Ancient Mars May Have Rained A Sugar Ingredient

A New Map Pinpoints Where Formaldehyde Rain May Have Fallen Hardest on Ancient Mars

Billions of years ago, rain may have fallen on Mars carrying the raw material for sugar. A new computer simulation maps where that rain came down hardest. The landing sites of NASA’s Curiosity and Perseverance rovers, which have both found organic matter in the ground, were not among the top spots.

Formaldehyde is that raw material, a toxic chemical that can set off a chain reaction in water, building sugars, amino acids, and other organic compounds. Geological evidence shows early Mars had liquid water, and researchers think short warm, wet spells interrupted a mostly cold climate roughly 3.6 to 3.8 billion years ago. A thick carbon dioxide atmosphere spiked with hydrogen could have let sunlight cook up formaldehyde during those spells. Earlier models looked at a single column of air, so none could say where on the planet that chemistry mattered most.

A team led by Shungo Koyama of the Institute of Science Tokyo built the missing map by linking a chemistry model to a climate simulation of ancient Mars. Their results, published in The Planetary Science Journal, show deposits varying enormously from place to place.

Humidity Controlled Formaldehyde Production on Ancient Mars

Two computer models did the work. The first tracked how formaldehyde forms in an atmosphere of mostly carbon dioxide with some hydrogen, across hundreds of combinations of temperature, humidity, air pressure, and ultraviolet light from the young sun.

Humidity won by a landslide. Sunlight breaks apart water vapor, and the leftover fragments fuel the reactions that build formaldehyde, so damper air meant more of it once moisture climbed past a low threshold. Stronger ultraviolet light helped too, while temperature and air pressure mattered far less. Formaldehyde lasts only about 10 hours in the air, so the model treats it as falling close to where it forms.

Next came a climate simulation with mountains, clouds, rain, and snow to supply the weather. It ran a Mars with an atmosphere twice as thick as Earth’s, a steeper tilt than today’s (about 40 degrees), and a hydrogen-boosted greenhouse effect. The team then tracked how much formaldehyde rain and snow washed down to the ground.

That virtual Mars turned out warm and wet, averaging about 55 degrees Fahrenheit. An ocean covered the northern hemisphere, and the giant Hellas basin in the south held a lake kept warm by its low elevation and the heat-holding power of water. The ocean came from the model’s starting assumptions, so it is not a confirmed feature of the real planet.

Mountain Slopes Received Roughly Ten Times the Average Formaldehyde on Ancient Mars

Deposition varied about a hundredfold across the globe. Humid air over the northern ocean and the Hellas lake held the most formaldehyde, and rain carried it down. Mountains did even better. Around Elysium Mons and the Tharsis region, rising terrain forced moist air upward, where it cooled and dumped its rain, delivering roughly ten times the global average.

Overall, the north collected several times more formaldehyde than the south. High southern terrain stayed frozen and received mostly snow, which delivered almost none, since formaldehyde dissolves far less readily in ice than in liquid water.

Curiosity and Perseverance Sites Fall Below Mars’ Average Formaldehyde Supply

Curiosity’s Gale Crater and Perseverance’s Jezero Crater received just over a third and a little over half of the global average, respectively. That does not clash with their organic finds. The authors say a supply at or below average would already have been enough to contribute. Rivers and groundwater could also have carried formaldehyde from the heavily dosed mountains south of Gale into the crater.

Amounts of organic matter found by the two rovers cannot be compared directly, because the sites differ in rock type, water history, and exposure to oxidants, chemicals that break down organic material. China’s Zhurong rover landed in southern Utopia Planitia, one of the highest-ranking rover sites, but it carries no instrument able to identify organic molecules. NASA’s Viking 2 and Phoenix landers touched down in spots that received more formaldehyde than Gale or Jezero. They sampled only modern surface soil, though, where salts called perchlorates, which disrupt heating-based tests for organic molecules, likely made any organics hard to detect.

None of this shows life ever existed on Mars. The simulation tracks supply, not survival. Sunlight destroys exposed formaldehyde quickly, so it would have needed shelter to last, such as being frozen inside ice, which preserves it in Earth’s polar ice cores.

Future missions, the authors say, should target basins beside persistently humid regions, where pooled water would have evaporated and concentrated the incoming chemistry. No rover has sampled the southern regions the model predicts got the least formaldehyde, and comparing organic matter from high-delivery and low-delivery areas would help test whether the atmosphere supplied it.

Mars went cold and dry long ago, but this model sketches a planet where the most formaldehyde fell on mountain slopes and around open water, though not at either rover’s landing site. Whether that rain ever led to life is still unknown, yet it hands future missions a shortlist of places to look.

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