Wu Qin Tong, Anderson Hannah, Watkins Aurland K, Arora Devyani, Barnes Kennedy, Padovani Marco, Shingledecker Christopher N, Arumainayagam Christopher R, Battat James B R
Department of Chemistry, Wellesley College, Wellesley, Massachusetts 02481, United States.
INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 5, 50125 Firenze, Italy.
ACS Earth Space Chem. 2023 Dec 14;8(1):79-88. doi: 10.1021/acsearthspacechem.3c00259. eCollection 2024 Jan 18.
We demonstrate for the first time that Galactic cosmic rays with energies as high as ∼10 eV can trigger a cascade of low-energy (<20 eV) secondary electrons that could be a significant contributor to the interstellar synthesis of prebiotic molecules whose delivery by comets, meteorites, and interplanetary dust particles may have kick-started life on Earth. For the energetic processing of interstellar ice mantles inside dark, dense molecular clouds, we explore the relative importance of low-energy (<20 eV) secondary electrons-agents of radiation chemistry-and low-energy (<10 eV), nonionizing photons-instigators of photochemistry. Our calculations indicate fluxes of ∼10 electrons cm s for low-energy secondary electrons produced within interstellar ices due to attenuated Galactic cosmic-ray protons. Consequently, in certain star-forming regions where internal high-energy radiation sources produce ionization rates that are observed to be a thousand times greater than the typical interstellar Galactic ionization rate, the flux of low-energy secondary electrons should far exceed that of nonionizing photons. Because reaction cross sections can be several orders of magnitude larger for electrons than for photons, even in the absence of such enhancement, our calculations indicate that secondary low-energy (<20 eV) electrons are at least as significant as low-energy (<10 eV) nonionizing photons in the interstellar synthesis of prebiotic molecules. Most importantly, our results demonstrate the pressing need for explicitly incorporating low-energy electrons in current and future astrochemical simulations of cosmic ices. Such models are critically important for interpreting James Webb Space Telescope infrared measurements, which are currently being used to probe the origins of life by studying complex organic molecules found in ices near star-forming regions.
我们首次证明，能量高达~10 eV的银河系宇宙射线能够引发一系列低能量（<20 eV）的二次电子，这些二次电子可能是星际合成益生元分子的重要贡献者，彗星、陨石和行星际尘埃颗粒对这些分子的输送可能启动了地球上的生命。对于黑暗、致密分子云内部星际冰幔的高能处理，我们探讨了低能量（<20 eV）二次电子（辐射化学的媒介）和低能量（<10 eV）非电离光子（光化学的激发因素）的相对重要性。我们的计算表明，由于银河系宇宙射线质子的衰减，星际冰中产生的低能量二次电子通量约为10个电子·厘米⁻²·秒⁻¹。因此，在某些恒星形成区域，内部高能辐射源产生的电离率比典型的星际银河系电离率高一千倍，低能量二次电子的通量应远远超过非电离光子的通量。因为电子的反应截面可能比光子大几个数量级，即使没有这种增强，我们的计算表明，低能量（<20 eV）二次电子在星际合成益生元分子中至少与低能量（<10 eV）非电离光子一样重要。最重要的是，我们的结果表明迫切需要在当前和未来的宇宙冰天体化学模拟中明确纳入低能量电子。这样的模型对于解释詹姆斯·韦布空间望远镜的红外测量至关重要，目前该望远镜正通过研究在恒星形成区域附近的冰中发现的复杂有机分子来探索生命的起源。
