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

Our mission, as it's planned, is to fly over some of the oldest terrain on Mars, very highly magnetized crustal material that dates back to the beginning of Mars, 4.6 billion years ago.

NPR: Revisiting the Viking Missions to Mars

We believe that in its early history - remember, Earth and Mars, the solar system, 4.6 billion years old. 4.6 billion years old the age of Earth and Mars and the solar system.

NPR: Revisiting the Viking Missions to Mars

Mars was bombarded with approximately 2.6 million of these pulses.

BBC: By BBC News Online Science Editor Dr David Whitehouse