Thiophenes on Mars: Biotic or Abiotic Origin?

Abstract

The question whether organic compounds occur on Mars remained unanswered for decades. However, the recent discovery of various classes of organic matter in martian sediments by the Curiosity rover seems to strongly suggest that indigenous organic compounds exist on Mars. One intriguing group of detected organic compounds were thiophenes, which typically occur on Earth in kerogen, coal, and crude oil as well as in stromatolites and microfossils. Here we provide a brief synopsis of conceivable pathways for the generation and degradation of thiophenes on Mars. We show that the origin of thiophene derivatives can either be biotic or abiotic, for example, through sulfur incorporation in organic matter during early diagenesis. The potential of thiophenes to represent martian biomarkers is discussed as well as a correlation between abundances of thiophenes and sulfate-bearing minerals. Finally, this study provides suggestions for future investigations on Mars and in Earth-based laboratories to answer the question whether the martian thiophenes are of biological origin.

The Dissipative Photochemical Origin of Life: UVC Abiogenesis of Adenine

“…Any planet around any star giving off light in the long wavelength UVC region, but with protection against shorter wavelength light which could destroy carbon-based molecules through successive ionization, should therefore have its own concentration profile of dissipatively structured carbon-based fundamental molecules (UVC pigments) whose characteristics would depend on the exact nature of the local UV environment and the precursor and solvent molecules available. Examples may include the sulfur containing UV pigments found in the clouds of Venus [138], the UV absorbing thiophenes [139] and the red chlorophyll-like pigments [140] found on the surface of Mars, the UVC and UVB absorbing poly-aromatic hydrocarbons (PAHs) found in the atmosphere and on the surface of Titan [141], on the surface of asteroids, and in interstellar space [12]. The observation that thiophenes and PAHs found on mars, on asteroids, and in space are of generally large size can be understood from within this non-equilibrium thermodynamic perspective since, without the possibility of vibrational dissipation through hydrogen bonding to solvent molecules, these molecules would have "grown" to large sizes through dissipative selection in order to support many low frequency vibrational modes which would increase dissipation by pushing the emitted photon energy towards the infrared.…”

UV-Induced Nanoparticles-Formation, Properties and Their Potential Role in Origin of Life

“…It is also not known if this compound can be substituted by another plausible molecule. However, several sulfur-containing molecules have been detected in the Gale impact structure on Mars [66] and the introducing of very complex sulfurcontaining organic compounds in prebiotic chemistry has been extensively explored starting with Wächtershäuser's hypothesis of the iron-sulfur world [67]. Acetic acid, whose Cd and Zn salt was used for the reaction described in this study, is also a common compound in Fischer-Tropsch processes and Wächtershäuser's prebiotic chemistry [68,69].…”

Formation of Thiophene under Simulated Volcanic Hydrothermal Conditions on Earth—Implications for Early Life on Extraterrestrial Planets?

“…The importance of sulfides in an origin-of-life scenario is emphasized by Wächtershäuser's Iron-Sulfur-World hypothesis [16]. Based on this hypothesis, we here report the facile abiotic formation of thiophene and some of its derivatives from acetylene and metallo-sulfides, especially NiS, under aqueous conditions at 105 • C. In context with the formation of potential building units and reaction networks for the emergence of metabolism under the same conditions [11,12] and capitalizing on recent hypotheses [2], the detection of extraterrestrial or terrestrial thiophenes could therefore indeed be indicative of early metabolic evolution under chemoautotrophic conditions.…”

“…Recent findings by the Curiosity mission have shown the existence of thiophene and some of its derivatives on Mars [1,2]. Within this context, different possibilities for their abiotic as well as biotic formation were discussed.…”

“…Within this context, different possibilities for their abiotic as well as biotic formation were discussed. Thiophene was even suggested as a biomarker in the search for life on Mars [2], whereas a hydrothermal abiotic origin was also considered [1]. Likewise, thiophenes are common pyrolysis products from meteoritic macromolecular materials.…”