domingo, 17 de marzo de 2013

"Could Life on Mars Have Existed Earlier than on Earth?" --New NASA Findings Provide Clues


“We have found a habitable environment that is so benign and supportive of life that probably if this water was around and you had been on the planet, you would have been able to drink it,” said John P. Grotzinger, the California Institute of Technology geology professor who is the principal investigator for the NASA mission. “What we have learned in the last 20 years of modern microbiology is that very primitive organisms, they can derive energy just by feeding on rocks."

"The range of chemical ingredients we have identified in the sample is impressive, and it suggests pairings such as sulfates and sulfides that indicate a possible chemical energy source for micro-organisms," said Paul Mahaffy, principal investigator of the SAM suite of instruments at NASA's Goddard Space Flight Center in Greenbelt, Md.

"A fundamental question for this mission is whether Mars could have supported a habitable environment," said Michael Meyer, lead scientist for NASA's Mars Exploration Program at the agency's headquarters in Washington. "From what we know now, the answer is yes."\

About three billion years ago, the conditions on Mars changed dramatically: with just one-tenth the mass of Earth, Mars lost most of its atmosphere. As a result, the inside of the planet cooled, the volcanoes stopped erupting, and the water froze or evaporated and escaped into space leaving Mars the cold and barren planet we see today.

Geological observations suggest rivers and seas dotted the martian surface 3.5 billion years ago. The amount of water has been equated to a planet-wide ocean half-a-kilometer deep or more. For the planet to have stayed warm enough for liquid water, scientists assume that Mars had a greenhouse "blanket" of carbon dioxide atmosphere at least 1000 times thicker than what Earth has now.

That carbon dioxide is mostly gone. So is the water. "Either they went up or they went down," says Dave Brain from UC Berkeley.

By "down," Brain is referring to the subsurface of the red planet. Water ice is known to be lurking underground, while vestiges of carbon dioxide can be found in the polar ice cap and in certain mineral deposits. But many scientists expect that a large fraction of the water-soaked atmosphere was sucked "up" into space.

"We know that escape is occurring today from the martian atmosphere and that it has occurred in the past," says Bruce Jakosky of the University of Colorado, Boulder.

The current loss rate of martian atmosphere is estimated to be around 100 tons per day, but this is based on incomplete data. Jakosky is leading a NASA mission called MAVEN that plans to fly to Mars in 2013 to measure all aspects of atmospheric escape.

MAVEN – which stands for Mars Atmosphere and Volatile EvolutioN – will not only provide a better handle on the current loss, but it will also provide a window on the past, by determining how the upper atmosphere controls the loss rate. NASA’s Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft is assembled and is currently undergoing environmental testing at Lockheed Martin Space Systems facilities, near Denver.

"The more we know about the loss rate now, the better we can extrapolate back in time when Mars was presumably warmer and wetter," says Michael Combi of the University of Michigan. Combi and his colleagues model the outer envelope of Mars' atmosphere, called the exosphere, where particles make their "jump" into space.As part of NASA's Mars Fundamental Research program, they are working on a full three-dimensional simulation that can use MAVEN's observations to say how much water Mars has lost to space.

The main ways that atmospheric particles can escape a planet's gravity are ion escape, neutral escape and impact erosion, explains Brain.

The last of these, impact erosion, was dominant around 4 billion years ago, when the terrestrial planets were bombarded with large pieces of space debris. Big "splashes" like these would have hurtled large volumes of atmosphere into space, while also introducing water and other material to the surface.

But Mars managed to hold onto a considerable amount of atmosphere throughout the bombardment. We know this because the evidence of martian water is 3.5 billion years old – when impacts had become less common. Scientists therefore have to look to other escape routes to explain where all the water went.

"The MAVEN mission is the first to have as its sole focus understanding the nature of the upper atmosphere and how it controls the escape rates," Jakosky says.

The $485-million MAVEN will carry eight instruments to measure ion and neutral escape, as well as the structure and composition of the upper atmosphere. Over its planned two-year mission, it will also monitor the solar wind, solar ultraviolet, and solar storms, which are the main drivers that influence the rate at which material is stripped off the martian atmosphere.

One of the challenges of past missions has been characterizing a "leak" that is spread over the entire 150,000 square kilometers on Mars' outer atmospheric surface. MAVEN's orbit will be varied in such a way that it samples the loss rate from a wide range of different latitudes, as well as at different times of the day. But the satellite can only be in one place at one time, so models like that of Combi's group are needed to fill in the gaps.

"These models are absolutely essential for us," Jakosky says. "They will allow us to take the MAVEN measurements that are made at discrete times and locations and extrapolate them to other times and places."

When trying to imagine loss rates long ago, researchers will have to account for changes in the solar output. By observing Sun-like stars at earlier stages in their lives, astronomers believe our Sun was more active in the past – with more storms and greater ultraviolet flux. Consequently, atmospheric escape should have been ramped up on high as well.

"We can't measure what the atmosphere was like billions of years ago," Jakosky says. "However, we can measure it today, measure how the processes that control it work, and then use models to extrapolate to other conditions."

So in the end, the models need the satellite to ground them in reality. And the satellite needs the models to stretch its reach to the beginning of martian history.

Only then will Mars reveal the secret of its lost water.
Clues to this potentially habitable environment announced this week come from data returned by the rover's Sample Analysis at Mars (SAM) and Chemistry and Mineralogy (CheMin) instruments. The data indicate the Yellowknife Bay area the rover is exploring was the end of an ancient river system or an intermittently wet lake bed that could have provided chemical energy and other favorable conditions for microbes. The rock is made up of a fine-grained mudstone containing clay minerals, sulfate minerals and other chemicals. This ancient wet environment, unlike some others on Mars, was not harshly oxidizing, acidic or extremely salty.
The patch of bedrock where Curiosity drilled for its first sample lies in an ancient network of stream channels descending from the rim of Gale Crater. The bedrock also is fine-grained mudstone and shows evidence of multiple periods of wet conditions, including nodules and veins. *"Clay minerals make up at least 20 percent of the composition of this sample," said David Blake, principal investigator for the CheMin instrument at NASA's Ames Research Center in Moffett Field, Calif.

These clay minerals are a product of the reaction of relatively fresh water with igneous minerals, such as olivine, also present in the sediment. The reaction could have taken place within the sedimentary deposit, during transport of the sediment, or in the source region of the sediment. The presence of calcium sulfate along with the clay suggests the soil is neutral or mildly alkaline.

Scientists were surprised to find a mixture of oxidized, less-oxidized, and even non-oxidized chemicals, providing an energy gradient of the sort many microbes on Earth exploit to live. This partial oxidation was first hinted at when the drill cuttings were revealed to be gray rather than red.

An additional drilled sample will be used to help confirm these results for several of the trace gases analyzed by the SAM instrument.

Scientists plan to work with Curiosity in the "Yellowknife Bay" area for many more weeks before beginning a long drive to Gale Crater's central mound, Mount Sharp. Investigating the stack of layers exposed on Mount Sharp, where clay minerals and sulfate minerals have been identified from orbit, may add information about the duration and diversity of habitable conditions.

NASA's Mars Science Laboratory Project has been using Curiosity to investigate whether an area within Mars' Gale Crater ever has offered an environment favorable for microbial life. Curiosity, carrying 10 science instruments, landed seven months ago to begin its two-year prime mission. 

Source: The Daily  Galaxy via NASA/JPL and New York Times

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