As NASA says, comets are “cosmic snowballs” of frozen gases, rocks and dust that include a good amount of organic materials. So scientists have long suggested that a comet brought these molecules to Earth and that they became the building blocks of life here. But a new study explains how that scenario might actually be impossible. It is very likely that comets carrying organic materials have crashed into Earth at some points in our distant past. The question, however, has to do with whether these fragile molecules survived the violent entry of their container, and the chances are very slim, as the researchers explained in a study published today in Scientific Reports. According to the paper, model simulations strongly suggest that comets travel too fast and at too acute an angle to Earth’s atmosphere for molecules to escape vaporization. “Historically, scientists thought comets were important to the origin of life on Earth because they carried molecules containing carbon and nitrogen,” Richard Anslow, lead author of the study and an astronomer at the University of Cambridge in the United Kingdom, said in a statement. “So these giant space snowballs were almost like a ‘get out of jail free card’ that carried all the molecules a planet would need to seed life. It’s a somewhat romantic idea, but our results suggest it’s very unlikely.” A bumpy ride In 2023, Anslow and his colleagues explored an idea that was arguably favorable to this “get out of jail free card,” as he puts it. Specifically, they explored whether comets could deposit similar organic materials on other planets in the galaxy, with the understanding that such systems would be promising candidates in the search for extraterrestrial life. However, an important conclusion of this study was that a comet would have to travel at speeds less than about 15 kilometers (9 miles) per second. Faster than that, the precious molecules would break apart due to the speed and temperature of the impact. Ideally, a comet could travel at the right speed in systems where groups of planets orbit very close to each other, so that a comet would essentially “bounce” between their orbits. Wait a second… Then, perhaps naturally, the team refocused on the solar system. Was it possible that something like this could happen in our neighborhood? To find out, the team ran simulations of hydrogen cyanide, a compound frequently found in comets in the solar system. The models incorporated the latest advances in simulations of comet impacts, comet-atmosphere interactions, and comet evolution. And the numbers didn’t look too promising. “It really would have to be a one-in-a-million occurrence,” Anslow said. “[N]Not only would an impact be needed at the right speed and angle, but we would also need everything to happen after that point, in terms of the concentration and speed at which the chemical origins of life were set in motion.” It would take a miracle. It’s possible that all of these factors miraculously aligned for it to work, but that would be “a really rare event; “at best, a one-time fluke,” Anslow said. And in the broader flow of things, the organic compounds that survived would be only an “insignificant source of the concentrated raw material molecules necessary for prebiotic chemistry.” [the synthesis of key molecular building blocks]”, according to the article. According to the study, scientists have proposed two main sources for prebiotic chemistry. One, an exogenous delivery (originating outside of Earth) via comets, asteroids, and interplanetary dust particles. Two, an endogenous synthesis (originating here on Earth) within Earth’s own atmosphere. To be clear, as the team explained in the article, there is still some conflicting evidence that made scientists hesitant to fully subscribe to the latter. vision. Unfinished business, but On a global scale, the “intact delivery” of hydrogen cyanide via comets is “orders of magnitude” lower than what the atmosphere can produce, even under the most unfavorable conditions, the team explained in the paper. That said, the study only rules out (mostly) comets, and the exogenous delivery field includes asteroids, which collided with Earth much more frequently than comets. scenario of delivering prebiotic chemistry to a comet might be more likely on Mars, since it is smaller and has a much thinner atmosphere than Earth, but that’s just a maybe, he added, the point being that, if something like this really happened on Earth, it would be strange if it didn’t happen on Mars. “Or the other alternative, perhaps more likely, is that it didn’t happen on any of the planets,” he said.
