Laboratory experiments to support the detection of organic substances on Mars, by the NASA Mars 2020 Perseverance rover and future planetary surface exploration missions

Mars is a primary target for astrobiological exploration. Its harsh environment is characterized by a mean temperature of -62°C, no magnetic field, and a thin carbon dioxide atmosphere that allows galactic cosmic rays (GCRs), solar energetic particles (SEPs) and UV radiation to sterilize the surface. Despite this, thanks to several Martian missions, now scientists are confident on a past habitability of the Red Planet. Indeed, over its first billion years, Mars had conditions similar to Earth, supporting the possibility that simple life forms developed, like those on early Earth. Nowadays, evidences of this possible past life might be preserved in ancient rocks, as Mars lacks plate tectonics. The NASA Mars 2020 mission, with the Perseverance rover, is exploring Jezero crater, a former river delta-lake system, to search for traces of past life. Perseverance uses a rotary drill to access rock interiors through abrasion patches, where organic molecules may have been protected from the harsh surface environment. When the rover performs an abrasion, for operational reasons, these exposed areas remain subject to the harsh environment for at least 1 sol (1 Martian day) before Perseverance's proximity scientific instruments can take measurements. In this time frame, any pristine organic material exposed due to abrasion could be degraded by UV irradiation, as it causes damage in characteristic timescales ranging from days to a few years, unlike GCRs and SEPs that have effects over hundreds of millions of years. The Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) proximity scientific instrument onboard the Mars 2020 Perseverance rover has so far detected Raman signals from the organic spectral regions on some abrasion patches. In the case of the Quartier abrasion (Issole outcrop in Jezero crater floor) the Raman signals were detected after 1 sol (first measurement) and even after 11 sol (second measurement) of surface exposure, in spatial co-location with hydrated sulfate minerals (e.g., hydrated magnesium sulfate). The Quartier organic hypothesis would be possible only in the presence of organic molecules which remain photostable, or photo-protected by minerals, for at least 11 sol. The major aim of this Thesis is to shed light on the Quartier SHERLOC organic hypothesis assessing the photostability of organic compounds that are plausibly on Mars: Polycyclic Aromatic Hydrocarbons (PAHs) and carboxylic acids. Specifically, 2,6-dihydroxynaphthalene (PAH), benzo[a]pyrene (PAH), phthalic acid (carboxylic acid) and mellitic acid (carboxylic acid) were selected. To reproduce the Quartier abrasion workspace and considering the sulfate mineral co-location of those Raman signals, magnesium sulfate hydrate (specifically, MgSO4-7H2O) was included in the experiments. At this point, molecular-mineral complexes (called Martian analog samples) were prepared, using a method that mimic natural processes that may have occurred over time in the Jezero crater. To assess the kind of molecule-mineral interaction, InfraRed (IR) spectroscopy, Raman spectroscopy and X-Ray Diffraction techniques were used, resulting in the adsorption of all the molecules except for the inclusion of phthalic acid in the mineral structure. Regarding the molecule photostability assessment, the average pure molecules half-lives after UV irradiation turns out to be between 0.7 sol≲t_(1/2)≲10 sol. This result not satisfied the 11 sol required for the Quartier Raman signals. Despite this, the addition of magnesium sulfate, by irradiating the Martian analog samples, drastically changed this result. In fact, both carboxylic acids and PAHs displayed greater photostability when adsorbed/included on hydrated magnesium sulfate: no significant degradation were observed over the full UV irradiation period, equivalent of a Martian time of UV irradiation largely greater than 11 sol. Beyond molecular degradation, photoproducts were detected for all the four molecules. Interestingly, in the presence of magnesium sulfate hydrate the molecular photoproducts or forms slowly than in the pure molecule experiments or in minor quantity. These findings represent a photoprotective behavior of hydrated magnesium sulfate, almost toward the molecules selected, underscoring the necessity of a broader and systematic characterization of mineral-mediated shielding processes. Notably, the laboratory evidence is consistent with SHERLOC in situ observations – such as those from the Quartier abrasion patch – where plausible organic signals were detected in close association with hydrated sulfates. This agreement between experimental and mission findings reinforces the idea that these minerals may play a key role in preserving molecular traces, potentially representing an important puzzle piece in the search for biosignatures on Mars.