They might look like trees on Mars, but they're not.
Groups of dark brown streaks have been photographed by the
Mars Reconnaissance Orbiter on melting pinkish sand dunes covered with light frost.
The above image was taken in 2008 April near the
North Pole of Mars.
At that time,
dark sand on the interior of
became more and more visible as the
spring Sun melted the lighter
carbon dioxide ice.
When occurring near the top of a dune,
dark sand may cascade down the dune leaving
dark surface streaks -- streaks that might appear at first to be
trees standing in front of the lighter regions, but cast no shadows.
Objects about 25 centimeters across are resolved on
this image spanning about one kilometer.
Close ups of some parts of this image show
indicating that the sand slides were occurring even
when the image was being taken.