From “Surface of Mars an Unlikely Place for Life After 600-Million-Year Drought, Say Scientists” (ScienceDaily, Feb. 3, 2012), we learn,
The results of the soil analysis at the Phoenix site suggest the surface of Mars has been arid for hundreds of millions of years, despite the presence of ice and the fact that previous research has shown that Mars may have had a warmer and wetter period in its earlier history more than three billion years ago. The team also estimated that the soil on Mars had been exposed to liquid water for at most 5,000 years since its formation billions of years ago. They also found that Martian and Moon soil is being formed under the same extremely dry conditions.
Satellite images and previous studies have proven that the soil on Mars is uniform across the planet, which suggests that the results from the team’s analysis could be applied to all of Mars. This implies that liquid water has been on the surface of Mars for far too short a time for life to maintain a foothold on the surface.
Given the high hopes, it’s a good bet that an assessment this blunt is fairly accurate for the surface. Some researchers, however, are looking underground.
See also: Unambiguous evidence for water on Mars?
Life moving from Mars to Earth idea to be tested by probe (live water bears aboard)