Compared to Earth, Mars has an incredibly harsh environment. Four decades of Mars exploration, which began in 1960, have demonstrated that the planet is an inhospitable, frigidly cold, arid place. Its surface is littered with craters and giant volcanoes and abused by planet-wide dust storms. Mars' atmosphere is so thin that the pressure is less than 1% of the atmospheric pressure at Earth's surface. The thin atmosphere on Mars contains virtually no oxygen, a requirement of life for almost all organisms on Earth, and offers no protection from incoming ultraviolet solar radiation. The planet experiences extreme fluctuations in temperature from day to night, and the average surface temperature is a frosty -63°C (-81°F). At these temperatures, water cannot exist in its liquid form, and all life, at least as we know it, requires liquid water.

Yet, Mars remains an alluring planet to explore for signs of extraterrestrial life. Despite the lack of liquid surface water, ice exists in the planet's polar ice caps and water vapor is present in the atmosphere. There is also evidence of subsurface water. More importantly, liquid surface water may have been present in the past. Previous missions to Mars have found indicators of water, such as channels that could have been carved by flowing water and characteristic erosion features in craters. This means Mars may have once been warm enough to have liquid water. Many scientists are convinced that where they can find water, there is a chance they may find life.

In 2003, the Mars Exploration Rover mission was launched with the goal of learning more about the geological history of the planet, particularly the role that liquid water may have played. To carry out this task, two robotic geologists were sent to explore the planet. The rovers, controlled by team members on Earth, had to weather the hostile environment to carry out their missions. Even for a robot, the extreme temperature changes and dusty conditions, along with the rocky and hazardous terrain, posed challenges. Nevertheless, Spirit and Opportunity landed in January 2004 and successfully completed their three-month-long primary missions. They delivered to scientists conclusive evidence that liquid water once existed on the surface of Mars. As of August 2005, the rovers were still operating, continuing their active exploration of the planet.

Investigate the variables that affect a parachute's rate of descent in this NOVA classroom activity.

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