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Huygens probe question
Steffen
post Mar 25 2006, 05:28 PM
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Which device made the Huygens probe spin underneath its satellite?
I read that's the way how 360° images were made during descent...
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centsworth_II
post Mar 25 2006, 05:39 PM
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I believe the probe had fins attached that were to spin it as it fell through the atmosphere of Titan - like the seed pods of a maple tree twirl as they drop, but much more slowly. As far as I know, it is still a mystery why the probe ended up spining in the opposite direction that it was designed to. It was unexpectedly windy (gusty?) during the descent. Now to hear from some more expert than I...

By the way, the Huygens probe spun beneath its parachute, not a "satellite".
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Guest_BruceMoomaw_*
post Mar 25 2006, 10:11 PM
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Yeah, it had small vanes on the outer edges of its lower surface, which can be clearly seen in the drawings. (This, I believe, was also the setup they used to make the Pioneer 13 Large Probe rotate in Venus' atmosphere.)

As to why it spun in the OPPOSITE direction: that's still a mystery. (Pre-launch photos show that they did manage to avoid the supreme humiliation of having installed the vanes backwards.) My own suspicion is that the swivel joint on the parachute line jammed, and the parachute itself was what spun in the opposite direction, dragging the probe with it.

And actually, you know, it rotated beneath both its parachute AND Cassini, which had some implications for maintaining the radio link properly.
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Decepticon
post Mar 26 2006, 04:00 AM
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^That works with me.


I never heard of the radio link issue!? The link between Cassini and Huygens would stop and start?
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Guest_BruceMoomaw_*
post Mar 26 2006, 05:44 AM
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They had to do some careful checking of the antenna lobe patterns on Huygens during its design to make sure that the probe's rotation (a crucial part of the DISR experiment) didn't interfere with its radio link to Cassini. That part they got completely right, even given the extent to which Huygens was violently tossed around by unexpected air turbulence in Titan's stratosphere.
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Guest_Richard Trigaux_*
post Mar 26 2006, 08:45 AM
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Any object under a parachute rotates naturally. They just relied on this to have 360° images.

I never heard of fins. Anyway even with fins it is not a mystery that air movements tossed the probe to make it rotate the other way.
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Guest_BruceMoomaw_*
post Mar 26 2006, 11:02 AM
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See page 15 of Lebreton's article on Huygens ( http://www.rssd.esa.int/SB/HUYGENS/docs/SP1177/lebreton.pdf ): "It is required that Huygens spins during the whole descent to provide the azimuth coverage needed by several sensors. The realtime spin information requirements are imposed by DISR and are very stringent for the final part of the descent for imaging the surface, in order to adapt the time delay between consecutive frames during the mosaic image-taking cycle. The spin is induced by a set of 36 vanes mounted on the bottom part of the foredome. The spin rate is measured by a set a system-provided accelerometers covering O-15 rpm with an accuracy of 0.1 rpm."

The same page shows two nice diagrams of the external appearance of Huygens, with the vanes clearly visible. Titan's winds alone should not have been capable of making the Probe rotate in the reverse direction -- unless something very unusual like a jammed chute-line swivel joint occurred.
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Guest_BruceMoomaw_*
post Mar 26 2006, 11:52 AM
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That same diagram revealed something else that I hadn't paid proper attention to: the heated inlet for the GCMS is squarely in the center of the probe's slightly convex bottom -- which, given that Huygens did land in fairly soft mud (albeit with pebbles in it) makes it highly likely that the inlet (heated to fully 80 deg C) DID come into direct contact with Titan's surface material. Indeed, Raulin and Owen's 2002 "Space Science Reviews" piece ( http://www.nasa-ksc.org/outgoing/SSR/raulin.pdf ) says: "The GC-MS has a heated inlet tube that extends several mm beyond the probe’s outer skin. A landing in a fluffy drift of aerosols would therefore provide access to an unusually concentrated sample of the organic compounds that the atmosphere of Titan has produced during many hundreds of millions of years. Even a successful touchdown on water ice could provide useful information."

And Hasso Niemann's piece in the same issue ( http://www-personal.umich.edu/~atreya/Arti...romatograph.pdf ) says: "If the Probe settles into a deposit of aerosols, one needs to extrapolate the accumulated information from the descent measurements to interpret the data. This would offer an opportunity to determine the level of chemical complexity achieved by chemical synthesis in the atmosphere, as even rare aerosols may accumulate inmeasurable concentrations on the surface. Here the GCMS heated inlet will ensure that the more volatile components of such aerosols reach the instrument. Landing on exposed ice could still permit a measurement of H2O ice ‘bedrock’ and a search for condensed CO2, measurements of fundamental importance to an understanding of atmospheric evolution. A determination of D/H in H2O on the surface would be of great interest for comparison with atmospheric values in CH4 and other species. It is recognized, however, that this is the most challenging landing scenario, both for Probe survival and for a good interface between the gas inlet and the surface...

"The most probable landing position is expected to be upright, which is also optimum for the instrument. In case of a landing on a liquid surface, the heated inlet tube will be submerged in the liquid, which will rapidly evaporate in the inlet tube and the vapors will flow through the inlet lines."

OK. So the odds actually look pretty good that that very hot GCMS inlet tube did come into actual contact with Titan's surface, or at any rate came so close that it very dramatically heated the surface. What are the implications? Well, this makes it easier to explain how the GCMS detected benzene on the surface, despite the latter's very high boiling point (80 deg C, exactly the same temperature as the inlet). On the other hand, it makes it more interesting that Huygens apparently detected relatively modest amounts of other substances with tremendously lower boiling points which were expected to exist there in quite large amounts: methane, ethane, acetylene. (Even solid HCN -- which theory predicted to have accumulated there in fairly large amounts -- has a boiling point of only 26 deg C; but apparently they still haven't confirmed beyond doubt that Huygens detected it on the surface at all. And while Huygens did detect a fair amount of cyanogen there, its boiling point is only -20 deg C.)

So -- if I'm interpreting the raw graphs of the GCMS data at all correctly ( http://www.nature.com/nature/journal/v438/...ature04122.html , Figure 1) -- the amounts of low-temperature volatiles on Titan's surface were much more modest than expected, but there was quite a bit of benzene there. Now, I'm not even remotely sure that I AM interpreting those GCMS graphs correctly -- if anyone out there knows more about the behavior of mass spectrometers than I do, I wish you'd take a look at them and report your own conclusions. Nevertheless, this is interesting -- especially since benzene is supposed to be one of the commonest polymers of acetylene, which was expected to exist in quite large amounts on the surface as frozen solid powder. Once again, I wonder if Titan's cryovolcanic activity isn't causing it to circulate the smog that lands at an incredibly slow rate on its surface down into Titan's interior, allowing those compounds to be exposed to the liquid water (or water/ammonia) down there and chemically modified before they are spat back up onto the surface. (In the "Nature" graphs, note all those intriguing little peaks over on the rightward part of the GCMS graphs implying the detection of some much heavier compounds on the surface.)

And, oh yeah, I can't resist annoying Alex further by reporting that while I was poking around in Google just now to find all this, I also stumbled across the fact that Prof. Erik Asphaug had reprinted my own lengthy description here of the ESA's post-landing Huygens press conference in toto to his grad students on his Solar System blog ( http://marsseminar.blogspot.com/ , Jan. 21 entry). So there: OTHER scientists appreciate me properly, Alex. Boy, will you be sorry some day you didn't treat me right. biggrin.gif It almost makes up for that little three-orders-of-magnitude mistake I've just discovered that I made in calculating Enceladus' ice loss rate... (It also kind of makes up for the fact that during my own sojourn at UCSC three decades earlier, the only class I took remotely related to planetary science was an introductory geology course in which I got an extremely lousy grade.)
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Guest_BruceMoomaw_*
post Mar 26 2006, 01:44 PM
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In this connection, one COSPAR abstract seems to say (although the phrasing is vague) that the DISR team MAY have come up with a model of Titanian surface composition that does match Huygens' puzzling near-IR spectra -- but, maddeningly, they don't say what it actually is:
http://www.cosis.net/abstracts/COSPAR2006/...006-A-02830.pdf
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Guest_PhilCo126_*
post Mar 28 2006, 03:38 PM
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Well, I'm still amazed how the Cassini-Huygens team managed to solve the issue with the frequency diffeence between the mothership and the probe ( Remember Cassini had to slow down in order to receive the Huygens data at a lower frequency ... O.K. there was another issue with chanels but they were happy to have the photos they received ! )
---O---
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The Messenger
post Mar 28 2006, 03:56 PM
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QUOTE (BruceMoomaw @ Mar 26 2006, 04:52 AM) *
So -- if I'm interpreting the raw graphs of the GCMS data at all correctly ( http://www.nature.com/nature/journal/v438/...ature04122.html , Figure 1) -- the amounts of low-temperature volatiles on Titan's surface were much more modest than expected, but there was quite a bit of benzene there. Now, I'm not even remotely sure that I AM interpreting those GCMS graphs correctly -- if anyone out there knows more about the behavior of mass spectrometers than I do, I wish you'd take a look at them and report your own conclusions.

Nice essay, Bruce.

I tried to get a more expert opinion from a GCMS chemist, (Dr. Waffle), and his answer was basically your guess is as good as mine. The problem is that the GC separates the chemicals according to both their molecular size and functionality. It is easy to break down what-is-what if you know all the controlling parameters: column length, packing material, temperature at (sample, inlet port, transfer) pressure, exit times of known standards, – and on and on. So we are really at the mercy of the Huygens GCMS specialists to produce meaningful data.

We went the rounds a little on this on Jason’s blog, but I need to revisit the possibility that the heat shield separated later than expected: First, look at the Doppler – the delta V during the first 6 – 15 minutes was much, MUCH greater than expected, and whether this was due to shear winds or other unexpected forces is academic: The 8 meter parachute was sized to provide a sufficient ‘jerk’ that a small spring action would separate the heat shield. The high rate of acceleration was not predicted, and may have glued the heat shield in place - thus the unpredictable aerodynamics and backward rotation.

If you look at the axial accelerometers (Nature article, supplementary material), the acceleration went off- scale in one direction, and stayed there for several minutes. Again, regardless of the cause, could the force causing this acceleration have buttresses the heat shield against the probe up until the second parachute jerk? Perhaps it was jammed, or weakly glued by the glassy phenolics bleeding from the heat shield. Like a dog running blinded by a frizbee held in its face by the wind, Huygens may have been sniffing her own vapors during the early part of the descent.

This would explain the warmer-than-expected readings on the “exposed” thermocouple early in the descent, and the out-of-focus ‘round cratered disk’ seen in some of the earliest Huygens visual images – this would be the heat shield finally falling away shortly after the camera started clicking ~ 15 minutes into the descent.

This would put benzene and other phenolic derivatives from the heat shield decomposition in the general vicinity of the sample port. (There was supposed to be about a thirty second delay between when the heat shield seperated and the GCMS port was exposed.) However, this could only be the primary source of benzene if the benzene peak faded rapidly before the methane peak increased, which was after landing.
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djellison
post Mar 28 2006, 04:09 PM
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Do you not think that if the descent profile were so very very far from nominal, they would have said so - if for no other reason than to say "look how different it was to what we expected, and it STILL worked great"

Anyway - the data is due into the PDS in a few months as I understand it - we can have fun then smile.gif

Doug
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The Messenger
post Mar 28 2006, 05:05 PM
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QUOTE (djellison @ Mar 28 2006, 09:09 AM) *
Do you not think that if the descent profile were so very very far from nominal, they would have said so - if for no other reason than to say "look how different it was to what we expected, and it STILL worked great"

Anyway - the data is due into the PDS in a few months as I understand it - we can have fun then smile.gif

Doug

Thanks, I will look forward to that!

All oddball physics asside, benzene presents a curious issue: If it is in the atomosphere, it implies venting - a heating source well above the surface temperature of Titan. If it was only detected in the surface measurements, as Bruce explained, the heated inlet tube could have extracted it from the surface matrix. (In which case, it could even be a fairly major consituent of the surface.)

But in either of these scenarios, if we found benzene, we can write-off ammonia and more volatile organics, at least in the vicinty of where the Huygens probe landed. Otherwise, they should have shown up along with the benzene. Since Benzene is a known product during pyrolisis of heat shield materials, it would be good if this possible source of contamination can be completely eliminated.

Edited to add:

I missed the significance of this on first reading of Niemann et al:

QUOTE
In principle, the water-ice value could have been measured directly by the GCMS after impact, if the end of the inlet tube had been in direct contact with surface ice. Evidently this was not the case, because the GCMS did not detect any H2O vaporized from the surface after probe impact.


The Vapor pressure of Benzene is pretty close to water (BP 80c), so if benzene was extracted from the surface, if ice water was also on the surface where Huygens landed, it should have been vaporized and detected by the GCMS as well.

The article clearly indicates Benzene was detected on the surface, but the article only contains the spectra averaged over the entire time the probe was on the surface. Did the benzene fraction curve behave like the methane concentration, or did it follow a different trend? Is the benzene curve consistent with a surface extraction? If it is, it is also reasonable to conclude there is no water ice directly beneath the GCMS inlet port!
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Guest_BruceMoomaw_*
post Mar 28 2006, 08:07 PM
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"In principle, the water-ice value could have been measured directly by the GCMS after impact, if the end of the inlet tube had been in direct contact with surface ice. Evidently this was not the case, because the GCMS did not detect any H2O vaporized from the surface after probe impact."

You know, I totally missed that passage when I read Niemann. Gadfry. Now I'll have to rethink everything I said on the subject.
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The Messenger
post Mar 29 2006, 08:47 PM
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I talked to Dr. Waffle again last night, about heat shield pyrolysis products – one of his specialties. Virtually all heat shields are fabricated with phenolic impregnated resins. Out-gassng of these resins starts at just above boiling, with the release of trapped water vapor and some light hydrocarbons. As the temperature increases, aliphatic phenols (tars) start to emerge, along with ‘a lot’ of gaseous hydrogen. As the temperature increases, the tars break down, releasing benzene and other, lighter derivatives, including acetylene and elemental carbon.

From the Nature articles, it is clear Huygens underwent unexpected accelerations and forces not yet fully characterized in the upper atmosphere. I have not seen any speculation that the heat shield did not properly detach (other than my own), but since benzene and acetylene are showing up where they are not expected, and there is quantitative disagreement between the Huygens’ isotope ratios in Titan’s atmosphere and ratio’s calculated from spectral analysis, I think the possibility of heat shield residue contamination of the GCMS port cannot be totally discounted.
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