938 nm, and depending on the sensor, "covered" is overstating it a little. For one sensor we commonly use, the sensitivity at 938 nm is about a factor of 6 lower at 938 nm than it is at say 600 nm. Still, you have to work with what you've got.

Nor, likely, would a casual analysis. There are a lot of variables here rather than an absolute categorical answer. Think about trying to see the Earth's Moon (or the Sun) through various degrees of cloud cover on Earth. This can depend on the altitude of the object above the horizon, the wavelength, whether we are talking about the human eye vs. the result of extensive processing. The Milky Way is by no means visible to the human eye from my urban location, but if I work at it, I can bring it out in long exposures.

Certainly if Saturn were the main source of light in a titanian night sky, it would provide detectible illumination. How sharp the image of Saturn would be would depend on the variables I mentioned and more.

Remember that Voyager managed to capture surface detail on Titan in visible wavelengths. The signal to noise ratio is not zero, but it is low.

Yes, exactly right. The sharpness will depend very much on wavelength - the optical depth decreases by a few between visible red and 938nm.

Note also that the haze is strongly forward-scattering, so some directional information is retained in each scattering event.

There are a few journal papers on the haze scattering function and its effects, this one (open access) looks at twilight
https://iopscience.iop.org/article/10.3847/...881/aae519/meta

There's also this simulation project which is pretty solid
https://www.cadfem.net/fileadmin/user_uploa..._Simulation.pdf

I'm not aware of any simulations of the night sky generally, nor of Saturn specifically

Ralph

Not to make this even more of an academic discussion, but Dragonfly's cameras don't even face the sky anyway, right? They seem to be pointed mostly down and at an angle in all the concept illustrations I've seen.

Fyi - my rules of thumb for seeing the sun through clouds (at least on Earth) are that an optical depth of <10 along the line of sight allows one to see sharp (albeit low contrast) edges (through cloud droplets). If there are crystalline clouds (like ice on Earth), the sun can be seen up to an optical depth of 25, though it is more of a frosted appearance without any sharp edges. Cloud ice on Earth though probably has a sharper forward scattering peak than the Titan haze and cloud liquid has a somewhat wider peak.

I've read that the optical depth in green light at the zenith on Titan is about 8, so by this analogy Saturn should be dimly visible at night if it's reasonably high in the sky. However the light from Saturn is more diluted than the sun case, since the angular diameter of Saturn seen from Titan is about 10 times that of the Sun from Earth. This lowers the surface brightness of Saturn relative to the overall scattered light illumination. The associated requirement would change to an optical depth <5 to be needed. This is just barely achieved in red light, so perhaps Saturn's disk as a sharp outline would only be seen visually with difficulty near the zenith, if one uses a red filter.

Conversely the Sun seen from Titan is a much smaller angular diameter and this would translate into being easier to see from Titan, compared with the same type of cloud/haze from Earth. The view in this landing video from a while back shows an interesting representation of the Sun at the 4:22 time mark: https://www.youtube.com/watch?v=9L471ct7YDo.

If I remember correctly, there are two cameras on the main antenna, which is used to point the cameras and in that mode will function much like the turret on Curiosity. So we will get views across the landscape and presumably they could be pointed to image the sky. (Perhaps images of the sun will be taken to estimate optical depth as on Curiosity.)

Interesting, I've never heard that. Always read that the surface was "completely obscured" or things like that.

Is there a map available on the public internet showing what the Saturn facing hemisphere and anti-Saturn "far side" hemisphere are of Titan? Its hard to visualize just how far over the horizon (or under its feet!) Saturn will be from the landing site of Dragonfly.

I believe that 0 degrees longitude is the sub-Saturn point and 180 degrees is the anti-Saturn point.

Yes it makes sense that at least large scale variations in surface albedo could be seen from space, since some diffused light passes through the clouds, then is either reflected back by the surface or it isn't. I suspect this wouldn't necessarily translate into seeing Saturn from the ground however.

Dtolman - this map has 0 degree longitude at the edges and would be the longitude best facing Saturn: https://solarsystem.nasa.gov/resources/1625...-map-june-2015/

I talk about this in my book "Titan Unveiled". When (during my postdoc at U. Arizona) we published the first maps of Titan based on the 1994 Hubble observations, there was some limited longitude coverage at visible red wavelengths (673nm) that seemed to show (albeit more faintly, maybe 2% contrast) the same surface features we saw in the near-IR at 940nm (~10% contrast)

Some years later, a grad student (Jim Richardson) worked with myself and Alfred McEwen on a project to see if we could pull out the same features (basically the western edge of Xanadu, where it has good contrast against the Shangri-La sand sea) in the Voyager orange filter images (~640nm). Indeed, there it was. You probably wouldnt be sure it was there if the Voyager data was all you had, but the exercise showed there was still a bit of signal leaking through the haze even at that wavelength. Jim's work was published in Icarus http://www.jerichardsonjr.info/Papers/jeri...on_ICAR2004.pdf

Correct. (I think the formal IAU definition refers it to the subsaturn point at a particular epoch). Because Titan's orbit around Saturn is eccentric (0.029), there is a 'libration' such that Saturn oscillates back and forth by about 3 degrees in longitude about that zero (and grows and shrinks by 3%)

Fun trivia (as I note in my 'Saturn's Moon Titan: Owners Workshop Manual" - the 14 looks at Titan that we got in 1994 with HST were selected to have 7 somewhat closely-spaced looks (somewhat over Xanadu), to measure possible cloud dynamics, and the rest to fill out the longitude range. The way things panned out with HST scheduling, there was one longer interval between images than the others, which meant there was a bit of a coverage gap, as it happens close to the sub-saturn longitude.

Because putting this in the middle of the maps would be rather unsightly, we centered our maps on 180 longitude, rather than zero. Then because much Cassini planning (especially radar, in which I was involved) relied on those HST maps, the radar maps used the same longitude convention. Similarly, the Cassini camera ISS looked at the same wavelengths as HST, so it was natural for their maps to also put 180 in the middle for comparison. This convention puts Xanadu to the right.

The VIMS team published most of their maps with 0 in the middle, so Xanadu appears to the left.

Thanks Ralph for the details. Tidally locked but significantly oscillating. If we could discern the glow, Saturn would be living in the sky to a certain extent during the 16 days of revolution around Saturn. A few days ago, I saw the Sun with a remarkably uniform appearance (a very nice white) without burning my eyes through a layer of clouds (or maybe a layer of fog I don't know...) in the sky. During a fraction of a second, I was wondering whether it was the Moon or the Sun.
Sometimes some physical phenomena can be really surprising!
Perhaps, there are some oasis or mirage effects with the disk of Saturn in the sky!

It has been a while since I've read that paper and I have to say, looking at figure 4, the comparison with the Cassini ISS map, with the margin of Xanadu, Adiri, and Dilmun is actually quite close. Like you said, if you didn't have a better data set to compare it to it would be hard to believe it, but still.

FWIW, another change that Saturn would display in the titanian sky would be phases. For something under half of Titan's diurnal cycle, Saturn would be above the horizon without daylight interfering, and there would be some changes in phase.

Per Voyager, we also would have had no idea at the time if variations in albedo were largely due to surface variations or variations in a cloud deck. A significant source of misconception in the early mapping of Mercury from Earth around 1880-1950 was the mistaken notion that clouds were moving around. This led observers to dismiss basically accurate visual observations that should have rejected the notion that Mercury was tidally locked.

Because Voyager only flew by Titan once (twice if you count the more distant Voyager 2) we had no way to check if the observable details stayed locked in place over time.