Today, the surface pressure on Mars is 6 millibars.

NPR: Revisiting the Viking Missions to Mars

The 2, 000-pound Mars rover Curiosity made its landing on Mars on August 6, 2012, and has been sending back fascinating images and data ever since.

CNN: Mars may have once supported life, NASA says

See more images sent by the rover:The 2, 000-pound Mars rover Curiosity made its landing on Mars on August 6, 2012, and has been sending back fascinating images and data ever since.

CNN: Mars may have once supported life, NASA says