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Mars’s polar ice preserves a record of how the planet’s climate shifted water between the poles and lower latitudes. Its layers offer evidence of ice ages and accumulation, but they are not a complete ledger of Martian water: some water is stored elsewhere, some escaped to space, and estimates of the ancient total depend on models and assumptions. Bright radar reflections beneath the south pole are not proof of a liquid lake.
What can Mars’s polar ice tell us about the planet’s water history?
The north and south polar layered deposits are stacks of ice and dust. Orbital radar from Mars Express’s MARSIS and Mars Reconnaissance Orbiter’s SHARAD reveals internal structure and deposit thickness, giving scientists evidence to reconstruct how ice accumulated and was redistributed over time. The south polar layered deposits at Planum Australe form a dome reported to reach about 3.7 km thick in its interior. The two poles differ in composition: a 2025 review reports less than 5% dust in the north polar deposits, compared with 5–12% and potentially higher estimates in the south. Those differences matter when interpreting both climate records and radar signals (Space Science Reviews, 2025).
The layers are not a simple calendar. A 2024 study found stratigraphic similarity and periodicities of about 40–50 m in radar reflectors from Korolev crater and the north polar layered deposits. The authors argue that the deposits may share a climate forcing mechanism, but say more work is needed to identify it and to connect small visible layers with radar reflectors. It would therefore be misleading to treat every layer as a precisely dated orbit cycle (Geophysical Research Letters, 2024).
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Changes in Mars’s axial tilt and orbit can produce major climate shifts over hundreds of thousands of years. When the poles become warmer relative to lower latitudes, polar ice can sublimate and migrate toward the equator, where it may accumulate as ground ice or glaciers. As conditions shift again, ice can build up at the poles while lower-latitude ice is depleted. This Mars-specific pattern is not simply an Earth-style glaciation moving ice from mountains or polar regions outward.
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NASA’s Mars Reconnaissance Orbiter overview describes present mid-latitude ice as remnants of the recent cycle. The polar deposits thus preserve part of a repeating exchange between high and middle latitudes, rather than a one-way story of water steadily disappearing (NASA Science, MRO Science Highlights).
A radar record of a recent glacial period
NASA/JPL reported radar evidence that a recent Martian glacial period ended about 400,000 years ago. The post-glacial upper unit reaches a maximum thickness of 320 m, equivalent to a 60 cm layer of ice spread across the globe. The unit’s form is consistent with model predictions of polar-deposit thickening after that glacial period; the thickness is a deposit measurement and its global-layer equivalent is a way to express the amount of ice, not a measured planet-wide sheet (NASA/JPL, 2016).
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Are there lakes under Mars’s south pole?
Not confirmed. Some bright basal reflections detected by MARSIS beneath Ultimi Scopuli were interpreted as possible liquid water, but the radar brightness alone does not establish that a lake exists. A wider survey found dozens of similar reflections, including in locations thought to be too cold for liquid water. MARSIS co-principal investigator Jeffrey Plaut said, “We’re not certain whether these signals are liquid water or not, but they appear to be much more widespread than what the original paper found” (NASA, 2021).
The interpretation remains disputed. A 2025 review notes that SHARAD has not detected bright basal reflections at the same location, that estimated temperatures are generally below the eutectic point of likely perchlorate brines, and that the presence or absence of CO₂ ice affects estimates of dust and thermal conditions. The review describes the question as unresolved; it does not establish that the reflections are liquid water or identify a confirmed subglacial lake (Space Science Reviews, 2025).
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How much water did Mars have in the past?
There is no single established total. Different approaches address different parts of the problem: isotope measurements help estimate atmospheric loss, while climate simulations test how much water a specified ancient ocean scenario would require. Neither method is a direct measurement of Mars’s complete ancient water inventory.
| Evidence | What it estimates or tests | How to interpret it |
|---|---|---|
| Isotope-based reconstruction reported by NASA in 2015 | At least 20 million km³ of ancient water; about 6.5 times the volume in today’s polar caps | A minimum estimate inferred from deuterium-to-hydrogen ratios in modern Martian water and an ancient meteorite, used to assess atmospheric water loss. The northern plains were proposed as a possible ocean location, not established as a settled ocean map (NASA, 2015). |
| Climate, ocean and ice-sheet simulation published in 2025 | About 700 m global-equivalent water for a stable-ocean scenario at 3 billion years ago, split roughly half between ocean and ice sheet | A model requirement under the authors’ scenario, not a measurement of the ancient water stock. The authors summarize the result as the water needed to maintain that ocean-and-ice-sheet configuration (NASA Goddard Institute for Space Studies, Schmidt et al., 2025). |
These figures should not be added together or treated as competing measurements of the same quantity. The isotope estimate concerns water loss inferred from atmospheric enrichment; the 700 m figure is a conditional model requirement for a particular climate scenario. A 2025 review also finds that estimates of present-day water vary widely with the models used, and identifies the ground-ice inventory as important but in need of direct mapping (Space Science Reviews, 2025).
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Where did Mars’s ancient water go?
The evidence points to more than one destination. Some water froze into polar deposits; climate shifts also moved ice to middle and lower latitudes, where remnants may remain underground or in glaciers. Another portion was lost to space through atmospheric escape. Isotope-based work helps constrain that loss, but it does not by itself inventory every surviving reservoir. The polar caps are therefore important archives and reservoirs, not a complete account of all water Mars once held or still holds.
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