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In the last ten years we have had confirmation that impacts of minor bodies on the planets, especially Jupiter and Saturn, are not events that happened only eons in the past, but are an ongoing phenomenon.
The number of small cometary nuclei and asteroids that cross planetary orbits is actually larger than thought up until the 80s and 90s, and this idea is now reinforced by two works published in the journal Science by two research teams led by Mark R. Showalter, Matthew M. Hedman and Joseph A. Burns (SETI Institute and Cornell University, Ithaca NY).
The two works, using images taken during the Galileo, Cassini e New Horizons missions, describe the perturbations in the ring systems of Jupiter (photo) and Saturn, caused by the passage of comet-sized bodies. Using dynamical models, they derive the epoch of the individual events.
When an object passes close to a ring it deflects it slightly from its axis. The action of the gravity of the planet then causes an initial warp to develop into an ever tighter spiral pattern. In time, this pattern becomes unrecognizable and the ring returns to its 'smooth' appearance.
Images taken of Jupier by the Galileo probe in 1996 and 2000 showed evidence of these winding spiral patterns, the strongest of which being the result of an initial perturbation of 2 km, which could be attributed to the celebrated impact of comet Shoemaker-Levy 9 in 1994. Two weaker spiral patterns were also identified in images taken by the New Horizons probe in 2007.
Similar structures, identified by Cassini, are also present in the C and D rings of Saturn. Of these, one particularly interesting feature was identified in 2009, thanks to the fact that the Sun was in the plane of the rings. This is a ripple, only between 2 and 20 metres high, with a wavelength of 30 to 80 km, that was caused by the passage of an unknown body, but probably a cloud of cometary debris, in 1983. The initial impact offset the D ring from its axis by about 100 metres.
Apart from the details of each event, it's interesting to note how the study of waves in planetary rings paints a picture of an unexpectedly dynamic solar system.
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