A year after reports that Nasa’s Perseverance rover had discovered organic compounds in Mars’ Jezero crater that could hint at ancient life, a new study in Science Advances is challenging these claims.1 While the rover’s mission is to uncover evidence of prebiotic chemistry, researchers now show that what was thought to be organic compounds might actually be inorganic material, opening up a debate about whether the Red Planet holds clues to past life or if the building blocks of life are still there below the surface.
Since Perseverance landed on Mars in February 2021, it has spent the last three years investigating the geology of the Jezero crater – a depression about 28 miles wide, which is thought to have once been flooded with water. The rover carries several data-collecting probes and instruments integrated into a robotic arm-mounted instrument called Sherloc (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals). Sherloc employs deep ultraviolet luminescence and Raman spectrometry to determine the geological history of the Jezero crater. The rover’s UV luminescence detection has higher sensitivity for aromatic organics, compared with Raman scattering, which can identify a broader range of organic and inorganic materials. Finding traces of organic molecules, which could contain potential biosignatures including ancient biological matter, in any samples collected by the rover would make them a high priority for collection and return to Earth for further study. Analysis of these samples in a laboratory would provide far more insight into some of the oldest potentially habitable environments in our solar system.
Thus far, the rover has drilled into and sampled rocks from the Máaz and Séítah formations on the Jezero crater floor that are between 2.3 billion and 2.6 billion years old. In 2022, a study in Science reported the presence of igneous materials like olivine and basalt in these rocks, with layering that indicated both their volcanic origin and repeated exposure to liquid water in the past.2 Aqueously altered rocks have a high probability of harbouring organic compounds, according to a follow-up study in Nature in 2023, which reported that signals from Sherloc were consistent with many classes of organic molecules.3
The study builds on observations based on data from Mars’s Gale Crater and Martian meteorites, revealing a sporadic distribution of organic material on Mars. ‘We found that the fluorescence signals retrieved by Sherloc were from organic materials – specific aromatic (ring) molecules can UV fluoresce in the wavelength range of Sherloc,’ explains Eva Scheller, a planetary scientist at the Massachusetts Institute of Technology (MIT) who was author on all three studies. Such small aromatic organics are typically found in extraterrestrial materials like carbonaceous meteorites and asteroids. Andrew Steele, an astrobiologist at Carnegie University who has worked on the data from Perseverance with Scheller, explains that ‘organic material is consistently detectable across the Martian surface [and] is associated with minerals that have undergone aqueous weathering and is present, but not ubiquitous, in Jezero crater’.
However, an alternative interpretation of these signals in the new Science Advances study points to UV fluorescent cerium ions, primarily concentrated in phosphate minerals. ‘Much of the phosphate likely formed when they crystallised from magma billions of years ago,’ adds Benjamin Weiss, a planetary scientist at MIT who contributed to this work. The fluorescent signals in the chemical data were tightly correlated to phosphate and other minerals, ‘suggesting that cerium(III) provides a very tangible interpretation to the fluorescence signals that we otherwise thought were from aromatic rings’, says Scheller. Sherloc has also not reported any Raman signatures of macromolecular carbon aside from a lone, ambiguous Raman feature at ~1600cm-1, which remains too faint to draw firm conclusions about its origin. ‘Any assumptions about organics that are based on the luminescence should be taken with a grain of salt,’ says Tanja Bosak, a geobiologist and group leader at MIT, who also participated in this new analysis.
The reason it is so hard to be certain what Perseverance has found on Mars lies in the inherent difficulty of conducting experiments using miniaturised, mobile laboratories in an extreme and unpredictable environment. ‘There is very little opportunity to perform replica experiments … when you actually go to an unknown place you will always encounter some problem that you hadn’t thought about or did not have the conditions to test on Earth,’ explains Scheller. ‘These are very difficult measurements to make, mainly due to the low concentrations of these materials and, therefore, requiring a very sensitive instrument that can still be miniaturised. This is a great open question, which I am sure may help direct the future of spacecraft instrument design.’
Even though the study contradicts previous results drawn from the chemical data sent by the rover, ‘this paper … proposes a more robust set of criteria that can be used to identify such organics from future observations’, suggests Weiss.
This ongoing debate of exactly which compounds lie under the surface of Mars, underscores the need for analysis back on Earth of the rock samples, many of which have been cached by Perseverance for potential return in the 2030s.