page_title: Hypothetical types of biochemistry
hierarchical_section_title: Hypothetical types of biochemistry
caption_attribution_description: This Cassini false-color mosaic shows all synthetic-aperture radar images to date of Titan's north polar region. Approximately 60 percent of Titan's north polar region, above 60 degrees north latitude, is now mapped with radar. About 14 percent of the mapped region is covered by what is interpreted as liquid hydrocarbon lakes. Features thought to be liquid are shown in blue and black, and the areas likely to be solid surface are tinted brown. The terrain in the upper left of this mosaic is imaged at lower resolution than the remainder of the image Most of the many lakes and seas seen so far are contained in this image, including the largest known body of liquid on Titan. These seas are most likely filled with liquid ethane, methane and dissolved nitrogen. Many bays, islands and presumed tributary networks are associated with the seas. The large feature in the upper right center of this image is at least 100,000 square kilometers (40,000 square miles) in area, greater in extent than Lake Superior (82,000 square kilometers or 32,000 square miles), one of Earth's largest lakes. This Titan feature covers a greater fraction of the surface, at least 0.12 percent, than the Black Sea, Earth's largest terrestrial inland sea, at 0.085 percent. Larger seas may exist, as it is probable that some of these bodies are connected, either in areas unmapped by radar or under the surface (see PIA08365). Of the 400 observed lakes and seas, 70 percent of their area is taken up by large "seas" greater than 26,000 square kilometers (10,000 square miles). The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL. The radar instrument was built by JPL and the Italian Space Agency, working with team members from the United States and several European countries. For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov/home/index.cfm. The original NASA image has been modified by rotating 90 degrees clockwise, cropping and reduction in size (the linear pixel density was reduced 50%). Some of the features in this image have been annotated in Wikimedia Commons.
context_page_description: Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons, it may be chemically similar; it is also possible that there are organisms with quite different chemistries—for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.
  The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction.
  The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent.
context_section_description: Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time. The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons, it may be chemically similar; it is also possible that there are organisms with quite different chemistries—for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.
  The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in synthetic biology and is also a common subject in science fiction.
  The element silicon has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the periodic table and, like carbon, it is tetravalent. Hypothetical alternatives to water include ammonia, which, like water, is a polar molecule, and cosmically abundant; and non-polar hydrocarbon solvents such as methane and ethane, which are known to exist in liquid form on the surface of Titan.
caption: False-color Cassini radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons.
  "The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth."—NASA Astrobiology Roadmap 2008[1]
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