"Old Soaker," a 4-foot long slab of mudstone bearing cracks that may have formed in drying mud over 3 billion years ago. (NASA/JPL-Caltech)
In its quest to find signs of water in the sediments of Mount Sharp, NASA's rover Curiosity has turned up some tantalizing clues to when and how the young, watery Mars began to dry up.
Images of geologic formations and measurements of mineral residues collected over two years tell a tale of a watery world caught in the process of drying up, and maybe not giving up without a fight.
A four-foot-wide patch of ancient mudstone called "Old Soaker," encountered late in 2016 within Mars' Gale Crater, may be a snapshot of the moment Mars began its transition from a wet and possibly lively planet to the cold, dry, apparently lifeless world we know today.
Bearing a network of cracks that may have formed in drying mud, Old Soaker shows that even as water was becoming scarce on Mars, it persisted in seeps, trickling streams and shallow desert lakes.
The moment captured in the Old Soaker mudstone over three billion years ago is one picture in a larger album that Curiosity has been assembling since it landed in 2012. Its compendium of Martian climatic history has captivated our imaginations.
Curiosity's Quest For Water
Did liquid water ever exist on Mars? When, and how much? Was the environment ever capable of supporting life?
These are the big questions Curiosity went forth to tackle.
So far, Curiosity's confirmed that liquid water once flowed into and pooled within Gale Crater, from very early in its history.
Imagery of geologic formations Curiosity captured in its earlier travels tell a captivating story of the young Gale Crater Lake. Sedimentary layering, lakebed mudstone, and aggregations of river pebbles and stones found in the oldest, lowest formations of Mount Sharp reveal that a wide deep lake, fed by rivers and streams, may have persisted in Gale Crater for many millions of years.
Taking Gale Crater and its ancient lake as an indicator of Mars' global environment, we know the atmosphere had to be much warmer and thicker than it is today. It almost certainly supported a water cycle of precipitation, runoff, pooling in lakes and seas, and evaporation similar to Earth's.
The crater formed 3.8-3.5 billion years ago when an asteroid hit Mars. It gradually filled though wind and water action with layer upon layer of sediments.
In more recent times after Mars dried up, wind eroded some of the infill, sculpting the multi-layered mountain Curiosity is doggedly crawling up today. As it visits each formation of sedimentary rock on its uphill climb, Curiosity is reading the pages of Mars's history.
Death Throes of a Drying World?
Now seven years into its mission, Curiosity has climbed to higher points on Mount Sharp, analyzing layers of rock that formed at different times and under different climatic conditions.
The story told by Old Soaker's mudstone cracks may be a page in a saga of tumultuous environmental change. Mars' environment dried up, became wet again, then swung back to dry in repeating cycles. Wetter periods preceded and followed the dry episode that formed this specimen, based on what Curiosity found at adjacent rock layers in the Mount Sharp stack.
Early in its mission Curiosity detected an abundance of clay minerals in the oldest layers of lake bed sediment. They indicated that those layers were deposited when lake waters were deep and plentiful. Freshwater conditions on Earth formed similar clays.
Higher on the mountain's slopes, the rover found chloride and sulfate salts in younger sediments, dated to about 3.5 billion years. Such mineral salts are known byproducts of bodies of water undergoing evaporation, like a lake drying up during a shift to a more arid climate.
Take a Walk Through a Mars-like Past—on Earth
If you've been to a place like Death Valley National Park, you may have witnessed evidence of long-gone water in that dry and desolate landscape.
Mineral salts, once dissolved in the waters of an ancient lake that filled today's Death Valley, now cover huge areas of the valley floor in thick, white crystalline deposits. When the drying climate east of the Sierra Nevada mountains reduced the 70-mile-long, 600-foot-deep Lake Manly to a salt-lined desert valley, it left behind the briny residue.
It's still possible to see, and even walk upon, ancient shorelines carved into the side of Shoreline Butte by the action of lapping waves.
In some side canyons of Death Valley are mudstone formations bearing the petrified imprints of ripples formed in the lake floor mud, now preserved in stone.
A few briny "springs" still issue seasonal seepage and offer a watery habitat for pupfish, the surviving descendants of that paleolake's fishy inhabitants.
But for all the signs of deep waters, flowing streams, and a once- thriving ecosystem, Lake Manly dried up thousands of years ago.
Curiosity is prospecting the Martian desert and turning up similar evidence of Mars's ancient waters. It's looking back three or more billion years, not just a few millennia.
No Signs of Life—Yet
One of Curiosity's mission goals is to assess Mars' past environment to determine whether it could ever have harbored some form of life. The result, so far, appears to be yes. When it more closely resembled Earth's conditions, Mars may have been hospitable to some form of life, if only single-celled organisms.
Scientists didn't equip Curiosity to look for actual signs of life—just the water it might have lived in.
Next year, NASA plans to launch its next mission to the Red Planet, the Mars 2020 rover. It will bookend Curiosity's mission by directly searching for the chemical residues left behind by any would-be Martian life.
So, get ready for the next chapter in the Martian saga.
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