[X] My Assistant

[Guest_AlexBlackwell_*]

The mission description document is now online (1.06 Mb PDF).

[MarcF]

I read in the T20 mission description document that a “high-altitude SAR” imaging observation of the Tortola Facula will be performed.
I hope the altitude will not be too high and that this observation will allow to confirm (or invalidate) the volvanic nature of Tortola Facula. I'm really impatient to see the result.

Marc.

[Olvegg]

The altitude will be about 100,000 km, judging from Geometry Table. This gives the resolution of 50 km, not too high...

[Phil Stooke]

SAR resolution does not scale with distance the way optical imaging does. Higher altitude means a weaker signal. To accomodate that, the data might be sampled differently, reducing resolution, but it's still not just linear scaling. I admit I know nothing about the expected resolution of the SAR under these circumstances.

Phil

[MarcF]

100,000 km seems really a great distance (it's even difficult to speak about altitude at that distance) !
The putative Tortola volcanic dome is only around 30 km in diameter.

Marc.

[Holder of the Tw...]

Something's wrong here. 100,000 km above Titan is way beyond the radar's ability to image. Two to three thousand km is more in line with an high altitude run.

[ugordan]

This is probably using a new method for distant SAR imaging the radar team have recently come up with. The resolution is naturally lower than low altitude passes, but it still does give useful data. So they say.

[volcanopele]

I would double check that 100,000 km number, but even at that distance, observations of Tortola's radar backscatter and dielectric properties could still be useful.

SAR has observed at 35,000 and 50,000 km before, and they looked just fine. I should point out that this would be using just the center beam, and not the other 4 beams.

[Big_Gazza]

What I have been wondering is that why have we not seen more of the high-res VIMS images like that of Tortola Facula (back on Ta?). Those images were far more detailed than anything ISS has/can produce, and if VIMS can produce images like this, I would expect the instrument to be assuming a higher visibility in a PR sense.

The field of view of the Tortola Facula images are very small, and the "volcano" is nearly central to the image. This, together with the lack of other released VIMS images, suggests two options:

1 - there are more such images which have not been released (hard to believe)
2 - one small area was investigated in detail

If 2 is correct (most likely), how did the imaging team know to look here? This small patch appears to be an-otherwise unremarkable region of the northern dark region, and this early in the mission there could not have been other investigations that led researchers to this feature.

Anyone have any insider info on the imaging of Tortola Facula?

Cheers,
Gary

[MarcF]

I think it's just by luck that they observed Tortola (but may be I'm wrong).
If I'm true, this could mean that such features may be very numerous on Titan surface to statistically observe one by chance during one rare very high resolution VIMS observation.
However, it's still not prooved that it's indeed a volcanic dome.
Marc.

[volcanopele]

There are a lot of little faculae in that region, so I don't think it was luck that they imaged Tortola, it just happened to be there. As to what Tortola is, currently my money is on impact crater...

The Tortola Facula image is the highest resolution image that VIMS has taken to this point, AFAIK. Other images have been taken at resolutions better than 10 km/pixel, most notably in eastern Xanadu from T9 where river channels are visible. The ISS team has given up its C/A prime coverage to VIMS since our images in that timeframe are usually not usable due to the high exposure times needed at Titan. Our mid-range coverage (unsummed pixel scales ~400-600 m/pixel) is actually pretty good if done right and we should get a great mosaic on this encounter.

[MarcF]

"It just happened to be there", this is what I wanted to say by using the word "luck", Jason. Sorry for my imperfect English. To be honest, I also think that Tortola is rather an impact structure, unlike Ganesa which really looks like a cryovolcanic dome in SAR images. However, I thought that Tortola's volcanic origin was quite well accepted by the specialists. This is the reason why I'm so impatient to see a SAR picture of Tortola. But I'm not convinced that we will get the answer during the coming T20 flyby.
Marc.

[volcanopele]

"It just happened to be there", this is what I wanted to say by using the word "luck", Jason. Sorry for my imperfect English.

I'm just being difficult. "Luck" presumes that there is something special about Tortola compared to the other faculae of about that size in the region, that VIMS was lucky to see Tortola and not the other losers in the region.

To be honest, I also think that Tortola is rather an impact structure, unlike Ganesa which really looks like a cryovolcanic dome in SAR images. However, I thought that Tortola's volcanic origin was quite well accepted by the specialists. This is the reason why I'm so impatient to see a SAR picture of Tortola. But I'm not convinced that we will get the answer during the coming T20 flyby.

It isn't well accepted, except perhaps by those on the VIMS team. Yes, I can see where they are coming from by saying that it looks like it might be a volcanic structure. But there is nothing in the VIMS data that says that it couldn't be something else, like a crater with bright ejecta that has been sculpted by aeolian forces. I've see a few features in ISS data that I thought might be a cryovolcano (one turned out to be the ejecta blanket for Ksa, a 30-km wide impact crater), but you don't see me shouting it from the roof tops. It could simply be an odd collection of hills...

In terms of answers from SAR, I agree, T20 probably won't solve it. The resolution is too low. But I seem to recall a flyby later in the mission where RADAR will get a better look at Tortola. I fear that was T16 and SAR turned on a few minutes too late for it to see Tortola. I'll have to check into that tomorrow.

[Holder of the Tw...]

SAR has observed at 35,000 and 50,000 km before, and they looked just fine. I should point out that this would be using just the center beam, and not the other 4 beams.

Good grief. Whenever did they pull that off? I hadn't head of anything like this.

Working from admittedly old mission documents (vintage 2004), I had the following parameters:
- High resolution SAR to 1,500 km altitude max
- Low resolution SAR to 4,000 km
- Altimetry to 10,000 km
- Scatterometry up to 32,000 km
- Beyond this, only radiometry

Knowing from earth based radar studies of asteriods that radar follows a fourth power law (double the distance, you cut the return signal sixteen fold), I would have thought any SAR attempt at 50,000 km to be right out of the question.

[Decepticon]