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Mars today is cold, dry and wrapped in an atmosphere too thin to keep liquid water stable on the surface for long. Yet the planet is covered with evidence that water once behaved very differently: branching valley networks, deltas, lake deposits and minerals that form in water. NASA’s Curiosity rover has documented long-lived rivers and lakes in Gale Crater, while Perseverance continues to investigate the complex hydrological history of ancient Jezero Crater.^[1][2]

A Japanese-led team has now asked a more chemically specific question: if early Mars had an active water cycle, where would rain have delivered molecules useful for prebiotic chemistry? Researchers led by Shungo Koyama, now a researcher at the Institute of Science Tokyo and a visiting researcher at Tohoku University, together with Arihiro Kamada of the Earth-Life Science Institute and Tohoku University Professor Naoki Terada, combined photochemical and global climate models for a warm period roughly 3.8 to 3.6 billion years ago. Their target was formaldehyde, H₂CO—a simple carbon-bearing molecule that can feed reactions producing sugars and other biologically relevant compounds. The study appeared in The Planetary Science Journal on September 30.^[3]

First, what the study did not discover

The work should not be described as a new direct detection of ancient Martian rainfall. The researchers modeled a warm, wet early-Mars climate consistent with existing geological and mineral evidence and then calculated how water vapor, ultraviolet radiation, atmospheric conditions and precipitation would affect formaldehyde production and delivery.

Rain is therefore part of the simulated hydrological cycle, not a newly observed fossil raindrop. The geological case for past surface water comes from other evidence—valley networks, deltas, lake sediments and water-altered minerals. The novelty here is connecting that hydrological cycle to the geographic delivery of a prebiotic feedstock.^[3]

Why formaldehyde matters

On Earth, formaldehyde is commonly associated with an irritating industrial chemical. In origins-of-life chemistry, however, it is also an unusually simple carbon compound that can participate in pathways toward much more complex molecules.

The formose reaction can convert formaldehyde into mixtures of sugars. One of those sugars, ribose, is a key component of RNA. Formaldehyde-linked chemistry can also feed wider networks that produce amino acids and other organic compounds relevant to life.^[4]

That does not mean formaldehyde naturally turns into life. Prebiotic chemistry also requires concentration, energy, catalysts, suitable pH, phosphorus, nitrogen and environments where reactions can persist rather than simply dilute away. But knowing where carbon-bearing feedstocks accumulate is an important part of identifying plausible chemical settings.

Water vapor helps make it; rain brings it down

Water vapor emerged as the key control in the model. Ultraviolet light can break water molecules apart, creating reactive hydrogen that helps drive atmospheric chemistry toward formaldehyde. Regions with more water vapor therefore favor H₂CO production. Precipitation then provides a mechanism for moving it out of the atmosphere and onto the surface.

This creates a double advantage for wetter regions: water supports both formation and deposition. The team predicted that the northern hemisphere could receive several times more formaldehyde than the south. Around highlands associated with Tharsis and Elysium, moist air rising over topography produced stronger rainfall in the model, creating local H₂CO deposition zones roughly ten times the global average.^[3]

The “ten times” figure refers to modeled annual delivery billions of years ago. It does not mean ten times more formaldehyde survives at those locations today.

The most interesting places may be downhill

Heavy deposition on a mountain is not automatically the best setting for chemistry. The researchers point instead toward nearby basins where water flowing off highlands could collect material delivered by the atmosphere.

Rivers could concentrate organic feedstocks in lakes or closed basins. Subsequent evaporation could raise concentrations further. That matters because dilution is one of the recurring difficulties in origins-of-life chemistry: making molecules is not enough if they never become concentrated enough to react efficiently.^[3]

A research program that began with “could Mars make it?”

The new paper is the third step in a line of work by Koyama, Terada and collaborators. In February 2024, the group used a photochemical model to show that formaldehyde could have been produced continuously in the atmosphere of warm Mars around 3.8–3.6 billion years ago. They proposed that delivery into oceans or lakes could have fed formose chemistry capable of producing sugars such as ribose.^[4]

Later that year, the group tackled a separate clue from Martian rocks: unusually low carbon-13 values in some organic material measured by Curiosity. Their atmospheric-evolution calculations showed that photochemical production of formaldehyde after ultraviolet breakdown of CO₂ could generate distinctive carbon isotope signatures consistent with some of those observations.^[5]

The progression is revealing. The first question was whether formaldehyde could be made. The second was whether its chemistry could help explain isotopic signatures in Martian organic matter. The 2026 study asks the geographical question: where would that formaldehyde have gone?

The warm-and-wet Mars problem is still unresolved

There is a famous difficulty at the heart of early-Mars climate science. The young Sun was fainter than it is today, yet ancient Mars preserves abundant geological evidence for running and standing liquid water. Climate models have long struggled to keep the surface warm enough for widespread rivers and lakes under the expected solar energy and atmospheric conditions.^[6]

A thick carbon-dioxide atmosphere alone may not provide sufficient warming in many models. Researchers have therefore explored additional hydrogen greenhouse effects, volcanic episodes, impact-driven warming, clouds, orbital changes and climates in which wet periods were intermittent rather than permanent.

For that reason, it is safer to speak of warm and wet periods than to imagine early Mars as a continuously Earth-like world. A cold baseline climate interrupted by episodes of snowmelt, rainfall or stronger greenhouse warming remains part of the scientific debate.

What is not controversial: water reshaped Mars

The mechanism that produced the warm periods may remain uncertain, but the geological evidence for ancient liquid water is extensive. Mariner 9 and Viking revealed huge channels and branching valleys in the 1970s. Mars Global Surveyor later returned much sharper images of valley networks carved into ancient terrains.^[7]

Curiosity found rounded pebbles that had been transported by flowing water and hundreds of meters of sediment deposited in long-lived lake systems at Gale Crater. NASA scientists have concluded that rivers and lakes persisted there for perhaps a million years or longer.^[2]

At Jezero Crater, Perseverance is exploring a preserved delta and ancient shoreline. In September 2026, NASA reported that rocks in the crater’s Margin Unit record a more complicated water system than expected, including clay, carbonate and multiple episodes of water-rock interaction.^[8]

A second October 1 study asked when the ocean retreated

On the same day Tohoku University announced the formaldehyde work, another Japan-linked team published a study in npj Space Exploration using laboratory tank experiments and Martian delta shapes to constrain the planet’s hydrological history. The authors argued that one class of deltas near proposed paleo-shorelines is consistent with falling water levels between roughly 3.8 and 3.2 billion years ago, potentially recording regression of a large Late Noachian–Hesperian ocean.^[9]

The study does not settle the long-running ocean debate, but it illustrates how the field is changing. The question is no longer merely whether Mars once had water. Researchers increasingly ask where it was, when it moved, how long it persisted and what chemistry it transported.

Habitability is not evidence of life

Water, carbon chemistry and energy sources can make an environment habitable. Habitability is not the same as inhabited.

Organic molecules have been detected on Mars, but organic chemistry does not require biology. Meteorites, atmospheric reactions and water-rock chemistry can all generate carbon compounds. Carbon isotope anomalies can also arise abiotically. Indeed, the Koyama group’s 2024 work is itself an example of how photochemistry may mimic signatures that might otherwise look biologically suggestive.^[5]

That makes studies of abiotic chemistry essential. To recognize a genuine biosignature, scientists first have to understand what a lifeless planet can produce on its own.

How the map could guide future missions

No one expects ancient atmospheric formaldehyde itself to have remained exposed on the Martian surface for nearly four billion years. Radiation, oxidation, erosion and chemical alteration are severe. Instead, the model can be tested indirectly.

Researchers can compare predicted high-deposition regions with mineralogy, preserved organic compounds and isotope measurements from rovers or future returned samples. If basins downstream of modeled high-H₂CO regions preserve chemical patterns expected from prebiotic processing, that would strengthen the hypothesis. If they do not, assumptions about atmospheric chemistry, rainfall or climate can be revised.

In that sense, the map is less a claim than a targeting tool. It identifies locations where a future mission might have a better chance of testing whether atmospheric carbon chemistry and surface water interacted in ways favorable to complex organic synthesis.

Rain as a planetary chemical conveyor

On Earth, rain does more than supply water. It scavenges molecules from the atmosphere, delivers them to soils and rivers, and transports them into lakes and oceans. If early Mars possessed a sufficiently active hydrological cycle, rain could have played the same role on another planet.

The new model links processes often studied separately: atmospheric photochemistry, water vapor, topographic rainfall, river transport, basin accumulation and evaporative concentration. Taken together, they form a planetary-scale chemical delivery system.

We still do not know whether life ever emerged on Mars. But if rain fell, rivers ran and lakes persisted roughly 3.7 billion years ago, that water may have done more than carve the landscape. It may also have carried some of the simple carbon chemistry from which more complex molecules could begin.