Where did Mars’ water go? Scientists uncover a missing link

Scientists know that rivers once carved channels across Mars, but exactly how the planet’s water cycled between the surface and subsurface has remained a mystery.

A new study led by graduate researchers at The University of Texas at Austin has recently filled a crucial gap in early Mars history. It reveals a link between rain-fed lakes and a deep groundwater reservoir thought to lie roughly a mile below the ancient surface.

Timing the movement of groundwater

Using a custom computer model, Mohammad Afzal Shadab and Eric Hiatt calculated infiltration rates for early Martian soils. They discovered that water percolated from the surface to the deep aquifer in anywhere from 50 to 200 years.

By terrestrial standards, that is glacially slow – the same journey on Earth usually takes only a few days because the water table sits far nearer the surface.

The results represent the first quantitative estimate of groundwater travel time for the red planet’s wetter period, roughly three to four billion years ago.

Missing surface water on Mars

The researchers then integrated infiltration rates over likely catchment areas. The model suggests that lost water could have covered Mars in at least 90 meters (300 feet) of global water depth.

Because Mars is believed to have started with an ocean perhaps a few hundred meters deep, the loss to groundwater would represent a large slice of its total early inventory.

“The way I think about early Mars is that any surface water you had – any oceans or large standing lakes – were very ephemeral,” Hiatt said. “Once water got into the ground on Mars, it was as good as gone. That water was never coming back out.”

No loop, no return

On Earth, evaporation sends surface water skyward; condensation drops it back as rain, replenishing rivers and lakes in a self-sustaining feedback.

Shadab and Hiatt’s findings reinforce an emerging alternative for Mars: a largely one-way route. Surface water drained, percolated and became locked in crustal pores or mineral lattices, while the thin atmosphere bled much of the remaining vapor to space.

“We want to implement this into [an integrated model] of how the water and land evolved together over millions of years to the present state,” Shadab said. “That will bring us very close to answering what happened to the water on Mars.”

Evaluating water stability

Knowing how long liquid persisted on the surface is central to evaluating past habitability. Many prebiotic reactions and microbial metabolisms need stable water for thousands of years – much longer than the model’s century-scale window.

Although Mars ultimately lost its atmosphere and surface seas, the groundwater modeled by Shadab and Hiatt would not have vented to space. Instead, it froze in place or reacted with subsurface rocks to form hydrated minerals.

For future human explorers, that deep reserve could be both a scientific archive and a practical asset.

Modeling the path of water on Mars

The researchers treated early Martian soils as a porous medium overlaying basaltic bedrock. They incorporated temperature profiles, gravitational acceleration, and likely permeability values derived from Martian meteorites and rover data.

By running probability algorithms across varying parameters, they identified travel times and cumulative volumes. The wide window – 50 to 200 years – reflects uncertainties in porosity, precipitation intensity, and surface temperature.

Mars’ lower gravity slows infiltration because pore-water pressure increases more gradually with depth. At the same time, cold surface temperatures reduce evaporation, allowing more runoff to soak in.

Together, these factors stretch journey times by two orders of magnitude compared with similar processes on Earth.

Finding Mars’ missing water

The work dovetails with orbital spectrometer findings that much of Mars’ crust is rich in hydrated minerals, and with radar detections of buried ice in mid-latitude deposits.

It also complements atmospheric escape measurements showing that more than half of Mars’ original water likely left the planet entirely.

By quantifying the movement of water from lakes to ground, the study helps close the mass balance. “Wet early Mars” models can now apportion initial inventories among oceans, glaciers, atmosphere, crustal water, and space loss with greater precision.

Mars’ water slipped underground

Shadab, now a postdoctoral researcher at Princeton University, plans to couple the infiltration model with global climate simulations that include rainfall, runoff, and volcanic outgassing.

This integrated framework could test scenarios from a long-lived northern ocean to short-term floods caused by impacts or volcanic activity.

Future missions may drill to one kilometer to sample the ancient aquifer and investigate Mars’ missing water. Isotopic signatures there could reveal how much water has remained sequestered versus how much reacted chemically with the crust.

For now, the new analysis paints Mars as a world where surface water was short-lived. The Red Planet’s hydrologic engine lacked the robust recycling pump that powers Earth’s blue marble. Once the water slipped underground, as Hiatt put it, “it was as good as gone.”

The study is published in the journal Geophysical Research Letters.

Image Credit: Ittiz/Wikimedia Commons

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