Cosmic Chemistry: How are molecules formed in the cold space clouds?
Whenever we hear the word space- we think of stars, planets, and lots and lots of vacuum. The word “vacuum”, as a young student, was quite a mystery to many (and still largely remains so!). Vacuum, we were told, meant just light-years and light-years of emptiness- no molecules, no atoms. We now know that while that is somewhat true as space does have large volumes of empty space. It also has a diverse range of environments that are in fact not so empty ranging over the lifecycle of a star, as shown in Figure 1.
Figure 1: Life cycle of a star, around which a planetary system forms
Over the various stages of a planetary system formation around a star, the molecular density, i.e. the number of molecules per unit volume, and the temperatures vary over a very wide range. The dense molecular clouds, as shown in Figure 1, are the starting point and moreover a key stage, where smaller dust grains and other smaller molecules which are well known to be abundant in the early universe like hydrogen, carbon monoxide, and oxygen come together to create a relatively dense environment with about 10000 molecules per cubic centimeter. I use dense in the context of space, as you can compare this with 100000000000000000000000000 molecules per cubic centimeter at the top of Mount Everest.
The huge advancements in the field of astronomy, especially radio astronomy and space telescopes like the Hubble  have allowed the identification of multiple molecular species in these molecular clouds, including many ionic species and radicals, which are uncharged molecules containing an unpaired electron. These extra-terrestrial environments also contain many strange chemical species, like the long carbon chain molecules e.g. HC11N, which interestingly were first detected in space and then made in the laboratories on Earth.
The small molecules like carbon monoxide and hydrogen in these molecular clouds still cannot justify the 200+ molecules  which have been detected in these environments, containing as many as 60 atoms. To create this wide variety of molecular species, reactions will have a crucial role to play, or in other words, the smaller species in these clouds need to collide to allow the reaction to proceed and form the bigger products. But if we ever tried to study the possible reactions with such low molecular densities, we would have such rare collisions, lasting over the time frame of days and months, that we would see no chemistry at all. So the mystery remains- if the molecular densities under these “vacuum” conditions are so low then how do we see so many complex and big molecules in these cold clouds?
See, when talking about space, one thing becomes very clear, we are all but a very very tiny part of this vast and great universe. The scales change drastically- both in terms of distance where a meter is replaced by a light year, which is almost 1016 meters, and time, where you now have millions of years for chemistry to occur. So while we lose out in terms of molecular density, we have extremely large periods of time for two species to meet, collide and form the product which we then detect.
So we know that we have enough time for the collisions to take place - what we need now are viable conditions in terms of temperature and avoid the harsh X-rays and gamma rays which would render any interesting chemistry impossible to take place as they have such high energies that they would instead completely break apart the atoms and molecules. The molecular clouds do not have uniform conditions throughout and have a significant variation as the outer regions are exposed to UV photons and cosmic rays which can penetrate the clouds to ionize atoms and dissociate molecules forming ions. The outer regions are also relatively higher in temperature compared to the inner regions which are protected from the UV photons and with temperatures in the range 5 K to 50 K. The inner regions have molecules both in the gas phase and frozen molecular species on dust grain surfaces. Given the extremely low pressures, liquid state molecules are not feasible. Towards the outer regions, the temperatures begin to rise and the frozen species like carbon monoxide and water begin to sublime off the grain surfaces, and most species will be present in the gas phase.
Now that we know about the conditions across the molecular clouds, let’s talk about the thermodynamics to see which types of reactions would be allowed to actually occur under these cold conditions. A typical potential energy surface for an exothermic reaction is shown in Figure 2, where the energy of the reactants is higher than the energy for the products, meaning energy is released as the reaction progresses. The other type, an endothermic reaction happens when the reactants’ energy is lower than the products’. Given the extremely low-temperature conditions in the molecular clouds, there is no energy available to drive a reaction forward, so only exothermic reactions are possible. However, as depicted in Figure 2, most reactions also go through a transition state. Since energy is needed to cross this barrier (transition state), such reactions will also not be able to proceed. This means the only reactions possible in the cold interstellar environments would be the ones that don’t require any energy to proceed.
Figure 2: A typical potential energy surface showing the reactants passing through a transition state to form the products
There are two major types of reactions that will form the bigger and complex molecules in the cold interstellar clouds: ice-phase reactions and via gas-phase. At the low-temperature conditions, the smaller molecules which have frozen out on surfaces can undergo reactions in two ways- Ely Rideal and Langmuir Hinshelwood mechanisms, as shown in Figure 3. Either one atom/molecule is on the surface while another species from the gas-phase comes and reacts to form the product, or both the species on the surface come close to each other and react. In both cases, the formed product will desorb off the surface into the gas phase. These reactions are generally exothermic and hence possible in these cold environments.
Figure 3: Two possible mechanisms for reactions on ice phase/surfaces
In the gas phase, there are two major possibilities- ionic species reacting with neutral, and neutral species reacting with each other. The ionic species have a major advantage as the charge allows long-range interactions which induce dipole moments in nearby neutral species (the electron cloud around a neutral species can shift as the positive ion attracts it). This forms an attractive force between the charged and the uncharged species, increasing the frequency of collisions, and hence the reaction probability. Reactions involving ions are also mostly exothermic and do not have a transition state allowing them to occur in the cold interstellar clouds. Molecular ions, thus, have had a key role in driving chemistry in the interstellar medium.
Since the cosmic rays and UV photons cannot penetrate into the innermost regions of the molecular clouds, the molecular ion density in the inner regions is very low. So, some other pathway is needed to explain the abundance of bigger molecules present in these regions. A key type of species in these cold regions are radicals. Radicals are neutral molecules that contain an unpaired electron and are a key component of many interstellar environments. Many of the reactions involving these radicals often proceed without any barrier or through an intermediate which then leads to a submerged barrier (a transition state whose energy is lower than that of the reactants). Due to this, the reactions between neutral species and radicals can also proceed significantly fast at low temperatures down to 5 K. This is contrary to the Arrhenius law which said that reactions become slower as we go down in temperature, and hence it was believed for a long time that reactions between neutral species in these molecular clouds would be impossible at the low-temperature conditions. On the other hand, reactions between neutral molecular species have been shown in the past few decades to proceed at a much higher rate at low temperatures as the low kinetics energies, due to low temperatures, actually, allow molecules to stay closer for a longer time frame.
Also, the extremely low temperatures allow quantum-mechanical tunneling for light atoms, especially H atoms, to dominate in many reactions for example the reaction between methanol and OH radical . A formation of a weak hydrogen-bonded intermediate in this case, allows H atom transfer to form products despite the presence of a barrier.
So, with all the interesting and the very much possible chemistry both in ices and has phase, we are now just starting to understand the molecular composition of the cold space clouds. Also, the changing abundance of different known species along with the computational chemical models (with various reactions and their rates) can help us predict the physical conditions of these clouds. A lot of mysteries still await us as we continue to understand the vast space.
Figure 4: Fullerenes have been detected while polyaromatic hydrocarbons are suspected to be present in space- their origins however are largely unknown.
I hope this article has helped you see space as beyond just a plain vacuum and admire the physics and chemistry which occurs in the cold and dark space clouds. So, next time someone says “vacuum”, go ahead and surprise them with all the interesting science, beyond nuclear physics, taking place in infinity and beyond.
Brett A. McGuire 2018 ApJS 239 17
Shannon et al. 2013; Nature Chemistry, 5(9), 745-749