Titan Surface Illumination?

[lpetrich]

Looking at the Huygens pictures, it was something of a surprise to me that so much sunlight gets through. Does anyone have any good numbers? I'm sure that it would be possible to calculate such numbers from Huygens's data.

And has anyone tried to find out how much sunlight makes it to Titan's surface without being scattered?

At first glance, the shadows of the surface rocks in Huygens's pictures may imply some sizable fraction, but when looking out my room's window at the overcast sky, I have second thoughts.

The sky looks bright, but the Sun cannot be seen behind the clouds. And the sky casts shadows, though rather diffuse ones and not sharp-edged ones.

And my attempts at radiative-transfer calculations show that this is the expected situation when the scattering of light is much greater than the absorption of light -- the light gets scattered out of its initial direction, but persists.

Is that also the case with Titan's atmosphere?

It also seems to be the case with Venus's atmosphere, at least judging from the Venera surface pictures.

[Cugel]

I have heard a number of 1% of the light we are used to here on Earth.
From that I concluded that a human wouldn't see anything on Titan but that doesn't seem to be the case. The Huygens picture is somewhat misleading because it carried a 20 watt lamp. I can't imagine the Sun casting any shadows on the surface of Titan. I think it is something like a moonless night on Earth.

[01101001]

I have heard a number of 1% of the light we are used to here on Earth.
From that I concluded that a human wouldn't see anything on Titan but that doesn't seem to be the case. The Huygens picture is somewhat misleading because it carried a 20 watt lamp. I can't imagine the Sun casting any shadows on the surface of Titan. I think it is something like a moonless night on Earth.
One-percent is still quite a lot of light, enough to see with.

Space.com: Huygens Probe Sheds New Light on Titan (http://www.space.com/missionlaunches/titan_update_050121.html)


[Principal Investigator] Tomasko described the process as "taking pictures of an asphalt parking lot at dusk."
Taking pictures of asphalt at dusk isn't simple, but it isn't impossible, and seeing asphalt at dusk is even easier.

A bright summer high-noon on earth is about 100,000 lux and a normal indoor room at night, might be illuminated at 100 lux, a factor of 1000. And that's a long ways from a moonless night.

Huygens' lamp didn't make much difference. It was only on at the very end, it couldn't reach far, yet the vast landscape was visible on descent -- below the thick clouds.

[lpetrich]

I have heard a number of 1% of the light we are used to here on Earth.That's the figure for sunlight arriving at the top of Titan's atmosphere. Saturn and Titan are on average 9.53 AU away from the Sun, thus yielding that 1% figure for incident sunlight.

But what I was really interested in is how much of that sunlight gets through Titan's atmosphere, and also how much of that gets through without being scattered by Titan's clouds and haze.

[Jerry]

I have heard a number of 1% of the light we are used to here on Earth.
From that I concluded that a human wouldn't see anything on Titan but that doesn't seem to be the case. The Huygens picture is somewhat misleading because it carried a 20 watt lamp. I can't imagine the Sun casting any shadows on the surface of Titan. I think it is something like a moonless night on Earth.
I don't think so - the light is aiming down, so the 'footprint' images are illuminated, but not the pebbles. 1% of the sun would still be brighter than a full moon, and well within the range of 1994 CCDs. I've notice that the latest 'natural light' pictures of Titan have a more reddish hue, as opposed to the flat 'Yellowstone' colors in the early images. The flatness is almost certainly a consequence of the low-light conditions.