Something on Mars

I've been browsing around moon.google.com and mars.google.com because they are there and to check out other planets for yourself is great fun. More than it sounds. It's like exploring a planet. When looking at the map of Mars, there are gray strips where the satellite missed areas of the ground. That's fine. Looking between those reveals a landscape with hills, valleys, riverbeds. It's quite astounding actually. Having a further scan around I found this. Now why is this rectangle grey? it's as if this bit of data has been removed. It's obviously very high because it casts an enormous shadow to the north. But the center of it looks like the gray that the satellite camera passes missed. Why gray it out? What is it? The mind boggles.