article_snippet.sent: This may be due to Martian weather or they blend in with the surface .
article_snippet.sent: It may be that Martian wind has swept them away, or Martian wind could be that the dark sand dug up by the wheels as them move is a similar colour to the surface, and thus indistinguishable.
highlights_snippet.sent: This may be due to Martian weather or that they are the same colour as the surrounding surface .