<P> The power law mentioned above allows estimation of the optical depth τ (\ displaystyle \ scriptstyle \ tau) of the main ring: τ l = 4.7 × 10 − 6 (\ displaystyle \ scriptstyle \ tau _ (l) \, = \, 4.7 \ times 10 ^ (- 6)) for the large bodies and τ s = 1.3 × 10 − 6 (\ displaystyle \ scriptstyle \ tau _ (s) = 1.3 \ times 10 ^ (- 6)) for the dust . This optical depth means that the total cross section of all particles inside the ring is about 5000 km2 . The particles in the main ring are expected to have aspherical shapes . The total mass of the dust is estimated to be 10 − 10 kg . The mass of large bodies, excluding Metis and Adrastea, is 10 − 10 kg . It depends on their maximum size--the upper value corresponds to about 1 km maximum diameter . These masses can be compared with masses of Adrastea, which is about 2 × 10 kg, Amalthea, about 2 × 10 kg, and Earth's Moon, 7.4 × 10 kg . </P> <P> The presence of two populations of particles in the main ring explains why its appearance depends on the viewing geometry . The dust scatters light preferably in the forward direction and forms a relatively thick homogenous ring bounded by the orbit of Adrastea . In contrast, large particles, which scatter in the back direction, are confined in a number of ringlets between the Metidian and Adrastean orbits . </P> <P> The dust is constantly being removed from the main ring by a combination of Poynting--Robertson drag and electromagnetic forces from the Jovian magnetosphere . Volatile materials, for example ices, evaporate quickly . The lifetime of dust particles in the ring is from 100 to 1000 years, so the dust must be continuously replenished in the collisions between large bodies with sizes from 1 cm to 0.5 km and between the same large bodies and high velocity particles coming from outside the Jovian system . This parent body population is confined to the narrow--about 1000 km--and bright outer part of the main ring, and includes Metis and Adrastea . The largest parent bodies must be less than 0.5 km in size . The upper limit on their size was obtained by New Horizons spacecraft . The previous upper limit, obtained from HST and Cassini observations, was near 4 km . The dust produced in collisions retains approximately the same orbital elements as the parent bodies and slowly spirals in the direction of Jupiter forming the faint (in back - scattered light) innermost part of the main ring and halo ring . The age of the main ring is currently unknown, but it may be the last remnant of a past population of small bodies near Jupiter . </P> <P> Images from the Galileo and New Horizons space probes show the presence of two sets of spiraling vertical corrugations in the main ring . These waves became more tightly wound over time at the rate expected for differential nodal regression in Jupiter's gravity field . Extrapolating backwards, the more prominent of the two sets of waves appears to have been excited in 1995, around the time of the impact of Comet Shoemaker - Levy 9 with Jupiter, while the smaller set appears to date to the first half of 1990 . Galileo's November 1996 observations are consistent with wavelengths of 1920 ± 150 and 630 ± 20 km, and vertical amplitudes of 2.4 ± 0.7 and 0.6 ± 0.2 km, for the larger and smaller sets of waves, respectively . The formation of the larger set of waves can be explained if the ring was impacted by a cloud of particles released by the comet with a total mass on the order of 2--5 × 10 kg, which would have tilted the ring out of the equatorial plane by 2 km . A similar spiraling wave pattern that tightens over time has been observed by Cassini in Saturns's C and D rings . </P>

Where is jupiter's ring located what is it made of and why