Cambridge scientists say comets probably didn't bring the building blocks of life to Earth
Researchers say the idea of 'giant space snowballs' seeding life is highly unlikely
Comets are unlikely to have brought the chemical ingredients for life to Earth, according to new research led by the University of Cambridge.
For close to a century, scientists have thought these icy objects from the outer solar system delivered vital molecules to the young Earth.
The new study suggests most of those fragile molecules would have been destroyed when the comets crashed into the planet.
The findings are published in the journal Scientific Reports.
Lead author Dr Richard Anslow, from Cambridge's Institute of Astronomy, said: "In this work, we were revisiting this long-standing idea that ice-rich comets formed in the outer regions of the solar system delivered key feedstock molecules to the young Earth. And so, although comets are rich in these carbon and nitrogen-bearing molecules that we think were required to produce the building blocks of life, what we found is that the majority of these fragile molecules were largely destroyed as these comets struck the Earth at tens of kilometres per second."
A 'romantic idea'
Comets are known to carry many of the molecules scientists believe are needed to make the building blocks of life.
That is why the idea that they "seeded" life on Earth has lasted for so long.
Dr Anslow said: "Historically, scientists thought that comets were important for the origin of life on Earth, because they carried carbon- and nitrogen-bearing molecules, so these giant space snowballs were almost like a 'get out of jail free card' that brought all the molecules a planet would need to seed life. It's kind of a romantic idea, but our results suggest it's highly unlikely."
What the researchers found
The team used computer modelling to test the idea.
They found comets travel too fast and hit Earth's atmosphere at too sharp an angle for the fragile molecules to survive.
In almost every case, the molecules would vaporise when the comet hit the ground.
The researchers focused on hydrogen cyanide, an important chemical ingredient that is regularly found on comets in our solar system.
Even in the rare cases where it survived, impacts delivering enough of it to be useful were extremely uncommon.
The number of comets hitting Earth also dropped quickly during the planet's first 500 million years.
That means there were, at most, a handful of chances for hydrogen cyanide to arrive on a comet.
Even if it did reach the surface, it would not have stayed concentrated for long.
That means the chemistry needed to start life would have had to happen very quickly after the impact, which the researchers say is highly unlikely.
Dr Anslow said: "It would really have to be a one-in-a-million occurrence, because not only would you need an impact at just the right speed and just the right angle, you would also need everything to happen after that point, in terms of the concentration and the speed at which the chemical origins of life got going. We can't rule out for certain that it happened that way. But it seems to be a really rare occurrence - at best, a one-off fluke."
'Bouncing' comets
In 2023, Dr Anslow and several of his co-authors suggested comets might slow down by "bouncing" from one planet to the next in systems where planets orbit close together.
That could let them drop off chemical ingredients along the way.
The new study looked at whether this could happen in our own solar system.
It found this would be very rare.
Dr Anslow said: "We're learning more and more all the time about how our Solar System and the Earth formed, and pulling this together with lots of different advances in modelling, which allowed us to investigate this idea that has been around for close to a century."
Could it have happened on Mars?
The researchers found Mars could be a slightly better candidate, although they say it is still highly unlikely.
Mars is smaller than Earth and has a much thinner atmosphere.
That means comets would have been more likely to reach its surface at a time when they were far more frequent visitors.
Dr Anslow said: "This is not to say it definitely happened on Mars, of course, but suggests it would be unlikely if it happened on Earth and didn't happen on Mars. Or the other, perhaps more likely, alternative, is that it didn't happen on either planet."
In an interview, he said: "But what we have to be really clear about is that we have lots more uncertainties about what the young Martian surface environments look like. So we're definitely not saying that this happened for sure on Mars or that this led anywhere near to the origins of life."
Where did life's ingredients come from?
The researchers say there are several other possibilities.
One is that the molecules formed after asteroids hit Earth, because asteroids struck the planet far more often than comets.
Another is that the ingredients formed through processes on Earth itself.
Dr Anslow said: "It could also be that a different mechanism entirely is responsible for the origins of life. And so if we can understand that better for the Earth, if maybe hydrothermal vents were really important, then this is something we could look at as well for other planets to understand, would these have similar environments and would this be a possible mechanism in as far as we understand these planets to envisage the origins of life there."
The search for life on other planets
The researchers say their results could help astronomers searching for life beyond our solar system.
That includes future telescopes that will follow the James Webb Space Telescope, such as the proposed Habitable Worlds Observatory.
Finding planetary systems where "bouncing" comets could exist could be one way to narrow the search.
Dr Anslow said: "And so I think what's nice about our results is that they can help us focus our attention on some of the other ways we could have maybe concentrated these key chemical ingredients on the surface of the Earth. And then looking sort of forwards, we could then try and understand how different planets might be better or worse suited to these other mechanisms that might or might not work better on the Earth."
'I'm still excited'
When asked if he was disappointed to have taken some of the romance out of the idea, Dr Anslow said: "No, I wouldn't say so. I'm still excited because I think this opens up these natural questions for me about how else could we have got this hydrogen cyanide? And there's lots of groups working on lots of really promising ideas to try and understand this. And so I think hopefully what our results will be able to do is help focus this attention and help us make this progress and unravelling a bit better these sort of environments where we think the origins of life might have happened on the Earth."
When asked if the research brings scientists a step closer to understanding how life began, he said: "I hope so. There's still lots of open questions, but I think our results suggest that comets might not be this robust source of these molecules that was once thought."
The research was supported in part by the Royal Society, the Leverhulme Centre for Life in the Universe and the Science and Technology Facilities Council, which is part of UK Research and Innovation.