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Saturn being at 10 A.U. means sunlight on Titan's cloud tops is about 1/100 that on Earth's. That's 4000 times the illumination of Earth's moon. Titan's atmosphere is described as opaque smog. If it were 99.97% opaque the surface would still be illuminated like a full moon. Photos from Huygens seem to show some light gets through.

Would a (warmly-insulated) astronaut see pitch black? A diffusely glowing brown sky? A foggy city streetlamp? I'm finding it hard to picture.

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    $\begingroup$ Related/starter question, if you're on Titan, the Sun and Saturn are in opposition, would Saturn be brighter if it was at your zenith, or likewise with the Sun, and by how much? $\endgroup$ – Nick T Jan 18 '15 at 23:35
  • $\begingroup$ Interesting, @NickT, similar to earthshine on the moon. Could any planet ever outshine its star? Guessing no, but how to prove it. What if it filled the sky and had albedo of 100%? Mirror finish? Please do make a question for this... $\endgroup$ – Bob Stein Jan 19 '15 at 0:07
  • $\begingroup$ A World much like Earth, very brownish. Another video on YouTube. $\endgroup$ – mins Jan 26 '15 at 18:51
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    $\begingroup$ @BobStein-VisiBone I think the Brightness Theorem would suffice to prove that a planet can never "outshine" its star. No passive optical system can increase the apparent brightness (luminosity per unit solid angle) of any light passing through it. Otherwise you can violate the 2nd law of thermodynamics. onlinelibrary.wiley.com/doi/10.1002/0471791598.app1/pdf $\endgroup$ – pericynthion Feb 16 '15 at 23:17
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Newbie Titan researcher here! I was wondering about this same question, but there are a few papers that have since answered your questions. I'll summarize their findings below:

TLDR: For an astronaut, the sunset would be quite underwhelming, much like being in a sand storm or thick smog. The sky will have no real change in its orange color. The Sun disappears well before reaching the horizon. You will need infrared goggles to see better sunsets.

Long answer

Overall, we actually observe that the twilight periods (sunrise/sunset) are brighter than the day on Titan from infrared to UV wavelengths due to the intense forward Mie scattering from the haze. This finding was found with Cassini images in various wavelengths (Garcia Muñoz et al. 2017) and confirmed with radiative-transfer models (Barnes et al. 2018).

The models of Barnes et al. 2018 provide useful insights into a standard sunny day on Titan. This figure shows the Sun setting from early afternoon and until after dusk in visible, near-infrared, 5 micron light. The image shows a "rolled-out" version of the sky as seen from the surface of Titan. The leftmost column shows the Sun and progresses toward the antisolar point to the right side. The top and bottom of the image are zenith and the horizon respectively. The ZD represents the angle between the Sun and zenith. The first diagram below explains the format of the "rolled-out" sky images in the 2nd figure: enter image description here

enter image description here We can note some key details about the sunsets:

  1. The 5 micron sunset is very similar to Earth due to the similar atmospheric optical depths of Titan at 5 microns and Earth at visible wavelengths.
  2. In the near-infrared, the sunsets resemble a Martian (PIA07997) or dusty terrestrial desert sunset. There is a distinct solar aureole that transitions color from white to "red" over the afternoon. The Sun would fade out just before it sets (<10° above the horizon). The interesting new detail is the fan-like aureole that develops above the Sun!
  3. The visible-light daytime and sunset might be quite disappointing for astronauts. The sky would be faintly and uniformly illuminated during the day, similar to looking through thick fire smoke on Earth (no issues with staring at it!). The sky would have a nice orange hue that just slowly fades away as the Sun nears the horizon. The daytime sky is likely 100-1000x dimmer than an afternoon on Earth.

Another crazy detail is that the twilight zone may extend 30° past the terminator line. This means that the surface visibility at infrared and visible wavelengths will be brighter than the full Moon for up to 1.25 days before sunrise or after sunset! This is about 20x longer than the usual twilight period on Earth.

One final result from the paper was the intriguing lack of near-horizon illumination at all wavelengths and times of day. On Earth, the opposite occurs due to Rayleigh scattering and its lower optical depth. However, this effect may be occurring since the paper explicitly did not consider Rayleigh scattering from liquid methane-ethane droplets.

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  • $\begingroup$ Awesome answer sir! Okay, a beautiful day on Titan is a very acquired taste. What is ZD, the angle between the sun and the zenith? Thinking a tiny diagram of a rolled-out sky labeling the 4 sides might be helpful. The bright-ish spots on the left is mister sun? $\endgroup$ – Bob Stein Apr 19 at 23:45
  • $\begingroup$ "...we actually observe that the twilight periods (sunrise/sunset) are brighter than the day on Titan..." this comes from Titan brighter at twilight than in daylight but it applies to the brightness of Titan seen from far away in space, not on Titan, doesn't it? $\endgroup$ – uhoh Apr 19 at 23:57
  • $\begingroup$ @BobStein It certainly would be! You got it; ZD stands for zenith angle such that zero is when the Sun at zenith. I updated the post to include a labeled diagram of the "rolled-out" sky. Sun is the white dot located on the left side and has a faint aureole or halo surrounding it at infrared wavelengths. $\endgroup$ – girtonoramsay Apr 20 at 14:15
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    $\begingroup$ @uhoh Yeah you're right. The Titan brighter at twilight than in daylight paper does only apply for viewing Titan from space. Also, Barnes et al. 2018 didn't find the twilight sky brighter than the daytime sky. However, they ignored Rayleigh scattering, so I think that we can't be certain of this effect until NASA's Dragonfly drone lands on Titan in the 2030s. It seems odd that twilight is brighter from space, but would not be brighter on the surface. $\endgroup$ – girtonoramsay Apr 20 at 14:35
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The Huygens probe that landed on Titan back in 2005 had a special flood-light affixed to its cameras because it's supposedly too dark to take pictures with any detail during Titans day without it.
Titan's surface from Huygens lnder (courtesy ESA/NASA/University of Arizona
Images from the DISR Side-Looking Imager and from the Medium Resolution Imager, acquired after Huygens' landing on Titan, were merged to produce this image.

Little sunlight reaches the surface, due to its thick haze, and large distance from the sun. I would imagine it being like an extremely overcast, late afternoon here on earth, everyday.

Huygen's cameras with spotlight.

Above is a picture of the cameras that Huygens used to snap the pictures. They were aided by the spotlight in the middle (the larger, goldish disc)

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It should be like about 5-10 minutes after sunset on Earth. At that time the global horizontal irradiance (with clear skies) is typically measured (e.g. with the NOAA SurfRAD network) to be about 1 W/m**2, about .001 times the mid-day value. Similarly, about 10% of the light incident on Titan reaches the surface via scattering, based on a green light optical depth of 8 and reasonable assumptions of backscatter fraction and single scattering albedo. This combines with a baseline value of 1% solar irradiance at the top of Titan's atmosphere compared with Earth's (due to being 10AU from the Sun).

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    $\begingroup$ Do you have any calculations or references to back up your assertion? $\endgroup$ – Organic Marble Jan 4 '18 at 1:23

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