NASA says:“The Defining Question for Mars Exploration: Life on Mars? …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?… Imagine how exciting it would be to answer, “Yes!!” Unquote (quoted from http://www.nasa.gov/mission_pages/mars/overview/#.UwR9YPmSxqU )