New 'Moons' of Saturn May Be Transient Objects
ADONIS Observes Pandora, S/1995 S6 and Others
19 January 1996
How many moons has Saturn, the second-largest planet in the solar system? Until recently, the best answer was eighteen, ranging from innermost Pan that circles the planet 75,000 km above the cloud tops in a little less than 14 hours, to distant Phoebe, 13 million km away in a reverse ('retrograde') 550-day orbit . Now the situation is less clear.
New observations have become available which raise some questions about the actual number and nature of small 'moons' near this planet. In particular, there is now evidence that some of the recent sightings may in fact refer to temporary condensations of material (dust clouds) in the inner rings rather than solid bodies.
Most of these observations have been made with the Hubble Space Telescope (HST), but important supplementary data  was also obtained with the high-resolution ADONIS camera at the ESO 3.6-m telescope.
When the Sun and Earth Cross the Plane of the Rings
Saturn is surrounded by a spectacular ring system in which a large number of small (probably cm- to m-size) icy bodies are moving. Soon after the invention of the telescope in the early 17th century, it was found to consist of an inner B- and an outer A-ring, separated by the dark 'Cassini division'. The faint F-ring was discovered further out by the Pioneer 11 spacecraft in 1979; it is separated from the A-ring by the 3000-km wide 'Pioneer division'.
All of these rings are very flat and quite thin. They are apparently no more than 2 kilometres thick in a global sense, and probably much less locally (10 - 100 metres). They all lie in the same plane which is inclined by 26.7 degrees, relative to the planet's orbital plane. One revolution of Saturn around the Sun lasts 29.455 years and twice during each orbital period, i.e. once about every 15 years, the Sun is situated exactly in this ring plane. This happened most recently on November 19, 1995. Astronomers refer to these relatively rare events as solar ring plane crossings (RPX) . At the corresponding times, the Sun illuminates the thin Saturnian rings exactly from the side.
Due to its own orbital motion around the Sun, the Earth will cross the ring plane either once or three times, just before and/or after a solar RPX event. In 1995, this happened on May 22 and August 10, and there will be a third Earth RPX event on February 11, 1996.
RPX Events Offer Improved Possibilities to Discover Faint Moons
The apparent brightness of Saturn's rings decreases dramatically around the time of a solar RPX event. It is then much easier to detect faint moons which would otherwise be lost in the strong glare of Saturn's ring system. Also, the edge-on view improves the chances of detecting faint and dilute rings . Moreover, numerous 'mutual events' (eclipses and occultations) occur between the moons during this period; exact timing of these events allows highly improved determination of the motions and orbits around Saturn of these objects.
The most recent Earth RPX event took place on August 10, 1995. At this time, Saturn was situated nearly opposite the Sun (in 'opposition'), as seen from the Earth, and conditions were very favourable for astronomical observations from both hemispheres. However, because of the longer nights during the southern winter, observing possibilities were particularly good in the south and thus at the ESO La Silla Observatory.
The ADONIS Observations
Here, a team of astronomers (Jean-Luc Beuzit, Bruno Sicardy and Francois Poulet of the Paris Observatory; Pablo Prado from ESO) followed this rare event during 6 half-nights around August 10, 1995, with the advanced ADONIS adaptive optics camera at the ESO 3.6-m telescope. This instrument neutralizes the image-smearing effects of the atmospheric turbulence and records very sharp images on an infrared-sensitive 256 x 256 pixel detector with a scale of 0.05 arcsec/pixel.
Most of the Saturn images were taken through the 'short K' filter with a central wavelength at 2.2 micron. This near-infrared wavelength region is characterised by strong absorption lines of molecular hydrogen (H2) and methane (CH4), two elements that are highly abundant in the planet's atmosphere. The planet therefore appears very dark at this wavelength, and its light interferes significantly less with the observations of the fainter rings and moons. In spite of a somewhat mediocre seeing, the ADONIS instrument enabled the observers to obtain images with an angular resolution as good as 0.3 arcsec, corresponding to about 1900 km at the distance to Saturn during the observations (1313 million km).
There were several scientific goals of these observations. First, they served to measure the brightness of the rings when seen from the side at Earth RPX in order to estimate their thickness, to determine their orientation and to detect a possible warp (deviation from flatness) of their plane. Secondly, they allowed to image for the first time again some of the small moons discovered in 1980/81 from the ground and by the Voyager 1 and 2 spacecraft and, if possible, to discover new ones. Thirdly, they offered the possibility to detect faint rings outside the main ring system. Numerous frames were obtained and many have still not been completely evaluated.
Imaging of the Smallest Moons
The accompanying, composite false-colour photo shows the rings, seen almost edge-on a few hours before the Earth RPX event at 20:54 UT on August 10, 1995. At this time, the Earth was slightly below the ring plane and the Sun was slightly above it, so that the dark side of the rings was visible. The observed brightness is then mainly due to the edge-on solar illumination of the ring system, i.e. to its thickness. To a lesser extent, it is also due to some solar light transmitted through the semi-transparent rings
Among the various moons seen in the ADONIS frames are Rhea, Thetys, Dione, Mimas, Janus, Epimetheus and Pandora. Janus, first discovered visually in 1966 by French astronomer Audouin Dollfus during an earlier RPX period, shares the same orbit as Epimetheus, 90,000 kilometers above the clouds. Epimetheus measures about 120 kilometres across, Janus 200 kilometres. Pandora, which was first seen on images obtained with the Voyager 1 spacecraft in 1980, is only half the size of Janus. The ADONIS images constitute one of the first sightings of this small and faint object with a ground-based telescope.
What is the Nature of S/1995 S6?
Of particular interest are the comparatively bright images in several ADONIS frames of an unresolved object. It has now been identified with the new 'moon' S/1995 S6 that was detected on frames obtained with the Hubble Space Telescope (HST), some hours after the ADONIS images were taken . It turns out that a total of nine pre-discovery images of S/1995 S6 were obtained at ESO, both East and West of Saturn, as this object moves around the planet. Based on all available observations, the orbital period has now been determined as about 14 hours 50 minutes. It is located at the same distance as the F-ring.
While there is little doubt that the object seen by HST and ADONIS is the same, there is a strange discrepancy in the measured brightness. From a comparison with the measurements of Thetys, the K magnitude of S/1995 S6 is found to be 14.9; this indicates a reflectivity that is about four times higher than that observed by the HST in the near-infrared part of the spectrum. This unexpected effect cannot yet be explained.
Moreover, it cannot be excluded that S/1995 S6 is not a real, solid moon, but just a transient clumping of material in the F-ring. This is because its observed motion corresponds exactly to that of the F-ring and especially since, considering its brightness, it ought to have been observed already by the Voyager spacecraft in 1980/81 if it were a permanent body. In that sense, S/1995 S6 is probably a 'transient moon'. The F-ring is very close to a zone of dynamical instability (the so-called 'Roche limit') near the planet, where condensations of the small bodies in the ring may form and again be disrupted in a matter of weeks.
The HST has seen a total of seven apparently new objects during the two Earth RPX events in 1995. An analysis has since shown that three of these are most likely images of already known moons, i.e. Prometheus (identified with two of the objects) and Atlas, one (S6) was also observed at ESO, and it cannot be excluded that the three others are also of a transitory nature.
The Thickness of the Rings
A provisional estimate from the measured brightness of the rings indicates an overall thickness of about 1.5 km, consistent with the results of the observations at the time of the RPX events in 1980. This figure, however, depends critically on an assumption about the reflectivity of the particles in the rings and it is therefore somewhat uncertain.
Most interestingly, the ESO observations also show that when seen edge-on, the Saturn ring system extends all the way out to the F-ring, and not just to the usually much brighter A-ring. A surprising result of these observations, confirmed by other observing teams, is thus that the edge-on brightness is actually dominated by the narrow and notoriously discreet, outer F-ring. This means that it is either relatively thick perpendicular to the basic ring plane, or it is inclined to this plane. It may also be that it hosts a significant number of small moons and clumps.
 Extensive information about the moons of Saturn may be found in the ``Gazetteer of Planetary Nomenclature (1995)'', published by the Working Group for Planetary System Nomenclature of the International Astronomical Union; the texts, tables and some images are also available on the Web (URL: http://www.worldcat.org/title/gazetteer-of-planetary-nomenclature-1994/oclc/32388441).
 This Press Release is accompanied by eso9603a , showing some of the ADONIS images.