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by Aaron L. Gronstal for Astrobiology Magazine
Pasadena CA (JPL) Dec 27, 2013
NASA's Curiosity rover touched down on Mars with dramatic style in August, 2012. Now that the rover has spent more than a year exploring the martian surface, scientific data from the mission is starting to make its way into journals and popular news here on Earth.
Astrobiology Magazine recently spoke with some of the researchers behind a series of recent Science articles in order to better understand how these findings relate to the study of life's potential on ancient Mars.
Pierre-Yves Meslin, ChemCam
Data from ChemCam is also directly useful for astrobiologists. Rocks and soil are formed and altered by physical and chemical processes at the surface. Studying the exact composition of Mars rocks can help astrobiologists determine if water played a role in shaping the structure of the martian surface at Curiosity's landing site, Gale Crater. This information helps scientists reconstruct what the ancient environment of Mars was like, and whether or not it could have been habitable for life as we know it.
"[ChemCam] is not the first instrument to measure chemical composition at the surface," Meslin noted. "On past missions scientists could use an instrument called APXS (Alpha Particle X-Ray Spectrometer), a version of which is also onboard Curiosity. What is new about ChemCam is that it is much more simple to operate because we don't have to deploy the robotic arm.
It's not easy to deploy the arm. It takes time, so it slows down the rover." ChemCam uses a laser to collect its data remotely, which is more simple than other instruments on Curiosity that need to scratch, dig and drill to collect information. Curiosity can basically just drive along, stop and zap things in its path in order to determine whether or not targets are of particular interest.
"One good thing about ChemCam is its operability. We can shoot many targets every day on Mars," Meslin continued. "The second new thing about ChemCam is the scale of the analysis. This is the first time that we have an insight into the chemical composition at the sub millimeter scale. With ChemCam we measure at a very small scale, so we can identify the different grains or minerals. This fine-scale analysis allows us to find and decipher some heterogeneous compositions, and it is important to know the relationship between grain sizes and composition."
The ChemCam team is an international effort with responsibilities mixed between the United States and France. Meslin is a scientist based at the University Paul Sabatier in Toulouse, France, and he also has an operational role as part of the ChemCam uplink team that selects the ChemCam targets.
A View from ChemCam
"In this first series of papers that was published in Science, we have made a recount of the chemical diversity seen in the soil," explains Meslin. "What we've found is that we have coarse grains - millimeter scale grains - that have a composition very specific to Gale Crater."
Meslin believes that these coarse grains are likely to originate from rocks, known as fluvial conglomerates, spotted around the Curiosity landing site, that appear to have been shaped by flowing water. These conglomerates are individual chunks of rock that are cemented together by a matrix of smaller grains (think bricks and mortar on a small scale). The 'fluvial' part of the name 'fluvial conglomerates' means that the clasts appear to have been moved and deposited in rivers and streams. Their outside edges are rounded, which is a telltale sign that they could have been smoothed and shaped by running water.
Curiosity spotted these rounded rocks early in its mission, and many scientists have cited them as evidence of an ancient streambed on the red planet. As the rocks were worn down, some of the matrix material and clasts broke away and mixed with the soil, explaining why its composition matches the coarse grains identified by ChemCam in the martian soil.
"So we find some material that is really characteristic of Gale and its rim," explains Meslin. "On the other hand we find that the dust -the very fine-grain material and the sand particles - have a composition which is different from the rocks in Gale. This fine grained material has probably been transported by winds from another place."
Meslin points out an interesting fact: the fine-grained material analyzed by ChemCam and also by the APXS has a composition that is very similar to the composition of materials analyzed by previous APXS instruments on missions like Viking, Pathfinder and the Mars Exploration Rovers. This means that the sand and dust in Gale Crater is a type of material that could cover the entire surface of Mars.
The second important finding is that the fine-grained material is hydrated. Over the last decade, satellites in orbit around Mars have been measuring hydrogen on the planet. These studies have also shown that the surface of Mars is hydrated globally, with 2% to 10% of the weight of materials being water-equivalent hydrogen. With Curiosity, the surface measurements are confirming the remote observations.
"For the last decade there was a kind of mystery," says Meslin. "We didn't know the nature of this hydrogen. We didn't know if it was subsurface ice, if it was clays, hydrated sulfates mixed in the soil or other things... so it was a mystery."
Bring in the Instruments
Curiosity camped out at Rocknest for a few weeks, taking the time to extend its robotic arm and collect samples. These samples were then analyzed with the instruments CheMin (Chemistry and Mineralogy X-Ray Diffraction) and SAM (Sample Analysis at Mars).
CheMin did not find any crystalline hydrated minerals, meaning that the water in the samples was not contained in clays or sulfates. Instead, the water is trapped in materials that are more amorphous, or do not have a crystalline structure. SAM showed that the water made up about 2% of the weight of samples - providing ground confirmation of the previous observations made by Mars orbiters.
SAM also found that the water in samples had the same hydrogen isotopic signature as water vapor in the atmosphere. This provides some clue as to where the water in soil comes from.
"It's probably not a very old alteration process that involves liquid water," explains Meslin. "It's probably a kind of processes that took a long time but that involved exchanges with the atmosphere, including possibly rock-ice interactions."
Curiosity's science team wanted to know if atmospheric exchange was something that controlled water content in the soil on a daily basis (think of the dew that forms on the surface of Earth in the morning as air temperatures change). They fired ChemCam's laser at targets both during the day and at night, searching for any sign that atmospheric water was being deposited on the surface.
"During the night we did not see much significant variation of the water content," reported Meslin. "This water has an isotopic signature that is similar to the atmosphere, but on a daily basis does not exchange much with the atmosphere."
The results show that the signature of global hydration on Mars - the water contained in the martian soil all over the planet - is not left over from ancient Mars when the planet was warm and liquid water flowed at the surface. Instead, it comes from the atmosphere, or it shares with the atmospheric water vapor a common and contemporary source reservoir. But because there isn't significant daily exchange of water between the atmosphere and soil, the process that led to hydration must have happened very slowly over Mars' more recent history.
"That's the next step now," says Meslin. "To try to better understand - to better identify this amorphous phase that contains the hydrogen, and then see what kind of process led to its presence and how much time it took to make this process effective."
This signature of water in Mars soils may not tell us about the ancient past of Mars, and the nature of liquid water at the planet's surface, but other sites do. Further down the road on Mars, Curiosity explored a site called Yellowknife bay.
"This is a very different kind of environment for habitability," said Meslin.
A second set of papers were published on Curiosity's results at Yellowknife Bay in Science on Dec 9, 2013.
A Rocky Close-Up with APXS
"It was the very first rock analyzed by the APXS instrument, so the team wanted to find a rock that looked homogeneous and would therefore be relatively easy to interpret and to compare with analyses made by ChemCam," says Megan Newcombe, a graduate student at the California Institute of Technology (CalTech). Newcombe works with data from APXS and is a contributing author on the paper in Science concerning work at Jake_M.
When the first APXS measurements came back, scientists realized how unique Jake_M actually was. They immediately noted similarities between Jake_M's elemental composition and that of a volcanic rock on Earth known as mugearite. It was the first example of this type of rock discovered on Mars.
According to Newcombe, Jake_M most closely resembles lavas found on the Spanish island of Tenerife, one of the Canary Islands off the coast of Africa. This comparison helped scientists come up with some initial ideas about the conditions in which Jake_M might have formed, and the possibility that water played a role.
"Water is commonly lost by degassing during volcanic eruptions, so it can be hard to quantify how much water was present during the formation of a volcanic rock, even in terrestrial studies," says Newcombe. "However, the concentration of water in magma affects the order and amount of crystallization of different minerals from the magma, and this in turn affects the evolution of the magma composition."
Determining the water concentration in magma based on crystallization requires many samples from the same volcano. Jake_M is only a single rock. But in terms of the Tenerife lavas, the team found that crystallization likely occurred at 4 kilobar (kbar) of pressure with magma that was starting with 1 % water by weight.
"This is only one of many possible ways of producing a composition like Jake_M," Newcombe notes. "Unfortunately, it is not possible to definitively answer this question with a single rock composition."
As Newcombe explains, the only way to determine water's role in the formation of Jake_M is to find many more closely related rocks. "If we could build up a picture of how the composition of the magma feeding this volcanic system evolved over time, then we could constrain the water concentration required to produce Jake_M without having to rely on terrestrial analogues like Tenerife."
"I personally think it would be interesting to analyze some more coarsely crystalline rocks, since these could represent magmas that crystallized and evolved even more than Jake_M," continues Newcombe. "However, every analysis made by Curiosity tells us something new and interesting, so it's easy to feel like a kid in a candy store sometimes!"
Comparative Planetology: The Importance of Mars' Past
"Basically, we have lost our memories on Earth," says Pierre-Yves Meslin. "It is difficult to access the conditions that were prevailing when life occurred on Earth. But we have another example in the Solar System. Mars."
Today Mars and Earth are very different planets, but these differences were not as pronounced in the early Solar System when the planets first formed. We now know that Mars once had liquid water at its surface, and it's possible that life gained a foothold on the red planet billions of years ago - around the same time that cells first appeared on Earth.
As the two planets aged and evolved, they became dramatically different worlds, and this could actually be a benefit for today's astrobiologists. Mars has no plate tectonics, meaning that the surface of the planet does not shift and change like the Earth's. The processes that have erased evidence of Earth's ancient history did not play out on Mars in the same way. This means that ancient evidence of Mars' environment still exists at the surface - just waiting for a robotic explorer to come along and pick it up.
"Our 'memory' may be on Mars," comments Meslin. "We have very old terrains on Mars because there is no plate tectonics. We can analyze these terrains and see for ourselves, not only activity of water, but 'were conditions agreeable for life to evolve' at that time."
By providing a view into the early days of a rocky planet's evolution, Mars could shed light on the history of life on Earth through the science of comparative planetology. This makes Mars doubly interesting for astrobiologists who are not only interested in life's potential on other planets, but also understanding of life's origin and evolution here on Earth.
Mars News and Information at MarsDaily.com
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