Life on Mars
Among our discoveries about Mars, one stands out above all others: the possible presence of liquid water on Mars, either in its ancient past or preserved in the subsurface today. Water is key because almost everywhere we find water on Earth, we find life. If Mars once had liquid water, or still does today, it's compelling to ask whether any microscopic life forms could have developed on its surface. Is there any evidence of life in the planet's past? If so, could any of these tiny living creatures still exist today? Imagine how exciting it would be to answer, "Yes!!"
Even if Mars is devoid of past or present life, however, there's still much excitement on the horizon. We ourselves might become the "life on Mars" should humans choose to travel there one day. Meanwhile, we still have a lot to learn about this amazing planet and its extreme environments. Since our first close-up picture of Mars in 1965, spacecraft voyages to the Red Planet have revealed a world strangely familiar, yet different enough to challenge our perceptions of what makes a planet work. Every time we feel close to understanding Mars, new discoveries send us straight back to the drawing board to revise existing theories. You'd think Mars would be easier to understand. Like Earth, Mars has polar ice caps and clouds in its atmosphere, seasonal weather patterns, volcanoes, canyons and other recognizable features. However, conditions on Mars vary wildly from what we know on our own planet. Over the past three decades, spacecraft have shown us that Mars is rocky, cold, and dry beneath its hazy, pink sky. We've discovered that today's Martian wasteland hints at a formerly volatile world where volcanoes once raged, meteors plowed deep craters, and flash floods rushed over the land. And Mars continues to throw out new enticements with each landing or orbital pass made by our spacecraft.

                                         Seek Signs of Life on Mars
To discover the possibilities for past or present life on Mars, NASA's Mars Exploration Program is currently following an exploration strategy known as "Seek Signs of Life."
This science theme marks a transition in Mars exploration. It reflects a long-term process of discovery on the red planet, built on strategies to understand Mars' potential as a habitat for past or present microbial life. Searching for this answer means delving into the planet's geologic and climate history to find out how, when and why Mars underwent dramatic changes to become the forbidding, yet promising, planet we observe today.
About 3.8-3.5 billion years ago, Mars and Earth were much more similar. Evidence from Mars missions suggest Mars may have been much warmer and wetter than we observe it to be today. In this ancient timeframe, scientists find the first evidence of microbial life on Earth. Did Mars provide similar environmental conditions for life long ago? If microbes were present on Mars in the planet's ancient past, could it exist in special regions today? And, even if microbial life never existed, might Mars provide a future habitat for human explorers someday in the future?
Because water is key to life as we know it, earlier Mars missions (2001 Mars Odyssey, Mars Exploration Rovers, Mars Reconnaissance Orbiter, Mars Phoenix Lander) were designed to make discoveries under the previous Mars Exploration Program science theme of "Follow the Water." Progressive discoveries related to evidence of past and present water in the geologic record make it possible to take the next steps toward finding evidence of life itself.
The Mars Science Laboratory mission and its Curiosity rover mark a transition between the themes of "Follow the Water" and "Seek Signs of Life." In addition to landing in a place with past evidence of water, Curiosity is seeking evidence of organics, the chemical building blocks of life. Places with water and the chemistry needed for life potentially provide habitable conditions. Future Mars missions would likely be designed to search for life itself in places identified as potential past or present habitats. Like all Mars Exploration Program missions, future missions will be driven by rigorous scientific questions that continually evolve from discoveries by prior missions. New and previously developed technologies will enable us to explore Mars in ways we never have before, resulting in higher-resolution images, precision landings, longer-ranging surface mobility and even the return of Martian soil and rock samples for studies in laboratories here on Earth.