August 3, 1993

Dear Colleague:

As you undoubtedly know, the next edge-on presentation of Saturn’s ring plane will occur in 1995/96. This apparition will be a fairly favorable one, with the solar crossing and one of three earth crossings occuring within two months of opposition in mid-September 1995. The Sun will cross the ring plane on 19 November 1995, and the Earth on 21 May and 11 August 1995, and again on 11 February 1996 (see Fig. 19 in Showalter et al. [1991] Icarus 94, 451).

As in 1980, these events will offer unusual opportunities for observations of the rings and inner satellites, as well as planetary eclipses and mutual occultations and eclipses by the larger satellites. In order to stimulate discussion of observational strategies and plans, we plan to organize a short workshop on this subject at the DPS meeting in Boulder, Colorado, during October 1993. The workshop will be held on Wednesday, October 20, from 2:00 - 4:00 pm, and will be listed in the third DPS mailing. It is intended that a more formal workshop will be organized under the auspices of the Planetary Data System Ring Node sometime in 1994, in Tucson.

A brief list of the kind of observations that may be made at this time follows, though it is undoubtedly not exhaustive. We have also included some references to reports of similar observations in 1966 and 1980, and would be pleased to hear of additional references on this subject, or of other types of observations being planned.

  • (1) Observations of the tenuous E ring will best be made very close to the moments of the earth’s crossing, especially on 11 August 1995, as well as during the period when the sun and earth are on opposite sides of the ring plane (see below). During the latter periods the scattered light from the main rings will be greatly suppressed. Most of our knowledge of the E ring’s vertical and radial structure comes from the analysis of CCD observations obtained by Baum et al. in 1980. It may even be possible to detect the fainter G ring.

    • Feibelmann [1967] Nature 214, 793.
    • Feibelmann & Klinglesmith [1980] Science 209, 277.
    • Baum et al. [1981] Icarus 47, 84.
    • Larson et al. [1982] Icarus 47, 288.
    • Dollfus & Brunier [1982] Icarus 49, 194.
    • Showalter et al. [1991] Icarus 94, 451 (E ring).
    • Showalter et al. [1993] Icarus 103, 124 (G ring).
  • (2) Observations of the photometric thickness of the rings, as seen edge-on, and of the precise time of the earth’s crossing of the ring plane will also best be made at the 11 Aug. 1995 crossing, although it may be possible to get data for 21 May 1995 and/or 11 Feb. 1996 also. It is now known that the thickness is probably due to bending waves and embedded satellites, or even the projected E ring, but it is definitely of interest to determine the time of ring plane crossing as accurately as possible. Recent stellar occultations have led to a tentative detection of the precession of the ring plane due to solar torques (French et al. 1993), and the observed rate implies an advance of ~2 hours in the time of ring plane crossing after a 15 yr interval (see Nicholson & French, 1993 DPS Abstract). Accurate timing of the crossing(s) could conceivably result in a confirmation of this precession rate, or at least provide an essential zero-point for future studies. Dollfus [1979] determined the time of the 18 Dec. 1966 earth crossing to +/-2 hrs, but I have been unable to find such an absolute determination from the 1980 events.

    • Focas & Dollfus [1969] A & A 2, 251.
    • Dollfus [1979] A & A 75, 204.
    • Fountain & Larson [1978] Icarus 36, 92.
    • Brahic & Sicardy [1981] Nature 289, 447
    • Sicardy et al. [1982] A & A 108, 296.
    • French et al. [1993] Icarus 103, 163.
  • (3) Observations of the ‘dark side’ of the main rings, and of the C ring, are possible during two extended periods when the sun and earth are on opposite sides of the ring plane: 21 May - 11 August 1995, and 19 November 1995 - 11 February 1996. During the latter period, the earth will be up to 2.5 degrees above the ring plane while the sun is in or below the plane, offering an excellent opportunity to observe light transmitted through the C ring and the Cassini Division.

    • Barnard [1910] A.J. 610, 79.
    • Focas & Dollfus [1969] A & A 2, 251.
  • (4) During the 1993 - 1996 period there will be a very large number of eclipses and occultations of the satellites by Saturn (and some by the rings), as well as an extensive set of mutual satellite occultations and eclipses in 1995. A listing of predicted eclipses and mutual events has been submitted by Arlot & Thuillot to _Icarus_. A&T; give data for 163 selected eclipses by Dione, Rhea, Titan, Hyperion and Iapetus. Apart from the two Iapetus eclipses by Saturn in May and July this year (Soma 1992), the eclipses commence in October 1994 and continue through to August 1996, but are most readily observed well away from opposition or conjunction. The mutual events, which occur only near the passages of the earth through the ring plane, last from January 1995 to September 1996, with the highest frequency in November 1995 - January 1996.

    There are a handful of early visual eclipse and mutual event observations, but the only published photoelectric observations of Saturnian satellite mutual events are by Aksnes _et al._ [1984], who report times for a total of 14 events involving Enceladus, Tethys, Dione and Rhea (S2 - S5). They report post-fit residuals of 80 km both in-orbit and out-of-plane, corresponding to approx. 0.01" in astrometric accuracy. These data, though few in number, are by the most accurate of all the earth-based observations of the Saturnian satellites used by Harper & Taylor [1993] in their recent orbital fits, and are probably comparable in accuracy to Voyager astrometry. A larger set of such mutual event observations in 1995 should lead to significant improvements in the orbits for all the inner satellites (S1 - S6), and may help, with the 1980 data, to establish better the amplitudes and frequencies of the resonant librations involving S1 - S4. Such observations may also permit mapping of albedo patterns on the satellites, much as has been done recently for Pluto and Charon, and studies of aerosols on Saturn and Titan (Smith _et al._ [1981], Combes _et al._ [1981]).

    • Aitken [1909] Lick Obs. Bull. 172, 169.
    • Comrie [1921] JBAA 31, 271.
    • Innes [1921] Circ. Union Obs. Sth Africa 53, 112.
    • Reitsema [1978] Nature 272, 601.
    • Combes et al. [1981] Icarus 47, 291.
    • Smith et al. [1981] Icarus 46, 424.
    • Dourneau [1982] A & A 112, 73.
    • Aksnes et al. [1984] A.J. 89, 280.
    • Soma [1992] A & A 265, L21.
    • Harper & Taylor [1993] A & A 268, 326.
  • (5) The small inner satellites, Prometheus, Pandora, Janus and Epimetheus, were all discovered during the ring plane passages of 1966 and 1980, supported by the Voyager encounters in 1980 and 1981. The only observations of any of these bodies acquired since then, to our knowledge, are near-IR observations of the coorbital satellites in July and August 1990 (Nicholson et al. 1992). The ring plane crossing will provide additional opportunities to observe all four objects near elongation, enhanced by our present capability to predict their appearances with reasonable precision. These data, especially for the F ring shepherds, may provide very important links between the Voyager observations in 1980/81 and Cassini observations to be made in 2004-08, from which it is hoped to measure the secular deceleration of Prometheus due to ring torques.

    • Dollfus [1967] C. R. Acad. Sci. Paris B 264, 822.
    • Fountain & Larson [1977] Science 197, 915.
    • Aksnes & Franklin [1978] Icarus 36, 107.
    • Fountain & Larson [1978] Icarus 36, 92
    • Larson et al. [1981] Icarus 46, 175.
    • Seidelmann et al. [1981] Icarus 47, 282.
    • Synnott et al. [1981] Science 212, 191.
    • Yoder et al. [1989] A.J. 98, 1875.
    • Nicholson et al. [1992] Icarus 100, 464.

P. D. Nicholson, S. M. Larson, M. R. Showalter

3 August 1993.