Frost line or ice line defines the boundary where simple molecules condense (dihydrogen H2, dinitrogen N2, dichlorine Cl2, water H2O, ammonia NH3, hydrogen sulfide H2S, carbon dioxide CO2, methane CH4, ethane C2H6). This line is a little less than 5 au (≈ 700 million km) from the Sun, well beyond the asteroid belt and just before the orbit of Jupiter. It marks the clear separation between the terrestrial planets and the gas planets. The event happens there is 4.5 billion years in the vicinity of a spiral arm of the Galaxy. In a fragment Nebula turning opaque gas, small clusters are formed by accretion. Among them, our future Sun emerges when his companions remain united and disperse in the Milky Way. Amid the collapse still gaseous, a protostar grows and turns in the center of a disc perpendicular to its axis of rotation. Assisted by the force of gravity, it contracts and will capture 99.86% of the total mass of the cloud. The central temperature increases and the cloud will reach temperatures of several million degrees kelvin. This warming of the heart will trigger the Getting Started thermonuclear reactor. In this phase the protons combine releasing energy under the effect of the nuclear force. It is the fusion of hydrogen into helium, which stops the contraction of the star and stabilizes its volume.
Our Sun was born!
The rest of the starting gaseous nebula, whose composition is identical to that of the Sun, continues to lose heat. It reaches the temperature at which certain chemical compounds are not stable in the gaseous state. These compounds will therefore condense, not into solid but into liquid, since the pressure is very low.
The nebula thus charge itself, solid particles of dust called condensates Solid beads, chemical compounds, and in the mineralogical condensed birth in the nebulae as a result of is called: the sequence of condensation. The first compounds that condense at 1300 ° C, are rich in oxides of titanium, aluminum and calcium. Toward 1050 ° C condense heavily metallic iron and then towards 950 ° C, in the first silicate, the magnesium silicate and iron. Toward 800 ° C, are forming silicates to stretched structure, feldspars and iron sulfide. At still lower temperatures are condensing a silicate containing water at 0 ° C and the water condenses into ice.. These are grains tossed by gravitation which will give birth to objects more bigger. The Sun was formed by the accretion of the central material attractive to it all the nearby gas and planets are formed on the disk from multiple collisions of solid particles and matter relics and remaining gas. This model of the nebula slowly turning on itself and collapses under the effect of gravity was first proposed in 1734 by Emanuel Swedenborg (1688-1772), followed in 1755 by Immanuel Kant (1724-1804) and explained in 1796 by Pierre-Simon Laplace (1749-1827). Planetary orbits are nearly coplanar (on the same plane), circular, concentric and planets all revolve in the same direction, which is also that of the Sun's rotation on itself.
In the protoplanetary disk, objects are formed in accordance with the abundances of chemical elements in the nebula. Hydrogen is preponderant (≈ 75% of the mass), followed by helium (≈ 23%), other elements (≈ 2%) are all less abundant heavy they are.
Near the Sun, below the line of ice were heavier elements such as silicates and metals. The high temperature does not allow condensation (from gaseous to solid state) of light molecules such as water, ammonia, hydrogen, carbon dioxide or methane sulfide. So at this point of the disc are formed small terrestrial objects of low mass (Mercury, Venus, Earth, Mars, asteroids). Scarce, the solid matter available has generated only small objects.
Beyond the line of ice, the temperature drops below 260 k and the water molecules, ammonia, hydrogen sulfide, carbon dioxide and methane are condensed. Thanks to the contribution of the ice, the amount of solid matter available is then more important. So at this point of the disc are formed large solid cores of more than 10 Earth masses. These rings were so massive that they have attracted the remaining gas of the nebula. The gravitational collapse on large nuclei, surrounding gas mostly hydrogen and helium, formed the giant planets (Jupiter, Saturn, Uranus, Neptune). Around these small systems, many small icy objects (satellites) were able to regroup. Jupiter attracted so gas that eventually reached 318 Earth masses. If Saturn has a mass substantially lower (95 Earth masses) is simply that it was formed a few million years after Jupiter, while there was less gas available. In this process of condensation, water relatively abundant, has played a key role among simple molecules, the water molecule is the first to condense when the temperature decreases (see picture). The condensation of water around 260 K, marks the limit of the "ice line" that separates the terrestrial planets of the gaseous planets.
NB: The temperature in the asteroid belt varies with the distance from the Sun. On the inner edge of the belt about the asteroid 2.2 AU, the dust particles have temperatures of about 200 K (-73 ° C), on the outer edge of the belt to about 3.2 ua, dust particles have temperatures of about 165 K (-108 ° C).
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Image: Saturation curves of some simple molecules such as water, ammonia, hydrogen sulfide, carbon dioxide or methane. From right to left: H2O, NH3, H2S, CO2, CH4. Water is the first molecule to condense when the temperature decreases to around 260K (-13 ° C). So the condensation of water, around 260 K, which marks the boundary of the "ice line" that separates the terrestrial planets of the giant planets. Credit: Thérèse Encrenaz, Ann. Rev. Astron. Astrophys. 46, 57, 2008.
Image: Decreasing abundance (from warmer to colder), chemical elements in the proto-solar nebula that condensed, there are 4.5 billion years.