The concept of precipitation on Mars is not a new one. Scientists have long believed that the series of valleys that cover the planet's surface was caused by some sort of precipitation. The hot topic of debate, though, concerns what sort of precipitation was responsible. Was it rain or snow? Some researchers at Brown University, however, think they now have an answer.
The study at Brown revolved around four specific valley formations on Mars that were believed to have been caused by run-off from rain or snow caused by orographic precipitation. This kind of precipitation occurs when moist air is lifted over a mountain range. Most of the precipitation falls upwind of the mountain range, but some precipitation also runs off of the mountain, eventually creating a valley over a great amount of time. This is a common occurrence on Earth, therefore scientists believe it could also apply to explaining much of Mars' own topography.
The scientists began by looking at specific mountain ridges and crater rims on Mars. Based on what scientists know about Mars' early atmosphere, a model was then formulated to simulate the direction of the winds in those regions. Using what we currently know about orographic precipitation, this model was used to figure out where such precipitation might fall. The result? Through the simulations, the scientists discovered that the precipitation, which they believe to have been snow, would have been heaviest in the valleys of those areas on the planet. The idea is that the snow melted and fell down that side of the mountain, carving the valleys into the planet's surface.