[X] My Assistant

[ngunn]

I hope the following very non-specialist post will interest some readers - and maybe even winkle out an expert reply or two.

Cassini is about to pull off a special trick. The next two Titan flybys are only 8 days apart. That single fact alone tells a great deal about what Cassini is up to. For Titan, 8 days is just half a month, meaning that the encounters happen on opposite sides of Titan’s orbit. Now it’s not that special for the orbits of two bodies to meet at two widely distant points. But for the two bodies to take exactly the same time to get from one point to the other is very special: it implies that their orbits must be twins. They must not only be the same size but the same shape too, in this case a fair approximation to a circle. The only way in which the orbits may (and do) differ greatly is in inclination. The double act will not last long, however, as Cassini’s orbit continues to be reshaped to meet it’s destined targets.

Over the coming weeks I hope this unusual event, and its implications for Cassini and the geometry of future Titan encounters, will be remarked upon at greater length by the Cassini team, or maybe by others here and elsewhere.

Watch out for the mission descriptions appearing on the Cassini website. Will we get a double-header for both flybys, as we did with Enceladus last year?

[Juramike]

From the mission description:
RSS observes bistatic scattering from Titan's surface on the outbound side. The observation probes the midnorthern latitude regions of Titan's surface (30-40 degs; ~190-200 degs west longitude). Same- and crosspolarized components of the quasi-specular surface echo, if detectable, provide information about the dielectric constant and physical state of the surface region probed.



This is in the mid-latitude region to the NW of the Dancing Monkey feature of Dilmun. (Or way to the N of Selk Crater).
This region may have also been imaged by SAR RADAR during the T43 pass (IIRC this will hit the PDS this April.)

Dielectric constant gives great information about the material properties of the surface stuff. Low dielectric constant stuff is more non-polar (like hydrocarbons) while higher dielectric constant stuff is more polar (like water).
But there's a catch: fluffy porous stuff lowers the apparent dielectric constant. So it's hard to tell if its solid wax (ca. 2.0-2.4), or fluffy snow (solid ice is 3.1, fluffy snow as low as 1.4). (Really rough angled surfaces can mess up this experiment too, but that doesn't seem to be a major problem on Titan - if it was, the SAR RADAR returns would be be really really bright).

Most of the blandlands have a pretty uniform dielectric constant of around 1.7. This is a smidgen higher than the global average of 1.6.
But based on a recently publication (Janssen et al., 2009), there is an much lower dielectric constant area near the bondary of the soon-to-be-observed region.
The low dielectic constant is about 0.9 - this is way to low to be any solid material. It has to be something likely organic AND fluffy (or porous).
Even more exciting, a region with the same dielectric properties is W Xanadu and Tui Regio.

I can't wait to see the T43 SAR Swath....

-Mike

BTW, my latest favorite article to carry around in my briefcase is:
Janssen et al., Icarus 200 (2009) 222-239. "Titan's surface at 2.2 cm wavelength imaged by the Cassini RADAR radiometer: Calibration and first results."
doi: 10.1016/j.icarus.2008.10.017