A new scientific discovery could challenge a decades-old theory about how life began on Earth.
For life to develop from non-living chemicals, early Earth would have needed a supply of organic molecules, along with the right conditions for those substances to combine.
One leading explanation has been that icy comets brought large quantities of concentrated chemicals to Earth’s surface.
But researchers at the University of Cambridge now say that scenario is highly unlikely.
Their concern is that most comets travel too quickly and strike the atmosphere at such steep angles that they would not survive the impact intact.
Smaller comets would be shattered by the intense shock, while the delicate organic molecules they carried would almost certainly vaporise before reaching the ground.
As a result, comets may have delivered the raw materials for life only through a rare “one-off fluke”, rather than through the regular bombardment of chemicals once proposed.
Lead author Dr Richard Anslow, of Cambridge’s Institute of Astronomy, said: “It’s kind of a romantic idea, but our results suggest it’s highly unlikely.”
Scientists have challenged the decades-old idea that icy comets from the outer Solar System delivered the building blocks of life to Earth. Pictured: Comet C/2022 E3 (ZTF)
Understanding how life emerged requires scientists to determine where the so-called “feedstock” molecules came from and how they were brought together.
Yet researchers have struggled to explain how these basic ingredients could have become concentrated in quantities high enough to support the chemical processes that led to life.
Dr Anslow told the Daily Mail: “Comets emerged as a promising delivery mechanism based on the observation of Solar System comets showing them to be rich in carbon- and nitrogen-bearing chemicals that we think are important for the origins of life.”
He said the giant icy bodies had effectively become a “get out of jail free card” for scientists trying to explain how Earth received the molecules needed to seed life.
The study, published in the journal Scientific Reports, examined hydrogen cyanide, a feedstock chemical containing hydrogen, carbon and nitrogen.
Hydrogen cyanide has attracted particular interest because it is especially abundant in comets from the outer Solar System and may be durable enough to withstand an impact.
However, new computer modelling indicates that comets could not have supplied anything close to the amount of hydrogen cyanide needed to trigger the chain of chemical reactions associated with the origins of life.
In earlier research, Dr Anslow proposed that comets might lose speed by “bouncing” through systems where planets orbit close to one another, potentially delivering organic molecules along the way.
Researchers found that comets travel too quickly to reliably survive an impact with Earth. Consequently, the number of arrivals that successfully deliver organic chemicals, shown on the y-axis, is too low
To test whether that process could have occurred in the Solar System, the team combined the latest comet-impact simulations with computer models examining how comets form and evolve.
The models enabled the researchers to calculate how comets would have approached Earth and determine what would happen when they entered the atmosphere and struck the planet.
‘We found that the typically high speeds that comets impact the Earth at lead to the destruction of the majority of these molecules, meaning that it is not straightforward for these to reach the surface intact,’ says Dr Anslow.
Any comet smaller than 330ft (100m) wide would ‘catastrophically fragment in explosive airbursts at high altitude’, while even kilometre-sized comets would be broken apart.
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After being dissolved in a planet’s worth of water, the concentrations of hydrogen cyanide delivered in this way would be orders of magnitude lower than what is needed to start life’s chemical reactions.
That suggests that comets are an unlikely source for the origin of life, meaning that the feedstock chemicals must have come from somewhere else.
Dr Anslow says: ‘There are many promising alternative hypotheses including their delivery by carbon-rich asteroids, or alternatively these molecules could have been synthesised on the young Earth, not requiring any impacts at all.’
Previous studies suggest that the heat generated by rocky asteroids’ passage through the atmosphere could provide the energy needed to forge these basic chemicals.
Mars, on the other hand, could have been seeded with chemicals from comets. Since its atmosphere is thinner and the planet is smaller, comets arrive slower and are more likely to survive the impact
On the other hand, some researchers propose that Earth’s basic chemical stockpile could have been created in hydrothermal vents deep under the oceans or near the surface.
However, although scientists say they have ruled out comets as the source of life on Earth, the situation is very different elsewhere in the Solar System.
In fact, the researchers identify Mars as a promising place where comets could have given life a helping hand.
Since Mars is smaller than Earth and currently has a much thinner atmosphere, comets would be much more likely to make it to the planet’s surface during the period of the Solar System’s development when comets were far more frequent visitors.
‘Because comets typically strike Mars at lower speeds than Earth, we found that — at face value — more of these molecules survive impact.
‘However, a big caveat is that there are many uncertainties about early Martian environments, meaning that this isn’t to say successful impacts occurred onto Mars!’
In the future, the researchers say that these findings could help telescopes like the James Webb Space Telescope or the proposed Habitable Worlds Observatory look for life.
By looking for planets where ‘bouncing’ comets could have safely reached the planet’s surface, scientists can narrow down the number of places they need to search.