(CNN) — In the search for life beyond Earth, NASA’s Curiosity rover has been on a nearly decade-long mission to determine whether Mars was ever habitable for living organisms.
A new analysis of sediment samples collected by the rover revealed the presence of carbon, and the possible existence of ancient life on the Red Planet is just one possible explanation for why it may be there.
Carbon is the basis of all life on Earth, and the carbon cycle is the natural process of recycling carbon atoms. On our planet, carbon atoms go through a cycle as they move from the atmosphere to the ground and back into the atmosphere. Most of the carbon is found in rocks and sediments, with the rest in the ocean, atmosphere and organisms, according to the National Oceanic and Atmospheric Administration (NOAA).
Therefore, carbon atoms –with their recirculation cycle– are tracers of biological activity on Earth. So they could be used to help researchers determine if life existed on ancient Mars.
When these atoms are measured inside another substance, such as Martian sediment, they can shed light on the planet’s carbon cycle, no matter when it occurred.
Learning more about the origin of this newly detected Martian carbon could also reveal the process of the carbon cycle on Mars.
A study detailing these findings was published Monday in the journal Proceedings of the National Academy of Sciences.
Secrets in the sediment
Curiosity landed in Gale Crater on Mars in August 2012. This 96-mile (154.5-kilometer) crater, named after Australian astronomer Walter F. Gale, was likely formed by a meteorite impact between 3,500 and 3.8 billion years. The large cavity likely once housed a lake, and now includes a mountain called Mount Sharp. The crater also includes layers of exposed ancient rock.
To get a closer look, the rover drilled to collect sediment samples throughout the crater between August 2012 and July 2021. Curiosity then heated these 24 dust samples to about 1,562 degrees Fahrenheit (850 degrees Celsius) to separate elements. This caused the samples to release methane, which was then analyzed by another instrument in the rover’s arsenal to show the presence of stable isotopes of carbon, or carbon atoms.
Some of the samples were low in carbon, while others were enriched. Carbon has two stable isotopes, which are measured as carbon 12 or carbon 13.
“The extremely carbon-13-depleted samples are a bit like samples from Australia taken from sediments that were 2.7 billion years old,” said Christopher H. House, the study’s lead author and a professor of geosciences at Pennsylvania State University, in a statement. release.
“Those samples were caused by biological activity when methane was consumed by ancient microbial mats, but we can’t necessarily say that on Mars because it’s a planet that may have formed with different materials and processes than on Earth.”
In lakes on Earth, microbes like to grow in large colonies that essentially form mats just below the surface of the water.
3 possible origins of carbon
The various measurements of these carbon atoms could suggest three very different things about ancient Mars. The origin of the carbon is probably due to cosmic dust, ultraviolet degradation of carbon dioxide, or ultraviolet degradation of biologically produced methane.
“All three scenarios are unconventional, unlike the usual processes on Earth,” according to the researchers.
The first scenario involves our entire solar system passing through a cloud of galactic dust, something that happens every 100 million years, according to House. The charged cloud of particles could trigger cooling events on rocky planets.
“It doesn’t have a lot of dust,” House said. “It’s hard to see any of these depositional events in Earth’s record.”
But it’s possible that during an event like this, the cosmic dust cloud would have lowered temperatures on ancient Mars, which could have had liquid water. This could have caused glaciers to form on the planet, leaving a layer of dust on top of the ice. When the ice melted, the sediment layer that includes the carbon would have remained. Although entirely possible, there is little evidence of glaciers in Gale Crater, and the study authors said more research would be needed.
The second hypothesis involves the conversion of carbon dioxide on Mars into organic compounds, such as formaldehyde, due to ultraviolet radiation. This hypothesis also requires further investigation.
The third way this carbon was produced has possible biological roots.
If this kind of depleted carbon measurement were done on Earth, it would show that microbes were consuming biologically produced methane. While Curiosity has previously detected methane on Mars, the researchers can only guess whether there were ever any large plumes of methane that were released from below the surface of Mars. If this were the case and there were microbes on the Martian surface, they would have consumed this methane.
It is also possible that the methane interacted with ultraviolet light, leaving a carbon trail on the Martian surface.
More drilling on the horizon
The Curiosity rover will return to the site where it collected most of the samples in about a month, giving it another chance to analyze sediments from this intriguing location.
“This research fulfilled a long-standing goal for the exploration of Mars,” House said. “It measures different carbon isotopes — one of the most important geological tools — from the sediments of another habitable world, and it does so by looking at nine years of exploration.”