Potential for photosynthesis on Mars within snow and ice

Exposed water ice and dusty snow are present on Mars, including mid-latitude exposures revealed by impacts and slope processes. Ice and snow transmit solar radiation to depth, enabling subsurface photic environments on Earth where organisms are shielded from UV yet receive photosynthetically active radiation (PAR). Mars lacks an effective ozone shield, so surface UV is stronger than on Earth, potentially damaging to life. Prior studies suggested radiative habitable zones (RHZs) could exist in martian snow and ice but often used optical assumptions better suited to fine snow or neglected the strong spectral variation of ice absorption and the influence of martian dust. The central question addressed here is whether, and at what depths, RHZs exist within exposed mid-latitude martian ice/firn where DNA-damaging UV is sufficiently attenuated and PAR remains adequate for photosynthesis, and whether these depths coincide with conditions where transient liquid water could occur. Establishing such zones would identify promising, accessible targets in the search for extant life on Mars.

Previous martian studies and Earth analog work showed that snow and ice can transmit solar radiation to depth, creating subsurface photic zones, and suggested RHZs might exist on Mars between decimeters and several meters. However, earlier martian models often assumed shallow absorption or used single broadband extinction values, underestimating energy at depth and not representing coarser-grained firn or glacier ice likely present on Mars. Observations show mid-latitude dust mantles can be removed to expose dusty ice; glacial and firn-like metamorphism is expected on timescales of decades to millions of years. Polar sites are generally too cold for melting. Earth-based measurements (e.g., in Greenland ice) and radiative-transfer developments (Delta-Eddington/adding–doubling approaches; SNICAR family) demonstrate the importance of wavelength-dependent absorption of ice and the strong absorbing nature of ferric-rich martian dust in UV–visible. Biological studies on Earth document microbial communities in ice/snow and cryoconite holes, illustrating the potential for phototrophic life in translucent ice with shielding from UV.

The study develops and applies a radiative transfer model tailored to martian dusty snow, firn, and glacier-like ice to assess subsurface radiative environments relevant to photosynthesis and UV damage. - Radiative transfer framework: Based on the SNI-ARC (Snow, Ice, and Aerosol Radiative Adding–Doubling) model, using a Delta-Eddington two-stream approach modified for vertically heterogeneous layers and varying refractive indices. The model can mix snow, firn, ice, and impurities (martian dust) and accounts for reflective boundaries. - Spectral treatment: Incorporates the large, order-of-magnitude spectral variation (about 10 orders overall across UV–NIR) in the imaginary (absorptive) refractive index of H2O ice to compute wavelength-dependent absorption and scattering, rather than using a single broadband extinction value. This is essential to correctly predict transmission depths. - Optical properties: For ice, a blended dataset for the imaginary part of the refractive index is used: Picard et al. (0.32–0.60 µm, extended to 0.2–0.32 µm using the 0.32 µm value) and Warren & Brandt (0.6–3 µm); real part from Warren & Brandt. Martian dust spectral absorption follows Wolf et al.; dust is orders of magnitude more absorbing than ice in UV–visible, mainly due to ferric iron. - Forcing spectra: The Planetary Spectrum Generator (PSG) provides the downward spectral solar flux at the martian surface using Mars–Sun distance 1.52 AU, solar zenith angle 0°, solar longitude 0°, at 0°N, 0°E, with a standard martian atmospheric profile and no aerosols. The spectrum is normalized and then scaled with peak annual broadband solar flux from the Mars Climate Database for each latitude to obtain surface spectral fluxes. - Media properties: Simulations span clean to dusty ice (e.g., 0.01% and 0.1% dust by mass), a range of grain radii/densities representing dense snow, firn, and bubble-bearing glacier ice, and latitudes/solar zenith angles relevant to mid-latitudes (30°–60°). - Diagnostics: Actinic flux (angle-integrated photon flux) with depth is computed across UV (0.2–0.4 µm) and PAR (0.4–0.7 µm). PAR is the spectrally integrated solar flux over 0.4–0.7 µm, evaluated against lower and upper limits for photosynthesis (about 10–20 µmol photons m−2 s−1; approximately 2.2–4.353 W m−2). DNA-damaging irradiance is computed by integrating 0.2–0.4 µm spectral flux weighted by a biological action spectrum (Cockell & Raven), with a conservative safety threshold of 0.1 W m−2. - Validation: Modelled downward spectral irradiances are compared with measurements in Greenland glacier ice (where available). Although impurity content was not measured in those field data, the model reproduces key spectral-depth behaviors, supporting its use for martian analog conditions. - Sensitivity analyses: Effects of dust content, grain size, latitude, and solar zenith angle on the depth and thickness of RHZs are explored. RHZs are defined where DNA-damaging irradiance is below 0.1 W m−2 and PAR lies between adopted lower and upper limits.

- Radiatively habitable zones (RHZs) exist within exposed mid-latitude martian ice despite intense surface UV. Depth and thickness depend strongly on dust content, grain size, and to a lesser extent on latitude and solar zenith angle. - Clean ice: Most solar radiation is attenuated within the top few meters; for clean ice at ~40° latitude and a low solar zenith angle, an RHZ occurs between about 2.15 and 3.10 meters (using the lower PAR limit). - Effect of martian dust: Even 0.01% dust by mass shifts deepest penetration toward longer wavelengths near 0.7 µm and drastically reduces penetration depth. Relative to pure ice, martian dust is ~7 orders of magnitude more absorbing in 0.2–0.4 µm, enhancing UV attenuation by a factor of ~25. - With 0.01% dust: PAR penetrates to ~40 cm; peak UV penetration occurs to ~15 cm. - With 0.1% dust: PAR penetration depth is ~5.5 cm; peak UV penetration is ~2 cm. - Grain size and density: Coarser grains and higher densities (transition from snow to firn to glacier ice) reduce internal boundary scattering and allow deeper penetration, moving RHZs deeper and often increasing their thickness. For bubble-bearing glacier ice, RHZ depths are on the order of tens of centimeters under dusty conditions. - Latitude: Changing latitude from 30° to 60° shifts RHZ depths by only a few centimeters because PAR and DNA thresholds are orders of magnitude lower than the variation in annual peak surface flux with latitude. - Solar zenith angle (SZA): Increasing SZA (Sun lower in the sky) reduces RHZ thickness; e.g., from ~5 cm at 0° to ~2 cm at 60° in representative cases. - High dust contents (approaching ~1%) can create very narrow RHZs (millimeters) or form opaque surface dust lags that block illumination, suppressing subsurface habitability. - Integration with thermal/melt models indicates that dense, dusty mid-latitude snowpacks can produce small amounts of subsurface meltwater (up to ~0.33 mm day−1 for ~50 days per martian year) centimeters below the surface, aligning with modeled RHZ depths.

The modeling shows that martian mid-latitude ice/firn can host subsurface zones where DNA-damaging UV is sufficiently attenuated while PAR remains adequate for photosynthesis. This directly addresses the research question by identifying depth ranges—from a few centimeters in slightly dusty ice (0.01–0.1% dust) to a few meters in cleaner ice—where phototrophic organisms could potentially function. Dust plays a dual role: it enhances UV shielding but reduces total transmission, compressing RHZs toward the surface as dust content increases. Grain metamorphism toward coarser firn/glacier ice further promotes deeper penetration, consistent with martian ice evolution. The weak dependence on latitude suggests RHZs are widespread across mid-latitudes when ice is exposed, while diurnal/seasonal changes in solar elevation modulate RHZ thickness. When coupled with independent models predicting transient subsurface melting within dusty snowpacks on steep mid-latitude slopes, the results identify plausible microhabitats where both radiative and liquid water requirements could be met episodically. Analogous to terrestrial cryoconite systems, dark dust and sediments can locally enhance absorption, create melt features, and sustain microbial communities under translucent ice lids while isolating them from the atmosphere. Therefore, exposed mid-latitude ice outcrops represent compelling astrobiological targets, potentially more accessible than polar regions that are too cold for melting at relevant depths.

Radiative transfer modeling tailored to martian dusty snow, firn, and glacier-like ice demonstrates the existence of radiatively habitable zones beneath exposed mid-latitude ice. These RHZs occur at depths of centimeters for ice containing 0.01–0.1% dust and extend to meters in cleaner ice. Martian dust substantially enhances UV shielding but reduces PAR penetration, setting a trade-off that determines RHZ depth and thickness. Grain coarsening increases penetration depth, while latitude exerts only minor control; higher solar zenith angles thin RHZs. When combined with predictions of transient subsurface melting in dusty mid-latitude snowpacks, the identified RHZs align with depths where liquid water may intermittently exist, highlighting these sites as priority targets to search for extant phototrophic life on Mars. Future work should include in situ measurements of spectral irradiance within martian ice, tighter constraints on grain size and dust content of exposed ice, improved treatment of atmospheric aerosols in surface spectra, and further validation using terrestrial analogs to refine RHZ predictions and guide exploration strategies.

- No direct measurements of spectral solar flux within martian ice exist; results rely on modeled spectra (PSG) scaled by Mars Climate Database outputs. - The PSG simulations assumed no atmospheric aerosols (dust or water ice clouds), which could alter the surface spectral flux and thus RHZ depths. - Optical constants of H2O ice in the UV–blue (shortward of ~0.6 µm) are uncertain; a blended dataset was used to mitigate this, but penetration depths remain sensitive to these values. - Validation with Greenland glacier ice is limited: impurity content and some structural properties were not measured, introducing uncertainties when comparing to model outputs. - Martian ice grain size, density, and dust content at exposed mid-latitude sites are poorly constrained; RHZ depth/thickness and melt predictions are sensitive to these parameters. - Surface dust-lag development can create opaque layers that block illumination, potentially eliminating subsurface photic habitats until re-exposure occurs. - Thermal models predicting subsurface melt are sensitive to dust content, grain size, slope, and insolation; thus, co-location of RHZs with liquid water is not guaranteed and may be episodic.