The ability to visualize organic and inorganic compounds in meteorites that have landed on Earth enables researchers to understand the formation of the solar system and the origins of both water and life on our planet. Recently, scientists from the IBM Research laboratory in Zurich, in collaboration with a multinational research team, studied organic molecules from meteorites using ultra-high-resolution atomic force microscopy (AFM) for the first time. The work demonstrated the technique's ability to resolve and identify single molecules of meteoric origin in unprecedented detail.
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Meteorites are fragments of asteroids or comets that have reached the surface of the Earth. Quite often, they are remnants of the early history of the solar system, and the molecules they contain can provide clues to the formation of our planet. The mineral content of these meteorites provides information about the history of their parent bodies, while the organic inclusions, which can represent up to 5% of their weight, give invaluable information on the organic materials that could have been delivered to the early Earth.
Witnessing the Early History of the Solar System
Among the most studied meteorites is the carbonaceous chondrite Murchison meteorite that landed in Australia in 1969 near Murchison, Victoria. The meteorite was relatively large, weighing more than 100 kg. The initial analysis after its discovery revealed that it is a remnant from the early days of the formation of the solar system.
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Most notably, the meteorite contains the oldest substance found on Earth to date. The silicon carbide particles found in the Murchison meteorite are estimated to be approximately 7 billion years old, predating the formation of our planet by approximately 2.5 billion years.
Subsequent studies have investigated fragments from the meteorite using a range of analytical methods to gain insight into the formation of planets and, in particular, the origin of the early organic molecules on Earth.
Ultra-High Resolution AFM Resolves Molecular Structures
Recently, a research group at IBM Research in Switzerland, led by Dr. Leo Gross, teamed up with scientists from NASA, the University of Santiago de Compostela in Spain, and other research institutions and took advantage of the unique strengths of the AFM to visualize and identify single organic molecules originating from the Murchison meteorite. The research was published in Meteoritics and Planetary Science.
Back in 2009, the IBM researchers developed a high-resolution AFM imaging technique that used a carbon monoxide-functionalized probe to resolve organic molecules at atomic resolution. The Zurich team took advantage of the atomic-level sensitivity of the functionalized AFM probes and demonstrated the ability to visualize the atomic structure of a wide range of complex organic molecules derived from crude oil and soot.
The carbon monoxide functionalization improved the AFM spatial resolution by enhancing the repulsive forces between the AFM probe and the molecules under investigation. The AFM data revealed the molecular structures as a contrast between the bright regions corresponding to the repulsive forces and the dark background resulting from the attractive forces between the probe and the underlying substrate. The repulsive forces are a consequence of Pauli's repulsion (a repulsive interaction resulting from Pauli's exclusion principle) between the carbon monoxide molecule on the tip and regions of increased electron density of the imaged molecule.
Visualization and Identification of Extraterrestrial Organic Molecules in Meteorites
In their most recent experiments, the Swiss researchers attempted to study molecules sublimed directly from unprocessed powder obtained from the Murchison meteorite. Although the meteorite powder contained a relatively small number of organic compounds suitable for AFM characterization, the researchers succeeded in resolving several species of organic molecules. These initial proof of principle experiments established that by using a carbon monoxide-functionalized probe, the AFM technique can detect and visualize very rare low-abundance species in meteorite samples.
Next, the researchers developed extraction methods targeted at planar polyaromatic and linear hydrocarbon chains present in the meteorite powder. This approach allowed the scientists to resolve multiple molecular species, such as 1-propyl naphthalene or pyrene, the presence of which was confirmed by mass spectrometry and other complementary characterization techniques.
The Next Frontier for Ultra-High-Resolution AFM
While the AFM-based approach has not resolved any novel molecules in this well-studied meteorite, the researchers at IBM envisage that, in the near future, the AFM technique might be used to reveal molecules that have not yet been found in meteorites and other samples from space missions. Such information is vital for tracking the evolution of organic matter and ultimately understanding the history of the formation of the solar system.
Owing to its single-molecule sensitivity, the same method can help with the identification of single molecules in a mixture, opening the possibility of identifying rare substances that can be hard to identify by the standard techniques for molecular structure elucidation, such as NMR and mass spectrometry.
References and Further Reading
Kaiser, K., et al. (2022), Visualization and identification of single meteoritic organic molecules by atomic force microscopy. Meteorit. Planet. Sci. 57, 644-656. Available at: https://doi.org/10.1111/maps.13784
L. Gross and K. Kaiser (2022) The first atomic-resolution images of extraterrestrial molecules. [Online] IBM Research Blog. Available at: https://research.ibm.com/blog/first-afm-of-extraterrestrial-molecules (Accessed on 15 March 2022)
M. Gross (2022) Chemical ecosystem of Murchison meteorite molecules revealed in snapshots. [Online] Chemistryworld. Available at: https://www.chemistryworld.com/news/chemical-ecosystem-of-murchison-meteorite-molecules-revealed-in-snapshots/4015195.article (Accessed on 15 March 2022)
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