probable that this satellite is the same as 1980S13 (Reitsema, 1980), which preceded Tethys by 6~4 on April 8, 1980. Harrington et al. (1981) first suggested that some observations in 1980 could be explained by satellites or clouds near the Tethys L4 and L5 triangular libration points. With the observations by Veillet (1981), Tethys is the first satellite known to have both L~ and L~ regions occupied by sizable objects (Reitsema, 1981b). After subtracting the light of the E ring, we obtain a magnitude of 18.5 (+_0.5) for 1981S1; this corresponds to a diameter of 30 km for a geometric albedo of 0.6. similar to that of Mimas, Dione, and Rhea (Smith et al., 1981).
AC KNOWLE DGMENTS We are indebted to E. Carder for assistance with the KPNO PDS microdensitometer, T. Duxbury and R. Doyle of JPL for providing accurate satellite positions and J. Gotobed for programming support. This work is supported by NASA Grant NGL 03-002-002.
RE FERENCES BAUM, W. A., KREIDL, T., WESTPHAL, J. A., DANIELSON, G. E., SEIDELMANN, P. K., PASCU, D., AND CURRIE, D. G. (1980). Profile of Saturn's E ring. Bull. A mer. Astron. Soe. 12, 702. (Abstract) BRAHIC, A., LECACHEAUX, J., AND SICARDY, B. (1980). Observations of Saturn's edgewise rings in France. Bull. A mer. Astron. Soc. 12, 700.
DOLLFUS, A., AND BRUNIER, S. (1980). Le coronagraphic focal et l'observation d'un nouvel anneau de Saturne. C. R. A c a d . Sei Paris Se t . B 290, 261-263. FEIBELMAN, W. A. (1967). Concerning the " D " ring of Saturn. N at ur e 214, 793-794. HARRINGTON, R. S., PASCU, O., AND SEIDELMANN, P. K. (1981). 1980SI3 - 1980S24 = 1980S25. IAU Circular No. 3583. LAMY, P. L., AND MAURON, N. (1980). The new satellite Dione B and outer ring of Saturn. Bull. Amer. Astron. Soc. 12, 728. LARSON, S. M., AND REITSEMA, H. J. (1979). A planetary coronagrph. Bull. Am e r . Astron. Soc. 11, 558. (Abstract) REITSEMA, H. J. (1980). Satellites ~f" Saturn. IAU Circular No. 3466. REITSEMA, H. J., SMITH, B. A., AND LARSON, S. M. (1980). The E ring of Saturn. Bull. Am e r . Astron. Soc. 12, 701. REITSEMA, H. J. (1981a). 1980S6. IAU Circular No. 3572. REITSEMA, H. J. (1981b). Orbits of the Tethys Lagrangian bodies, in preparation. SMITH, B. A. (1981). Satellites o f S a t a r n . 1AU Circular No. 3593. SMITH, B. A., SODERBLOM, L., BEEBE, R., BOYCE, J., BRIGGS, G., BUNKER,A., COLLINS, S. A., HANSEN, C. J., JOHNSON, T. V., MITCHELL, J. L., TERRILE, R. J., CARR, M., COOK, A. F., CUZZl, J., POLLACK, J. B., DANIELSON, G. E., INGERSOLL, A., DAVIES, M. E., HUNT, G. E., MASURSKY, H., SHOEMAKER, E., MORRISON, D., OWEN, T., SAGAN, C.. VEVERKA,J., STROM, R., AND SUOMI, V. E. (1981). Encounter with Saturn: Voyager I imaging science results. Science 212, 182-184. VEILLET, C. (1981). Satellites o f Saturn. IAU Circular No. 3593.