Mars exploration. Artwork of astronauts examining an outcrop of sedimentary rock on a Martian dune field. Their Mars rover vehicle is in the background. Mars is a rocky desert world with no surface water. Its gravity is about one third of that on Earth. The atmosphere is mostly carbon dioxide and surface temperatures are well below freezing. Martian astronauts will have to breathe their own air supply and wear heated spacesuits.
Mars exploration, artwork. Photograph. Britannica ImageQuest. Encyclopædia Britannica, Inc., 25 May 2016.
http://quest.eb.com/search/132_1352630/1/132_1352630/cite. Accessed 11 Aug 2016.
In general, the sources you use in your research can be divided into a few groups depending on their author, intended audience, publication, and other factors.