QUOTE (Mark Adler @ Jul 24 2007, 10:25 PM)
"No" is a bit extreme. Also in Emily's article is the formula for this, which is that e^-tau of the direct light comes through. So at tau = 5, you're still getting about 0.7% of the direct light you'd get through a vacuum. At typical Mars taus of 0.5 to 1, you're getting 40% to 60% of the direct light.
This is right as written, but especially for high tau, the phase function of the dust
can matter a lot (as it does e.g. for Titan's haze*). If some of the light is scattered forward
rather than being absorbed, you can still push a lot of light through several optical depths
(IIRC Chris McKay explained to me once it goes as 1/(1+tau) or something.) Of course,
the phase function and optical depth are different for different wavelengths, so you may end
up knocking out all the blue light, but red still leaks through.
As descriptors for atmospheric optics go, tau is a good number, but it is only one number
and so may not tell the whole story.
* I think Titan's haze was measured by DISR to have an optical depth of 3 or more, which
shouldnt allow Cassini ISS to see the surface at all. But because the haze has such a strong
forward-scattering peak, the light preserves some information on where it came from.