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UNIVERSEin the

A summary of recentdevelopments in thefield of astrobiology

LIFE

The Eagle Nebula, photographed by the Hubble Space Telescope in 1995, covers an area roughly twolight-years bottom to top. Projections at top edges of nebula are gaseous globules surrounding newly

formed stars; the nebula as a whole is shaped by photoevaporation under the radiation from hot stars.

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HPaul D . Lowman J r .

ow widespread is life in the universe? The optimism of the late Carl Sagan,expressed through a radio astronomer in his novel Contact, is widely shared:“The origin of life seemed to be so easy—and there were so many planetarysystems, so many worlds, and so many billions of years available for biological

evolution—that it was hard to believe the Galaxy was not teeming with life and intelli-gence” (1985, p. 57). Spending a few minutes studying the Milky Way with binocularsprovides a feel for the strength of Sagan’s belief.

We still have no indisputable evidence of life outside Earth. However, this situationmay change within a few years or even months because four unmanned missions—twoAmerican, one European, and one Japanese—are now on the way to Mars, the mostlikely site for extraterrestrial life. In addition, the Mars Global Surveyor and Mars Odysseyspacecraft, both orbiting Mars, are still returning data. The field of astrobiology has beenrevolutionized within the last few years, and the following offers a concise summary ofrecent developments, which cross many scientific disciplines.

A frame of referenceTo begin, a few fundamental assumptions must be laid out, the first being a defini-tion of life. For the purpose of this discussion, I define life as a form of mattercharacterized by metabolism, reproduction, mutation, and multigenerational trans-mission of the mutations. This subject is covered well by Lynn Margulis and DorionSagan in an excellent summary of modern biology (2000).

A second fundamental assumption asserts that extraterrestrial life must be ofnatural origin. Artificial intelligence, broadly speaking, has already been produced,and artificial life is in principle achievable. However, this discussion covers only lifeand intelligence produced naturally, by whatever mechanisms, leaving other possi-bilities to science fiction writers.

The chemical requirements for life can be specified with some confidence.First, life must be based on carbon. Knowledgable students who know the peri-odic table may suggest silicon-based life, recognizing that silicon is chemicallysimilar to carbon. Although it makes for a good classroom discussion, this possibil-ity can be dismissed. Carbon is simply a far more versatile element, in particularbecause of its ability to form carbon–carbon covalent bonds. This versatility isdemonstrated by the presence of many millions of organic compounds, in contrastto the dozen or so major rock-forming silicate minerals (generally solid solutionsof several end-members). Silicon does not form complex, high-molecular-weightcompounds in nature. Silanes and silicones, for example, do not occur naturally.The most obvious argument against silicon-based life is the simple question,where is it? There is plenty of silicon on Earth, the Moon, Mars, and in themeteorite parent bodies, yet there is no silicon-based life anywhere. Students whoknow their biology may object, citing radiolarians as an example of silicon-basedlife. Radalorians have siliceous shells but are not made of silicon. The subject ofbiogenic minerals is thoroughly covered by Margulis and Sagan (2000).

Significantly, dozens of organic compounds have been found in many differentregions of space, chiefly by millimeter-wave radio astronomy. All major types oforganic compounds have been identified, including amino acids, in molecular cloudsfrom which stars and eventually planets form. The prebiotic material for carbon-based life is therefore abundant in the universe as a whole. It is alsoabundant in the outer reaches of our Solar System, occasionally fallingtoward the Sun as comets. I will revisit this subject when discussing theMoon. A useful reference is The Molecular Origins of Life (Brack 1998).

A second chemical requirement for life is water-based, a materialwith a wide pressure-temperature range as a liquid and a strong dipolemoment, which makes it an effective solvent. Liquid ammonia mightN

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serve as a solute or medium, as could hydrogen sulfide.But taking a broad view, water is made of cosmicallyabundant elements and is the dominant liquid medium onEarth and Mars and in the Jovian satellites. Realistically,therefore, it must be assumed that life will involve water.

Could life arise within solid rock or in gases likethose that comprise Jupiter’s atmosphere? The answeris essentially no. Solid–solid chemical reactions, such asthose of regional metamorphism, are slow, taking thou-sands or millions of years. Chemical reactions in gasesare generally rapid, but gases are inherently disordered.Gaseous compounds cannot retain and transmit theenormous amount of information carried by the DNA/RNA mechanism.

A fundamental requirement for life related to thechemical ones is the time needed for the formation ofnecessary elements. This is a large topic. Ward andBrownlee (2000) argue that life in general is probablywidespread but that complex multicellular life is rare. Partof the basis for this conclusion is our knowledge of howelements heavier than hydrogen form, hydrogen havingoriginated in the Big Bang some 15 billion years ago. Ele-ments up to iron in atomic weight can form by nucleosyn-thesis in massive stars. However, supernovas are requiredfor the formation of heavier elements. The heavy elementsin our Solar System, for example, were produced not morethan 100 million years before the Solar System itself, prob-ably by one or more nearby supernova explosions. (High-energy x-rays in the primordial nebula may also have con-tributed.) The time needed for life to arise, then, mustinclude the time needed for star formation, star evolution,and supernova occurrence. These events, added to the riseof complex life some 500 million years ago, took nearly 5billion years on Earth, or one-third the age of the universe.(A discouraging implication of this figure is that extremelydistant galaxies, more than 10 billion light-years away, areprobably devoid of complex life because they have simplynot, as we see them today in the first few billion years aftertheir formation, been in existence long enough.)

Life in EarthThe title of this section emphasizes ournew knowledge of the incredible rangeof habitats life occupies on Earth. Endo-lithic organisms were first confirmedfrom rocky cliffs in Antarctica—bacteriaand fungi living a few centimeters inside

the rock between mineral grains. Since then, endolithicorganisms have been found in many other locations, asdeep as 3 km in solid rock on land and 1.5 km below theocean floor. The term extremophiles has been coined forsuch life-forms. An obvious question is: Did life evolveelsewhere and migrate into these exotic niches or could ithave arisen in them? Black smokers, for example, on themid-ocean ridges, are rich in energy and the chemical

requirements for life and are increasingly being consid-ered as a possible site for the origin of life. Stable isotoperatios in 3.8-billion-year-old rocks from Greenland indi-cate that life had already arisen then, possibly in a deep-sea volcanic environment, as suggested by Mojzsis andHarrison (2000). At such an early point in Earth’s historythere could hardly have been a surface biosphere fromwhich life could have migrated into other niches. Thissupports the possibility that life arose in extreme environ-ments. Darwin’s “warm little pond” is now starting tolook rather unlikely for the first life on Earth. In anyevent, the discoveries of the last decade or so have pro-vided a totally new perspective on the origin of life.

The MoonNo life or even prebiotic compoundswere found in samples returned from theMoon, and it seems clear that this dry,barren satellite never had any indigenouslife. However, new discoveries since theApollo missions suggest that we may yet

learn much about the origin of life from the Moon. TheClementine and Lunar Prospector missions have shown alarge amount of water ice in the permanently shadedcraters at the lunar south and north poles. This ice wasprobably deposited from infalling comets, and it may berich in prebiotic compounds as well as water. Added tothe probable contribution from carbonaceous chondrites,which are rich in complex organic compounds, the polarregions of the Moon have suddenly become very interest-ing from a biological viewpoint. The same considerationsapply to Mercury, on which Earth-based radar surveyshave indicated probable polar ice caps.

VenusSurface conditions on Venus are nowwell known, and they appear at firstglance to be utterly hostile to life—tem-peratures around 400°C, surface pressuresaround 100 bars, and an atmosphere con-sisting largely of carbon dioxide, perhaps

with clouds of sulfuric acid. There is no direct evidenceof water on the surface, and there are no landforms indi-cating a past hydrosphere.

Despite these conditions, Schulze-Makuch andIrwin (2002) suggest that life might have arisen veryearly on Venus, as it did on Earth, and later adaptedto changing environmental conditions by migratingunderground or perhaps into the atmosphere or tohigh elevations near the poles. Further speculation atthis point is not useful for this discussion. However, itshould be kept in mind that Venus has abundant en-ergy, an active interior, a dynamic atmosphere, andprobably a wide range of chemical compounds. Dog-matic pessimism is therefore unwarranted.

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MarsThe subject of life on (or in) Mars is ex-panding explosively, and I recommendkeeping abreast of developments throughthe Internet. The international squadronof spacecraft now approaching Mars willpresumably keep the red planet on the

front pages. Good general summaries of Martian geologyand the possibilities of life can be found in The New SolarSystem (Beatty, Peterson, and Chaikin 1999). The redplanet has traditionally been considered the most promis-ing Solar System site for life outside Earth, and recentdiscoveries have strengthened this view.

Life on Mars may have been discovered by the two Vikingmissions of 1976, each of which landed three biological instru-ments. All three initially produced positive results, generatingenormous interest as described by Cooper (1980). However,after several years of occasionally acrimonious discussionamong the Viking scientists, it was generally concluded thatthe results were caused by highly oxidized soil, not life. Thesubject of life on Mars subsided for several years, one reasonbeing the apparent scarcity of water on Mars in recent times.However, the general belief that the two Viking landers of1976 failed to find life is not universally accepted. The positiveresults of the labeled release instrument, in particular, inwhich soil was dropped into a carbon-14 tagged nutrientsolution, have been argued by the initial investigators Levinand Straat (1979) and by DiGregorio (1997) to show actuallife, not just highly oxidized chemistry.

This view, though a minority one, is indirectly sup-ported by recent discoveries from Mars missions of thelate 1990s and later, and by the study of Martian meteor-ites. More direct support comes from the discovery byMiller of Martian circadian rhythm in the radioactive gasevolution from the Viking experiment (Oliwenstein2002). By reanalyzing the original labeled release data,Miller found that the gas evolved by the labeled releaseinstrument followed a 24.7-hour rhythm, the length of aMartian day. This could not have been caused by day-light, because the nutrient was in darkness, but it couldhave been caused by the day-night temperature cycle.Miller concluded that the odds were 90 percent that thelabeled release instrument had in fact detected life.

The biologically important discoveries are as follows.First, a Martian meteorite, called Allan Hills 84001, dis-covered in the Antarctic ice in 1984, was found in 1996 tocontain microscopic structures resembling terrestrial fos-sil bacteria (Figure 1). This discovery, front page newseverywhere, was critically viewed by many scientists,who apparently agreed with Sagan’s view that “extraor-dinary claims require extraordinary evidence” (1995).One criticism, for example, was that these meteoriticstructures, only about 100 µm long, were simply toosmall to contain DNA and other structures found in ter-restrial bacteria. This criticism should not be casually

dismissed, but there are many comparably sizednanobacteria known in terrestrial rocks. These are notnecessarily complete bacteria but are possibly bacterialparts or features produced in some way by bacteria.

It was also argued that the magnetite crystals found inthe Allan Hills structures were high-temperature inorganicforms. The NASA scientists who made the original discov-ery have countered both arguments. Of particular interest istheir demonstration that about one-quarter of the magnetitecrystals in the meteorite structures have all six attributes(morphology, crystalline structure, and so on) of terrestrialmagnetite crystals found in magnetotactic bacteria. A re-lated finding is that, contrary to previous views that Marshad no magnetic field (and thus presumably nomagnetotactic bacteria), there are very strong local remnantmagnetic anomalies. These imply that Mars had a globalmagnetic field early in its history. The controversy contin-ues and cannot be covered in depth here except to note thatall findings since 1996 have been on the positive side, sup-porting a biological origin for the Allan Hills structures.

More general in nature, discoveries from the Mars GlobalSurveyor, the Sojourner Mars rover, and most recently theOdyssey mission have drastically changed our overall conceptof the Martian environment (Figure 2, p. 44). Most important,extensive subsurface ice deposits were found, which occasion-ally melt and give rise to liquid water eruptions on the sur-face. These eruptions are so recent in some places that alluvialfans overlie sand dunes. There is little doubt that the liquidinvolved was water, since neutron spectroscopy from the Od-yssey mission has confirmed the existence of huge areas withhydrogen concentrations in the south polar regions. Themany high-resolution pictures of the Martian surface haveshown large areas of stratified rock reminiscent of the GrandCanyon. The scientists responsible for the Mars Orbiter Cam-

F I G U R E 1

Scanning electron microscope view of objects in Allan Hillsmeteorite ALH84001, interpreted as life-forms or fragmentsthereof; 200 nm long scale bar.

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era, Malin and Edgett, have argued convincingly that theseare in fact water-deposited sedimentary rocks. The smoothnorthern plains, now studied by laser altimetry that producesphotographlike images, have been interpreted as sedimentsdeposited from a former Mars ocean. In summary, a primerequirement for life—water—is now known to exist in Marsand occasionally on its surface.

Opinions on the geologic evolution of Mars have alsobeen drastically changed in the last few years by recentdiscoveries. In particular, the former view of Mars as ageologically inactive planet with an extremely ancientMoonlike terrain has now been discredited. Basaltic mete-orites from Mars have been radiometrically dated at 160million years, showing that the planet was internally activeuntil at least that time. Furthermore, high-resolution pic-tures of the surface have shown substantial areas that havebeen dated, from crater counts, at about 10 million years.

The petrologic evolution of Mars has also been unrav-eled to a surprising degree by space missions and meteoritestudies. Most general is the finding that the early crustalevolution of Mars may have been similar to that of earlyEarth (when life arose here). Earth’s continental crust has anoverall composition close to that of the volcanic rock andes-ite, about 60 percent silicon dioxide (Lowman 2002). TheSojourner Mars rover produced the first in situ analyses ofrocks on Mars. All five analyzed turned out to be andesite, adiscovery that stunned most geologists. Andesites on Earthform by plate tectonic mechanisms, in particular over sub-duction zones, but it is agreed that Mars never reached theplate tectonic stage. In any event, it now seems likely thatthe early crust formation on Mars was like that on Earth, sothe general chemical environment may have been similar onboth planets. However, more recent studies of the surfacecomposition of Mars from the Mars Global Surveyor have

F I G U R E 2

Left: View of hemisphere of Mars from Mars Global Surveyor. White clouds over Tharsis volcanoes and Olympus Mons are water ice.

Right top: Color-coded topographic maps of Mars produced by the Mars Orbiter Laser Altimeter; blue areas low, red areas high.

Right center: Viking view of terrain in Martian highlands, with white rectangle showing location of bottom figure.

Right bottom: High-resolution Mars Orbiter Camera (Mars Global Surveyor) view of area in rectangle, showing gullies interpretedas formed by geologically recent seepage and runoff of liquid groundwater.

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produced puzzling results, showing that the southern high-lands are basaltic and the northern plains andesitic, just thereverse of what had been expected after the Sojourner analy-ses. We clearly cannot label this situation “case closed.”

Mars, in short, is now known to be an internally active,geologically evolved planet, with water, internal energy,and a crust with roughly Earthlike chemistry. There maybe no life on the surface, at least not visible life, but thepossibility of a subsurface biosphere of single-celled life,perhaps prokaryotic, is being given serious thought. TheESA Mars Express mission, with a lander (appropriatelynamed Beagle), may produce evidence of such life.

Satellites of JupiterLike Mars, the four large satellites of Ju-piter—or Galilean satellites, in honor oftheir discoverer—are turning out to holdconsiderable biological interest. Workingoutward, they are Io, Europa, Ganymede,and Callisto. First imaged clearly withthe Voyager mission in 1979, these planet-sized bodies are strikingly different, re-flecting their distance from Jupiter. Io is aspectacular sight, with a surface of brightyellows and oranges, and is studded withactive volcanoes. The volcanic activity isevidently produced by internal frictioncaused by Io’s gravitational interactionwith Jupiter and Europa (see the chapterby T.V. Johnson in The New Solar System(Beatty, Peterson, and Chaikin 1999). Io isan extremely unlikely site for life of anysort, but it helps us understand Europa.

Europa is an utterly alien body, lacedwith a network of lineaments and very fewimpact craters. High resolution picturesshow a terrain resembling the ArcticOcean, and reflection spectroscopy shows

that the surface is water-ice. A finding of the Galileo mis-sion was that Europa apparently has a magnetic field,probably generated by the motion of an electricity-con-ducting material through the magnetic field of Jupiter.These findings have been interpreted as the expression ofan ocean under the icy surface that is kept liquid by tidalinteractions with Jupiter and Io. This ocean has stimulatedintense interest as a possible site for life of some kind.Even Ganymede and Callisto may have subsurface liquidwater and thus cannot be ruled out as sites for life.

The outer solar systemEvery planet but Pluto has been visited by spacecraft,and our knowledge of the outer solar system has ex-panded enormously. What are the prospects for life be-yond Jupiter? Thirty years ago the answer would havebeen quick and definite: zero. However, the astounding

discoveries from the Jovian system warn us that at thisstage we simply do not know enough to rule out life inthe outer solar system (probably on the satellites of thegiant planets). Again, the suggestion is to stay tuned.

This review has barely touched on an enormous and excit-ing field. I have not, for example, even mentioned the Gaiatheory, which is rapidly evolving from a vague, semireligiousconcept into a quantitative, testable hypothesis with potentialapplications to problems such as deforestation (Margulis1998). The subject of extremophiles is already a major field ofstudy in itself. Theories of the origin of life are changingmonthly. I hope some of this excitement is conveyed to class-room teachers who in turn will convey it to their students. n

Paul D. Lowman Jr. is a geologist at NASA’s GoddardSpace Flight Center, Greenbelt, MD 20771; e-mail:lowman@core2.gsfc.nasa.gov.

References

Beatty, J.K., C.C. Peterson, and A. Chaikin. 1999. The New SolarSystem. 4th ed. Cambridge, Mass.: Sky Publishing Companyand Cambridge University Press.

Brack, A., ed. 1998. The Molecular Origins of Life. Cambridge:Cambridge University Press.

Cooper, H.S.F., Jr. 1980. The Search for Life on Mars. New York:Holt, Rinehart and Winston.

DiGregorio, B.E. 1997. Mars, The Living Planet. Berkeley, Calif.:Frog, Ltd.

Levin, G.V., and P.A. Straat. 1979. Laboratory simulations of theViking labeled release experiment: Kinetics following secondnutrient injection and the nature of the gaseous end product.Journal of Molecular Evolution 14:185–197.

Lowman, P.D., Jr. 2002. Exploring Space, Exploring Earth. Cam-bridge: Cambridge University Press.

Malin, M.C., and K.S. Edgett. 2000. Sedimentary rocks of earlyMars. Science 290:1927–1937.

Margulis, L. 1998. Symbiotic Planet. New York: Basic Books.Margulis, L., and D. Sagan. 2000. What Is Life? Berkeley: Univer-

sity of California Press.McKay, D.S. et al. 1996. Search for past life on Mars: Possible relic

biogenic activity in Martian meteorite ALH84001. Science273:924–927.

Mojzsis, S.J., and T.M. Harrison. 2000. Vestiges of a beginning:Clues to the emergent biosphere recorded in the oldest knownsedimentary rocks. GSA Today 10(4): 1–6.

Oliwenstein, L. 2002. A day in the life on Mars. USC HealthWinter:32.

Sagan, C. 1985. Contact. New York: Pocket Books.Sagan, C. 1995. The Demon-Haunted World. New York: Random

House.Schulze-Makuch, D., and L.N. Irwin. 2002. Reassessing the

possiblity of life on Venus: Proposal for an astrobiology mission.Astrobiology 2(2): 197–202.

Ward, P.D., and D. Brownlee. 2000. Rare Earth: Why Complex LifeIs Uncommon in the Universe. New York: Copernicus.