Vast longitudinal dunes up to 330 feet (100 m) in height in the equatorial deserts of Saturn’s moon Titan are the Solar System’s most monumental surface structures, but the chemical composition of their dark organics remains a fundamental, unsolved enigma, with acetylene ice detected near the dunes implicated as a key feedstock. In a series of lab experiments, a team of researchers at the University of Hawaii at Manoa has demonstrated that aromatic hydrocarbons such as benzene, naphthalene, and phenanthrene — prospective building blocks of the organic dune material — can be efficiently synthesized via galactic cosmic ray exposure of low-temperature acetylene ice on Titan’s surface.
The moons of our Solar System are brimming with unusual landscapes. However, sometimes they look a little more familiar, as in this radar image from NASA’s Cassini orbiter. The image shows dark streaks carved into dunes reminiscent of those we might find on a beach on Earth, or raked with flowing lines in a Japanese Zen garden — but this scene is actually taking place on Saturn’s moon Titan. While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye. While previous images have spotted these eerily familiar patterns on Titan’s dunes, this new image (colorized) shows them in greater detail. The image was obtained by Cassini’s radar mapper on July 10, 2013. The vertical seam near the center is an artifact of radar image data processing. Image credit: NASA / JPL-Caltech / Sci-News.com.
“Titan’s dunes represent the dominating surface sink of carbon in Titan’s organic chemistry,” said Matthew Abplanalp, a researcher at the W. M. Keck Research Laboratory in Astrochemistry and the Department of Chemistry at the University of Hawaii at Manoa.
“Therefore, unraveling the origin and chemical pathways to form this organic dune material is vital not only to understand Titan’s chemical evolution, but also to grasp how alike the chemistries on Titan and on Earth might have been like before life emerged on Earth 3.5 million years ago.”
In the study, Abplanalp and colleagues exposed acetylene ice at low temperatures to proxies of high-energy galactic cosmic rays.
The process successfully converted simple acetylene molecules to more complex organic molecules.
“We show that polycyclic aromatic hydrocarbons (PAHs) such as naphthalene and phenanthrene, along with its precursors (benzene, phenylacetylene, and styrene), can be synthesized via a cosmic ray-mediated nonequilibrium chemistry in low-temperature acetylene ices on Titan’s surface and may act as a critical molecular feedstock to the organic dune material,” they said.
“These processes eventually furnish the molecular building blocks not only for Titan’s organic dunes, but also for organics on airless bodies in general such as on Kuiper Belt objects like dwarf planet Makemake,” said Dr. Ralf I. Kaiser, a physical chemist in the W. M. Keck Research Laboratory in Astrochemistry and the Department of Chemistry at the University of Hawaii at Manoa.
“The low temperature synthesis of PAHs from acetylene ices represents a fundamental shift from currently accepted perceptions that PAH formation takes place solely in the gas phase at elevated temperatures of a few 1,000 K such as in combustion processes.”
“Overall, this study advances our understanding of the complex organics and fundamental chemical processing of simple molecules in deep space and provides a scientifically sound and proven mechanism of formation of aromatic structures in extreme environments in low temperature ices,” he added.
“Since Titan is nitrogen-rich, the incorporation of nitrogen in these PAHs may also lead to carbon-nitrogen moieties (parts of a molecule) prevailing in contemporary biochemistry such as in DNA and RNA-based nitrogen-bases.”
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Matthew J. Abplanalp et al. 2019. Low-temperature synthesis of polycyclic aromatic hydrocarbons in Titan’s surface ices and on airless bodies. Science Advances 5 (10): eaaw5841; doi: 10.1126/sciadv.aaw5841
