section_path: Suitable geological environments/Surface bodies of water
section_name: Surface bodies of water
paragraph_text: Surface bodies of water
  
  Surface bodies of water provide environments able to dry out and be rewetted. Continued wet-dry cycles allow the concentration of prebiotic compounds and condensation reactions to polymerise macromolecules. Moreover, lake and ponds on land allow for detrital input from the weathering of continental rocks which contain apatite, the most common source of phosphates needed for nucleotide backbones. The amount of exposed continental crust in the Hadean is unknown, but models of early ocean depths and rates of ocean island and continental crust growth make it plausible that there was exposed land. Another line of evidence for a surface start to life is the requirement for UV for organism function. UV is necessary for the formation of the U+C nucleotide base pair by partial hydrolysis and nucleobase loss. Simultaneously, UV can be harmful and sterilising to life, especially for simple early lifeforms with little ability to repair radiation damage. Radiation levels from a young Sun were likely greater, and, with no ozone layer, harmful shortwave UV rays would reach the surface of Earth. For life to begin, a shielded environment with influx from UV-exposed sources is necessary to both benefit and protect from UV. Shielding under ice, liquid water, mineral surfaces (e.g. clay) or regolith is possible in a range of surface water settings. While deep sea vents may have input from raining down of surface exposed materials, the likelihood of concentration is lessened by the ocean's open system.