It seems, these white blotches are reflective enough that they caused oversaturation of a spot of image taken from a satellite at the L1 lagrangian point (click to open at max resolution, the purple spot is squat in the middle of the picture).

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Clear quartz crystals has been found nearby ( link ), but could it be this? Can quartz so clear and reflective appear in such abundance? Or is it some other cause?

(I thought it might be frost, which shows up on the desert in the mornings, but the photo taken from L1 places the point at peak of the summer with the Sun precisely in zenith above it; I doubt any frozen water could withstand that. Also, Wikimapia has some photos taken nearby with what looks like frost, but the daytime is obviously different).

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    $\begingroup$ Salt or other evaporites? White rocks like chalk or limestone? $\endgroup$ – Gimelist May 24 '17 at 11:39
  • $\begingroup$ @Michael: I seriously doubt salt / evaporites in the middle of Sahara. Although I'm definitely a layman. $\endgroup$ – SF. May 24 '17 at 11:57
  • $\begingroup$ Interesting. This geological map of Libya shows the three igneous bodies on the Egypt-Sudan border, but misses the smaller just to the north of that, so I'm not certain how reliable it is. $\endgroup$ – Gimelist May 24 '17 at 12:10
  • $\begingroup$ Reading a bit through the link you gave and looking up some things online, seems that this is white sandstone. Completely white sandstone is unusual, but it exists. White rocks are very reflective and look completely white in satellite images. $\endgroup$ – Gimelist May 24 '17 at 12:28
  • $\begingroup$ Can you provide a latitude-longitude for the centre of the upper image? Have you tried looking at the topography to see if the white areas are formed in depressions? $\endgroup$ – haresfur May 25 '17 at 1:23

The white areas appear to be formed on the edge of elevated parts of the landscape and in low-lying flat areas. You can see this if you use Google Earth and looking at the elevation profiles along paths you sketch across the landscape. They don't appear to form at a particular elevation or other pattern that would indicate they are bedrock outcrops.

I think the most likely possibility is that they are caliche deposits (calcium-carbonate cemented sediments), formed as water ran off of higher elevations, mobilizing calcium and evaporating to cause calcite saturation. They could be halite but I would expect more extensive deposits in the broader drainage areas if they were salt. They could have formed under wetter climate conditions. If you look at 23 degrees 14.053' North, 24 degrees 25.22' East, you can see deposits being covered by sand dunes.

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  • $\begingroup$ I'm not knowledgeable about that stuff, but I think any older drainage area deposits could have been covered by sand blown over from the desert; slopes around these may remain clear. $\endgroup$ – SF. May 26 '17 at 17:39

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