Passage:
In 2005, spectral data gathered by space probes orbiting Saturn revealed that the icy moon Enceladus ejects plumes of water vapor and ice grains from fractures in its south polar terrain. Early chemical analyses confirmed the presence of simple organic molecules, but planetary scientists remained divided over whether the subsurface source of these plumes could sustain conditions suitable for prebiotic chemistry. Subsequent high-resolution mass spectrometry of the ice grains detected sodium and potassium salts at concentrations comparable to Earth's oceans. Because volatile gases egested from subterranean reservoirs would freeze rapidly without dissolving minerals unless liquid water was in prolonged contact with a rocky core, the presence of these salts indicated that the plume material originates from a global subsurface ocean rather than isolated, localized pockets of melting ice.
Furthermore, recent detection of native molecular hydrogen () within the plume matrix provided key insights into the thermodynamic state of this subsurface ocean. In terrestrial oceanic systems, dissolved is generated primarily through serpentinization—a geochemical reaction in which ultramafic mantle rocks are oxidized by water at elevated temperatures, releasing energy and chemical reductants. Planetary geologists reasoned that if serpentinization occurs on Enceladus, it would generate alkaline hydrothermal environments rich in dissolved minerals. Crucially, while high salt salinity demonstrates extensive water-rock leaching over long timescales, the presence of volatile molecular hydrogen—which escapes rapidly into space once exposed—proves that serpentinization activity must be ongoing in the present epoch, rather than a legacy of ancient geothermal activity. Synthesizing these observations suggests that Enceladus possesses both a sustained fluid ocean and an active hydrothermal energy source capable of fueling chemosynthetic metabolisms.
Based on the passage, which of the following inferences regarding the subsurface environment of Enceladus is most strongly supported?
- AThe presence of sodium and potassium salts in the plume ice grains is sufficient by itself to demonstrate that hydrothermal reactions are actively occurring in the present epoch.
- BChemosynthetic organisms on Enceladus actively utilize molecular hydrogen to produce complex organic molecules at rates comparable to deep-sea hydrothermal vents on Earth.
- The liquid water body on Enceladus is currently undergoing high-temperature geochemical interactions with a rocky core rather than existing as an isolated, inert layer of melted ice.Answer
- DThe volatile gases ejected from Enceladus originate from isolated, localized pockets of subsurface ice that briefly melt during periodic geothermal events.
- EPlanetary scientists universally agree that the thermodynamic environment on Enceladus renders the emergence of extraterrestrial life an inevitable outcome.