I've been busy lately, and haven't looked at the MER raw images in a while. But this morning I'm procrastinating. Browsing the Navcam images, I came across this, which gave me quite a start:

(Click for link)

Notice that the sky is BLACK. This just isn't possible! My mind quickly rejected a couple of explainations:

1.) The image is filtered weirdly -- no, this is a Navcam image. No filters.
2.) The image somehow became negative -- no, the shadows of the rocks are black.
3.) It's very early in the day, and the shadow of the planet is still visible in the atmosphere (similar to the "belt of venus" effect that happens here) -- no, the shadows are short and it's just after 2pm local time.

Finally, I think I found the solution: the ground is very, very shiny. The sun is behind the camera, causing a ground reflection which is considerably brighter than the sky. Due to the histogram stretch, the sky ends up being black. Here's another image which demonstrates the effect to a lesser degree:

Also, notice that the light is intensifying directly around the shadow of the camera mast. This is an effect known as "heiligenschein" which typically occurs on dewey grass (although I've also noticed it occuring on icy asphalt). Here's an image:

The fact that this is occuring in a dry desert is rather puzzling, and I'm not sure how to account for it. A human explorer standing on the surface would notice the heiligenschein effect immediately -- it's rather dramatic and etherial, and would definitely not let you mistake Mars for any place on Earth.

Cheers,

Nathan