WHEN did Mars have lakes and rivers?

Mars has no surface liquid water today, but Slide show 4.62 shows clearly that it had lakes and rivers in the past. But when?

Scientists can answer this question, at least approximately, by studying impact craters in and around the dried up lakes and rivers on Mars. Notice, for example, that image below (repeated from Slide Show 4.64) shows a few impact craters located on top of a dried up river bed on Mars. This tells us that these impact craters are from impacts that occurred after the rivers dried up, since otherwise they would have been erased (at least partially) by the river flow. Now, remember that scientists can estimate the age of a surface from the number of impact craters. It is not an exact estimate, but based on the number of impact craters in and around the dried up lakes and rivers on Mars, scientists have concluded that the water dried up at least 2 to 3 billion years ago.