Water Origin Enigma Solved

Earth Begun Development Beyond the “Frost Line”

“Why Earth has water is a mystery, for “especially early on in the solar nebula, the area where the Earth formed was too hot for water to incorporate into a solid body,” Ciesla said. Meteorites may have delivered at least part of Earth’s water, although that remains open to debate, he added.

The scenario also suggests how so much organic material has survived in the carbonaceous chondrite meteorites. If water reacted with the fine dust in the solar nebula as the new research suggests, temperatures in the meteorites would have remained low enough for organic molecules to survive and be delivered, along with water, to Earth.

Although the idea that shock waves formed the hydrated rock and chondrules found in the most primitive meteorites stands up to quantitative analysis, scientists are still speculating about where the shock waves come from.”



“Although the outer solar system contains many tens of Earth masses of water, located within the giant planets, their moons, and objects in the Kuiper belt and Oort cloud, it is the water in the terrestrial-planet region that has garnered the most persistent interest from the astrobiological point of view [despite the possibility of a subsurface ocean on Europa (Chyba et al., 1998)]. The roughly 0.001 M of water in the Earth’s crust and mantle is the critical factor in the existence of life — together with a surface temperature salubrious for liquid water. Evidence continues to accumulate that Mars once possessed a surface water budget equivalent to somewhere between 0.01 and 1 times that of Earth (Baker,2001). The highly enriched deuterium abun-dance in the atmosphere of Venus suggests that this nearly Earth-sized planet once contained amounts of water com-parable to Earth (Hunten et al., 1989). Thus, although the amounts of water are small relative to those in the outer solar system, the implications for the question of planetary habitability are large, and hence the question of the origin of water on the terrestrial planets deserves close attention.

It is possible, based on the temperature profiles shown in Fig. 1, that water was directly available in the form of ice or water of hydration in the nebula at or near 1 AU when grains were still small. However, temperature profiles low enough for pure ice to condense are extreme ones, and likely existed (if at all) in the very late stages of disk accretion, when silicate bodies were already large (kilometer-sized or larger). The constraints on the temperature profile are less severe for water of hydration, but as discussed in section 2, doubts have been raised as to whether hydration of silicates could have occurred in the nebula. Were hydration to be widespread, we might expect to find a chondritic composition for planetesimals in the 1 AU region, and hence for Earth itself. However, Earth itself cannot contain more than a few percent, perhaps only 1%, of chondritic material (Drake and Righter, 2002). An alternative is that small bodies migrated inward from a zone of hydration at 2–3 AU via gas drag in the disk and retained sufficient water to supply Earth’s inventory (Ciesla et al., 2004). Adsorption of water vapor onto dry silicate grains that have the isotopic and ele-mental composition of Earth is also possible, but the grains must have a highly fractal nature in order to adsorb suffi-cient water (Stimpfl et al.,2004). Both the adsorption and the inward migration model create constraints on the prop-erties of planetesimals at the time of the growth of Earth (size distribution, porosity, ambient gas density) that have yet to be explored in detail.”



“In astronomy or planetary science, the frost line, also known as the snow line or ice line, refers to a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains. Depending on density, that temperature is estimated to be about 150 K. The frost line of the Solar System is around 5 AU.”

“The lower temperature in the nebula beyond the frost line makes many more solid grains available for accretion into planetesimals and eventually planets. The frost line therefore separates terrestrial planets from jovian planets in the Solar System. However, gas giant planets have been found inside the frost line around several other stars (so-called hot Jupiters). They are thought to have formed outside the frost line, and later migrated inwards to their current positions.”



“Beyond the asteroid belt, water ice is abundant. It is a minor component of Jupiter’s moon Europa, but constitutes almost half the mass of Jupiter’s moons Ganymede and Callisto and Saturn’s moon Titan. It is the dominant, or at least key, constituent of the intermediate-sized moons of Saturn (e.g., Enceladus), the moons of Uranus, Neptune’s moon Triton, and the Kuiper belt object Pluto and its moon Chiron.”


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