NASA has sent over 40 different unmanned space probes to Mars since the beginning of the space age. Of these, only 10 have ever returned images or data back to Earth. Their main aims have been to investigate the Martian atmosphere, look for the possibility of life, signs of ancient life, search for surface water, and discern if Mars may one day be a habitable planet for humankind.
These missions have discovered that Mars is a very dry planet with an arid landscape. However, there are some clues to habitable conditions like frozen carbon dioxide deposits which could possibly support microbial life.