Snowline

Ice condensation in the solar nebula occurred outboard of the snowline, defined as the radial distance along the nebular mid-plane beyond which water ice was stable. Calculation of the snowline radius is complicated, and its exact position likely varied with time (Lunine, 2005). 

Ices formed as mantles on silicate grains in interstellar space, trapping noble gases and providing sites for the synthesis of organic compounds. As the solar system formed, these ices were vaporized, particularly in the warmer regions near the Sun. Water ice recondensed outside the snowline and combined with rocky material and surviving interstellar material to form planetesimals. 

S-complex asteroids, which include the older E, S, and M groups, dominate the inner and middle belt out to 2.95 AU, C-complex asteroids are most common in the outer belt, and X-complex bodies, which include the P and D classes, are most common at about 3 AU (Fig. 11.7b). Note that this distribution represents only a part of the main asteroid belt shown in Figure 11.7a. Because of uncertainties in the interpretation of S-complex objects as either ordinary chondrites or achondrites, we can no longer say that the innermost asteroids are differentiated but we can infer that S-complex bodies were at least heated (recall from Chapter 6 that ordinary chondrites are mostly metamorphosed). Cl and 2 chondrites have suffered extensive aqueous alteration, suggesting they formed beyond a snowline marking the condensation of ice that later melted that snowhne likely marks the transition to C-complex objects at about 3 AU. 

Although planetesimals that formed beyond the snowline are composed of relatively primitive materials (chondritic solids and ices), their compositions are variable. That should not be surprising, because objects now in the asteroid belt, the Kuiper belt, and the Oort cloud formed in different parts of the outer solar system and were assembled at different temperatures. In a systematic study of the spectra of 41 comets, A Heam el al. (1995) recognized two compositional groups, one depleted in carbon-chain (C2 and C3) compounds and the other undepleted (Fig. 12.18). NH compounds in the same comets show no discemable trend. The depleted group represents comets derived from the Kuiper belt, whereas the undepleted group consists of Oort cloud comets. 

Asteroids that formed beyond the snowline represent rock and ice accreted inside the orbit of Jupiter. The most distant asteroids may still contain ices, but many asteroids have been heated. Melting of ice produced aqueous fluids, which reacted with chondritic minerals at low temperatures to form secondary minerals (phyllosilicates, carbonates, sulfates, oxides). The alteration minerals can be discerned in asteroid spectra and characterized by analyses of chondrites derived from these bodies. 

Planet formation unfolds differently beyond the snowline, where water condensation enhances the surface density. Here massive cores (> 5-10 MEarth) may form rapid enough to accrete directly and retain nebular gas. These massive cores, if formed prior to the dispersal of the gas disk, rapidly reach Jupiter masses, forming giant planets. An alternative mechanism that may be responsible for the formation of some giant planets is gravitational instability in a massive, marginally unstable disk (e.g. Boss 2007 Mayer etal. 2007). 

The innermost radius in a protoplanetary disk where temperatures are low enough for a particular chemical species to exist as a solid has traditionally been termed the condensation front of that species. The most commonly used example of such a front is the snowline, which represents the point inside of which water exists as a vapor and outside of which it exists as ice. This term largely was used in static disk models where dynamical processes were ignored. 

Ices are present in the cold, outer disk region beyond the snowline, at 100 K. The major component is water ice, with an abundance of about 10-4 (relative to the total hydrogen density). Usually water ice is intermixed with other, more volatile ices of e.g. CO, CO2, NH3, CH4, H2CO, and HCOOH (Zasowski et al. 2007). Typical abundances of these minor constituents are about 0.5-10% of that of water. Trapping of the volatile ices depends on the crystallinity of the water ice and the history of thermal processing (Sonnentrucker et al. 2008). 

Figure 10.4 A simulation of oligarchic growth in the inner region of a proto-planetary disk around a solar-mass star. In the inner disk, embryos grow to 0.1 Earth-masses in <106 years. Growth then slows dramatically. Embryos continue to grow larger beyond the snowline at 2.5 AU. The simulation uses the semi-analytic model of Chambers (2008) with Esom ccl/a.

Earth and Mars clearly contain H2O. Venus s atmosphere is very dry, and composed mainly of CO2, but the high D/H ratio of the small amount of water present suggests Venus was once much wetter than today (Zahnle 1998). Mercury is perhaps too small and too close to the Sun to have acquired and retained water. Water may have been present in much of the material that accreted to form the Earth. Small amounts of water may have been adsorbed onto dust grains at 1 AU by physisorp-tion or chemisorption (Drake 2005). Once Jupiter formed, substantial amounts of water could have been delivered to the growing Earth in the form of planetesimals and planetary embryos from the Asteroid Belt (Morbidelli et al. 2000). It is also possible that Earth lay beyond the snowline at some point during the evolution of the solar nebula (Chiang et al. 2001) so that local planetesimals contained ice. 

The fringes of the Alps are much more humid than the central part. The climate of the northern slopes is vastly different from that of the southern slopes. Consequently the boundaries of soil and vegetation zones in the Alps shift like the snowline in interior parts of the mountains the snowline lies 3(X)-500 m above that in the outer zones. 

A glacier may be defined as a mass of ice that is formed from recrystallized snow and refrozen melt water, and that moves under the influence of gravity. Glaciers develop above the snowline, that is, in regions of the world that are cold enough to allow snow to remain on... 

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