Search for Life Beyond Earth Lec 12 Post Midterm No copyright infringement is intended by the creation or use of these notes Astrobiology s Core Questions o How does life begin and evolve o Does life exist elsewhere in the Universe o What is the future of life on Earth and beyond Mantras of Astrobiology to search for life elsewhere o Traditional approach Follow Water o New approach Follow Nitrogen and energy source o Assumptions Life is Carbon based Sun like stars provide energy o A theoretical area and certain distance from a star where a planet can have liquid water on its surface Habitable Zone Following Nitrogen o Nitrogen is very abundant in our atmosphere we have nitrogen cycle o Nitrogen cycle nitrogen being converted into different forms typically by microbes to be used Inorganic N2 g Organic NH4 NH3 NO3 o Since Nitrogen is so abundant it is easier to find on different planets if they have similar atmospheres rather than carbon a lot less abundant mostly in crust mantle Following Energy o Energy requirements are common for life o Energy requirements consist of two components Power energy per unit time Voltage energy per unit event Both have a minimum requirement that if not present seems impossible for biological activity Ex Microbes only go through reactions if making ATP is likely o We are looking for disequilibrium for potential energy that an organism can use ex volcanism hydrothermal activity Goals of Astronomy Astrophysics in Astrobiology o Find earth like planets o Determine basic composition of an extrasolar planet s atmosphere and surface Goals of Biology in Astrobiology o Study extreme environments origin of life and evolution of life o An alternative biochemistry of alien life forms radically different from Alternative Biochemistry those of earth Chirality Same molecules but different structure mirrored On Earth o Amino acids in proteins are left handed L o Sugars in DNA RNA and metabolic pathways are right handed D Aliens could posses reversed structures chirality Carbon Chauvinism Not considering non carbon based life Humans are carbon based beings and life has not been encountered beyond earth Special things about carbon o Can bond with itself buildability o Can form long chains length o Can form rings stability Other prospective element based life include o Silicon o Nitrogen phosphorous o Arsenic o Chlorine o Sulfur Silicon Biochemistry Most popular alternative to carbon biochemistry b c of its similarity properties same periodic table group carbon group Problems o Molecular size Silicon has a larger molecular size and atomic radius than carbon difficult to form double or triple covalent bonds less stability and necessary for biochemical systems Silicones Alternating O and Si atom polymers are more common stable however they are still less stable than carbon chains still o Silicon dioxide alternative to CO2 Non soluble solid at temps where water is liquid thus hard to interact with water based biochemical systems Also it would have to be product of aerobic respiration exhaling solid issue Silicon vs Carbon o Many molecules found in the interstellar medium are carbon based o Cosmic abundance of carbon to silicon is 10 1 o Earth and other terrestrial planets are silicon rich and carbon poor yet terrestrial life is still carbon based o Conclusion Even though carbon is rare in terrestrial planet life is still carbon based Silicon must not be suitable biochemistry for terrestrial like planets Silicon still plausible for planets with different temperatures and pressures different from surface of terrestrial planets Nitrogen and Phosphorous biochemistry Can bond together and form stable covalent bonds Can form many molecules long chain and ring shaped included Biochemistry useful in a NO2 or NH3 atmosphere b c N2 g is inert has strong triple bond and requires large energy to fix convert to usable organic form In NO2 atmosphere o NO2 equivalent to CO2 o Plants would take in NO2 and released O2 o Built biomass on phosphorous and nitrogen molecule In NH3 atmosphere o Plants take in NH3 and release H2 o Built biomass on NH3 phosphorous o Arsenic Chlorine and Sulfur biochemistry Arsenic can replace phosphorous in sugars or in the Chlorine could replace Oxygen in both carbon based or Sulfur can form long chain molecules but also highly backbone of DNA non carbon based life reactive Difference of other planets from Earth Life o Non green photo synthesizer plants The chlorophyll of earth s planets are green b c they absorb red and blue light and reflect green light frequency If other stars were in a different state than our sun and if light frequencies that reached the planet were different then plants would have different photosynthetic pigment plants different color Conclusion Atmospheric and sun conditions determine the wavelength of light that enters a planet different pigments different colored plants Different planets may have different atmospheres thereby affecting o Atmospheric Composition the biochemistry of life o Non Water Solvents Advantages of Water Liquid at wide range of temperatures High heat capacity temp regulation and heat of vaporization Less dense as ice protects oceans from freezing Capable of dissolving many compounds Advantages of Ammonium NH4 Capable of numerous chemical reactions Dissolves many organic molecules at least as much as Can dissolve many metals water cannot dissolve as much Disadvantages of Ammonium NH4 Only stable in a reduced atmosphere anoxic non oxygen water rich Hydrogen bonds between ammonium molecules are weaker than those of water mean o Lower heat of vaporization 1 2 of H2O o Lower surface tension 1 3 of H2O o Reduced ability to concentrate non polar molecules through hydrophobic effect oils and water less separated Ammonium replacing water Ammonium boiling and freezing pt low at normal pressure bad for biochemical reactions 78 to 33 C Ammonium boiling and freezing pt at high pressure 60 bar for ex good for biochemical reactions 77 to 98 C Ammonium mixed with water remains liquid at temperatures below freezing temperature of pure water good for planets outside habitability zone Other proposed liquids non water solvents HF most aggressive acid not active acid if no water Methanol Hydrogen sulfide chloride not liquid if binds to minerals iron manganese etc Mixture of hydrocarbons such as methane ethane o Need polarity to dissolve irons but other life might not need it Formamide used to kill and preserve but possible liquid