Curiosity rover inventories key life ingredient on Mars

Scientists using data from NASA’s Curiosity rover have measured total organic carbon — a key component in the molecules of life — in Mars rocks for the first time.

“Total organic carbon is one of many measurements [or indices] that help us understand how much material is available as feedstock for prebiotic chemistry and possibly biology,” said Jennifer Stern of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We found at least 200 to 273 parts per million organic carbon. This is comparable to or even more than the amount found in rocks in places of very low Earth life, such as parts of the Atacama Desert in South America, and more than has been detected in Martian meteorites.”

Organic carbon is carbon bonded to a hydrogen atom. It is the basis for organic molecules, which are made and used by all known forms of life. Organic carbon on Mars, however, does not prove the existence of life there, as it could also come from non-living sources, such as meteorites and volcanoes, or be formed on site through surface reactions. Organic carbon has been found on Mars before, but previous measurements only provided information about certain compounds, or represented measurements that captured only some of the carbon in the rocks. The new measurement gives the total amount of organic carbon in these rocks.

Although the surface of Mars is now inhospitable to life, there is evidence that billions of years ago, the climate was more Earth-like, with a thicker atmosphere and liquid water flowing into rivers and seas. Because liquid water is necessary for life as we understand it, scientists believe that life on Mars, if it ever evolved, could have been sustained by key ingredients such as organic carbon, if present in sufficient quantity.

Curiosity is advancing in astrobiology by investigating the habitability of Mars and studying its climate and geology. The rover drilled samples from 3.5-billion-year-old mudstone rocks in the Yellowknife Bay formation of Gale Crater, the site of an ancient lake on Mars. Mudstone at Gale Crater formed as very fine sediment (from physical and chemical weathering of volcanic rock) in water that settled at the bottom of a lake and was buried. Organic carbon was part of this material and was incorporated into the mudstone. In addition to liquid water and organic carbon, Gale Crater had other conditions conducive to life, such as chemical energy sources, low acidity, and other elements essential to biology, such as oxygen, nitrogen, and sulfur. “Basically, this location would have provided a habitable environment for life, if it ever existed,” said Stern, lead author of a paper on this study published June 27 in the Proceedings of the National Academy of Sciences†

To make the measurement, Curiosity delivered the sample to its Sample Analysis at Mars (SAM) instrument, where a furnace heated the powdered rock to increasingly higher temperatures. This experiment used oxygen and heat to convert the organic carbon into 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 carbon with oxygen to form CO. to make₂† Some carbon is trapped in minerals, so the furnace heats the sample to very high temperatures to decompose those minerals and release the carbon to convert it to CO₂† The experiment was conducted in 2014, but took years of analysis to understand the data and put the results in context with the mission’s other discoveries at Gale Crater. The resource-intensive experiment was conducted just once during Curiosity’s 10 years on Mars.

This process also enabled SAM to measure the carbon isotope ratios, helping to understand the source of the carbon. Isotopes are versions of an element with slightly different weights (mass) 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. Because heavier isotopes tend to react slightly more slowly than lighter isotopes, the carbon from life is richer in carbon-12. “In this case, the isotopic composition can really only tell us what part of the total carbon is organic carbon and what part is mineral carbon,” Stern said. “While biology cannot be completely ruled out, isotopes also cannot really be used to support a biological origin for this carbon, as the range overlaps with igneous (volcanic) carbon and meteoritic organic matter, which are most likely the source of this.” organic carbon.”