Thermochemical Equilibrium in Solar Nebula

In the first place, we must consider whether the kinetics of the chemical reactions are sufficiently rapid for the dust to reach chemical equilibrium with the gas of the nebula. The chemical kinetics of a gas of solar composition has

HELIOCENTRIC DISTANCE

Fig. 2.2. The same diagram as in Fig. 2.1 is enlarged here in the vicinity of the Earth, with a view to improving our knowledge of the temperature in the Earth's zone, when dust sedimented from the nebular gas. Of course, all clues on this temperature have been erased by the igneous differentiation of our planet. The best cosmothermometer for this epoch lies in bodies that have never differentiated in the asteroid belt. It tells us that at the time of sedimentation, all ordinary chondrites were at a temperature higher than 450 K, and all carbonaceous chondrites at a temperature lower than that. Reflection spectra and colors of asteroids tell us that 2.6 AU represents the separation distance. Since the temperature gradient was in the vicinity of-1, possibly -0.9, but definitely larger (in absolute value) than -0.75, it is concluded that the temperature in the Earth's zone was such that the dust did not contain any volatiles. All water remains as steam in the nebular gas, carbon was in CO, and nitrogen in gaseous N2. Hence the Earth accreted from an outgassed material, completely depleted of volatiles.

HELIOCENTRIC DISTANCE

Fig. 2.2. The same diagram as in Fig. 2.1 is enlarged here in the vicinity of the Earth, with a view to improving our knowledge of the temperature in the Earth's zone, when dust sedimented from the nebular gas. Of course, all clues on this temperature have been erased by the igneous differentiation of our planet. The best cosmothermometer for this epoch lies in bodies that have never differentiated in the asteroid belt. It tells us that at the time of sedimentation, all ordinary chondrites were at a temperature higher than 450 K, and all carbonaceous chondrites at a temperature lower than that. Reflection spectra and colors of asteroids tell us that 2.6 AU represents the separation distance. Since the temperature gradient was in the vicinity of-1, possibly -0.9, but definitely larger (in absolute value) than -0.75, it is concluded that the temperature in the Earth's zone was such that the dust did not contain any volatiles. All water remains as steam in the nebular gas, carbon was in CO, and nitrogen in gaseous N2. Hence the Earth accreted from an outgassed material, completely depleted of volatiles.

been very carefully and thoroughly discussed by Lewis et al. (1979) and Lewis and Prinn (1980).

The largest time constants that turn out to be significant are in the range of one century near 1,000 K. Since, after sedimentation to the midplane, the grains need 103-104 years to agglomerate into large objects (Weidenschilling, 1988), there is no doubt that thermochemical equilibrium will be reached in the Earth's zone before solids are completely separated from the gas phase. This means that all dust (which is originally in submicrometer-sized grains) has been completely dehydrated, degassed, and reduced (except a fraction of the silicate grains) before its incorporation into larger and larger planetesimals and their later integration into planetary bodies.

Lewis et al. (1979) have, in particular, considered the carbon problem, and they have rightly been puzzled by the retention of carbon in the terrestrial planets. Searching for a mechanism of carbon retention, they correctly deduce that the only way to imprison carbon in the solid phase is to put it in a solid solution inside reduced (metallic) iron grains. In spite of their efforts, they reach an amount two or three times smaller than the observed amount on the Earth (carbonates) or on Venus (CO2); they do not address the question on how to extract the carbon from iron and bring it to the surface of the Earth or Venus.

However, in their efforts to reach an amount of carbon retention in the grains as large as possible, their choice of the adiabat in the solar nebula has put the Earth's zone near the peak of graphite activity (see Fig. 2.3). For any adiabat, graphite activity goes through a maximum in the vicinity of the line separating the domains of CH4 and CO in Fig. 2.3. Bringing the Earth's zone here removes the 2.6 AU zone from the same location, whatever the model. Therefore, Lewis et al. (1979) quite reasonable efforts to bring enough carbon on the Earth have removed the possibility of explaining the observed separation of the C asteroids (carbonaceous chondrites) from the S asteroids (ordinary chondrites) at the distance of 2.6 AU.

Using Lewis et al. (1979) own data, if a proper adiabat (between C and CD on Figure 2.3) is used to get the 2.6 AU heliocentric distance close to the line separating CH4 and CO on Fig. 2.3, the amount of carbon available in solid solution in the iron grains becomes in the Earth's zone several hundred times smaller than the observed amounts in the carbonates of the Earth, and the situation is even much worse for the CO2 present on Venus.

Hence if we accept the accretion temperatures of chondrites as the best available cosmothermometer at the epoch of dust separation from gas, we have also to accept that the bulk of the observed carbon in the terrestrial planets has an exogenous origin.

The same conclusion can be reached for water, because at those temperatures no hydrated silicates exist, hence the total amount of water is in steam in the nebula; in the same fashion, all nitrogen is in gaseous N2 . The presence of the atmosphere and of the oceans on the Earth would therefore be quite mysterious, if the formation of the giant planets had not created the unavoidable mechanism that was going to put a veneer of volatile material on the terrestrial planets.

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