CAMBRIDGE, United Kingdom, Nov 1, 2026 - New research concludes slow moving asteroids are the most plausible source of intact organic molecules delivered to early Earth, ruling out comets as a viable carrier for these fragile compounds.^[1] Simulations run for the study found organic molecules carried by icy comets would have been destroyed entirely on impact with Earth.^[2] The same work notes ancient Mars experienced lower comet impact velocities. On that planet, comets may have successfully delivered carbon molecules.^[3][4] Impact Insight. Organic molecules are the carbon-based building blocks required for all known life. For decades researchers have debated whether these compounds formed naturally on planetary surfaces, or arrived from space via impacts. This result narrows the candidate delivery mechanisms for Earth, and highlights that conditions for prebiotic chemistry may have developed very differently on early Mars.