Thursday, October 08, 2009

New mega ring around Saturn discovered using Spitzer

Infrared observations using the Spitzer Space Telescope, published by Verbiscer et al (2009, Nature), have revealed the largest known ring around Saturn, an annulus of very tenuous material extending between 6 million and 18 million kilometers from Saturn, and tilted by 27 degree from the plane of the traditional rings (which only extend out to ~240,000 km).

The material in the new ring comes from the battered and cratered moon Phoebe. Of more interest, this new dust ring explains why the leading side of Iapetus is so much darker than the rest of it - the dark front surface of Iapetus is material from the ring swept up by Iapetus as it orbits at the inner edge of the new ring.

In truth a link between the dark front of Iapetus and Phoebe has been suspected before now, as the composition of the dark material is very similar to that of Phoebe based on near IR spectroscopy with Cassini. What the Spitzer observations reveal is the presence of the dust ring and hence the mechanism of material transfer from Phoebe to Iapetus.

Although the ring is physically huge, with a volume of ~5e21 km^3 (this is my BOTE calculation. As far as I can tell Verbiscer et al do not quote a volume), it is incredibly tenuous, and if all the material within it were collected back into one place it would possibly only occupy ~ 1 km^3 of rock, i.e. the volume of a crater on Phoebe.

The other interesting thing is that the material migrates inwards under the influence of radiation pressure. From Verbiscer et al:

On long timescales, collisions and inward transport become important. Collision with Phoebe, the dominant loss mechanism for particles larger than several centimetres in size, takes on the order of 1010 years. Re-radiation of absorbed sunlight exerts an asymmetric force on dust grains, causing them to spiral in towards Saturn with a characteristic timescale of 1.5 times 105rg years where rg is the particle radius in micrometres. This force brings all centimetre-sized and smaller material to Iapetus and Titan unless mutual particle collisions occur first. The rate of mutual collisions depends on the size distribution of the ring particles and optical depth; if the ring were comprised entirely of 10 mum grains, then the collisional timescale would be tens of millions of years, which is comparable to the inward drag timescale. Most material from 10 mum to centimetres in size ultimately hits Iapetus, with smaller percentages striking Hyperion and Titan3.


Verbiscer, A., Skrutskie, M., & Hamilton, D. (2009). Saturn's largest ring Nature DOI: 10.1038/nature08515

BBC article published by Jonathan Amos. 2009/10/08 (the source of the nice graphic shown above).

Spitzer press release, 2009/10/06.

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