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 (Caltech) 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 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.
A More Dynamic Red Planet
NASA’s Mars Reconnaissance Orbiter (MRO) has revealed to scientists slender dark markings — possibly due to salty water – that advance seasonally down slopes surprisingly close to the Martian equator.
“The equatorial surface region of Mars has been regarded as dry, free of liquid or frozen water, but we may need to rethink that,” said Alfred McEwen of the University of Arizona in Tucson, principal investigator for MRO’s High Resolution Imaging Science Experiment (HiRISE) camera.
Tracking how these features recur each year is one example of how the longevity of NASA orbiters observing Mars is providing insight about changes on many time scales.
Researchers at the American Geophysical Union meeting Tuesday in San Francisco discussed a range of current Martian activity, from fresh craters offering glimpses of subsurface ice to multi-year patterns in the occurrence of large, regional dust storms.
The seasonally changing surface flows were first reported two years ago on mid-latitude southern slopes. They are finger-like features typically less than 16 feet (5 meters) wide that appear and extend down steep, rocky slopes during spring through summer, then fade in winter and return the next spring. Recently observed slopes stretch as long as 4,000 feet (1,200 meters).
McEwen and co-authors reported the equatorial flows at the conference and in a paper published online Tuesday by Nature Geoscience. Five well-monitored sites with these markings are in Valles Marineris, the largest canyon system in the solar system. At each of these sites, the features appear on both north- and south-facing walls. On the north-facing slopes, they are active during the part of the year when those slopes get the most sunshine. The counterparts on south-facing slopes start flowing when the season shifts and more sunshine hits their side.
“The explanation that fits best is salty water is flowing down the slopes when the temperature rises,” McEwen said. “We still don’t have any definite identification of water at these sites, but there’s nothing that rules it out, either.”
Dissolved salts can keep water melted at temperatures when purer water freezes, and they can slow the evaporation rate so brine can flow farther. This analysis used data from the Compact Reconnaissance Imaging Spectrometer for Mars and the Context Camera on the MRO as well as the Thermal Emission Imaging System experiment on NASA’s Mars Odyssey orbiter.
Water ice has been identified in another dynamic process researchers are monitoring with MRO. Impacts of small asteroids or bits of comets dig many fresh craters on Mars every year. Twenty fresh craters have exposed bright ice previously hidden beneath the surface. Five were reported in 2009. The 15 newly reported ones are distributed over a wider range of latitudes and longitudes.
“The more we find, the more we can fill in a global map of where ice is buried,” said Colin Dundas of the U.S. Geological Survey in Flagstaff, Ariz. “We’ve now seen icy craters down to 39 degrees north, more than halfway from the pole to the equator. They tell us that either the average climate over several thousand years is wetter than present or that water vapor in the current atmosphere is concentrated near the surface. Ice could have formed under wetter conditions, with remnants from that time persisting today, but slowly disappearing.”
Mars’ modern climate becomes better known each year because of a growing set of data from a series of orbiters that have been studying Mars continually since 1997. That has been almost nine Martian years because a year on Mars is almost two years long on Earth. Earlier missions and surface landers have added insight about the dynamics of Mars’ atmosphere and its interaction with the ground.
“The dust cycle is the main driver of the climate system,” said Robert Haberle of NASA’s Ames Research Center in Moffett Field, Calif.
One key question researchers want to answer is why dust storms encircle Mars in some years and not in others. These storms affect annual patterns of water vapor and carbon dioxide in the atmosphere, freezing into polar ice caps in winter and replenishing the atmosphere in spring. Identifying significant variations in annual patterns requires many Martian years of observations.
The data emerging from long-term studies will help future human explorers of Mars know where to find resources such as water, how to prepare for hazards such as dust storms, and where to be extra careful about contamination with Earth microbes.
Launched in 2005, MRO and its six instruments have provided more high-resolution data about the Red Planet than all other Mars orbiters combined. Data is made available for scientists worldwide to research, analyze and report their findings.
NASA’s Jet Propulsion Laboratory built Curiosity and manages the mission for NASA’s Science Mission Directorate, Washington.
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Posted by: Soderman/SSERVI Staff