Photoemission of spin-polarized electrons from aligned grains and chiral symmetry breaking

Abstract

The unique biosignature of life on Earth is the homochirality of organic compounds such as amino acids, proteins, and sugars. High-energy spin-polarized (spin-up or spin-down) electrons (SPEs) from the $\beta$ decay of radioactive nuclei were proposed as a source of symmetry breaking, leading to homochirality; however, their exact role is much debated. Here we propose magnetically aligned dust grains as a new source of SPEs due to photoemission of electrons having aligned spins by the Barnett effect. For the interstellar UV radiation field of strength $G_{\rm UV}$, we found that the SPE emission rate is $\Gamma_{\rm pe}^{\rm SPE}\sim 10^{-14}G_{\rm UV}$ electrons per second per H, the fraction of spin-polarized to total photoelectrons is $\sim 10\%$, and the SPE yield (photoelectron number per UV photon) can reach $\sim 1\%$, using the modern theory of grain alignment. SPEs emitted from aligned grains could play an important role in chiral-induced spin selectivity-driven reduction chemistry in the icy grain mantles, producing an enantiomer excess of chiral molecules formed on the grain mantle. Finally, we suggest magnetically aligned grains could directly impact the enantioselectivity through the chiral-induced spin-selective adsorption effect and exchange interaction. We estimated the disalignment of electron spins and depolarization by elastic scattering using the Mott theory and found that these effects are negligible for low-energy SPEs, so that the spins of SPEs remain well aligned during their jurney through dust grains and the gas. Our proposed mechanism might explain the chiral asymmetry of prebiotic molecules detected in numerous comets, asteroids, and meteorites.