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Retention of potential biological evidence through deposition, burial, alteration and exposure. Organic molecules, microscopic textures and isotope fractionation have different sensitivities to heating, oxidation and ionizing radiation. A former habitable environment can lack a recoverable biosignature if its record was destroyed; a preserved feature still requires tests against nonbiological origins and contamination.

The relative abundance of two specified isotopes. Radiogenic growth, mixing and isotope fractionation can modify different ratios in different ways.

Prioritize accessible ancient lacustrine deposits at a preserved river delta, especially fine-grained, clay mineral-bearing mudstones, with nearby evidence of groundwater or hydrothermal circulation. The target should combine past habitability, good biosignature preservation and practical access to minimally altered rock.

An inlet valley, a river delta and a stratified basin together can establish a connected fluvial–lake paleoenvironment on Mars. A long-lived lake offers liquid water, chemical transport and a sustained habitat, while a river delta concentrates material from a larger catchment. Distal mudstone is particularly attractive: fine particles can adsorb organic molecules, burial reduces exposure, and quiet deposition can preserve small-scale structures. Rover observations have independently established ancient lake and river delta settings on Mars, so this is a geological search strategy grounded in observed environments.^[ref][ref]

A second high-value setting is an ancient hydrothermal deposit, particularly silica-rich precipitates, spring terraces or mineralized groundwater fractures near a heat source. Such environments combine water-rock interaction, chemical disequilibrium and the possibility of rapid mineral entombment. Distal, moderate-temperature parts of a hydrothermal circulation system are preferable to strongly reheated zones where organic molecules would be destroyed. Orbital observations identify candidate ancient hydrothermal deposits on Mars, although their geological interpretation requires testing on the ground.^[ref] Carbonate-bearing sedimentary deposits can also preserve textures and chemical records of former water, but precipitation of carbonate alone is not a biosignature.

Choose ancient deposits with good stratigraphy, limited later metamorphism and relatively recent exposure by erosion, or sample beneath the irradiated surface. Present exposed organic molecules can be destroyed by cosmic rays, radiolysis and oxidizing surface chemistry, so a drill, fresh scarp or recently excavated material improves the preservation prospects. Laboratory irradiation experiments on Martian analogue materials demonstrate why shallow ancient surface exposure can erase an organic record.^[ref]

At the site, relate every sample to its sedimentary facies and mineral context. Seek several independent, spatially associated biosignatures: plausible microscopic structures, distinctive organic molecules, and isotope fractionation consistent with a biological process. Each has nonbiological alternatives, so test water-rock interaction, mineral self-organization and contamination before inferring life. Include control samples from adjacent unpromising rocks and track terrestrial contamination. A safe landing ellipse with traversable exposures of several related sedimentary facies is more useful than a spectacular but inaccessible single outcrop. The desired outcome is contextual evidence that can distinguish former life from merely former habitability.

Probably yes in connected fractures or transiently opened pore space, especially where a transform fault admits circulating seawater; a uniform water-filled porous layer several kilometres thick is not guaranteed. Porosity and permeability of a porous medium generally decrease as cracks close or seal, while ongoing faulting can reopen pathways. Fluid can occur at high pressure and temperature, including a supercritical state; it need not resemble cold surface water.

Several complementary tests address presence and origin:

No one anomaly uniquely identifies water. Seismic velocity can also reflect cracks, composition and temperature; electrical conductivity can reflect melt or conductive minerals. Serpentinization demonstrates incorporation of water into minerals, which must be distinguished from fluid currently occupying pore space. At an oceanic transform fault, an external seawater supply is the natural first hypothesis, but isotope ratios and chemical budgets can test contributions from deep fluids rather than presuming the source. A research example of coupled deformation and hydrothermal alteration is Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif by Chiara Boschi and others; a detachment system illustrates the methods without proving identical flow at every transform fault.