QUESTION:
Is it known how much light will be available for imaging the surface of Titan 
by the Huygens probe?  I would think that it would be quite dark under a 
cloud deck so far from the sun.  Also, will the probe make any images of 
saturn during the descent through Titan's atmosphere, as shown in several 
pieces of Cassini artwork? That could make some spectacular images!


ANSWER from Jim Frautnick on June 20, 2000:
The Huygens probe that will parachute through Titan's atmosphere carries
the Descent Imager/Spectral Radiometer (DISR). This instrument will make a
range of imaging and spectral observations using several sensors and fields
of view. By measuring the upward and downward flux of radiation, the
radiative balance (or imbalance) of the thick Titan atmosphere will be
deduced. Solar sensors will measure the light intensity around the Sun due
to scattering by aerosols in the atmosphere. This will permit the
calculation of the size and number density of the suspended particles. Two
imagers (one visible, one infrared) will observe the surface during the
latter stages of the descent and, as the probe slowly spins, build up a
mosaic of pictures around the landing site. There will also be a
side-looking visible imager to get a horizontal view of the horizon and the
underside of the cloud deck. For spectral measurements of the surface, the
weak sunlight will be augmented by a lamp that will switch on shortly
before landing.

The level of sunlight at the top of Titan's atmosphere is about 1/90 of
Earth's.  This is still quite bright by the standards of illumination:  on
a rainy day in winter the ground is illuminated by perhaps 1/16 to 1/32 of
the light reaching the ground on a sunny day, and the light cast on the
ground by the full moon is about 1/60000 of full sunlight on the Earth's
surface.  Of course, we are sending the probe to Titan to learn more about
its atmosphere, including the thickness of its clouds.  If Titan's clouds
are as dense as a winter storm's clouds, we would estimate Titan's surface
illumination to be roughly 1/1500 to 1/3000 of full sunlight on Earth,
still much brighter than illumination by the full moon.  Just how dark it
is at the surface is a result that will be of great interest to many people.

Steve Edberg for Jim Frautnick