Comparing with Earth’s conditions and our understanding, driven by the still limited knowledge of the origin of life on Earth, results in a list of numerous requirements for favourable conditions for past and present habitability and development of biological signature hosting materials: (1) properties of available water like salinity, pH, and temperature; (2) the energy of water in the environment (e.g., quiet vs. energetic), which has implications for the stabilisation of microbial communities; (3) availability of raw materials necessary for life including the so-called CHNOPS elements and a source of electron donors, presence of mineral suites of mixed valence states for redox energy; (4) proximity to a paleo-surface to enable photosynthesis; (5) radiogenic elements for radiolysis; (6) protection from radiation like that provided by a planetary magnetic dipole field; (7) and the rate of burial, for example, in a lacustrine setting, which has implications for the viability and stability of microbial communities. Thus, we search for places in the Solar System for which such conditions comply or did comply in the past with the preserved geological record or current environmental state.

The variety of aqueous minerals identified to date are a testimony to a complex past water history and the alteration of crust-forming minerals on Mars, since these minerals span a wide range of geochemical environments. Their study in localised regions, or globally with a statistical approach, has furthermore revealed variations in time and space. A correlation with morphological features indicates that it is possible to decipher their original environments, based on the knowledge we gathered about Earth environments. Despite this diversity, some key minerals on Mars have not been detected yet, or have not been satisfactorily characterised. Future landing missions with the adequate measurements proposed here, are paramount to achieve this.

The alteration of rocks on Earth and Mars is driven by geochemical, mechanical and sometimes biological processes. On Earth, these processes can be tested and evaluated under laboratory conditions, as well as in nature, while for Mars and other planetary bodies we mainly rely on remotely detected mineral distributions and the interpretation of surface morphology to yield insight in the geological and climatic evolution of Mars or the respective planetary body. Merging these two very different lines of investigation is the goal of the PTAL project.