1 from molecular clouds to the origin of life€¦ · 1 from molecular clouds to the origin of life...

1 From Molecular Clouds to the Origin of Life

Pascale Ehrenfreund and Karl M. Menten

In our Milky Way and in external galaxies, the space between the stars is filled with aninterstellar medium (ISM) consisting of gas and dust. The ISM can be divided in vari-ous different components with very different physical parameters, ranging from a veryhot (106 K), dilute (<10-2 particles cm-3) component heated by supernova explosions,which fills more than half of its volume, to molecular clouds with temperatures from10 to 100 K and molecular hydrogen (H2) densities from a few hundred to 108 parti-cles cm-3, which make up less than 1% of the ISM's volume. Approximately 1% of itsmass is contained in microscopic (micron-sized) interstellar dust.

While accounting for only a small fraction (~1%) of the Galaxy's mass, the ISM isnevertheless an important part of the Galactic ecosystem. Gravitational collapse ofdense interstellar clouds leads to the formation of new stars, which produce heavierelements in their interiors by nucleosynthesis. The main reaction in stellar interiors isthe nuclear fusion of H into He. In a later stage of stellar evolution C, N and O areformed. Further nucleosynthesis occurs in massive stars leading to elements as heavyas iron. At the end of their lifetime which is mainly determined by their initial mass,stars return material to the interstellar environment by mass outflows, forming ex-panding shells and envelopes, as well as by violent explosions. Thus, the ISM repre-sents an environment, in which atoms, molecules, and solid matter undergo strongevolution and recycling.

Observations at radio, millimeter, sub-millimeter, and infrared wavelengths have ledto the discovery of well over a hundred different molecules in interstellar clouds andcircumstellar shells (see Table 1.1.). Many of these are organic species of considerablecomplexity, with HC11N [1] and diethyl ether [(C2H5)2O] [2] being the largest detectedso far. In all cases, accurate line frequencies predicted by laboratory spectroscopy en-sure an unambiguous identification. These molecules, which have abundance ratiosrelative to molecular hydrogen of less, and in many cases much less than 10-4, are im-portant tracers of the physical conditions and chemistry of many different interstellarand circumstellar environments. Despite their relatively low abundances, the varietyand complexity of organic compounds currently detected in space indicates an activechemistry and ubiquitous distribution [3, 4]. Extraterrestrial organics may have playeda role in the origin and evolution of life [5-7].

The dense cold phases of the interstellar medium also host icy dust grains, whosemolecular composition has recently been well constrained by observations from theInfrared Space Observatory (ISO) [8, 9]. These dust particles are important chemicalcatalysts and trigger molecular complexity in the interstellar gas and dust. In contrast tothe case of gas phase molecules, the exact nature of interstellar solids cannot be unam-biguously identified by astronomical observations but can be constrained by laboratory

8 P. Ehrenfreund and K. M. Menten

techniques. Our knowledge of the carbonaceous solid state and gas phase inventoriesof molecular clouds has recently been reviewed [4].

Numerous protoplanetary disks have been imaged with the Hubble Space Telescope(HST), indicating that the formation of extrasolar systems similar to our own solarsystem (Fig. 1.1) is a rather common process [10, 11, see Chapt. 2, Udry and Mayor].During star formation, interstellar molecules and dust become the building blocks forprotostellar disks, from which planets, comets, asteroids, and other macroscopic bodiesform [12].

Fig. 1.1 Planetary systems now forming in Orion (Credit: C. R. O'Dell and S. K. Wong, RiceUniversity, and NASA). The Orion nebula is a star-forming region located in the constellationOrion the Hunter about 1500 light-years away. The optically visible nebula is excited by one ofthe young massive stars that formed here about one million years ago together with thousands oflower mass stars. Many of the low mass stars are still surrounded by disks of placental cloudmaterial of gas and dust that formed during the protostellar collapse. Using the Hubble SpaceTelescope, various of such protoplanetary disks have been detected in silhouette against thenebular emission background. The above mosaic shows several examples. In the bottom leftinsert the relative size of our own Solar System is shown for comparison. The discovery ofprotoplanetary disks around other stars provides strong evidence for the paradigm of solarsystem first proposed by Kant and Laplace.

1 F

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Orig

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9

Tab

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nter

stel

lar a

nd c

ircum

stel

lar m

olec

ules

as

com

pile

d pe

r Oct

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12

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the

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m.

Num

ber

of A

tom

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34

56

78

910

1112

+H

2C

3c-

C3H

C5

C5H

C6H

CH

3C3N

CH

3C4N

CH

3C5N

?H

C9N

C6H

6A

lFC

2Hl-C

3HC

4Hl-H

2C4

CH

2CH

CN

HC

OO

CH

3C

H3C

H2C

N(C

H3)

2CO

HC

11N

AlC

lC

2OC

3NC

4Si

C2H

4C

H3C

2HC

H3C

OO

H ?

(CH

3)2O

NH

2CH

2CO

OH

?PA

Hs

C2

C2S

C3O

l-C3H

2C

H3C

NH

C5N

C7H

CH

3CH

2OH

C60

+

CH

CH

2C

3Sc-

C3H

2H

C3N

CH

CO

CH

3H

2C6

HC

7NC

H+

HC

NC

2H2

CH

2CN

HC

3OH

NH

2CH

3C

8HC

NH

CO

CH

2D+

?C

H4

HC

3SH

c-C

2H4O

CO

HC

O+

HC

CN

HC

3NH

C3N

H+

CO

+H

CS+

HC

NH

+H

C2N

CH

C2C

HO

CP

HO

C+

HN

CO

HC

OO

HN

H2C

HO

Csi

H2O

HN

CS

H2C

HN

C5N

HC

lH

2SH

OC

O+

H2C

2OK

Cl

HN

CH

2CO

H2N

CN

NH

HN

OH

sCN

HN

C3

NO

MgC

NH

2CS

SiH

4N

SM

gNC

H3O

+H

2CO

H+

NaC

lN

2H+

NH

3O

HN

2OSi

C3

PNN

aCN

CH

3SO

OC

SSO

+SO

2Si

Nc-

SiC

2Si

OC

O2

SiS

NH

2C

SH

3+

HF

H2D

+


Comets are agglomerates of frozen gases, ices, and rocky debris, and are likely to bethe most primitive bodies in the solar system. Comets are formed in the outer solarsystem from remnant planetesimals that were not integrated into planets. Such cometnuclei were thrown into large high inclination orbits by perturbation of the major plan-ets into the so-called Oort cloud at ~50 000 astronomical units (AU) from the sun.Comet nuclei that were formed near the plane of Neptune and beyond reside in theEdgeworth-Kuiper belt [13]. When comets are perturbed and enter the solar system,solar radiation heats the icy surface and forms a gaseous cloud, the coma. During thissublimation process, "parent" volatiles are subsequently photolysed and produce radi-cals and ions, the so-called "daughter" molecules. Space missions to comet Halley andrecent astronomical observations of two bright comets, Hyakutake and Hale-Bopp,allowed astronomers to make an inventory of cometary molecular species [14, 15].Small bodies in the solar system, such as comets, asteroids, and their meteoritic frag-ments carry pristine material left over from the solar system formation process, thussampling the molecular cloud material out of which the sun and planets formed.

1.1 The Search for Large Organic Moleculesin Dense Clouds

To date, more than 120 molecules have been detected in the interstellar medium andthe circumstellar environments of red giant stars (i.e. stars at the end of their life cycle,see Table 1.1.). Most of these have rotational spectra at radio-, millimeter-, and sub-millimeter wavelengths, which can be efficiently observed with modern telescopes anddetector equipment. Many interstellar molecules only exist in dense clouds, where theyare sufficiently shielded from UV irradiation. In particular, a number of prebiotic spe-cies such as H2CO, HCN and NH3 have been identified.

1.1.1 The Search for Amino Acids in the Interstellar Medium

For more than two decades, considerable effort has been devoted to various attemptsof detecting the simplest amino acid, glycine (NH2CH2COOH) in interstellar molecu-lar clouds. Glycine exists in a variety of conformations [16] and the extensive labora-tory microwave spectroscopy of conformers I and II [17] allows meaningful astro-nomical searches. The very large partition function of a species as complex as glycineresults in relatively weak lines which are difficult to detect in dense, warm molecularcloud cores due to confusion produced by a "forest'' of weak lines from a large numberof species that are also present in the region in question. Consequently, none of thesearches conducted so far has resulted in an unambiguous detection; the best upperlimits on the glycine-to-H2 abundance ratio are of order 10-10 [18-25].

As discussed by Snyder [23], the use of high spatial resolution interferometric ob-servations may bring some relief to the confusion problem as various classes of mole-cules (e.g., O-rich vs. N-rich species) have different spatial distributions and/or radialvelocities [26].

1 From Molecular Clouds to the Origin of Life 11

The formation routes of amino acids in interstellar gas and dust are not yet estab-lished. Charnley [27] proposed that protonated glycine could be formed in hot cores byreaction of protonated aminomethanol and HCOOH.

OHCOOHCHNHHCOOHOHCHNH 22222 +→+ ++ (1.1)

Amino acids could also be formed in the solid phase by irradiation of interstellargrain mantles with ultraviolet photons [28]. However, amino acids are highly suscepti-ble to UV photo-destruction, even under exposure to UV photons of relatively lowenergy [29]. Though low concentrations of amino acids may be detected by astronomi-cal observations at radio wavelength in UV shielded environments, such as the hotcores and the inner cometary coma, all environments with an elevated UV flux shouldhave merely traces of these compounds. This may explain the lack of detection of thesecompounds in space.

1.1.2 Organic Molecules in our Galactic Center

(Sub)millimeter line surveys of well known sources, such as the dense cores in theOrion and Sagittarius molecular clouds [30-32], show that molecules of considerablecomplexity can be found in these regions and others might yet have to be detected [25].A large part of the molecular complexity found in these regions is due to gas-graininteractions. The sources mentioned above are examples of so-called “hot cores”,dense (106 particles cm-3) hot (~200 K) regions in the immediate vicinity of massiveprotostars. The high abundance of many molecular species found in hot cores cannotbe explained by gas phase chemistry and one must invoke molecule formation on cata-lytic icy grain surfaces [3, 4] during the cold dark cloud phase of these cores. Heatingby the newly formed protostar and/or energetic processes such as outflows producingshock waves lead to evaporation of the grain mantles into the gas, which is followed bygas-phase reactions. Pathways to the formation of larger molecules include alkyl cationtransfer [25].

Figure 1.2 shows Berkeley-Illinois-Maryland Array (BIMA) observations near 3mm wavelength of a methanol (CH3OH) transition, the dust continuum emission, andthe 120,12-111,11 transition of methyl-ethyl-ether (MeOEt) toward the massive densemolecular cloud core Sagittarius B2(North) [Sgr B2(N)] near the Galactic center.These interferometer maps show a clear concentration of methanol around the SgrB2(N) hot core (Fig. 1.2, top). The Sgr B2(N) regions is also known as the “LargeMolecule Heimat“ [23] due to the high concentrations and rich diversity of the com-plex organic molecules detected there. By contrast, few large organics are detected atSgr B2(M) and this is generally believed to be because B2(M) is much more dynami-cally and chemically evolved; Sgr B2(N) appears to have only recently `switched on'(less than about 104 years) and the contents of its evaporated mantles have not yet beendestroyed in gas phase reactions. This situation is evident in Fig. 1.2 where MeOH andMeOEt are only present in the northern source. The map shown in Fig. 1.2 (bottom)shows that the MeOEt emission has a distribution that is very similar to that of themethanol, as one might expect if these molecules are chemically linked [25]. TheBIMA data indicate [MeOEt/H2] abundance ratios of 10-10-10-9 in Sgr B2(N).


Fig. 1.2 BIMA maps of CH3OH and MeOEt in Sgr B2(N) [25]. The contours roughly representthe strength of the molecular radiation. The size of the synthesized beam in each case is given bythe ellipses in the lower-left corner. Top: CH3OH 13-3-14-2 at 84.4 GHz (contours) plus 3-mmcontinuum (color scale). For the 3 mm continuum the grey scale range is 0.51-6.5 Jy/beam. ForMeOH, the sythesized beam was 21''.91 x 4''.74 and the contours are at 3, 4 and 5 sigma, with 1rms=0.07 Jy/beam. Bottom: MeOEt 120,12-111,11 at 79.6 GHz (contours) plus CH3OH 13-3-14-2(color scale). The number on the upper-left corner is the LSR velocity. For MeOEt, the sythe-sized beam was 24''.32 x 4''.88 and the contours are at 3, 5, 7, 10, 20, 28 sigma, with 1rms=0.15 Jy/beam. For MeOH, the gray scale range is 0.2-2.3 Jy/beam.


Radioastronomical observations are vital for the identification of large organicmolecules in the interstellar medium and in cometary comae. Such observations helpreconstructing the gas-grain chemical pathways in such regions.

1.2 Molecules in Protoplanetary Disks

In current scenarios of low mass star formation a protostar with an accretion disk andstrong mass outflow is formed after gravitational collapse of a molecular cloud on atime-scale of 104-105 years [33, 34]. In its early evolutionary phases this protostar isstill embedded in its placental cloud material. It then evolves into a T Tauri star andafter ~106-107 years reaches the main sequence. During this phase, a planetary systemmay form. T Tauri stars are considered to resemble our sun when it was a few millionyears old. Studies of their surrounding gas and dust can therefore provide importantclues on the early evolution of the solar nebula.

Infrared and millimeter surveys have shown that most T Tauri stars have circum-stellar disks with masses of 10-3-10-1 solar masses and sizes of 100-400 AU [35-37].Such disks provide a reservoir of gas and dust for the formation of potential planetarysystems [38]. For the purpose of discussing their chemistry, one may divide proto-planetary disks in a multi-layer structure consisting of a midplane, an intermediateregion, and a surface region [39]. The densities and radiation fields in the surface re-gion are similar to interstellar photodissociation regions. In the warm upper atmos-phere of the disks exposed to both the central star and the interstellar radiation field,the molecules are rapidly destroyed by photodissociation [40]. The intermediate regionhas conditions similar to dense molecular clouds where ionmolecule and neutral-neutral reactions occur in addition to photochemistry. The midplane of the disk cannotbe penetrated by UV photons at all and molecules freeze out onto dust grain surfaces.

Determining the molecular composition of such protostellar disks provides infor-mation about the protosolar nebula and the evolution of our solar system. Dutrey et al.[41] reported the detection of CN, HCN, HNC, CS, HCO+, C2H and H2CO (ortho andpara) in the protoplanetary disks of DM Tau and GG Tau. These systems are only 140pc away and located in the outskirts of the Taurus molecular cloud complex. DM Tauis one of the oldest T Tauri stars in this region, whereas GG Tau is a young binary star.It was found that many of the above mentioned molecules are under-abundant withrespect to standard dense clouds. The relatively large abundances of CN and C2H indi-cate a rich photon-dominated chemistry [41]. It is worth pointing out that the smallangular dimensions of the emission regions in question preclude detailed studies of thechemical and physical conditions on solar system size scales (i.e. less than 0.5 arcsec atthe Taurus molecular cloud distance) with current millimeter-wavelength equipment.Future instruments such as the Atacama Large Millimeter Array (ALMA) combine thenecessary spatial resolution and high sensitivity to allow such observations resolvingsize scales of a few astronomical units [42].


1.3 Diffuse Interstellar Clouds

Diffuse clouds are dominated by photochemistry. Such clouds have moderate extinc-tions (<1 mag) and densities of roughly 100-300 cm-3. Interstellar extinction is meas-ured in magnitude (mag) at a given wavelength and reflects the starlight which is ab-sorbed and scattered by dust grains and reaches the observer dimmed. The HST hasprovided many accurate ultraviolet measurements of metals like C, O, N, Mg, Si, S, Cr,Fe, Ge, Zn, and Kr, greatly advancing our knowledge on the metallicity of diffuse gasclouds [43]. In particular, measurements of the carbon abundance provide strong con-straints on the budget available to form grains with carbonaceous mantles. Sightlinesthrough diffuse clouds also allow measurements of extinction curves [44]. With manysuch measurements available, it has become clear that the long wavelength part of theextinction curve (>2500 Å) varies little from sightline to sightline, but that the con-verse is true for the short wavelength behavior, including the 2200 Å bump. Theseuncertainties very likely reflect changes in the composition and size distribution of(small) interstellar dust grains [45].

Organic molecules present in the diffuse medium can originate via gas phase reac-tions, either in situ [46] or by reactions in circumstellar envelopes followed by subse-quent mixing into the diffuse medium, or by photoreactions of carbonaceous particlesand sputtering by grain-grain collisions. Among the large organic molecules observedor suspected in diffuse clouds are polycyclic aromatic hydrocarbons (PAHs), fullere-nes, carbon-chains, diamonds, amorphous carbon (hydrogenated and bare), and com-plex kerogen-type aromatic networks. The formation and distribution of large mole-cules in the gas and solid state is far from understood. In the envelopes of carbon-richlate-type stars, carbon is mostly locked in CO and C2H2. C2H2 molecules are precur-sors for soot formation where PAHs might act as intermediates [47]. The ubiquitouspresence of aromatic structures in the ISM and in external galaxies has been welldocumented by numerous observations with the Infrared Space Observatory, or ISO(see special volume 315 of Astronomy & Astrophysics, 1996). Possible evidence forcarbon-chains and fullerenes arises from the characterization of the Diffuse InterstellarBands (DIBs) [48-51]. Diamonds were recently proposed to be the carriers of the 3.4and 3.5 µm emission bands observed in planetary nebulae [52] and hydrogenatedamorphous carbon (HAC) seems to be responsible for the 2200 Å bump in the inter-stellar extinction curve [53].

Carbonaceous dust in the interstellar medium may show considerable diversity andmay include amorphous carbon (AC), hydrogenated amorphous carbon (HAC), coal,soot, quenched-carbonaceous condensates (QCC), diamonds, and other compounds[54]. The coexistence of PAHs and fullerenes together with complex carbonaceousdust suggests a common link and an evolutionary cycle that is dominated by energeticprocessing [55, 56]. A detailed comparison of solid-state carbonaceous models ofcosmic dust has been summarized in [57]. Figure 1.3 displays the chemical structure ofsome carbon compounds which are likely to be present in diffuse clouds and otherspace environments.


1.4 The Evolution of Organic MoleculesDuring Solar System Formation

In comets and outer solar system asteroids, organic molecules formed in the presolarinterstellar nebula may have survived solar system formation in relatively pristineform. Therefore, these small bodies carry important evidence on the formation of thesolar system. A number of now extinct short-lived nuclides provide important infor-mation about the sources of solar system material and, moreover, are useful chro-nometers of chemical processes, which have occurred in the early solar nebula [58].Primitive meteorites contain dust grains that predate the solar system. Isotopic analysisshows that such grains are formed in stellar atmospheres and thus represent samples ofancient stardust [59]. Isotopic analyses (e.g., of C, N, 26Al, Sr, Zr, Mo etc.) of presolargrains allow reconstruction of the nucleosynthetic Processes, which occurred in theenvironment in which such particles were formed. Presolar material has been altered bychemical and physical processes in the solar nebula before becoming incorporated intosmall bodies. Dust and molecules have been affected by different processes dominatingat various radial distances from the sun. It is assumed that the outer solar nebula wasan environment of low temperature and pressure. Whereas heating and thermochemicalreactions were important in the inner nebula, the outer solar nebula where cometsformed, was mainly dominated by UV photochemistry and ion-molecule reactions[60].

Fig. 1.3 Some of the various forms of carbon that are likely present in gaseous and solid state inthe interstellar medium and in solar system material.


1.4.1 Comets

Small bodies of the solar system were formed in the region of the giant planets andbeyond from remnant planetesimals which were not assembled into planets. Comets areamongst the most pristine objects and studies of their composition are thus of obviousinterest for all models of the early solar system. The only way to measure the nuclearcomposition of a comet directly is via in-situ measurements by a space probe such asthe Giotto mission to comet Halley [61]. Observations of the coma allow us in princi-ple to deduce the molecular inventory of the nucleus, see Fig. 1.4. Remote sensingobservations of comets Hyakutake and Hale-Bopp have revolutionized our under-standing of the volatile chemical inventory of comets and the interstellar-comet con-nection. Many new cometary molecules were discovered by IR and radio observations[62-64, 14, 15]. Recent detections include SO, SO2, HC3N, NH2CHO, HCOOH, andHCOOCH3 [65]. Interesting to mention are also the upper limits for organic moleculessuch as glycine and ketene, which indicate that such molecules are not abundant in thistype of comets [66]. With a few exceptions such as ethane (C2H6), N

+2, CO+, all mole-

cules in the cometary coma are also observed in the interstellar medium.

Fig. 1.4 The spectacular appearance of comet West showing the ion and dust tails observedfrom the Observatoire de Haute Provence, France. Comets as bright as this appear only once ortwice in a decade. Future space missions to rendezvous a comet will strongly improve ourknowledge on the organic inventory, which may have seeded the early Earth.


Most of the current cometary inventory has been determined from remote sensing ofthe long period comets Halley, Hale-Bopp and Hyakutake. Much less is known aboutthe composition of short-period comets. Interestingly, recent observations of suchobjects indicate significant chemical diversity in the giant planets region. Observationsof comet 21P/Giacobini-Zinner show deficiencies of ethane and CO and comet Lee isdepleted in CO compared to the long-period comets Hale-Bopp, and Hyakutake [67].

A large amount of information is expected to be obtained from space missions cur-rently on their way or on the launching pad for a rendezvous with a comet such asSTARDUST, CONTOUR and ROSETTA (see Chap. 24, Foing). On-board instrumen-tation on the ROSETTA spacecraft will measure the physical properties of comet Wir-tanen and the chemical composition of its coma, but there will also be an attempt toland for the first time on a comet nucleus to perform in situ measurements. Such un-precedented encounters will strongly increase our knowledge on the chemistry andcomposition of comets.

1.4.2 Meteorites

Over a century ago, it was established that some meteorites contain carbonaceousmaterial. These carbonaceous chondrites contain a few percent of carbon and some ofthem exhibit a large variety of organic compounds [68]. The best studied carbonaceouschondrite to date is the Murchison meteorite, of CM type, which fell in Australia on 28September 1969. Subsequent analyses using a variety of methods have shown that theMurchison meteorite contains over 70 different amino acids, the majority of whichhave no known terrestrial occurrence [68]. It is generally thought that most meteorites,and in particular the CM type carbonaceous chondrites, originate from the asteroidbelt. In fact, powdered samples of the Murchison meteorite heated up to 900 °C showstrong similarities in their reflectance spectra to C and G type asteroids (C-type in-cludes more than 75% of known asteroids; they are extremely dark, albedo 0.03),which points to an asteroidal origin [69]. However, based on mineralogical and chemi-cal evidence, it has recently been suggested that both CI and CM meteorites could alsobe fragments of comets [70, 71]. The distinction between comets and asteroids is nolonger clearly drawn, and several objects have currently a dual designation [72]. Un-derstanding the link between small bodies such as comets, asteroids and their fragmentsenables us to reconstruct the processes occurring during planet formation [73].

1.5 Implications for the Origin of Life on Earth

Right after the formation of the Earth, about 4.5 x 109 years ago, the planet providedvery hostile conditions for life to develop. Volcanic eruptions from the heated interiorand external heavy bombardment by small bodies may have extinguished emerging lifeon a rapid time scale. The heavy bombardment phase, which has been scaled from thelunar record, ended about 3.8 x 109 ago. The first evidence for life follows 300 millionyears later and is provided by microfossils [74]. Ion probe measurements of the carbonisotope composition of carbonaceous inclusions (within apatite grains) from the oldestknown sediments (banded-iron formation (BIF) from Isua) showed isotopically light


carbon, indicative of biological activity even 300 million years earlier [75]. Thoseresults provide evidence for the emergence of life on Earth ~3.8 x 109 years beforepresent, just at the end of the late heavy bombardment phase. This leaves very littletime for life to develop.

Today, research on the origin of life is an interdisciplinary field, which, apart frombiologists, involves chemists, physicists, geologists and astronomers. Numerous theo-ries for the origin of life exist which are based either on a terrestrial or an extraterres-trial origin [76]. Ideas for a terrestrial origin of life are focussed on the spontaneousformation of stable polymers out of monomers. It has been shown that amino acidsspontaneously form polypeptides in aqueous solution under certain conditions andRNA oligomers spontaneously form on inorganic substances such as clay structures.For detailed information on the organic chemistry leading to higher complexity andlife, we refer to the Chaps. 22, Heckl, and 23, Schidlowski.

An extraterrestrial origin of life on Earth via cosmic delivery of living organisms(panspermia), as proposed already in 1903 by Arrhenius, appears unlikely. Indeed,some organisms (and in particular their spores) are able to survive in extreme condi-tions of temperature and radiation on Earth and it has been argued that such speciescould survive interplanetary travel (see Chap. 4, Horneck et al.). However, recentstudies have shown that the survival potential for living entities embedded in comets,asteroids, and cosmic dust impacting on the early Earth is negligible [77]. In contrast,the possible transport of extraterrestrial organic material via infalling comets and as-teroids is a serious possibility [5, 6]. The very narrow window between the end of theheavy bombardment phase and the evidence for primitive organisms favors the idea thatimpacting prebiotic matter could have been the first step to life. Though it cannot beexcluded that organics and living organisms have developed locally in protected areason the Earth's surface or within the oceans, a substantial fraction of the Earth's prebi-otic inventory of organic molecules and water may have been of extraterrestrial origin.Impact studies show that in particular small particles can be gently decelerated by theEarth's atmosphere and may have brought intact organics to the early Earth [78, 79].

The origin and development of life must be strongly dependent on the conditions ofits host environment. One of the most important events in early Earth evolution is theformation of an atmosphere. The primitive atmosphere originating from the accumula-tion of gases released from the surface must be related in composition to volcanicemissions, whose composition, in turn, depends on the internal structure of the planetsuch as the oxidation state of the upper mantle [80]. Current evidence strongly favorsthe hot accretion model, in which the Earth essentially formed in a differentiated state[80]. In this case, non-reducing or mildly-reducing emissions, composed of CO2 andN2 (and only traces of other species), are predicted from the earliest times. Such anatmospheric composition is not favorable for the formation of abundant prebioticmolecules in contrast to the conditions for the Miller-Urey experiments [81]. Theseexperiments showed that important biological molecules, including sugars and aminoacids could be formed by spark discharge in an atmosphere of reducing conditions(containing CH4, NH3, H2, H2O). Exogenous delivery of organics may have been there-fore more important than the endogenous production of organics on Earth. Even today,tons of organic material is brought to Earth via small particles, so called micrometeor-ites. More than 120 major craters found on Earth show the effects of violent impactsfrom space, see Fig. 1.5. For example, the Cretaceous-Tertiary (C/T) boundary sedi-


ments are sedimentary deposits that accrued from the end of the Cretaceous period tothe beginning of the Tertiary period. They are distributed world wide and are recog-nized as a unique signature of a large asteroidal impact event near Chicxulub in Mex-ico [82].The discovery of high concentrations of extraterrestrial Ir in KTB sedimentsand Cr ratios are consistent with a chondritic type impactor [83].

The early atmosphere contained little or no free oxygen. The oxygen concentrationincreased markedly near 2.0 x 109 years ago due to photosynthetic activity of microor-ganisms. Greenhouse warming by small amounts of CH4 in the atmosphere may haveformed an organic haze layer, which cooled the climate and protected primitive lifefrom UV irradiation in the period 3.5-2.5 x 109 years [84]. Recent analysis of precam-brian sedimentary rocks have revealed a profound change in chemical reactions in-volving S and O in the atmosphere that occurred between 2.1 x 109 and 2.5 x 109 yearsago [85]. During this exact period, the oxygen levels in the atmosphere strongly in-creased. The increase of oxygen in the atmosphere was a major step in the evolution oflife on Earth (see Chap. 14, Cockell).

Fig. 1.5 49 000 years ago a large meteor created the Barringer Meteor Crater near Flagstaff,Arizona (credit D. Roddy, LPI). In 1920, it was the first feature on Earth to be recognized as animpact crater. Today, over 100 terrestrial impact craters have been identified.

1.6 Conclusions

The biogenic elements (H, C, N, O, S, P) and organic matter are some of the majorconstituents of the universe. Observations over the entire electromagnetic spectrum


revealed a rich diversity of organic matter which is formed in interstellar and circum-stellar regions. The route from a diffuse cloud to a self-gravitating molecular cloudcore may take tens of millions of years, see Fig. 1.6. During this time, the interstellargas undergoes strong chemical changes. To understand the process of star formation,one needs to comprehend the combined thermal and chemical balance of diffuse anddense interstellar gas clouds as they make their way from stellar winds to proto-stellarobjects. The discoveries of proto-planetary disks around other stars suggest that theprocesses, which occurred in our solar nebula are common and the formation of solarsystems like ours is not unique. The recent advances in the search for planets confirmthis picture. The role of comets and other planetesimals in contributing organic matterto the primitive Earth and the prebiotic synthesis of biochemical compounds are major

Fig. 1.6 The cycle of a low-mass star (taken from [86]).questions, which remain to be answered in the future. Impacts may have led to extinc-tion but may have also brought vital molecules to planet Earth, including organics andsome H2O. Valuable information may be obtained by future space missions investi-gating the existence of extinct and extant life on Mars, Europa and other bodies of thesolar system and the search for new planets. Many of these questions will be answered


by research groups, which are participating in the NASA Origins program. To provideanswers to the questions how life originated is of vital importance in the frame ofrecent planetary detections and the possible emergence of life elsewhere.

Acknowledgement. The authors especially want to thank Malcolm Walmsley for acareful reading of the manuscript. This work was supported by the Netherlands Re-search School for Astronomy (NOVA). We thank Y. Kuan and S. Charnley for permis-sion to show their BIMA maps of SgrB in advance of publication and Y. Pendleton forpermission to reprint her figure from Sky & Telescope (1994).

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