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. 2016 Aug;16(8):561-653.
doi: 10.1089/ast.2015.1460.

The Astrobiology Primer v2.0

Free PMC article

The Astrobiology Primer v2.0

Shawn D Domagal-Goldman et al. Astrobiology. .
Free PMC article

Erratum in


<b>FIG. 1.</b>
FIG. 1.
A periodic table showing the major nucleosynthetic sources of naturally occurring elements. The three lightest elements formed mostly in the first three minutes after the Big Bang (Big Bang nucleosynthesis). Other lightweight elements formed in fusion reactions in stars, and these elements in turn served as fuel for the production of even heavier elements in larger stars. Slow neutron capture in massive stars synthesized elements beyond iron. Still heavier elements formed in explosive environments, such as in a supernova, through a rapid neutron-capture reaction. (Credit: Aditya Chopra)
<b>FIG. 2.</b>
FIG. 2.
Chronology of nucleosynthesis in the Universe. (Credit: Aditya Chopra)
<b>FIG. 3.</b>
FIG. 3.
Elemental composition of the Sun (a typical star) by mass. The word “metal” here is used in the way commonly applied to stellar compositions and includes all elements heavier than H and He. (Credit: Martin Asplund)
<b>FIG. 4.</b>
FIG. 4.
The abundance of elements (by number of atoms normalized to silicon) in Earth's crust. The crust is depleted in siderophilic (“iron-loving”) elements relative to the bulk Earth as siderophiles were partitioned into the Earth's core. Compared with the mantle, Earth's crust is enriched in lithophilic (rock-forming) elements. (Credit: Aditya Chopra)
<b>FIG. 5.</b>
FIG. 5.
Onionlike structure caused by progressive nucleosynthesis in stars. Nearest to the core are the sites of the hottest and heaviest nuclear fusion, while regions farther away near the surface host the coolest and lightest nuclear fusion reactions. (Credit: NASA)
<b>FIG. 6.</b>
FIG. 6.
Life cycle of stars, for both Sun-like stars and massive stars. (Credits: NASA and the Night Sky Network)
<b>FIG. 7.</b>
FIG. 7.
General schemes of possible stages of prebiotic evolution. (Credit: K. Adamala, adapted from scenario proposed by Eigen and Schuster, 1982)
<b>FIG. 8.</b>
FIG. 8.
Amino acid synthesis in the Strecker reaction.
<b>FIG. 9.</b>
FIG. 9.
Example of a prebiotic sugar synthesis: elements of the formose reaction system.
<b>FIG. 10.</b>
FIG. 10.
Phosphonic acid synthesis based on inorganic sources of phosphorous; pathway proved by the existence of vinyl phosphonic acid in the Murchison meteorite (De Graaf et al., 1997).
<b>FIG. 11.</b>
FIG. 11.
Examples of prebiotic syntheses of major organic molecules. 1: nucleobase synthesis, a: formamide condensation (Saladino et al., 2001), b: cyanoacetaldehyde condensation (Robertson et al., 1996); 2: amino acid synthesis, c: Strecker reaction (Li et al., 2003); 3: lipid synthesis, d: Fisher-Tropsch-type syntheses on mineral catalysts (McCollom et al., 1999); 4: phosphorylation of organic compounds, e: inorganic phosphates yield phosphonic acid derivatives (De Graaf et al., 1997); 5: carbohydrate synthesis, f: formose reaction (Breslow, 1959). Red: nucleic acid building blocks; blue: peptide building block; green: membrane building blocks.
<b>FIG. 12.</b>
FIG. 12.
Non-enzymatic template-directed RNA synthesis (Joyce et al., 1984). The incoming base (green), activated with 2-methylimidazole (red), binds to the template region (blue).
<b>FIG. 13.</b>
FIG. 13.
Nodes and branches compose a phylogenetic tree. Taxonomic units are represented by modern organisms located at the tips/leaves of branches. Nodes are the connection points before the divergence and represent common ancestors before bifurcation of two species. Branches can be flipped freely about the nodes; therefore the determination of which taxon appears in which branch of the node is arbitrary. There are various visual representations of trees; shown here are the (a) rooted “rectangular phylogram,” (b) rooted “rectangular cladogram,” and (c) bifurcating unrooted tree. The branch lengths are proportional to evolutionary distance in phylograms. In cladograms, all branches are equal in length. Unrooted trees specify taxonomic relationships without making assumptions about the evolutionary path. (Credit: Betül K. Kaçar)
<b>FIG. 14.</b>
FIG. 14.
Energy for metabolism is provided by pairing oxidation reactions (in the gray shaded portion of the figure) with reduction reactions (in the nonshaded area). Electrons are donated by the reductants (left) to the oxidants (right). The energy made available through electron transfer is determined by the relative electron affinities of the reaction pairs, portrayed schematically in this figure as vertical distance: the greater the difference in electron affinity between reaction pairs, the more energy there is to do work. Note that here only a small number of known possibilities are shown, and any thermodynamically favorable redox couple is possible to do metabolic work. (Credits: Jeff Bowman and Shawn McGlynn)
<b>FIG. 15.</b>
FIG. 15.
Comparison of the atmosphere of Earth (Rakhecha and Singh, 2009) with its neighbors Mars and Venus (William, 2011). Note the similarity in relative composition in the atmosphere of Venus and Mars versus the biologically cycled atmosphere of Earth. (Credit: Carie Frantz)
<b>FIG. 16.</b>
FIG. 16.
Energy delivery in a simple circuit and a similar conceptual energy delivery in an organism. (Credit: Kennda Lynch)
<b>FIG. 17.</b>
FIG. 17.
Top equation: electrical power calculation, where I = current and V = voltage. Bottom equation: maintenance energy calculation, where J = substrate utilization flux and ΔG = Gibbs free energy of the given redox reaction. (Credit: Kennda Lynch)
<b>FIG. 18.</b>
FIG. 18.
The habitable zone distance changes as a function of stellar temperature (vertical axis) and the amount of energy from the star that hits the planet (horizontal axis). This plot shows the limits for both the “conservative habitable zone,” which are based on one-dimensional climate model calculations (Kopparapu et al., 2013), and for the “optimistic habitable zone,” which are based on observations that Mars once had liquid water at the surface and Venus used to have more water, possibly contained in oceans. (Credit: Chester “Sonny” Harman, using planet images published by the Planetary Habitability Lab at Aricebo, NASA, and JPL)
<b>FIG. 19.</b>
FIG. 19.
Overview of important astrobiology targets on Mars based on the possibility of past or present water, here represented on a simplified cross-sectional profile from the south (left) to the north (right) pole. The features are generalized and not to scale. Dark shading with diffuse outline marks the locations where melted water may have resided in pore spaces in the past. Topographic depressions of gullies and valleys are marked with dotted lines. (Credit: Akos Kereszturi)
<b>FIG. 20.</b>
FIG. 20.
Timeline of major geological time periods and events for Venus (Section 6.1), Earth (Chapter 4), and Mars. *The Late Heavy Bombardment is thought to have brought a large flux of impactors to all three bodies (Johnson and Melosh, ; see Section 3.2). (Credits: Akos Kereszturi and Kelsi Singer, with input from Colin Goldblatt)
<b>FIG. 21.</b>
FIG. 21.
(Top) Predicted interior structure of Europa, showing it is composed of (from the inside out) a metallic core, a rocky mantle, a global subsurface ocean and outer ice shell. (Bottom) Close-up of the outer ∼100–150 km of Europa illustrating the global ocean and overlying ice shell (kilometers to tens of kilometers thick). This ocean is potentially heated by hydrothermal activity and may be an abode for life. Europa is the fourth-largest satellite of Jupiter, with a radius of ∼1565 km, and it has a surface area similar to that of the continent of Africa. (Credit: NASA/JPL)
<b>FIG. 22.</b>
FIG. 22.
(Left) Titan, as seen by the Cassini spacecraft in UV and IR wavelengths, which revealed the surface through Titan's dense atmospheric haze. (Right) The surface of Titan as viewed by the Huygens descent probe. The rounded appearance of the pebbles is consistent with long-distance transport along a riverbed (Tomasko et al., 2005). The surface area of Titan is approximately twice that of the continent of Asia. (Credits: NASA/JPL/ESA/Space Science Institute/University of Arizona)
<b>FIG. 23.</b>
FIG. 23.
A cross-section of the south pole of Enceladus that depicts the differentiated (layered) internal structure. Hydrothermal vents at the water-rock interface could provide habitable conditions by creating energy gradients analogous to vents on the ocean floor of Earth. The radius of Enceladus is ∼252 km, resulting in a surface area of ∼2 times that of the state of California. (Credit: NASA/JPL; more information and images are available at NASA's online Planetary Photojournal)
<b>FIG. 24.</b>
FIG. 24.
Neptune's moon Triton exhibits active nitrogen plumes and exotic surface geology, such as icy volcanic features, and the hummocky cantaloupe terrain seen at upper left. The same internal energy that allowed for recent geological activity also creates the possibility for a subsurface ocean on Triton. The radius of Triton is 1350 km, giving it a surface area equivalent to that of North America. (Credits: NASA/JPL/USGS and Smith et al., 1989)
<b>FIG. 25.</b>
FIG. 25.
Distribution of exoplanet semimajor axes (a measure of the distance of the planet from its star) and mass in terms of Jupiter masses. Large, gas giant planets are red circles, super-Earths to Earth-mass planets (1–10 Earth masses) are blue diamonds, and planets less massive than Earth are shown with green boxes. Additionally, the planets in the Solar System are marked for reference with open circles, and the yellow triangles represent the directly imaged HR 8799 system. Data are confirmed planets with mass estimates (, downloaded November 19, 2015).
<b>FIG. 26.</b>
FIG. 26.
Examples of some of the planetary analog sites described in Table 6. (A) Microbial growth (pink color) on glacial ice, Svalbard (photo credit: Katherine Wright). (B) Microbial mats in Yellowstone Hot Springs (photo credit: Katherine Wright). (C) Sand dunes, Morocco (photo credit: Katherine Wright). (D) Hematite concretion, Utah (photo credit: Katherine Wright). (E) Río Tinto, Spain (photo credit: Damhnait Gleeson). (F) Elemental sulfur deposits on the surface of Borup Fiord Pass Glacier, Canada (photo credit: Katherine Wright).
<b>FIG. 27.</b>
FIG. 27.
Stromatolites are still present today in a few places on Earth, such as Shark Bay in Western Australia (left). These modern stromatolites are very useful for understanding structures that were formed much longer ago (right). [Credits: Richard Arculus (left) and NASA Ames (right)]
<b>FIG. 28.</b>
FIG. 28.
Many biological molecules, like amino acids, are chiral; they exist in two mirror-image versions (just like our left and right hand), but biochemistry uses only one enantiomer. (Credit: NASA)
<b>FIG. 29.</b>
FIG. 29.
(Left) Martian meteorite ALH84001, discovered in Allan Hills, Antarctica. Ruler and 1 cm cube for size reference. (Right) Scanning electron micrograph showing one of the very small tube-like structures discovered in ALH84001 that resemble fossilized bacteria. (Credits: NASA/JSC/Stanford)
<b>FIG. 30.</b>
FIG. 30.
The spectra of Earth and the Sun. In the visible portion of the spectrum, indicated by the rainbow, light from the planet is simply reflected sunlight, whereas the emission spectrum of the warm planet itself can be observed in the IR. Observing in the IR also offers a far more favorable contrast ratio between the intensity of the planetary emission and that of its star (106) compared to visible wavelengths (1010). The spatial resolution of the terrestrial spectrum shown is similar to that being considered for TPF-C and Darwin missions. (Credit: Tyler Robinson)

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