Justin Kelly writes: [[I find nothing strange about this. Cosider you have this cold landscape with CO2 frost on the top surface (or perhaps H2O) the dust slides and there goes the crystalized frost. . . you then have the darker natural soild exposed underneath. Which then slowly has the frost deposited on it again as time goes on and its get lighter.]]

We were aware of that possibility, although we had been thinking in terms of the streaks being covered over with airborne dust rather than frost. But either way, you're right that it wouldn't be unusual for the streaks to lighten with time in that way. What I think is unusual is that some streaks are actually lighter than the underlying ground. There was one example of this in the Mars Society paper in Figure 6. There's another image that is also interesting at:

If you look in the lower left hand corner of this subframe, you will see a streak that is lighter than the color of the underlying slope next to another streak that is much darker. The dark streak appears to be on top of another lighter-than-background streak originating from the same source as the black streak The source of both appears to be a small knob. If the streaks are dark rock surfaces exposed by dust slides, I don't see how they could become lighter than the adjacent undisturbed surfaces by being re-covered in dust or frosted over.