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Because of the proximity and likeness of Mars to Earth, scientists have long speculated about the possibility of life on Mars. As early as the mid-17th century, Astronomers observed polar caps on Mars. And by the mid-19th century, scientists discovered certain similarities to Earth, including the length of the day and axial tilt. But in 1965, photos taken by the Manner 4 probe revealed Mars without rivers, oceans, or signs of life and in the mid-1980s it was discovered that Mars, unlike earth, no longer possessed a substantial global magnetic field, allowing radiation to reach the planet's surface, and solar wind to eliminate much of Mars's atmosphere even in the course of several billion years. More recent probes have focused on whether there was once water on Mars. Some scientists believe that their question is definitely answered by the presence of certain geological landforms. Others offer different explanations, such as wind erosion or carbon dioxide oceans that may be responsible for this formation. Mars's Rovers, Opportunity and Spirit, which have been exploring the surface of Mars since 2004, have both discovered geological evidence of past water activity. These finding substantially bolster claim that there was once life on Mars.