The Curiosity rover recently found sequestered carbon deposits in the Gale Crater on Mars. The CO2 was found in 3.5-billion-year-old siderite, suggesting that an ancient water-related carbon cycle once took place on Mars.
Ample evidence suggests that Mars once had flowing, liquid water, prompting the search for Martian CO2. For liquid water to exist on Mars, unlike its icy form today, the planet had to be much warmer. CO2 is a likely solution to this need as it causes a heat-amplifying greenhouse effect within the atmosphere. This effect was crutial to ancient Mars, as the sun was dimmer and thus imparted less heat than today.
Ana Gonzalez-Nayeck, PhD, is an assistant professor in the Natural Sciences department at Baruch. She stresses the relationship between CO2 levels and the presence of liquid water. “When we look at the rocks on Mars, we only see 10% of the CO2 needed to trap that amount of liquid water,” she said.
The missing CO2 is especially unexpected, considering CO2 is a heavy gas, unlikely to have escaped into space. The discovery of siderite suggests that the missing CO2 is presently beneath the Martian surface, concealed under layers of another rock called sulfate. In a carbon cycle, the siderite would be rained on and release its carbon back into the atmosphere. On Mars, this cycle stopped, likely due to sulfate buildup. Martian sulfate likely formed through acid rain depositing minerals onto siderite, eventually building up into a solid surface. This covered the siderite, preventing it from releasing its CO2 back into the atmosphere. This process was especially damaging when coupled with the loss of Mars’ magnetic field around 4.1 billion years ago.
Subsequent atmospheric thinning from solar wind and trapped CO2 may have led to a positive feedback loop. The atmospheric CO2 loss led to less liquid water, and vice versa. In turn, the lack of water stopped sulfate formation. Gonzalez-Nayeck explains that “…once you lose that liquid water, potentially because you have buried all this carbon, you can no longer form [sulfate].” The sulfate layer, followed by siderite, is ultimately left as an informative tomb for the death of Mars’ once-active carbon cycle.
Initial surveys looked for sequestered CO2 where it would usually appear on Earth: on the surface. However, the carbon cycle on Earth today is very different from the halted one on Mars. Carbon-containing rocks are exposed to rain on Earth, letting them release carbon much like early Mars. On Mars, these rocks are buried and undetectable via orbital scans.
“I think this is a really good example of how we have these biases from being Earthlings that prevent us from considering how climate can opate on different planets. For the whole time, we were looking for carbon on the surface of Mars, because the carbon rocks on our planet are primarily on the surface,” Gonzalez-Nayeck said.
“We didn’t appreciate how on a planet like Mars, that the carbon could be buried under the surface.”
