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Does this map show an ancient ocean floor on Mars?
New evidence says "maybe". But it's a pretty good maybe!
It's been argued for years that Mars may have once had oceans of water, billions of years ago. Some catastrophe dried them up, making the evidence for them difficult to detect. Topographic (relief) maps look like there may have been two oceans in one spot, for example, separated by some time. There appear to be two separate shorelines, with one smaller, later ocean existing where there once had been a much larger ocean.
But that's circumstantial. More direct evidence is needed.
So some scientists speculated a bit. Rocks containing elements like potassium, thorium and iron would get made in the highlands (near volcanoes), then get transported down into the lowlands. If there were an ocean there, those elements would get leeched out of the rocks by the water. Then, when the water evaporated, those elements would be deposited in a thin layer on the surface.
On board the orbiting probe Mars Odyssey is the Mars Gamma-Ray Spectrometer, a device which can measure the abundance of elements on the Martian surface. When it was trained on the lowlands of Mars, it found evidence to support the existence of those oceans! The elements in question were most abundant below the shorelines, as expected, when compared to regions outside (above) the shorelines. The regions with higher concentrations of potassium are marked in red and yellow in the map above, right where the lowlands are.
While this doesn't prove an ocean as big as the United States once occupied a large chunk of Martian real estate, it's further evidence of it. Scientists are still arguing over whether Mars had long-lasting, ponded water, or if it was released in short, transient events, only to evaporate quickly away. But either way, we're not arguing over whether Mars had water, just how long it lasted.
Either way, Mars is an incredible planet. It may have once been much more Earth-like, but then something went wrong. Maybe it was the formation of the giant volcanoes (indicated by the red arrows in the above image), or the loss of its magnetic field that exposed its atmosphere to erosion by the solar wind. As we study Mars more, we get closer to figuring this out. And make no mistake: knowing where Mars went wrong gives us great insight into our own planet. If you think we're wasting money on researching Mars, then I suggest you take a cold, hard look at that cold, hard planet, then look out your window at our own home world.
There but for the grace of science may go us.