Curiosity dates rock, finds potential good news for astronauts and search for life
By David Szondy
December 9, 2013
The chances of life having once existed on Mars got a boost this week alongside good news for astronauts on any future expeditions to the Red Planet. Six papers from Curiosity team members presented to the autumn meeting of the American Geophysical Union in San Francisco revealed that they had directly dated their first Martian rock, gave details of an ancient lake where life may once of existed, and found new evidence about the radiation hazards that explorers and colonists may one day face.
The highlight of the announced results was the first geological dating of a rock on Mars. It came from "Cumberland," the second rock Curiosity drilled. This was not only the first sample to be dated on Mars, but also the first dating to be carried out on-site on another planet.
The method is a standard geological test called potassium-argon (K-Ar) dating. It’s a 60-year old radioisotope dating method that is the only one capable of dealing with extremely old rocks up to four billion years old. It’s based on the decay of the radioactive potassium-40 isotope, which has a half-life of 1.28 billion years, into argon-40.
The clever bit is that argon is a gas and in igneous rocks this gets trapped in the rock’s crystalline structure. This means that, first, the laboratory inside of Curiosity can measure the ratio of potassium-40 to argon-40 and, since the rate of radioactive decay is constant, measure how old the rock is, The other brilliant thing is that when the rock melts, all the argon can escape. When the rock recrystallizes, it can trap the gas again – effectively resetting the clock. This provides geologists with a lot of confidence as to the date. In the case of Cumberland, the sample of was of mudstone, which is a sedimentary rock made up of weathered igneous rock particles, which can be dated.
The latest estimates put the age of Cumberland at 3.86 billion to 4.56 billion years old. This tallies with the estimate already made of the region of Gale Crater, which was based on counting the local impact craters and comparing them with crater densities on the Moon. This acts as a good baseline, thanks to dates recovered from the rock samples brought back by the Apollo astronauts.
"The age is not surprising, but what is surprising is that this method worked using measurements performed on Mars," says Kenneth Farley of the California Institute of Technology (Caltech) in Pasadena "When you're confirming a new methodology, you don't want the first result to be something unexpected. Our understanding of the antiquity of the Martian surface seems to be right."
Weathering rock and organic compounds
Another interesting point about the dating of the Cumberland drill sample is that it helped to not only find out how old it is, but how fast it weathered over the ages. Cosmic rays hitting the rocks can also produce gases, which can be measured. The deeper the rock, the fewer cosmic rays can penetrate. This means that by measuring three target gases at different depths, the Curiosity team was able to deduce how fast the rock wore away, exposing deeper layers to the rays.
In the case of Cumberland, the exposures dated between 60 million and 100 million years, indicating that the sand scours the rock very quickly – at least in geological terms. Scientists estimate the rate of wear at 1 m (3 ft) per million years.
"The exposure rate is surprisingly fast," Farley says. "The place where you'll find the rocks with youngest exposure age will be right next to the downwind scarps."
These very young rocks makes NASA optimistic about finding organic compounds in the rock deposits. Such compounds are the building blocks of life and would be a necessary condition for the presence of past life. However, it wouldn’t be conclusive because organic compounds can form naturally and Mars is constantly receiving them from space. And, of course, there’s the danger of such compounds having been brought along with Curiosity and contaminating the samples. However, the Cumberland drilling found increased amounts of organics compared to other samples.
"We're making progress on the path to determining whether there are Martian organics in there," Doug Ming, of NASA's Johnson Space Center, Houston says about the Cumberland sample. "We detect organics but can't rule out that they might be brought along from Earth."
A lake with life?
Ten months ago, NASA announced evidence that the "John Klein" drill sample in the "Yellowknife Bay" area showed positive signs that Yellowknife Bay is composed of an ancient clay-rich lakebed where life might once have existed billions of years ago.
Scott McLennan of Stony Brook University in New York says that the way the minerals that make up Yellowknife Bay weathered and were deposited from their upstream source shows a lot about the conditions of early Mars. One example is the presence of smectite
"Smectite is the typical clay mineral in lake deposits," says David Vaniman of the Planetary Science Institute in Tucson, Arizona. "It is commonly called a swelling clay – the kind that sticks to your boot when you step in it. You find biologically rich environments where you find smectites on Earth."
More to the point, NASA says that these favorable conditions lasted for millions, if not tens of millions of years, where the subsurface regions remained wet while the lakes and rivers disappeared. The presence of clay shows that Mars had fresh water about four billion years ago. Scientist say that as Mars dried, deposits of water became too acidic and briny to support life.
Curiosity’s findings indicate that the lake that made up part of Gale Crater formed before this happened, improving the chances that life once existed there. In addition, the clay is partly composed of iron- and sulfur-containing minerals that acted as electron acceptors as they oxidized and so could have provided an energy source for microbial life.
"This habitable environment existed later than many people thought there would be one," says John Grotzinger of Caltech. "This has global implications. It's from a time when there were deltas, alluvial fans and other signs of surface water at many places on Mars, but those were considered too young, or too short-lived, to have formed clay minerals. The thinking was, if they had clay minerals, those must have washed in from older deposits. Now, we know the clay minerals could be produced later, and that gives us many locations that may have had habitable environments, too."
It isn’t just fossil microbe hunters who have more cause to be optimistic. Curiosity has also collected daily measurements of radiation on the surface of Mars and NASA says that this is potential good news for any astronauts who visit the planet. According to the space agency, the average dosage from August 2012 to June 2013 was 0.67 millisieverts per day, though this comes with the caution that the Sun has been uncharacteristically inactive in recent years, so there haven’t been any major solar storms since Curiosity landed. Therefore, this is a measurement of background radiation and cosmic rays.
To put the dosage into perspective, a typical chest X-ray is about 0.02 millisievert. NASA estimates that a crew on a round trip voyage to Mars would receive a total radiation dose of 1,000 millisieverts. This isn’t a trivial dose and someone exposed to this would have a five percent greater risk of developing a fatal cancer in their lifetime. NASA says its current risk threshold for astronauts is three percent, but it says that the new data will help engineers in planning a manned Mars mission and that the agency is collaborating with the Institute of Medicine of the National Academies to address the ethics, principles and guidelines for health standards for such a mission.
"Our measurements provide crucial information for human missions to Mars," says Don Hassler of Southwest Research Institute in Boulder, Colorado. "We're continuing to monitor the radiation environment and seeing the effects of major solar storms on the surface at different times in the solar cycle will give additional important data. Our measurements also tie into Curiosity's investigations about habitability. The radiation sources that are concerns for human health also affect microbial survival as well as preservation of organic chemicals."
The six papers presented at the San Francisco meeting were published online by Science Express.
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