Curiosity, NASA’s rover, arrived on Mars on Aug. 6, 2012, and has been roaming Gale Crater ever since, collecting samples and transmitting them back to Earth for experts to analyse. Researchers have three plausible explanations for the carbon’s origin based on analysis of carbon isotopes in sediment samples taken from half a dozen exposed locations, including an exposed cliff: cosmic dust, ultraviolet degradation of carbon dioxide, or ultraviolet degradation of biologically produced methane.
“All three of these situations are unorthodox, unlike processes typical on Earth,” the researchers write in Proceedings of the National Academy of Sciences today (Jan. 17).
The stable isotopes of carbon are 12 and 13. Researchers may discover information about the carbon cycle that occurred by looking at the levels of each in a material, even if it occurred a long time ago.
“The quantities of carbon 12 and carbon 13 in our solar system are the same as they were when the solar system was formed,” said Christopher H. House, a Penn State geosciences professor. “Both present in everything, but since carbon 12 reacts faster than carbon 13, the carbon cycle may be revealed by looking at the relative levels of both in samples.”
Curiosity has spent the past nine years studying a region of Gale Crater that has unearthed layers of ancient rock, guided by NASA’s Jet Propulsion Laboratory in Southern California. Drilling into the surface of these strata allowed the rover to retrieve samples from the subsurface sedimentary layers. To separate any compounds, Curiosity heated the samples in the absence of oxygen. Spectrographic investigation of a sample of the reduced carbon generated by this pyrolysis revealed a broad range of carbon 12 and carbon 13 levels, depending on where and when the initial sample was created. In carbon 13, some carbon samples were very depleted, while others were abundant.
“The samples that are exceptionally low in carbon 13 are similar to samples from Australia derived from 2.7 billion-year-old sediment,” House added. “Those samples were generated by biological activity when ancient microbial mats devoured methane, but we can’t assume the same thing about Mars since it may have developed out of different materials and processes than Earth.”
The researchers propose three explanations for the unusually depleted samples: a cosmic dust cloud, UV radiation breaking down carbon dioxide, or ultraviolet destruction of biologically produced methane.
According to House, the solar system travels through a galactic molecular cloud every few hundred million years.
“It doesn’t produce a lot of dust,” House said. “Any of these deposition episodes are difficult to find in the Earth record.”
The cosmic dust cloud would have first decreased the temperature on a Mars that still had water and generated glaciers in order to develop a layer that Curiosity could study. The dust would have accumulated on top of the ice and would have to stay there until the glacier melted, leaving a layer of soil containing the carbon.
So yet, there’s just a smattering of evidence of former glaciers at Mars’ Gale Crater. “This theory is feasible,” the researchers say, “but it needs more investigation.”
The UV conversion of carbon dioxide to organic molecules like formaldehyde is a second probable cause for decreased levels of carbon 13.
According to House, “there are articles that indicate UV might create this kind of fractionation.” “However, we need additional experimental data that reveal this size fractionation before we can rule this theory in or out.”
The biological foundation for the third way of obtaining carbon 13 deficient samples.
On Earth, a paleosurface with a high carbon 13 depletion would imply that previous organisms devoured microbially generated methane. Large plumes of methane may have been emitted from the subsurface of ancient Mars, where methane production would have been energetically advantageous. The released methane would then be eaten by surface bacteria or would react with UV radiation and be deposited on the surface.
However, since there is presently no sedimentary evidence of surface bacteria on the past Mars landscape, the biological explanation proposed in the report depends on UV light to position the carbon 13 signature on the ground.
“All three scenarios hint to a carbon cycle unlike anything seen on Earth today,” House said. “However, additional evidence is required to determine whether of these explanations is right. It would be ideal if the rover could detect a massive methane plume and test the carbon isotopes from it, but although methane plumes do exist, they are typically tiny, and no rover has sampled one large enough to quantify the isotopes.”
Finding the remnants of microbial mats or evidence of glacial deposits, according to House, might also help to clear things up.
“We’re being careful in our interpretation,” House said, “which is the proper route for researching another universe.”
Curiosity is currently collecting and analyzing materials, and in approximately a month, it will return to the pediment where it discovered some of the samples for this research.
“This discovery fulfilled a long-held ambition for Mars exploration,” House stated. “It looks at 9 years of exploration to quantify distinct carbon isotopes — one of the most essential geology tools — from sediment on another livable planet.”
Gregory M. Wong, a recent Penn State geosciences PhD graduate, was also involved in the experiment.
At NASA Jet Propulsion Laboratory, Christopher R. Webster, a fellow and senior research scientist; Gregory J. Flesch, a scientific applications software engineer; and Amy E. Hofmann, a research scientist; and at NASA Goddard Space Flight Center’s Solar System Exploration Division, Heather B. Franz, a research scientist; Jennifer C. Stern, a research assistant; Alex Pavlov, a space scientist; Jennifer L. Eigenbrode, a research assistant; and Daniel P. Glisson,
This project was backed by NASA.