[meteorite-list] NASA Curiosity: First Mars Age Measurement and Human Exploration Help

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.php?release=2013-356

NASA Curiosity: First Mars Age Measurement and Human Exploration Help
Jet Propulsion Laboratory
December 09, 2013

NASA's Curiosity rover is providing vital insight about Mars' past and 
current environments that will aid plans for future robotic and human 
missions.

In a little more than a year on the Red Planet, the mobile Mars Science 
Laboratory has determined the age of a Martian rock, found evidence the 
planet could have sustained microbial life, taken the first readings of 
radiation on the surface, and shown how natural erosion could reveal the 
building blocks of life. Curiosity team members presented these results 
and more from Curiosity in six papers published online today by Science 
Express and in talks at the Fall Meeting of the American Geophysical Union 
in San Francisco.

The Age of 'Cumberland'

The second rock Curiosity drilled for a sample on Mars, which scientists 
nicknamed "Cumberland," is the first ever to be dated from an analysis 
of its mineral ingredients while it sits on another planet. A report by 
Kenneth Farley of the California Institute of Technology in Pasadena, 
and co-authors, estimates the age of Cumberland at 3.86 billion to 4.56 
billion years old. This is in the range of earlier estimates for rocks 
in Gale Crater, where Curiosity is working.

"The age is not surprising, but what is surprising is that this method 
worked using measurements performed on Mars," said Farley. "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."

The analysis of Cumberland from a sample drilled by Curiosity was a fundamental 
and unprecedented measurement considered unlikely when the rover landed 
in 2012. Farley and his co-authors adapted a 60-year-old radiometric method 
for dating Earth rocks that measures the decay of an isotope of potassium 
as it slowly changes into argon, an inert gas. Argon escapes when a rock 
is melted. This dating method measures the amount of argon that accumulates 
when the rock hardens again.

Before they could measure rocks directly on Mars, scientists estimated 
their ages by counting and comparing the numbers of impact craters on 
various areas of the planet. The crater densities are correlated with 
ages based on comparisons with crater densities on the moon, which were 
tied to absolute dates after the Apollo lunar missions returned rocks 
to Earth.

Farley and co-authors also assessed how long Cumberland has been within 
about an arm's reach of the Martian surface, where cosmic rays that hit 
atoms in the rock produce gas buildups that Curiosity can measure.

Analyses of three different gases yielded exposure ages in the range of 
60 million to 100 million years. This suggests shielding layers above 
the rock were stripped away relatively recently. Combined with clues of 
wind erosion Curiosity observed, the exposure-age discovery points to 
a pattern of windblown sand chewing away at relatively thick layers of 
rock. The eroding layer forms a retreating vertical face, or scarp.

"The exposure rate is surprisingly fast," Farley said. "The place where 
you'll find the rocks with the youngest exposure age will be right next 
to the downwind scarps."

>From Rocks to Building Blocks?

Finding rocks with the youngest exposure age is important in the mission's 
investigations of whether organic chemicals are preserved from ancient 
environments. Organic chemicals are building blocks for life, although 
they also can be produced without any biology.

"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, 
said of the Cumberland rock sample. "We detect organics but can't rule 
out that they might be brought along from Earth." Curiosity detected higher 
amounts in Cumberland than it did in in either test runs with Martian 
soil samples or analysis of empty sample cups. Increasing the amount of 
rock powder in the test cup increased the amount of organic content detected.

Favorable for Life

Ming is the lead author of a new report about a site called "Yellowknife 
Bay." The team reported 10 months ago that the first rock Curiosity drilled 
there, nicknamed "John Klein," yielded evidence that met the mission's 
goal of identifying a Martian environment favorable for microbial life 
long ago. Yellowknife Bay's clay-rich lakebed habitat offers the key chemical 
elements for life, plus water not too acidic or salty, and an energy source. 
The energy source is a type used by many rock-eating microbes on Earth: 
a mix of sulfur- and iron-containing minerals that are ready acceptors 
of electrons, and others that are ready electron donors, like the two 
poles of a battery.

Not only has Curiosity accomplished its primary goal of finding evidence 
for an ancient environment that could have supported life, but it also 
has provided evidence habitable conditions existed more recently than 
expected and likely persisted for millions of years.

Additional new results from Curiosity are providing the first readings 
of radiation hazards at Mars' surface, which will aid planning of human 
missions to Mars. Other findings will guide the search for evidence of 
life on Mars by improving insight about how erosion may expose buried 
clues of molecular building blocks of life.

New estimates of when habitable conditions existed at Yellowknife Bay 
and how long they persisted come from details of rocks' composition and 
layering. It is thought that Mars had enough fresh water to generate clay 
minerals -- and possibly support life -- more than 4 billion years ago, 
but that the planet underwent drying that left any remaining liquid water 
acidic and briny. A key question was whether the clay minerals at Yellowknife 
Bay formed earlier, upstream on the rim of Gale Crater where the bits 
of rock originated, or later, downstream where the rock particles were 
carried by water and deposited.

Scott McLennan of Stony Brook University in Stony Brook, N.Y., and co-authors 
found that chemical elements in the rocks indicate the particles were 
carried from their upstream source area to Yellowknife Bay and that most 
chemical weathering occurred after they were deposited. The loss of elements 
that leach easily, such as calcium and sodium, would be noticeable if 
the weathering that turns some volcanic minerals into clay minerals had 
happened upstream. Scientists did not notice such leaching.

David Vaniman of the Planetary Science Institute in Tucson, Ariz., and 
co-authors found supporting evidence in a separate mineral analysis of 
sedimentary rocks at Yellowknife Bay. They noticed a lack of olivine and 
an abundance of magnetite, which suggests the rocks turned to clay after 
they washed downstream. The presence of smectite tells about conditions 
where the clay formed.

"Smectite is the typical clay mineral in lake deposits," Vaniman said. 
"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."

John Grotzinger of Caltech and co-authors examined physical characteristics 
of rock layers in and near Yellowknife Bay and concluded the habitable 
environment there existed at a time "relatively young by Martian standards." 
It was a part of Martian history called the Hesperian Era, when parts 
of the planet were already becoming drier and more acidic, less than 4 
billion years ago and roughly the same time as the oldest evidence for 
life on Earth.

"This habitable environment existed later than many people thought there 
would be one," Grotzinger said. "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."

Research suggests habitable conditions in the Yellowknife Bay area may 
have persisted for millions to tens of millions of years. During that 
time rivers and lakes probably appeared and disappeared. Even when the 
surface was dry, the subsurface likely was wet, as indicated by mineral 
veins deposited by underground water into fractures in the rock. The thickness 
of observed and inferred tiers of rock layers provides the basis for estimating 
long duration, and the discovery of a mineral energy source for underground 
microbes favors habitability throughout.

Implications for Human Explorers

Today's reports include the first measurements of the natural radiation 
environment on the surface of Mars. Cosmic rays from outside our solar 
system and energetic particles from the sun bombarded the surface at Gale 
Crater with an average of 0.67 millisieverts per day from August 2012 
to June 2013, according to a report by Don Hassler of Southwest Research 
Institute in Boulder, Colo., and co-authors. For comparison, radiation 
exposure from a typical chest X-ray is about 0.02 millisievert. That 10-month 
measurement period did not include any major solar storms affecting Mars, 
and more than 95 percent of the total came from cosmic rays.

Results from the surface-radiation monitoring provide an additional piece 
of the puzzle for projecting the total round-trip radiation dose for a 
future human mission to Mars. Added to dose rates Curiosity measured during 
its flight to Mars, the Mars surface results project a total round-trip 
dose rate for a future human mission at the same period in the solar cycle 
to be on the order of 1,000 millisieverts.

Long-term population studies have shown exposure to radiation increases 
a person's lifetime cancer risk. Exposure to a dose of 1,000 millisieverts 
is associated with a 5 percent increase in risk for developing fatal cancer. 
NASA's current career limit for increased risk for its astronauts currently 
operating in low-Earth orbit is 3 percent. The agency is working with 
the Institute of Medicine of the National Academies to address the ethics, 
principles and guidelines for health standards for long duration and exploration 
spaceflight missions.

The radiation detected by Curiosity is consistent with earlier predictions. 
The new data will help NASA scientists and engineers create better models 
to anticipate the radiation environment human explorers will face, as 
the agency develops new technologies to protect astronauts in deep space.

"Our measurements provide crucial information for human missions to Mars," 
Hassler said. "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."

If any organic chemicals that are potential signs of life did exist within 
rocks at about 2 inches (5 centimeters), the depth of Curiosity's drill, 
Hassler estimated they would be depleted up to 1,000-fold in about 650 
million years by radiation at the exposure rate measured in Curiosity's 
first 10 months. However, the Cumberland rock that Curiosity sampled with 
its drill at Yellowknife Bay had been exposed to cosmic rays' effects 
for only about 60 million to 100 million years, by Farley's estimate. 
Researchers calculate that, with such a young exposure age, enough organic 
material could still be present in Cumberland to be detectable. Even if 
Mars has never supported life, the planet receives organic molecules delivered 
by meteorites, which should leave a detectable trace.

NASA's Jet Propulsion Laboratory built Curiosity and manages the mission 
for NASA's Science Mission Directorate, Washington.

For more information about the mission, visit: http://www.jpl.nasa.gov/msl 
, http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl

Guy Webster 818-354-6278
Jet Propulsion Laboratory, Pasadena, Calif.
guy.webster at jpl.nasa.gov

Dwayne Brown 202-358-1726
Headquarters, Washington
dwayne.c.brown at nasa.gov

2013-356