In the absence of an actual visit, much of the evidence for water on Mars rests on careful study of satellite images such as this one.

The light comes from the upper right. The remains of two dendritic patterns are obvious: one across the center of the photo plus a much smaller twin just to the right of the first. Because such patterns characterize dendritic drainage systems that are found all over the Earth's continents, it seems highly likely that these too were formed by running water. In the cases above, it appears that the drainage ran from the upper left to the lower right.

When did the water run? This is difficult to estimate precisely without a visit and detailed sample collection. Relative ages can be assigned by studying the distribution and morphology of meteorite impact craters relative to the dendritic drainages. For example, the large flat crater in the upper left appears to be quite worn down and thus is relative old. Note that a couple of canyons seem to originate from this crater. In contrast, a number of small craters in the lower left and a medium-sized one in the upper right are distinct and well-formed, suggesting relative youth. There are a few such well-formed craters that interrupt the drainage pattern, suggesting that this feature has been around long enough for it to have been hit by the occasional meteorite. How long is this? It is difficult to say precisely, but it is certainly on the order of millions or tens of millions of years.

Evidence for water flowing on Mars within the past few million years comes from some recently discovered drainage features that have not been around long enough to have been hit by meteorite impacts. Such recent wetness is highly interesting and unexpected!

Many questions need better answers: What is the history of water on Mars? When did most of it flow? Where did it come from? Where did it go? How much is left? Was it around long enough for life to evolve? It is clear that sending geologists to Mars would result in a great deal of important and interesting science.