Some Lunar Thoughts

Those of you who know me will know where I am going to on this.

Can someone please explain why command modules on the Apollo lunar missions became so worn and discolored simply by traveling into outer space? This vehicle did not land on the moon. It remained in orbit. But it also re-entered the earth's atmosphere, just like today's space shuttle missions. But the space shuttles don't come back looking brown and beat up from re-entry.

Here's the before and after photos from Apollo 17 (the final lunar landing mission:)

The command module before the mission...

...and how it looked following splashdown and recovery.

And, just for good measure, space shuttle Discovery.
Photo Source: The NASA Apollo and Space Shuttle archives.