Ammonia as a parameter shaping habitability on icy moons

The search for life now extends beyond the traditional habitable zone to include the icy moons of Jupiter and Saturn. These moons feature ice-covered surfaces overlying substantial oceans formed primarily of liquid water and other potential constituents, such as ammonia. On Earth, ammonia supports biochemistry at low concentrations by providing nitrogen but becomes disruptive at higher concentrations. Ammonia could therefore influence the habitability of extraterrestrial oceans, yet this topic has received limited attention in the literature. This review synthesises current research on ammonia in Saturn's icy moons, Enceladus and Titan, and its effects on terrestrial life. We summarize the celestial incorporation, speciation, and phase behaviour of ammonia and review data on its occurrence and concentration in icy moon oceans. We examine the role of ammonia in prebiotic chemistry, biochemistry, and toxicity. Focusing on bacteria, we compare known survival limits in ammonia to estimated ammonia concentrations on Enceladus and Titan. We find that bacterial survival limits exceed concentrations estimated on Enceladus, but are below those estimated on Titan, and propose that ammonia measurements are crucial for assessing extraterrestrial habitability. Finally, we highlight outstanding knowledge gaps and challenges that influence our understanding of how ammonia shapes the potential for life beyond Earth.