in data from the Galileo orbiter and is clearly visible in back-scattered images obtained from New Horizons in February–March 2007. The early observations by Hubble Space Telescope (HST), Keck and the Cassini spacecraft failed to detect it, probably due to insufficient spatial resolution. However the fine structure was observed by the Keck telescope using adaptive optics in 2002–2003. Observed in back-scattered light the main ring appears to be razor thin, extending in the vertical direction no more than 30 km. In the side scatter geometry the ring thickness is 80–160 km, increasing somewhat in the direction of Jupiter. The ring appears to be much thicker in the forward-scattered light—about 300 km. One of the discoveries of the Galileo orbiter was the bloom of the main ring—a faint, relatively thick (about 600 km) cloud of material which surrounds its inner part. The bloom grows in thickness towards the inner boundary of the main ring, where it transitions into the halo. Detailed analysis of the Galileo images revealed longitudinal variations of the main ring's brightness unconnected with the viewing geometry. The Galileo images also showed some patchiness in the ring on the scales 500–1000 km. In February–March 2007 New Horizons spacecraft conducted a deep search for new small moons inside the main ring. While no satellites larger than 0.5 km were found, the cameras of the spacecraft detected seven small clumps of ring particles. They orbit just inside the orbit of Adrastea inside a dense ringlet. The conclusion, that they are clumps and not small moons, is based on their azimuthally extended appearance. They subtend 0.1–0.3° along the ring, which correspond to –. The clumps are divided into two groups of five and two members, respectively. The nature of the clumps is not clear, but their orbits are close to 115:116 and 114:115 resonances with Metis. They may be wavelike structures excited by this interaction. Spectra and particle size distribution Spectra of the main ring obtained by the HST, Keck, Galileo and Cassini have shown that particles forming it are red, i.e. their albedo is higher at longer wavelengths. The existing spectra span the range 0.5–2.5 μm. No spectral features have been found so far which can be attributed to particular chemical compounds, although the Cassini observations yielded evidence for absorption bands near 0.8 μm and 2.2 μm. The spectra of the main ring are very similar to Adrastea and Amalthea. The