In order to find the temperature of the dust when it sedimented out of the nebular gas, we must find a cosmothermometer working at the right time and place. We know that igneous differentiation processes have erased any fossil trace of this temperature on the Earth and, for that matter, on all the planets. Chondrites are the only undifferentiated objects known in our neighborhood. Hence we have no choice and must use chondrites as cosmothermometers.
We mentioned earlier that chondrites come from the asteroid belt. From the reflection spectra of asteroids in the visible and in the infrared, there is consensus among astronomers that (dark) carbonaceous chondrites come from (dark) C asteroids, and (light) ordinary chondrites come from (light) S asteroids (Gaffey and McCord, 1979). But asteroids C outnumber asteroids S only beyond 2.6 AU (Morrison, 1977). It is true that the distribution of S asteroids extends beyond 2.6 AU, but their fraction falls off with a scale length of 0.5 AU (Zellner, 1979). The present distribution can be interpreted as resulting from the secular orbital diffusion through the original 2.6 AU limit, which was sharper 4.5 billion years ago.
To know the accretion temperature of chondrites, the best cosmother-mometer is probably the partial loss of their volatile metals Pb, Bi, Tl, and In, which causes their fractionation. Using these latter elements, Larimer (1967) and Anders (1971) find, for a nebular pressure of 10~4 bars, a mean accretion temperature of 510 K for ordinary chondrites and of 450 K for the C3 type of carbonaceous chondrites (the other types of carbonaceous chondrites give slightly lower temperatures). If we take into account lower pressures used in recent models of the nebula, these values would become 480 and 430 K. Several other assessments are consistent with adopting a 450 K temperature to separate the two classes.
We can deduce that, at the epoch of dust sedimentation from the nebular gas, the temperature at 2.6 AU, in the central plane of the nebula, was 450 K. Since the temperature gradient predicted by theory for that epoch has been empirically confirmed, we conclude that the accretion temperatures of the planetesimals that were going to form the Earth were between 900 (at 1.3 AU) and 1,400 K (at 0.8 AU), during the 10 thousand years needed to grow from dust to meter- and kilometer-sized bodies; this removes them from thermochemical equilibrium with the nebular gas. Figure 2.2 summarizes the situation that we have just described.
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