What Is This Curious Cloud On Mars?

Here’s a stumper for any Mars experts. While processing my Mars images from last night, I found a strange feature over Acidalia (top left of the animation below). I made this 5-frame animation of the green-light images. The feature appears in all the channels, but is most visible in blue and green and least visible in IR. Also, it moves with the planet (ruling out dust motes on the sensor) and seems to rise over the limb. Fog rolled in after this, so there is no additional data later than this. If anyone caught Mars after 2:15UT last night, please check your images… particularly after 2:51UT.

Any ideas or thoughts on what this might be? E-mail me or use the contact button above to let me know. Also, here’s what it looks like in RGB…