QUESTION:
How bright is daylight on Mars? At noon on a clear day, is it as bright as a
cloudy day on Earth? Is noon on Mars like twilight or dusk, since Mars is so
much further away?

ANSWER from Jim Murphy on July 18, 1997:
The brightness of the sun on Mars, were there to be a clear day, is about
half the brightness of a similar day here on Earth.  I arrive at the value
of one-half simply by knowing that the brightness of an object decreases by
the square of the distance, and since mars is on average 1.5 times as far
from the Sun as earth, 1.5 times 1.5 is 2.25, and 1 divided by 2.25 is 0.44,
or 44 percent as bright.

Now, just because the apparent brightness is half, that does not necessarily
mean that the scene will be half as bright on Mars.  Mars' less massive
atmosphere will mean less scattering of light, and thus the clear martian
sky would not be as blue as here on earth, and all the scattered light which
we see here on Earth, which makes the sun appear a bit less of a flashlight
beam than it is would all conspire to make the Martian sky appear darker.

As for comparing the martian brightness to the brightness of a cloudy day
here on earth, I'm not sure what a good analogous situation is.  That is a
good question, however.

Jim Murphy
Mars Pathfinder ASI/MET Science Team

ANSWER from John Moreau on July 25, 1997:
Some of the images taken by the Pathfinder have undergone enhancement in
Adobe Photoshop to sharpen features or bring out contrast, but any image
processing is minimal.  The Martian surface is really that bright on the
dayside.  Compare these images to images of the Viking Lnading site taken
back in the 70's and you will see the similarity in brightness.