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Last update: August 29, 2025

Frost Line: The Boundary That Shaped the Planets

Diagram of the frost line in the Solar System

Frost Line: The Invisible Frontier That Sculpted the Solar System

The frost line, or snow line, represents the critical distance from the Sun at which volatile compounds such as water, ammonia, carbon dioxide, and methane can condense into ice. This thermodynamic boundary played a decisive role in the formation and differentiated composition of the planets.

The frost line is located approximately at 2.7 AU for water in the Solar System, but this value varies depending on the Sun's luminosity. It separates regions where solid matter is dominated by rocks from those where ice becomes abundant, influencing the size and composition of the planets. The position of the frost line is determined by the balance between the radiant flux of the young Sun and the ability of molecules to condense.

For water, the frost line is around 170 K (-103°C), the temperature at which water vapor condenses directly into ice in the vacuum of space. This critical temperature corresponds to a heliocentric distance of about 3 AU in the primordial Solar System.

Role in Planet Formation

The frost line fundamentally influenced the architecture of our Solar System:

Temporal Evolution of the Frost Line

The frost line is not fixed over time. Its position has evolved throughout the history of the Solar System depending on solar luminosity, the temperature of the protoplanetary disk, and internal dynamic processes. During the early stages of formation, when the Sun was less luminous, the frost line was closer to the Sun.

Over time, as solar luminosity increased and the disk gas dissipated, the frost line moved outward, changing the condensation zone of water and other volatile compounds. This migration had a direct impact on the mass that planets could accumulate and on the distribution of icy bodies.

This temporal evolution explains why some planets and satellites contain large amounts of ice despite their relatively close current position to the Sun, and why the distribution of icy bodies is not uniform.

Table: Comparison of Condensable Compounds

Condensation temperatures of major volatile compounds
Compound NameCompoundCondensation Temperature (K)Approximate Distance from the Sun (AU)Role
WaterH2O~170~2.7Formation of gas giants and comets
AmmoniaNH3~80~5Component of outer body ices
MethaneCH4~30~10Present in gas giants and icy satellites
Carbon dioxideCO2~70~4–5Constituent of comet and icy satellite ices
Carbon monoxideCO~20~15Present in comets and the outer protoplanetary disk
NitrogenN2~25~15–20Main constituent of Triton's atmosphere and some comets
ArgonAr~30~20Trace of rare ices in the outer Solar System

Sources: NASA Solar System Formation.

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