---
title: "Comets Travel Too Fast to Have Seeded Life on Earth, Cambridge Model Finds"
description: "100,000 simulated impacts show the fragile chemistry comets carried mostly didn’t survive the landing"
author: "Luis Goa"
published: 2026-10-31T10:00:36.520Z
modified: 2026-10-31T13:18:57Z
url: https://rews.cc/a/comets-travel-too-fast-to-have-seeded-life-on-earth-cambridg-7ec236
language: en
tags: ["comets", "origin-of-life", "panspermia", "astrobiology", "simulation", "science"]
publisher: "Rews (https://rews.cc)"
---

# Comets Travel Too Fast to Have Seeded Life on Earth, Cambridge Model Finds

*100,000 simulated impacts show the fragile chemistry comets carried mostly didn’t survive the landing*

By Luis Goa · October 31, 2026 · https://rews.cc/a/comets-travel-too-fast-to-have-seeded-life-on-earth-cambridg-7ec236

## In brief

- Cambridge simulation: 54% of 100,000 modeled comet-bombardment histories deliver zero prebiotic HCN to early Earth
- Median successful HCN delivery across simulations is zero; mean is about one strike per billion years
- Comets need to be >100m, slow, hit at under ~15° angle on land to deliver intact HCN; above ~15 km/s survival is near zero
- Mars’s lower impact velocities (~5 km/s vs Earth’s ~11 km/s) give it roughly 5x more successful HCN deliveries than Earth
- Outside reviewers say the result doesn’t rule out comets delivering other, less volatile organic compounds

The number that matters is 54%. Run 100,000 Monte Carlo simulations of Earth’s early cometary bombardment and in 54,000 of them, not one comet strike delivers enough hydrogen cyanide (HCN) to get prebiotic chemistry going, according to a paper by Richard Anslow and colleagues at the University of Cambridge’s Institute of Astronomy, [published in Scientific Reports](https://www.nature.com/articles/s41598-026-74214-y) on October 28. The median outcome across the simulations is zero successful deliveries over more than a billion years of bombardment. The mean is about one. That is the headline result against a theory that has stood, in one form or another, since 1961, with the broader panspermia idea older still, dating to a 1908 proposal.

HCN is the molecule the model tracks because it is a feedstock compound: combined with water it can form amino acids, the building blocks of proteins, and it sits upstream of several proposed routes to RNA precursors and sugars. It has been detected in dozens of comets, including Halley’s and Hale-Bopp. The question the paper asks is narrow and answerable: given what’s known about comet composition, early Earth’s surface conditions, and impact physics, how often would a comet strike have delivered HCN intact and in a concentration high enough to matter, rather than vaporized, diluted or chemically destroyed on the way in.

## Why speed kills the chemistry

Earth’s escape velocity is about 11 km/s; a comet falling in from deep space arrives faster than that, and the paper’s HCN-survival rate drops to essentially zero above impact velocities of roughly 15 km/s. For intact delivery, Anslow’s team found the impactor needed to be larger than 100 meters across, moving slowly, and hitting at a shallow angle, below roughly 15 degrees from horizontal, rather than head-on. “The typical velocities of these comets when they impact the Earth meant that actually very few, if any, would have been able to deliver intact organic molecules, so comets actually don’t seem to be as useful for prebiotic chemistry as was previously thought,” Anslow said, as reported by [CNN](https://cnn.com/2026/10/31/science/origin-life-theory-comet-simulation).

Even a successful strike isn’t the end of the problem. An ocean impact dilutes surviving HCN too fast for it to drive the following chemistry, so the comet has to land on terrain, not water, on a planet that is mostly water. And HCN alone isn’t sufficient: prebiotic chemistry also needs phosphorus- and sulfur-bearing compounds that comets don’t routinely carry, so a successful strike has to happen in a spot that already has the rest of the recipe. Anslow put the compound probability bluntly: a workable impact “would really have to be a one-in-a-million occurrence,” since it needs the right speed, the right angle, the right location and everything to proceed quickly afterward, according to coverage in [ScienceAlert](https://www.sciencealert.com/scientists-ran-100000-simulations-of-comets-striking-earth-the-results-are-a-surprise).

That chart is the mechanism in one picture. Mars’s escape velocity is about 5 km/s against Earth’s 11 km/s, so comets arrive slower and the thinner Martian atmosphere does less to shock-heat and shatter them before they reach the ground. Venus is the opposite case: its thick atmosphere destroys almost every comet smaller than 2 kilometers across before it lands, and the paper models an average Venusian impact velocity above 36 km/s, which leaves successful HCN delivery there confined to a vanishingly small slice of possible impact parameters. The net result, across all three planets modeled, is that Mars saw substantially more successful HCN deliveries than Earth, by Anslow’s count roughly five times as many, while Venus saw the fewest.

“Because Mars is much smaller than Earth and further away from the sun, the typical impact velocity of comets is much lower, so while these impacts on Earth were destroying most of the HCN, this wasn’t the case so much for Mars,” Anslow told CNN. He was careful not to oversell the inference: “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.” The other reading, which he called perhaps the more likely one, is that it happened on neither planet.

## What the model doesn’t settle

Two scientists not on the paper flagged the same limit on what a null result like this can claim. Queenie Chan, an Earth scientist at Royal Holloway University of London, told CNN that HCN could react before a comet ever enters the atmosphere, producing less volatile, more impact-resistant compounds that the model doesn’t track.

> Therefore, the inefficiency of delivering intact HCN does not completely rule out a potential role for comets in supplying other important organic compounds that may have contributed to Earth’s prebiotic organic inventory before the emergence of life.

Nicolle Zellner, a physicist at Albion College, said she wasn’t surprised by the result and that the authors had put reasonable numbers around something researchers had long suspected qualitatively. Anslow’s own paper draws the same line, distinguishing a low probability for one mechanism from ruling out comets altogether, and the team states explicitly that its Earth result matches earlier work by Todd and Öberg on the same question.

The paper’s own alternative candidates are asteroids and in-situ chemistry. Asteroids hit Earth more often and at lower average speeds than comets, and sample-return missions to asteroids Bennu and Ryugu have already found most of the protein-building amino acids in their returned material, giving that route an existing physical sample rather than a simulation. The other option is that Earth’s own early surface chemistry, the primordial-soup model that competed with panspermia from the start, synthesized what was needed without extraterrestrial delivery at all. Neither alternative gets a Monte Carlo treatment in this paper; that is the obvious next model to build, for Earth, and, on this paper’s own numbers, for Mars first.

## See also

- [Comets didn't seed life on Earth after all, new study suggests](https://www.cam.ac.uk/research/news/comets-didnt-seed-life-on-earth-after-all-new-study-suggests) — cam.ac.uk · University of Cambridge's own release on the study
- ['Bouncing' comets could deliver building blocks for life to exoplanets](https://www.cam.ac.uk/research/news/bouncing-comets-could-deliver-building-blocks-for-life-to-exoplanets) — cam.ac.uk · Anslow's 2023 prior work on comet delivery between closely orbiting planets