1. Mars

Mars, the Red Planet, is so far away that it is difficult to study from Earth. Photos of Mars like this one, taken by the Hubble Space Telescope, are the best ever taken from Earth’s orbit. To get more detail and learn more, we have sent spacecraft to Mars: The first was Mariner 4 in 1963, and then the two Viking missions in 1976. In 1997, Mars Global Surveyor and Mars Pathfinder spacecraft both arrived at Mars, providing some of the best images of the surface obtained so far.

Although Mars is now a freezing desert, its climate has changed enormously; early in its history, Mars was much warmer and wetter (slides #2, #4, #5, #7) and life might have flourished. The idea that Mars’ climate could change was reinforced when astronomers turned the Hubble Space Telescope on the planet in 1995. They found that Mars had become colder and dryer than it was in the late 1970s at the time of the Viking missions. A change of just 2°F would make a major difference to life on Earth, but (on average) Mars had become 36°F colder than 20 years ago. For scientists looking for signs of life now or signs of former life on Mars, this evidence of dramatic climate change over a short period of time gives greater credibility to theories that Mars has undergone major climate changes in the past.

Hubble Space Telescope Wide-Field Planetary Camera 2 image, February 1995. Philip James (University of Toledo), Steven Lee (University of Colorado, Boulder), NASA.

Click here to view a high-resolution version of the image (6.96 MB)