Like the Earth, Mars is roughly 4.5 billion years old. Scientists believe that, at one time, Mars' surface flowed with liquid water, and its surface, like Earth's, was active with the forces of tectonic drift. Now, unlike our lush planet of oceans and forests, Mars is a dead world.

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Under Mars' current configuration, cold on the surface, with low air pressure, water can either evaporate or freeze, but cannot stay stable enough to flow as a liquid, says Sanjoy Som, writing for Nature. Certainly at some points throughout Mars' history, there was water on the planet's surface. But according to a new study a watery Mars would have been the exception.

Roughly 4 billion years ago, Mars started to lose its atmosphere. Like the Earth's, Mars' atmosphere would have insulated the planet, warming the surface above its current freezing temperatures. Over time, however, the solar wind—a flow of charged particles streaming from the Sun—blasted the planet's atmosphere into space. In the past 4 billion years Mars has lost as much as 95 percent of its air.

A thinner atmosphere allows smaller meteorites to penetrate it. And based on an analysis of the sizes of meteors that made it down to Mars' surface, says Liat Clark writing for Wired UK, researchers suggest that, throughout most of its history, Mars' atmosphere was likely too thin to support persistent surface water.

“The team does, however, offer an alternative theory for all the evidence of flowing water sources on the planet: "transient warming by eruptions, impacts, or infrequent orbital conditions could unfreeze the surface and shallow subsurface, allowing runoff, but would not last long enough to unfreeze ground at less than 1km depth,” says Clark.