NASA's Perseverance Mars rover has been working in Jezero Crater for five years, looking for "chemical footprints" left behind by various geological or chemical processes on Mars billions of years ago. It has previously found organic carbon inside several rocks, but it required drilling or abrasion to expose it.The latest results show that in an outcrop area called Neretva Vallis on the edge of an ancient river channel, Perseverance detected complex macromolecular carbon directly on the surface of exposed rocks.
Ashley E. Murphy, lead author of the study and a researcher at the Planetary Science Institute in Tucson, Arizona, said: "As far as we know, this is the case where organic matter has been detected at the shallowest level on the surface of Mars." On Earth, such abundant and macromolecular carbon materials usually imply that they have biological origins. However, it is still not possible to draw conclusions about what kind of carbon is on this rock named "Bright Angel" and where it comes from. To really find out the answer, I am afraid that the sample needs to be brought back to the Earth laboratory.
UV Raman instrument locks into “macromolecular carbon” signal
The key to this discovery lies in the ultraviolet Raman spectrometer on the "Perseverance" robotic arm - SHERLOC, which stands for "Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals". The instrument emits a deep ultraviolet laser onto a target and then analyzes the tiny shifts in energy of the reflected light to identify the presence of specific chemical bonds.
Between Martian days (sol) 1180 and 1218, Perseverance pointed its ultraviolet laser at four observation points in the Bright Angel area. One of the rocks, called Steamboat Mountain, was treated as a normal rock and used as a control sample. The spectral signals of three other rocks, named "Cheyava Falls", "Apollo Temple" and "Walhalla Glades", all show the presence of macromolecular carbon. This signal is called the "graphitic band" (G-band), which is characterized by a complex network cross-linked by a large number of reduced carbon atoms. It has strong chemical and thermal stability and is not easy to decompose.
Within the accuracy of Perseverance's instruments, the material is similar to kerogen on Earth. But the research team deliberately avoided using the term "kerogen" because on Earth, kerogen is almost entirely derived from biological material, mainly the remains of microorganisms that have been buried for millions of years. Murphy explained: "The word 'kerogen' has an obvious biogenic meaning, and we prefer to use 'macromolecule carbon' to indicate that its origin is uncertain and may be either a biological process or a non-biological process." The team particularly emphasized that the macromolecular carbon currently found on Martian rocks is entirely possible to be produced by non-living processes.
Eliminate "instrument artifacts" and "contaminated passengers"
In such sensitive detection work, an abnormal result usually raises two primary questions: Is this an artifact of the instrument itself? Could this be pollutants brought from the earth? The research team also followed this idea and investigated one by one.
First, scientists were concerned that the detected signal originated from SHERLOC's own fused quartz front window, rather than the rock surface. It is worth noting that "Bright Angel" is the location where SHERLOC conducted its first scientific observations after the dust cover failure. Since the focusing mechanism was forced to deactivate, the team had to adopt a new working model. To confirm the performance in the new mode, SHERLOC deputy principal investigator Kyle Uckert of the Jet Propulsion Laboratory (JPL) in the United States and his colleagues conducted spectroscopic tests on the backup flight optics in the ground laboratory, and repeatedly observed blank areas and known calibration targets on Mars to verify whether the instrument was operating normally.
The final confirmation came from the comparison target "Steamboat Hill". Uckert said: "There are no G-band spectral signals from other nearby rock targets." This shows that the graphite band signal on the "Bright Angel" rock does not come from the instrument hardware, but is indeed related to the material on the surface of the specific rock.
The second question is the risk of contamination: could these organic matter be "hitchhikers" brought by the Mars rover from Earth? Scientists pointed out that the drill bit used by "Perseverance" to abrade the rock surface has been strictly sterilized before launch, and such a strong G-band signal has never been seen before when it was used on many rocks in Jezero Crater. More importantly, the rock "Cheava Falls" has never been directly contacted by any hardware, and the rover only removes dust on its surface through nitrogen injection. The control rock "Steamboat Mountain" again showed a blank - no spectral evidence of any organic matter. Uckert emphasized: "There is no evidence of organic matter in its spectrum." Based on these exclusion steps, the team believes that the macromolecular carbon signal on the "Bright Angel" rock is more likely to be local Martian material rather than contamination from the Earth.
Associated with different minerals, suggesting multiple “carbon sequestration” events
After basically confirming that the signal is authentic and reliable, the research team further analyzed the mineral combinations near these macromolecular carbons, hoping to infer the formation and enrichment process of carbon materials. "These associated chemical environments suggest that carbon insertion may have occurred in at least two separate events in geological history," Murphy said.
In the Temple of Apollo rocks, macromolecular carbon signals are concentrated with carbonate and sulfate minerals—minerals that are typically the product of water flowing inside older rocks and settling in the pores. In the "Valhalla" rocks, macromolecular carbon is distributed in siliciclastic-rich sediments. Murphy believes this difference likely represents at least two storage windows: first, when organic matter was buried into the rocks along with muddy sediments at the bottom of ancient lakes; second, when groundwater reflowed through these buried rocks at a later time, leaving new carbonate and sulfate minerals in place along with the carbon material.
However, the key question of whether the carbon on Bright Angel's rocks is truly a remnant of ancient Martian life will remain open in the short term. Uckert said: "The design purpose of the scientific payload of 'Perseverance' is not to directly distinguish between abiotic and biological processes, but to identify the most valuable and indicative rock samples on Mars to prepare for possible future sample recovery missions."
Earth Lab needs to give a clearer answer
Kevin P. Hand, chief scientist of the "Perseverance" project and JPL researcher, pointed out that although the current instruments are quite advanced, their capabilities are still limited compared with "world-class analytical technology" on Earth. "The combination of instruments we have on board the rover is excellent, but it pales in comparison to the top-notch technology available in our ground-based laboratories," Hand said.
Hand is particularly interested in the isotopic signature of the carbon in the Bright Angel rocks because the isotope ratios are expected to provide clues about the involvement of life. He also hopes to analyze the chirality of these carbon-related molecules in the future - in life systems on Earth, a molecule's preference for a certain "chiral" direction is a strong biological signal. Hand added: "If we have the opportunity to bring samples back to Earth, we can also use the most powerful microscopes to search for possible microbial fossils, thereby providing more intuitive evidence of past life activities on Mars."
At the same time, the research team also emphasized that there is currently no shortage of non-living production mechanisms for this type of macromolecular carbon. In some environments, reactions between fluids and rocks can synthesize organic compounds without the involvement of life at all. Murphy points out that on Earth, carbon found near carbonate minerals can sometimes be traced to water-rock chemical reactions and sometimes to microbial activity, depending on the geological environment. Hand expressed the hope that Perseverance can find more such rocks on Mars that are worthy of in-depth study before the samples are transported back to Earth.
"Right now we're exploring the area outside of Jezero Crater - and the rocks in our current path are likely to be some of the oldest that a rover has ever studied," Hand said. "If life existed on Mars early in its evolution, we may find some clues in these ancient rocks." This research result has been published in Science Advances (Science Advances), the paper number is 2026 adx0047.