NASA’s Perseverance rover spent five years traversing Jezero Crater, searching for chemical remains of whatever processes took place on Mars billions of years ago. The rover found organic carbon, but it was mostly in rocks that had to be dug or eroded to expose it. But now, at the edge of an ancient river channel called the Neretva Vallis, Perseverance has discovered complex macromolecular carbon sitting on the rock surface.
“To our knowledge, this is the shallowest detection of organic matter on the surface of Mars to date,” said Ashley E. Murphy, a researcher at the Planetary Institute in Tucson, Arizona, and lead author of the Bright Angel rock study. On Earth, this much macromolecular carbon usually indicates a biological origin. But we may have to return the samples to Earth to find out what this Bright Angel carbon is and where it came from.
Carbon in rocks
The Bright Angel carbon detection was achieved by SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), a UV Raman spectrometer mounted on Perseverance’s robotic arm. SHERLOC fires a deep ultraviolet laser at a target and reads the light bounced back at shifted 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 (called Cheyava Falls, Temple of Apollo, and Valhalla Glades) returned a spectroscopic signature of macromolecular carbon. 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 limits of precision of the Perseverance instruments, the material roughly matches the surface kerogen. The researchers decided that the word “kerogen” should not be used. On Earth, kerogen is made almost exclusively of biological matter, mostly fossilized microbes buried millions of years ago. “The term kerogen means a biogenic source,” Murphy said. “We don’t know if macromolecular carbon is of biotic or abiotic origin.”
