Frost line (astrophysics)

• 1. Introduction

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  The frost line is an invisible boundary inside the disk of gas and dust around a young star. Cross it, and water turns to ice and stays that way. Astronomers also call it the snow line or ice line. It marks the minimum distance from a protostar where temperatures drop low enough for volatile compounds to condense into solid grains. Those volatiles include water, ammonia, methane, carbon dioxide and carbon monoxide. The resulting grains can then accrete into planetesimals, the building blocks of planets. Beyond the line, abundant gaseous compounds condense easily enough to build gas giants and ice giants. Within it, only heavier compounds remain, forming the typically much smaller rocky planets. Where exactly this line sits, whether it stays in place, and what becomes of the ice trapped inside it are the questions ahead.

• 2. A Line With Many Names

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  In soil science, long before astronomers borrowed the term, a frost line meant something different: the maximum depth below the surface where groundwater can freeze. Every volatile substance in a solar nebula has its own frost line. Carbon monoxide, nitrogen and argon each condense at their own characteristic temperature, so each has a distance where it starts to freeze. Because of this, it matters which material's frost line is being discussed, though the qualifier is frequently dropped, especially for water. Some materials are hard to detect directly, so researchers rely on a tracer gas instead. Diazenylium, for instance, stands in for carbon monoxide when carbon monoxide itself is difficult to detect directly.

• 3. Locating The Line

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  In 1981, the astronomer Hayashi calculated the water ice frost line at 170 K, sitting 2.7 astronomical units from a young star. Later estimates moved the number: Podolak and Zucker, in 2010, put it anywhere from 143 K at 3.2 AU to 150 K at 3 AU. Martin and Livio, in 2012, arrived at a distance of 3.1 AU. D'Angelo and Podolak, in 2015, split the difference by grain size, finding roughly 150 K for micrometer-sized grains and roughly 200 K for kilometer-sized bodies. These numbers shift because different volatile compounds condense at different temperatures and partial pressures within the nebula. The frost line itself does not sit still. Over time, its radius for a solar-mass star can grow to a maximum of 17.4 before it begins to shrink again.

• 4. A Line In Motion

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  During the formation of the Solar System, the water frost line sat at about 5 astronomical units, a position current conditions no longer match. Back then, the solar nebula was an opaque cloud, keeping temperatures low even close to the Sun. The young Sun itself also gave off less energy than it does today. As the system took shape, ice already present got buried under falling dust and has stayed stable ever since, a few meters beneath the surface. Expose that buried ice today, say by a crater within 5 AU, and it sublimates away fairly quickly once sunlight reaches it. Out of direct sunlight, ice can persist on the surfaces of asteroids, the Moon and Mercury, provided it sits in a permanently shadowed polar crater. On the Moon, such craters can hold temperatures as low as 30-40 K for the entire age of the Solar System.

• 5. Evidence In The Asteroid Belt

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  The asteroid belt, orbiting between Mars and Jupiter, offers physical evidence for where the water frost line once stood. Researchers including Abe and colleagues in 2000, and separately Morbidelli and colleagues the same year, identified the outer asteroids as icy C-class objects. The inner asteroid belt, by contrast, is largely devoid of water. That split suggests the frost line sat at around 2.7 astronomical units from the Sun when planetesimals were forming. The dwarf planet Ceres, with a semi-major axis of 2.77 AU, lies almost exactly at that lower estimate. Ceres appears to carry an icy mantle and may even hide a water ocean beneath its surface. Water ice has also turned up on 24 Themis, an asteroid that orbits the Sun at an average distance of 3.1 AU.

• 6. Shaping The Planets

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  Earth orbits at less than a quarter of the distance to the Sun's frost line, yet it never became a giant planet. Around other stars, though, giant planets called hot Jupiters have been found sitting well inside their own frost lines, despite that boundary. Astronomers think these worlds formed farther out, beyond the line, and later migrated inward to their present orbits. Earth's gravity, meanwhile, is strong enough to hold onto methane, ammonia and water vapor rather than losing them to space. Methane and ammonia remain rare in Earth's atmosphere only because they are unstable in an oxygen-rich atmosphere. That atmosphere is a byproduct of photosynthesis, whose underlying chemistry implies both gases were once far more plentiful. Liquid water and ice, chemically stable in that same oxygen-rich air, now form much of Earth's surface.

• 7. Dust Rings And Belts

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  Rebecca Martin and Mario Livio proposed that asteroid belts tend to cluster near a star's frost line, disrupted from forming planets by nearby giant worlds. To test the idea, they analyzed the temperature of warm dust surrounding roughly 90 stars. In most cases, that dust, and any asteroid belt it implied, sat close to the frost line. The pair suggested an underlying mechanism: the frost line itself may be thermally unstable on timescales of 1,000-10,000 years. That instability could periodically deposit dust into narrow rings circling the star. Such a signature might still be detectable in future surveys of other planetary systems.