The dark side of Titan

[rthorvald]

I am trying to get an idea of what the dark areas on Titan??s surface actually is. It isn??t necccarily liquid, though some parts of it might be. I also imagine that any liquid areas ought to be relatively shallow. The Huygens and Cassini images are full of texture, and not all of it seems to be artifacts... These statements are only speculation based on what i have read and what images i have seen...

So, why are it almost black? Can it be deposits of stuff carried down to lower ground by rain and melting material, that gets a different consistency than the rest, and so acquires different (almost no) reflective properties? Sort of like sand... Dirty ice gravel... Or mud?

Any opinions and even wild speculation is very welcome.

-rthorvald

[Evil Dr Ganymede]

it's not necessarily black... it's just dark. We only really have one wavelength to look through the atmosphere, and that's near IR. Can't really get a good idea of the colour of something from that, it just means that whatever it is absorbs near-IR wavelengths.

As to what it is... maybe it's similar to whatever makes Callisto's and parts of Ganymede's surface dark - dust or rocky material. But given the environment I think it's probably more likely that it's some kind of hydrocarbon, perhaps?

[rthorvald]

it's not necessarily black... it's just dark. We only really have one wavelength to look through the atmosphere, and that's near IR. Can't really get a good idea of the colour of something from that, it just means that whatever it is absorbs near-IR wavelengths.

Right, no i don??t expect it to be black close to the surface. Maybe dark greenish or something... It is an interesting question. Is there any way to make an informed guess on that?

As to what it is... maybe it's similar to whatever makes Callisto's and parts of Ganymede's surface dark - dust or rocky material. But given the environment I think it's probably more likely that it's some kind of hydrocarbon, perhaps?

Maybe. The ESA site features images, which they say shows "islands" in the middle of a flowing darker landscape. They assume the islands are mostly water ice, rising above plains of darker stuff. The river channels are probably not liquid, at least not all the time, but it does seem to either collect the dark material or maybe the liquid cleans the surface down to it. If so, it is "rock"...

-rthorvald

[t00fri]

Runar,

after the Huygens landing, we had some very interesting and plausible report by ESA scientists about how they interpret now Titan's 'landscape' and the 'weather' forming that landscape.

In case you can read some German, here is a partial summary with some information from ESA. If not, I can translate the most essential points. Let me know.

Bye Fridger
http://www.tagesschau.de/aktuell/meldungen/0,1185,OID3992226_TYP6_THE3901404_NAV_REF_BAB,00.html

[rthorvald]

In case you can read some German, here is a partial summary with some information from ESA. If not, I can translate the most essential points. Let me know.

Thank you, Fridger
My very basic school german is so many years ago, it has evaporated almost completely
I ran the text through a systran translator, and it didn??t do too bad a job - i recognize parts of it from other ESA material i have read. So, i??ll go over it in more detail, and instead of you needing to translate, i??ll ask questions if something is completely garbled...

This line is praticularily interesting:

Under the surface is a substance, which has the consistency of wet sand.

That could mean the dark stuff is indeed a sort of sand, composed of methane, ammonia, water ice, and the rains keeps it muddy. If so, the shorelines aren??t too fixed, either; they must freeze and become narrower some places, and get eroded away in others... I wonder how fast that process are.

If the dark stuff consists of - more or less - the same material as the bright, maybe it works sort of like swamps on Earth, and the darkness is explained just by it being a wet and gooey mix.

- rthorvald

[Evil Dr Ganymede]

I do wish people would stop calling them "shorelines" - there's no evidence that this is what they really are. They're boundaries between light and dark terrain, that's all we can say.

I'd also point people to:
http://saturn.jpl.nasa.gov/multimedia/i ... ageID=1165

This shows that at least part of the dark terrain seems to be more like the so-called "grooved terrain" on Ganymede in texture.

[t00fri]

Runar,

let me add some statements that I remember from the senior ESA scientists during a TV briefing about one week after the Huygens landing:

1) Clear evidence for a temporarily "wet" landscape: right now the drainage channels and extended "dark" areas were said to be probably dry. Clear evidence that the drainage channels lead /downwards/ into the "dark" areas. The scientists stated that there was strong evidence for 'weather' on Titan with occasional strong liquid methane rainfall that would fill the drainage channels temporarily. They went on speculating that the dark color in the 'river beds' and extended dark areas were carbo-hydrate sediments carried along during the "wet" phase.

2) There is direct evidence for a swampy|soft consistency of the landing area. Moreover the dissipated heat of the landed Huygens caused a /violent/ evaporation of (liquid) /methane/ in the ground close to the surface. This was proven by the Huygens instruments.

3) They emphasized that by combination of the different kinds of instruments abord of Huygens with optical observations, a consistent picture of the 'landscape' and the 'weather' shaping it had emerged!

4) Clear evidence for active vulcanism, however, with /water ice/ being ejected instead of lava and rocks on earth...

So far so good...

Bye Fridger

[Evil Dr Ganymede]

1) Clear evidence for a temporarily "wet" landscape: right now the drainage channels and extended "dark" areas were said to be probably dry. Clear evidence that the drainage channels lead /downwards/ into the "dark" areas. The scientists stated that there was strong evidence for 'weather' on Titan with occasional strong liquid methane rainfall that would fill the drainage channels temporarily. They went on speculating that the dark color in the 'river beds' and extended dark areas were carbo-hydrate sediments carried along during the "wet" phase.

4) Clear evidence for active vulcanism, however, with /water ice/ being ejected instead of lava and rocks on earth...

I don't know if it was the same interview, but the team later seems to have changed their mind about this. It seems that the drainage channels may be linked to the cryovolcanic activity, and not caused by rain after all - apparently the channels are too short and stubby to be caused by rainfall. The current idea is that the channels are caused by the cryovolcanic activity melting methane below the surface and causing it to erupt as Titan's equivalent of "hot springs" (except really they're cold!).

That's not to say that no rain occurs at all on Titan, it's just that it may not have caused the channels. Bear in mind that at the moment we only have images of the surface that are high enough resolution to see the channels for a small part of Titan's surface, so this may not even be something that is commonly seen elsewhere on Titan.

[t00fri]

1) Clear evidence for a temporarily "wet" landscape: right now the drainage channels and extended "dark" areas were said to be probably dry. Clear evidence that the drainage channels lead /downwards/ into the "dark" areas. The scientists stated that there was strong evidence for 'weather' on Titan with occasional strong liquid methane rainfall that would fill the drainage channels temporarily. They went on speculating that the dark color in the 'river beds' and extended dark areas were carbo-hydrate sediments carried along during the "wet" phase.

4) Clear evidence for active vulcanism, however, with /water ice/ being ejected instead of lava and rocks on earth...

I don't know if it was the same interview, but the team later seems to have changed their mind about this. It seems that the drainage channels may be linked to the cryovolcanic activity, and not caused by rain after all - apparently the channels are too short and stubby to be caused by rainfall. The current idea is that the channels are caused by the cryovolcanic activity melting methane below the surface and causing it to erupt as Titan's equivalent of "hot springs" (except really they're cold!).

That's not to say that no rain occurs at all on Titan, it's just that it may not have caused the channels. Bear in mind that at the moment we only have images of the surface that are high enough resolution to see the channels for a small part of Titan's surface, so this may not even be something that is commonly seen elsewhere on Titan.

I have nowhere seen any different statements from what I reported, but it is not excluded, of course.

I know that some effects of water ice "eruption" have been attributed to channelling effects as well, like this one



But as the dominant mechanism it does not really make too much sense to me. We know for sure that just a little below the surface of the Huygens landing site, there were massive resources of liquid methane. Where should they have come from if not via some recent torrential rainfall...We also know about this intensive methane layer in the atmosphere between 18 and 20 km altitude. It must have been 'feed' by significant evaporating methane resources near Titan's surface...The rainfall scenario just matches.

[Evil Dr Ganymede]

But as the dominant mechanism it does not really make too much sense to me. We know for sure that just a little below the surface of the Huygens landing site, there were massive resources of liquid methane.

Do we know this for sure? I haven't heard anything definitely saying that "massive resources of liquid methane was just below the surface". Though it does sound like the surface is 'squelchy', implying it might be "muddy" because of liquid methane in it.

(and furthermore, the same interview revealed that the 'creme brulee' interpretation of the surface may have been misleading - the 'crust' may have actually been the lander hitting one of those solid 'cobbles' of ice that we saw in the view from the surface, and then settling in the squelchy stuff rather than breaking through a hard crust.

Where should they have come from if not via some recent torrential rainfall...We also know about this intensive methane layer in the atmosphere between 18 and 20 km altitude. It must have been 'feed' by significant evaporating methane resources near Titan's surface...The rainfall scenario just matches.

Not particularly. If there is liquid methane mixed in with the surface, then it could easily have come from repeated eruptions as it could have come from rain or just seeped up from below because of "geothermal heating". And methane rain is actually likely to evaporate before it hits the ground if it falls from a height of about 20 km on Titan. So while it could indeed rain on Titan, it might not actually hit the ground!

[t00fri]

Do we know this for sure? I haven't heard anything definitely saying that "massive resources of liquid methane was just below the surface". Though it does sound like the surface is 'squelchy', implying it might be "muddy" because of liquid methane in it.
...

I am referring to the erruptive evaporation of a massive amount of liquid methane after the heat from Huygens penetrated the ground. This effect is undoubtedly recorded (in writing).

[t00fri]

And methane rain is actually likely to evaporate before it hits the ground if it falls from a height of about 20 km on Titan. So while it could indeed rain on Titan, it might not actually hit the ground!

That is not at all clear. It depends on the "condensation point" which is a strong function of temperature. I am talking about the limiting amount of humidity that the atmosphere is able to sustain (in gaseous form) for a given temperature before it starts to condense. At low temperature, condensation takes place much earlier.

If you'd live in Hamburg you would surely know what I mean. Here the humidity can be so high that nothing likes to evaporate anymore . The humidity in the air tends to condense everywhere: at the windows, on the car's window screen etc.

Anyway, Hamburg's vegetation tends to like it...Similarly on Titan as you can see here
http://www.shatters.net/forum/viewtopic.php?t=6547&start=127

[Evil Dr Ganymede]

I am referring to the erruptive evaporation of a massive amount of liquid methane after the heat from Huygens penetrated the ground. This effect is undoubtedly recorded (in writing).

But then that could also have been from the melting of a methane 'permafrost' in the ground by Huygen's residual heat. Which apparently wasn't that much, since it spent several hours dropping through an upper atmosphere that was COLDER than the surface (about 70 K).

Either way, I'm not sure you could call it "a massive amount". It's not like it all gushed out in a torrent...

That is not at all clear. It depends on the "condensation point" which is a strong function of temperature. I am talking about the limiting amount of humidity that the atmosphere is able to sustain (in gaseous form) for a given temperature before it starts to condense. At low temperature, condensation takes place much earlier.

Well, I'm going by research done before the landing by Ralph Lorenz (one of the leading authorities on Titan), published in a book called "Lifting Titan's Veil". I haven't heard anything yet from the Huygens team so far that indicates that this might be wrong, but then I haven't heard if there's any new data on the humidity at Titan's surface either. But considering that the rain would take several hours to fall from a height of about 25 km in Titan's low gravity, there is ample time for something to happen to it on the way down.

[andersa]

While both NASA and ESA have been quick to disseminate photos from Titan, I have seen comparably little other data, in particular from
Cassini's radar equipment which is supposed to help mapping Titan's topography. There were some graphs showing radar measurements made during the October flyby, but since then I have neither seen nor heard anything about it. NASA has links to the websites of most other research teams involved, but none for the radar team. Isn't there any elevation data ready for publication yet? When can we expect to see it, and what level of detail?

[Tetzauh]

While both NASA and ESA have been quick to disseminate photos from Titan, I have seen comparably little other data, in particular from
Cassini's radar equipment which is supposed to help mapping Titan's topography. There were some graphs showing radar measurements made during the October flyby, but since then I have neither seen nor heard anything about it. NASA has links to the websites of most other research teams involved, but none for the radar team. Isn't there any elevation data ready for publication yet? When can we expect to see it, and what level of detail?

As far as I have read During that Flyby only 1% of titatn's surface was mapped with the radar and it does not even match the areas imaged visually, so there is still not enough information get some concusions. The next thing i would expect is that they will relase a short animation of Titan terrain with a texture of the IR photos cassin'st taken. Remeber that most of us rely on vision and therefore numbers and graphics don't make it to the papers headlines while blurry pictures of hypotetical shorelines do. I am not in the least surprised that they are using their space on the web to make avaliable things that can be shown off rather than raw data that needs a phd to be interpreted.

[t00fri]

Tetzauh,

so it seems you are living in the periphery of Mexico City (Tesistan?)?

Bye Fridger

[andersa]

As far as I have read During that Flyby only 1% of titatn's surface was mapped with the radar and it does not even match the areas imaged visually, so there is still not enough information get some concusions. The next thing i would expect is that they will relase a short animation of Titan terrain with a texture of the IR photos cassin'st taken.

Cassini has some 40 more encounters with Titan ahead of it over the next three years, so perhaps we shouldn't expect a complete map anytime soon. However, it would be nice to know how they are progressing.

Remeber that most of us rely on vision and therefore numbers and graphics don't make it to the papers headlines while blurry pictures of hypotetical shorelines do. I am not in the least surprised that they are using their space on the web to make avaliable things that can be shown off rather than raw data that needs a phd to be interpreted.

That's true for the JPL main Cassini website itself, but they also provide links to most of the science teams that are responsible for the instruments onboard Cassini. Some of them, such as the Radio and Plasma Wave Science team do try to show what kind of data they have been getting lately, while others don't seem to have updated their websites at all during the past two years (but the CAPS team provides copies of mission plans and specifications that may be of interest to some of us). They sure don't sprinkle their websites with recent photos from Titan.

However, it seems the RADAR team doesn't even have a website.

[Spaceman Spiff]

What I'm puzzled about with SAR imagery from Titan is that the October 2004 flyby scanned about 1,500km worth of surface. Yet, the three images ever released only cover 100km -ish each, and two of them overlap. The rest isn't released yet.

There was actually a release of altimetry data (a graph can be seen at http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=1172) and this seems to show Titan as rather flat: 150m relief over a 500km swath, and even that 150m slopes over just one third of that swath...

Spiff.

[andersa]

What I'm puzzled about with SAR imagery from Titan is that the October 2004 flyby scanned about 1,500km worth of surface. Yet, the three images ever released only cover 100km -ish each, and two of them overlap. The rest isn't released yet.

I looked a bit further, and found the website of Ralph Lorenz. He has a page with those images (I think) and indicates he will be adding more, but he is appearantly busy with press conference preparations or whatever...