Comets Almost Never Delivered Usable Cyanide to the Early Earth, Simulations Find

In 100,000 simulated histories of Earth's early comet bombardment, the median number of impacts that delivered hydrogen cyanide to dry land in a concentration that prebiotic chemistry could use was zero. Richard J. Anslow of the University of Cambridge's Institute of Astronomy and colleagues report that result, published Oct. 28, 2026.

Their paper in Scientific Reports tests an idea more than a century old: that comets stocked the early Earth with the reactive molecules that the chemistry leading to life would have needed. The authors use hydrogen cyanide as the test case, reporting that it is the only one of those molecules observed in comets at a useful concentration, and the most resistant to being destroyed by the heat of a high-speed impact.

The team set the bar at 10 mM, the lowest concentration that recent experiments have used to build sugars, fats and the building blocks of DNA and RNA, then sent comets through a model of atmospheric entry and a published model of how cyanide is destroyed on impact. Only slow, highly glancing strikes got their cyanide to the ground intact, and that window is narrow enough that most runs produced no qualifying impact at all. In the 46% of runs that produced at least one, it fell overwhelmingly in the planet's first few hundred million years. The authors say they cannot rule out one fortunate slow, glancing comet having delivered nearly all of its cyanide.

In the same simulations, Mars does far better: the authors count 744 successful impacts there, against 87 for Earth and 24 for Venus. Those three counts set aside how much dry land each planet had. For Earth, the authors report, the scarcity of exposed land on the earliest surface takes the median from 87 to zero.

The authors attribute Mars's advantage to its lower escape velocity and thin atmosphere, which together let almost every comet large enough to survive entry reach the ground. By their account, Venus's dense atmosphere destroys most of the comets that arrive, including some several kilometers across.