Tuesday, June 22, 2021

An overview of Astrochemistry

Let’s start our astrochemical voyage by first getting used to the basic definition/overview of Astrochemistry!

So, very broadly speaking, astrochemistry is the study of chemical composition of universe, abundance of different elements in the universe, the chemical composition of stars planets and Interstellar medium. In astrochemistry, we study how atoms, molecules, ions, radical etc interact outside of the Earth’s atmosphere.

Astrochemistry, is an interdisciplinary field, precisely an overlap between Chemistry and Astronomy, though it takes help from other disciplines as well, like planetary science, chemical biology, physics, and computational science. Astrochemistry  contributes to our understanding of geological processes on other planets, and it helps explore conditions under which life might form by examining molecules on other planets and in outer space.️ ️

Formation, destruction and excitation of molecules in astronomical environments and their influence on the structure, dynamics and evolution of astronomical objects are the areas of our interest when it comes to Astrochemistry

Molecules(our focus of study for astrochemistry) are found throughout the universe   Molecular clouds, evolved stars, planetary nebulae, protoplanetary disks, stellar and (exo-) planetary atmospheres, solar photosphere, comets, galaxies (nearby to high z), …..

The experimental conditions under which we study Astrochemistry are very different from those normally encountered in lab on Earth. Astrochemistry is also strongly driven by observations, thus it has a heavy reliance on technology.

 The study of Interstellar clouds are of particular interest to us as they are birthplaces of new stars, so naturally abundances of various kinds of molecules is present there.

About 150 different molecules firmly identified via Astrochemistry out of which, Ordinary molecules include NH 3, H 2O, H 2CO, CH 3CH 2OH, …. While Exotic molecules include HCO +, N 2 H +, HCCCCCCCN, ….

➡Why is study of molecules in space important though?

Clearly  studying the abundance of various elements, ions, and molecules in space helps us to understand Earth’s geochemistry. The deuterium:hydrogen ratio in Earth’s oceans can be taken as one example. Scientists have examined that ratio in various comets and found that one has the same ratio as that in Earth’s oceans. One could imagine that the deuterium:hydrogen ratio in Earth’s oceans resulted from the early bombardment of Earth by meteorites and is a relic of the early solar system. It is interesting to note that the deuterium:hydrogen ratio in organic compounds in space is higher than (or different from) that in water.

➡Hydrogen Chemistry in the early universe:

As atoms combined to form the first molecules, the universe was finally able to cool and began to take shape. Scientists have inferred that helium hydride was this first, primordial molecule.

Once cooling began, hydrogen atoms could interact with helium hydride, leading to the creation of molecular hydrogen — the molecule primarily responsible for the formation of the first stars. Stars went on to forge all the elements that make up our rich, chemical cosmos of today. The problem, though, is that scientists could not find helium hydride in space. This was indeed the first step in the birth of chemistry. 

Helium hydride is a finicky molecule. Helium itself is a noble gas making it very unlikely to combine with any other kind of atom. But in 1925, scientists were able to create the molecule in a laboratory by coaxing the helium to share one of its electrons with a hydrogen ion.

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Cosmic evolution is the tale of progressive transition from simplicity to complexity. The newborn universe starts with the simplest atoms formed after the Big Bang and proceeds toward ‘astronomical complex organic molecules’ Understanding the chemical evolution of the universe is one of the main aims of Astrochemistry, with the starting point being the knowledge whether a molecule is present in the astronomical environment under consideration and, if so, its abundance

Astrochemists perform experimental, analytical and computational laboratory research to generate data for explaining astronomical observations, to provide input data for models, and to test theories about the formation and evolution of large and small molecules ️in various astrophysical environments.

They also use Earth-based telescopes, satellites, ️and space vehicles to collect spectroscopic data.

Astrochemistry is based on the molecular ️emission patterns in space ️, which is the process in which photons are emitted and absorbed when a molecule changesenergy states. By studying molecular emission and absorption, the chemical composition and physical properties of astronomical objects 🪐may can be measured..

There have been observed, several patterns of molecular emission, astrochemistry uses those patterns. 

The chemistry of the interstellar medium  depends on the local density, temperature, ionization and radiation conditions and molecules with well understood chemistry can be used as instruments to understand various astrophysical phenomena.

 Molecules ️can also regulate  aspects of star and planet formation.

Finally, molecules such as water and organic ones are interesting in their own because of the possible connection between them and origins of life.

Astrochemists ‍ carry out millimeter and infrared observations of molecules, spectroscopic and kinetic experiments, and astrochemical theory to explain these broad topics. 

➡Challenges in Astrochemistry:

The hunt for molecules: The progress of observational studies revealed chemical diversity in space, the source-to-source variation in chemical composition which led to the concept of chemical evolution. Despite the more than 200 molecules that have been discovered in the ISM and circumstellar shells, a significant number of features in radioastronomical spectra are still unidentified. This means that we are far from a complete census of the interstellar molecules. How many molecules have escaped our knowledge? And what about chemical complexity?

➡Chemical Reactivity:

As already mentioned, understanding the chemical processes in space is one of the main aims of astrochemistry. Molecular complexity and the formation of a star proceed hand in hand. In the early 1970s, gas-phase ion-molecule reactions were employed to (successfully) explain the molecular abundances observed in interstellar clouds. Later on, it was recognized the importance, even in low-temperature regimes, of gas-phase neutral-neutral reactions. However, the advance of observational capabilities has led to the detection of molecules in regions where gas-phase reactions could not contribute significantly to chemical reactivity. This marked the beginning of grain-surface chemistry, i.e., the modeling of chemical reactions occurring on dust grains. However, there is still much to be understood about the formation of molecules and, often, the proposed reaction mechanisms are controversial and inconclusive. 

We are in the presence of a gigantic puzzle with a huge number of pieces that are still missing.

➡This was a very broad overview of Astrochemistry, the discipline itself is very complex, and all of it’s contents were not covered, we’ll cover more aspects in of Astrochemistry in the future and might revisit some of the topics discussed here in the future for a detailed insight.

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