section_path: Astrobiology
section_name: Astrobiology
paragraph_text: Astrobiology
  
  The characteristics of ocean worlds or ocean planets provide clues to their history, and the formation and evolution of the Solar System as a whole. Of additional interest is their potential to form and host life. Life as we know it requires liquid water, a source of energy, and nutrients, and all three key requirements can potentially be satisfied within some of these bodies, that may offer the possibility for sustaining simple biological activity over geological timescales. In August 2018, researchers reported that water worlds could support life.
  
  An ocean world's habitation by Earth-like life is limited if the planet is completely covered by liquid water at the surface, even more restricted if a pressurized, solid ice layer is located between the global ocean and the lower rocky mantle. Simulations of a hypothetical ocean world covered by five Earth oceans' worth of water indicate the water would not contain enough phosphorus and other nutrients for Earth-like oxygen-producing ocean organisms such as plankton to evolve. On Earth, phosphorus is washed into the oceans by rainwater hitting rocks on exposed land, so the mechanism would not work on an ocean world. Simulations of ocean planets with 50 Earth oceans' worth of water indicate the pressure on the sea floor would be so immense that the planet's interior would not sustain plate tectonics to cause volcanism to provide the right chemical environment for terrestrial life.
  
  On the other hand, small bodies such as Europa and Enceladus are regarded as particularly habitable environments because the theorized locations of their oceans would almost certainly leave them in direct contact with the underlying silicate core, a potential source of both heat and biologically important chemical elements. The surface geological activity of these bodies may also lead to the transport to the oceans of biologically-important building blocks implanted at the surface, such as organic molecules from comets or tholins, formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane, often in combination with nitrogen.