Radiogenic heating of comets by Al-26 and implications for their time of formation

The effect of radiogenic heating on the thermal evolution of spherical icy bodies with radii between 1 km and 100 km was investigated by means of numerical simulations. The isotopes considered were Al-26, K-40, Th-232, U-235, and U-238; the Al-26 abundance was varied, while that of the others was kept constant. The initial models were homogeneous ice spheres with uniform temperatures of 10 K. It is found that the influence of Al-26 dominates the effect of other radioactive species for bodies with radii up to about 50 km. If the ice is retained in amorphous form, an upper limit of about 4 x 10 to the -9th is obtained for the initial Al-26 abundance in comets, a factor of 100 lower than that of inclusions in the Allende meteorite. A lower limit for the formation time of comets may thus be derived. The possibility of molten cometary cores and extended amorphous ice mantles is ruled out. 36 references