If there is life on Mars, scientists believe it’s likely to be tiny organisms that can survive below the planet’s surface, without sunlight or oxygen, nourished by the minerals available even in that harsh environment. In other words, said Ricardo Amils Pibernat, a researcher at the Center for Astrobiology in Madrid, past or present life on the red planet could well resemble the unusual microbes that populate Spain’s Rio Tinto. The 58-mile-long river, which flows through one of the world’s largest deposits of pyrite, or fool’s gold, has a pH similar to that of automobile battery acid and contains virtually no oxygen in its lower depths.