Organic chemicals, mainly composed of carbon and hydrogen, are predominant in all life. They are also widespread in the Universe, so they cannot be considered a clear signature of the presence of life. That creates an alarming situation regarding the search for evidence of life on Mars, which clearly has some organic chemicals despite the harsh environment.
But we don’t know if these are the right kinds of molecules to be signs of life. Currently, we also lack the ability to split Martian rocks, isolate molecules, and determine what they are. In the meantime, our best option is to get some rough information about them and find out the context in which they can be found on Mars. And a big step has been taken in that direction with the publication of results from imaging performed by the Perseverance rover.
The instrument key to the new work has a name that pretty much tells you that it was designed to handle this specific question: Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) . SHERLOC has a deep UV laser to excite the molecules to become fluorescing, and the wavelengths they fluoresce can tell us about the molecules present. It also acquired the hardware to perform Raman spectroscopy simultaneously.
In general, these two capabilities indicate what types of molecules are present, although they do not usually identify specific chemicals. And, critically, SHERLOC provides spatial information, which tells us where specific sample signals are coming from. This allows the instrument to determine which chemicals are located in the same area of a rock and therefore likely formed or deposited together.
SHERLOC can sample rocks just by holding them close. The new results are based on a set of samples from two rock formations found on the floor of the Jezero crater. In some cases, imaging is done by pointing it directly at a stone; in others, the surface of the stone, and any dust and contaminants it may contain, is removed by Persistence before imaging is completed.
SHERLOC identified different signatures of potential organic material in these samples. There are some cases where it is technically possible that signatures are made of a specific chemical that lacks carbon (mainly cerium salts). However, given the choice between several organic molecules or a specific salt, researchers prefer organic materials as a source.
One thing that is clear is that the level of organic material present has changed over time. The deeper, older layer called Séítah has only a tenth of the material found in the Máaz rocks that formed above it. The reason for this difference is unclear, but it indicates that the production or deposition of organic material on Mars has changed over time.
Between the different samples and the ability to resolve different regions of the samples, the researchers were able to identify different signals that each occurred in several samples. Although it is not possible to identify the specific molecule responsible, they have said a fair bit about them.
A signal comes from samples containing organic compounds, including sulfates. The most common signal comes from a two-ring organic molecule, and is associated with various salts: phosphate, sulfate, silicates, and possibly a perchlorate. The other probably has a benzene ring associated with iron oxides. A different ring compound was found in two of the samples.
Overall, the researchers concluded that these differences were significant. The fact that different organic chemicals are often associated with different salts suggests that there are many different ways of synthesizing organics or that they were deposited and preserved under different conditions. Many of the salts found here are also associated with water-based deposition or water-driven chemical alteration of the rock-again, according to the processes associated with change over time.
Overall, the researchers say this argues against organic chemicals being delivered to Mars by a meteorite.
Also, 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 involved living things. But that could eventually change because some of the rocks examined by SHERLOC are being used to obtain material for the planned sample return to Mars. If that mission happens, then there will finally be a chance to isolate and study Earth’s chemicals.
Nature, 2023. DOI: 10.1038/s41586-023-06143-z (Part of DOIs).