Analysis - Enthusiasts lit up social media recently with images of what appeared to be a "doorway" into a hillside on Mars. Was it, some wondered, evidence that the red planet could be, or have been, inhabited by aliens? The "door" was imaged by Nasa's Curiosity rover on May 7 on the slopes of Mount Sharp, the central massif within Gale crater, where it landed in 2012. Described on one website as a "pharaonic tomb door", because of its resemblance to some ancient Egyptian remains, it is in fact only about one foot hi