M1001115 season etc.
The MSSS page for the image is:
M1001115
Use http://marsglobalsurveyor.com to find images (up to about month 12 - M12xxxxx)
Reading the info we find Ls~260 which is very late spring.
The dark streaks imply that either dust has been laid down on top of the snow, or the snow has been cleared. In either case, this is along a thin streak less than 5m wide.
If it were just dust, you would need a source of dust at the top of the streak, but there is no obvious collapse feature.
my model for these features is here.