09:05 〜 09:25
[MIS06-01] Geochemical perspectives on habitability and the origin of life in the outer solar system
★Invited Papers
キーワード：Ocean worlds, Enceladus, Europa
The potential to find evidence of extraterrestrial life has never been greater than it is now. In the past twenty-some years, we have come to the recognition that icy worlds beyond the asteroid belt are perhaps the most promising places to host extant life in the solar system. These bodies are icy and cold on the outside, but evidence indicates that many of them harbor subsurface water oceans. Liquid water appears to be common out there. We are starting to reach a point where geochemistry moves to the forefront, as we begin to contemplate and investigate the forms and quantities of bioessential elements and energy sources that may be available to support life. These considerations have coalesced at Enceladus, a Saturnian moon that serves as an example for revealing conditions and processes that can occur inside an icy ocean world. We found that Enceladus hosts a diverse organic chemistry (Postberg et al., 2018), and hydrothermal activity that generates chemical disequilibrium in its ocean (Waite et al., 2017). More recently, geochemical modeling based on Cassini data has shown that the potential metabolic landscape may be richer than previously thought, thanks to the formation of oxidants that are derived from water radiolysis (Ray et al., 2021). Geochemical modeling also suggests that phosphate, one of the final "missing" ingredients of habitability, should be relatively abundant in Enceladus's ocean (Hao et al., submitted). These findings suggest that the ocean of Enceladus is habitable.
We have reached a juncture of three pathways moving forward. One pathway leads to the question of the origin of life on Enceladus. Habitable does not imply inhabited unless life first emerges. If atmospheric synthesis of prebiotic building blocks or the presence of dry land is essential for abiogenesis, then Enceladus's ocean will be lifeless. If, on the other hand, hydrothermal processes are central to the origin of life, especially the strong drive for abiotic organic synthesis under dynamic H2-rich conditions, then finding life on Enceladus might provide fresh clues to how life began on Earth. Such insight from a natural laboratory that has operated for at least tens of millions of years is one reason why many of us are eager to proceed down the second pathway that culminates with a return to Enceladus. Mission concepts are now being formulated to search for biomolecules in the plume or on the surface of Enceladus. Lastly, there is a third pathway of comparative planetary oceanography. The next destination will be Jupiter's moon Europa, which will be explored in great detail by the Europa Clipper mission. This will provide an important opportunity to look for similarities and differences in the geochemistry of icy ocean worlds, and give us new perspectives on the role of geochemistry in their habitability and the possible origin of life on such bodies.
We have reached a juncture of three pathways moving forward. One pathway leads to the question of the origin of life on Enceladus. Habitable does not imply inhabited unless life first emerges. If atmospheric synthesis of prebiotic building blocks or the presence of dry land is essential for abiogenesis, then Enceladus's ocean will be lifeless. If, on the other hand, hydrothermal processes are central to the origin of life, especially the strong drive for abiotic organic synthesis under dynamic H2-rich conditions, then finding life on Enceladus might provide fresh clues to how life began on Earth. Such insight from a natural laboratory that has operated for at least tens of millions of years is one reason why many of us are eager to proceed down the second pathway that culminates with a return to Enceladus. Mission concepts are now being formulated to search for biomolecules in the plume or on the surface of Enceladus. Lastly, there is a third pathway of comparative planetary oceanography. The next destination will be Jupiter's moon Europa, which will be explored in great detail by the Europa Clipper mission. This will provide an important opportunity to look for similarities and differences in the geochemistry of icy ocean worlds, and give us new perspectives on the role of geochemistry in their habitability and the possible origin of life on such bodies.