Schematic
representation of the new method: Two reactants R1 and R2 are added to a
helium droplet. The energy released in the resulting reaction decreases
the droplet’s size. The decrease in size can be measured, and allows
researchers to deduct the reaction energy. Image: Krasnokutskiy / MPIA
Two astronomers from the Max Planck Institute for Astronomy and from the University of Jena have found an elegant new method to measure the energy of simple chemical reactions, under similar conditions as those encountered by atoms and molecules in the early solar system. Their method promises accurate measurements of reaction energies that can be used to understand chemical reactions under space conditions – in cluding those reactions that were responsible of creating organic chemicals as the raw material for the development of life.
In order for life to form, nature needed plenty raw materials in the shape of complex organic molecules. Some of those molecules are likely to have formed long before, in space, during the birth of the Solar System. Systematic studies of the necessary chemical reactions, which take place on the craggy and convoluted surfaces of dust grains, were and are hampered by a lack of data. Which elementary reactions, involving which individual reactants are possible? What temperature is required for a reaction to take place? Which molecules are produced in those reactions? Now, Thomas Henning, director at the Max Planck Institute for Astronomy (MPIA), and Sergiy Krasnokutskiy of the MPIA’s Laboratory Astrophysics Group at the University of Jena have developed an elegant method to study such elementary surface reactions – using minute liquid helium droplets.
In the early solar system, long before the formation of Earth,
complex chemical reaction took place, creating substantial amounts of
organic molecules. The cosmic laboratory for these works of chemical
synthesis was provided by grains of dust – clusters of mostly silicates
and carbon, covered with a mantle of ice, with complicated and delicate
tendrils and ramifications, and on this basis with one crucial property:
A comparatively large surface on which chemical reactions could take
place. In the millions of years that follows, many of those dust grains
would cluster together for form ever larger structures, until finally,
solid planets emerged, orbiting the young Sun.
Creating the raw ingredients for life
While all of the organic compounds synthesized on the grain surfaces
would be destroyed by the unavoidable heat during planet formation, some
of the molecules remained in waiting, encapsulated in, or clinging to
the surface of, smallish grains or lumps of rock, as well as in the icy
bodies of the comets. By one account of the history of life, once
Earth’s surface had cooled sufficiently for liquid water to form, it was
these grains and rocks, hitting Earth’s surface in the shape of
meteorites, some of them landing in warm, small, ponds, that provided
the chemical basis for life to form on our home planet.
In order to understand the early natural chemical experiments in our
universe, we need to know the properties of the various reactions. For
instance, do certain reactions need a specific activation energy to
happen? What is the eventual product of a given reaction? Those
parameters determine which reactions can happen under what conditions in
the early Solar system, and they are key for any realistic
reconstruction of early Solar system chemistry.
Yet precise data on these reactions is surprisingly scarce. Instead, a
substantial part of chemical research is dedicated to the study of such
reactions in the gaseous phase, with the atoms and molecules floating
freely, colliding, and forming compounds. But the crucial chemical
reactions in space needed to build up larger organic molecules take
place under markedly different conditions – on the surfaces of dust
grains. This changes even the basic physics of the situation: When a new
molecule is formed, the energy of the chemical bond formation is stored
in the newly created molecule. If this energy is not passed on to the
environment, the new molecule will quickly be destroyed. This prevents
the formation of many species of in the gas phase. On a surface, or in a
medium, where energy can readily be absorbed by the additional matter
present, the conditions for certain types of reactions building complex
molecules, step by step, are much more favorable.
Henning and Krasnokutskiy developed an elegant method for measuring
the energetics of such reactions. Their mock-ups of cosmic laboratories
are miniature helium droplets, a few nanometers in size, drifting in a
high vacuum. The reactants – that is, the atoms or molecules meant to
take part in the reaction – are brought into the vacuum chamber as
gases, but in such minute amounts that helium droplets are
overwhelmingly likely to pick up either a single molecule of each
required species or none, but not more. The helium droplets act as a
medium that, similar to the surface of a dust grain, can absorb reaction
energy, allowing reactions to happen under similar conditions to those
in the early Solar system. This reproduces a key feature of the relevant
surface chemistry (although other properties, such as catalytic
properties of a specific dust surface, are not modelled).
Nanodrops as measuring devices
Furthermore, the two astronomers used the helium nanodrops as energy
measuring devices (calorimeters). As reaction energy is released into
the drop, some of the Helium atoms will evaporate in a predictable
fashion. The remaining drop is now smaller than before – a difference in
size that can be measured using two alternative methods: an electron
beam (a larger drop is easier to hit than a smaller one!) or a precise
measurement of the pressure in the vacuum chamber created by Helium
droplets hitting the wall, where larger droplets produce greater
pressure. By calibrating their method using reactions that had been
studied in detail beforehand, and whose properties are well-known, the
two astronomers were able to increase the method’s accuracy
considerably. All in all, the new method provides an elegant new way of
investigating the formation pathway of complex organic molecules in
space. This should enable researchers to be more specific about the raw
materials nature had to work with in the run-up of the emergence of life
on Earth. But there is more:
The first measurements using the new technique confirm a trend that
had already been visible in other recent experiments: On surfaces, at
low temperatures, carbon atoms are surprisingly reactive. The
researchers found a surprisingly high number – almost a dozen – of
reactions involving carbon atoms which are barrierless, that is, which
do not require extra energy input to proceed, and hence can occur at
very low temperatures. Evidently, the condensation of atomic gas at low
temperatures is bound to lead to the formation of a large variety of
organic molecules. But that large possible variety also means that
molecules of each specific species will be very rare.
This, in turn, suggests that astronomers might be drastically
underestimating the amount of organic molecules in outer space. When it
comes to estimating abundances, astronomical observations examine the
trace signatures (spectral lines) of each molecular species separately.
If there are many different species of organic molecules out there, each
separate species can “fly under the radar.” Its molecules might be
present only in amounts too minute for astronomers to detect, and in
addition, even the tell-tale signatures of the molecules (more generally
those of specific functional groups common to different types of
molecules) could be slightly altered, making the molecule evade
detection. But added up, it is possible that all these separate species
of molecule together could make up a substantial amount of matter in
outer space – a hidden outer-space world of organic chemistry.
Background information
The results presented here have been published as Henning, Th. &
S. A. Krasnokutski 2019, “Experimental Characterization of the
Energetics of Low-temperature Surface Reactions” in the journal Nature
Astronomy.
Journal article
Science contact
Sergiy Krasnokutskiy
Email: sergiy.krasnokutskiy@uni-jena.de
PR contact
Markus Pössel
Public Information Officer
Phone:+49 6221 528-261
Email: pr@mpia.de
