In a new study published in the journal Nature Astronomy, a team of planetary researchers from the United States and France studied the photochemical role of hydrogen sulfide (H2S) in warm carbon dioxide-rich exoplanet atmospheres.
Venus in real colors, processed from Mariner 10 images. Image credit: Mattias Malmer / NASA.
“We found that just a small presence of sulfur in the atmosphere, less than 2%, can have major impacts on what, and how many, haze particles are formed,” said first author Dr. Chao He, an assistant research scientist in the Department of Earth and Planetary Sciences at the Johns Hopkins University.
“This entirely changes what scientists should look for and expect when they examine atmospheres on planets beyond our Solar System.”
While planetary scientists already know that sulfur gases influence the photochemistry of many planets within the Solar System such as Earth, Venus and Jupiter, not much is known about sulfur’s role in the atmospheres of exoplanets.
Due to its role as an essential element for life on Earth — emitted from plants and bacteria, and found in several amino acids and enzymes — scientists propose to use sulfur products to search for life beyond Earth.
“Understanding whether sulfur exists and how it affects these atmospheres can help scientists determine whether sulfur gases could be used as a source for life to originate,” Dr. He said.
Dr. He and colleagues performed two sets of experiments using carbon dioxide, carbon monoxide, nitrogen, hydrogen, water and helium as a guide for their initial gas mixtures
One experiment included 1.6% sulfur in the mix and the other did not. The researchers performed the simulation experiments in a specially designed Planetary HAZE (PHAZER) chamber.
Once in the chamber, they exposed the gas mixtures to one of two energy sources: plasma from an alternating current glow discharge or light from an ultraviolet lamp.
Plasma, an energy source stronger than UV light, can simulate electrical activities like lightning and/or energetic particles, and UV light is the main driver of chemical reactions in planetary atmospheres such as those on Earth, Saturn and Pluto.
After analyzing for solid particles and gas products formed, the team found that the mixture with sulfur had three times more haze particles, or solid particles suspended in gas.
Most of these particles were organic sulfur products rather than sulfuric acid or octasulfur, which researchers previously believed would make up the majority of sulfur particles on exoplanets.
“This new information means that if you’re trying to observe an exoplanet’s atmosphere and analyze its spectra, when you previously expected to see other products, you should now expect to see these organic sulfur products instead,” Dr. He said.
“Or, at least, you should know that it wouldn’t be unusual for them to be there. This would change researchers’ explanation and interpretation of spectra they see.”
Similarly, the findings should direct researchers to expect more haze particles if they are observing exoplanet atmospheres with sulfur, as just a small bit of sulfur increases haze production rate by three.
Again, this would change how they interpret their findings and could be critical for future observation of exoplanets.
“The last major implication of our findings is they push for heightened awareness that many sulfur products can be produced in the lab, in the absence of life, so scientists should be caution and rule out photochemically-produced sulfur before suggesting sulfur’s presence as a sign for life,” Dr. He said.
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C. He et al. Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres. Nat Astron, published online April 6, 2020; doi: 10.1038/s41550-020-1072-9
