For about 15 million years, up until 50 million years or so ago, the area now marked on the maps as the corners where the U.S. states of Wyoming, Utah and Colorado intersect was covered by vast bodies of water known as the Green River Lake System. Today the area is semi-arid, a rolling expanse of scrub brush, hills, canyons and flat-top buttes. But below the surface lies a fossil wonderland where the rich life that thrived in, around and above the lakes is preserved in stone.
The remains of birds, animals and fish that ended up on the lake bottoms were covered in what the National Park Service calls a “gentle rain” of calcium carbonate that eventually solidified into limestone, a whole ecosystem from the Eocene epoch that paleontologists have studied for more than a century.
Just as there is no surface evidence today of the rich Green River biosphere set in stone below, the surface of Mars has yet to yield solid proof that life ever existed in the vast seas that once flowed on the surface. The Curiosity rover edging its way across the floor of Gale Crater has found plenty of evidence that the area once was habitable, running with streams of water that Mars Science Laboratory Chief Scientist John Grotzinger has famously declared would be drinkable by today’s humans. But the Martian water we can detect today is frozen, and the dry rivers, streams and lakes found there have not been wet for 3.7 billion years.
Mars is Earth’s next-door neighbor, but time, distance from the Sun, and such other factors as the state of the two planets’ internal cores—solid versus molten—have created two very different environments. This month NASA’s $671 million Mars Atmosphere and Volatile Evolution (Maven) arrives at the planet with a set of instruments designed to help scientists understand why it is red (illustration), while Earth is blue. Also arriving will be India’s Mangalyaan Mars Orbiter Mission (MOM), that country’s first exploration beyond the Moon. And next month an Oort Cloud comet will swing past Mars, possibly stirring up the atmosphere in ways that may shed light on how it changes over time.
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