Water on Mars

Water on Mars spans a 4.5-billion-year hydrological history during which the fourth planet from the Sun evolved from a world hosting dendritic river networks, hundreds of open- and closed-basin paleolakes, and extensive groundwater systems into its present cold, hyperarid global cryosphere. Because liquid H₂O is the universal solvent for terrestrial biochemistry, reconstructing the timing, pH, salinity, and longevity of Martian aqueous environments is the central pillar of Martian astrobiology and planetary habitability assessment.

On modern Mars, pure liquid water is thermodynamically unstable across nearly the entire planetary surface. Mars's mean surface atmospheric pressure is approximately 6.1 mbar (610 Pa)—sitting right at the triple point of H₂O (P_tp = 611.65 Pa, T_tp = 273.16 K)—while the global mean surface temperature is ~210 K (−63 °C). Under these conditions, pure surface ice sublimates directly into water vapor upon heating, and transient pure liquid water simultaneously freezes and boils unless stabilized at low elevations (such as Hellas Planitia at 1,150 Pa) or in concentrated deliquescent brines of calcium and magnesium perchlorate (Ca(ClO₄)₂ and Mg(ClO₄)₂), which depress the eutectic freezing point of aqueous solutions to 198–205 K.

The Phyllosian Era (~4.1 to 3.7 Ga; Early–Middle Noachian)

In 2006, Jean-Pierre Bibring and the Mars Express OMEGA imaging spectrometer team established a three-era mineralogical chronology of Martian water—subsequently refined by the CRISM spectrometer aboard the Mars Reconnaissance Orbiter—that maps directly onto Mars's classical geological epochs. The earliest stage, the Phyllosian Era, corresponds to the Early and Middle Noachian epoch (~4.1 to 3.7 Ga). During this period, sustained neutral-to-alkaline (pH 6 to 9) chemical weathering of Mars's primary basalt crust produced widespread Fe/Mg-smectite phyllosilicate clays (including nontronite, saponite, and montmorillonite), aluminum-rich kaolinite, carbonates, and hydrothermal serpentinization assemblages.

Geomorphological evidence from the Phyllosian/Noachian transition includes branching dendritic valley networks (such as Warrego and Nanedi Valles) incised by precipitation-fed surface runoff and groundwater sapping, feeding more than 400 documented paleolake basins. Prominent examples include Jezero Crater (explored by the Perseverance rover) and Gale Crater, where the Curiosity rover's drill campaigns in the Sheepbed and Murray mudstones demonstrated that an ancient neutral-pH, low-salinity lacustrine environment persisted for millions to tens of millions of years. Geomorphic and mineralogical budgets indicate that Early Mars possessed a surface and near-surface water inventory equivalent to a Global Equivalent Layer (GEL)—the uniform depth of water if spread over the entire planet—of 100 to >500 meters.

The Theiikian and Siderikian Eras (<3.7 Ga to Present)

Around 3.7 Ga, at the Noachian–Hesperian boundary, intense volcanic construction of the Tharsis igneous province outgassed vast quantities of sulfur dioxide (SO₂) and hydrogen sulfide (H₂S) into the atmosphere. Photochemical oxidation of these sulfur gases into sulfuric acid (H₂SO₄) initiated the Theiikian Era (~3.7 to 3.0 Ga), driving a global transition to acidic (pH 2 to 4), high-ionic-strength groundwater and evaporative playas. This era deposited thick layered hydrated sulfates, including monohydrated kieserite (MgSO₄·H₂O), polyhydrated sulfates, gypsum (CaSO₄·2H₂O), and the acid-diagnostic potassium iron hydroxysulfate mineral jarosite (KFe₃(SO₄)₂(OH)₆), which was discovered alongside diagenetic concretions of grey hematite ("blueberries") by the Opportunity rover at Meridiani Planum in 2004.

During the Hesperian epoch, thickening of the global permafrost cryosphere pressurized deep aquifers until volcanic dikes or tectonic faults ruptured the cryospheric seal, unleashing catastrophic outflow channels (such as Kasei, Ares, Tiu, and Maja Valles). These megafloods discharged peak fluxes of 10⁶ to 10⁹ m³ s⁻¹ toward the northern lowlands (Vastitas Borealis), potentially pooling into a transient northern ocean (Oceanus Borealis) bounded by the putative Arabia and Deuteronilus paleoshorelines. By ~3.0 Ga, Mars entered the Siderikian Era (Amazonian epoch), characterized by anhydrous atmospheric oxidation of surface rocks into nanophase ferric oxides (hematite, α-Fe₂O₃) that impart Mars's characteristic rust-red color.

Atmospheric Escape vs. Crustal Hydration Sequestration

For decades, planetary scientists attributed the disappearance of Mars's ancient water primarily to atmospheric escape to space. After Mars's core dynamo magnetic field shut down at ~4.0 to 4.1 Ga, solar ultraviolet photolysis split atmospheric H₂O vapor into H and OH radicals, while solar wind pick-up ions and photochemical escape stripped lightweight hydrogen from the upper atmosphere—a process directly measured by NASA's MAVEN orbiter. Because protium (¹H) has half the mass of deuterium (²H or D) and a larger atmospheric scale height, lighter ¹H escapes preferentially, enriching the remaining Martian water inventory to a deuterium-to-hydrogen (D/H) ratio of 5 to 8 times Earth's Vienna Standard Mean Ocean Water (VSMOW), as measured by the Curiosity SAM spectrometer.

However, in 2021, Eva L. Scheller, Bethany L. Ehlmann, and colleagues combined isotopic D/H mass-balance modeling with MAVEN escape rates and orbital mineral mapping to demonstrate that atmospheric escape accounts for only a fraction of Mars's lost water: between 30% and 99% of Mars's initial Noachian water budget was permanently sequestered into the crust via irreversible hydration of mafic minerals into structurally bound OH⁻ and H₂O in clays, sulfates, and zeolites. Unlike Earth, where plate tectonics subducts hydrated oceanic crust into the mantle and recycles H₂O back to the atmosphere via arc volcanism, Mars is a stagnant-lid planet whose hydrated crust acts as a one-way hydrological sink.

Polar Ice Caps and the Mid-Crustal Liquid Reservoir

Today, Mars's observable surface cryosphere stores approximately 20 to 30 meters GEL of water ice within the North and South Polar Layered Deposits (Plana Boreum and Australe, where up to 3 km of H₂O ice is capped by seasonal or perennial solid CO₂ dry ice and potential clathrate hydrate zones), alongside another ~5 to 10 m GEL in mid-latitude debris-covered glaciers (lobate debris aprons) and high-latitude ground ice probed at 5 cm depth by the Phoenix lander in 2008. Even larger volumes reside in the deep subsurface: in August 2024, Vashan Wright, Michael Manga, and colleagues analyzed seismic P- and S-wave velocities and bulk rock conductivity recorded by NASA's InSight lander to reveal that Mars's fractured igneous mid-crust between 11.5 and 20 km depth is saturated with liquid water—representing a deep subsurface reservoir equivalent to a 100 to 200 meter Global Equivalent Layer that could provide a stable refuge for chemolithoautotrophic subsurface life.

How could early Mars maintain liquid surface water when the young Sun was 25–30% fainter?

During the Noachian epoch (~4.0 Ga), solar luminosity was only ~70–75% of its modern value (the Faint Young Sun paradox). Pure CO₂–H₂O atmospheres condense into reflective CO₂ clouds above ~2 bar, failing to warm Mars above 230 K. Climate models resolve this by invoking transient collision-induced absorption (CIA) warming from 1% to 5% outgassed molecular hydrogen (H₂) and methane (CH₄) generated by crustal serpentinization, volcanic SO₂ greenhouse episodes, or episodic subglacial melting beneath thick icy highlands.

What are Recurring Slope Lineae (RSL), and are they flowing water?

Recurring Slope Lineae (RSL) are narrow (0.5–5 m wide), dark streaks that lengthen down steep crater walls during warm Martian summer seasons when surface temperatures reach 250–300 K. Initially interpreted in 2015 from CRISM hydration bands as seeping perchlorate brines, subsequent high-resolution HiRISE stereophotogrammetry demonstrated that RSL terminate precisely at the 28°–35° angle of repose for dry granular sand, indicating they are primarily dry dust and sand avalanches triggered by gas-lubrication or trace deliquescent salts.

Could there be subglacial lakes beneath Mars's South Pole?

In 2018, MARSIS radar sounding aboard Mars Express detected specular basal radar reflections at 1.5 km depth beneath the South Polar Layered Deposits at Ultimi Scopuli, initially interpreted as a hypersaline perchlorate subglacial lake. However, subsequent laboratory dielectric measurements and SHARAD/MARSIS modeling showed that At ~200 K basal temperatures, conductive iron-bearing smectite clays, saline ice, or constructive radar interference across thin CO₂/H₂O layers reproduce the bright reflections without requiring liquid water.

1. Bibring, J.-P., Langevin, Y., Mustard, J. F., Poulet, F., Arvidson, R., Gendrin, A., Gondet, B., Mangold, N., Pinet, P., Forget, F., & the OMEGA Team (2006). "Global mineralogical and aqueous mars history derived from OMEGA/Mars Express data." Science, 312(5772), 400–404. https://doi.org/10.1126/science.1122659
2. Grotzinger, J. P., Sumner, D. Y., Kah, L. C., Stack, K., Gupta, S., Edgar, L., Rubin, D., Lewis, K., Schieber, J., Mangold, N., Milliken, R., Conrad, P. G., DesMarais, D., Farmer, J., Siebach, K., Calef, F., Hurowitz, J., McLennan, S. M., Ming, D., Vaniman, D., Crisp, J., Vasavada, A., Edgett, K. S., Miceli, M., & the MSL Science Team (2014). "A habitable fluvio-lacustrine environment at Yellowknife Bay, Gale Crater, Mars." Science, 343(6169), 1242777. https://doi.org/10.1126/science.1242777
3. Scheller, E. L., Ehlmann, B. L., Hu, R., Adams, D. J., & Yung, Y. L. (2021). "Long-term drying of Mars by sequestration of ocean-scale volumes of water in the crust." Science, 372(6537), 56–62. https://doi.org/10.1126/science.abc7717
4. Wright, V., Morzfeld, M., & Manga, M. (2024). "Liquid water in the Martian mid-crust." Proceedings of the National Academy of Sciences, 121(35), e2409983121. https://doi.org/10.1073/pnas.2409983121