[X] My Assistant

[Juramike]

Flyby page is up (mission description not yet posted): http://saturn.jpl.nasa.gov/mission/flybys/titan20090505/

[ngunn]

'Looking ahead':
http://ciclops.org/view/5620/Rev110

[Juramike]

From the Looking Ahead entry "This [ISS] mosaic, HIRESNA001, will focus on a region near 65 degrees south latitude, 260 degrees west longitude. Several dark features have been previously observed in this region, though these features do not have a low enough albedo to be considered lake feature candidates. Their current origin is unknown"

Cool! This is an area to the SW of the T53 Mosaic.

The red plus sign shows the approximate center of the focus area in relation to the T53 Mosaic and the global map.

[ngunn]

Mission Description:
http://saturn.jpl.nasa.gov/files/20090505_...description.pdf

[alan]

some images up, this one is interesting
http://saturn.jpl.nasa.gov/photos/raw/rawi...?imageID=191166

[MahFL]

Personally I have never been able to get excited about Cassini's images of Titan. They alwasy seem so fuzzy. Maybe some artist's interpreatations might be better ? I loved the Hugans images though.

[Ian R]

Here's my attempt at processing a global view:

[Ian R]

Here's another view of the southern hemisphere, with Elba Facula standing out like a sore thumb:



Mezzoramia can also be seen near the terminator.

[ngunn]

Very nice! (and the previous).

[Juramike]

Image of Titan in eclipse with stars in the background:

[Juramike]

Animated sequence of a stellar occultation of Titan's haze layers by a bright star:

(click to animate)

The CL1/VIO filtered Wide angle camera images were placed in the blue channel.
The CL1/VIO filtered Wide angle camera images were placed in the red and green channels.

The images were realigned to show the shortest path of the star through the atmospheric layers.

A static image showing contrast-enhanced zoom of the atmospheric layers as the star sets can be found here: http://www.flickr.com/photos/31678681@N07/3516686323/

(the animated GIF is also in my photostream).

-Mike

[ngunn]

Image of Titan in eclipse with stars in the background

I wonder what's providing the backlighting there. Hyperion? Iapetus? Just starlight?

[nprev]

I'd guess the rings; they're not completely edge-on yet.

[ngunn]

I'd guess the rings; they're not completely edge-on yet.

I presume that's what's providing the front lighting.

[ugordan]

I'd guess the rings; they're not completely edge-on yet.

Could be the rings, but indirectly. They're almost edge-on as seen from Titan and probably not very bright in high phase geometry. It might be ringshine onto Saturn's nightside which in turn illuminated Titan.

A flip GIF between the 3 better frames here.

[ngunn]

Maybe my original question wasn't clear. Since Titan is in eclipse presumably the main source of illumination, from the left, is sunlight scattered by the rings or bounced between rings and planet in some way as you say. Fine. I was asking about the illumination from the right that is allowing us to see the opposite limb. Presumably that is well within the anti-Saturn hemisphere so there should be no light arriving there at all from either rings or planet. Some very rough calculations tell me that, seen from Titan, Hyperion and Iapetus at their best are only about as bright as brightish stars so I don't think they are responsible. With the Sun behind Saturn the anti-Saturn hemisphere of Titan should be as dark as if it were in interstellar space. Are we really seeing this part of Titan imaged by starlight? That would be amazing enough. Until I know, I'll try not to think about alternatives . .

[ugordan]

I can't find any plausible source except for Phoebe which, being 13 mil km away doesn't strike me as likely. Both Hyperion and Iapetus are behind the moon as seen from Cassini. Note that these are very probably very long exposures so it wouldn't surprise me if it was indeed starlight. Certainly appears uniform enough.

[Ian R]

The latest view of Titan's southern hemisphere:

[ugordan]

May 10:

[Phil Stooke]

Re: the illumination question - light refracted through Saturn's atmosphere? I assume that if you were at Titan when it's in eclipse, you would see a ring of light around Saturn's disk caused by refraction. Seems more likely than stars or distant moons.

Phil

[ngunn]

This is an amazingly stubborn issue - or is that just me?

What you propose is fine for the sub-Saturn hemisphere which is on the left in this view of the eclipsed moon. But for the anti-Saturn hemisphere which we also see at the right hand side of the image there can be no light coming from Saturn, its atmosphere or its rings. Its outer moons don't help either - it has to be starlight. I find this astonishing too, but also inescapable unless you want to postulate that the atmosphere of Titan is faintly luminescent. I don't, and here's why.
Imagine for a moment that Titan is a perfect reflector: albedo 1. In that case it should emit what it receives. Look at the image again (I'll try to link to it) and imagine how many background stars the disc of Titan covers. If it were a perfect reflector we should see all of that light evenly distributed over its disc. Very roughly, that's what we do see, I think. Of course it's not a perfect reflector, more like one third of the incident light is reflected. All the same it does seem to me there is enough starlight there to illuminate the darker side of this already darkened moon.

The implications of this are interesting. Could New Horizons image the night side of Pluto? Could cold interstellar objects one day be imaged by reflected distant starlight?

I have a question - is this a first, or are there earlier examples of 'planetary' surfaces imaged by starlight alone?

http://www.unmannedspaceflight.com/index.p...st&id=18027

[Juramike]

Actually, the anti-Saturnian side of Titan can still have light coming from Saturn.

The "central flash" of Titan occurs at the midpoint of stellar occultation and results from starlight being focused all the way around the atmospher and coming out on the backside. (The star itself is totally obscured by Titan's bulk).

I assume that light is refracted around Saturn and hitting Titan's saturn facing hemisphere whis then is itself refracted around Titan and scattered away in the haze. I suppose normally the higher contrast due to sunlight hitting the illuminated side would blow out this effect. The only time we would see this is when Titan is pretty dark and the contrast is less (i.e. only during Titan eclipse season)

Here's some links to the central flash of Titan:
http://sci.esa.int/science-e/www/object/in...fobjectid=38200
http://cassini-huygens.jpl.nasa.gov/news/f...ccult/index.cfm

[ngunn]

Only if you're viewing from distance and looking towards the light source. You can't make the light bend all the way round and impact Titan on the backside of the globe.

Good grief, come to my rescue, somebody!

[Juramike]

Only if you're viewing from distance and looking towards the light source. You can't make the light bend all the way round and impact Titan on the backside of the globe.

Sure. Not all the light refracts around and makes it out directly opposite.
Some if it hits haze particles and scatters away at orthogonal angles. That is the light I think we are seeing.

(Picture shining a flashlight into a cloudy aquarium. Some of the light will come out the sides perpindicular to the original beam.)

[john_s]

It should be possible to test the scattered-around-to-the-darkside theory by comparing this image to sunlit images. The brightness ratio between the direct light on the illuminated side and the scattered light on the dark side should be much the same regardless of the source of the direct illumination- sunlight or Saturn night light. The only difference is that Saturn and its rings are a larger source, so the direct light can wrap a bit further round to the dark side, but that should be a fairly small effect at Titan's distance from Saturn.

My memory is that normally-exposed sunlit Titan images at this phase angle do *not* show illumination of the dark side, so the darkside/brightside ratio is much higher in these eclipse images than in the sunlit images. In that case something else must be lighting the night side, as ngunn says.

If it's starlight, that's indeed very interesting (though all the options are very interesting...), and indeed we should think about whether starlight can usefully illuminate the night side of Pluto. We are planning some very long exposure images of Pluto's night side with New Horizons to look for details in Charon-light, but it's possible that starlight could help us out too- we need to do the math on that.

John

[ugordan]

I was going to bring up the point about the brightness ratio regardless of the source brightness, but John already covered that. I'd just say the dark side illumination here seems to be pretty bright compared to the "dayside" illumination, it also appears pretty uniform, which I don't think I'd expect for some refractory effects.

Regarding atmospheric luminescence, maybe it's not such a far-fetched idea after all. Recall the VIMS discovery early in the mission of CO luminescence on the dark side. It's a very narrowband emission so if the clear filter exposure is really long enough and the eclipse lighting faint, both could (?) be visible at the same time.

For comparison, here's a look at the contrast between sunlit and saturnlit Titan: http://www.flickr.com/photos/ugordan/3372388543/

Whatever the dark side illuminant is, it's not radically darker than forward illumination.

EDIT: Ah, scratch the luminescence, it's actually stratospheric thermal emission (http://saturn.jpl.nasa.gov/photos/imagedetails/index.cfm?imageId=1132) and at a wavelength too long to be picked up by ISS.

[rlorenz]

Some very rough calculations tell me that, seen from Titan, Hyperion and Iapetus at their best are only about as bright as brightish stars so I don't think they are responsible.

I don't know what the geometry is here (i.e. whether any satellite is particularly well-placed to
illuminate the nightside) but in fact the satellites are quite bright - I worked out how bright they
would be once for astronavigation of balloons etc on Titan. If my calculations are correct, the
sources are as follows (so perhaps are bright enough for the observed surface brightness..)

Object Mag1 Size2 Comment

Saturn -14.2 5.5o 11x bigger, 10x brighter than Moon seen from Earth
*Tethys -5.4 3.9’ ~10x brighter than Venus from Earth
*Dione -5.7 4.7’
*Rhea -6.2 7.9’ 1/3 size of Moon seen from Earth
Hyperion -3 2.3’
Iapetus -2 2.0’ as bright as Jupiter seen from Earth
Earth -4.5 (tiny)
Jupiter -0.4 0.7’

*Orbit inside Titan’s, so visibility not very different from Saturn’s
1 Magnitude under best geometry. Brightness~10(-1.0*magnitude/2.5)
2 Largest apparent size in arcminutes, except Saturn

[ngunn]

Thanks - it's great to have those authoritative figures. I was a couple of magnitudes out for both Hyperion and Iapetus (perhaps not surprising as I figured them out whilst driving home from work).

However, I note that even at mag -2 Iapetus would not dominate over aggregated starlight. Hyperion at -3 would do so by a modest margin, but it would have to be perfectly placed. In an earlier post Gordan stated that the geometry rules them both out anyhow. I don't doubt him, but it would be nice to have independent confirmation of that.


BTW I find that figure of -4.5 for Earth strange.

[remcook]

About the brightness ratio of dark side/illuminated side that John and Gordon mentioned: There might be a difference here since the main illumination source (was it the rings?) can be extended in size and so illuminates Titan from different angles at the same time. This in contrast to the Sun, which is tiny as seen from Titan and light rays come in parallel to each other. Some more light might be diffusely scattered throughout the atmosphere in this case relative to the illuminated side. But I'm just guessing here.

[ngunn]

Believing it's starlight becomes quite painless after a short while - honest. A bit like swimming off Llandudno.

[Jason W Barnes]

Believing it's starlight becomes quite painless after a short while - honest. A bit like swimming off Llandudno.

If VIMS was riding along, then we could solve this. If Titan's haze in eclipse suddenly has giant H2O absorptions, then it's the rings. What's the time of the observation?

- Jason

[ugordan]

What's the time of the observation?

May 7, I'd say between 6:30 and 10:30 UTC, the reported Titan distance suggests around 09:30 UTC.

[rlorenz]

Thanks - it's great to have those authoritative figures.
.....
BTW I find that figure of -4.5 for Earth strange.

Dunno about authoritative - I think I got them using Starry Night..

Now you mention it about Earth, I agree. Might have got the sign wrong.

[ngunn]

If VIMS was riding along, then we could solve this.

That would be a superb way to distinguish the possibilities. I'll be surprised, though, if the ISS team can't answer the question directly. They must know what absolute level of illumination their pixel values represent, and both the intensity of ambient starlight and Titan's albedo are well known parameters. Perhaps it's just a matter of waiting for the calibration etc. to be carefully gone through. Wherever this nugget of information finally makes a definitive appearance I hope it's in a paper to which I have access, and I hope I notice it!

[ngunn]

Aha! The 'Looking ahead' for T55 tells us what they were looking for with the T54 eclipse images:

QUOTE:
"Following the T55 flyby, Cassini imaging will be focused on Saturn's ring and satellite systems. Cassini will turn its cameras to Titan for an observation on May 23, occurring while the satellite is in Saturn's shadow. Unlike a similar observation in Rev110, the exposure times are not optimized for looking at secondary illumination on Titan's atmosphere from secondary light sources such as the rings, atmospheric emission, and other satellites."

[ngunn]

A note on the brightness of starlight. I found a paper that gives the total light of all stars as being equivalent to a single star of magnitude -6.1. Here is the relevant page (I'll link to the whole thing in due course):

http://articles.adsabs.harvard.edu/cgi-bin...p;filetype=.gif

That would provide illumination on Titan roughly equivalent to having one mag.-4 star directly overhead, still brighter than any of the outer moons of Saturn. That differential could close somewhat due to differences between the wavelengths used for the ISS image and those that determined the 'photographic magnitudes' referred to in the paper. (Sunlight is redder than starlight.) Nevertheless it seems that if they were looking for moonlight on Titan's anti-Saturn hemisphere they would inevitably have found starlight as welll, or instead. It's interesting, though, that the 'looking ahead' mentions atmospheric emission but not starlight.

Link to full starlight article:

http://adsabs.harvard.edu/full/1914MNRAS..74..446C

EDITED - Nobody pulled me up on my oversimplifications (errors!) - aside from VP putting "light" in quotation marks - so I've fixed a couple in post.

[volcanopele]

I should note that the list of potential sources of "light" isn't meant to be exclusive. So don't think that other sources like reflected light from the rings off Saturn or starlight are being excluded.

[ngunn]

Mention of atmospheric emissions sent me off to do some reading on that as well. As a complete beginner I found this article useful:

http://astroprofspage.com/archives/445

It seems that on Earth sunlight is the main source of energy for natural airglow, with cosmic rays providing a smaller but still considerable contribution. In Titan's case one might expect that situation to be reversed, perhaps enormously reversed with a big Cerenkov component, but I'm only guessing there. How bright is it relative to ambient starlight? I haven't a clue, but maybe it could dominate after all. A comparative set of Rhea eclipse images (when Titan is out of the way) would be interesting. I wonder whether that will figure in future plans, or if indeed existing Rhea images (at their full dynamic range) could come into play here. Where was Titan when those shots were taken? Questions, questions.

I think I'll give up at this point (sighs of relief all round) and wait to see what the professionals come up with. I've learned enough to know that their task is not straightforward.
_ _ _

EDIT: a terrestrial comparison, QUOTE from here http://www.albany.edu/faculty/rgk/atm101/airglow.htm (my highlighting):

Airglow is commonly divide into:
Dayglow (when entire atmosphere is illuminated by the Sun) is the brightest airglow due to the importance of RESONANT and FLUORESCENT processes (see below) but it is overwhelmed by direct and scattered sunlight
Twilightglow (when only the upper atmosphere is illuminated) is the most readily observable airglow from the ground since the observer is in darkness (and Rayleigh scattering of sunlight by the dense lower atmosphere is absent) while the airglow region of upper atmosphere is still illuminated
Nightglow (when entire atmosphere is in darkness) is not as bright as dayglow since CHEMILUMINESCENCE (see below) is the dominant process; however contributes more light than starlight to the total luminosity of the night sky