A decades-old theory suggesting that icy comets delivered the essential building blocks for life to Earth is likely incorrect, according to new research from the University of Cambridge. Scientists claim that the high speeds at which comets typically impact Earth would cause them to disintegrate and vaporize, destroying the fragile organic molecules they carry.
For life to emerge from lifeless chemicals, Earth required a supply of organic molecules and the conditions to assemble them. The prevailing theory posited that comets, rich in carbon- and nitrogen-bearing compounds, provided this vital "feedstock." However, computer modeling by the research team indicates that comets could not have delivered sufficient quantities of key molecules, such as hydrogen cyanide, to initiate life's chemical reactions.
"It's kind of a romantic idea, but our results suggest it's highly unlikely," said lead author Dr. Richard Anslow of Cambridge's Institute of Astronomy. The study, published in Scientific Reports, focused on the fate of hydrogen cyanide during comet impacts. Previous research had considered hydrogen cyanide a promising candidate because of its abundance in comets and its potential resilience.
However, the simulations revealed that most comets would fragment explosively in the atmosphere. Even comets several kilometers wide would break apart. After dissolving in Earth's water, the concentration of delivered hydrogen cyanide would be significantly lower than what is believed necessary to spark life's chemical chain reactions. Any comet smaller than approximately 330 feet (100 meters) would be completely destroyed by airbursts.
This new analysis suggests that comets were unlikely to be the primary source of life's initial ingredients. Dr. Anslow noted that alternative hypotheses are more promising. These include the delivery of organic molecules by carbon-rich asteroids, which may generate sufficient heat during atmospheric entry to synthesize necessary chemicals, or the synthesis of these molecules directly on young Earth, possibly in hydrothermal vents.
While comets may not have been the source for Earth, the researchers suggest they could have played a role on other planets, such as Mars. Due to Mars's thinner atmosphere and smaller size, comets would have impacted at lower speeds, increasing the likelihood of molecule survival. This distinction could be crucial for future searches for extraterrestrial life, potentially guiding telescopes like the James Webb Space Telescope to focus on planets where such "bouncing" comet impacts might have occurred.
