Thank you all for your responses. Stimulated by them, I did some
searching (which I should have done before writing in the first
place). For those who have not looked at the photos, I suggest
Astronomy Picture of the day, April 6, 2003. Studying that, I think,
will show anyone that those lines were not produced by any sort of
impact. There are also some excellent pictures in NASAs Planetary
Photo Journal. And lastly, I recommend you read the following
document. Lunar and Planetary Science XXXIV 2003. These authors show
that that object is nearly completely made up of layers. And they did
it in a much more scholarly manner than I did. They did not, however,
speculate, as I did, that what we are looking at is sedimentary rock.
As I see it, that feature alone makes Phobos potentially the most
valuable place to visit in the solar system.

As the article sited above points out, there are at least four
asteroids with similar layers, but Phobos is very readily available.
It might easily be possible to bring home a couple of samples. Do our
scientists know how to sterilize such samples? George

[Mod. note: reformatted -- mjh]