In the early days of the Curiosity mission on Mars, scientists were excited by what they found in what was once a mud-flat they called Yellowknife Bay. After months of drilling and testing, the mission team concluded that the site once had the roughly neutral water, an array of chemicals that could support metabolism and the organic carbon compounds needed for life. So Yellowknife Bay and the surrounding Gale Crater were deemed to have once been “habitable.”
The finding of organic carbon was a major step forward because it is essential as a building block for the emergence of life as we know it. The readings were clear that the organic carbon was present, but it has taken a decade to produce the first measurement of how much of the precious organic carbon was present.
The results, published late last month in the Proceedings of the National Academy of Sciences, show higher organic carbon levels than in some “low-life” environments on Earth. But those levels are still quite reduced and point to an unwelcoming Mars even in an area declared to be habitable billions of years ago when Mars was wetter and warmer.
“Total organic carbon is one of several measurements that help us understand how much material is available as feedstock for prebiotic chemistry and potentially biology,” said Jennifer Stern of NASA’s Goddard Space Flight Center.
“We found at least 200 to 273 parts per million of organic carbon. This is comparable to or even more than the amount found in rocks in very low-life places on Earth, such as parts of the Atacama Desert in South America, and more than has been detected in Mars meteorites.”
The Atacama is one of the driest places on Earth, but it does support some life — bacteria under the surface of the desert and even some desert flowers in areas that experience fog. Not surprisingly, NASA and other scientists often use the Atacama when they study conditions on ancient Mars.
This carbon data has been a long time coming.
As explained in a NASA release, the experiment was performed in 2014 but required years of analysis to understand the data and put the results in context of the mission’s other discoveries at Gale Crater. (One of the key issues was to eliminate from the measurement any of the carbon in the MTBSTFA contaminant — an organic chemical — that was found to have leaked into the instrument used to measure organics.)
The experiment was very resource-intensive and has been performed only once during Curiosity’s 10 years on Mars, but Stern is hoping for another try in the months ahead. Because the sample came from Yellowknife Bay — that had been a lake and thus a catchment for the surrounding region — Stern said that it most likely had a higher concentration of preserved organic carbon than other parts of Gale Crater.
“To me, the most interesting thing is that there appears to be more carbon in Mars surface rocks than in meteorites,” Stern said. There was 10 times more organic carbon in the Yellowknife Bay sample than in most Mars meteorites found on Earth.
Organic carbon is carbon bound to a hydrogen atom. It is the basis for organic molecules, which are created and used by all known forms of life.
Organic carbon has been found on Mars before, but prior measurements only produced information on particular compounds, or represented measurements capturing just a portion of the carbon in the rocks. The new measurement gives the total amount of organic carbon in these rocks, from minerals to macromolecules to meteorites.
To make the measurement, Curiosity delivered the sample to its Sample Analysis at Mars (SAM) instrument, where an oven heated the powdered rock to progressively higher temperatures.
This experiment used oxygen and heat to convert the organic carbon to carbon dioxide (CO2), the amount of which is measured to get the amount of organic carbon in the rocks. Adding oxygen and heat allows the carbon molecules to break apart and react with oxygen to make CO2.
Some carbon is locked up in minerals, so the oven heats the sample to very high temperatures to decompose those minerals and release the carbon to convert it to CO2.
This process also allowed SAM to measure the carbon isotope ratios, which help to understand the source of the carbon. Isotopes are versions of an element with slightly different weights (masses) due to the presence of one or more extra neutrons in the center (nucleus) of their atoms.
For example, carbon-12 has six neutrons while the heavier carbon-13 has seven neutrons. Since heavier isotopes tend to react a bit more slowly than lighter isotopes, the carbon from life is richer in carbon-12.
But while an earlier PNAS paper highlighted this excess of carbon-12 on Mars as a potential biosignature — among other explanations –Stern did not see her results as a compelling sign of potential long-ago life.
“In this case, the isotopic composition can really only tell us what portion of the total carbon is organic carbon and what portion is mineral carbon,” said Stern.
“While biology cannot be completely ruled out, isotopes cannot really be used to support a biological origin for this carbon either, because the range overlaps with igneous (volcanic) carbon and meteoritic organic material, which are most likely to be the source of this organic carbon.”
Although the surface of Mars is inhospitable for life now, Curiosity and other NASA mission have found clear evidence that billions of years ago the climate was more Earth-like, with a thicker atmosphere and liquid water that flowed into rivers and seas.
For the samples used in Stern’s research, Curiosity drilled into 3.5-billion-year-old mudstone rocks — which were formed as very fine sediment (from physical and chemical weathering of volcanic rocks) in water settled on the bottom of a lake and was buried.
Organic carbon was part of this material and got incorporated into the mudstone. Besides liquid water and organic carbon, Gale crater had other conditions conducive to life, such as chemical energy sources, low acidity, and other elements essential for biology, such as oxygen, nitrogen, and sulfur.
“Basically, this location would have offered a habitable environment for life, if it ever was present,” said Stern, lead author the Martian organic carbon paper published June 27 in the PNAS.
But if there was life, it would have to be simple life if the levels of organic carbon measured by the Curiosity instrument are broadly indicative of those long-ago and best-case conditions that made Gale Crater “habitable.”
Asked if the amount of organic carbon measured by SAM is roughly equal to the amount present in the wetter and warmer days 3.5 billion years ago, Stern replied that “We have no idea.” The instruments don’t provide that information and the process by which organic carbon is degraded on Mars is not well understood.
Further testing with SAM could provide some related insights.
“This experiment is more resource intensive than our normal evolved gas experiment, which is why it has only been performed once. We’d really like to perform it again now that we are in a completely different location representing different environmental conditions to see if it is different. ”
But they are looking for ways to make the experiment less time and resource consuming to allow for a fuller picture of organic carbon on ancient Mars to be drawn.