[X] My Assistant

[jsheff]

Have you seen these stunning descent images from Huygens?

Huygens views

[volcanopele]

I started a new thread with this info, so I am just going to merge it into this one. Here was my original post:

A whole slew of Huygens DISR views have been released on the Planetary Photojournal:

http://photojournal.jpl.nasa.gov/new

The big release is a movie showing the decent to the surface of Huygens using images from DISR, colorized by DISR spectrometer data. Views from different altitudes and projections were also released as well as views of the surface that have gone through more rigourous processing in the last few months.

This data release by DISR, I believe, is part of, maybe not quite a special issue, but at least a series of articles on Titan coming very soon in a well known journal that puts new issues online on Thursdays...

EDIT: Never mind. Completely unrelated.

[Rakhir]

ESA press release

http://www.esa.int/esaSC/SEMKVQOFGLE_index_0.html

[ljk4-1]

How long before there is a similar print on Titan?

http://photojournal.jpl.nasa.gov/catalog/PIA08115

[helvick]

The big release is a movie showing the decent to the surface of Huygens using images from DISR, colorized by DISR spectrometer data.

Now that is extremely slick. Anyone know if a higher res movie is going to be made available?

[volcanopele]

A high-resolution version of the movie (both silent and with sound), can be found at http://www.lpl.arizona.edu/DISR/Multimedia/Titan_Movies.htm. Please note that these movies are in Windows Media Video format.

[JRehling]

How long before there is a similar print on Titan?

http://photojournal.jpl.nasa.gov/catalog/PIA08115

Oh, it's covered with them -- Huygens just didn't land near one.

[paxdan]

awesome doesn't do justice. It is a hell of a set of products.

[helvick]

A high-resolution version of the movie (both silent and with sound), can be found at http://www.lpl.arizona.edu/DISR/Multimedia/Titan_Movies.htm. Please note that these movies are in Windows Media Video format.

I had problems playing these with Windows Media Player 10 despite the fact that they are WMV format - VLC to the rescue

[ElkGroveDan]

How long before there is a similar print on Titan?

http://photojournal.jpl.nasa.gov/catalog/PIA08115

Like this?

Attached thumbnail(s)

 

[Guest_Richard Trigaux_*]

What is interesting is that at last they suceeded in processing the complete image, including the one of the bottom camera, which was very hard because of uneven lighting. So we have the complete view now.

[tedstryk]

Massive release on the photojournal.

http://photojournal.jpl.nasa.gov/catalog/PIA08117
http://photojournal.jpl.nasa.gov/catalog/PIA08118
http://photojournal.jpl.nasa.gov/catalog/PIA08116
http://photojournal.jpl.nasa.gov/catalog/PIA08115
http://photojournal.jpl.nasa.gov/catalog/PIA08114
http://photojournal.jpl.nasa.gov/catalog/PIA08113
http://photojournal.jpl.nasa.gov/catalog/PIA08112
http://photojournal.jpl.nasa.gov/catalog/PIA08427
http://photojournal.jpl.nasa.gov/catalog/PIA08119

Here is my Phil-O-Vision take on the after-landing image.

[tfisher]

Interesting: in the descent movie you can see they used an elevation model to add relief. I'm surprised though by how much they have the light colored 'islands' standing out above the surrounding dark colored 'sea'. I had always pictured that being much flatter looking at the images.

[The Messenger]

http://photojournal.jpl.nasa.gov/catalog/PIA08117

"During its descent, the Descent Imager/Spectral Radiometer took 3,500 exposures. "

???
There were about 600 visual images in the raw image catalogue. If half the exposures were lost with channel A, does that mean that there were ~1000 spectral images?

[Guest_BruceMoomaw_*]

Most of those "exposures" were simply spectra -- not images of any sort. (I've even seen an exact count of them somewhere, although I can't remember where.) Don't forget that a single CCD array was used for all of DISR's data: images, spectra, and even photometry.

[Matt]

How long before there is a similar print on Titan?

http://photojournal.jpl.nasa.gov/catalog/PIA08115

Oh, it's covered with them -- Huygens just didn't land near one.

J, although you don't actually know it, there is a deckchair and a rolled up newspaper just out of shot in the huygens surface image....

[Bob Shaw]

J, although you don't actually know it, there is a deckchair and a rolled up newspaper just out of shot in the huygens surface image....

And the surface below Huygens is actually a towel, kindly left there by a German tourist. I still say it's always best if you know just where your towel is!

Bob Shaw

[ugordan]

Looking at another thread showing the surface of Venus as it would appear under Earth-like illumination conditions, I was wondering if the same could be done to the Huygens surface image? Obviously, it would still be a uniformly hued image, but would the surface actually look any different if the atmosphere weren't filtering specific wavelengths out? Is the scenery orange due to the atmosphere or accumulated organics (tholins, ...) or both?

[The Messenger]

Is the scenery orange due to the atmosphere or accumulated organics (tholins, ...) or both?

Iron-rich sand is my guess - very Mars-like. We should have a better handle, constraints upon composition from the polarized bistatic radar after T-14 this week.

[ngunn]

While we're still free to guess here's mine. Iron-rich sand seems rather too dense a material (if you mean silicate sand, that is) to be blowing around on the surface of an ice-world. I'd go with some kind of fluffed-up organic material - sort of sooty micro-snow with a nanostructure that does funny things to the reflection spectrum. The colour question is fascinating too. I go for most of the orange/brown colouration being due to the ambient lighting, which would make it very hard to distinguish between hues of grey and brown in the surface material itself.

[Bob Shaw]

While we're still free to guess here's mine. Iron-rich sand seems rather too dense a material (if you mean silicate sand, that is) to be blowing around on the surface of an ice-world. I'd go with some kind of fluffed-up organic material - sort of sooty micro-snow with a nanostructure that does funny things to the reflection spectrum. The colour question is fascinating too. I go for most of the orange/brown colouration being due to the ambient lighting, which would make it very hard to distinguish between hues of grey and brown in the surface material itself.

Yup. I can't see any reason for much in the way of iron, or sand as such - but cryogenic 'sooty micro-snow', packed full of vitamins and goodness (I jest!), certainly.

Bob Shaw

[ngunn]

Yup. I can't see any reason for much in the way of iron, or sand as such - but cryogenic 'sooty micro-snow', packed full of vitamins and goodness (I jest!), certainly.

Bob Shaw

Indeed - a land of milk and honey (caramelised) . . .

[Guest_BruceMoomaw_*]

Bar-Nun's pre-landing lab tests repeatedly concluded that virtually all of Titan's smog particles completely harden (due to cross-linking between their organic molecules) into totally hard, non-sticky dust grains very quickly after their formation, and long before their descent onto the surface. An interesting question is whether liquid methane rain can dissolve any of the smog's materials even a little bit to stick the grains together -- or, whether, when the methane dries out completely, the landed organic smog turns back into a totally powdery and loose, extremely fine powder.

[ngunn]

Some more interesting questions:- How much of the dark stuff is brought down by rain and how much comes down dry? Does a rainstorm clear the air creating a temporary and localised blue sky? If so would rare periods of comparatively unfiltered sunlight and/or clear nights have observable effects on the surface?

[Guest_BruceMoomaw_*]

It may clear it to some extent -- but the raindrops form at a much lower altitude than most of the haze, so it probably does only a very modest job of clearing the total amount of haze.

It does now appear -- given the incredibly slow rate at which haze particles actually form in Titan's air -- that virtually all of them get carried down to the surface in raindrops rather than just drifting slowly down to the surface in a dry state, which may very well have implications for the extent to which they stick together after landing. (They accumulate on the surface as a total layer of just a few microns per century.)

[The Messenger]

Bar-Nun's pre-landing lab tests repeatedly concluded that virtually all of Titan's smog particles completely harden (due to cross-linking between their organic molecules) into totally hard, non-sticky dust grains very quickly after their formation, and long before their descent onto the surface. An interesting question is whether liquid methane rain can dissolve any of the smog's materials even a little bit to stick the grains together -- or, whether, when the methane dries out completely, the landed organic smog turns back into a totally powdery and loose, extremely fine powder.

All well and good, but the color is wrong. On second thought, not so well and good: heavier oils dissolved in methane shouldn't ball-up, they should normally harden into tars and heavy sludgy oils. There may be some thios and NCO's and such, but they should show up clearly in the IR. Larry Soderblom made it clear in his early presentations that water-ice as we know it cannot be dominating the surface of Titan. So we have orange where there should be black, dusty where it should be waxy, and only traces of ammonia. We teased methane and possibly benzene out of the soil, but not water and ammonia. Either the chemistry is not understood, or the materials are different from what was expected. Or both.

[ngunn]

It may clear it to some extent -- but the raindrops form at a much lower altitude than most of the haze, so it probably does only a very modest job of clearing the total amount of haze.

It does now appear -- given the incredibly slow rate at which haze particles actually form in Titan's air -- that virtually all of them get carried down to the surface in raindrops rather than just drifting slowly down to the surface in a dry state, which may very well have implications for the extent to which they stick together after landing. (They accumulate on the surface as a total layer of just a few microns per century.)

Thanks for this very useful picture. Just one thing about the height of rain formation - my impression of the white clouds seen earlier was that they were pretty tall affairs, even compared with the large scale height of Titan's atmosphere. Richard suggested a plausible convection mechanism arising from the lower density of methane-rich air. Unlike the case of thermal convection a rising column of intrinsically less dense air would not simply cool and equalise its density with the surroundings but presumably go on rising until most of the methane had condensed out. As condensation nuclei are scarce, especially lower down, I imagined this happening quite high up, perhaps directly to the solid state as snow or hail which would then melt on the way down. Am I completely wrong about this? (If so a one word answer will suffice!)

[Guest_BruceMoomaw_*]

I would now be very hesitant to say that we didn't pick up any traces of water or ammonia evaporated from the surface. We could identify benzene very easily because of its particular AMU (78), which had no other compounds with similar AMUs to confuse the results -- but, when you look more carefully at Niemann's graph, the actual amount of the stuff that Huygens detected evaporated out of the surface was very tiny. The same is true for cyanogen (52 daltons). By contrast, water and ammonia both have AMUs very close to the very strong, dominating 16-dalton peak for methane -- and acetylene and HCN similarly have peaks very close to the even stronger 28-dalton nitrogen peak.

So it may have been seriously premature for me to say what I was saying earlier -- namely, that really freakish amounts of cyanogen and benzene were found on the surface while there were shortages of the other expected compounds. It may well just be that those expected compounds really do exist in much bigger amounts on Titan's surface, but were simply concealed from clear GCMS detection by Titan's CH4 and N2. I still haven't talked to Niemann himself on this.

Meanwhile, some French lab simulations have -- if I remember correctly -- found that tholins containing nitrogen tend to be orange, while those just consisting of hydrocarbons tend to be black. I'll track this abstract down.

Thanks for this very useful picture. Just one thing about the height of rain formation - my impression of the white clouds seen earlier was that they were pretty tall affairs, even compared with the large scale height of Titan's atmosphere. Richard suggested a plausible convection mechanism arising from the lower density of methane-rich air. Unlike the case of thermal convection a rising column of intrinsically less dense air would not simply cool and equalise its density with the surroundings but presumably go on rising until most of the methane had condensed out. As condensation nuclei are scarce, especially lower down, I imagined this happening quite high up, perhaps directly to the solid state as snow or hail which would then melt on the way down. Am I completely wrong about this? (If so a one word answer will suffice!)

I doubt that this effect is any more important for Titan than for Earth. We're talking about just a few percent of methane in Titan's air (and just a few percent of water vapor in Earth's air), and the density difference between that gas and the dominant gases in the air is similar for both worlds. In both cases, I think we're simply dealing with a wet-adiabat phenomenon -- that is, humid air rises until it's reached a cool enough level that its condensable gas (CH4 or H2O) condenses into liquid, releasing a fresh burst of stored heat energy that rewarms the air to loft it still higher. But this phenomenon must be less dramatic for Titan than for Earth simply because water releases much more heat energy when it condenses than methane does. And Titan's clouds are still very low in its overall atmosphere (about 12 km, I believe).

J, although you don't actually know it, there is a deckchair and a rolled up newspaper just out of shot in the huygens surface image....

I remember predicting in 1969 that the first thing Neil Armstrong would see on the Moon would be a beer can.

[ngunn]

I think we're simply dealing with a wet-adiabat phenomenon -- that is, humid air rises until it's reached a cool enough level that its condensable gas (CH4 or H2O) condenses into liquid, releasing a fresh burst of stored heat energy that rewarms the air to loft it still higher. But this phenomenon must be less dramatic for Titan than for Earth simply because water releases much more heat energy when it condenses than methane does. And Titan's clouds are still very low in its overall atmosphere (about 12 km, I believe).

Thanks, that answers my question completely as regards the observed clouds and their inability to 'clear the air'. But here's another (completely hypothetical) query: Suppose a cryovolcano released a large plume of almost pure methane into Titan's atmosphere. Might it not rise considerably higher than 12 km before condensing? If the plume was initially warm enough to contain water vapour too then presumably the rapid condensation of the water would provide even more heat to drive it up.

[Bob Shaw]

Thanks, that answers my question completely as regards the observed clouds and their inability to 'clear the air'. But here's another (completely hypothetical) query: Suppose a cryovolcano released a large plume of almost pure methane into Titan's atmosphere. Might it not rise considerably higher than 12 km before condensing? If the plume was initially warm enough to contain water vapour too then presumably the rapid condensation of the water would provide even more heat to drive it up.

Oddly enough, today's 'New Scientist' has a discussion of a similar phenomenon on Earth - I'll scan the images later!

Bob Shaw

[Guest_BruceMoomaw_*]

First, on the altitude of Titan's clouds, see Sushil Atreya's piece in the Feb. "Planetary and Space Science" ( http://www-personal.umich.edu/~atreya/Arti...tan_Methane.pdf ). Huygens' GCMS pretty firmly detected a very rarified methane cloud layer at 8 to ~20 km altitude. (Tomasko reports the Side-Looking Imager seeing an apparent thin cloud layer at 21 km altitude.) "A lack of widespread cloud activity at these latitudes [Huygens' 10 deg S. landing site] is consistent with it being a dry season in this region. Titan's southern summer solstice was in October 2002, with each season on Titan lasting roughly 7.4 years. Clouds extending from approximately 20 km to 46 km are now being detected at mid-southern latitudes of 37 – 44 deg S, with the Cassini Visual and Infrared Mapping Spectrometer and from the Keck Telescope as the southern summer moves northward." (pg. 7) Even that maximum altitude is still way below the top altitude of significant air density and organic smog formation on Titan -- the tropopause is fully 44 km up, at a pressure level of about 0.1 bar, and Titan's scale height is about 50 km. (You're quite right, however, that a cryovolcanically produced cloud, by dint of being much warmer than anything else in Titan's atmosphere, could soar to very high altitudes, just as they do on Earth.)

As for the color of Titan's organic compounds: I'm still looking up material on that, but see http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2183.pdf on a recent study showing the two different-colored types of tholins produced in lab tests (and even including a nice color photo). There is a hell of a lot of orange tholin material produced by virtually all lab experiments. I was wrong about the black ones being nitrogen-free, though -- their differences are subtler than that -- but there does seem to be some indication that tholins with higher nitrogen content often tend (depending on their formation mechanism) to be redder ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2105.pdf , and pg. 263-64 of http://nai.arc.nasa.gov/institute/general_...bstractBook.pdf ). Note also that frozen acetylene, and acetylene polymers, can sometimes be whitish or even "silvery" in color.

So, colorwise, we're dealing with very complex phenomena, but there is no apparent clash between what Huygens has told us about Titan's surface chemistry and the fact that it's covered in orange stuff.

[ngunn]

Fascinating stuff, thanks Bruce. Just one nit-pick: I thought that scale height meant the one-over-e point of the exponential decrease of pressure. By that reckoning the scale height for Titan would be about 20km. If as you say the pressure at the height of the cloud tops is only around 0.1 bar then the clouds are indeed towering through over 90 percent of the mass of the atmosphere. The equivalent on Earth would be a cumulus cloud reaching about 20 km up, which they just don't do - except over volcanoes.

[helvick]

Fascinating stuff, thanks Bruce. Just one nit-pick: I thought that scale height meant the one-over-e point of the exponential decrease of pressure. By that reckoning the scale height for Titan would be about 20km.

I seem to recall that the scale height for Titan's atmosphere is often listed as 50km - this 2003 AIAA\NASA paper on Titan Aerocapture Guidance Systems by Masciarelli and Queen puts it at 42.5-53km, compared to 9km on earth. Assuming that is true then the pressure at 50km would be 0.55bar and ~63% of the atmospheric mass would be below that height.
If the pressure is 0.1 bar then the altitude of the cloud tops would need to be at around 135km. So one or other of the numbers is incorrect.

[ngunn]

I think someone is using a definition of scale height based on 1 over 10 instead of 1 over e. By my reckoning the scale height for Titan is only about 2.5 x Earth's whichever definition you use.

[remcook]

The scale height changes with altitude, since temperature changes as well. At low altitudes (clouds), the scale height is roughly 20 km, whereas at high altitudes (aerocapture) it's more like 50 km. So both are probably right.

[Guest_BruceMoomaw_*]

Yep, I did get the scale height figure for Titan from a document on aerocapture. Not once did it occur to me that scale height depends on atmospheric temperature. *sigh*

[helvick]

I think someone is using a definition of scale height based on 1 over 10 instead of 1 over e. By my reckoning the scale height for Titan is only about 2.5 x Earth's whichever definition you use.

Good point. I went digging and found a formula to check it and realised just how temperature sensitive pressure scale height is. Plugging in the numbers into H=kT/mg (Scale height = Boltzman const * Temperature/(molecular mass of atmosphere * gravitational acceleration) gives a scale height of 20km at the surface (T=94K) however ths Huygens atmospheric temperature profile shows the temperature varying from around 75K to 210K depending on altitude - that seriously mucks around with scale height, pushing it from a minimum of 16 to a maximum of 46.

All I can say is good luck to the atmospheric modelling folks.

[Bob Shaw]

Here are the images from today's New Scientist showing a plume punching through a temperature inversion, as observed from above.

I don't know about Titan, but Triton certainly springs to mind - and perhaps Pluto.

Bob Shaw

Attached thumbnail(s)

 

[ngunn]

Good point. I went digging and found a formula to check it and realised just how temperature sensitive pressure scale height is. Plugging in the numbers into H=kT/mg (Scale height = Boltzman const * Temperature/(molecular mass of atmosphere * gravitational acceleration) gives a scale height of 20km at the surface (T=94K) however ths Huygens atmospheric temperature profile shows the temperature varying from around 75K to 210K depending on altitude - that seriously mucks around with scale height, pushing it from a minimum of 16 to a maximum of 46.

All I can say is good luck to the atmospheric modelling folks.

Look again. The big temperature swings are all well above the tropopause and hence well above the 1 over e point. In fact the temperature in the first 20km is actually lower than the surface temperature, which would tend to LOWER the 1 over e point slightly from the simplistic value based on constant T. The upper atmosphere is indeed much inflated due to both temperature and inverse square law for g, but that doesn't affect the lower atmosphere up to the 1 over e point.

[helvick]

Yep - you're dead right there. The scale height in the region under discussion is 20km or lower which puts the 0.1 bar level at 24-30km.

Ah well - at least I have a better understanding of it now - it's good to be wrong.

[djellison]

Here are the images from today's New Scientist showing a plume punching through a temperature inversion, as observed from above.

I saw the very same thing on a flight from East Midlands to Malaga, I think possibly Didcot Powerstation was to blame.

Doug

[ngunn]

Yep - you're dead right there. The scale height in the region under discussion is 20km or lower which puts the 0.1 bar level at 24-30km.

Ah well - at least I have a better understanding of it now - it's good to be wrong.

Almost there now The scale height is around 20 Km which puts the 0.1 bar level at around 50 km (0.1bar is about one 15th of surface pressure which is roughly one over e to the 2.5).

[The Messenger]

Another curiosity in this report is the electrical conductivety:

http://cisas.unipd.it/hasi/welcome.html

Notice that it is absolutely flat from 40km to the surface, then increases at about the same time that the Huygens GCMS picked up an increase in methane. Methane is so non-polar, I don't see why this would happen, especially if there really is no change in conductivity from 40km to the surface, where the methane mole count should change quite drastically.

So if all the altitude information is correct; it would seem that some molecule other than methane was released from the surface after landing - something more polar, like ammonia or water.

[Guest_BruceMoomaw_*]

Quoting the HASI team's article in the Dec. 8 "Nature" (which is open-access -- http://www.nature.com/nature/journal/v438/...nature04314.pdf ):

"The complex permittivity of the surface material is measured after impact with the PWA mutual impedance probe, at five frequencies. As a first estimation, the mean relative permittivity within the sensor range (radius 1m, depth 2m) is of the order of 2, in reasonable agreement with the measurements performed with the radar on board Cassini...

"The measured relative permittivity (of the order of 2) constrains the soil composition. No evidence for the presence of liquid phase on the surface was returned by the signal of the radar altimeter."

I seem, however, to recall seeing somewhere recently an article that actually DID express puzzlement at the high electrical conductivity of Titan's surface as measured by HASI. This could easily be a false memory on my part -- it's a very faint recollection -- but I'll hunt around a little.

The "Nature" article also says a bit more about Huygens' radar altimeter:

"In addition to providing altitude (Fig. 8), the Radar Altimeter measures the signal backscattered within the footprint of the beam, whose diameter is 0.14 times the altitude. This signal is strong and smooth with small variations over the ground track, indicating a surface with little relief. The atmosphere was scanned and return signal from droplets was searched for, but no significant signature of rain could be found." That beam is probably wide enough to explain why the deep gullies seen by DISR didn't show up in the radar altimetry. Note also, however, the sonar altimetry from the SSP package during the final part of Huygens' descent ( http://www.nature.com/nature/journal/v438/...nature04211.pdf ):

"The Acoustic Properties Instrument–Sonar (API-S) recorded the approach to the surface on final descent (Fig. 1). API-S is a pulse send–receive sonar, where the time of flight gives distance (and hence final descent speed). The probe vertical speed just before landing was determined as 4.60+0.05m/sec. The peak width and signal strength are influenced by surface topography, probe position and acoustic reflectivity according to the usual radar equation for an extended target.

"As Huygens descended towards the surface the sensor footprint shrank, and a smaller area of terrain was illuminated. Owing to variation in probe tilt and wind drift during descent, the sensor illuminated different areas of ground for each pulse, with partial overlap. Initial derivation of surface acoustic reflectivity shows no significant variation as a function of altitude, implying that the landing site as seen by Huygens is typical of the local surroundings (the maximum area sampled by API-S, for the highest altitude of around 90 m, is approximately a circle of 40 m diameter).

"For all returns the peak widths are typically 30–50 milliseconds wide, showing no trends. This implies that the surface is topographically similar over all sampled beam footprints. However, this width is greater than would be expected for a purely flat surface, implying that some small-scale vertical topography is present.

"The final peak immediately before impact is at a height of 14.4 m at the time of pulse transmission, with a beam footprint of ~26 square meters (equivalent to a circle of ~2.9 m radius). This final peak is recorded by the SSP at higher time resolution than previous ones, giving more information on surface structure (Fig. 1 inset). The relatively broad shape of this peak indicates that the surface cannot be completely flat, or concave over the footprint. However, the flat top of the peak also requires that there be some local height variation over the surface sampled within the footprint. Rock size determined from the postlanding surface images will provide a good starting point for further collaborative analysis.When averaged to a lower time resolution, the width of the final peak is entirely comparable to the width of the higher-altitude peaks, implying that they are seeing very similar terrain.

"Together these data suggest a surface that is relatively flat but not completely smooth; such an interpretation is compatible with the Descent Imager and Spectral Radiometer (DISR) surface images, suggesting that perhaps the DISR images show a typical surface that probably surrounds the probe in all directions. The fact that slight horizontal and vertical topographic variation is seen over the footprints, rather than a completely flat plain, implies a certain level of complexity during the history of surface formation in the region of the landing site."

[Guest_BruceMoomaw_*]

Here's the peculiarity noted after landing by the electrical sensors of HASI -- mentioned in two places on the Web. First, Emily mentions it in her blog entry on the first day of last September's DPS meeting ( http://www.planetary.org/blog/article/00000227/ ):

"There were two little mysteries that Marcello [Fulchignoni] mentioned, but didn't explain. First, his instrument included a permittivity sensor. Permittivity is one physical property you can measure for a substance, having to do with how much it resists the flow of an electric charge. Well, HASI recorded an increase in the permittivity of the Titan surface at 12 minutes after impact. They don't know what it means, and are working first to rule out anything that might be coming from within Huygens to cause that observation. Second, he mentioned that they MAY be seeing some electrical discharge events (read: lightning) in their data. Again, though, they must first rule out anything that was happening aboard the spacecraft as the cause."

The same thing is mentioned on a Powerpoint slide at last September's Titan Conference ( http://www.lpl.arizona.edu/titanconference...ts2/wrk.062.ppt ), with a graph:

"The study of the dielectric properties of materials in the ELF range provides useful information about the surface of planetary environments. Furthermore, the permittivity of many solids and liquids is function of frequency and temperature. From Figure 4, it is possible to identify the impact on the surface and the variation of both amplitude and phase shift. Furthermore, 12 min after landing, a sudden variation both in phase shift and amplitude has been observed, more visible on the phase shift and at lower frequencies...

"The nature of the event observed 12 min after impact on the surface is not understood. Several possibilities are studied, namely mechanical, electrical and surface properties variations."

OK. Was any other event seen by Huygens at that time? Well, there seems to have been no shift in the rate of the very slight, slow change in the craft's post-landing tilt angle at that time -- and, while, the methane level sensed by the GCMS rose dramatically during the first 2 minutes after landing, it then levelled off completely again for the rest of the probe's stay on the surface ( http://www.nature.com/nature/journal/v438/...nature04122.pdf , Fig. 3).

But: there is one other change that does show up at about the same time -- which can be seen on pg. 21 of Dan Harpold's presentation on the GCMS results at last September's Harsh-Environment Mass Spectrometry Workshop ( http://cot.marine.usf.edu/hems/workshop/Wo...day/Harpold.pdf ): the amount of ethane detected started rising at about the same time, and the amount of CO2 just a few minutes later. I have no idea what this might signify, but it is definitely worthy of note.

[Guest_BruceMoomaw_*]

And here's some more recent stuff on the last-second altitude measurements from Huygens' sonar sensor ( http://www.lpi.usra.edu/meetings/lpsc2006/pdf/1567.pdf ):

"The terrain derived indicates a terrain height variation of 1-2 m over 20 m or so [horizontal distance], in agreement with DISR imagery. Such a terrain is geologically plausible given the descent and surface imagery as seen in figures 3 and 4, and may represent hummocks or channels (the wavelength seen is comparable with terrestrial analogues) –- however, it is important to note that vertical variation in values derived are within the error estimates for the sensor, and should not be overinterpreted."

[ngunn]

The only probe-induced event I can think of is downward-conducting heat reaching some significant layer in the subsuface after 12 minutes.

If it's not probe related we'd have to assume that it's something electrical that happens at this location pretty frequently - ground currents ?? Does Titan have underground 'weather' too ???

[ljk4-1]

Here's the peculiarity noted after landing by the electrical sensors of HASI -- mentioned in two places on the Web. First, Emily mentions it in her blog entry on the first day of last September's DPS meeting ( http://www.planetary.org/blog/article/00000227/ ):

"There were two little mysteries that Marcello [Fulchignoni] mentioned, but didn't explain. First, his instrument included a permittivity sensor. Permittivity is one physical property you can measure for a substance, having to do with how much it resists the flow of an electric charge. Well, HASI recorded an increase in the permittivity of the Titan surface at 12 minutes after impact. They don't know what it means, and are working first to rule out anything that might be coming from within Huygens to cause that observation. Second, he mentioned that they MAY be seeing some electrical discharge events (read: lightning) in their data. Again, though, they must first rule out anything that was happening aboard the spacecraft as the cause."

Does Cassini have any way to detect lightning on Titan? Would it be the same
as detecting lightning on Saturn?

[The Messenger]

It is curious that there was not a significant change at the time that the probe first contacted the ground - as a general rule, any enviromental change will effect permittivity - although the sensitivity in any given medium is highly correlated with the resonant frequency.

Permittivity is a notoriously fickle measurement, so battery drain, or thermal equilibrium within Huygens could be systemic show stoppers that produced the peak. Another possibility, though, is that the tip of the permittivity probe was deep enough that it was recording an increase in ground temperature due to conduction from Huygens, or perhaps due to the lamp, (but I think the surface the lamp was some distance from the permittivity probe).

[Bob Shaw]

It is curious that there was not a significant change at the time that the probe first contacted the ground - as a general rule, any enviromental change will effect permittivity - although the sensitivity in any given medium is highly correlated with the resonant frequency.

Permittivity is a notoriously fickle measurement, so battery drain, or thermal equilibrium within Huygens could be systemic show stoppers that produced the peak. Another possibility, though, is that the tip of the permittivity probe was deep enough that it was recording an increase in ground temperature due to conduction from Huygens, or perhaps due to the lamp, (but I think the surface the lamp was some distance from the permittivity probe).

One of the joys of this forum is the exposure to new things: 'permittivity' is not a word I'd ever come across before!

http://en.wikipedia.org/wiki/Permittivity

Bob Shaw

[Guest_BruceMoomaw_*]

Does Cassini have any way to detect lightning on Titan? Would it be the same
as detecting lightning on Saturn?

Yes, they can listen to possible emissions from lightning bolts (the famous "whistlers") using its RPWS experiment. Voyager 1 did so during its flyby and heard nothing, but Cassini of course is far more sensitive and can listen for longer periods -- although I don't know whether it has yet heard anything suggestive. (This may depend on whether and when it's finally lucky enough to fly directly over one of Titan's rare local concentrations of rainstorms.) They are nowhere near certain whether the emissions that HASI heard during Huygens' descent had anything to do with lightning dischrages, or were simply other types of electric-current phenomena in Titan's atmosphere.

[edstrick]

Obviously, the permittivity change and the gas emissions seen 12 min after landing were the result of a titanian cow wandering up to stand on the upwind side of the spacecraft, out of view of the camera.

[Guest_BruceMoomaw_*]

Either that, or that penguin who turns up in a couple of the photos was unusually flatulent.

[The Messenger]

I ran the permittivity question past a guru, who has actually designed and built permittivity sensors. He confirmed that it is highly unlikely that there would be no change in permittivity as the atmosphere thickened, and virtually no chance the permittivity did not change when the probe entered the 'soil'. He did say that at high impedences, the 'real' component may not change, but the 'imaginary' should. So this leaves at least three possibilities:

1) The ESA is reporting only the real phase, this would mean the increase ~15 minutes after landing could be the result of a decrease in the resistance of the ground, and/or, lightning or some other ionizing event.

2) The probe is malfunctioning - (There was at least one ESA article where they speculated that one of the probes was bent, and I think it was this one.)

3) The timing on the X axis is off. I doubt this - as I have perused other charts, it appears to me that if the ESA has any doubt as to the time frame, they leave clocking off of the chart.

4) The probe was not sensitive enough to detected weak disturbances.

5) After ~15 minutes, the Titanian found the probe unpleasant

[JTN]

An idle query:
In the narrated video release which started this thread, just after the landing, the narrtor says

Some seconds after impact, the shadow of Huygens' parachute drifts across the scene.

Is this just artistic licence, or is the parachute's shadow actually evident in the returned data?

[tfisher]

An idle query:
In the narrated video release which started this thread, just after the landing, the narrtor says
Is this just artistic licence, or is the parachute's shadow actually evident in the returned data?

Artistic licence.

[edstrick]

Since the sun is minimally (if at all) visible at RGB wavelengths, any shadows would be very diffuse, corresponging to the amount and direction of skylight blocked. That's somewhat directional, since the horizion will be darker than directly above (due to absorbtion of light by the haze and it's high optical thickness). Basically, shadows will be like those on a cloudy, stratus-clouds day on Earth.

[ngunn]

I've spent some time trying to imagine how a human observer would perceive the quality of light on Titan (with a view to possibly producing a painting at some point). Since longer wavelengths are scattered less than short I had expected the sun to appear as a fuzzy bright red spot in the middle of a larger duller orangish glare, in turn surrounded by a brown sky. I was delighted to see it depicted in the new Huygens movie as appearing red, but surprised that they show it with a sharp edge. (A sharp edge would mean some red light gets through the atmosphere unscattered - but then surely our views from Cassini would be sharper?) Anyhow if red light travels the most directly then the eye ought to perceive any diffuse shadows as having a distinct greenish tinge. For this reason alone I think someone standing on Titan would enjoy a more colourful spectacle than the renderings show, especially when the sun is at somewhat lower elevations. Add to that the scope for variety of colour among the various icy and organic surface materials below the resolution of the Huygens instruments and interesting images come to mind. When we get a better look I think we'll find that Titan is a far from monochrome world.

[ugordan]

I guess it all comes down to contrast. The sun is much, much brighter than dark space. Even through all that atmospheric scattering, it's plausible it would still have a pretty well-defined limb (even if surrounded by a very diffuse red glow and even more diffuse shorter wavelength glow). Titan's atmosphere is hardly a perfect 'blur' filter, a photon has a chance of coming through unscattered. The chance getting better and better with increasing wavelength. Shove a huge number of photons (such as from a bright, starlike object) and a good number of them might pass unscattered. Naturally, this would also lead to a bright diffuse glow as well.
Cassini's images through the CB1 filter (which is about centered on red) already hint at the surface of Titan which, arguably, is much less contrasted than the sun appearing against a pitch-black sky.
There's a paper somewhere by the CICLOPS team trying to model the haze and predict the visibility of surface features to ISS, pre-arrival of course. I'll try to dig it out to see if they have better models for light transmission than this 'chance' stuff.

[ngunn]

Shove a huge number of photons (such as from a bright, starlike object) and a good number of them might pass unscattered.

I'd really like to know if this is true for any part of the visible spectrum. That would make the stars visible too (to sufficiently sensitive eyes), at least from the outward facing hemisphere at night.
I take your point about contrast - if we had a very bright source of light on the surface Cassini could perhaps resolve it as a point source but in reality we have scattered light coming from neighbouring areas swamping the direct image of each point.

If a measurable fraction of the photons do penetrate the atmosphere unscattered perhaps it would be possible to scan the night-time surface with a laser and use some kind of polarisation trick to filter out the scattered photons. That could yield a higher resolution albedo map from orbit than direct daylight photography. It would also reveal specular reflection from even quite small pools of liquid.

[djellison]

It's a cloud day outside - your average large dustbin lid isn't going to cast any visible shadow when it's more than about 2m off the ground. I find it near impossible to believe that in even more hazy conditions Huygens would have caught an image of its own shadow. PERHAPS it's own heatshield after jetison if images were taken soon enough after that, but certainly not it's own shadow...it would have had to be only a few M above the ground, and I don't believe even the last image before impact shows that.

Hopefully we'll see some Huygens data on the PDS this summer, and we can be more informatively speculative

Doug

[The Messenger]

I'd really like to know if this is true for any part of the visible spectrum. That would make the stars visible too (to sufficiently sensitive eyes), at least from the outward facing hemisphere at night.
I take your point about contrast - if we had a very bright source of light on the surface Cassini could perhaps resolve it as a point source but in reality we have scattered light coming from neighbouring areas swamping the direct image of each point.

If a measurable fraction of the photons do penetrate the atmosphere unscattered perhaps it would be possible to scan the night-time surface with a laser and use some kind of polarisation trick to filter out the scattered photons. That could yield a higher resolution albedo map from orbit than direct daylight photography. It would also reveal specular reflection from even quite small pools of liquid.

I think we have a little more than a guess: Huygens couldn't find the sun in near-infrared to lock the imaging sequence; this is pretty good evidence that the deep Huygens atmosphere eliminates the solar limb in the visible spectrum. It is still possible that the 'shadow' of the parachute could have been observed as a general darkening of the complete image, but I have not found any Huygens reports that support this.

[ngunn]

I think we have a little more than a guess: Huygens couldn't find the sun in near-infrared to lock the imaging sequence; this is pretty good evidence that the deep Huygens atmosphere eliminates the solar limb in the visible spectrum. It is still possible that the 'shadow' of the parachute could have been observed as a general darkening of the complete image, but I have not found any Huygens reports that support this.

That's what I thought originally, but then why do they show the solar limb clearly in the simulation at the end of the descent movie? More artistic license? If so it's highly misleading and I wouldn't have thought they'd do that.

[climber]

Dear All,
I received recently my monthly copy of "Ciel & Espace" magazine. Inside was a CD called "Huygens probe, Landing on an Earth-like world".
Two video are available in 10 languages : English-French-German-Finnish-Polish-Greek-Spanish-Irish-Hungarian-Czech.
First video start from Saturn orbit down to landing with commentary.
Second one show the landing (no comments but noises instead) showing the landing with a lot of parameters like this (screen capture) :  Huygens.doc ( 493K ) Number of downloads: 327
showing on the sreen. Quite nice actualy.
Quality is good and I've enjoy the landing several time.
Trouble is that I don't know if the CD is available elsewhere. It has been given by ESA & Europlanet but that's all what is said.
If somebody knows and people here are interested please tell us.
In any case, if interest, I'll call "Ciel & Espace".

[djellison]

Is it the same as this:
http://sci.esa.int/science-e/www/object/in...fobjectid=40030

Doug

[climber]

Is it the same as this:
http://sci.esa.int/science-e/www/object/in...fobjectid=40030

Doug

Yes Doug
I didn't try on the web but I thought it would have been better on a CD. Seeing how big the files are, not that sure now.