Three Siblings, Three Fates: Earth, Mars, and Venus

To exist, life needs liquid water, a form of energy and CHNOPS. Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus and Sulphur. If we want to find life elsewhere, these are first three things we are looking for. If they do not exist, life as we know it cannot form. This is what NASA’s Curiosity rover looked for on Mars. When it landed in Gale Crater in 2012, one of its key science goals was to “assess past habitability”. It looked for

• Carbon (C): in the form of organics, carbonates, or CO₂ locked in minerals.
• Hydrogen (H): usually as water or hydrated minerals.
• Nitrogen (N): in nitrates, a bio-available form.
• Oxygen (O): bound into oxides, sulphates, or water molecules.
• Phosphorus (P): a key ingredient in phosphates, necessary for DNA/RNA/ATP.
• Sulphur (S): in sulphates and sulphides, important for microbial metabolism.

Curiosity confirmed that Gale Crater once had water-rich environments with clays and sulphates, and it detected organic carbon in Martian rocks. Nitrogen in the form of nitrates was also found. This means Mars once had all six CHNOPS elements available at least intermittently — so the raw ingredients for life were there. But they were not on early Earth!

CHNOPS from space

A new study from the University of Bern reveals that our early Earth began as a dry, volatile‑poor rocky world: essential life-building ingredients like water, carbon, and sulphur were virtually absent during its formation in the inner Solar System, which occurred within the first three million years after the Solar System formed (~4.568 billion years ago). By analysing isotope and elemental data, using the decay of radioactive manganese‑53 as a precise chronometer, the researchers found Earth’s chemical composition was effectively locked in early and unsuitable for life on its own. They propose that only a later colossal impact, likely with the water‑rich planetary embryo Theia, delivered the volatile ingredients needed to make Earth habitable, marking life’s possible beginning as a cosmic accident rather than a natural progression. This is incredible once again, because so many things had to happen for life as we know it today to exist. Mass extinctions triggered by supernovae, cold weather and even an asteroid impact that wiped the dinosaurs, all had to happen for us to exist! And now this discovery, that if it was not for Theia, we might not be here at all, not us, not any life on Earth!

Manganese-53 Chronometry – How we know

The new study relies on the decay of the short-lived radioactive isotope manganese-53 (⁵³Mn), which has a half-life of 3.7 million years. By looking at the ratio of its daughter isotope chromium-53 in ancient meteorites and comparing it with Earth’s mantle composition, scientists can pinpoint when Earth accreted and locked in its volatile (or rather, volatile-poor) inventory. The results show that Earth was essentially formed dry within ~3 million years of the Solar System’s birth. That’s extremely early on a planetary scale.

This evidence rules out the idea that water, carbon, and sulphur were “sweated out” gradually from Earth’s interior. Instead, it implies that volatiles had to arrive later, by a massive collision with a water-rich body like Theia, which also gave us the Moon.

Early Earth ≈ Venus?

Venus is often called Earth’s twin albeit evil. This new data shows that might not be too far from the truth. Early Earth, stripped of water and carbon, would have resembled a dry, rocky Venus-like planet. Both worlds likely formed inside the Solar System’s “snow line,” where it was too hot for water ice to survive during accretion.

• Venus probably followed the same script as early Earth: rapid formation, no initial water.
• The key difference: Earth got “lucky” with the Theia collision, importing volatile-rich material.
• If Venus never experienced a similar water-delivering impact, it may have remained dry from the start.

This puts a twist on the long-standing idea that Venus once had oceans. Some models (and hints from deuterium/hydrogen ratios in its atmosphere) suggest it could have had water that was later lost to the runaway greenhouse effect. But this new chronometry suggests maybe Venus never had much water at all — or at least, not enough to form oceans.

Implications for Habitable Worlds

• Earth’s habitability = cosmic accident. Without that late colossal impact, Earth might have ended up as barren as Venus.
• Exoplanets: It’s not enough for a rocky planet to be in the habitable zone; it also needs the right delivery of volatiles at the right time.
• Mars: Smaller and farther from the Sun, Mars may have captured volatiles differently, which explains why it once had water despite losing most of it now.

Where Mars’s CHNOPS came from?

Inherited from the Solar Nebula: Mars formed farther from the Sun than Earth and Venus — closer to or just beyond the “snow line”, where ices and volatiles could survive. This means Mars’s building blocks (planetesimals) were richer in volatiles (H₂O, CO₂, NH₃, organics) than Earth’s. So from the start, Mars likely accreted with a higher inventory of CHNOPS.

Delivered by Impacts: Just like Earth, Mars was bombarded by asteroids and comets in the Late Heavy Bombardment (~4.1–3.8 Ga). Many of those impactors came from the outer Solar System (carbonaceous chondrites, comets), which are rich in water and organics — bringing in extra C, H, N, P, S.

Geological Cycling: Mars’s early volcanism released sulphur, CO₂, H₂O, and nitrogen from the mantle into the atmosphere. Curiosity’s discovery of nitrates and sulphur compounds suggests Mars had an active geochemical cycle early on, making these elements accessible for potential life.

Why Mars kept its CHNOPS (At least for a while)?

Mars is small, so it cooled quickly thus less geological recycling than Earth, but also less volatile loss during its formation. Being farther out meant less solar radiation so early Mars could hold onto water and CO₂ longer. But eventually, with its weak gravity and loss of magnetic field, the atmosphere was stripped by the solar wind, and most volatiles were lost to space.

The three siblings

• Earth: Formed dry inside the snow line, needed a giant impact (Theia) + later bombardment to gain CHNOPS. Has life.
• Mars: Formed farther out nearer the snow line, naturally incorporated more volatiles from the start + got extra delivery via comets/asteroids in the Late Heavy Bombardment. Mars had water, atmosphere, and all CHNOPS ingredients during its first one billion years. Could have had life?
• Venus: Formed inside the show line, hot and volatile-poor. Never stood a chance?

Ancient Mars had lakes, rivers, and all six CHNOPS — a habitable window early in its history. While Earth had to “import” the CHNOPS later, Mars was “made” with them.

Takeaway for astrobiology

Life on Earth might only exist because of a lucky planetary billiards shot 4.5 billion years ago. Earth’s habitability might be a cosmic accident with Theia colliding into early Earth. Mars’s habitability was a time bomb, it only lasted one billion years. And Venus? Still the evil twin.

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*In planetary science, “volatiles” means basic life-ingredients like water, carbon, nitrogen, and sulfur — not the complex organics (like PAHs) we sometimes call “organic volatiles” in astrobiology.

Earth, Mars, and Venus are three siblings with very different destinies. Earth got lucky with a giant impact that delivered CHNOPS, Mars was born with them but lost its habitability, and Venus never stood a chance. Explore these three planetary stories side by side in teacher-friendly cards.

Update – Was Earth Born With Water?

A recent modelling study suggests the Solar System didn’t have a sharp “snow line.” Instead, water molecules could stick to dust grains inside Earth’s orbit, pointing to a more gradual distribution of water sources — not just delivery from icy bodies beyond the snow line.

On top of that, a new meteoritic analysis (using Icarus data) shows that Earth’s building materials—specifically enstatite chondrites—already included abundant hydrogen in their mineral matrix. This implies that Earth may have built-in potential for water formation from its own accreted matter, with later impacts serving as enrichment, not the sole source.