The moment the news was released that the Mars Reconnaissance Orbiter had discovered opal on Mars, I knew that at one time, there had once been persistent masses of liquid water on the Martian surface. Hydrated silica, better known as an opal, is a gel which contains as much as 21% molecular water by weight. Essentially, opal is quartz, with water trapped in its crystal structure: and it can only be created when basaltic rock is dissolved in water, which then evaporates.
Not only has the Orbiter spectrometer discovered opal, it has discovered opals across large regions of Mars.
Water is life. No form of life as we know it is able to sustain its entire life cycle through reproduction and a new generation in the absence of water. The absence of water on Mars had once been one of the major stumbling blocks to any hypothesis that life had ever existed on Mars. No more.
The evidence is now strong that Mars had flowing water some three billion years ago, perhaps two billion years ago. Now, the possible discovery of opal moves the likely timeline when water still flowed openly on Mars forward to within a few hundred million years. The longer liquid water existed on Mars, the longer the window during which Mars may have supported life.
Even these estimates may be erring on the conservative side. Some geological evidence seems to suggest that there may have been open geysers within the past decade.
We don't yet know whether past water was truly persistent or the result of odd flooding events. Just because there was flowing water on the surface of Mars at some point in the past does not mean it remained on the surface of Mars throughout. That could make a difference to whether life on Mars was able to get a true foothold.
Yet many terrestial species at the bacterial level are able to survive extended periods without water. Even some vertebrate species can survive a decade or so of drought conditions. For every Martian environment we have discovered thus far, be it acid, alkaline, volcanic, saline, low air pressure, minimal or no oxygen, complete absence of light, or just plain dry: we know of entire terrestial ecosystems capable of making their home in it. All that is needed is a minimal amount of water at some point in the life cycle. All the evidence increasingly suggests that Mars has that, and has had it for several billion years.
Meteorites believed to have originated on Mars show evidence of having been touched by water. Some show evidence that they may have been touched by bacterial life.
An indirect piece of evidence in favour of there still being a large liquid water reservoir under the poles comes to us from a study of Lake Vostok in Antarctica. The comparable heat flow patterns suggest that any lake deeper than 63 metres could have survived subsequent surface glaciation without itself freezing to the bottom: which further implies that organisms living in that lake could have survived to the present day.