Origins of Life

Musings from Nuclear Physics, Astrophysics and Astrobiology

Vlado Valkovic

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000470802
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

The primary purpose of this book is to prepare the ground for coordinated efforts aiming to answer the question: where and when life originated. The appearance of life involves three successive stages: i) the formation of chemical elements and their combination to simple molecules, which is the concern of physicists; ii) the evolution of organized complexity in biomolecules and their reactions, which falls within the field of chemistry; iii) the onset of Darwinian evolution after the appearance of the first cell-like structure, which is studied by biologists. This bookfocuses on the first two steps of this process with chapters exploring topics such as chemical element abundances; galaxies, galactic magnetic fields and cosmic rays; galactic chemical evolution.

Key Features:

• Contains extensive lists of reference and additional reading.

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• Includes new hypotheses concerning the origin of life.

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• Combines consideration from nuclear physics, astrophysics, astro- and geochemistry.

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Despite its interdisciplinary nature, this book remains accessible to nonexperts, and would be a valuable companion for both experts and laypeople.

1 Chemical Elements Abundances

DOI: 10.1201/9781003181330-1

1.1 Element Synthesis

1.1.1 Big Bang Nucleosynthesis

Big Bang nucleosynthesis (BBN) is very different from stellar nucleosynthesis that produces heavier elements. It is a non-equilibrium process that took place over a few minutes in an expanding radiation-dominated plasma with high entropy (10⁹ photons per baryon) and many free neutrons (Burles et al., 1999).

In the first 100 seconds after the Big Bang, when the Universe was very hot (T ≥ 10¹⁰K), any nucleus heavier than proton and neutron formed at this stage was immediately dissociated by high-energy photons. For example, deuterons produced from the reaction p + n → d + γ were destroyed in the reaction d + γ → p + n. When the temperature dropped to T ≈ 10⁹K, deuterons started to accumulate, and further reactions proceeded to produce more massive nuclei. This process lasted until ∼10³ seconds after the Big Bang, when almost all remaining free neutrons (with a half-life of about 14.7 minutes) have decayed via reaction n → p + e⁻ + ν_e. The process of nuclei production during this period is called Big Bang nucleosynthesis, and the produced nuclei are called primordial nuclei.

By now, it is accepted that the element synthesis in the Universe starts with the p–p chain illustrated by a sequence of nuclear reactions as follows:

$$\begin{array}{l}{}^{\text{1}}{\text{H +}}^{\text{1}}\text{H}{\to }^2{\text{H + e}}^{\text{+}}\text{+}{\nu }_{\text{e}}\\ {}^{\text{2}}{\text{H +}}^{\text{1}}\text{H}{\to }^3\text{He +}\gamma \\ {}^3{\text{He +}}^{\text{3}}\text{He}{\to }^4{\text{He +}}^{\text{1}}{\text{H +}}^{\text{1}}\text{H}\end{array}$$pp-1

or

$$\begin{array}{l}{}^3{\text{He +}}^{\text{4}}\text{He}{\to }^7\text{Be +}\gamma \\ {}^7\text{Be}+{\text{e}}^{-}{\to }^7\text{Li +}{\nu }_{\text{e}}\\ {}^7{\text{Li +}}^{\text{1}}\text{H}{\to }^4{\text{He +}}^{\text{4}}\text{He}\end{array}$$pp-2

or

$$\begin{array}{l}{}^7{\text{Be +}}^{\text{1}}\text{H}{\to }^8\text{B +}\gamma \\ {}^8\text{B}{\to }^8{\text{Be + e}}^{\text{+}}+{\nu }_{\text{e}}\\ {}^8\text{Be}{\to }^4\text{He}{+}^4\text{He}\end{array}$$pp-3

The relative importance of pp-1 and pp-2 chains (branching ratios) depends on conditions of H-burning (T, ρ and abundances). The transition from pp-1 to pp-2 occurs at temperatures above 1.3 × 10⁷ K. Above 3 × 10⁷ K, the pp-3 chain dominates over the other two, but another process takes over in this case.

The helium produced is called the “ash” of this thermonuclear “burning”. It cannot participate in the fusion reactions at these temperatures or even substantially higher temperatures. A nuclear physics prevents this process in our Universe; namely, stable isotopes (of any element) having atomic masses of 5 or 8 do not exist.

A step, which might be of interest, is

$${}^{\text{1}}{\text{H +}}^{\text{2}}\text{H}{\to }^3{\text{H + e}}^{\text{+}}\text{+}{\nu }_{\text{e}}$$

or

$${}^{\text{2}}{\text{H +}}^{\text{2}}\text{H}{\to }^3{\text{H +}}^{\text{1}}\text{H}\left(\text{Q=4.03 MeV}\right)$$

which can lead to one or more neutron-producing reactions

$${}^{\text{2}}{\text{H +}}^{\text{2}}\text{H}{\to }^3\text{He + n}\left(\text{Q=2.45 MeV}\right)$$

$${}^2{\text{H +}}^{\text{3}}\text{H}{\to }^4\text{He}\left(\text{3.5 MeV}\right)\text{+ n}\left(\text{14.1 MeV}\right)\left(\text{Q=17.590 MeV}\right)$$

$${}^3{\text{H +}}^{\text{3}}\text{H}{\to }^4\text{He + n + n}\left(\text{Q=11.3 MeV}\right)$$

The reaction ²H + ³H→⁴He + n is peaked in the total cross section (σ_T = 5 barns) at a deuteron energy of approximately 100 keV (= 1.16 × 10⁹ K), while the reaction ²H + ²H→³He + n is peaked in the total cross section at a deuteron energy of around 2.0 MeV.

The existence of the 400 keV resonance in ⁷Li in the reaction: ³H + ⁴He→⁷Li + γ (Alpher and Herman, 1950) could help overcome the non-existence of nuclei with atomic weights 5 and 8. However, no such resonance was found to exist, and attempts to bridge the gap at mass 5 were abandoned. Only at extremely high temperatures (order 10⁸ K) can this bottleneck be overcome by an unlikely reaction. The fusion of two ⁴He nuclei takes place at those temperatures, and the result is highly unstable ⁸Be. However, it is formed at a fast enough rate resulting in a minimal equilibrium concentration of ⁸Be at any moment (Salpeter, 1952). According to the BBN theory, the universal abundances of ²H,³He,⁴He and ⁷Li are fixed in the first few minutes, as schematically shown in Figure 1.1, indicating the relative mass fractions as a function of time and temperature (see, for example, Burles et al., 1999).

FIGURE 1.1 Schematic representation of the relative abundances of light elements as a function of temperature and time after the Big Bang.

The elemental composition of the Universe remained unchanged until the first stars were formed 100–250 million years later and until nucleosynthesis has commenced in stars. Note that stars are classified into seven main types in the order of decreasing temperatures: O, B, A, F, G, K and M. Therefore, for all of the light elements, systematic errors are a dominant limitation to the precision with which primordial abundances can be inferred.

In their report, Kneller and McLaughlin (2003) considered the possible contribution of the effect of bound di-neutron upon BBN. Although the bound state of two neutrons does not exist, the known effects of the final state interaction in reactions with three particles in the final state suggest that it may result in reactions of the type (²n, x) on some light nuclei.

1.1.2 Li–Be–B Abundance Depletion

During BBN, nearly all neutrons end bound in ⁴He, which is the most stable light element. In addition, heavier nuclei are not formed in any significant quantity. There are two reasons: the absence of stable nuclei with mass number 5 or 8 (which impedes nucleosynthesis via n + ⁴He, p + ⁴He or ⁴He + ⁴He reactions), and the large Coulomb barriers for other reactions. The solar system shows the very low relative abundance of Li, Be and B compared to other elements (see Figure 1.2).

FIGURE 1.2 Very low abundances of elements Li, Be and B in the solar system. (After Lamia 2013.)

The so-called cosmological lithium problem is one of the most important unresolved problems in nuclear astrophysics (Cyburt et al., 2004). It refers to the large discrepancy between the abundance prediction by the standard BBN theory for primordial ⁷Li and the value deduced from measurements, as seen in the so-called Spite plateau for halo stars. The predictions of the BBN theory successfully reproduce the observations of all primordial abundances except for ⁷Li. The abundance of Li isotope is overestimated by more than a factor of 3. Among the possible resolutions to this discrepancy are (i) ⁷Li depletion in the atmosphere of stars, (ii) systematic errors originating from the choice of stellar parameters – most notably the surface temperature – and (iii) systematic errors in the nuclear cross sections used in the nucleosynthesis calculations.

The primordial ⁷Li is mainly produced by β-decay of ⁷Be (t_½ = 53.2 days). Therefore, the abundance of ⁷Li is essentially determined by the production and destruction of ⁷Be. The results of the measurements of reactions on ⁷Be induced by charged particles rule out the removal of ⁷Be during BBN. The neutron-induced reactions can also play a role in the destruction of ⁷Be, in particular the ⁷Be(n, α)⁴He reaction in the energy range of interest for BBN, in particular between 20 and 100 keV (Barbagallo et al., 2014). Their calculations have shown that the ⁷Be(n, α)⁴He reaction may account for a substantial reduction in the primordial ⁷Li abundance (a factor of 2), thus partially solving the ⁷Li problem. On the other hand, Broggini et al. (2012) claim that it is unlikely that the ⁷B...