Chemistry · Planetary Science & Astrobiology · The Search for Life

Did Mars Have Oceans? The Chemistry of Water Across the Solar System

DRAFT — review before publishing to students

Water is such an ordinary part of daily life that it's easy to miss how strange it actually is as a molecule — and how much of modern planetary science, from rovers on Mars to spacecraft headed for the moons of Jupiter, is really organized around one question: where else has this specific, unusual chemistry happened? Getting to a real answer means understanding what water actually does chemically, what counts as evidence rather than speculation, and what a very real, very serious calculation says about how rare or common the result might be.

The strangest common molecule in the universe

Water's bent shape and polarity let its molecules form hydrogen bonds with each other — a weak but consequential attraction between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of the next. That single property produces a list of behaviors almost no other small molecule shares: water has an unusually high specific heat capacity, meaning it absorbs a large amount of energy before its temperature changes, which is why oceans buffer climate and why your own body (about 60% water) resists rapid temperature swings. Water is also one of the only common substances whose solid form is less dense than its liquid form — ice floats, insulating the liquid water beneath it instead of sinking and freezing a lake from the bottom up, which would be catastrophic for aquatic life. And water's polarity makes it an exceptional solvent, capable of dissolving the huge range of ionic and polar substances that biochemistry depends on. Chemist Philip Ball's book H₂O: A Biography of Water (1999) makes the case that these aren't incidental quirks — they're close to the specific reason chemistry can build something as complicated as life at all, on a planet positioned where this particular molecule can stay liquid.

The Goldilocks zone, and the equation scientists actually use

A planet's habitable zone (sometimes called the "Goldilocks zone") is the range of orbital distances from a star where a planet could maintain liquid water on its surface — not so close that it boils away, not so far that it stays permanently frozen. Mars sits near the outer edge of our sun's habitable zone today; the evidence in the next section suggests it may once have been further inside it. But "could a planet have liquid water" is only the first question. In 1961, astronomer Frank Drake proposed a way to organize the much bigger question — how many civilizations capable of communicating across space might actually exist — into a single equation:

N = R_* × f_p × n_e × f_l × f_i × f_c × L

Each term is its own real scientific question: R_* is the rate of star formation; f_p is the fraction of stars with planets; n_e is the number of planets per star that could support life; f_l, f_i, and f_c are the fractions of those planets where life, then intelligence, then detectable technology actually appear; and L is how long such a civilization keeps producing a detectable signal. Multiplying all seven together gives N, the estimated number of communicating civilizations in the galaxy right now. The SETI Institute's own explanation of the Drake Equation is candid about what it can and can't do: several of its seven terms, especially the later ones, are still essentially unknown, so the equation was never meant to output one confident number. Its real value is as a checklist — a way of seeing exactly which unknowns astrobiology still has to chip away at, one term at a time. Two of those terms (the fraction of stars with planets, and roughly how many of those planets could support life) have gotten dramatically better since 1961, thanks to the thousands of confirmed exoplanets discovered since 1995. That progress is part of why the search for water elsewhere in our own solar system isn't a side curiosity — it's a direct input into one of the equation's least-understood terms.

Did Mars actually have oceans?

The evidence is chemical and geological, not just a guess based on the planet looking dry today. Orbiters and rovers have mapped widespread clay minerals (phyllosilicates), which only form through long-term chemical weathering of rock in the presence of liquid water. NASA's Perseverance rover specifically landed in Jezero Crater because orbital images show an unmistakable river delta fanning out where a channel once cut through the crater rim — the same shape a delta makes on Earth, preserved for billions of years. NASA's own Mars Exploration science goals page describes this as the central strategy: look for the places liquid water was stable long enough to leave a chemical fingerprint, then look for what that water environment might have preserved. Isotope chemistry adds a second, independent line of evidence: Curiosity's onboard instruments measured the ratio of deuterium (hydrogen with an extra neutron) to ordinary hydrogen in Martian water-bearing minerals at roughly three to four and a half times the ratio found in Earth's oceans, and Mars's present-day atmosphere shows a ratio around six times Earth's. Because ordinary hydrogen escapes a planet's gravity into space more easily than heavier deuterium does, a steadily rising ratio over billions of years is exactly what you'd expect if Mars lost most of its original water to space as its atmosphere thinned — current estimates suggest at least 80% of it. Interestingly, more recent research has complicated that picture in a genuinely useful way: some of Mars's missing water may not have escaped to space at all, but instead become chemically locked into hydrated minerals in the crust itself — a live, ongoing scientific debate about exactly where the water went, not just whether it was ever there.

The robots that found the evidence

Every piece of that case came from a specific mission. Viking 1 and 2 (1976) were the first successful Mars landers. Pathfinder and its small rover Sojourner (1997) proved a rover could operate on the surface at all. Spirit and Opportunity (2004) found small hematite-rich spherules — nicknamed "blueberries" — that form in liquid water. Curiosity (landed 2012, Gale Crater) drilled into ancient lakebed mudstone and found the chemical building blocks of life alongside the isotope evidence described above. Perseverance, with its helicopter Ingenuity (landed 2021, Jezero Crater), is caching drilled rock samples for an eventual return to Earth as part of the planned Mars Sample Return program — an ambitious, multi-spacecraft relay that NASA and its partners have been actively redesigning to manage cost and schedule, so treat any specific target date for it as provisional.

Water as the resource that could get humans to Mars

Water on Mars isn't only a clue about the past; it's a practical resource for the future, under a strategy called in-situ resource utilization (ISRU) — using what's already on-site rather than launching every needed material from Earth. Perseverance carried an experiment called MOXIE (Mars Oxygen In-Situ Resource Utilization Experiment), which successfully produced breathable oxygen directly from Mars's carbon-dioxide atmosphere across 16 separate tests, demonstrating a method future astronauts could use to make both breathing air and rocket propellant without hauling it from Earth. Subsurface and polar ice would fill a similar role for drinking water and, split by electrolysis, more rocket fuel. NASA's own Human to Mars planning describes this as one of several major open engineering problems — alongside radiation shielding and the sheer transit time — still being worked through, generally as a follow-on to NASA's Artemis Moon campaign rather than a near-term, dated mission. Reasonable people, including NASA and private efforts like SpaceX's Starship program, currently disagree on the timeline; none of it currently has a confirmed date, and that's worth saying plainly rather than guessing at one.

What counts as finding life somewhere else

Mars isn't the only place in the solar system with a serious water case. Jupiter's moon Europa and Saturn's moon Enceladus both show strong evidence of liquid, salty oceans hidden beneath an icy shell — Europa's may hold more than twice the liquid water of every ocean on Earth combined. NASA's Europa Clipper spacecraft, launched in 2024, is on its way to Jupiter (arrival 2030) specifically to determine whether conditions there could support life. Farther out, astronomers search for biosignatures — chemical signs, like an atmosphere with both oxygen and methane persisting together in a combination that would otherwise react away, which is difficult to explain without ongoing biological activity replenishing it. The James Webb Space Telescope can already break starlight passing through a distant exoplanet's atmosphere into its chemical components looking for exactly this kind of evidence. NASA's overview of the search for life in the universe is a good single starting point for how all of these efforts — Mars rovers, icy-moon missions, and exoplanet spectroscopy — connect back to the same handful of Drake Equation terms.

None of this is only nonfiction. Andy Weir's 2021 novel Project Hail Mary — adapted into a film starring Ryan Gosling as a science teacher who wakes up on an interstellar spacecraft, released in March 2026 — is built directly on top of real astrobiology: an alien microbe that treats starlight as fuel, a search for life defined by chemistry rather than appearance, and first contact handled as a problem to reason through rather than a spectacle. It's fiction, but the chemistry questions underneath it — what would count as evidence, and what would a truly different biochemistry even look like — are the same ones this entire reading has been building toward.

Hydrogen bond: A weak attraction between a slightly positive hydrogen atom on one molecule and a slightly negative atom on another; responsible for many of water's unusual properties.
Habitable (Goldilocks) zone: The range of distances from a star where a planet's temperature could allow liquid water to exist on its surface.
Drake Equation: A 1961 formula estimating the number of detectable civilizations in the galaxy, used mainly to organize which unknowns about life's prevalence remain to be answered.
Phyllosilicate (clay mineral): A mineral that forms only through long-term chemical weathering of rock in the presence of liquid water; its presence is strong evidence of a past wet environment.
In-situ resource utilization (ISRU): Using materials already present at a destination (such as Martian CO2 or ice) rather than transporting everything needed from Earth.
Biosignature: A chemical or physical sign (such as an atmospheric gas combination that wouldn't persist without ongoing biological activity) used as evidence for present or past life.

Check your understanding

Explain how hydrogen bonding produces water's density anomaly (ice floating on liquid water), and why that property matters for life in a lake or ocean. (SOL CH.2)
List the seven terms of the Drake Equation and explain why the equation is better described as a "checklist of unknowns" than a tool that produces one confident number. (SOL CH.1)
Explain how a rising deuterium-to-hydrogen ratio over time provides evidence that Mars lost much of its water to space, and describe one alternative explanation current research is investigating for where that water actually went. (SOL CH.2, CH.1)
Explain what in-situ resource utilization means, and describe how MOXIE demonstrated it on Mars. (SOL CH.1)
Explain what a biosignature is and why an atmosphere containing both oxygen and methane together is considered more scientifically interesting than either gas alone. (SOL CH.1, CH.2)

Sources: Ball P., H₂O: A Biography of Water, 1999; Shostak S., "The Drake Equation," SETI Institute (seti.org); NASA Science, "Mars Exploration Science Goals," "Mars 2020: Perseverance Rover," "Mars Science Laboratory: Curiosity Rover," "Mars Sample Return," "Europa Clipper," and "The Search for Life in the Universe" (science.nasa.gov, mars.nasa.gov); Rapin W. et al. and Mahaffy P.R. et al., Curiosity SAM instrument deuterium/hydrogen measurements, various dates; Wernicke L. et al. and related crustal-water-sequestration research; Weir A., Project Hail Mary, 2021, and the 2026 film adaptation. DRAFT — verify current Virginia Science SOL codes with the current Curriculum Framework before publishing.

All external links above point to NASA/JPL and SETI Institute pages current as of July 2026. If any of these move or change, flag it and I can help track down the replacement URL to keep this page current.