[X] My Assistant

[Guest_AlexBlackwell_*]

Since Emily posted in her LPSC notes:

"So, so much for Titan. All in all, there wasn't a lot that was new, either new data or new insight. I wish now that I'd skipped the latter part of the Titan session and gone to the rovers, but it was too late for that. The radar story will get more interesting, because after a long hiatus in the acquisition of radar data they are going to be getting a lot more radar passes beginning with the T13 flyby on April 30, so there is much to look forward to there."

I guess I'll go ahead and post the science highlights for the T13 flyby:

T13 - Science Highlights

Imaging Science Subsystem (ISS) – will observe particle properties, vertical distributions (~6 km/px. 0.6 Mbit/frame with 2x2 summing). ISS will also examine wind/cloud motions; (3-12 km/px, 3 images/timestep in CB1 filter to increase SNR), and search for and monitor lightning/aurora. (high-resolution imaging, 50-200 m/px, special targets, emission angles < 45 prefer IR-polarizer (phase 45-110)).

Cassini Plasma Spectrometer (CAPS) – will investigate large-scale and distant aspects of the Titan interaction by observing during entire period around an encounter from 10 to 25 RS.

Composite Infrared Spectrometer (CIRS) –will obtain information on trace constituents in Titan's stratosphere. Integrate on limb at two positions POINTING: Obtain information on CO, HCN, CH4. Integrate on disk at air mass 1.5-2.0. POINTING: -y to Titan, x away from sun.

Ultraviolet Imaging Spectrometer (UVIS) – will observe star Beta Ori as it becomes occulted by Titan’s atmosphere.

Visible and Infrared Mapping Spectrometer (VIMS) – will obtain new high resolution images that will help understand Titan's geology and the fate of CH4.

Magnetometer (MAG) – will investigate large-scale and distant aspects of the Titan interaction by observing during entire period around an encounter from 10 to 25 RS. (23TI (T13)) T13 is an equatorial wake flyby under plasma conditions near Saturnian local midnight with 1852 km altitude at CA. Thus it is very similar to T11 even according to local time.

Magnetospheric Imaging Instrument (MIMI) – will investigate micro-scale and near aspects of the Titan interaction by observing during about one hour period around an encounter. With -Y pointed toward Titan, when within 30 minutes of the targeted flyby, optimize secondary axis for co-rotation flow as close to the S/C -X, +/- Z plane as works with the other constraints on pointing. Also, measure Titan exosphere/magnetosphere interaction by imaging in ENA with INCA (when sun is not in INCA FOV).

Ion and Neutral Mass Spectrometer (INMS) – will obtain data regarding Titan’s atmospheric and ionospheric composition and thermal structure. INMS will also observe the magnetospheric/ionospheric interaction.

Radio and Plasma Wave Spectrometer (RPWS) – will perform observations in the immediate vicinity of Titan, including thermal plasma density and temperature measurements with the Langmuir probe, search for lightning and other radio emissions, characterization of plasma wave spectrum, search for evidence of pickup ions. Langmuir probe within 90 degrees of spacecraft ram at closest approach, co-rotational ram outside of +/- 15 minutes.

RADAR – will perform low and high resolution SAR (Synthetic Aperture RADAR) imagine of Titan’s surface. Additionally, RADAR will collect Altimetry, Radiometry, and Scatterometry Data.

[Phil Stooke]

Impacts are not concentrated on the leading hemisphere - except possibly in statistically trivial amounts. The other synchronously rotating moons are equally cratered at all longitudes except where some geological process might have covered or erased one region. People have looked for the effect you described but never found anything convincing. (Of course somebody will probably find a claim of such an effect now, but just look at Mimas if you want a test of the idea).

Phil

[Thorsten]

Impacts are not concentrated on the leading hemisphere - except possibly in statistically trivial amounts.

Thanks a lot for the clarification, Phil. I was just puzzled, because, for example, the Cassini-Huygens website on Dione states that “in contrast to what scientists had expected much of the heavily cratered terrain [of Dione] is located on the trailing hemisphere, with the less cratered plains existing on the leading hemisphere. This anomaly suggests that during the period of heavy meteors bombardment, Dione was tidally locked to Saturn in the opposite orientation” and that a large impact could have spun the satellite. Apparently, this idea has become obsolete today!

One last question. Volcanopele mentioned in another thread, that the T17 (Sept. 7) flyby includes a half-SAR pass, covering northeast Xanadu. Guessing from the tracks shown on the Cassini Cam page, this pass should allow an overlap with the western part of the T13 SAR-pass. Can any additional information be gained by such an overlap, for example on altimetry?

[helvick]

Just a thought on impact distribution on Titan. Since it has an extremely dense atmosphere and has a synchronous orbit then the combination of the two should selectively concentrate impacts at the centre of the leading hemisphere. Meteorite impacts through an atmosphere are dependant on angle of incidence - the atmopsheric depth that has to be travelled has a major effect and deflection effect of hitting at an angle will both significantly affect the results. The long term stability of the synchronous rotation is probably not sufficient to keep the evidence in once place but I am pretty sure the odds of a cratering impact at the centre of the leading hemisphere are much higher than at the edge.

It's just a thought but it makes sense to me...

[volcanopele]

Emily has a nice article up on the Planetary Society website on the recent paper on Titan's dune fields and on the recent T13 SAR swath:

http://www.planetary.org/news/2006/0523_Ca...by_Another.html

She goes into a discussion of the river systems seen in Xanadu. The drainage pattern of the systems in western Xanadu suggest flow to the south and initially, away from the bright-dark boundary. While this may seem puzzling, keep in mind the dendritic channels seen by Huygens. Initially, those channels indicate flow away from the bright-dark boundary, but eventually, they recurve back to "shore". I have a feeling that is what we are seeing here. The channels flow away from a near-"shore" headland, then the flow is south and past the sar swath, back to the south and west.

With regards to the lack of a tectonic pattern, I think we again have to be cautious about interpreting the mountain observations. There is a clear sign that the topographic highs in Xanadu have been heavily modified by fluvial activity which may mask the original tectonic framework that is so clear in Adiri and in Antilla Faculae.

[Thorsten]

Hmmm, I have the impression that some of the prominent river/ channel features shown by SAR in Western Xanadu (PIA08428) can also be discerned by ISS (Scrutinizing Titan's Surface, PIA06204), for example the confluence (A) and the sinuosities (B and C). Of course, it is much easier to see something when you already know where and what to look for!
In the ISS the river/ channel feature appears to flow further southwards (D) and to broaden into several dark patches, like some that can be seen in Shikoku.

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[ngunn]

QUOTE(ngunn @ May 5 2006, 04:15 AM)

Regarding the circular features: Impact craters and volcanic calderas are mentioned here, but are these the only possibilities? What sort of appearance would we expect from the remains of an ice/clathrate diapir that slowly made it all the way to the surface, rather like a terrestrial salt dome? (A kraken rather than a vampire?)

hmm, kinda like those lenticulae on Europa. Not quite sure how those would look in RADAR.

A bit of follow-up on the above. I've just read this paper linked by Alex today in 'Venus Express':
http://www.blackwell-synergy.com/doi/abs/1...04.2006.47316.x

The Venus coronae sound like the sort of thing I was trying to suggest as a possibility for Titan. They are very varied but mainly circular-ish with different concentric arrangements of high and low topography. Apparently the rising diapir hypothesis can account for the various forms by varying such parameters as crustal thickness/layering and the stage the process has reached. Richard, you mentioned circular forms on Titan that might be diapirs also. Have you read about the Venusian coronae and do you think there could be a parallel?