Evidence for the existence of 107 Pd in the early solar system

The concentration and isotopic composition of Ag and Pd were measured in the Santa Clara iron meteorite. This meteorite has Pd/Ag of about 10 4 . The isotopic composition of Pd is identical to terrestrial Pd within 0.1%. The 107 Ag/ 109 Ag ratio was 4% greater than the terrestrial value and suggests the in situ decay of 107 Pd(τ ½ = 6.5 × 10 6 y) in this meteorite or its parent body. From the observed 107 Ag*/ 110 Pd ratio (* ≡ excess) we infer an initial ( 107 Pd/ 110 Pd) ⊙ ⩾2 × 10 −5 . These data are incompatible with an interval of ∼ 10 8 y between the end of nucleosynthesis and the formation of planetary objects but are compatible with a last‐minute injection of material. The inferred existence of 107 Pd and 26 Al indicates that the late injection included freshly synthesized material of both intermediate and low atomic weight on a similar time scale. We estimate the fraction of freshly synthesized material to be between 10 −5 to 10 −4 in the region of Z ∼ 50. The 244 Pu and some of the 129 I present in the early solar system may be due to earlier contributions. The highly granular nature of stellar nucleosynthesis is made apparent by these considerations. The 107 Pd‐ 107 Ag chronometer is of particular importance as it can measure the time interval between the last injection of freshly synthesized material and the formation of differentiated planetary‐size objects. Our data suggest that the time scale for the differentiation of some planets into large scale metallic and silicate segregations is as short as ∼10 6 y.