In this Messenger image:
Looking Toward the South Pole of Mercury (click to enlarge)
There's a very odd, plumelike, extension of daylight at the leftward terminator.
If it's caused by sunlight reflecting off a tall ridge, it's a mighty tall, oddly shaped ridge. Of course, it might be a large fumerole, rather than the exhaust from an alien smokestack, but whatever it is, it's odd.