NASA’s Perseverance rover has spent five years traversing Jezero Crater in search of chemical remains of whatever processes were at work on Mars billions of years ago. The rover has found organic carbon, but it’s mostly found within rocks that had to be drilled or weathered to expose it. But now, at an outcrop on the edge of an ancient river channel called the Neretva Vallis, Perseverance detected complex macromolecular carbon sitting right on the surface of the rock.
“To our knowledge, this is the most superficial detection of organic matter on the Martian surface to date,” said Ashley E. Murphy, a researcher at the Planetary Institute in Tucson, Arizona, and lead author of the study of the rock, which was found at a site called Bright Angel. On Earth, this amount of macromolecular carbon usually suggests a biological origin. But to know what this Bright Angel carbon is and where it comes from, we may need to bring samples back to Earth.
Carbon in rocks
The Bright Angel carbon detection came from SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), a UV Raman spectrometer installed on Perseverance’s robotic arm. SHERLOC fires a deep ultraviolet laser at a target and reads the light that bounces back with displaced energies, a signal that allows scientists to identify specific molecular bonds.
Between sols 1180 and 1218, the rover aimed this UV laser at four targets in Bright Angel. One, called Steamboat Mountain, was an ordinary rock that the team used as a control. The remaining three (named Cheyava Falls, Apollo Temple and Walhalla Glades) returned a macromolecular carbon spectroscopic signature. This signal, called the graphitic band (G-band), indicates the presence of a tangled, cross-linked network of mostly reduced carbon atoms that is resistant to chemical and thermal degradation.
At least within the precision limits of Perseverance’s instruments, the material roughly matches terrestrial kerogen. However, the researchers decided that the word “kerogen” could not be used. On Earth, kerogen is composed almost exclusively of biological matter, mainly fossilized microbes that were buried millions of years ago. “The term kerogen implies a biogenic source,” Murphy explained. “Macromolecular carbon means that we don’t know if its origin is biotic or abiotic.”
