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 a potential explanation for why it may be there.
That's why carbon atoms - with their recycling cycle - are traces of biological activity on Earth. So they could be used to help scientists determine if life existed on ancient Mars.
When these atoms are measured inside another substance, such as Mars sediment, they can shed light on a planet's carbon orbit, no matter when it occurred.
Learning more about the origin of this newly discovered carbon from Mars can also reveal the process of carbon orbit on Mars.
Secrets of the sediment
Curiosity landed in Gale Crater on Mars in August 2012. The 96-mile (154.5-kilometer) crater, named after Australian astronomer Walter F. Gale, was probably formed by a meteor impact between 3.5 billion and 3.8 billion years ago. since. The large cavity probably once held a lake, and now it includes a mountain called Mount Sharp. The crater also includes layers of exposed ancient rock.
For a closer look, the rover drilled to collect samples of sediment across the crater between August 2012 and July 2021. Curiosity then heated these 24 powder samples to about 1,562 degrees Fahrenheit (850 degrees Celsius) to separate the 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 carbon isotopes or carbon atoms.
Some of the samples were depleted of carbon, while others were enriched. Carbon has two stable isotopes, measured as either carbon 12 or carbon 13.
"The samples extremely depleted in carbon 13 are a bit like samples from Australia taken from sediment that was 2.7 billion years old," Christopher H. House, lead author and professor of geoscience at Pennsylvania State University, said in a statement.
"These samples were caused by biological activity when methane was consumed by ancient microbial mats, but we can not necessarily say that on Mars because it is a planet that may have been formed by materials and processes other than Earth."
In lakes on Earth, microbes like to grow in large colonies that essentially form mats just below the water surface.
3 possible carbon origins
The different measurements of these carbon atoms could suggest three very different things about ancient Mars. The origin of carbon is probably due to cosmic dust, ultraviolet decomposition of carbon dioxide or the ultraviolet decomposition of biologically produced methane.
"All three of these scenarios are unconventional, as opposed to processes common on Earth," according to the researchers.
The first scenario involves our entire solar system passing through a galactic cloud of dust, something that happens every 100 million years, according to House. The particle-heavy cloud can trigger cooling events on rocky planets.
"It does not deposit a lot of dust," House said. "It's hard to see any of these deposition events in the Earth's record."
But it is possible that during an event like this, the cosmic dust cloud would have lowered the temperature of ancient Mars, which may have had liquid water. This could have caused glaciers to form on Mars, leaving a layer of dust on top of the ice. When the ice melted, the sediment layer including carbon would have remained. While fully possible, there is little evidence of glaciers in Gale Crater, and the study's authors said it would require further research.
The second scenario involves the conversion of carbon dioxide on Mars into organic compounds, such as formaldehyde, due to ultraviolet radiation. That hypothesis also requires further research.
The third way in which this carbon was produced has possible biological roots.
It is also possible that the methane interacted with ultraviolet light, leaving a trace of carbon on the surface of Mars.
More drilling on the horizon
The Curiosity rover returns to the site where it collected the majority of the samples in about a month, allowing for another chance to analyze sediment from this exciting site.
"This research achieved a long-term goal for Mars exploration," House said. "Measuring various carbon isotopes - one of the most important geological tools - from sediment in another habitable world, and doing so by looking at nine years of exploration."