The chemistry of the extraterrestrial carbonaceous stone “Hypatia”: A perspective on dust heterogeneity in interstellar space

An extraterrestrial origin for a carbonaceous, diamond-bearing and silicate-free stone named “Hypatia”, found in 1996 in southwest Egypt, was determined in 2013 and confirmed in 2015 on the basis of argon, helium, neon, xenon and nitrogen isotope characteristics. Petrography and Raman spectroscopy has indicated that the parent body was primitive, i. e. not thermally processed in the solar nebula. Here we report the results of in situ chemical analyses of this object using proton induced X-ray emission (PIXE) spectroscopy. The stone has a bimodal matrix. Type 1 is essentially devoid of elements heavier than oxygen, whereas matrix 2 shows a unique, consistent element abundance pattern from aluminum to zinc, up to a sum of 4.5 wt%. Fe is dominant and Si markedly depleted, with a CI-chondrite normalized Si/Fe ratio of c. 0.1. Element abundance ratios to Fe show several correlations, with one group (Al, Si, K, Ca, Ti and Cu) being negatively correlated to another one (P, S and Ni). Since the parent body was primitive, magmatic differentiation cannot be invoked to account for any part of this dataset. Considering published Raman spectroscopic data and interstellar dust models, we hypothesize that Hypatia's parent body consisted of interstellar dust. However, Hypatia's matrix 2 chemistry does not fit predictions for the average interstellar dust composition based on absorption spectroscopic studies of element depletions in gas along stellar lines of sight. Instead, it shows similarities to the nucleosynthesis yields of specific supernova Ia models, whereby the element abundance correlations and the strong depletion of Si may be understood as resulting from nuclear photodisintegration. It has been known for decades that the chemistry of interstellar space is made up of many components, each bearing the signature of the nucleosynthesis processes giving rise to them. Now it appears from the case of Hypatia that an individual component was preserved in dust, and delivered to a region of the solar nebula where mixing did not obliterate it prior to the coagulation of larger bodies.