Water’s Origin: Chemistry Reveals Gradual Loss During Solar System Formation

Research indicates Earth’s water likely originated locally within the protoplanetary disc, rather than from distant sources beyond the traditionally defined snowline. Modelling utilising a gaussian distribution of water binding energies on dust grains reproduces observed water abundances and hydration trends in chondrites, suggesting early, in-situ acquisition.

The enduring question of Earth’s water source receives renewed attention in new research suggesting a substantial portion may have originated closer to home than previously thought. Conventional models posit that Earth formed from dry materials within the proto-solar nebula, necessitating delivery of water from beyond a defined ‘snowline’ where ice could survive. However, Lise Boitard-Cr´epeau, Cecilia Ceccarelli, and colleagues, from the Université Grenoble Alpes and the University of Torino, challenge this view. Their work, titled ‘Was Earth’s water acquired locally during the earliest phases of the Solar System formation?’, utilises recent advances in chemical understanding of ice binding energies to model a more gradual distribution of water across the early solar system, potentially accounting for observed water abundances without invoking distant delivery mechanisms. The team’s modelling reproduces hydration trends observed in chondrite meteorites, supporting the hypothesis of a local water source.

Earth’s Water Likely Originated Within the Early Solar System’s Protoplanetary Disc

The enduring question of Earth’s water origin remains central to understanding planetary formation, with current models often positing delivery from beyond the early Solar System’s snowline. Recent research challenges this assumption by focusing on the chemical processes governing water’s behaviour within the disc itself, investigating the possibility that a significant portion of Earth’s water originated locally during the Solar System’s formative stages. This work proposes a refined understanding of water’s distribution, moving beyond simplistic models reliant on a sharply defined snowline and incorporating a more nuanced understanding of chemical binding energies.

The traditional understanding of the snowline relies on a single temperature threshold of approximately 180 Kelvin, often neglecting the complex interplay between water molecules and the dust grains upon which they reside. Crucially, the strength with which water adheres to these grains, known as the binding energy, significantly influences its sublimation point and distribution within the disc. This binding energy isn’t a fixed value but follows a Gaussian distribution, implying a more gradual transition in water abundance, fundamentally altering the concept of a sharp snowline and replacing it with a broader “water transition zone” where water sublimation occurs over an extended radial distance.

This distribution of binding energies allows for a more nuanced understanding of water’s distribution, combining this refined understanding of water’s chemical behaviour with established models of the protoplanetary disc’s structure. Researchers leverage data from chondrites, ancient meteorites considered remnants of the early Solar System, to validate their model by analysing the hydration levels within different chondrite groups and inferring the conditions under which these materials formed. By comparing the model’s predictions regarding water abundance with the observed hydration trends in chondrites, scientists reassess prevailing theories regarding Earth’s water origin and demonstrate the potential for a locally sourced water inventory.

To model this behaviour, researchers estimate the radial distribution of adsorbed ice – water molecules clinging to the surfaces of dust grains – across the protoplanetary disc, incorporating the understanding that water doesn’t simply appear or disappear at a fixed distance from the sun. This modelling accounts for a gradual process of adsorption and sublimation influenced by the strength of chemical bonds, effectively simulating how water molecules would behave on dust grains at varying distances from the early sun. The significance of this approach lies in its ability to reproduce the observed range of water abundances on Earth, a figure that has long presented a puzzle for planetary scientists.

By incorporating the Gaussian distribution of binding energies, researchers demonstrate that a substantial portion of Earth’s water could have originated locally, within the region where Earth formed, rather than being delivered from distant, icy bodies beyond the classical snowline. This challenges the prevailing narrative of a late veneer of water-rich material impacting Earth after its initial formation and aligns with hydration trends observed in different types of chondrites, primitive meteorites considered remnants of the early solar system. These meteorites exhibit varying degrees of water content, and the model accurately predicts these hydration levels based on their estimated formation distances from the sun, reinforcing the idea that the distribution of water in the early solar system was far more complex and nuanced than previously assumed.

The composition of Earth’s water presents a long-standing challenge to conventional models of Solar System formation, which typically posit a dry inner protoplanetary disc. Researchers now investigate the role of water binding energies on dust grains within this disc, challenging the traditionally defined ‘snowline’ as a sharp boundary for water ice stability. This study moves beyond the single condensation temperature approach, employing a Gaussian distribution of binding energies to model a more gradual sublimation process of water across the protoplanetary disc, calculating the radial distribution of adsorbed ice on dust grains and considering the varying energies required to bind water molecules to their surfaces.

This modelling demonstrates the capacity to reproduce the observed range of water abundances currently estimated for Earth, crucially aligning with hydration trends identified in various chondrite meteorites. Researchers link their water content to their predicted formation distances within the early Solar System, demonstrating that a substantial portion of Earth’s water may originate from local sources within the inner protoplanetary disc. This challenges the prevailing hypothesis that Earth’s water was primarily delivered by icy planetesimals originating from the outer Solar System, offering a refined perspective on the origin of terrestrial water.

By incorporating a chemically informed understanding of water binding, this work successfully reconciles theoretical predictions with observational data from meteorites, providing a compelling argument for a more nuanced understanding of early Solar System processes. This research highlights the importance of considering chemical properties when reconstructing the formation history of planets and their volatile inventories, demonstrating that the early stages of solar system formation were characterised by a gradual hydration process.

This compilation of citations demonstrates a robust and interconnected research landscape spanning planetary science, astrochemistry, and geochemistry, with current investigations actively pursuing understanding of the solar system’s formation and evolution. Foundational work such as that of Weidenschilling (1977) and the Nice model developed by Morbidelli et al. (2000, 2022) focuses on understanding planetesimal growth and the subsequent migration of giant planets. Researchers consistently examine isotopic signatures within meteorites to reconstruct the building blocks of planets and trace their origins, focusing on the origin and distribution of water within the solar system.

Investigations into chondrules and refractory inclusions, such as Calcium-Aluminum-rich Inclusions (CAIs), remain crucial for deciphering the conditions present in the early solar system, with analyses of chondrites providing insights into parent body processes and the evolution of planetary building blocks. Simultaneously, astrochemistry plays a vital role, with observations from facilities like ALMA detailing the molecular inventory of protoplanetary disks and the chemical processes occurring within them. The integration of analytical techniques, notably mass spectrometry and spectroscopy, with chemical modelling strengthens the ability to interpret observational data and simulate astronomical environments.

This interdisciplinary approach allows researchers to test hypotheses regarding the formation and evolution of planets, asteroids, and comets, actively exploring the composition and evolution of terrestrial planets, including Earth and Mars, alongside studies of smaller bodies to build a comprehensive picture of the solar system’s history. Future work should prioritise refining models of protoplanetary disk chemistry, incorporating more complex grain surface processes and non-mass-dependent isotope effects (NMIE). Further investigation into the distribution of organic molecules in meteorites and protoplanetary disks will be essential to understanding the potential for prebiotic chemistry.

Expanding the range of isotopic systems studied, and improving the precision of isotopic measurements, will also refine our understanding of the sources and evolution of solar system materials, with continued collaboration between observational astronomers, analytical chemists, and modellers crucial for advancing our knowledge of planetary origins and the conditions necessary for habitability. This research demonstrates that a comprehensive understanding of planetary formation requires a nuanced approach that integrates chemical properties, observational data, and sophisticated modelling techniques, ultimately providing a more complete picture of our solar system’s origins and evolution.