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Mars Orbiter Finds Evidence For Ancient Rivers, Lakes
Cowards Anonymous points out news that studies based on data from the Mars Reconnaissance Orbiter have found that vast regions of Mars contained rivers and lakes when the planet was young. The studies also suggest that the water existed for quite some time, often in standing pools, which are conducive to the formation of basic organic matter. NASA provides a color-enhanced photo of a delta within a crater. Quoting:
"The clay-like minerals, called phyllosilicates, preserve a record of the interaction of water with rocks dating back to what is called the Noachian period of Mars' history, approximately 4.6 billion to 3.8 billion years ago. This period corresponds to the earliest years of the solar system, when Earth, the moon and Mars sustained a cosmic bombardment by comets and asteroids. Rocks of this age have largely been destroyed on Earth by plate tectonics. They are preserved on the moon, but were never exposed to liquid water. The phyllosilicate-containing rocks on Mars preserve a unique record of liquid water environments possibly suitable for life in the early solar system."
They had plenty of greenhouse gases. The problem was that after the geomagnetic field of Mars was lost, the solar wind was able to strip away the atmosphere, leaving it today at about 5 to 10 millibars (in contrast with the Earth which is about 1000 millibars).
Martian geological time is subdivided into a number of time periods based upon major geomorphological features seen from orbit -- major crater basins, the density of craters (generally speaking, crater frequency was higher in the deep past -- as on the Earth's Moon), canyons and channels such as Valles Marinaris, and volcanoes. While it isn't possible to determine their exact numerical age, it is possible to figure out their relative age (i.e. the order of the events that made them). For example, the overlapping shapes of craters tells you which impact formed first. If a volcano has a crater on it, then obviously the volcano formed first and then the crater. If a channel is eroded into a crater, then the channel came after. That kind of thing. So, there's a reasonably detailed relative chronology for events on Mars, and this is divided into eras known as (from oldest to youngest) the Noachian, the Hesperian, and the Amazonian.
Using crater densities and the fact that rocks were recovered and dated on the Moon, it is possible to link the better-known chronology of the Moon to that of Mars. There are significant uncertainties of course, but generally speaking that allows people to estimate that the Noachian was from about 4.6 billion to about 3.5 billion years ago, essentially the time when the cratering frequency started to drop off on the Moon. There is ample evidence that at this time on Mars there was freely-flowing water on the surface, hence, "Noachian".
The pages cited above has some really nice charts and descriptions, and the wikipedia page has a map showing the distribution of the deposits of different ages.