A new study has challenged the long-standing theory that comets may have delivered the chemical ingredients needed for life to emerge on Earth, suggesting that the high-speed impacts could have destroyed most of these compounds before they had a chance to contribute to the development of life.
Researchers from the University of Cambridge used 100,000 computer simulations to examine whether comets could have delivered sufficient quantities of hydrogen cyanide, an important chemical involved in the formation of life's building blocks, to early Earth. The findings suggest that successful delivery was extremely unlikely, raising questions about the role comets played in the origin of life on our planet.
The study, led by Richard Anslow of the University of Cambridge, was published in the journal Scientific Reports on October 28. It examined whether hydrogen cyanide (HCN), a compound containing hydrogen, carbon and nitrogen, could have survived the extreme conditions generated when comets struck early Earth.
Explaining the findings, lead researcher Richard Anslow told CNN that the speed at which comets typically strike Earth makes it difficult for organic molecules to survive the impact. "The 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," Anslow said, adding that comets may not be as useful for prebiotic chemistry as scientists had previously believed.
The simulations considered a period of intense bombardment during Earth's early history, when more than 100,000 comets larger than 100 metres may have struck the planet. In 54 per cent of the simulations, not a single impact successfully delivered the required concentration of hydrogen cyanide. The remaining 46 per cent recorded at least one potentially successful impact, but scenarios involving multiple successful deliveries were extremely rare.
The researchers also found that the most favourable conditions for successful delivery may have existed around 4.41 billion years ago. However, the overall results suggested that comets were an unlikely source of the chemical ingredients needed for life to emerge on Earth.
What are comets
Comets are small celestial bodies made primarily of frozen gases, water ice, dust, rock and organic compounds. Often described as “dirty snowballs”, they are remnants of the formation of the solar system around 4.6 billion years ago, according to NASA. Unlike asteroids, which are mainly rocky or metallic, comets contain large amounts of ice and frozen gases. Most comets originate from two distant regions of the solar system: the Kuiper Belt, beyond Neptune, and the much more distant Oort Cloud. They generally travel around the Sun in elongated orbits, sometimes taking hundreds or even millions of years to complete one revolution. As they approach the Sun, their ice begins turning into gas, releasing dust and forming the bright cloud and tail commonly associated with comets.
Comets do not deliberately head towards Earth. Their orbital paths can change because of the gravitational influence of planets such as Jupiter and Neptune, as well as other gravitational disturbances. These changes can push some comets into paths that bring them into the inner solar system, where their orbits may cross Earth's orbit. If a comet and Earth reach the same point at the same time, a collision can occur. However, such impacts are rare. Most comets pass through the solar system without coming close to Earth, while smaller particles shed by comets can enter Earth's atmosphere and burn up, producing meteor showers.
