Overview: Exobiology in Solar System Exploration

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 processes of life in their pursuit to understand its origin, evolution and ubiquity.

Through the science of exobiology, we seek to understand 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; assist us in modeling the ensureavital, exciting,and processes that occur on 2)Prebioticevolution; planets and smaller solar system bodies. 3)Originandearlyevolution of life;and

4)Evolutionof advancedlife. The Inner Compared with Earth, the If we plan to use lunar Moon is depleted of hydro- resources to further planetary Planets gen, carbon, nitrogen and exploration, we first need to their simple compounds. understand the Moon in very The inner planets and the Small amounts of water and close detail. During the Moon have been explored other volatiles, as well as proposed Lunar Orbiter more intensely than the outer minuscule amounts of carbon Mission, data will be collected planets or small bodies due to found in fine-grained mate- to compile a global lunar their proximity and similarity rial, may be vestiges of im- map. From this data we to the Earth. Missions have pacting bodies. No clear should be able to deduce the included flyby and orbiter evidence of complex organic composition of the lunar missions, soft-landings of compounds has been found, surface, the presence of spacecraft on Mars, probes but traces of methane and condensed water, and other into the Venusian atmo- carbide-like species have. This volatiles. It has been sug- sphere, and manned missions suggests a synthesis from gested that a lunar base would to the Moon with samples interactions between the solar be the ultimate location to returned. and the lunar surfaces. In fact, study the origin and history solar wind provides a major of the Moon, which may portion of the Moon's hydro- better help us understand the The Moon gen, carbon, and nitrogen. early history of the Earth. Investigators found that the From this base investigations parts per billion (ppb) using the radio-quiet far side The U.S. Lunar Program amounts of amino acids in could search for evidence for began in 1964 with Ranger lunar soils were not indig- extraterrestrial intelligence VII's photography-based enous, but rather produced and conduct other astronomi- mission, and culminated in during analyses from trace cal investigations. Addition- 1972 with the Apollo project. amounts of chemical precur- ally, scientific investigations Apollo landed men on the sors of amino acids. Appar- could take advantage of the Moon, returned lunar samples ently, solar and cosmic Moon's high vacuum and from six sites, and permitted a irradiation, meteorite bom- extreme changes in quantum leap in our under- bardment, and volatile loss temperature. standing of spaceflight and have severely constrained the lunar evolution. Although Moon's chemical behavior life apparently did not evolve and the evolution of bio- on the Moon, studies help us to better understand Earth chemically significant elements. Questions still to be and conditions and events answered include, How old that occurred more than are the lunar impact craters? 3.5 billion years ago. Did impacts occur in any particular pattern, or were they random? What are the compositions of meteorites on the lunar surface? Venus those waters for nearly a Mars billion years. The possible presence of lightning may Venus is too hostile today to In 1976 and 1977, after have been an important harbor life; temperatures are several Mariner flybys and energy source for the produc- very high as clouds cover after many remote images had tion of organic chemical 100% of the surface. If life did been taken of the Martian compounds. Early environ- exist, it did so far in the past surface, two Viking Landers mental conditions on Venus when conditions were more touched down on Mars. Earth-like. The United States may have been favorable for Because the search for life on the development of life, but if began studying the planet Mars was a major driver for Venus in 1962 with the launch the impact flux was as high as this Mission, three microbio- it was on Earth, early life on of the first successful inter- logical experiments were Venus may have been repeat- planetary probe, Mariner II. performed. However, no In addition to this and edly wiped out. The study of unequivocal evidence was Venus may give us insight Pioneer Venus, various Soviet found for metabolic activity into the physical limitations Venera missions have pro- in any of the surface samples of the habitability of an vided data concerning the analyzed, no organic matter Earth-like planet. properties of the surface of was detected, and no images Venus. Although controver- were returned suggesting Presently, data collected sial, data collected during signs of life. These data led us during the Magellan Mission these missions suggest the to believe the possibility of are being used to create a possibility that ancient lakes extant life on Mars was quite global map of Venus. may have once spread across slim, unless it existed in the surface of Venus. Models Magellan entered orbit hidden, protected niches. around Venus in April 1990. also imply that Venus may The probe's mapping radar have possessed a sizable water However, even if there is no has already mapped over 95% inventory, as well as a cooler extant life on Mars, it is still of the planet and has revealed climate, in its early history. reasonable to hypothesize that about 85% of the planet that life may have once is covered by volcanic rocks, These putative lakes (or existed there. Geologic and mostly lava flows that form oceans) could have remained climatologic studies suggest the great plains. Detailed for several hundred million that the early Martian envi- studies of the surface features years, until they boiled away ronment may have been and their characteristics will as temperatures rose because similar in many respects to allow us to address questions of a runaway greenhouse that of the early Earth. Both on the age, origin, and history effect. There is also a sugges- planets show histories of of Venus. tion that comets, thick with liquid surface water, warmer ice, could have replenished temperatures, and cometary and meteoritic impacts, as well as relatively thick carbon dioxide and nitrogen atmospheres and volcanic activity. Therefore, the elements needed for chemical evolution may have been present on Mars,eventhoughsubse- Oneplannedmissionisthe TheMarsEnvironmental quentplanetaryeventsdid MarsObserverMission(MO), Survey(MESUR)Network not favorthecontinuing scheduledfor launchin Missionisplannedfor several evolutionof life. September1992.TheMission launchopportunitiesbegin- will continuemoredetailed ning in 1999.TheMission Regardlessof whetherlife studiesof theRedPlanetby objectiveisto establisha originatedon Mars,the planet sendingaspacecraftplatform globalnetworkof stationson maycontainwithin its sedi- to orbit Mars.Remotesensing the surfaceof Marsto concur- mentaryrecordtheearly instrumentswill observeand rently collectandreturn historiesof both Marsand maptheentireMartian scientificdataoveramini- Earth.Becauseof Earth's surfaceandatmospherefor mumof oneMartianyear. geologicprocesses,unaltered atleastoneMartianyear, Thefull networkwill consist rocksthat datebackto collectingdataon surface of 16 stations providing pole- 3.5-4billion yearsagoare materials,volatilesanddust; to-pole coverage of Mars. rare.On Mars,however,over topographyandthegravita- Each station will be identical, half theplanet(thesouthern tionalandmagneticfields; weigh less than 150 kilograms, hemisphere)datesbackto the andthe structureandcircula- and house instrumentation to latebombardmenteventof tion of theatmosphere.MO characterize the planet's about3.8billion yearsago. datawill constituteabaseline meteorology, internal seismic StudyingMars,therefore,not for futureMarsmissions. activity, and local surface only helpsusunderstand MO'smostimportantcontri- properties. Using data eventsthat occurredwhilelife bution to exobiologywill be obtained during the Mission, on Earthwasoriginating,it improvingourunderstanding exobiologists will gain insight alsoallowsusto investigate of climatechangeandits into the geological and the possibilityof anextinct consequenceson anypast atmospheric conditions in the Martianbiota. biotaandbiologicalprocesses.past and present, and Mars' ability to sustain a potential Thestudyof Marsprovidesus past biota. MESUR is viewed with cluesto betterdiscrimi- by the scientific community natebetweenconflicting as the logical, evolutionary theoriesof solarsystem step after Viking and before formation,revealingto us sample return/human explo- newcluesabouthowterres- ration missions. trial planetswith atmospheres formedandevolvedand allowingusto morefully understandplanetEarth.New missionsto Marswill study Martianatmosphericand surfacematerialsandpro- cesses,andwill searchbelow theplanet'ssurfaceforburied cluesof past,andperhaps evenpresent,biological activity. TheMarsRoverSample The Outer example, extend ten thou- Return(MRSR)missionis sand to several tens of thou- currentlyproposedfor the Planets sands of kilometers into the late1990s.Recommendedby planets' interiors, thereby the SSECandtheNASA's The outer planets and small representing a significant Officeof ExplorationTask bodies have been explored fraction of a planet's entire Group,chairedbySallyRide, less intensely than the inner mass. These atmospheres are theMRSRmissionwill: gather planets and the Moon. A highly reducing environ- datato betterunderstandthe recent accomplishment in ments, where methane is origin andevolutionof Mars space exploration is the converted into more complex andtheconditionspresenton successful reconnaissance of molecules. veryearlyMars;searchfor Voyager 2 with all the giant evidenceof formerlife;and planets during its grand tour Some exobiologists believe improveourknowledgeof the of the solar system. the chemical steps leading to Martianenvironmentin life involved the production preparationfor human The giant planets include of complex carbon-containing exploration.TheRoverwill Jupiter, Saturn, Uranus, molecules from simple mol- sampledifferentmaterialsat Neptune, and Pluto. They ecules in a highly reducing differentlocationsandwill formed in the same flattened environment. If this is so, performarangeof in situ disk of gas and dust, the solar then atmospheres of the giant measurements. Because the nebula, as did the terrestrial planets represent natural MRSR mission will only planets, but are strikingly laboratories for observing and return 5-10 kilograms of different. Thought to have understanding initial chemis- material, selected samples retained much of the original try that leads to production of must represent the planet's solar nebular material, the organic molecules. Energy full variety. Once returned to giant planets are composed of sources driving the atmo- Earth, the samples can be three basic materials: gas spheric chemistry include analyzed with the benefit of (mostly hydrogen and solar ultraviolet radiation, tools and techniques, such as helium), ice (containing lightning, and high-energy sample dating and trace mixtures of water, carbon charged particles that are element analyses, that are too and nitrogen-containing precipitated into auroral complicated to be performed materials), and rock (mixtures zones. Lightning may be remotely. Data taken during of silicon, magnesium, iron, important in producing the MRSR mission will help oxygen, and other heavy certain disequilibrium species answer questions about how elements). such as hydrogen cyanide and the surface of Mars evolved, carbon monoxide. If carbon what the structure of the The giant planets are bigger and other biologically rel- interior is, whether or not life and more massive than the evant materials came to Earth once existed on the planet, terrestrial planets, and have from the outer solar system, what the planet's present numerous rings and moons the giant planets may have thermal and dynamic states surrounding them. The giant played a fundamental role in are, and what role water planets themselves exhibit a transferring this material to played in the evolution of the wide variation in basic prop- the inner solar system by Martian surface. erties. Atmospheres, for virtue of their gravitational interactions with small bodies.

IO Jupiter low temperatures on the The Galileo Mission will also protocellular stages of pre- help elucidate questions of biological evolution. Some exobiological interest: Is there Our understanding of life as exobiologists believe that if abundant organic material a planetary phenomenon is any Europan biota exists, it present on Europa? Do the based upon Earth-like planets. would have to be anaerobic fractures on the smooth, icy However, there are environ- and prokaryotic. surface indicate the presence ments in the solar system of liquid water beneath the where liquid water, com- The Galileo Mission will take surface? Are there internal monly believed to be a a close look at Jupiter and sources of energy that would prerequisite for biological Europa. It was launched from promote life? Can prebiotic activity, may exist in a dis- the Space Shuttle in October chemistry occur at such cold tinctly non-Earth-like environment. Such an environ- 1989 and follows a complex temperatures? The monitor- ing of Europa for vapor and ment may occur on Jupiter's looping orbit to arrive at Jupiter in late 1995. Once in frost clouds that could result moon, Europa. There, surface the Jovian vicinity, it will from fracturing events may water ice, organic molecules reveal whether liquid water and biogenic elements (avail- spend many years examining the Jovian moons and lies beneath its icy surface. able as precursors for the Jupiter's atmosphere. Radar sounding could also synthesis of biocompounds), detect liquid water if it is are thought to exist along Laboratory research indicates present. High resolution with a continuous supply of that Jupiter's copper and gold imaging of Europa's surface energy (either through the colors are most likely due to could elucidate processes dissipation of tidal energy as an atmosphere rich in bio- involved in the formation of heat or through hydrothermal genic elements, such as its linear features, as well as in activity or solar radiation). phosphorus, methane, variations of its coloration ammonia, and hydrogen gas. and albedo. Europa formed from the Discovery of large storms on Jovian nebula, rich in water Jupiter during the Voyager Future missions using landing and other biogenic elements; project set the stage for a spacecraft to penetrate the therefore, early Europa may have been favorable for the natural laboratory for the surface and perform in situ study of chemical evolution. surface and subsurface mea- capture and in situ formation The Galileo Mission will allow surements in Europa will be of different organic com- us the opportunity to take required to find out what pounds, including biochemi- advantage of this natural actually lies beneath the cal monomers and polymers. laboratory once again. surface. The low-temperature environment may have also facili- tated the preservation and further interaction of labile biochemical polymers. Whether these processes proceeded to a more advanced stage is not known; no experi- mental work has been done at

I1 Saturn Titan is Saturn's largest of Titan's atmosphere should satellite. Titan is an icy, rocky provide more information object that possesses a dense about the primordial nebula Saturn's rings are composed nitrogen-methane atmo- from which Titan was formed. of particles ranging in size sphere. Although its size and from tiny pebbles to large atmospheric density might Information about material boulders and include a stimulate comparisons with presently impacting Titan sprinkling of fine dust. The terrestrial planets, at such a may also be obtained. Because material is mostly icy, and great distance from the Sun, hydrogen cyanide has been contains impurities of Titan is much colder and detected in Titan's atmo- unknown origin. The Satur- richer in ices. The surface sphere, syntheses of as yet nian satellites have dark temperature is about 94K, undetected organics may have coatings presumed to be only a few degrees above the occurred in the atmosphere. carbon-rich organic material. melting point of methane. It Clearly, synthesis of some The source of these coatings is has been postulated that lakes abiotically useful compounds unknown, but further analysis or oceans of liquid methane should have taken place, may help to elucidate the or other hydrocarbons might albeit anhydrously. This is relationship among icy dominate the surface, and likely to have been the case, satellites, comets, and primi- that dense methane clouds as evidenced by compounds tive carbonaceous meteorites. exist in the lower atmosphere already detected. Therefore, By continuing studies of the below the visible orange haze. careful inventories of the outer planets and by compar- various organic compounds ing their structure and behav- Although the conditions on in the atmosphere of Titan ior, we gain insight into the are essential to expand formation and evolution of present day Titan are quite different from those that are exobiology's knowledge of the solar system and can learn thought to have existed on prebiotic evolution of the what chemistry might have early Earth, Titan provides a precursor molecules necessary taken place here on Earth natural setting, on a planetary for the origin of life. Many during the first mysterious scale, for the study of pro- questions remain unan- years. cesses involved with chemical swered: What kind of prebi- evolution. The low tempera- otic chemistry is taking place tures found on Titan preclude in Titan's lower reaches? Do the existence of significant these processes provide clues amounts of water vapor; as to how precursor molecules however, abiotic chemical led to life on Earth? evolution, leading to complex hydrocarbons and nitriles, can still take place. Also, Titan's hazes and ubiquitous colored clouds indicate the presence of aerosols. These aerosols can interact with the atmospheric gases and with each other, thereby influenc- ing the path of atmospheric evolution. Chemical analyses

12 The Cassini Mission is a vital detail. The probe will be Comets, component of an exciting followed by a four year orbital exploration program. While tour of all parts of the Satur- Asteroids, and much of what we know about nian system, including Other Objects Saturn's and Titan's atmo- numerous flybys of Titan and spheres has come from the other main "icy" satellites Comets are known to contain Voyager flyby data, the (Mimas, Enceladus, Tethys, a relatively large amount of Cassini Mission to Saturn and Dione, Rhea, Hyperion, and organic molecules. They are Titan will address fundamen- Iapetus). vehicles potentially capable tal questions about the formation and evolution of NASA will build the orbiter as of delivering to the planets products of interstellar pro- the solar system, prebiological the second spacecraft in its cesses, such as organic com- organic chemistry, plasma Mariner Mark I| series; these pounds produced in space by physics, atmospheric dynam- are modular spacecraft nonequilibrium reactions ics, and virtually every other designed for exploration of (catalysis and radiation aspect of space science. Data the outer planets and small processing of condensed retrieved from Saturn may bodies. The European Space volatiles). Despite their provide insight into the Agency (ESA) will provide the apparent diversity, comets dynamics and nature of Titan Huygens Probe, which represent a population of Saturn's ring particles, will be launched with the essentially homogeneous Saturn's atmospheric compo- orbiter. The Huygens Probe objects that show strong sition and the nature of its will begin sampling the analogy with carbonaceous magnetosphere. Additionally, atmosphere and transmitting chondrites. Saturn's moons will be data at an altitude of about explored and the surface 170 kin. The descent from Comets eventually decay into composition and geologic this level to the surface will gas and dust, becoming a history of these moons will be last approximately 2.5 hours. source of interplanetary determined. Together with Because of the thick haze particles containing carbon. the Galileo Mission to Jupiter, layer, the surface of Titan has Comets may also be a major these missions represent a never been observed; its source of planetary atmo- systematic and comprehen- precise nature will remain spheres. It is believed that sive approach to the study of unclear until the Huygens Earth may have received most the outer planets and will Probe is able to resolve it. of its ocean and organic provide tremendous insight into the formation and molecules from heavy comet bombardment as the solar structure of the solar system. system formed. These simple

The Cassini Mission will and complex organic molecules may still be preserved consist of two parts. The first in comets today. Despite will be the delivery of a probe invaluable data gathered from into the atmosphere of Titan. the Comet Halley flyby, many The probe will sample and questions remain: What is the analyze chemical and physical characteristics of the atmo- complete inventory of organic materials present in comets? sphere and haze particles in How has this material been

13 synthesized into complex, Exobiology is also interested organic complexes from the biologically interesting in the continuing study of methane that is detected molecules? ls there any asteroids and meteorites. Most spectroscopically. Alteration evidence that substantial asteroids, particularly those of these organics to more amounts of material from toward the outer parts of the complex solids by the action comets have been delivered to asteroid belt, are objects of of ultraviolet and cosmic ray the planets? very low-surface reflectivity. flux probably occurs. Tele- Similarly, the surfaces of scopic searches for the CH The concept of orbital diffu- cometary nuclei appear to be spectral signature strongly sion throws a new light on very black. Some planetary suggest its presence on dark, exobiology. It suggests the satellites are also black on a hydrous asteroids, but confir- existence of a cosmic mecha- global scale or contain black mation and further explora- nism, working everywhere, matter mixed with surface tion are needed. that can transfer prebiotic ices. Macromolecular carbon, compounds to ubiquitous found in meteorites and Questions still remain about rocky planets so as to provide interplanetary dust particles the origin of meteoritic a proper environment to (IDPs), is a plausible low- organic matter, particularly initiate life. However, not albedo constituent of these concerning the role played by enough is known about dark (asteroid, comet, and presolar, interstellar processes cometary chemistry for a full satellite) surfaces. and factors controlling the understanding. The study of extent of chemical prebiotic Comet Halley has brought a The presence of organic evolution on different solar wealth of new information, matter in cometary dust has system bodies. Using data but the fast flyby velocity been established. How much obtained during the October destroyed significant mol- organic material do asteroids 1991 Galileo encounter of ecules that were thus not contain? To what extent, and Gaspra, and the planned, characterized. An active, short how, has the organic material fast flyby of Ida in August period comet that has entered in asteroids and meteorites 1993, we will gain insight the inner solar system only been synthesized into com- into the nature of asteroids very recently is the best target plex, biologically interesting and the composition and for exobiology studies. Such a molecules? The source of the source of their low-albedo comet should still contain a organic matter in the meteor- component(s). substantial supply of volatiles, ites and asteroids is unclear, and a high level of activity but isotopic composition should assure a sufficient (deuterium enrichment) amount of gas and dust indicates formation in low- reaching the spacecraft for temperature interstellar studies. molecular clouds. Organic synthesis, in the form of photochemical smog, is currently in progress in some planetary atmospheres Gupiter, Saturn, Uranus, Titan) and on planetary and satellite surfaces (Pluto, Triton), producing yellow and brown-colored

I4 Dust is a ubiquitous compo- accumulated in distant re- logical organic chemistry, nent of our galaxy and solar gions of the solar nebula plasma physics, and atmo- system. Interstellar dust is the and that were later accreted spheric dynamics. This predominant form of con- onto planets. Mission will also help provide densable elements in the evidence as to whether galaxy that are not in stars. Also contributing to the substantial amounts of Grains form in gas outflows understanding of interstellar cometary material have been from stars and they are and interplanetary dust is the delivered to the planets, processed, destroyed and Long Duration Exposure particularly Earth. reformed in the interstellar Facility (LDEF). LDEF is a medium and molecular large, reusable, unmanned, CRAF will study a typical clouds. Interplanetary dust is free-flying spacecraft accom- short-period comet, Kopff, debris recently liberated from modating technology, sci- over more than half its orbit, comets and asteroids within ence, and applications experi- following aphelion near the the solar system and is rela- ments for long-term exposure orbit of Jupiter through tively rich in volatiles, a to the space environment. perihelion and beyond. In source of abiotic organic The LDEF was placed in Earth 2000, CRAF will reach its molecules. The interstellar orbit in April 1984 by Chal- destination with Comet Kopff medium offers a rich variety lenger, and retrieved from and be inserted in an orbit of environments in which orbit in January 1990 by around the Sun that precisely carbon chemistry can occur. Columbia. Several experi- matches that of the comet. A wide variety of chemical ments on board the LDEF CRAF will map the entire processes provides for the studied the chemistry and surface of Kopff with imaging production of rich assem- morphological characteristics and spectroscopic instrumen- blages of biogenically interest- of micrometeoroids and tation. It will identify differ- ing compounds. interplanetary dust. Data ent ices and minerals on the obtained during those experi- comet's surface, and will use A strong link exists between ments should provide an accurate radio tracking to interplanetary and interstellar understanding of the biogenic determine the mass of the dust. Prior to the formation of significance of such extrater- comet. The spacecraft and the solar nebula, most atoms restrial materials. comet will then travel heavier than helium were together through one or more contained in interstellar Due to federal budget cuts, complete orbits. Detailed grains. Some grains were the Comet Rendezvous analyses will be made of the incorporated into comets and Asteroid Flyby (CRAF) Mission gas and dust flowing from the asteroids. Both interstellar and was canceled in February comet's nucleus. A year after interplanetary dust seeded 1992. If resurrected, the CRAF the spacecraft's arrival, a early Earth with elements and Mission will perform measure- penetrator carrying a set of compounds. Collection and ments aimed at a deeper instruments will be placed analysis of extraterrestrial dust understanding of chemical directly into the surface of the through the use of dust- evolution. It will address nucleus to measure elemental collection facilities on Earth- fundamental questions about composition, temperature, orbiting vehicles will provide the formation and evolution and surface strength and to information about the sources of the solar system, prebio- determine the physical state of biogenically significant and composition of the icy elements and compounds that organic mix below any crust on the surface.

15 En route to comet Kopff, Comparison of data with Summary CRAF will fly by and observe results of analyses of inter- a primitive, carbonaceous- planetary dust and meteorites Many fundamental questions type, main-belt asteroid, will elucidate relations remain about the origin of 449 Hamburga, and make between these different solar life, chemical evolution, and remote sensing measure- system bodies, and should the formation and composi- ments. Data obtained will give a clearer picture of how tion of our solar system. A address questions concerning primitive bodies, comets, number of spacecraft missions elemental and isotopic asteroids, and interplanetary have been proposed that will abundances, chemical com- debris may have contributed certainly provide very impor- to the origin of life on Earth. position, ice phases, and the tant clues and answers to mineralogic forms and micro- some of these outstanding scopic physical structures of The data anticipated from questions. For exobiology, the these primitive solar system CRAF will yield much insight highest priorities for future materials which are of great on comets, but ultimately a missions should include a interest to exobiologists. comet sample return mission, thorough investigation of a such as the proposed Rosetta comet and a future cometary The CRAF Mission will pro- Sample Return Mission, will nucleus sample return mis- vide an opportunity for be needed to provide the sion. In addition, it is recom- significant advances in the range and detail of analyses mended that a comprehensive knowledge of primitive (yet needed to definitively charac- study of Mars be pursued and complex) material in short- terize a comet nucleus. that a sample return mission period comets and the surface to Mars, with intelligently of an asteroid. The Mission chosen samples from carefully will obtain new information selected sites, be a high concerning the early environ- priority. ment of the solar system and the extent of chemical evolu- Future exploration objectives tion in such environments. should be to determine the present nature of the solar system, its planets and primitive bodies to understand how the solar system and its objects formed, evolved, and (in at least one case) produced an environment that could give rise to and sustain life. Another objective should be that of comparative planetol- ogy: to better understand the Earth by determining the general processes that govern all planetary development and by understanding why

16 terrestrial planets differ so • Scientific exploration of the With a national commitment much from each other. Moon and Mars, with the first to human presence in the piloted lunar landing in 2003 solar system endorsed, and National goals for space and extensive robotic activity reports such as the Synthesis exploration were outlined in on both bodies; Report and the Ride Commis- the Space Exploration Initia- sion Report "Leadership and tive (SEI). First announced in • A permanent Moon base America's Future in Space" 1989 and established by the followed by human excur- recommending Mission to Bush Administration in 1990, sions to Mars; Planet Earth, future lunar SEI provided the basic frame- bases, expanded planetary work for future exploration: • A Moon-oriented approach exploration, a series of Mars back to the Moon and then a toward returning solar energy Sample Return missions, and piloted mission to Mars. The and other natural resources to the human exploration of 90-Day Study, 1989, con- Earth, with a self-sufficient Mars, we are preparing for a tained explicit suggestions for Mars base initiated in 2016. future of national leadership the successful exploration of and activity beyond Earth the Moon and Mars. More To achieve these objectives orbit and into the solar recently, the Synthesis Report, and advance exploration of system. 1991, was the first serious the solar system at the look at how to fulfill the healthy pace recommended President's stated objectives in by the SSEC, we must space. The Report contains strengthen our ground-based the findings of a year-long planetary science research survey of the U.S. aerospace effort, put energy into major engineering community and technological developments, the general public, and build up our research and features four basic options for data analysis capabilities, and returning humans to the increase our potential for Moon and proceeding toward telescopic observations and Mars. These options include: remote analysis capabilities. Additionally, we need to • A trip to Mars in 2014 by six develop and foster relation- astronauts, with testing of ships with other nations related technology on and to further international around the Moon beginning cooperation. in 2005;

17 ORIGINAL _-_,AGE COLOR PHOTOGRAPH

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