Earth's building blocks contained water before it was a full planet

About 4.56 billion years ago, when our Sun was still in its infancy, the solar system was nothing more than a swirling disk of gas and rocky dust. It was within this cosmic nursery that the fundamental building blocks of Earth and other inner planets, known as “planetesimals,” gradually formed.

These kilometer-sized entities emerged from the accumulation of dust over millions of years, growing steadily like snowballs gaining mass in a winter landscape.

Understanding planetesimal formation

Scientists have long been intrigued by the conditions under which these planetesimals originated. A question of particular interest is the presence of water during their formation.

Was our planet inherently wet, or did it acquire its water later?

This question is pivotal in understanding not only the history of Earth but also the evolution and the building blocks of our entire solar system.

A recent breakthrough in this field comes from a study combining meteorite analysis with advanced thermodynamic modeling.

The study was led by Damanveer Grewal, a former Caltech postdoctoral scholar, and the research was conducted in the laboratory of Paul Asimow.

Their findings suggest that the earliest planetesimals in the inner solar system were formed amidst water, challenging many existing theories about our solar system’s infancy.

Cosmic time capsules

Iron meteorites, remnants of the metallic cores of these ancient planetesimals, serve as time capsules preserving details from the solar system’s earliest days.

These meteorites, which managed to evade incorporation into growing planets and instead journeyed through the solar system before landing on Earth, are crucial in this study.

Their chemical compositions provide clues about their formation environments, particularly whether they originated near the cooler, water-rich regions far from the Sun, or closer to the Sun where heat would have prevented the existence of water ice.

Role of water in Earth’s building blocks

Interestingly, while these meteorites contain no water today, scientists can deduce its former presence by examining its influence on other elements.

Water, composed of hydrogen and oxygen, can induce oxidation — a process where the oxygen atom from water interacts with other elements.

A classic example is iron (Fe) reacting with water (H2O) to form iron oxide (FeO), leading to further oxidation products like Fe2O3 and FeO(OH) found in rust. Mars, with its iron oxide-rich surface, exemplifies this process, hinting at its past watery conditions.

Chemical signatures yield clues

Damanveer Grewal, a former Caltech postdoctoral scholar and the lead author of the study, has delved deep into the chemical signatures of iron meteorites to unearth secrets of the early solar system.

Although any original iron oxide has long disappeared, the research team could estimate the extent of iron oxidation by analyzing the proportions of nickel, cobalt, and iron in these meteorites.

Any deviation in the expected ratios of these elements suggests that some of the iron was oxidized and therefore indicates the historical presence of water.

“Iron meteorites have been somewhat neglected by the planet-formation community, but they constitute rich stores of information about the earliest period of solar system history, once you work out how to read the signals,” says Asimow.

“The difference between what we measured in the inner solar system meteorites and what we expected implies an oxygen activity about 10,000 times higher.”

Rethinking the origin of Earth’s water

The team discovered that meteorites thought to originate from both the inner and outer solar system showed similar amounts of ‘missing’ iron metal.

This consistency suggests that planetesimals from both regions formed in environments where water was present, implying that the foundational elements of planets like Earth were exposed to water from the very beginning.

This evidence challenges many current astrophysical models. If these planetesimals formed in Earth’s current orbital position, then the inner solar system must have been cooler than what models currently propose.

Alternatively, they might have originated further out in the solar system, where cooler conditions prevailed, before migrating inward.

Implications and future study

Grewal highlights the broader implications of this finding. “If water was present in the early building blocks of our planet, other important elements like carbon and nitrogen were likely present as well,” says Grewal. “The ingredients for life may have been present in the seeds of rocky planets right from the start.”

Asimow adds a caveat. He explains, “The method only detects water that was used up in oxidizing iron. It is not sensitive to excess water that might go on to form the ocean.”

Asimow continued, “So, the conclusions of this study are consistent with Earth accretion models that call for late addition of even more water-rich material.”

In summary, this important study reshapes our understanding of the solar system’s early days, while also opening new avenues for exploring the origins of life on Earth and potentially other planets.

The full study is published in the journal Nature Astronomy.

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