How the Building Blocks of Life Arrived on Earth Through Primitive Asteroids

Volatiles are compounds or elements that easily transition into vapor at low temperatures, such as water and the six key elements found in living organisms. This study focuses on how zinc, one of these volatile elements, found in meteorites, can be used as a marker to determine the origins of Earth's volatiles. Zinc has isotopic signatures unique to different regions of the solar system, enabling researchers to trace the material back to its source.

The study, published in 'Science Advances', builds on prior work by the same team, which found that Earth's zinc originated from two distinct areas: roughly half came from regions beyond Jupiter, and the rest from materials closer to Earth. These findings were crucial to understanding how Earth, a rocky planet, came to host the elements necessary for life.

The Role of Planetesimals
Dr. Rayssa Martins, lead author and researcher at Cambridge's Department of Earth Sciences, explained, "One of the most fundamental questions about the origin of life is where the materials we need for life to evolve came from. If we can understand how these materials were delivered to Earth, we might uncover clues to how life could emerge elsewhere."

Planetesimals, the small bodies from which planets like Earth formed, were central to this research. These bodies form through a process called accretion, where particles orbiting a young star stick together to form progressively larger objects. However, the composition and history of these planetesimals vary widely.

Early planetesimals that formed in the presence of radioactive isotopes experienced intense melting, which caused them to lose a significant portion of their volatile elements. In contrast, those formed after radioactive decay ceased were able to retain their volatile contents. This difference created two distinct categories of planetesimals: those that melted and lost their volatiles and those that preserved these critical compounds.

Tracing Zinc's Journey
Martins and her team examined a wide range of meteorites derived from different types of planetesimals to reconstruct Earth's accretion history. Their analysis revealed that while planetesimals that underwent melting contributed approximately 70% of Earth's overall mass, they only provided around 10% of Earth's zinc. The remaining zinc, and presumably other volatiles, came from unmelted, primitive planetesimals that had not lost their volatile content.

"We know that a planet's distance from its star is crucial for maintaining liquid water," Martins continued. "But our findings suggest that even if a planet is the right distance from its star, it may not always have the right materials to support water and other volatiles in the first place."

The study provides further support for the idea that unmelted asteroids were the key suppliers of volatiles to Earth. According to the researchers, these primitive materials were essential for creating the conditions necessary for life, particularly because melted planetesimals were depleted in volatile elements like water, carbon, nitrogen, and sulfur.

Implications for the Search for Life Beyond Earth
Understanding the processes that brought life-essential volatiles to Earth has profound implications for astrobiology. The ability to trace zinc and other elements through billions of years of planetary evolution could become a critical tool in the search for habitable planets. By identifying similar processes in other young planetary systems, scientists might better predict which planets are most likely to host life.

"Similar conditions and processes are likely in other young planetary systems," Martins commented. "The roles these different materials play in supplying volatiles is something we should keep in mind when looking for habitable planets elsewhere, especially beyond our Solar System."

As detailed in their paper, the researchers used sophisticated modeling to simulate Earth's accretion process over tens of millions of years. By combining these models with their new data on zinc isotopes, they were able to estimate how different types of planetesimals contributed to Earth's final volatile inventory.

The study highlights that while a planet may be in the "habitable zone" of its star, ensuring the right material mix - particularly of volatile elements - is another crucial factor for the potential emergence of life.

Broader Astrobiological Implications
The research also has broader implications for understanding how planetary bodies outside our solar system may develop habitable conditions. In our own system, the presence of volatiles on Earth likely depended on a delicate balance between early formed planetesimals that lost their volatiles and later accreted primitive material that retained them.

"Whether we are looking at Mars, icy moons, or exoplanets in other solar systems, this ability to track volatile sources across such vast timescales could prove essential in our quest to find life beyond Earth," Martins said.

The findings were supported in part by Imperial College London, the European Research Council, and UK Research and Innovation (UKRI). This growing understanding of planetary formation adds a vital piece to the puzzle of how planets like Earth acquire the building blocks of life.

Research Report:Primitive asteroids as a major source of terrestrial volatiles