It’s a sad thing when a planet dies young. Earth has been pulsing with life for close to 4 billion years—a thumping example of what can happen when everything goes right. Mercury and Venus were stillborn—one a dry, airless, roasted rock; the other suffocated by a dense, overheated atmosphere that ruled out life before it could even get started. And then there’s Mars—once balmy, once wet, once protected by a blanket of air. It may once too have been home to life—and it still could be, though likely in the form of underground microbes.
But even in its cold and dessicated state, Mars stirs still. Its tenuous winds blow, its ice caps expand and shrink, tracks that appear to be a result of trickling springtime water form on mountain faces. Now, it seems that Mars has had a lot more than that going on lately. According to a new study in Science, the planet was the scene of a recent—in geological terms—flood, and if the analysis is correct, it could easily happen again. Never mind the idea of Mars as a planetary has-been, there’s at least a chance it could be a still-is.
The new study, led by geologist Gareth Morgan of the Smithsonian Institution in Washington, DC, relied on new readings returned by NASA‘s Mars Reconnaissance Orbiter (MRO), which has been circling the Red Planet since 2006. Some of the spacecraft’s most intriguing surveys have involved the dry sea-beds, riverways and flood channels etched all over the planet, which testify to a wet and turbulent past. All of those features date from long ago—about 3 billion years, which means that the planet has been largely without water for about two-thirds of its 4.5 billion year history.
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That, at least, is the conclusion to be drawn from the waterways that can be seen, but there are some that can’t be. Mars has flowed with lava as well as water over its long life, and the tracks cut by floods have sometimes been be covered up by eruptions. One of the most mysterious of the obscured waterways is the Marte Vallis formation in the planet’s northern hemisphere. All that’s visible of Marte Vallis is its outflow channel, but where it begins, how deep it is and how old it is have been mysteries. MRO, however, can look straight through the lava, thanks to its Shallow Radar (SHARAD) system.
The SHARAD imager has been used before, but principally to study the polar caps. This was the first time mission planners had used it to study ancient flood patterns, and that turned out to be a very good idea. For starters, Marte Vallis is a lot bigger than anyone realized—about 600 mi. (965 km) long, 62 mi. (100 km) wide and 260 ft. (80 m) deep. The depth, especially, was a surprise. “This is about twice what we had thought,” says Morgan, “and the channels could be deeper still.”
More important was what the shape and topography of the channels revealed. Initially, the floods etched a central waterway, with narrower tributaries threading alongside it; these later merged into larger, variegated channels. There were also tear shaped islands that stood for a short period, before being eroded away on top. “They’re similar in some ways to visible channels elsewhere on Mars and we were able to compare them,” says Morgan. “At Martes Vallis, we identified multiple stages of flooding.” The ruggedness of the waterways also suggest that the flooding played out quickly and violently, on the order of weeks or months.
The biggest surprise, though, was the source of all the water. The prevailing theory had been that a large lake or a small sea had breached its barriers and spilled over the surrounding landscape. This is similar to what happened during the Missoula floods in North America’s Pacific northwest when ice dams repeatedly ruptured late in the last Ice Age, 13,000 to 15,000 years ago. The alternative explanation for Martes Vallis was that Marsquakes or volcanoes had cracked open the ground above subsurface reservoirs and released the water. “A fracture system could have been responsible,” says Morgan, “but in the past, the fractures we could see nearby did not cut across the region.”
The SHARAD readings changed that. The telltale cracks, it turns out, indeed radiate precisely where they should to produce the kind of flood the channels suggest. “The floods, fractures and lava could all be linked to underground forces,” says Morgan. “The lava pushes its way up, forms dikes and then breaks through the surface.”
Most dramatic of all was not the how of the flooding but the when—and the when was extremely recent. The age of surface features on worlds can be determined by the simple expedient of counting craters. We know how heavy the cosmic bombardment was during the early days of the solar system and how it tailed off over time. Very heavily cratered surfaces are thus typically old; ones with fewer craters have been resurfaced comparatively recently and then lightly tattooed anew. In the case of Martes Valles, there appear to have been two episodes of volcanic resurfacing—one that occurred 500 million years ago and one just 10 million years ago. The flood channels cut into the older of the two and are partly obscured by the younger. “So the flooding is framed by those dates, taking place sometime between 10 and 500 million years back,” says Morgan.
Given the size of Mars, it’s all but certain that its core is still radioactively and gravitationally hot, meaning that volcanism should be continuing up to the present, and indeed some eruptions have been dated from as little as 2.5 million years ago. This makes future eruptions likely—and means future floods are a real possibility. Judging partly by the volume of water spilled during the Martes Vallis event and partly by the fact that the upwelling flood did not cause the surrounding landscape to collapse, Morgan speculates that there could be a layer of water stretching across much or even all of Mars, at a stable depth of 5 to 10 mi. (8 to 16 km). It could be ice, slush or water, depending on how much lava and heating any one region gets. And as we’ve learned from Earth’s polar regions, as long as you’ve got water, extreme cold is no bar to life.
Mars will surely never catch up to us in our solar system’s biology steeplechase. But if the new findings indicate anything, it’s that the Red Planet may at least still have a horse in the race.