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The article An Impact-Free Mechanism to Deliver Water to Terrestrial Planets and Exoplanets, which was recently published in Astronomy & Astrophysics (A&A, 692, A70, 2024), presents a novel approach to explaining how terrestrial planets and exoplanets might acquire water without relying on traditional impact theories. The research, led by Quentin Kral and colleagues, offers a detailed exploration of an alternative mechanism based on the sublimation of icy asteroids and the formation of a gaseous water disk that spreads within planetary systems.
The origin of Earth’s water remains a topic of considerable scientific debate. The prevailing theory suggests that water was delivered through asteroid impacts. However, Kral et al. propose that primordial icy asteroids could have sublimated to form a gas disk, distributing water to planets without direct collisions.
The study introduces a model that simulates the sublimation of icy asteroids over gigayear timescales, accounting for changes in the Sun’s luminosity. The resulting gas disk undergoes viscous diffusion, spreading inward and outward within the solar system. The model evaluates two scenarios: one where the asteroid belt began with its current mass and another where it started with a significantly higher mass and later depleted. The authors conclude that their mechanism could explain Earth’s water content and correct deuterium-to-hydrogen (D/H) ratio, with most water arriving between 20 and 30 million years after the Sun’s birth, coinciding with a sharp increase in solar luminosity.
The proposed mechanism is potentially universal, applying to exoplanetary systems where similar conditions may exist. For example, gaseous water disks could form from sublimating exo-asteroid belts, detectable using facilities such as the Atacama Large Millimeter/submillimeter Array (ALMA).
The researchers argue that viscous water transport is a more generic and inevitable process than impact-based delivery. They emphasize that most planetary systems likely possess the necessary conditions for this mechanism: a proto-planetary disk initially cold enough to form ice, followed by a shifting snow line that induces sublimation after the disk’s dissipation.
In their simulations, the authors demonstrate that large icy asteroids in the young asteroid belt would have released significant quantities of water vapor, forming a gaseous disk. This disk would spread due to viscous forces, with some gas accreting onto terrestrial planets. Their results indicate that this process could supply Earth with the equivalent of its current water mass, consistent with observed isotopic signatures.
The study also suggests that this mechanism could be observed in exoplanetary systems. For instance, observations of the star HD 69830 indicate the presence of water ice in an exo-asteroid belt, hinting at ongoing sublimation processes that could deliver water to orbiting exoplanets.
The researchers call for further observational studies using high-resolution instruments to detect water vapor disks in young exoplanetary systems. Such discoveries would provide valuable insights into planetary formation processes and the prevalence of water in habitable zones.
This alternative model challenges the traditional impact-based view of water delivery, offering a broader perspective on how planetary systems may distribute vital resources necessary for life.
