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Scalding Rains, Flash Floods and Worse Plagued Ancient Mars

Mars in the popular imagination is a planet that was once warm and wet, a place that might have fostered life. But new research shows how these imagined pleasant periods were brief, hellish, and punctuated by utter catastrophe.

[...] Roughly 25 space rocks between 60 and 150 miles in diameter (100-250 kilometers) gouged the Martian surface every 10-20 million years back then. An impact of this size would rock the planet, fueling quakes and volcanic activity. But that would not be the end of it. A typical impact, Segura said, would have generated enough water in the ensuing years to bury the entire planet under "tens of meters" of water, or more than the height of a two-story building.

[...] Segura described the details of a typical large impact, one roughly ten times bigger than the 6-mile-wide (10-kilometer) asteroid suspected of killing off the dinosaurs. Vaporized rock and ice from the incoming object and the impact site expands ballistically in a huge cloud, she said, filling the global atmosphere.

Eventually, the atmosphere cools enough to condense the water. Heavy rains ensue -- up to six feet of scalding rain every year -- lasting for perhaps hundreds of years. Meanwhile, the hot rock layer melts ice in the soil of Mars. [...] "We find that globally, Mars will be above the freezing point of water -- all water will be in the liquid phase -- for years to millennia for the largest objects."

[...] While some scientists have theorized that the wet periods on Mars fueled longer lasting, greenhouse-like climates that would have been kind to evolution, Segura and her team think otherwise. They envision a cold and dry planet, "an almost endless winter, broken by episodes of scalding rains followed by flash floods."