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Astrobiology

INTERNATIONAL

SPACE

SCIENCE

INSTITUTE

Published by the Association Pro ISSI No 16, July 2006

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Editorial

Impressum

SPATIUMPublished by the

Association Pro I S S Itwice a year

Association Pro I S S IHallerstrasse 6, C H -3012 BernPhone +41 (0)31 631 48 96seewww.issi.unibe.ch/pro-issi.htmlfor the whole Spatium series

President

Prof. Heinrich Leutwyler,University of Bern Publisher

Dr. Hansjörg Schlaepfer,legenda schläpfer wort & bild,C H-8173 Neerach Layout

Marcel Künzi, marketing · kom-munikation, CH -8483 Kollbrunn Printing

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Front Cover: An image of the Eagle nebula (Credit : NASA) iscombined in this artist’s impres-sion with a colony of bacteria.

Are we the proud subjects of our will or the wretched victims of our drives? Charles Darwin, the

British clergyman and naturalist,found the origin of the species –including men – in the continued natural selection process ruling therandomly mutating gene pools of living systems. In other words, thevariety of species – including men – is seen as the result of a simplealbeit very efficient trial and error process adapting the species todifferent and ever changing en-vironmental conditions. Are wenothing else than the output of a soulless evolution machinery?

Sigmund Freud, the Austrian psy-chiatrist, identified in us an ego

struggling for a niche between thesuper-ego coined by the authori-ties we experienced in our youth and the id, the instinctual heritagefrom our distant ancestors. This isgood news, as the ego is, at least in principle, able to anticipate the fu-ture and to act responsibly in thehere and now, while the evolution process is strictly governed by theconditions prevailing in the past. If we take, however, the number of

species living on this planet as themeasure of success, then we find no much remains of the proud re-sponsible ego: the rate of extinc-tion caused by human activitiesis estimated to be similar that of the major mass extinctions in the

Earth’s long history.

Sometimes we have to go far tocome close. Astrobiology tells uswhat an isolated and outstanding value life represents in the uni-verse: against all efforts so far, notraces of life have been found be-

yond the Earth. Oliver Botta, theyoung scientist at ISS I, presented the fascinating quest for life in theuniverse in the frame of the ProISS I lecture on 25 October 2005.We are indebted for his kind per-mission to submit our readersherewith a revised version of his talk. May the present issue of Spa-tium on astrobiology enhance our awareness of the unique value of life here on Earth!

Hansjörg Schlaepfer Zürich, June 2005

INTERNATIONAL

SPACE

SCIENCE

INSTITUTE

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Introduction

Natural science has made over-whelming progress in the past few centuries: Galilei, Newton,

Einstein are among the namesstanding for quantum leaps in our knowledge of the physical world.Yet, we are far from understand-ing one of the most common

phenomena here on Earth: life.Charles Darwin’s 2 concept of evo-lution based on random mutationof genetic properties and selectionby the environment was a major leap forward (Fig. 1). Watson 3 and

Crick4

found the double helix structure of the deoxyribonucleic acid (DNA) helping to under-stand these processes on the mo-lecular level. But despite all this progress , the most fundamentalquestions remain unanswered:What is life? How did life emergeon Earth? What are the conditionsfor life to emerge? How did theenvironment influence the evo-lution of life on Earth and how

did life in turn influence the envi-ronment? These and many other questions are open: the stage isset for astrobiology. It requires aninterdisciplinary approach with contributions from all branchesof science to unravel the answersto these questions. Astrobiology

(sometimes also called xenobiol-ogy or exobiology) is the attempt to bring together scientists to com-bine their knowledge with a two-fold goal: to understand the originof life on Earth and to search for life outside of the Earth.

Astrobiology 1

Dr. Oliver Botta,NASA Goddard Space Flight Center and International Space Science Institute

1 This article is based on a Pro ISSI lecture by Dr. Oliver Botta on 25 October 2005. In addition, we owe Dr. Frank Rutschmann,University of Zurich, significant contributions regarding biology.

2 Charles Darwin, 1809 Shrewsbury–1882 London, clergyman and natural scientist 3 James Dewey Watson, 1928 Chicago, biochemist, Nobel laureate 19624 Francis Harry Compton Crick, 1916 Northampton– 2004 San Diego, physicist, Nobel laureate 1962

Fig. 1: Charles Darwin (1868)

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What is Life?

Life is everywhere on Earth: heat-loving micro-organisms (called hyperthermophiles) populate hot springs with temperatures of more than 100 °C, alkaliphiles(alkalic-environment-preferring organisms) are found in soda lakes, piezophiles (pressure-resistant or-ganisms) have been reported fromthe Mariana Trench, the deepest sea floor on Earth, some 10’000metres below sea level. Life is eve-rywhere, yet the most fundamen-tal problem is our current inabil-

ity to provide an all-encompassing definition, which we could use todirect the search for life elsewhere.In this situation, NASA adopted for its current roadmap for astro-biology the formulation of life as a self-sustained chemical system capable of undergoing Darwinian evolution. Inthe following chapters, we will ad-here to this definition.

There are two basically different

approaches to the understanding of the origin of life on Earth. The “top-down” approach looks at the cur-rent organisms, in particular inextreme environments, and also at the fossil record, to understand themolecular and phenological histo-ry of life and to attempt to extra- polate this knowledge towards theearliest forms of life on this planet.The “bottom-up” approach triesto understand how the planetary

system, including the Earth, hasformed and what the conditions on the surface were in the first bil-lion years of its history. One alsotries to understand which molecu-lar building blocks of life could have been present on the early

Earth, and how and where they were formed. From that knowl-edge one attempts to extrapolatetowards the earliest traces of lifewhich can be observed today.

The Building Blocks of Life

From atoms and molecules.. .

On the level of atoms, life isnothing else than a complex ar-

rangement of a very limited set of chemical elements. This set comprises the biogenic elementscarbon C, hydrogen H, nitrogenN, oxygen O, phosphorus P and sulphur S. Of every 200 atomsin our body, for example, 126 arehydrogen, 51 oxygen, 19 carbon,three are nitrogen and the re-maining atom is one of all other elements. We have inherited allthe hydrogen atoms, 63% of the

material constituting our body ,directly from the Big Bang four-teen billion years ago. The other elements up to iron have beensynthesized by the main sequencestars burning hydrogen to heliumand helium to carbon, etc., whilethe heaviest elements were creat-ed and expelled into space during supernova explosions 5. As the first supernovae probably occurred just

500 million years after the Big

Bang, life has had, at least in prin-ciple, plenty of time and space toevolve since then.

Amongst the six biogenic elements,carbon plays a central role thanks toits outstanding ability to form complex macromolecules containing thousands of atoms, which are stablein the temperature range wherewater is a liquid. On the other hand, the energy content of thecarbon-carbon bond is low enough allowing water to dissolve and re-arrange the carbon compounds.

Two other biogenic elements, name-ly hydrogen and oxygen, are impor-tant when combined to the water molecule H 2O. Water is a key in-

gredient for life as we know it as a solvent allowing molecules to betransported and chemical reactionsto take place. Water has someunique properties including the fact that its solid form (ice) is lighter than its liquid state. In addition,water is very common in the uni-verse: water is produced in the cold molecular clouds of the interstellar medium, where the oxygen atomsfreeze on the surface of silica dust

grains and eventually capture twohydrogen atoms to form the water molecules. Comets consist of a mixture of dust grains embedded in water ice. As they are the most pristine objects in the solar system,they testify that water has been present in remarkable quantitiesin the early solar system already.

In addition to water, a variety of

other solvents, such as formamide(CHONH 2 ) or ammonia (N H3 ) ,have similar solubility propertiesunder certain conditions, but as

5 See Spatium 13: Woher kommen Kohlenstoff , Eisen und Uran , Ruedi von Steiger,November 2004

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they are less abundant in the uni-verse than water, they did not play a role for the emergence of life on

Earth.

In search for experimental evi-dence that the elements and simple compounds present on theearly Earth can form complex or-ganic molecules, Miller and Urey

(1953) conceived an experiment (Fig. 2) which produced a wealth of complex organic molecules fromwater, methane, ammonia and hydrogen. In his apparatus, Miller boiled water and circulated thevapour through a glass vial, past anelectric spark, then through a cool-ing jacket that condensed the vapour and directed it back into theboiling flask. This process yielded for the most part black, tarry or-ganic residue, which was difficult to characterize back in these days.Most surprisingly, however, Miller

was able to identify in this mix-ture the presence of amino acids,which are the very basic building blocks of proteins. This was thefirst experimental evidence that biologically relevant moleculescould be synthesized from simple gases under conditions as they were thought to have occurred onthe early Earth.

Later, it was found that the early atmosphere of the Earth was probably composed of other gases,from which under the same condi-tions significantly lower amountsof amino acids and other organic molecules are formed. This find-ing led to an increased interest among astrobiologists in the pos-

sibility that extraterrestrial organic matter could have been delivered to the early Earth by impacts of comets, asteroids, meteorites and interplanetary dust particles tocontribute the first prebiotic build-ing blocks of life.

...over macromolecules...

The next major step consists of

combining many relatively simplemolecules, such as for exampleamino acids, to complex macro-molecules. In modern biology, the formation of macromolecules,which involves many individualsteps, requires a sophisticated bio-chemical machinery. The synthe-sis of these molecules under pre-biotic conditions has turned out tobe one of the biggest challenges in

the understanding of the origin of life. Biological macromolecules inall forms of life on Earth come inthe following classes:

The carbohydrates are macro-molecules based on a carbonstructure with many hydroxylgroups (–OH) attached. Largecarbohydrate structures are used for energy storage (for examplesugar or starch) as well as provid-ing structural support in living systems (for example cellulose inthe cell wall of plants).

The proteins are the most di-verse and functionally versatilebiological macromolecules. The proteins consist of a chain of amino acids. Every amino acid is connected to one of 20 differ-ent residues, small compoundssuch as for example CH3 or CH 2OH . Most proteins of living

systems on Earth are composed of the same 20 different aminoacids. Proteins are either impor-tant structural molecules of cellsor possess catalytic properties.In the latter case they are called enzymes.

The nucleic acids are the largest macromolecules existing in liv-ing organisms. They consist of individual nucleotides linked to-

gether to form long linear poly-mers. A nucleotide is a chemicalcompound that consists of a het-erocyclic base, a sugar, and oneor more phosphate groups. Thenucleic acids come in two fla-vours, the deoxyribonucleic acid (DNA, see Figure 3) and the ribo-nucleic acid (RNA) discussed below.

Both the DNA and the RNA con-sist of two complementary strandsthat coil about each other like a spiral staircase to form a double

Fig. 2: The original laboratory equip-

ment used by Stanley Miller (1953).

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helix. Hydrogen bonds between

the nucleotides form the steps of the staircase (Fig. 3). They consist always of pairs: adenine in onestrand is always linked to thyminein the other, and guanine in onestrand is always paired with cy-tosine in the other. The bases areattached to a backbone made out of sugar molecules, which in turnare connected together along theexterior of the helix by phosphategroups.

Intriguingly, the DNA of allknown forms of living systemsis based on only these four basesconstituting the universal genetic code. The code is expressed inthe form of the specific sequenceof the base molecules. The ge-netic code directs the produc-tion of proteins needed for thestructure and the functions of the

cell and for its own replicationmachinery. It represents the en-tire construction plan of a living organism.

...to self-replication...

Replication is one of the core prin-ciples of life. It allows overcoming the limited lifetime of all materialentities. In addition, replication iscentral for evolution by providing a set of similar copies of an origi-nal on which the process of selec-tion can act. Under the effect of a large number of enzymes, the DNA can rep-

licate in a very complicated pro-cess. In the vast majority of cases,the replication process yields anexact copy of the original DNA ,so the replica is identical to theoriginal. Sometimes, however, thesequence of the nucleotides of thecopy is different from the original.It is this accidental change which isthe underlying mechanism for theDarwinian evolution. A sequenceof such random mutations may

become prevalent in the gene poolof a population, if it is favoured by the natural selection process.

Fig. 4: The basic building blocks of a simple cell. (Credit: I. Gilmour, M. A. Sephton, Editors: An Introduction to Astrobiology, Cambridge University Press, 2003)

DNA 0.1 µm

cytosol cell membrane

Fig. 3: The structure of the DNA

double helix. (Credit: I. Gilmour, M. A. Sephton, Editors : An Introductionto Astrobiology, Cambridge University Press, 2003)

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In contrast to the DN A , some RNA molecules were found to possesscatalytic properties enabling to rep-licate themselves (self-replication).In a complicated la bor atory proced-ure called in-vitro selection, some RNA molecules were found that showed the unique capacity to catalyze their own replication. It is thought that a selection process favoured RN A thanks to its replication poten-tial. This led to the “RNA World Hypthesis”, stating that on the early Earth the first replicating systemswere RNA molecules. Based on itsreplication and evolution capacity,the RNA is seen as the most basic living system as per the definitionabove. That system did not containa cell, but solely existed in suit-

able media providing the necessary supply of organic matter. Eventu-ally, according to the RN A World Hypthesis, the genetic storagefunctionality of RNA was takenover by the more stable DN A , and the catalytic activity by the pro-teins, which have a wider range of functionalities. However, there stillremain (major) problems even inthis relatively straightforward sce-nario, for example how the build-

ing blocks of RN A , the ribonu-cleotides, were synthesized under prebiotic conditions. Another un-solved question is how the very reactive ribonucleotides were pro-tected from simply being hydro-lyzed in water thereby losing their reactivity.

... to living systems...

While the DN A in modern or-ganisms stores its genetic infor-mation, it cannot self-replicate

without being provided with monomeric building blocks toconstruct a second copy. Thesebuilding blocks have to be syn-thesized by the organism or sup-

plied by the surrounding medi-um. It is the cell, which providesthe DNA the required infrastruc-ture for its replication. The next giant step forward in the history of life was the encapsulation of bio-molecules inside compart-ments, where they are protected and concentrated (Fig. 4).

The function of the cell can becompared with a factory, inwhich the DNA acts as the con-struction plan contained in theDNA and the necessary raw

material. The DNA acts as thefactory’s master plan storing thegenetic code and controlling the processes in the cell. The DNA issurrounded by the cytosol, a liq-uid containing thousands of dif-ferent types of proteins constitut-ing the raw material. The cytosolacts as the factory’s assembly lineallowing the raw material to betransported to the places whereit is needed. The cell volume fi-

nally is enclosed by a cell mem-brane made from lipids and pro-teins.

To replicate, the cell goes through a process called mitosis. First, theentire DNA of the cell is repli-cated. Then, the original and itscopy move to different parts of the cytosol. Then, the cell be-gins to divide, separating prop-

erly the twoDNA

-containing regions. The result of this pro-cess are two identical daughter cells.

.. . to Darwinian evolution

If the replication process would work perfectly and would pro-duce identical replicas exclusively,no evolution would be possible.The term evolution designatesrandom mutation of the genetic properties of the individuals cou- pled with natural selection. Inorder to be selected by the envi-ronment, the genetic mutationmust translate into a phenologicaladvantage of the individual. In hisepochal oeuvre “On the Originof Species by Means of NaturalSelection, or the Preservation of Favoured Races in the Strugglefor Life” Charles Darwin saw thenatural selection process driven

for example by climate change or by competition with other speciesfor limited resources. As a further important selection mechanismhe identified the sexual selection:Failing to mate prevents the indi-vidual to contribute to the evolu-tion of the population’s gene poolwhich is equivalent to eliminat-ing its genetic properties from thegene pool. While these selection processes act on the level of the

individual, they cause the popula-tion’s collective gene pool to drift continuously beyond the lifetimeof the single individual.

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The Evolution of Life on Earth

The Sun and its planets are theresult of the gravitational col-lapse of an interstellar cloud some4’600 million years ago. The plan-ets were formed from a debrisdisk that circulated the early Sunthrough accretion of dust grainson relatively short time scales of some tens of million years. Theexact environmental conditionson the early Earth at that time ,called the Hadean, are not known,

but it is thought that the surface of the Earth cooled down to a levelwhere water is liquid after about 200 million years. The heavily cratered surface of the Moon, lunar samples and meteorites have pro-

vided evidence for the early impact history of the Earth–Moonsystem. While impacts of cometsand asteroids may have brought water and organic compoundsto the early Earth, these impactswere devastating events that may have sterilized the whole surfaceof the planet and even boiled away the water on Earth. Eventu-ally, though, the inventory of largebodies crossing the Earth’s orbit declined, so that the frequency of these impacts decreased paving the way for the evolution of lifeon Earth.

The conditions under which prebiotic chemistry on the early Earth occurred that would have

eventually led to the first self-rep-licating organisms are not known.It is assumed that sandy beachesand volcanic mud pools may have provided the required milieu that protected prebiotically synthe-sized organic compounds fromultraviolet radiation and oxidation.Superheated columns of mineral-rich water that gush from ventsin the seafloor, the so-called blacksmokers, may have been another

potential environment for life toemerge.

The oldest traces of life on Earth are fossil cyano-bacteria that werefound in 3.5 billion years old rocksin Western Australia, although there is a strong controversy about this topic in the scientific commu-nity. More secure morphologicaltraces appear in the form of stro-

matolites in around 2.5 billionyears old rocks. Stromatolites arethe solidified (“lithified”) rem-nants of microbial mats, in par-

ticular of cyano-bacteria. Activestromatolites can be found today in hyper-saline lakes and marinelagoons (Fig. 5). It can be said that these modern forms of cyano-bac-teria constitute the most ancient organisms of life on Earth.

The history of life on Earth isalso a history of mass extinctions.Life has been endangered or evennearly extinct many times sinceit emerged by massive climatechanges, cosmic impacts or major volcanism events. It is estimated that more than 99.9% of all spe-cies that have ever lived are now extinct. An especially critical phase seems to have been the period between 750 and 580 mil-

lion years ago when the Earth was completely covered by heavy ice sheets with surface tempera-tures as low as –50 °C according to the “Snowball Earth Theory”.Such environmental conditionswould have erased all but a smallfraction of existing life deep onthe floors of the oceans. Becausethe ice cover could not stop thecontinued output of the green-house gases by volcanoes, the

Earth regained a moderate climatein a very short timeframe lead-ing to hot environments accord-ing to the “Hot House Theory”,now endangering the existing living systems which had success-fully adapted to low temperatureclimates before. However, thesubsequent phase of a warm,relatively stable climate allowed virtually every major phylum of

animals to evolve within a rela-tively short time period of some40 million years (the Cambrianexplosion).

Fig. 5: These living stromatolites havean age of around 3’500 years. Thewarm, shallow waters of Western Aus-tralian’s Shark Bay favour the growth of micro-organisms, particularly cyano-bacteria. Fossil stromatolites – literally layered rocks – also represent the oldest evidence for life on Earth. (Credit: Uni-versity of South Carolina, Geology De-

partment)

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Mass extinctions have led to theloss of many species, but they stimulated also the evolution of life by creating unoccupied eco-logical niches where the remain-ing organisms could proliferate.The last known mass extinctionoccurred some 65 million yearsago. It is thought to be caused by the impact of an asteroid of thesize of 10 kilometres near what is now the north-western part of Mexico’s Yucatan peninsula. Thisimpact and the subsequent dust in the atmosphere led to a dec-ade-long winter and to the extinc-tion of 60 to 80% of all then living species, including the dinosaurs.Interestingly, some animal classeswere less affected, like for exam-

ple the amphibians and the mam-mals. The latter were present inthe form of rat-like animals. After the cease of the dinosaurs’ domi-nance, they were able to prolifer-ate and to develop into the most complex animals on this planet,including men.

Co-evolutionof Life and its Environment

According to the operational de-finition cited above, life is charac-terized as a self-sustaining chem-ical system with the ability toundergo Darwinian evolution.Through its chemical processeslife is in constant exchange of matter and energy with its envi-ronment causing life to alter theenvironmental conditions lead-

ing to a co-evolution with its en-vironment. This aspect is gain-ing central importance when it comes to searching for life in theuniverse as specific atmospheric biosignatures can be used as a proxy for biological activit ies ona planet.

The Earth’s early atmosphere issupposed to have been dominat-ed by carbon dioxide, nitrogen

and water vapour. At that time,oxygen was present in the atmosphere only in trace amountsnamely as a product of the break-down of water vapour by the Sun’sultraviolet radiation or out-gassing from the interior. Therefore, noozone layer was there to shield thesurface of the Earth against incom-ing high energy radiation. This isone of the arguments in support

of the hypothesis that the first living systems evolved in moresheltered environments such assediments or deep waters.

The atmospheric concentration of oxygen rose to modern levels only about 2 billion years ago. Howev-er, oxygen was produced by cyano-bacteria on the Earth through pho-tosynthesis supposedly since 3.5billion years. This leaves about 1.5billion years of time on the Earth (the Archean epoch) when lifewas present, but the atmospherewas still anoxic. In fact, one of thekey questions associated with theevolution of life on Earth is why it took so long for oxygen to reach these higher levels. Only this riseof oxygen in the atmosphere pro-vided enough harvestable energy required by multicellular organ-isms.

Fig. 6: The white dwarf star in thecentre of the planetary nebula NGC 6369 radiates strongly at ultraviolet wavelengths and powers the expanding nebula’s glow. The nebula’s main ring structure is about a light-year across. It contains the three biogenic elementsoxygen, hydrogen, and nitrogen, which are coloured blue, green, and red respec-tively. In a remote future some of theseelements eventually may help to consti-tute a living system. (Credits: HubbleHeritage Team, NASA)

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Life from Space, Life into Space

The possibility that the emer-gence of life on Earth was based on organic molecules delivered tothe Earth from space has become a subject of increasing interest. Theanalysis of carbonaceous meteor-ites has demonstrated that organic molecules, including amino acids,are formed in extraterrestrial en-vironments. Astronomical obser-vations have also shown that someof these molecules were formed already before the sun even start-

ed to form, and were incorporated into planets and smaller solar sys-tem objects such as comets and asteroids. Due to the extremely high temperatures on the planetsduring their formation, no organic molecules survived this epoch.The organic compounds necessary for the origin of life were formed either later in the atmosphere(e.g. through Miller-Urey-typereactions), or were delivered to

the early Earth by large asteroid and comet impacts (as discussed above) or by their fragments, themeteorites and interplanetary dust

particles . The study of meteor-ites, particularly the carbonaceouschondrites that contain organic matter up to 5% by weight, has al-lowed close examination of extra-terrestrial organic material. One of the most famous sources of extra-terrestrial material is the Mur-

chison meteorite named after the Australian town where it reached the Earth in 1969 (Fig. 8). Among other classes of organic com-

pounds, more than eighty differ-ent amino acids have been identi-fied in the Murchison meteoriteat abundances of more than 1 part- per-million (ppm), eight of which are identical to those used in lifeon Earth as building blocks of pro-teins and enzymes. Also, the pres-ence of sugar-related moleculesand nucleobases was confirmed.

Another source of information isthe collection of micrometeoritesthat have been extracted from

Antarctic ice. Many of these t iny objects in the 50–100 µm size

Fig. 8: The Murchison meteorite. This is the most intensively investigated car-bonaceous chondrite. It fell near the town of Murchison in Australia on 28 Sep-tember 1969, only three months after the first lunar landing of Apollo 11. Fortunate-ly, all the techniques that were developed at NASA to investigate the lunar samplesfor organic compounds could be applied to the meteorite as well. This allowed pub-lishing the first report about the extraterrestrial amino acids in this meteorite shortly after its impact.

Fig. 7: This chicken embryo has beenbred for seven days. Like its remoteancestors and like the majority of fos-sil and living tetrapodes, it will developfeet with five digits in the next few days,of which one however will disappear again to produce an individual with only four digits on its feet like all the current-ly living birds. This is an example of a vestigial trait, showing that the evolu-tion of this individual (its ontogeny)mirrors the evolution of the birds in gen-eral (its phylogeny). (Credit: Hansjörg Schlaepfer)

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are unmelted chondritic micro-meteorites, indicating that they had crossed the terrestrial atmosphere without suffering drastic thermal shock. In February 2006,the NASA Stardust spacecraft hasreturned, for the first time, samples from beyond the Earth-Moonsystem in the form of cometary dust particles, which will give sci-entists a new window on the composition of comets.

It is thought that the various extra-terrestrial sources may have de-livered about 10 20 g of carbon tothe Earth during the first 600 mil-lion years (the Hadean period).This delivery represents morecarbon than the amount engaged

in the current biomass, which isestimated at 1018 g. Whether or not these extraterrestrial compounds were the major source of organic molecules on the early

Earth cannot be established evenqualitatively since there are cur-rently no estimates of the contri-butions from the other, indigenoussources.

If meteorites can transport organic

matter to the Earth, the next logi-cal question relates to whether living systems could have beendelivered to the Earth. Of course,such specimens would experi-ence the severe environmentalconditions of space. Still, under-standing survival in extreme con-ditions is essential for evaluating the potential for the interplan-etary transfer of viable micro-or-

ganisms, and thus the potentialthat any life elsewhere in the solar system might share a commonorigin with life on Earth (or vice

versa). Conditions in space and on other worlds are much moreextreme than those encountered by any of the habitable extremeenvironments on Earth. There-fore, studies of survivorship be-yond the Earth belong to the important tests of the resilience of Earth-originated life towards ex-treme conditions and its potentialfor dissemination in space.

Another option for life to leave itshome planet is that of a civilized society using advanced technology to travel into space. This is what we call manned spaceflight (Fig. 9).

The fact that all the niches on Earth are occupied by some formsof life suggests that it might be theultimate destiny of life to leaveits cradle and to proliferate intospace.

Fig. 9: Launch of the NASA Space Shuttle Columbia on STS-109 on 1 March 2002.Currently, rockets with their inefficient chemical fuels are our on ly way to leave the

Earth and to tr avel into space. I f this journey is the destiny of mankind, we have only achieved the very first steps so far. A major leap forward, comparable with the in-troduction of jet engines in aeronautics that revolutionized air-travel, is required totravel anywhere beyond Mars. (Credit: NASA)

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Strategies for Life Detection

When it comes to finding strate-gies for the detection of extrater-restrial life, two cases have to bedistinguished depending on wherethe target is located:

Within the solar system, detectionof living systems may be done by sending a spacecraft there with instruments to take in-situ measure-ments. Another option is remotesensing like for instance mapping the methane distribution in the

Martian atmosphere from an orbit -ing spacecraft. The remote sens-ing approach allows only the de-tection of an active biosphere by finding gases in the atmospherein concentrations that are beyond any abiotic processes. In contrast,the in-situ strategy allows thesearch for biosignatures that areeither based on morphological, mo-lecular or isotopic patterns fromnot necessarily currently living

systems. There is not one singlebiomarker that can be used to un-ambiguously identify a signatureof life. Therefore, the recognitionof patterns, for example in theform of an overabundant presenceof a certain group of molecules,becomes the favourite approach since life as we know it makes useof building blocks that are used re- petitively to form larger structures.

This approach is attractive alsobecause it allows the recognitionof patterns that do not necessarily have to be similar to terrestrial life.

Beyond the sola r system, the dis-tances are by far beyond the reach of any spacecraft. The quest for life, therefore, must be based onremote sensing techniques exclu-sively. Of primary interest are exo-solar planets. So far, over 200 ex-trasolar planets have been found,but most of them are very differ-ent from what we expect to be a habitable planet. The test whether such a planet may harbour life willbe based on so-called atmospheric biosignatures, similar to the spec-troscopic approach mentioned above.

The European Space Agency (ESA ) is currently planning a mission called Darwin (Fig. 10) to

search for gases in the atmosphereof exosolar planets such as ozone,a proxy for the presence of largeamounts of molecular oxygen,

ozone, water, carbon dioxide, and methane. This approach, however,is extremely challenging since theluminosity of the star is orders of magnitudes higher than that of the planet. The analyses can bedone by means of very sensitivelong based spectrometers, which split the light reflected by a plan-et’s atmosphere into its spectrum,where the content of these tracegases can be observed. The important clue here would comefrom the fact that some of thesegases can be present in detectableamounts in the atmosphere of anexosolar planet only if there is a constant re-supply maintained by living systems. Any gas mixturethat is in disequilibrium in an at-

mosphere to such an extent that it can not be explained by abiotic re-actions would indicate biologicalactivity on the planet.

Fig. 10: One of the three space telescopes of the European Space Agency’s Darwinmission. (Credit: ESA )

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Life in theUniverse

So far we have discussed the prop-erties of life, how life evolved on

Earth and how we can maximizethe chance to detect life in theuniverse. Now, we address thequestion where we should search for life.

The Prerequisites for Life

Although no convincing signs of life have been found outside the

Earth so far, a set of conditionscan be compiled which must befulfilled by a star and its planetary system in order to allow the devel-opment of life as we know it:

The central star must not be toomassive (no more than about 1.5times the mass of the Sun). Moremassive stars burn their hydrogenfaster, and the time span in which the star is in a stable state is prob-

ably too short for life to originateand evolve on its planets. On theother hand, stars with less thanabout 10% of the solar mass willtidally lock planets into synchro-nous orbits. Such a planet would have a very hot side towards thestar and a very cold side in the op-

posite direction, similar to Mer-cury in our own solar system. Pro-vided the planet does not have the

mass range of Jupiter-type plan-ets, it will probably not be able tokeep an atmosphere over periodsof hundreds of millions of years.

It is also highly unlikely that such a planet contains liquid water. Inaddition, the planet would be ex- posed to high energy solar flares providing a deadly environment for prebiotic chemistry and life.

For each star it is possible todetermine the so-called habitablezone, which depends of course onhow one defines the term “habit-able”. For example one could cor-relate “habitability” with the pres-ence of liquid water on the surfaceof a planet with an appreciableatmosphere. If such an atmosphere contains a certain amount of greenhouse gases, such as methaneor carbon dioxide, the habitablezone is widened towards the outer

range of the stellar system. Sincethe luminosity of a star tends toincrease during its lifetime, thehabitable zone is being shifted outwards slowly.

Finally, the size of the planet iscritical as well. The mass of a po-tentially habitable planet should not be too small; otherwise itsgravitational field would be tooweak to maintain a dense atmos-

phere over hundreds of mill ionsof years. Also the planet should bemassive enough to maintain ac-tive plate tectonics or at least someform of volcanism throughout itslifetime. Plate tectonics continually recycle oceanic crust back into themantle at subduction zones and continually regenerate it at oceanridges by the solidification of fresh magma. Without such a process,

the planet does not have a feed-back mechanism that would allow it to stabilize its climate and the physical conditions on the surface.

While these prerequisites apply to planets on which life could po-tentially develop, there are placesin our solar system that are consid-ered as potential habitats for someforms of life, but do not fulfil allthese criteria. Of particular in-terest are the moons of the giant planets Jupiter and Saturn, wherethe lack of solar irradiation may be compensated by other mecha-nisms such as tidal heating to pro-vide the required energy. Space-craft data from the Galileo and Cassini missions have supported the notion that liquid water, and perhaps oceans, could be present under the icy surfaces of theseobjects.

Astrobiological Explorationof the Solar System

The solar system is of principleinterest in the quest for extrater-restrial life. Most importantly, thedistances to the other planets aresuch that, with our current tech-nology, we have the capability toexplore these worlds with robotic missions to perform in-situ meas-

urements and eventually bring samples back for analysis here on

Earth. Although we have set foot on the moon almost 40 years ago,we have not yet made the next step, which is the manned missionto Mars. And it seems that it willtake at least another 30 years for this to happen. Of all the planetsand small objects in our solar sys-tem, most are outside the habit-able zone (as defined above). For example, Mercury, the innermost planet in the solar system, is far too close to the Sun and there-

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fore too hot to harbour life. Thesurface temperatures on Mercury range from about –180 °C at thebottom of craters near the polesto about 400 °C at the sub-solar point. In combination with theabsence of a dense atmosphere onMercury, life as we know it could not develop there.

Venus

Venus is the most similar planet tothe Earth in the solar system. It isoften called the Earth’s sister, sinceit has almost identical diameter and mass. Very early in its history,

its environmental conditions may have also been similar to those of the Earth, and perhaps favourablefor the emergence of life. Howev-er, due to its closer distance to theSun, Venus has developed surfaceconditions, including temperaturesof around 460 °C and permanent thick clouds of H

2

SO4

droplets,which make the current presenceof any form of life highly unlikely.

The Earth’s Moon

The Moon is thought to have beenformed as a result of a collision be-tween a very early, semi-molten

Earth and a planet-like object with the size of Mars. The samples re-turned from the Apollo and Luna missions have shown that thereare no signs of life and not evenorganic compounds present onthe Moon’s surface. The steriliz-ing UV and particle radiation it isexposed to would prevent any or-ganic molecule to build up. How-ever, the Moon may have been a crucial element in the develop-ment of li fe on the Earth. With theexception of Pluto and its moonCharon, Earth is the only planet in the solar system that has such a massive satellite relative to its ownmass (the mass of the Moon is

Fig. 11: Nanedi Valles, a roughly 800-kilometre valley extending southwest–northeast and lying in the region of Xanthe Terra,southwest of Chryse Planitia of Mars. In this view, Nanedi Valles ranges from approximately 0.8- to 5.0-kilometre wide and extends to a maximum of about 500 metres below the surrounding plains. The valley’s origins remain unclear, with scientists de-bating whether erosion caused by ground-water outflow, flow of liquid beneath an ice cover or collapse of the surface in associationwith liquid flow is the responsible mechanism. This image was captured by the High-Resolution Stereo Camera (HRSC) onboard ESA ’s Mars Express. (Credit: ESA) .

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about 1/ 7 of the Earth’s mass). Thishigh mass ratio has the effect that the Earth’s rotation axis has alwaysbeen relatively stable at its current inclination of 23.5°, which in turn

provided a stabilized climate onthe surface over billions of years.

Mars

Mars is the primary target for thesearch of traces of past or present life in the solar system. From themany Mars exploring spacecraftsand rovers there is convincing evidence that water existed insubstantial quantities on its sur-face at some earlier epoch. Wedo not know, however, whether

it was present on the surface for a sufficiently long period of timeand if it is perhaps still present insubsurface aquifers. Based on themorphological evidence for the presence of surface water in theearliest epochs, including largeoutflow regions (Fig. 11) and lay-ered sedimentary records, Marsinitially had a dense atmosphere.However, due to the smaller massof Mars as compared to the Earth,

it lost its atmospheric gases to

space, leaving it with the current thin CO 2-rich atmosphere.

The two Viking landers, which touched down on the Martiansurface in 1976, carried biologi-cal life detection instruments aswell as a gas chromatograph-massspectrometer. These instrumentswere used to analyze soil samplesin the immediate vicinity of thelanders (Fig. 12), but they failed todetect organic molecules in any of these samples. This finding isthe primary evidence that pointsto an absence of life on the sur-face and immediate subsurface of Mars today. However, given therecent discovery of flourishing biospheres at 1’000 metres below

the Earth’s surface in South Afri-can gold mines, it is conceivablethat a similar microbial communi-ty might be present in the deeper subsurface of Mars.

The Giant Planets Jupiter and Saturn

The atmospheres of Jupiter and Saturn (as well as Uranus) are

mainly composed of hydrogen

and helium, with a noticeablefraction of methane and a lower contribution of ammonia. Theastrobiological interest of these planets is limited as they have nosolid surface. In contrast, the inter-est in the giant planets’ satellites israpidly growing currently.

The Jovian Moons

The Galilean moons of Jupiter were visited several times by theGalileo spacecraft while orbiting

Jupiter for almost 10 years. Mag-netometer data have provided evi-dence for an ocean of liquid water beneath the icy crust of Europa,Fig. 13. Similar conditions may also

exist on Ganymede and Callisto.While their distances from the Sun prevents these objects to receivesufficient solar radiation to main-tain liquid water on their surfaces,their orbital geometries give riseto tidal forcing of their interior that provides a heat source which is probably sufficient to melt someof the subsurface ice layers. Al-though liquid water is supposed tobe present on Europa, the chances

for life to have originated and de-

Fig. 12: A Martian landscape acquired by the Viking 2 camera in 1976. While some parts of the lander are visible in the fore-ground, a rocky environment is seen in the background. (Credit : Edward A. Guinness, Washington University, St. Louis, USA)

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veloped in this subsurface oceanare remote because it is not clear

where organic compounds would have come from. Although someorganics may have survived an impact on Europa, the transport of this material through the ice shellis not at all understood. Becauseof these uncertainties, Europa is still a prime target for futuremissions, which could include a ground penetrating radar instru-ment or even a sample returnmission.

The Saturnian Moons

Since 1944, when Gerhard Kuiper detected methane in Titan’s at-mosphere, this moon was sus- pected to harbour li fe, as on Earth methane is a product of organic metabolism. Titan has a denseN 2 atmosphere rich in organics

in both gas and aerosol phases. It represents, therefore, a naturallaboratory for studying the forma-tion of complex organic molecules

on a planetary scale and over geo-logical times. Despite the fact that

Titan’s surface temperatures aremuch lower than those found at the Earth’s surface and that liquid water is totally absent therefore,the satellite provides a unique mi-lieu to study the products of thefundamental physical and chemi-cal interactions driving a planetary organic chemistry. However, nei-ther the Cassini orbiter nor theHuygens lander have found any sign of life so far.

Recently, another moon of Sat-urn, the tiny Enceladus, has made

scientific headlines when NASA announced the detection of hugewater geysers on Enceladus by theCassini orbiter (Fig. 14). Plumes of icy material extend far above itssouthern polar region. It is be-lieved that the plumes stem fromgeysers erupting from pressurized subsurface reservoirs, potentially containing liquid water, while thesurface cover is ice at a tempera-ture of around –200 °C.

Fig. 13: Jupiter’s icy moon Europa (left) and Agenor Linea (right), a bright white band on its surface obtained by NASA ’s Gali-leo spacecraft. Along this portion of Agenor is a “triple band,” flanked by dark, reddish material of uncertain origin. On the right side of this image, Agenor splits into two sections. While these and other details of Europa’s surface formations remain mysterious,the general results of Galileo’s exploration of Europa have supported the idea that an ocean of liquid water lies beneath the cracked and frozen crust. (Credit: NASA).

Fig. 14: Plumes of icy material extend above the southern polar region of Enceladus.(Credit: NASA/JPL/ Space Science Institute)

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Objects on the Outer Rim

of the Solar System

The icy giants Uranus and Neptuneas well as Pluto and the Kuiper belt objects are too far away from theSun to have liquid water on their sur-face; however, the exploration of these planets is immensely impor-tant to our general understanding of

the formation of planetary systems.

Comets

Comets are believed to be Kuiper belt or Oort cloud objects whoseorbit has been changed by gravita-tional pull from the outer planetsbringing them into the inner solar system. As stated above, cometscontain large amounts of water 6.

For example, two thirds of the ma-

terial in Comet Halley’s nucleusare water ice, and the rest is madeout of silicate grains and organic particles. Substantial amounts of organic molecules including for example formaldehyde, methanol,etc. were detected in the comas of several comets. Improving detec-tion capabilities of scientific probesallowed for the identification of

many more organic substances incometary comas, like for exampleammonia, methane and acetylene,up to more complex moleculeslike cyano-acetylene. Unfortunately,we have no direct measurementsabout the composition of the nu-cleus yet. The European Space

Agency’s Rosetta mission with itslander Philae is on its way to comet 67 P / Churyumov- Gerasimenko

and will address this question with its scientific payload (Fig. 15).

Outlook

Searching for new worlds hasbeen one of mankind’s very basic endeavours since its earliest days.

Astrobiology is nothing else thanthe continuation of this effort intonew dimensions. But at the sametime it provides us with a deep-ened knowledge of the condi-tions of life on our own planet. Inthat sense, astrobiology may helpus also to become aware of theunique value of life prompting usto safeguard the richness of spe-cies on our Earth.

Fig. 15: An artist’s view of the Rosetta mission and its lander Philae heading for the comet 67 P/Churyumov-Gerasimenko. Rosetta is under way since spring 2004and will reach the orbit of its target comet in spring 2014. After close inspection of thecomet, the probe will deliver the vehicle Philae, which is to touch down on the comet some days later. The probe itself will continue to circle around the comet for another year during its closes approach to the Sun. (Credit: ESA)

6 Spatium 4 : Kometen by Kathrin Altwegg, October 1999

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Oliver Botta was born in Liestal,Kanton Baselland, where he vis-ited the elementary and second-ary schools. Fascinated by theoverwhelming wealth of mole-cules that can be composed froma limited set of atoms he decided to study chemistry at the Univer-

sity of Basel. He concluded thestudies with the Ph.D. degree inorganic chemistry in 1999. Thenhe moved to the Scripps Institutionof Oceanography at the Univer-sity of California at San Diego,US A , where he was engaged inorganic trace analysis. This activ-ity brought him in contact with the analysis of meteorites, thuscombining his chemical back-ground with his fascination of space. He also was involved in the early development of an ana-lytical instrument for the robotic

exploration of solar system bod-ies and that is now part of the science package of the European

ExoMars miss ion.

From 2002 to 2004 Oliver Botta was engaged by the EuropeanSpace Agency ESA as an external

Post Doctoral Fellow at the Uni-versity of Leiden in the Nether-lands, where he not only contin-ued his activities in the area of organic trace analysis of meteor-ites, but was also involved in as-tronomical observations of organ-ic molecules in space. From April1st 2004 onwards he was engaged at the International Space Sci-ence Institute with the primary task to organize an internationalforum on Astrobiology, leading the way for the Institute to in-crease its activities in this inter-

disciplinary scientific field. Last year, Oliver Botta then moved to the United States again, wherehe was engaged at the NASA Goddard Space Flight Center inMaryland to support the devel-opment of the Sample Analysisat Mars (SAM) instrument, a gaschromatograph-mass spectrom-eter for the future NASA MarsScience Laboratory rover mission.On July 1st 2006 he returned to

ISSI to lead the Institute’s futureactivities in Astrobiology.

Astrobiology is not really a new scientific field, but more a rigor -ous attempt to combine the know l- edge and the methodologies of the classical branches of sciencewith the goal to lead to answersto some of mankind’s most funda-mental questions relating to theemergence of life on the Earth and

the possibilities for life at other places in the solar system and beyond. This interdisciplinary approach requires scientists to com-municate in a common language,and that is where Oliver Botta sees his role in his future engage-ment at ISSI . Together with hiswife, Oliver Botta has two sonsand a daughter, with whom helikes to play Lego, a concept that

is comparable to nature’s to build up complex systems by repeating and re-arranging (relatively) simple building blocks.

The Author

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