Solar System Science

Abstract

tem began in spectacular style with the close flyby of Saturn’s outer moon, Phoebe, on 11 June. Phoebe appears to be a heavily cratered body comprising a primordial mixture of ice, rock and carbon-containing compounds. This composition indicates Phoebe formed in the outer solar system and could be a Kuiper Belt object. Cassini manoeuvred into orbit around Saturn on the night of 30 June – 1 July. Five science instruments remained active as the spacecraft flew within 15 000 km of Saturn’s main rings, in a region of space that had not been previously observed. This was also the closest passage to Saturn of the entire mission – just 19 980 km from the cloud tops. One early surprise was that, while Saturn’s rings are almost exclusively composed of water ice, the broad Cassini Division is relatively dirty, with particles remarkably similar to the dark material on Phoebe, suggesting that the rings might be the remnants of a moon. Another instrument on Cassini has detected large quantities of oxygen at the edge of the rings. One suggestion is that the oxygen may be left over from a collision that occurred as recently as January 2004. Cassini also found two new moons and an object that might be a moon, or just a clump of ring material. Cassini’s examination of Saturn’s atmosphere showed that near-equatorial winds decrease dramatically with altitude above the cloud tops, falling off by as much 140 m s over an altitude range of 300 km in the upper stratosphere. This was the first time winds have been measured so high in Saturn’s atmosphere. Non-imaging instruments also returned some interesting results. Cassini encountered Saturn’s bow shock much further from the planet than any previous mission. The 27 June bow shock crossing occurred at a distance of 49.2 Saturn radii (2.97 million km), more than twice the distance recorded by the Pioneer spacecraft in the 1970s. The difference is because of the different flight trajectories, since Cassini was coming in from the side of the planet, rather than “head on”. Another pre-encounter observation left scientists puzzling over Saturn’s rotation period, which appeared about 6 minutes (1%) longer than the radio rotational period measured by the Voyager spacecraft in 1980 and 1981. Scientists believe that there has been some kind of slippage between the deep interior of the planet and the magnetic field, which controls the charged particles responsible for the radio emission. A major finding of the magnetospheric imaging instrument was the discovery of a new radiation belt just above Saturn’s cloud tops, up to the inner edge of the D ring, detected by the emission of fast neutral atoms. With this discovery, the radiation belts were shown to reach much nearer to the planet than previously known (http://saturn.jpl.nasa.gov).