A Low Abundance of 135 Cs in the Early Solar System from Barium Isotopic Signatures of Volatile-depleted Meteorites

Abstract

Precise knowledge of the abundances of short-lived radionuclides at the start of the solar system leads to fundamental information about the stellar environment of solar system formation. Previous investigations of the short-lived system ( t 1/2  = 2.3 Ma) have resulted in a range of calculated initial amounts of 135 Cs, with most estimates elevated to a level that requires extraneous input of material to the protoplanetary disk. Such an array of proposed 135 Cs/ 133 Cs initial solar system values has severely restricted the system’s use as both a possible chronometer and as an informant about supernovae input. However, if 135 Cs was as abundant in the early solar system as previously proposed, the resulting deficits in its daughter product 135 Ba would be easily detectable in volatile-depleted parent bodies (i.e., having sub-chondritic Cs/Ba) from the very early solar system. In this work, we show that angrites and eucrites, which were volatile-depleted within ∼1 million years of the start of the solar system, do not possess deficits in 135 Ba compared to other planetary bodies. From this, we calculate an upper limit for the initial 135 Cs/ 133 Cs of 2.8 × 10 −6 , well below previous estimates. This significantly lower initial 135 Cs/ 133 Cs ratio now suggests that all of the 135 Cs present in the early solar system was inherited simply from galactic chemical evolution and no longer requires an addition from an external stellar source such as an asymptotic giant branch star or SN II, corroborating evidence from several other short-lived radionuclides.