Water on Earth: Thanks to a Little Help from Sulfur

Water on Earth: who knew sulfur helped? Discover the surprising link to life.

Earth is an oceanic planet with a complex water cycle. However, how did it get so much water? For years scientists have been puzzled by this. Sulfur can be the key, according to recent studies.

We can see the unexpected role sulfur played in making Earth water-rich by employing astrobiology concepts to examine Earth’s geological and chemical past.

Understanding chemical events that took place in Earth’s atmosphere during the Archean era—before the atmosphere became oxygenated approximately 2.5 billion years ago—requires mapping the bonds and vibrational modes of molecules containing sulfur isotopes.

Geologically speaking, the Archean era spanned 4 billion years to 2.5 billion years ago. Earliest living forms on Earth appeared during this period, but they were anaerobic, meaning they did not need oxygen to survive. In reality, there was no molecular oxygen in the Earth’s atmosphere during the Archean. Rather, other elements such as carbon and sulfur in particular were abundant in the atmosphere.

Volcanic activity released sulfur that was present in atmosphere of Archean Earth. The distinct isotopes of sulfur, which are sulfur atoms with the same number of protons but different amounts of neutrons, got enriched in a way that does not correlate with their mass through a process known as mass-independent fractionation.

Surface deposits from Archean period contain evidence of this activity. These sulfur isotopes were broken down by early bacteria into compounds like sulfur dioxide (SO2) and hydrogen sulfide (H2S), which released oxygen during process. Great Oxygenation Event, as it is called, started process of oxygenating the Earth’s atmosphere.

Sulfur is rapidly oxidized and eliminated from the atmosphere by precipitation and flow into the ocean in an oxygen-rich environment. As a result, the sulfur chemistry of early Archean life gradually disappeared. However, scientists can gain additional insight into the pre-oxygenated Earth’s atmosphere and the circumstances surrounding the earliest life on Earth by comprehending the mass-independent fractionation process.

Sulfur’s mass-independent fractionation mechanism is yet unknown. Processes between elemental sulfur and photolysis—dissolution of molecules by ultraviolet light from Sun—are two basic theories. Professor of Physical Chemistry & Molecular Physics at Marquette University in Milwaukee, Wisconsin Dmitri Babikov adds, “Yet the actual phenomenon, reaction or process is still to be identified.”

Molecular bonds of sulfur

Babikov coauthored a new work in journal Molecular Physics with Igor Gayday and Alexander Teplukhin, two of his colleagues at Marquette. Study looks at a sulfur-4 (S4) molecule’s molecular bonds and how they impact molecule’s vibrational modes, which may have impact on mass-independent fractionation process.

They found previously unidentified connection that forms S4 by joining S2 molecules, each of which has two sulfur atoms. “This second bond prevents two S2 molecules within S4 from easily rotating and holds molecule tightly in a trapezoid-shaped arrangement,” according to Babikov. The way sulfur atoms are arranged in S4 molecule dictates their motion.

Potential energy surface of S4 molecule, which depicts the energy of isotopes arranged in a trapezoid configuration and way chemical processes alter potential energy of that system, have an impact on vibrational states or frequencies of S4 molecule.

In addition to influencing reaction rate, the number of vibrational modes—which include stretching and compressing the bonds between S2 molecules—may also be sensitive to a particular isotope, which may assist determine the nature of the chemical reaction behind mass-independent fractionation. But this is still only a hypothesis, according to Babikov.

In addition to providing insights into pre-oxygenation environment on Earth, our understanding of mass-independent fractionation in sulfur chemistry of Archean Earth may also point to possible biosignatures for conditions akin to Earth’s on other worlds.

According to Babikov, “sulfur isotopes may be able to act as a marker of the conditions that gave rise to life on Earth.” He does point out that it is exceedingly challenging to precisely ascertain the isotopic makeup of an exoplanet’s atmosphere with the telescopic technology we currently have.

Conclusion: Mystery of water on Earth: Surprising role of sulfur

Presence of sulfur isotopes in Earth’s atmosphere shows unexpected similarity between Earth and Archean Earth. This sulfur’s mass-independent fractionation boosted its various isotopes, and it played a critical role in forming Earth’s water-rich environment. This knowledge may offer biosignatures for Earth-like conditions on other worlds, although precisely identifying the isotopic composition of an exoplanet’s atmosphere is still difficult.