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Formation of Astrobiologically Important Molecules in Extraterrestrial Environments
  By Sébastien Dupraz, Trent Dupuy, & Sandrine Bottinelli
Based on the presentation at the Astronomy 734 Astrobiology Seminar by Ralf Kaiser, Fall 2004

1. Introduction

1.1. The interstellar medium

The interstellar medium (ISM), material in the space between stars, consists of gas and dust that represent 20-30% of the mass of our Galaxy. The ISM consists of

  • Gas (99%): about 90% hydrogen, 10% helium, and trace amounts of other elements, especially oxygen, carbon, and nitrogen.
  • Solid particles (1%): These are typically smaller than 1 µm (a millionth of a meter) and have a carbonaceous or silicate-based core surrounded by an icy mantle of (H2O), carbon dioxide (CO2), or ammonia (NH3).

The first diatomic molecules (compounds of two atoms, e.g., methylidyne, CH, or cyanogen, CN) were discovered in the ISM almost 70 years ago. To date, more than 120 species have been reported in the ISM, and the most complex molecules have 13 atoms. The list includes some astrobiologically important molecules such as the simplest sugar, glycolaldehyde, and possibly the simplest amino acid, glycine, which is a kind of molecule involved in the production of proteins like DNA.

The exact composition of the ISM and the chemical reactions going on in it are of great interest to astronomers, biologists, and chemists. Indeed, it is in the densest regions of the ISM (called dense cores) that stars, eventually surrounded by a planetary system, form. Therefore, some of the molecules necessary for life have their origins in the ISM, so knowing the chemistry of dense cores and how it evolves during star formation would help us understand how life appeared on Earth and whether it could form somewhere else. In fact, we already know that there is a link between chemical processes in dense cores and cometary volatiles; therefore, establishing in detail the connection between interstellar, cometary, and meteoritic material could provide constraints on the composition and evolution of the protoplanetary nebula from which the solar system formed.

The extreme conditions prevailing in the ISM are challenging to study: temperatures range from ten to thousands of Kelvin (absolute zero at 0 K corresponds to -273°C or about -460°F), and densities range from 100 to 108 molecules per cm3. The densest regions of the ISM contain fewer particles per unit volume than an ultra-high vacuum on Earth (1010 molecules per cm3), and for further comparison the density of air in a room on Earth is 5 x 1019 molecules per cm3. Therefore, it is only through a combination of astronomical observations and laboratory experiments on chemical reactions that we will be able to fill in the gaps in our understanding of the cycle of matter in the ISM (Figure 1) and get a complete picture of how it impacts the formation of biologically important molecules in interstellar space. Finally, combining these results with biological investigations on Earth will help us understand how these molecules reached Earth and how they reacted further to lead to the appearance of life.


Figure 1. Cycling of matter through the different extraterrestrial environments. (Credit: Kaiser 2002)

1.2. Environments for interstellar molecules

As mentioned above, the ISM can be very hot, in which case molecules cannot survive because their chemical bonds will be easily broken by thermal energy. Therefore, molecules are confined to distinct regions or environments of the ISM whose properties are summarized in Table 1 and described below:

  • Diffuse clouds: These clouds have a low density (hence their name) and can easily be penetrated by interstellar ultraviolet (UV) radiation. This type of radiation is important for the chemistry of these regions because it is energetic enough to dissociate some molecules. It can also ionize atoms and molecules by "tearing" an electron off a given species.
  • Dense clouds: Also called molecular, dark, or cold clouds, they are the densest and coldest type of concentration of particles in the ISM. UV radiation cannot penetrate the central parts of these regions, where molecules that are more complex can be formed without being destroyed. Visible light (less energetic than UV light) cannot pass through these clouds either, so they appear dark to an observer who sees them in front of a field of background stars. They are mainly composed of molecular hydrogen (H2), followed in abundance by carbon monoxide (CO) and hydroxyl (OH), and there are almost no ionized species. In the central parts of dense clouds, where the density is the highest and temperature the lowest, some molecules stick to grain mantles where they can undergo chemical reactions.
  • Hot cores: When dense clouds collapse under their own gravity, the process of star formation begins. The dense clouds break up into clumps that continue to accrete infalling material. These protostars radiate energy in the form of heat, which evaporates the grain mantles and releases molecules into the gas phase where they can react further. This is the hot core stage, characterized by the presence of complex organic molecules in the gas phase rather than in ices, and by enhancement in the abundance of deuterated species (molecules in which a hydrogen atom has been replaced by its isotope deuterium, also called heavy hydrogen, for example, HDO instead of H2O).
  • Circumstellar envelopes (CSE): At the end of its life, a star loses a large amount of matter because stellar winds expel it, and the star is surrounded by a large, expanding shell called a circumstellar envelope. The lost matter consists of elements heavier than hydrogen that were produced by the fusion of lighter elements inside the star. Some of the ejected matter will eventually end up in a dense cloud that will form another star, where even heavier elements will be produced and ejected again into the ISM, thereby further enriching it. CSE are also the site of dust formation.
  • Planetary nebulae (PN): As the star is dying, the mass loss process becomes very efficient because of high-velocity stellar winds. The radiation from the central star photodissociates and ionizes the material in the CSE that, when pushed away by the winds, yields the beautifully colored ring structures seen in PN.

Table 1. Summary of physical parameters in extraterrestrial environments.

2. Chemical Reactions in the Interstellar Medium

2.1 General concepts of reactions

Before we can understand what occurs in the interstellar medium, we must first consider what a chemical reaction is. By definition, it is the cleavage (splitting) of a chemical bond with or without the creation of another chemical bond. The reaction is constrained in two ways: kinetically due to the repulsion force between the molecules that are interacting, and thermodynamically due to the difference in energy between the molecules that react together (reactants) and the molecules that are produced (products). These two constraints are always applied together: a reaction that is possible kinetically and not possible thermodynamically will not occur in nature, and the inverse is also true.

If a reaction is possible thermodynamically, that means that the products should have less energy than the reactants. In general, two terms define this energy: the enthalpy, which is the intrinsic energy of the molecules and atoms, and the entropy, which is the variation of the energy due to the "order." The more ordered a system is, the more energy it is able to release by becoming disordered. But the entropy depends strongly on the temperature, and because the temperature is very low in the cold ISM, we will consider that the energy of a system is dependent only on the enthalpy. When the energy of the products is smaller than the sum of the energies of the reactants, we say that it is an exoergic reaction, and when it is larger, it is called an endoergic reaction.

Some reactions are possible thermodynamically but never occur. This is mainly because the molecules have to cross an energy barrier to react together. This barrier depends on many parameters that are related to the repulsion force between molecules. The shape and the orientation of the molecules are two examples of these parameters. A familiar example of a reaction that is thermodynamically possible but kinetically impossible is a match that won't burn until it is struck. The flammable chemicals present at the tip of the match could thermodynamically react with air to burn into ash, but this will never happen unless you help the process by striking the match. When you do that, you merely help the reactants (air and the flammable chemicals) to cross the kinetic barrier by furnishing them with a supply of energy.

In addition to the above two constraints on reactions, we can classify reactions in two important classes: direct and indirect (see Figure 2).

Figure 2. Diagram representing the different types of reactions for the reactants A and BC. Possible products are
AB+C (exoergic reaction) and AB'+C (endoergic reaction; AB' is an isomer of AB). Exoergic reactions can be direct
(upper pathway) or indirect (lower pathway). Indirect reactions can have an entrance barrier (TSin) and/or an exit
barrier (TSout). See text for details. (Credit: Ralf Kaiser)

  • Direct reactions have one and only one transition state, [ABC]‡, but no intermediate between reactants and products.
  • Indirect reactions have at least one intermediate, [ABC]*, which can be formed either via an entrance barrier, TSin (between the reactant and the intermediate), or without barrier.

This could be compared to the delivery of a package. To reach the delivery point in the first case, the deliveryperson must climb up to a platform where she or he cannot stay. In second case, the deliveryperson must jump over a fence before giving the package to another deliveryperson.

As can be seen in Figure 2, the intermediate has less energy than the reactant. The intermediate carries this difference in energy by rotating or vibrating, and it is said to be in an excited state. This state is not stable enough to exist for long, so the intermediate generally reacts again in one of the following ways:

  • It can decay to form AB+C or AB'+C, via an exit barrier TSout, or without a barrier. (The second deliveryperson reaches the delivery point directly or by jumping over another fence.)
  • If the density of the surrounding medium is large enough, the intermediate can collide with another molecule, M, that will carry away the excess energy so that the intermediate will be in a stable state. (The second deliveryperson is given another package and goes to another destination.)
  • It could react back to the reactants. (The package goes back to its starting point.)

Although unstable, intermediates can be isolated and studied in a laboratory. However, the transition states are not real, identifiable molecules but rather represent a theoretical energy barrier between reactants and products. Transition states cannot be identified, let alone isolated, in the lab, so we are reduced to making assumptions about their properties.

Finally, reactions can be classified by the number of participating reactants. Binary reactions have two reactants, ternary if there are three reactants, etc.

2.2 Reactions in the solid phase

The solid phase in the ISM consists of small (less than 1 µm) particles of dust at 10 Kelvin with a core composed of silicate and carbonaceous material that is surrounded by a layer of ice. The first property that affects their chemistry is that many molecules, for example, water (H2O), carbon monoxide (CO), ammonia (NH3), methane (CH4) , and carbon dioxide (CO2), easily stick to the ice layer when they collide with the dust particle. There are, however, some exceptions, such as hydrogen and helium atoms that are light enough to escape if they are absorbed by the dust. This results in a mantle of ice made of these particles that is a favorable place to have reactions, because many different molecular species can interact together. Secondly, the molecules buried in the ice mantle are ensured protection from very destructive UV radiation. As a result, these grains are sometimes compared to molecular nurseries.

However, the temperature of the solid medium is a strong limitation, for it requires that, for any reaction to occur, it must be exoergic with no or a very low energetic barrier. This means that it is impossible to have a direct reaction (which always has one transition state) in these conditions, because the transition barrier cannot be overcome. Only indirect reactions with no entrance or exit barrier are possible on dust grains.

Hydrogen reacts easily on the grains because it is light and mobile, which enables it to jump over the irregularities of the grain surface ("hopping" in Figure 3). It also has special properties that permit it to easily move through the molecular structures of the grain surface ("tunneling" in Figure 3). Two models have been proposed to explain the formation of molecular hydrogen (H2) from two hydrogen atoms: the Langmuir-Hinshelwood mechanism, which permits two absorbed hydrogen atoms to recombine by hopping or tunneling, and the Eley-Rideal mechanism, which allows the formation of H2 after the collision of a hydrogen atom from the gas phase with a hydrogen atom on the dust grain.

Figure 3. Two possible theories explain the formation of H2 on the surface of dust
grains in molecular clouds. (Credit: Kaiser 2002)

The hydrogen atom can also react with other atoms, like oxygen (O), nitrogen (N), and carbon (C) while on the dust grain. This is a good explanation for the formation on the ice mantles of very basic molecules such as H2O, CH4, and NH3. Nevertheless, this basic chemistry cannot explain all the molecular diversity found in the grains, so there must be a source of energy that allows other reactions to occur. While molecular clouds are for the most part shielded from UV radiation because of their high density, cosmic rays that are just energetic protons (H+) and alpha particles (these are helium nuclei, He2+) can easily penetrate a cloud. This may be an important source of energy that could be exploited by the molecules in the grain's ices mantles. The interaction of cosmic rays with the molecular cloud leads to an important release of energy that raises the energy state of molecular species. Consequently, these "excited" molecules are allowed to take chemical pathways that were previously forbidden. This new field of chemistry is called "nonthermal chemistry" because reactions are independent of temperature, which provides thermal energy. Nonthermal reactions allow the formation of many new molecules in the cold grain ices. (See Figure 4.)

Figure 4. The same diagram as in Figure 2, but now both the direct and indirect pathways, as well as
the formation of the products AB'+C (now exoergic), are possible. This is because the reactants have
extra initial energy. This is the sort of reaction that might occur on dust grains if an energy source is
available. (Credit: Ralf Kaiser)

2.3 Chemistry of the gas phase

The main difference between the solid and the gas phases is that the gas phase is much less dense. As a result, no ternary reactions (reactions involving three reactants) are possible, because the probability of having a collision of three particles at the same time is infinitesimal: about one collision every one billion years. When reactions happen in the solid phase, it is often because there are neighboring molecules near the excited product or an intermediate that is able to take away the excess energy released during the reaction. If no third molecule or atom is close enough to carry away this excess of energy, the energy is used to reverse the reaction, so you are left just with reactants. That is why, for instance, the formation of H2 is very difficult in the gas phase and must occur on dust grains by the two processes discussed previously. Also, gas phase reactions cannot draw energy from cosmic rays because a molecule that is split by the energy is not likely to collide with another molecule and react before it recombines into its previous state.

For the reasons mentioned above, the only reactions occurring in the gas phase are binary (two reactants), exoergic, and indirect (no entrance barrier). In the 1970s, only reactions between ions (positively or negatively charged species) and molecules were known to have no entrance barrier. In this case, cosmic rays help with the creation of ions by ripping off electrons from molecules, and these can then react with the un-ionized (neutral) molecules. Much of the molecular synthesis occurring in molecular clouds was explained this way, but the production of some molecules, like cyanoacetylene (HCCCN), was still unexplainable. In the 1990s, the existence of barrier-free neutral-neutral reactions was envisaged. It was demonstrated that several species of radicals (atoms or molecules with one supplemental electron) and atoms in an unexcited state are able to interact together in the ISM without having strong repulsion forces. This provided an explanation for the existence of several molecules like nitriles and unsaturated carbon molecules. However, ion-molecule and neutral-neutral reactions in the gas phase cannot explain all the complex molecules seen in the ISM, so they are believed to be created by nonthermal dust grain reactions.

3. Simulations of Complex Molecule Production in the ISM

3.1 A case study in solid phase reactions

If cosmic rays are responsible for nonthermal reactions occurring on the ice mantles of dust grains in cold molecular clouds, then perhaps the process can be simulated in a laboratory on Earth. Dr. Ralf Kaiser at the University of Hawaii has designed and carried out an experiment to show that high-energy particles can indeed process molecular ices into more complex molecules while in the solid phase. For this case study in ice processing, he chose to track the production of C2H4O isomers (molecules with identical chemical formulas but different structural arrangements of chemical bonds) that have been observed in space: acetaldehyde, ethylene oxide, and vinyl alcohol (Figure 5 ). The experimental setup is designed to measure the rate of production of each isomer simultaneously, which gives an empirical prediction of the relative abundance of each isomer resulting from cosmic-ray processing in cold molecular clouds.

Figure 5. Schematic diagrams of the three chemical isomers of C2H4O studied in Kaiser's laboratory. The spheres of different colors represent different atoms: oxygen (red), carbon (gray), and hydrogen (blue). (Credit: Ralf Kaiser)

3.2 Creating a little bit of the ISM on Earth

The conditions in a dense molecular cloud are simulated in the experiment mentioned in section 3.1 by enclosing a volume that can be cooled down to as low as 10 K while at a pressure of 8 × 10-11 torr. This pressure is about ten trillion times lower than the pressure of the air we breathe, and this would be considered an adequate vacuum for most scientists' experiments. A special surface in the enclosure is available for molecular ices to condense onto as the chamber is bombarded by high-energy particles from an ion gun.

The abundance of molecules in the gas phase is measured by a mass spectrometer, and the abundance of molecules in the solid phase is measured by spectroscopy of the infrared light reflected off the molecular ices. The latter technique employs the same principles that astronomers use to study molecular ices in space: different molecules will absorb light of different frequencies that correspond to the difference between a pair of electron energy levels in the molecule. If absorption is detected at a certain wavelength of light, then that indicates the presence of one type of molecule, and the amount of light absorbed reveals how much of that molecule is present.

The cosmic rays that are theorized to impart the energy needed to cleave molecular bonds and allow chemical reactions to take place on cold dust grains are protons that carry 10 MeV in kinetic and mass energy. Computer calculations show that, out of these 10 MeV, 1 MeV is absorbed by dust grains and is used for cleavage of chemical bonds (up to 10 eV), ionization, and production of secondary electrons. However, on Earth, protons of such high energy can be produced only in giant particle accelerators called cyclotrons, and there are only two in the world suited for this task. But simulations have shown that lower-energy 5 keV electrons would reproduce the relevant characteristics of cosmic rays in Kaiser's experiment. The advantage is that there is no need for a costly particle accelerator since they can easily be produced directly by an ion gun.

3.3 Measuring ice processing by energetic particles in the lab

By comparing the irradiated and nonirradiated ice reflection spectra, the reactions used to produce C2H4O isomers from simpler molecules can be studied. In the case of the first isomer studied, acetaldehyde, a mixture of methane (CH4) and carbon monoxide (CO) molecules, was introduced into the chamber, some of which froze out of the gas phase onto a cold surface. The effect of irradiation by high-energy electrons was to separate the methane ice molecule into CH3 and H since this bond is much weaker than the bond linking carbon to oxygen in CO. After irradiation, three molecules were present as ice, and there are only two possible chemical pathways to form an intermediate molecule: formyl (HCO) or acetyl (CH3CO). The intermediate that forms fastest can then join with the remaining CH3 or H, respectively, to make C2H4O (Figure 6).

Kaiser observed the absorption line of HCO ice and not of CH3CO, so this implied a single reaction path via an HCO intermediate for the formation of acetaldehyde. Supporting the experiment's findings, Osamura et al. (2004) showed theoretically that the transition state that H+CO must pass through to form HCO has a lower energy than the transition state that CH3+CO must pass through to become CH3CO.

Figure 6. This diagram using the structural representations of molecules shows the two reaction pathways that can produce acetaldehyde. Methane is broken up into CH3 and H by an energetic electron. After this step, one pathway produces the intermediate CH3CO, whereas the second pathway produces the intermediate HCO. (Credit: Ralf Kaiser)

The production of the C2H4O isomer ethylene oxide from carbon dioxide (CO2) and ethylene (C2H4) ices has also been studied. In this case, the initial bond cleaved by the energetic electron is the oxygen-carbon bond in CO2, but there are two intermediate pathways that are more or less equally preferred because their reaction rates are more or less equal. Both intermediate pathways involve the oxygen atom joining the ethylene, but in each case the oxygen bonds to a different part of the ethylene, so the two intermediate states are isomers of each other. In the end, both intermediates react with the remaining CO to form ethylene oxide. The third C2H4O isomer vinyl alcohol has not yet been studied.

3.4 Astrochemical predictions

Since the number of high-energy electrons irradiating the ices can be controlled in the laboratory, the time it takes for reactions to occur can be scaled down from the timescales found in the interstellar medium, where it might take millions of years to produce a significant number of complex molecules. In fact, a million years of ice processing in the interstellar medium can be done in about half an hour in Kaiser's laboratory. This capability allows the reaction rates for producing C2H4O isomers to be measured directly by counting the number of molecules present in the ice mixture at intervals of a few minutes. Once empirical values for the reaction rates of complex molecule production are established, the expected abundances of the complex products and their intermediates that are frozen onto dust grains in the ISM can be predicted. At some point, the material frozen onto dust grains is released when a nearby young star begins heating up. By studying chemical abundances of molecules in both the solid and gas phases, astronomers can use the predictions provided by laboratory experiments on Earth to understand the astrochemical history of the clouds of gas and dust from which stars and planets form.

4. Implications for the Solar System and Future Studies

Laboratory experiments are important to help us understand our observations, such as the type of molecules we find in different astronomical environments and in what quantity. Indeed, being able to explain the presence of certain molecules in different parts of our solar system could help us understand its chemical evolution and in particular, Earth's chemical evolution.

4.1 Comets and meteorites

Comets are thought to be the most primitive bodies in the solar system. They are made of frozen gases, ices, and rocky materials (they are often called "dirty snowballs") and were formed in the outer regions of the protoplanetary solar nebula. When a comet enters the inner solar system and comes close enough to the Sun, the solar radiation heats up the icy material, which sublimates. Not only does this process offer breathtaking views to people on Earth who witness the production of the two tails of gas and dust like the ones shown in Figure 7 , but it also provides a rich mine of information for astronomers. For example, several spacecraft were sent to make direct measurements of the comet Halley, which showed the presence of a large amount of dust and organic material. Recent observations of comets Hyakutake and Hale-Bopp led to the identification of more than 25 molecular species. The diverse composition of comets is a result of the chemical processes going on in the protosolar and protoplanetary nebula; hence knowing this composition in detail would provide checks to laboratory simulations and allow us to better understand these processes.

Figure 7. Comet Hale-Bopp over Indian Cove in April 1997. (Copyright ©1997 Wei-Hao Wang. All right reserved.)

Meteorites, which are asteroids that have fallen to Earth, also contain a wide range of organic species. Large amounts of these organic species may have been delivered to Earth through cometary or meteoritic impacts. Only samples of carbonaceous meteorites are currently available, but future spacecraft missions will allow direct, in situ measurements as well as sample returns from other types of asteroids. It has already been shown that deuterium enrichment and the 13C/12C ratio in meteorites are similar to those found in extraterrestrial environments, proving that interstellar matter is incorporated in asteroids.

4.2. Planets and their satellites

Laboratory studies of reactions involving cyanogen (CN) and ethynyl (C2H) show that these two molecules react with hydrocarbons to produce the nitrogen-bearing molecules and the complex hydrocarbons in the hydrocarbon-rich atmospheres of the planets and their satellites, such as the atmosphere of Titan, one of Saturn's moons (Figure 8). These chemical processes could also be occurring in the atmospheres of Jupiter, Saturn, Uranus, Neptune, Pluto, and Neptune's moon, Triton.

Studies aiming at understanding chemistry in extraterrestrial environments have only been done for reactions between two molecules, since the densities in the ISM are so low that the probability of a three-particle encounter is negligible. But in the denser atmospheres of these planets and moons, collisions between three molecules are more likely; therefore, more investigation is needed in this area to fully understand reaction pathways available in such reactions.

Figure 8. False-color view of Titan recorded by the Cassini spacecraft as it approached its first close flyby of Saturn's moon on October 26, 2004. Red and green colors represent specific infrared wavelengths absorbed by Titan's atmospheric methane, while bright and dark surface areas are revealed in a more penetrating infrared band. Ultraviolet data showing the extensive upper atmosphere and haze layers are seen as blue. (Source: Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA)
A combination of experimental results from the laboratory and measurements from the Cassini-Huygens probe that recently entered the atmosphere of Titan will allow us to build a database of reaction products and a chemical pathways network. It will be possible to predict the presence and abundance of currently unobserved molecules so future missions could be designed to check these predictions. Making use of the powerful combination of laboratory experiments, theoretical modeling, and observations is the most efficient way of improving our understanding of chemical processes in extraterrestrial and solar system environments and will be relied upon frequently in the future.


Ehrenfreund, P., & Charnley, S. B. 2000. Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth. Annual Review of Astronomy and Astrophysics, 38, 427-483.

Kaiser, R. I. 2002. Experimental Investigation on the Formation of Carbon-Bearing Molecules in the Interstellar Medium via Neutral-Neutral Reactions. Chemical Reviews, 102, 1309-1358.

Osamura, Y., Roberts, H., & Herbst, E. 2004. On the Possible Interconversion between Pairs of Deuterated Isotopomers of Methanol, Its Ion, and Its Protonated Ion in Star-Forming Regions. Astronomy & Astrophysics, 421, 1101-1111.


absolute zero: Absolute zero (0 K) corresponds to −273.15°C or −459.67°F.

amino acid: Large molecules that are the building blocks for proteins. Twenty of them are also present in the genetic code.

AU (astronomical unit): The Earth-Sun distance, approximately 150 million km or 93 million miles.

circumstellar envelope (CSE): The shell of material expelled by an old star and surrounding that star.

cold cloud: See molecular cloud.

cosmic rays: Highly energetic particles that are made of protons (H+, hydrogen atoms that have lost their electron) or alpha particles (He2+, helium atoms that have lost their two electrons).

dark cloud: See molecular cloud.

dense cloud: See molecular cloud.

dense core: High-density region of a molecular cloud that can collapse under its own weight to lead to the formation of a protostar.

deuterated molecule: A molecule in which a hydrogen atom has been replaced by its isotope deuterium.

diffuse clouds: Low-density clouds in the ISM that can easily be penetrated by energetic radiation such as ultraviolet radiation.

dust: Generic term for grains of conglomerate interstellar material with silicate (SiO2) or carbonaceous (containing carbon) cores. The grains tend to be "sticky" when colliding with some molecules. Dust is primarily responsible for the preferential scattering of blue light, which is the reason why molecular clouds are only transparent to red wavelength light.

Eley-Rideal mechanism: One of the two models (the other is the Langmuir-Hinshelwood mechanism) explaining the formation of molecular hydrogen (H2). In this mechanism, a hydrogen atom from the gas phase collides directly with a H atom that is already on the surface.

endoergic: Process in which energy is captured.

enthalpy: The heat content of a specific amount of substance.

entropy: A measure of the disorder of a system.

exoergic: Process in which energy is liberated.

glycine ( NH2CH2COOH): The simplest of the 20 amino acids, it makes up one third of collagen, the protein responsible for the skin's elasticity.

glycolaldehyde ( CH2OHCHO): The simplest sugar, it can combine with other molecules to form glucose, the sugar found in fruits, and ribose, a building block of the genetic code carriers, RNA and DNA.

hot core: a small (102-104AU), dense (>107 cm3) and warm (>100 K) region surrounding a new born protostar.

hydrocarbons: Molecules made of hydrogen and carbon atoms.

interstellar medium (ISM): The term astronomers to describe all gas and dust in the plane of the galaxy that exists "between the stars." The ISM can be ionized if it is diffuse and near a radiation source (such as a star), but the denser more isolated regions of the ISM typically consist entirely of molecules both in the gas phase and frozen onto dust grains.

ion: An atomic particle that is electrically charged, either negative or positive.

isomer: Molecules with the same chemical formula (i.e., same number of atoms of a given type), but in which atoms are arranged differently.

isotope: Atoms are made of protons, electrons, and neutrons (respectively, positively charged, negatively charged, and neutral particles). The isotope of a given atom contains the same number of protons and electrons, but not of neutrons. In the case of hydrogen (1 proton, 1 electron), its isotope deuterium contains an extra neutron, making its weight greater than that of hydrogen, hence its name, heavy hydrogen.

Kelvin (K): Temperature scale for which the zero is absolute zero. The temperature in Kelvin,TKis related to the temperature in degree Celsius, TC, by TK = TC – 273.15, and to the temperature in degree Fahrenheit , TF, by TK = (TF+459.67)/1.8

Langmuir-Hinshelwood mechanism: One of the two models (the other is the Eley-Rideal mechanism) explaining the formation of molecular hydrogen (H2). In this mechanism, two hydrogen atoms are already on the grain-surface, and one of them reaches the other either by jumping over the bumps of the grain-surface or by crawling through the molecular structure of the surface.

mantle (of dust grain): Outer layer made of water, carbon dioxide, or methane ice that surrounds the silicate or carbonaceous core of a dust grain.

MeV: One million electronvolt (eV). The eV is a unit of energy used in physics to express more conveniently a tiny amount of energy. Indeed, 1 MeV particles are considered to be highly energetic, but they are roughly 2x1020 times smaller than an average daily intake of 2,000 kilocalories.

molecular cloud: A cold (10 K), dense (102-109 cm3) region of the ISM opaque to visible and UV light where all of the gas is in molecular form, mostly of hydrogen (H2), but also carbon monoxide (CO) and other species.

nitrile: A type of molecules containing the –CN (cyanide) group; sometimes labeled as XCN.

organic: Used to refer to any compound or molecule that contains one or more carbon atoms.

photodissociation: A process by which, following the absorption of radiant energy (light), a molecule breaks up into parts capable of recombining under other conditions.

planetary nebula : The colored ring structure surrounding a dying star.

protostar: Object that has condensed from the collapse of a molecular cloud and that radiates heat, warming up its surroundings.

radical: An atom or group of atoms that contains one or more unpaired electrons (usually very reactive species).

tunneling: The ability of an atom to pass through the molecular structure of the material surrounding it (like a child going through a jungle-gym to reach the other side of the playground).

volatiles: Species in a solid form that easily become gases when heated. Usually refers to the gas released by comets as they are heated by the Sun.



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