next up previous
Next: Latent heat of crystallization Up: Key Physical Quantities in Previous: Thermal conductivity of an

Heating by decay of radioactive nuclides

Radioactive nuclides contained in silicate cores of grains heat a cometary nucleus when they decay. We assume that cometary silicate has similar abundance of radioactive nuclides as that of carbonaceous chondrites, which are believed to be one of the most pristine materials in the solar system (see Jessburger in this volume). Then the major species that contribute the heating are tex2html_wrap_inline1508 K, tex2html_wrap_inline1510 Th, tex2html_wrap_inline1512 U, and tex2html_wrap_inline1514 U. Table 4 lists their initial abundance tex2html_wrap_inline1516 (i.e. abundance at the time of formation of the solar system), decay rate tex2html_wrap_inline1518 (i.e. the probability of decay per unit time), and heat H released at decay per unit mass together with the heating rate tex2html_wrap_inline1522 per unit mass of the silicate.

It should be noted in Table 4 that tex2html_wrap_inline1508 K has the largest heating rate tex2html_wrap_inline1522 , and contributes most to the radiogenic heating when tex2html_wrap_inline1384 Al is not taken into account.

To see the effect of the radiogenic heating, let us estimate the temperature increase tex2html_wrap_inline1530 when heat conduction is ignored. Denoting specific heat of a cometary nucleus by tex2html_wrap_inline1532 , we have tex2html_wrap_inline1530 as:

eqnarray283

for tex2html_wrap_inline1536 , a typical value for slicates. This estimate indicates that, though only a small amount of heat is released per year, the radioactivity can heat a cometary nucleus to very high temperature for a long time on the order of tex2html_wrap_inline1538 yr if the heat loss is ignored.

tex2html_wrap_inline1384 Al is another possible heat source if its initial abundance was comparative with that in Ca-Al rich inclusion found some carbonaceous chondrites. The relevant data for tex2html_wrap_inline1384 Al are also listed in Table 4. The life time tex2html_wrap_inline1544 of tex2html_wrap_inline1384 Al is as short as tex2html_wrap_inline1548 yr from its nucleosynthesis, so tex2html_wrap_inline1384 Al must be incorporated within this time interval in order for tex2html_wrap_inline1384 Al-heating to be effective; otherwise tex2html_wrap_inline1384 Al would decay in the gas phase or in the grains before incorporation into a cometary nucleus. In the following discussion, we shall ignore tex2html_wrap_inline1384 Al. See Haruyama et al. (1993) for the effect of the tex2html_wrap_inline1384 Al-heating under the low thermal conductivity of amorphous ice.


next up previous
Next: Latent heat of crystallization Up: Key Physical Quantities in Previous: Thermal conductivity of an

Operator
Mon Sep 16 16:23:29 JST 1996