# How much sunlight gets to the surface of Titan? What would astronauts see?

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.

• 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? – Nick T Jan 18 '15 at 23:35
• 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... – Bob Stein Jan 19 '15 at 0:07
• – mins Jan 26 '15 at 18:43
• A World much like Earth, very brownish. Another video on YouTube. – mins Jan 26 '15 at 18:51
• @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 – pericynthion Feb 16 '15 at 23:17

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.

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.

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)

• Floodlights would illuminate the foreground more than the background wouldn't they? Not seeing that in any Huygens photos. Overcast at twilight, that helps. – Bob Stein Jan 17 '15 at 19:06
• So the question is, what would it be like to stand on Titan with all the lights off? – Bob Stein Jan 18 '15 at 22:22

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).

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