All about the shadow of Spirit's 'head' in Image 20 of the MER Top 25 images (25 raw image picks) there is a startlingly-clear dry heiligenschein (the shiny halo). Interestingly, the heilgenschein is displaced slightly upwards from the anti-solar point (which lies directly behind Spirit's head). Is that displacement just an accident, due to the presence of a cluster of better-reflecting rocks on the surface just above the shadow of Spirit's head, or is something else going on?
Are there crystals on the surface of some sort (platy crystals....feldspar maybe?) that are refracting sunlight to some extent, or are there maybe little spheres of some crystalline material on the surface?
There even seems to be a heiligenschein streak trending off to the upper right of the image. Why is that there? The apparent orientation of the rocks is horizontal (they lie on their sides), not vertical, and not trending to the upper right of the image. So why not a horizontal heiligenschein? Is the heiligenschein established just by the accident of the distribution of the rocks on the surface? Why? Why? Why?
(reference: Robert Greenler's Rainbows, Halos, and Glories, 1991 reprint, p. 146-149).
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