Overview: Exobiology in N93-18546 Solar System Exploration Glenn C. Carle and Deborah E. Schwartz ow, why, where, and when did life arise? What is the relationship between life and I Iongoing processes in the universe? These questions are perhaps some of humankind's oldest philosophical inquiries and still resonate today. Philosophers ponder these most difficult and profound subjects from a metaphysical standpoint, while scientists have and will continue to conduct methodical studies of the physical and chemical pro- cesses of life in their pursuit to understand its origin, evolution and ubiquity. Through the science of exobiology, we seek to under- stand the origin and evolution of life and life-related processes and materials throughout the universe. We know a great deal about contemporary living systems and the way they function, but as we ask about earlier and PRECEDING P.3G_ bL,,I_K riO'i FILMED earlier life forms, there are about the interplay between are very similar to the con- fewer known facts. Much has environment and prebiotic temporary microbial mat been learned about exobiol- process remains to be under- communities often found in ogy, but considerably more stood. Some of the questions shallow waters. The fossil knowledge is left to be gained exobiologists seek to answer stromatolites were, as the before the subject is well are: What were the sources microbial mats are today, understood. and complexities of the complex communities com- prebiotic molecules? What prising a number of different Understanding the relation- role did the environment play types of microorganisms. ship between the origin and in mediating or directing the While the fossil stromatolites evolution of life and the chemical evolution that are very ancient, they were origin and evolution of the fostered Earth's first replicat- highly evolved ecosystems solar system is one of the ing system? ls Earth the only and certainly do not represent main obiectives of exobiol- place in the solar system (or Earth's earliest life forms. ogy. Clearly, life is intimately in the universe, for that connected with its environ- matter) where life exists? Given that evidence of the ment. With respect to the Could life have originated earliest epochs has not yet origin and subsequent evolu- and survived elsewhere? been discovered and may not tion of life on Earth, the be available on Earth, exobi- environment played a critical While pursuing the answers ologists have turned outward role. When life first arose on to these weighty questions, to the rest of the solar system Earth, the biogenic elements exobiologists extrapolate from to understand the conditions (carbon, hydrogen, nitrogen, the single example they have that fostered life. Further- oxygen, phosphorus, sulfur) access to: life on Earth. They more, in other bodies of the and traces of many other study contemporary biology solar system, they may find elements must have been in the laboratory by way of additional environments available in appropriate the conventional disciplines where life now exists or where amounts. Temperatures must such as microbiology, bio- it once existed. Even though have been moderate enough chemistry, and biophysics. the Viking spacecraft found for water to exist as a liquid, They also study the Earth's no evidence of life existing on but not so hot as to destroy geological and fossil record by Mars, if early Mars was like organic molecules. The way of the disciplines of the early Earth, life may have environment must have geology and paleontology. begun on Mars, also. Fortu- been conducive to prebiotic Earth itself complicates this nately, substantial areas on organic chemistry (i.e., the work. The ancient rock record, Mars have had little or no evolution of simple chemicals which may have once held geological recycling since its to complex molecules neces- the key to chemical evolution earliest period. Sediments sary for development of living and the origin of life, has containing fossils, or other systems); otherwise, the apparently been destroyed by biomarkers, have not been prebiotic precursors to life ongoing geological processes. subducted, ground up, or could not have formed. Much The oldest fossil evidence of remelted as they have been life are stromatolites, which date back to about 3.S billion years ago. Although only slightly younger than Earth's oldest rocks, these life forms on Earth.Therefore,the Continuing Mars. While neither investiga- possibilityexistsfor apre- tion found evidence for servedgeologicalrecordof the Inquiries extraterrestrial life, each originof life on Mars.In fact, gathered a wealth of informa- Marsmayhold theonly In 1958, the Congress of the tion about these and other existinggeologicalevidence United States enacted the bodies in the solar system. of theorigin of life in the National Space Act. This act This information is still being solarsystem.A missionto validated the importance of interpreted. Today, many new Marswith exobiology-specific exploring space and estab- opportunities lie on the experimentsandinstruments lished, for the first time, an horizon. In the United States mayfind thisevidence. organization that could and the Commonwealth of provide direct access to the Independent States (formerly Exobiologistsalsoseekknowl- solar system. This organiza- the Soviet Union), as well as edgeof theearliestenviron- tion was the National Aero- in Europe and Japan, missions menton Earthbyinvestigat- nautics and Space Administra- are being planned to comets, ingthe rolethat cometsmay tion (NASA). In 1984, the the outer planets and their haveplayedin deliveringto Subcommittee on Space satellites, and Mars. It is Earthbiogenicmaterials Science and Applications appropriate, therefore, to necessaryfor chemicalevolu- reviewed the Space Act and reassess the current status of tion andtheorigin of life. concluded that knowledge about the bodies Scientistsask,Whatroledid of the solar system and cometaryimpactsplayin the A healthy space science identify the role currently origin of life?Whatwasthe program is essential to planned missions can play in compositionof Earth'satmo- continued strength and expanding our understanding sphereduringstagesof vitality of the space of exobiology. chemicalevolutionandthe program...(and that) NASA origin of life?Accordingto should initiate enhance- In August 1988, the NASA onetheory,cometsintro- ments in the space and Ames Research Center held a ducedessentialorganicsinto earth sciences including three-day symposium in theatmospherespurring pursuit of planetary explora- Sunnyvale, California, to developmentof biopolymers. tion through a renewed discuss the subject of exobiol- Recentdatafrom encounters commitment to exploration ogy in the context of explora- with Halley'sCometsupport of the solar system, and tion of the solar system. theseclaims,showingthat expansion of human Leading authorities in exobi- cometscontainsubstantial knowledge of the Earth and ology presented invited amountsof waterandorganic phenomena in the atmo- papers and assisted in setting compounds.Theexactnature sphere and space. future goals. The goals they of theseorganicsisasyet set were to: unknownandtheir contribu- NASA has carried out many tionsto chemicalevolution important steps in this • Review relevant knowledge arealsounknown.Missionsto pursuit. Two notable accom- learned from planetary othercometsmayprovide plishments are the Apollo exploration programs; informationneededto under- Missions with the concurrent standEarth'searliestchemical search for life on the Moon andphysicalprocesses. and the Viking Missions which searched for life on • Detailsomeof theinforma- scientificallyvaluableeffort NASA'sprogramisdesigned tion that isyet to beobtained; throughtheturn of the to traceevolutionarypath- century.TheSSEC'sgoals waysof theuniverse,includ- • Describefuturemissions include: ing synthesisof biogenic andhowexobiologists,aswell elements,evolutionof plan- asother scientists,canpartici- • Determiningtheorigin, etarysystems,origin of life, pate;and evolution,andpresentstate andevolutionof intelligent of thesolarsystem; life. • Recommendspecificways exobiologyquestionscanbe • UnderstandingEarth To attainthesegoals,relevant addressedon futureexplora- throughcomparativeplan- studiesof theplanetsand tion missions. etologystudies;and otherbodiesof our solar systemarenecessary.The Thesegoalsarein agreement • Revealingthe relationship SSEC'sstrategyfor explora- with thoseof the SolarSystem betweenthechemicaland tion isto beginwith aglobal ExplorationCommittee physicalevolutionof the viewof the solarsystem (SSEC)of theNASAAdvisory solarsystemandtheappear- bodies,andonly latermove Council.Formedin 1980to anceof life. to detailedobservationsand respondto theplanetary measurementsin selected explorationstrategiessetforth TheSSEC'sgoalsareconsis- regions.Althoughmuch by theSpaceScienceBoardof tent with theover-arching informationisgainedfrom theNationalAcademyof goalof NASA'sExobiology first generationreconnais- Sciences'Committeeon Program,whichprovidesthe sancemissions,in situ atmo- PlanetaryandLunarExplora- criticalframeworkandsup- spheric and surface measure- tion (COMPLEX),theSSEC's portfor basicresearch.The ments and the analysis of mainfunctionisto reviewthe researchisdividedinto the returned samples provide the entireplanetaryprogram.The followingfourelements: specific details needed for committeeformulatedalong- interpretation and under- termplan(within acon- 1)Cosmicevolutionof the standing. Such missions also strainedbudget)thatwould biogeniccompounds;