Organic compounds, made mainly of carbon and hydrogen, that are present in all living things. They are also everywhere in the Universe, so they cannot be taken as a clear signature of the existence of life. This makes it a bit annoying to look for evidence of life on Mars, which clearly has biological properties despite the danger.
But we don’t know if these are the right types of molecules to be life signals. At this point, we can no longer tear apart the Martian rocks, apart from the molecules, and determine what they are. For now, our best bet is to find out more about them and see how they are found on Mars. And a big step has been made in this direction with the publication of the results of the photography done by the Perseverance rover.
Ask SHERLOCK
The most important tool in this new project has a name that tells you that it was created to answer this question: Raman & Luminescence Spot Analysis of Organics & Chemicals (SHERLOCK). SHERLOC comes with a deep-UV laser to excite molecules into fluorescing, and the wavelengths of light can tell us something about the molecules present. It also has tools to do it Raman spectroscopy at the same time.
All in all, these two forces indicate the types of molecules present, although they cannot detect the actual chemical. And, counterintuitively, SHERLOC provides location information, telling us where the sample signals come from. This allows the tool to determine which compounds are at the same location in the rock and thus are formed or grouped together.
SHERLOC can test rocks just by being held close to them. The new results are based on samples from two rocks found at the bottom of the Jezero Valley. Sometimes, the picture was taken by pointing it directly at the rock; among other things, the rock, and any dust and dirt it contained, was removed by Perseverance before the picture was made.
SHERLOC identified a variety of potential signatures in these samples. There were a few cases where it was technically possible for signatures to be made with real chemicals that didn’t contain oxygen (especially cerium salts). But, when choosing between many types of natural molecules or very special minerals, the researchers prefer organic materials as a source.
One thing that was clear was that the amount of material available changed over time. A deep, ancient cave called Séítah contained one-tenth of the material found in the Máaz rocks that formed above it. The reason for this difference is unknown, but it suggests that the production or deposition of material on Mars has changed over time.
Regional differences
Between the different samples and the ability to solve different areas of the samples, the researchers were able to identify different symptoms that each occurred in many samples. Although it was not possible to identify the exact molecule that causes it, they were able to make a clear statement about it.
One indicator came from samples that contained organic compounds, including sulfates. The most common sign comes from a two-dimensional molecule, and it has been associated with various minerals: phosphate, sulfate, silicates, and possibly perchlorate. One apparently had a benzene ring attached to iron oxides. A different circular pattern was found in two samples.
Overall, the researchers find that this difference is significant. The fact that different chemicals are associated with different minerals indicates that there were several different methods of organic production or that they were stored and preserved under different conditions. Many of the minerals seen here are also associated with water deposition or water-driven chemical changes in the rock—again, consistent with changes that occur over time.
Together, the researchers say this contradicts the natural chemical that is only delivered to Mars on meteorites.
Again, the instrument cannot tell us what the chemicals are, so there is no way to know if any of the processes involved in making these deposits affect organisms. But that could change because some of the rocks examined by SHERLOC were used to find material to return to Mars. If that mission happens all the timethen we will have the opportunity to isolate ourselves and study the medicine on Earth.
Nature, 2023. DOI: 10.1038/s41586-023-06143-z (About DOIs).