\n Life on Earth requires complex organic molecules, but it\u2019s unclear whether our early planet had enough of the raw materials needed for prebiotic chemistry \u2014 the chemical reactions that produce the building blocks of life. Comets are made of dust and ice but are also known to contain organic material. This composition, which has earned the celestial bodies the nickname \u201cdirty snowballs,\u201d<\/a> prompted scientists to suggest that comets could have deposited water and organic molecules on Earth\u2019s surface.\n <\/p>\n \n Early in its history \u2014 over 4 billion years ago \u2014 Earth may have experienced a period of heavy bombardment from more than 100,000 comets larger than 100 meters (328 feet), according to the new study, published Wednesday in the journal Scientific Reports<\/a>. Those impacts, however, were mostly too heavy for life-starting molecules to withstand.\n <\/p>\n \n The team used the latest models of comet formation and evolution to simulate this early planetary period 100,000 times. \u201cGiven what we know about surface environments on the early Earth, on average, our simulations found that there were no successful impacts,\u201d said lead study author Richard Anslow, who conducted the research while completing his doctorate at the Institute of Astronomy of the University of Cambridge in the United Kingdom.\n <\/p>\n \n The comets contained large concentrations of key molecules needed for life, but the simulation found the objects were coming in too fast to successfully deposit them. \u201cThe 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\u2019t seem to be as useful for prebiotic chemistry as was previously thought,\u201d Anslow said.\n <\/p>\n \n More precisely, 54% of the simulations showed no impact at all, while the remaining 46% had at least one and a \u201ctiny minority\u201d returned up to three collisions, Anslow said. \u201cWe can\u2019t rule out confidently that there was never a successful impact \u2014 there could have been one of these fluke impacts.\u201d However, such flukes had to occur in a \u201cperfect location\u201d that already met several important conditions, including the presence of liquid water and other chemicals needed to support life. That combination makes the overall process unlikely, he added.\n <\/p>\n \n The study focuses on a naturally occurring chemical called hydrogen cyanide or HCN, an important precursor in prebiotic chemistry that has been detected<\/a> in dozens of comets, including the notable Halley\u2019s and Hale-Bopp comets.\n <\/p>\n \n Composed of hydrogen, carbon and nitrogen, HCN can lead to the formation of amino acids \u2014 molecules that make up proteins \u2014 when combined with water; other studies<\/a> have previously identified HCN as a potential key ingredient for the origin of life on Earth.\n <\/p>\n \n \u201cWhen comets strike Earth, they\u2019re shocked to high temperatures and pressures, and recent studies have found that a lot of these precursor molecules can be destroyed by this high pressure and temperature as well as the following impacts,\u201d Anslow said.\n <\/p>\n \n The study authors argue that even if a successful impact occurred, it\u2019s unlikely that it would have delivered the chemical in a useful concentration, and in the case of an ocean impact, it would have quickly dissolved.\n <\/p>\n \n To set the stage for prebiotic chemistry to occur, many other precursor molecules are also needed, such as compounds that contain phosphorus and sulfur, but these are not routinely found in comets, Anslow said. \u201cComets are really good for delivering hydrogen cyanide, but given it\u2019s random where a comet impacts onto Earth, we\u2019re not going to be guaranteed to have everything else that we need for this chemistry right in that spot.\u201d\n <\/p>\n \n If comets didn\u2019t deliver the building blocks of life on Earth, Anslow said there are several other promising avenues to explore. Another long-held, competing theory is the possibility that Earth\u2019s primordial soup could have synthesized the molecules needed for life on its own. An alternative cosmic source of organic material could have been asteroids, which typically struck Earth more slowly and in greater numbers. Anslow pointed to recent findings on samples from asteroids Bennu and Ryugu,<\/a> which contain the majority of protein-building amino acids.\n <\/p>\n![\"Comets](\"https:\/\/www.newsbeep.com\/il\/wp-content\/uploads\/2026\/10\/esa-rosetta-navcam-20150131-mosaic.jpg\")
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