[X] My Assistant

[ngunn]

I was struck by the first paragraph on p.166 relating to Huygens:

The SSP tilt sensors and the HASI accelerometer both seemed to indicate the probe was tilted by several degrees relative to the local vertical. But the horizon on the DISR images was pretty horizontal. It seemed unlikely that the ground would be sloping so steeply. Were some of the sensors wrong - and if they were, why did they agree with other factors? Maybe the probe had bent out of shape, changing the relative alignment. In fact, it would be impossible to know.

Reading this makes me want to view for myself the surface image oriented as the instruments indicate it should be, rather than on the assumption of a horizontal skyline. Just how implausible would it look? Presumably the imaging team did this, but perhaps that was before we knew how surprisingly high some of the 'pebble banks' near the landing site are. We only see one short section of the skyline after all.

Could Ralph or anyone provide a version of the image rotated accordingly?
EDIT - Or a version of the surface image annotated with the position of the theoretical horizon determined from SSP and HASI?

[rlorenz]

I was struck by the first paragraph on p.166 relating to Huygens:

[i]The SSP tilt sensors and the HASI accelerometer both seemed to indicate the probe was tilted by several degrees relative to the local vertical. But the horizon on the DISR images was pretty horizontal.

Wow, seize upon the least interesting thing from the whole encounter........ The DISR images were studied to
death by Erich Karkoschka - see his Planetary and Space Science paper, from which I quote below

8.3. Attitude of Huygens after landing
The best constraint from DISR about the rest attitude
comes from the comparison of the sky brightness in SLI
images taken before and after landing at similar azimuths,
taking into account the gradual decrease of brightness with
altitude. This yields a pitch after landing of 3  1 and a
roll within a few degrees of zero.
More accurate are measurements by the SSP tilt sensors
(Lorenz, private communication). Since there are concerns
that one of the tilt sensors may have had a constant offset,
we will only use relative tilt data here. For the last 20 SLI
exposures, we measured an average pitch of 0:3 and an
average roll of 0:3. These data are the most reliable
because of the visibility of the horizon in these images. The
average SSP tilt data for these 20 instances are 0:3 for
TIL-X and 7:6 for TIL-Y (Lorenz, private communication).
Considering the directions of the tilt sensors, we used
this relationship between pitch/roll and SSP tilt data to
convert the SSP tilt data measured after landing, which are
2:0 for TIL-X and 8:6 for TIL-Y, into pitch of 3:1
and roll of 0:9. This means DISR is 3:1 looking up and
0:9 rotated clockwise in the viewing direction. These data
are estimated to be accurate to about 0:5. They are
consistent with the data using the sky brightness. In other
words, SSP detected a change of average tilt of 3 at
impact, which corresponds to a decrease of pitch of 3 and
an increase of roll of 1. Since pitch and roll averaged near
zero before landing according to our investigation, they
must have been 3 and 1, respectively, after landing.
Because the horizon on SLI images after landing seems
to be depressed by 1:5, and because the pitch was 3:1,
the horizon was actually raised about 1:6, which indicates
moderate relief south of the landing site. Because the
horizon in SLI images after landing does not seem to be
tilted either way, but the roll was 0:9, the horizon actually
was sloping up toward the left by about 0:9, a very
gentle slope. Images taken before landing suggest higher
hills to the east and lower hills to the west, simply judging
from the contrast of features, which is consistent with such
a slope.

As for rotating images, they are on the PDS.... have at it.

[ngunn]

Thanks for that very detailed reply. If I understand it correctly (despite some font problems with the quote) the rotation would only be about 0.9 degrees and the theoretical horizon from the instruments would be below the observed skyline at all points, leaving open the question of determining the absolute truth, as you said in the passage from the book which I quoted, but also rendering it largely irrelevant to interpretation of the scene and therefore of limited interest, as you point out here!

There are lots of wonderful things about the Huygens landing - it was my favourite moment in the whole history of space exploration - I just want to know as much as I can about it and to understand what I'm looking at to the best of my limited amateur ability. Everything you post here, whether reflections, corrections or pointers to papers helps immensely.

Now I'm off downstairs to carry on with the book . .

[ngunn]

For the record my close interest in gradients at the landing site is connected with the idea that exceptional storm winds could play a role, alongside gravity, in moving sediment-laden floods around on Titan's plains. This could result in flows from different directions leaving their marks at a single location, as we seem to see here. One could even imagine a flow front being pushed uphill, as happens with terrestrial storm surges. I'd like to hear from anyone with thoughts (or references) on this in relation to Titan. I don't have easy access to the literature so undoubtedly miss a lot.

[Juramike]

Stom surge...tsunami...

formed by winds, storms...and...earthquakes, tectonic thrusts, and impacts.

Brilliant!!!!


That gives a possible mechanism for causing the sudden rush of liquid (methane, nitrogen?) through many channels at once from basin to the other. A sudden splash could also cause overspill along many lower breaches simultanaeously.

A slow rise in liquid (rise< rate of canyon valley erosion) would've caused only one major canyon to dominate. In contrast, a catastrophic release (or splash, or slosh) would allow multiple simultaneous breaches to occur due to sudden overspill of the basin. We see multiple interbasin breaches in central Quivira and E Quivira (T23 RADAR Swath channels between Fensal and Aztlan [Equatorial Sand Seas thread, post 325).

[This makes much better sense than my previous idea of an sporadic ice-dam releases. This is tough because you would have to explain blockage of many channels to isolate an area AND I'm hard pressed to think of a solid that would be sufficiently insoluble in methane or nitrogen, but could then dissappear without a trace. (Acetylene or frozen methane?) I was also in love with the ice-dam release idea because then I could use the word jokulhlaup on regular basis.]

Titan's gentle slopes and narrow channels would have also contributed to some really impressive waves and surges during these events.

Wow! Just imagine the massive surge ripping down Fensal basin that could have occurred during the Menrva impact.

-Mike

[Stefan]

Wow, seize upon the least interesting thing from the whole encounter........

Historical note: Back then this was considered a very interesting discrepancy leading to heated debates, with the SSP team gradually adjusting to (the uncomfortable) reality.

[rlorenz]

For the record my close interest in gradients at the landing site is connected with the idea that exceptional storm winds could play a role, alongside gravity, in moving sediment-laden floods around on Titan's plains. This could result in flows from different directions leaving their marks at a single location, as we seem to see here. One could even imagine a flow front being pushed uphill, as happens with terrestrial storm surges. I'd like to hear from anyone with thoughts (or references) on this in relation to Titan. I don't have easy access to the literature so undoubtedly miss a lot.

In a bizarrely Adamsesque 'fundamental interconnectedness of all things' link, I agree it is possible,
and I actually have an interest in the moving rocks at Racetrack Playa in Death Valley National Park
(so far just had a paper submitted and rejected on the topic, early days yet...)

One day the calibration data for the Huygens radar altimeter will get recovered from the VMS system
they are imprisoned on and that may get published. We may also get some SARtopo out of T41 which
goes across the landing site area.

[rlorenz]

Historical note: Back then this was considered a very interesting discrepancy leading to heated debates, with the SSP team gradually adjusting to (the uncomfortable) reality.

Yes. I think if I were to be writing Titan Unveiled now, I don't know if I'd have bothered including
the discrepancy discussion (which maybe, from a historical perspective, is a good reason for
having done so then...)

SOME of the SSP team were reluctant to accept that the tilt meter had an offset, but it always
seemed the most obvious explanation to me (recall the tilt sensors are little electrolytic cells
with a slug of potassium iodide solution whose position is read by where it bridges a pattern
of electrodes plated onto the inside of a glass vial in which the solution sits........ makes you
think about how music was once recorded by scratches on a disc of plastic, read mechanically...
for tilt these days you would just take the MEMS accelerometer out of a Wii and use that -
much easier and more robust!)

There was also the puzzle that the HASI accelerometer reads a slightly low surface g value - in
apparent cosine agreement with the (probably dodgy) SSP tilt result.

Of course, the data would all be consistent if the experiment platform (onto which all of these
sensors are bolted) bent. But at this point we should heed Francis Crick's advice
"Any theory that fits all the facts is bound to be wrong, because some of the facts will be wrong"

[ngunn]

One day the calibration data for the Huygens radar altimeter will get recovered from the VMS system
they are imprisoned on and that may get published. We may also get some SARtopo out of T41 which
goes across the landing site area.

Nice to know we'll get more news from that very special location - our one sure foothold in the outer solar system.

(and hats off to Adams for promoting lateral thinking in the funniest possible way)

All the same I think caution is in order. In line with the Francis Crick quote you could say "If you've tied up all the loose ends you've got it wrong because you haven't got all the ends yet."