Recurring Slope Lineae

Recurring slope lineae (universally abbreviated RSL) are narrow, low-albedo (dark) surface markings—typically 0.5 to 5 meters wide and tens to hundreds of meters long—that emerge annually near bedrock outcrops on steep, sun-facing slopes on Mars, lengthen incrementally downslope during the warmest months of the Martian year, fade during autumn and winter, and recur in nearly the same locations over multiple consecutive Martian years. Discovered in 2011 by planetary geologist Lujendra Ojha, Alfred S. McEwen, and colleagues using 25-cm-per-pixel time-lapse imagery from the High Resolution Imaging Science Experiment (HiRISE) camera aboard NASA's Mars Reconnaissance Orbiter (MRO), recurring slope lineae have been documented at hundreds of mid-latitude and equatorial sites, particularly within Valles Marineris, southern highland craters (such as Hale, Horowitz, Newton, and Palikir craters), and Acidalia Planitia.

Within astrobiology, the discovery of recurring slope lineae—combined with orbital near-infrared detections of hydrated perchlorate and chlorate salts (Mg(ClO₄)₂, NaClO₄, Ca(ClO₄)₂) at RSL sites in 2015—triggered a major debate over whether transient, freezing-point-depressed liquid brines still percolate through the shallow Martian regolith today, or whether RSL are driven predominantly by dry granular flows stopping at the dynamic angle of repose of cohesionless sand.

Unlike slope streaks—broad (10–200 m wide), bright- or dark-toned dust avalanches in high-albedo equatorial regions that form instantaneously in a single event, take decades to fade, and show no seasonal preference—recurring slope lineae exhibit four strict morphological and environmental correlations mapped by HiRISE:

1. Steep Bedrock-Headed Slopes (27° to 40°): RSL originate almost exclusively at rocky ledges or small gullies on steep crater walls, central peaks, and canyon scarps with inclinations between 27° and 40°, terminating near the transition where the talus apron flattens below 28° to 35°.
2. Seasonal Insolation and Diurnal Orientation Shift: In the southern mid-latitudes (30° S to 50° S, across the cratered highlands south of the Martian dichotomy), RSL form exclusively on equator-facing (north-facing) slopes during late southern spring and summer (solar longitude L_s = 250°–360°), when afternoon surface brightness temperatures measured by Mars Odyssey THEMIS exceed 250 K (−23 °C) to 300 K (+27 °C). In equatorial canyons of Valles Marineris, RSL switch aspect over the course of the Martian year: active on north-facing walls when the Sun is north of the equator (L_s = 0°–180°) and shifting to south-facing walls when the Sun moves south (L_s = 180°–360°).
3. Incremental Downslope Growth: Rather than appearing all at once like a landslide, individual RSL lengthen gradually downslope at rates of 0.5 to 2.0 m per Martian sol over several weeks to months before halting and fading as surface temperatures drop below ~240 K (−33 °C), likely as windblown bright ferric dust settles back over the darkened substrate.
4. Absence of Topographic Volume Changes: Repeated HiRISE stereo digital terrain models (DTMs) show no measurable topographic trenching or depositional lobes (>10 cm vertical relief) after multiple years of RSL activity, indicating that each seasonal episode mobilizes a very thin surface veneer (millimeters to centimeters thick).

1. The Wet Hypothesis: Perchlorate Deliquescence and Cryogenic Brines

Pure liquid water (H₂O) is thermodynamically unstable across almost all of modern Mars because the mean atmospheric surface pressure (~610 Pa, near the 611.65 Pa triple point of water) and low relative humidity cause pure water to either freeze solid below 273.15 K or boil/evaporate rapidly when heated. However, orbital spectroscopy by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard MRO (Ojha et al., 2015) detected diagnostic 1.4, 1.9, and 2.15 μm H₂O/OH vibrational absorption bands of hydrated magnesium perchlorate (Mg(ClO₄)₂·6H₂O), sodium perchlorate (NaClO₄·xH₂O), calcium perchlorate (Ca(ClO₄)₂), and chlorates (ClO₃⁻) spatially correlated with active RSL slopes in Palikir, Horowitz, Hale, and Coprates Chasma.

Perchlorate and chloride salts (CaCl₂) possess two remarkable physicochemical properties:

• Extreme Eutectic Freezing-Point Depression: Whereas a saturated NaCl brine freezes at 252 K (−21 °C), eutectic aqueous solutions of Mg(ClO₄)₂ remain liquid down to 205 K (−68 °C), and Ca(ClO₄)₂ brines remain liquid down to 198 K (−75 °C) (with metastable supercooled brines persisting even lower).
• Atmospheric Deliquescence: Hygroscopic perchlorate and CaCl₂ salts spontaneously absorb water vapor directly from the Martian atmosphere when relative humidity exceeds their deliquescence relative humidity threshold, forming microscopic liquid brine films around regolith grains that darken the surface albedo by optical wetting (reducing index-of-refraction scattering at grain boundaries).

However, the purely wet seepage model faces severe mass-balance and hydrological challenges: the Martian atmosphere contains only ~10–20 precipitable micrometers of water vapor, completely insufficient to supply continuous downslope percolation of macroscopic liquid streams via deliquescence alone, while deep fresh-water aquifers cannot easily discharge from isolated crater rims and central peaks hundreds of meters above the regional water table.

2. The Dry Hypothesis: Granular Sandflows and Knudson-Pump Gas Lift

In 2017, Colin M. Dundas, Alfred S. McEwen, and colleagues analyzed the exact geometric stopping angles of 151 RSL across 10 sites using HiRISE digital elevation models and found that almost every RSL terminates when the local slope angle drops to 28° to 35°—precisely the dynamic angle of repose of dry, cohesionless sand and silt (whereas liquid water or fluidized mud slurries continue flowing down slopes as gentle as 5° to 15°). Under the dry granular flow hypothesis:

• Dark basaltic sand and silt grains sliding down steep slopes sort by grain size or cast away bright surface dust coatings (composed of nanophase ferric oxide Fe₂O₃), exposing darker coarse grains beneath; during autumn and winter, regional dust storms and saltation redeposit a millimeter-thin bright dust coat that erases the streak until the following spring.
• To explain why dry granular flows occur only during warm seasons (T > 250 K), laboratory experiments under 600 Pa Martian atmospheric pressure demonstrate two thermal triggers: (a) Thermal Creep / Knudsen Pump Gas Lift, where strong solar heating of the top millimeters of porous regolith creates a steep thermal gradient across pore spaces weaker than the gas mean free path, pumping CO₂ gas downward to pressurize and levitate surface sand grains until they avalanche; and (b) Hygroscopic Salt Hydration/Dehydration Cycling, where trace deliquescent perchlorate salts swell and shrink as they absorb and release micrometers of water vapor, breaking inter-grain cohesion and triggering dry sand cascades down the slope.

From the perspective of terrestrial extremophiles and planetary habitability (as opposed to shielded deep subsurface environments such as planetary lava tubes), even if transient perchlorate brines are present in the top millimeter of RSL to trigger granular flows, their thermodynamic water activity (a_w = P_vap,brine / P_{vap,pure H2O}) and chaotropicity pose formidable biochemical barriers:

• Water Activity Limit of Terrestrial Life: All known terrestrial halophilic archaea, bacteria, and xerophilic fungi require a water activity of a_w ≥ 0.585 (and typically a_w > 0.61) alongside temperatures above −20 to −25 °C (248–253 K) for cellular division and metabolic replication.
• Eutectic Perchlorate Brine Conditions: At the temperatures (< 240 K) and relative humidities where Ca(ClO₄)₂ and Mg(ClO₄)₂ deliquesce on Mars, the resulting brines have water activities of a_w ≈ 0.20 to 0.50 and high chaotropic ion strengths that denature hydrogen-bonded protein structures unless diluted during warmer transient disequilibrium pulses.

Nevertheless, out of caution against forward terrestrial microbial contamination, the Committee on Space Research (COSPAR) and NASA Office of Planetary Protection classify recurring slope lineae as Uncertain Regions Treated as Special Regions—mandating Category IVc spacecraft bioburden sterilization (equivalent to Viking-level dry-heat terminal bioburden reduction) before any landed rover or helicopter may make physical contact with an active RSL slope.

1. McEwen, A. S., Ojha, L., Dundas, C. M., Mattson, S. S., Byrne, S., Wray, J. J., Cull, S. C., Murchie, S. L., Thomas, N., & Gulick, V. C. (2011). "Seasonal Flows on Warm Martian Slopes." Science, 333(6043), 740–743. https://doi.org/10.1126/science.1204816
2. Ojha, L., Wilhelm, M. B., Murchie, S. L., McEwen, A. S., Wray, J. J., Hanley, J., Massé, M., & Chojnacki, M. (2015). "Spectral evidence for hydrated salts in recurring slope lineae on Mars." Nature Geoscience, 8(11), 829–832. https://doi.org/10.1038/ngeo2546
3. Dundas, C. M., McEwen, A. S., Chojnacki, M., Milazzo, M. P., Byrne, S., McElwaine, J. N., & Urso, A. (2017). "Granular flows at recurring slope lineae on Mars indicate a limited role for liquid water." Nature Geoscience, 10(12), 903–907. https://doi.org/10.1038/s41561-017-0012-5
4. Rummel, J. D., et al. (2014). "A New Analysis of Mars 'Special Regions': Findings of the Second MEPAG Special Regions Science Analysis Group (SR-SAG2)." Astrobiology, 14(11), 887–968. https://doi.org/10.1089/ast.2014.1227