Wednesday, 9 April 2014

Ripples in the rings of Jupiter.

Although fainter and considerably less famous, the planet Jupiter has a system of rings similar to that of Saturn, between the orbits of the small moons Metis and Adriastea. In 1996 the Galileo spacecraft observed a series of ripples within these rings, with material moving as much as 2 km from the plain of the ring. These ripples formed a pair of superimposed and apparently independent spiral patterns. These ripples were again observed by Galileo in 2000, and by the New Horizons spacecraft in 2007.

In a paper published in the journal Science on 6 May 2011, Mark Showalter of the SETI Institute and Matthew Hedman and Joseph Burns of Cornell University proposed that these rings were caused by the interaction of the rings with comet Shoemaker-Levy 9, which broke into a number of pieces following a close encounter with Jupiter in July 1992, and then impacted the planet in a series of collisions in July 1994.

(A) Image taken by the Galileo spacecraft on 9 November 1996, showing the tip of the Jovian ring system. (B) The same image expanded vertically, with two similar frames added to improve signal-to-noise, and a reversed duplicate subtracted. Neutral grey areas are on the plain of the ring, lighter areas raised and darker areas lowered relative to this ring. Hedman et al. (2011).

Hedmen et al. propose that the tilt was caused by particles ejected from Shoemaker-Levy 9 directly impacting the ring, and note that the pattern of ripples observed in 2000 was appeared to have evolved somewhat from that seen in 1996.

The path of particles derived from the comet Shomaker-Levy 9 past Jupiter and its ring system. Particles on the heavy dashed line would pass directly through the ring, and many would impact particles within the ring system, leading to the creation of ripple patterns within the gravitationally bound ring. Hedman et al. (2011).

While this explanation accounts for the observed ring patterns seen in 1996 and 2000, the patterns observed by New Horizons in 2007 require further explanation. The collision between Shomaker-Levy 9 and Jupiter was originally seen as a once in a century event, a similar collision was observed between Jupiter and an unnamed comet on 19 July 2009, suggesting that such events are much more common than was previously thought. Hedman et al. also note that comet 16P/Brooks 2 is thought to have passed very close to Jupiter in 1886, and comet P/Gehrels 3 to have made a close pass of the planet in 1970. Thus they propose that the reinforced ring pattern observed by New Horizons in 2007 could conceivably have been caused by the interaction of the rings with ejecta from an unobserved comet that made a close pass of Jupiter between 2000 and 2007. They further note that similar ripples have been observed in the rings of Saturn by the Cassini space probe, and that these probably have a similar cause.

Ripples on the Jovian ring system observed by New Horizons in 2007. Hedman et al. (2011?).

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