A NASA photo taken on Mars shows a possible Unidentified Flying Object hovering on the horizon.
It has a dark disk and is not an anomaly stuck to the lens. I checked other photos from the same camera and it is not there, therefore its not on the lens.
According to Ufologists, The dome-like object seems to be inconsistent with the surrounding terrain and incredibly stands out in the image taken by the Opportunity rover.
Potential explaination, but check the comment from UFO Sightings Daily above: