Exploring an ancient lakebed on Mars -- a now-vanished fresh-water lake that increasingly confirms the past habitability of the red planet -- NASA's Curiosity rover is looking for areas where erosion may have uncovered pristine layers in which organic compounds -- and possibly remnant traces of life -- might still be found, scientists said Monday.
"Really what we're doing is turning the corner from a mission that is dedicated to the search for habitable environments to a mission that is now dedicated to the search for that subset of habitable environments which also preserves organic carbon," Principal Investigator John Grotzinger told reporters Monday. "That's the step we need to take as we explore for evidence of life on Mars."
In papers published Monday in the journal Science, the Curiosity team unveiled critical new findings, including measurements of the radiation environment at Mars, which does not have an active magnetic field to shield the surface from its harmful effects. As it turns out, cosmic rays from deep space can penetrate the upper few feet of martian rocks and soil, breaking apart organic compounds and effectively erasing evidence of past life or the materials necessary for life as it is known on Earth.
To find that evidence, scientists are looking for places along Curiosity's planned route where wind erosion over many millions of years has uncovered underlying beds in the relatively recent past, before energetic cosmic rays have had time to destroy whatever organic compounds might be present.
"Our measurements show that the organics could be preserved at a depth of one meter, even life could possibly, if it existed, survive at a depth of roughly one meter on Mars," said Robert Wimmer-Schweingruber, a co-investigator with the Radiation Assessment Detector on Curiosity.
"It also shows us that as radiation penetrates into the soil ... it reaches the natural background at a depth of roughly three meters. In the top surface layers of four to five centimeters, which [Curiosity] can drill into, it would, with this radiation, reduce the preserved organics by a factor of roughly 1,000 over about 650 million years. So if you want to find organics, you need to find places where it hasn't been exposed for such a long time."
Scientists were able to date a lakebed rock Curiosity drilled into earlier by measuring how an isotope of potassium decayed into argon. The rock in question was formed 3.86 billion to 4.56 billion years ago. It was once buried under many feet of rock and soil, but the martian winds slowly eroded the upper strata, bringing it into reach of Curiosity's drill.
Ken Farley, a Curiosity researcher with the California Institute of Technology, said an analysis of the drill sample showed the erosion happened relatively recently, over the past 60 million to 100 million years, and that about three feet of strata is removed every million years or so.
The relatively recent uncovering of the lakebed clay "suggests that there will be some organic degradation, but perhaps not extensive organic degradation," Farley said. "But more importantly, we now have a model of where to look for the least cosmic ray irradiated rock we can get to. We simply drive to the downwind scarp and drill at the base of that scarp."
By drilling within three feet or so of a scarp, or ridge line, where the martian wind has uncovered lower layers, "we might get surface exposure ages, cosmic ray dosages, of only about a million years."
If so, Curiosity would have a much better chance of detecting complex organic compounds.
"Our hypothesis is that we can decrease the surface exposure age by drilling right up at those edges," Farley said. "And then we can test that hypothesis by obtaining the surface age date. That's our goal as we go forward here and we think the big step for the mission that takes us closer to the search for life on Mars is being able to reduce this risk of radiation, which is a very Mars-unique process."
Curiosity landed in Gale Crater in August 2012. Since then, it has been slowly making its way toward a towering mound of layered rock in the heart of the crater known as Mount Sharp, stopping along the way to investigate intriguing formations.
One such site is known as Yellowknife Bay, where Curiosity's power drill collected samples confirming a once-habitable environment. Scientists do not yet know the full extent of the lake they now believe existed there, but it likely stretched at least 30 miles around the base of Mount Sharp.
"Imagine something, an environment you might have had back on Earth about .... 10,000 years ago," Grotzinger said. "Cool, cold, maybe even ice available at the time. ... The size of these lakes would have been like the small finger lakes of upstate New York, something like that.
"The important thing we learned about the chemistry, with the clay minerals forming there, we have a moderate to neutral pH. Also we know from the absence of salt in the rock ... that lake didn't have a lot of dissolved salt in it. And finally, we have the kinds of chemicals and minerals that would have allowed simple micro-organisms to live in that environment."
In the absence of oxygen, such micro-organisms likely would have had to get by with a process known as chemolithotrophy, or "eating rock" as a summary in Science put it.
But finding actual fossils, or any remnants of ancient microbial life, assuming any are there to be found, will remain a major challenge.
"The key thing here is that ... if you go back into rocks that are billions of years old and ask what remnants of life there are, it is rare, rare, rare to find an actual microfossil," Grotzinger said. "It is a little bit less rare to find a large organic molecule. We call those chemofossils.
"And so, the trick is to make sure you have enough of the good minerals and as little as possible of the bad chemical compounds that will (alter) them. That's the game we're now weighing in on, in addition to quantifying radiation exposure."
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