Definition of the Subject
Astrobiology is concerned with questions regarding possible origins of life on Earth and elsewherein the Universe. Although to date there has beenno detection of extraterrestrial life, it is generally assumed that life could be widespread, provided certain conditions of habitability are met.A common implicit hypothesis in astrobiology is that life can emerge spontaneously once certain environmental conditions are met. This implies thatthere may well have been multiple geneses, separated only by global extinction events, such as major impacts by other celestial bodies [11].
Four important discoveries can be named that have provided impetus to the field of astrobiology.
- 1.
More than 300 extrasolar planets have been discovered since 1995, providing explicit targets for detecting life outside the solar system.
- 2.
Recent Mars missions have provided evidence for liquid water on the surface of Mars in the past and possibly even in the present time. This has fostered...
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Notes
- 1.
Under the assumption of local isotropy (Eddington approximation) radiative equilibrium implies that the flux is proportional to the negative gradient of the radiative energy density aT 4, where a is the radiation‐density constant, so
$$ \boldsymbol{F} =-\frac{1}{3} \text{c} \ell\nabla(aT^4 ) \,. $$Here, c is the speed of light and ℓ is the mean free path of photons. The latter can be expressed in terms of the opacity κ and the density ρ as \( { \ell=(\kappa\rho)^{-1} } \). Hydrostatic equilibrium can be written in the form
$$ \boldsymbol{g}=-{{\mathcal R}\over\mu}\nabla T \,, $$where g is the gravitational acceleration, \( { {\mathcal R} } \) is the universal gas constant, and μ is the mean molecular weight. These equations can be solved by a polytrope, i. e., \( { T=T_0(1-z/H) } \) and \( { \rho=\rho_0(1-z/H)^3 } \), where z is the distance from the surface and H is the vertical pressure scale height. This leads to a condition of the form Eq. (48) where \( { \ell_\mathrm{crit}=3H/16\approx0.19H } \) is the critical mean free path of photons
- 2.
Hydrostatic equilibrium can be written as \( -\rho^{-1}\nabla p-\nabla\phi=0 \), where p is the pressure and \( \phi=gz+\mathrm{const} \) is the gravitational potential. Using the thermodynamic relation \( -\rho^{-1}\nabla p=-\nabla h+T\nabla s=0 \), where \( h=c_{\text{p}}T \) is the specific enthalpy and \( \nabla s=0 \) for adiabatic stratification, we have \( \text{d} (c_{\text{p}}T)/\text{d} z=g \)
Abbreviations
- Chiral, achiral and racemic:
-
A molecule is chiral if its three‐dimensional structure is different from its mirror image. Such molecules tend to be optically active and turn the polarization plane of linearly polarized light in the right- or left-handed sense. Correspondingly, they are referred to as d- and l-forms, which stand for dextrorotatory and levorotatory molecules. An achiral molecule is mirror-symmetric and does not have this property. A substance is racemic if it consists of equally many left- and right-handed molecules. A polymer is said to be isotactic if all its elements have the same chirality.
- Enantiomers and enantiomeric excess:
-
Enantiomers are a pair of chiral molecules that have opposite handedness, but are otherwise identical. Enantiomeric excess, usually abbreviated as e.e., is a normalized measure of the degree by which one handedness dominates over the other one. It is defined as the ratio of the difference to the sum of the two concentrations, so e.e. always falls between −1 and +1.
- Epimerization and racemization:
-
Epimerization is a spontaneous change of handedness of one sub-unit in a polymer. Racemization indicates the loss of a preferred handedness in a substance.
- Catalysis and auto‐catalysis:
-
Catalysts are agents that lower the reaction barrier. A molecule reacts with the catalyst, but at the end of the reaction, the catalyst emerges unchanged. This is called catalysis. In auto‐catalysis the catalyst is a target molecule itself, so this process leads to exponential amplification of the concentration of this molecule by using some substrate. Biological catalysts are referred to as enzymes.
- Nucleotides and nucleic acids:
-
Nucleotides are monomers of nucleic acids, e. g., of RNA (ribonucleic acid) or DNA (deoxyribonucleic acid). They contain one of four nucleobases (often just called bases) that can pair in a specific way. Nucleotides can form polymers, and their sequence carries genetic information. One speaks about a polycondensation reaction instead of polymerization because one water molecule is removed in this step. Other nucleotides of interest include peptide nucleic acid or PNA. Here the backbone is made of peptides instead of sugar phosphate.
- Peptides and amino acids:
-
Amino acids are molecules of the general form NH3-CHR-COOH, where R stands for the rest, which makes the difference between different amino acids. For glycine, the simplest amino acid, we have R=H, so two of the bonds on the central C atom are the same and the molecule is, therefore, chiral. For alanine, R=CH3, so all four bonds on the central C atom are different, therefore this molecule is chiral. A peptide is a polymer generated through a polycondensation reaction of amino acids. Peptides are also referred to as proteins.
- Solar constant and albedo:
-
The solar constant is the total flux of energy from the Sun above the Earth's atmosphere. Its current value is \( { S=1.37\,\mathrm{kW}\,\mathrm{m}^{-2} } \), but it was about 30% lower when the solar system was young (108 yr ago, say), so S is not a constant. The albedo A is the fraction of the Sun's energy that is reflected from the surface of the Earth, e. g., by clouds and snow and, to a lesser extent, by land masses and oceans.
- Photosynthesis and carbon fixation:
-
Photosynthesis uses light to reduce CO2 and to produce oxygen either as free molecular oxygen or in some other chemical form. This process removes CO2 from the atmosphere and produces biomass, which is written in simplistic form as (CH2O) n . This process is referred to as carbon fixation.
- Life:
-
A preliminary definition of life involves replication and death, coupled to a metabolism that utilizes any sort of available energy. Life is characterized further by natural selection to adapt to environmental changes and to utilize available niches. A proper definition of life is difficult given that all life on Earth can be traced back to a single common ancestor. Any definition of life may need to be adjusted if extraterrestrial or artificial life is discovered.
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Brandenburg, A. (2009). Exobiology (theoretical), Complexity in. In: Meyers, R. (eds) Encyclopedia of Complexity and Systems Science. Springer, New York, NY. https://doi.org/10.1007/978-0-387-30440-3_195
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