Abstract
Abstract Liquid water oceans are at the center of our search for life on exoplanets because water is a strict requirement for life as we know it. However, oceans are dynamic habitats—and some oceans may be better hosts for life than others. In Earth’s ocean, circulation transports essential nutrients such as phosphate and is a first-order control on the distribution and productivity of life. Of particular importance is upward flow from the dark depths of the ocean in response to wind-driven divergence in surface layers. This “upwelling” returns essential nutrients that tend to accumulate at depth via sinking of organic particulates back to the sunlit regions where photosynthetic life thrives. Ocean dynamics are likely to impose constraints on the activity and atmospheric expression of photosynthetic life in exo-oceans as well, but we lack an understanding of how ocean dynamics may differ on other planets. We address this issue by exploring the sensitivity of ocean dynamics to a suite of planetary parameters using ROCKE-3D, a fully coupled ocean–atmosphere general circulation model. Our results suggest that planets that rotate slower and have higher surface pressure than Earth may be the most attractive targets for remote life detection because upwelling is enhanced under these conditions, resulting in greater nutrient supply to the surface biosphere. Seasonal deepening of the mixed layer on high-obliquity planets may also enhance nutrient replenishment from depth into the surface mixed layer. Efficient nutrient recycling favors greater biological activity, more biosignature production, and thus more detectable life. More generally, our results demonstrate the importance of considering oceanographic phenomena for exoplanet life detection and motivate future interdisciplinary contributions to the emerging field of exo-oceanography.
Highlights
Water is an essential ingredient for life as we know it (McKay 2014)
We hypothesize that planets with more efficient nutrient recycling via ocean upwelling will be better hosts for photosynthetic life than planets where nutrients will be sequestered at depth
Ocean circulation throttles the accumulation of biological products in planetary atmospheres and is an important consideration for the oxygenation of our planet and the detectability of exoplanet life
Summary
Water is an essential ingredient for life as we know it (McKay 2014). For this reason, the potential existence of a liquid water ocean defines the Habitable Zone concept that guides our search for life in the Universe (Kasting et al 1993). Recent studies have investigated the importance of considering ocean heat transport for regulating climate and elucidating the boundaries of the Habitable Zone (Hu & Yang 2014; Cullum et al 2014, 2016; Yang et al 2019), but the significance of ocean circulation is not limited to climate influences. Ocean circulation is a primary control on the distribution of biological activity at Earth’s surface. Life in Earth’s ocean is concentrated in the shallow sunlit portion of the water column where photosynthesis is viable. The chemical reaction corresponding to photosynthesis can be represented as: CO2 + H2O −h−v→ CH2O + O2
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