Comets probably didn’t seed life on Earth, and Mars was a better target

Comets carry one of the raw ingredients that origin-of-life chemists prize most, hydrogen cyanide, and they carry plenty of it. New simulations of the young Earth’s bombardment find that comets almost never landed it on the surface in a usable state. In more than half of the simulated histories, not a single comet did.

That cuts against an idea more than a century old: that a rain of icy visitors stocked the early planet with the building blocks of life. If the result holds, the hunt for life’s chemical starter kit turns back to Earth itself, to asteroids, or to Mars, which the same models rank as a far easier target.

The study was led by Richard Anslow of the University of Cambridge’s Institute of Astronomy, with colleagues at Cambridge and the University of Edinburgh, and appears in Scientific Reports. “Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules,” Anslow said. “It’s kind of a romantic idea, but our results suggest it’s highly unlikely.”

Hydrogen cyanide matters because its triple bond between carbon and nitrogen stores energy that can drive later reactions. Lab experiments using it at about 10 millimolar, roughly a quarter of a gram per liter of water, have produced lipids, sugars and the building blocks of RNA and DNA. Comets hold the molecule at about 0.1 percent of their water ice, so a comet that arrived intact could, in principle, leave a local pool at around 30 millimolar. The team treated it as the best case, because its tough bond should survive impact better than more fragile molecules.

How 100,000 simulated histories put comets to the test

Most earlier studies counted the Moon’s craters to estimate how many comets struck the early Earth, but those craters may record mostly asteroids. This team took impact rates from a dynamical model by David Nesvorný and colleagues that follows one million icy bodies born beyond Neptune as planets scatter them inward. Small comets tend to spin apart before reaching the inner Solar System, which cut the number of Earth impacts by roughly a factor of ten.

Each incoming comet then had to survive the air. Using a code called atmosentry, the researchers flew the comets through a one-bar atmosphere, equal to today’s surface pressure and probably gentler than the early Earth’s. Comets narrower than about 100 meters blew apart in high-altitude airbursts, scattering their cargo over huge areas. The atmosphere also preferentially destroyed comets coming in at shallow angles, the very impacts gentle enough to spare fragile molecules.

For every comet that reached the ground, a separate model calculated how much hydrogen cyanide survived the heat of a collision at many kilometers per second. The team ran the whole bombardment 100,000 times in steps of a million years, counting only hits on dry land, since cyanide dropped in the sea dilutes at once. As a check, the simulated comets added up to about 2.6 billion billion kilograms, in line with estimates from noble gases in Earth’s atmosphere.

The numbers behind a near-zero result

Earth swept up more than 100,000 comets wider than 100 meters during its early bombardment. Yet the typical run produced zero impacts that left at least 10 millimolar of hydrogen cyanide on land. In 54 percent of the simulations, no comet ever did it. The successful hits needed a comet larger than 100 meters, moving unusually slowly and striking at a shallow angle.

Globally, the picture is no better. All the cyanide that comets delivered intact, about 26.5 trillion kilograms, would reach roughly 0.75 micromolar if dissolved in a single ocean. That is more than ten thousand times weaker than the lab recipes, and below what sunlight-driven chemistry in the atmosphere could make on its own.

Timing works against comets too. When a successful hit happened at all, the most likely date was about 4.41 billion years ago, more than 100 million years before the estimated age of the last universal common ancestor of all life, around 4.2 billion years ago. Fewer than 5 percent of runs recorded one later than 4.03 billion years ago, and back then Earth had very little exposed land. “It would really have to be a one-in-a-million occurrence,” Anslow said. “We can’t rule out for certain that it happened that way. But it seems to be a really rare occurrence, at best, a one-off fluke.”

Mars was the easier target

Run the same bombardment on the neighbors and the ranking flips. Ignoring how much land each planet had, Mars received 744 impacts delivering useful cyanide concentrations, against 87 for Earth and 24 for Venus. Mars is smaller and farther from the Sun, so comets hit it more slowly, on average at 19.7 kilometers per second, and its thin air lets nearly every comet wider than 100 meters reach the ground. Even with a one-bar Martian atmosphere, the count stays at 448. Venus’s thick air destroys almost every comet under two kilometers across.

“This is not to say it definitely happened on Mars, of course, but suggests it would be unlikely if it happened on Earth and didn’t happen on Mars,” Anslow said. “Or the other, perhaps more likely, alternative, is that it didn’t happen on either planet.”

What the simulations do not settle

This is a model of one molecule and one delivery route. César Menor-Salván, an astrobiologist at the University of Alcalá in Spain who was not involved, told Spain’s Science Media Centre that the paper “does not prove, it calculates” that intact delivery of concentrated cyanide is unlikely. He notes that impacts can also create hot-water systems where chemistry continues, and that the study leaves out how cyanide can be stockpiled in minerals. The authors accept that storage in iron-cyanide salts is possible, but argue it would erase the main advantage of comets, a ready-made concentrated dose.

Several inputs remain poorly known: the pressure of the early atmosphere, how much land stood above the sea, and how likely chemistry is to cross the line into life. The authors set each one in the comets’ favor and argue they still overestimate cyanide survival, because their impact model ignores the porosity and strength of real comets. The study also covers only short-period comets; long-period comets strike faster, which would make survival harder still.

Nor does the work claim that comets brought nothing: it does not address water or the many other compounds comets carry, and a single lucky, slow, glancing impact cannot be excluded. Gordon Osinski, a planetary geologist at Western University in Canada who was not involved, told Science News the study “definitely will make people think twice about comets being the originator for the delivery of organics.”

The alternatives are closer to home. Asteroids struck the early Earth far more often than comets, carbon-rich meteorites called carbonaceous chondrites carry organic material, and hydrogen cyanide can form in the atmosphere and rain onto exposed land in a slow, steady supply. Volcanoes and hydrothermal vents offer other options.

Common questions about comets and the origin of life

Did comets bring the ingredients for life to Earth?

Probably not in any useful amount, according to this study. Across 100,000 simulated histories of the early Earth, 54 percent produced no comet impact that left concentrated hydrogen cyanide on dry land, and the typical run produced none.

Why is hydrogen cyanide important for the origin of life?

Its energy-rich triple bond can drive the reactions that build more complex molecules. Lab experiments with hydrogen cyanide at about 10 millimolar have produced lipids, sugars and the building blocks of RNA and DNA.

Could comets have delivered life’s ingredients to Mars instead?

More easily than to Earth: the simulations gave Mars 744 such impacts against 87 for Earth, because comets hit Mars more slowly and its thin air lets them through. The lead author stresses that this does not mean it happened there.

Next, a search for planets where comets bounce

The paper was published on 28 October 2026. Anslow and several co-authors proposed in 2023 that in tightly packed planetary systems, comets could slow down by bouncing from one planet to the next. The team suggests that future telescopes such as the proposed Habitable Worlds Observatory look for systems where that could happen. Osinski hopes others will reproduce the result and run a similar test for carbonaceous chondrites. For Earth itself, the search is moving back to the ground.

Reference: Anslow et al., “Cometary impacts an unlikely pathway towards the origins of life on Earth,” Scientific Reports, 2026. DOI: 10.1038/s41598-026-74214-y

Tags: astronomy, Mars, comets, University of Cambridge, hydrogen cyanide, origin of life