Abiotic formation of organic molecules Mars rovers have found complex organic molecules in the ancient rocks exposed on the planet’s surface and methane in the modern atmosphere. It is unclear what processes produced these organics, with proposals including both biotic and abiotic sources. Steele et al . analyzed the nanoscale mineralogy of the Mars meteorite ALH 84001 and found evidence of organic synthesis driven by serpentinization and carbonation reactions that occurred during the aqueous alteration of basalt rock by hydrothermal fluids. The results demonstrate that abiotic production of organic molecules operated on Mars 4 billion years ago. —KTS

We present and analyze a three-dimensional (3D) volume of nanoscale helium bubbles in a tritium-exposed palladium alloy that we have reconstructed by transmission electron tomography. Helium nanobubbles commonly form within metals during exposure to radiation and radioactive substances. The radioactive decay of tritium stored in metal tritides often results in a high density of these nanoscale helium bubbles. A persistent question about the mechanisms of bubble nucleation and growth has been the role of lattice defects and impurities. To address this matter, we have determined the 3D positions of helium nanobubbles in a palladium–nickel alloy exposed to tritium for 3.8 years. We introduce methods to determine the 3D shapes, volumes, and spatial positions of helium bubbles as small as 1 nm within solids. We find that the size and spacing of observed nanobubbles are not correlated. Our results suggest that previous models, which hypothesize initial, rapid homogeneous nucleation of nanobubbles followed by diffusion-limited growth as helium atoms join the nearest bubble, are inadequate. We propose that the lack of size and spacing correlation is due to traps of atomic helium in the metal lattice that allow bubbles to nucleate even at low average helium concentration. This work will facilitate the development of high-fidelity models of helium nanobubble formation in radiation-exposed metals.

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