Features on border of a crater & ravine on Mars

Question

This image of the Sirenum Fossae Trough - MGS MOC Release No. MOC2-248 of Mars shows spots on the northern lip of the crater that draw my attention.

See the Mars Global Surveyor - Mars Orbiter Camera page linked above for the original 768x1536 px GIF, here are some zooms and crops of the images to highlight the 3 dark spots. Most zooms are showing the image from further away (smaller) but the last shows the section at double size (2x zoom in).

All of the spots are on the border between the crater and ravine. One is near the right of the last image, while the other two are over to the left.

The only things I could think them to be are either:

• Glitches in the image (unlikely in such a pattern).
• Craters from smaller meteorites (unlikely pattern of hits, and they seem far too dark).

Does anyone know what they are?

Answer 1

I did further processing on a 80x40 sub-image covering the anomalous pixels:

All images are seen at a browser based 8X zoom scale (FF will dither it smoothly, which makes the lines less sharp, but almost as clear).

The information in pixel x/y, RGB & 'gray scale difference' between that pixel and the surrounding pixels was calculated and shown in a sortable table (of 3200 rows).

Each titled section shows an image, as well as the first six rows of the table, indicating both the sort keys used, and the rows selected.

Image of interest - nothing selected.

The 3 darkest pixels seem obvious, without any further prompt..

X   Y   RGB     Diff.   Select
0   0   192     20.75   -
1   0   186     21.625  -
2   0   174     25.625  -
3   0   177     23.375  -
4   0   175     24.375  -
5   0   181     24.25   -
6   0   189     19.75   -

Darkest Pixels

But here are the darkest pixels high-lit with green cross-hairs just to check our logic.

X   Y   RGB ▲   Diff.   Select
16  25  0       240.625 TRUE
6   30  0       240.125 TRUE
74  7   6       230.875 TRUE
78  31  112     37.875  -
79  34  112     22.75   -
75  37  115     31.375  -

Next Darkest Pixels

The difference in gray scale between the 'next darkest' pixels and their immediate neighbors is much less obvious. Not nearly as obvious as the darkest 3 pixels.

X   Y   RGB ▲   Diff.   Select
16  25  0       240.625 -
6   30  0       240.125 -
74  7   6       230.875 -
78  31  112     37.875  TRUE
79  34  112     22.75   TRUE
75  37  115     31.375  TRUE

Next Biggest Difference

The next biggest difference in pixels by gray scale is similarly nondescript. The difference drops from over 200 pixels to less than 50.

X   Y   RGB     Diff. ▼ Select
16  25  0       240.625 -
6   30  0       240.125 -
74  7   6       230.875 -
37  18  205     47.875  TRUE
31  20  209     45.375  TRUE
59  13  196     44.875  TRUE

Conclusion

Based on the data it seems those 3 darkest pixels are outliers & anomalous, so I'd say "image glitch" failing a more definitive answer.

Code

See this answer on SO for the Java code used to generate the images. It hot-links to the first, cropped image.