Exobiology in the Solar System & The Search for Life on Mars - ESA

More documents

Recommendations

Info

SP-1231 102 vidual compounds is such that <strong>the</strong> simplest <strong>in</strong> terms of structure are most abundant. This implies an abiogenic orig<strong>in</strong>, i.e. built up from s<strong>in</strong>gle carbon atoms. <strong>The</strong> same k<strong>in</strong>d of effects are also seen <strong>in</strong> short-cha<strong>in</strong> aliphatic hydrocarbons, mono and dicarboxylic acids (Yuen & Kvenvolden, 1973) and polyaromatic hydrocarbons. <strong>The</strong> above <strong>in</strong>terpretation is confirmed by <strong>the</strong> <strong>in</strong>vestigation of <strong>the</strong> isotopic composition of homologous series (Yuen et al., 1984; Gilmour & Pill<strong>in</strong>ger, 1994), where isotope fractionation effects are <strong>in</strong> keep<strong>in</strong>g with <strong>the</strong> greater strength of 12 C- 13 C bonds as might be expected if <strong>the</strong> compound were constructed from simple precursors by a cha<strong>in</strong>build<strong>in</strong>g process. Biosyn<strong>the</strong>sis, while produc<strong>in</strong>g isotopic differences between oxidised and reduced carbon, does not seem to show regular fractionation between small and large homologues. Although <strong>the</strong> orig<strong>in</strong>al breakthrough <strong>in</strong> recognis<strong>in</strong>g <strong>in</strong>digenous compounds <strong>in</strong> Murchison came from <strong>the</strong> discovery of racemic mixtures of am<strong>in</strong>o acids, <strong>the</strong> most recent activities <strong>in</strong> this area have <strong>in</strong>volved show<strong>in</strong>g that <strong>in</strong> some <strong>in</strong>stances <strong>the</strong>re are slight excesses (5-10%) of L enantiomers <strong>in</strong> some compounds (Cron<strong>in</strong> & Pizzarello, 1996). <strong>The</strong> great pa<strong>in</strong>s taken with extraction procedures suggest that this is a real effect ra<strong>the</strong>r than a result of contam<strong>in</strong>ation. One proposed explanation of <strong>the</strong> results is that UV synchrotron circularly polarised light, emitted by neutron stars, might <strong>in</strong>troduce an antiomeric bias. If this explanation turns out to be valid, <strong>the</strong>n while it could account <strong>for</strong> <strong>the</strong> orig<strong>in</strong> of optical activity, it possibly calls <strong>in</strong> to question <strong>the</strong> whole philosophy of us<strong>in</strong>g chirality as a means of detect<strong>in</strong>g exobiology <strong>in</strong> extraterrestrial environments <strong>in</strong> <strong>the</strong> early <strong>Solar</strong> <strong>System</strong>. Although <strong>the</strong> organic compounds, which are solvent-extractable from carbonaceous chondrites, are treated somewhat warily because of contam<strong>in</strong>ation issues, <strong>the</strong> macromolecular material that comprises <strong>the</strong> bulk of <strong>the</strong> carbon is usually considered as <strong>in</strong>digenous. <strong>The</strong>re is no plausible way that such a large accumulation, up to 5wt‰, could have accrued from contam<strong>in</strong>ation. In any case, even <strong>in</strong> <strong>the</strong> early 19th century, bulk carbon contents were measured soon after <strong>the</strong> fall of specimens such as <strong>the</strong> CI chondrite Alais, and high carbon contents were recorded at that time. <strong>The</strong> carbon isotopic composition of this material is remarkably constant (Smith & Kaplan, 1970; Kerridge, 1985) and relatively high <strong>for</strong> terrestrial contam<strong>in</strong>ation anyway. Nitrogen isotopic composition is well outside of <strong>the</strong> terrestrial range (Kerridge, 1985). It should be noted that when compared to <strong>the</strong> carbonates found <strong>in</strong> <strong>the</strong> same samples, <strong>the</strong>re is a relatively large fractionation, 50-80‰, <strong>in</strong> <strong>the</strong> same sense as used by Schidlowski as an <strong>in</strong>dicator of biogenicity (Schidlowski, 1987). Like kerogen <strong>in</strong> <strong>the</strong> Earth’s sedimentary rocks, meteorite macromolecules are very <strong>in</strong>tractable, yield<strong>in</strong>g <strong>in</strong><strong>for</strong>mation only about <strong>the</strong>ir general structure when broken down by various pyrolytic or oxidative techniques; <strong>the</strong> build<strong>in</strong>g blocks appear to be substantially aromatic. <strong>The</strong>re is a strong likelihood that <strong>the</strong> material constitut<strong>in</strong>g <strong>the</strong> bulk of <strong>the</strong> carbon <strong>in</strong> carbonaceous chondrites and unequilibrated ord<strong>in</strong>ary chondrites orig<strong>in</strong>ates <strong>in</strong> dark <strong>in</strong>terstellar clouds. <strong>The</strong> key results <strong>in</strong> this respect are <strong>the</strong> recognition of substantial deuterium enrichments (Kerridge, 1993; McNaughton et al., 1981). Deuterium to hydrogen ratios approach<strong>in</strong>g unity are encountered <strong>in</strong> small <strong>in</strong>terstellar molecules because of ion-molecule reactions, which encourage H to D exchange (Geiss & Reeves, 1981). In addition to <strong>the</strong> organic chemistry discussed above, <strong>the</strong>re have been <strong>in</strong>stances when <strong>the</strong> existence of microfossils were suggested <strong>in</strong> carbonaceous chondrites (Nagy et al., 1972), but such reports have been strongly criticised, sometimes justifiably, but perhaps not always. It should not be considered unscientific to <strong>in</strong>dulge <strong>in</strong> detailed characterisation of <strong>the</strong> organic matter <strong>in</strong> carbonaceous chondrites from <strong>the</strong> po<strong>in</strong>t of view of exobiology (Nagy et al., 1961) but as already stated this is almost a taboo area. As can be seen from <strong>the</strong> am<strong>in</strong>o acid studies above, <strong>in</strong>vestigations are almost always motivated by abiogenic <strong>in</strong>terests, with large quantities of <strong>in</strong><strong>for</strong>mation ignored because contam<strong>in</strong>ation is perhaps too readily <strong>in</strong>voked. Methods to recognise contam<strong>in</strong>ation should be fur<strong>the</strong>r <strong>in</strong>vestigated, us<strong>in</strong>g <strong>for</strong> example isotopic order, ra<strong>the</strong>r than discard<strong>in</strong>g unconsidered large quantities of possibly useful data. Huge amounts of carbonaceous matter from carbonaceous chondrites have been destroyed <strong>in</strong> <strong>the</strong>
quest <strong>for</strong> <strong>in</strong>terstellar gra<strong>in</strong>s. Although important <strong>in</strong> <strong>the</strong>ir own right, <strong>the</strong>se studies might be better designed <strong>in</strong> future to preserve material <strong>for</strong> o<strong>the</strong>r studies. It may be ironic that martian meteorite studies could reawaken an <strong>in</strong>terest <strong>in</strong> <strong>the</strong> <strong>in</strong>vestigation of <strong>the</strong> organic material <strong>in</strong> carbonaceous chondrites. Even more so, if <strong>the</strong> connection could be made because of <strong>the</strong> search <strong>for</strong> martian sediments on Earth. Although it is highly speculative, a possible association between martian meteorites and type CI chondrites has been suggested (Brandenburg, 1996). Brandenburg’s <strong>the</strong>sis, that Mars is <strong>the</strong> parent body of <strong>the</strong> CI chondrites, is based on several l<strong>in</strong>es of thought and both isotopic and geochemical arguments. Top of <strong>the</strong> list is <strong>the</strong> similarity <strong>in</strong> <strong>the</strong> ∆ 17 O displacements <strong>for</strong> martian samples and <strong>the</strong> CI chondrites relative to <strong>the</strong> terrestrial fractionation l<strong>in</strong>e. O<strong>the</strong>r favourable comparisons made by Brandenburg concern <strong>the</strong> δD, δ 13 C, δ 15 N and noble gas isotopic compositions of martian samples, <strong>in</strong>clud<strong>in</strong>g m<strong>in</strong>erals with<strong>in</strong> this such as carbonates and various equivalent components <strong>in</strong> carbonaceous chondrites. He also po<strong>in</strong>ts out that chemical analyses of <strong>the</strong> Mars regolith by Vik<strong>in</strong>g call <strong>for</strong> an <strong>in</strong>put of CI material and makes <strong>the</strong> po<strong>in</strong>t that Urey (1968) suggested that CI meteorites might have come from an ext<strong>in</strong>ct planetary seabed, such as <strong>the</strong> Moon. Martian meteorites (Burgess et al., 1989) like CIs (Burgess et al., 1991) also conta<strong>in</strong> both sulphide and sulphate m<strong>in</strong>erals. <strong>The</strong> list of similarities is <strong>for</strong>midable and <strong>the</strong> longer it gets one assumes <strong>the</strong> less chance that serendipity is <strong>in</strong>volved. <strong>The</strong> Brandenburg idea appeared just be<strong>for</strong>e <strong>the</strong> revelations regard<strong>in</strong>g putative martian fossils (McKay et al., 1996) so <strong>the</strong> most obvious comparison, with <strong>the</strong> work of Nagy and co-workers on Orgeuil (Nagy et al., 1972), was not mentioned. <strong>The</strong> most crucial test of a relationship between CI chondrites and martian meteorites is <strong>the</strong> ∆ 17 O value, which Brandenburg suggests as ‘similar’. From prelim<strong>in</strong>ary reappraisal of only three bulk CI samples, to obta<strong>in</strong> results of <strong>the</strong> highest precision, a l<strong>in</strong>k looks tenuous: <strong>the</strong> ∆ 17 O <strong>for</strong> Argal, Ivuna and Alais has been found to be 0.64±0.03 (Franchi et al., 1997b), i.e. outside <strong>the</strong> error limits <strong>for</strong> <strong>the</strong> martian oxygen l<strong>in</strong>e 0.320±0.013 (Franchi et al., 1997a). However, <strong>the</strong> case will not be fully closed until more def<strong>in</strong>itive measurements with separated and identified fractions have been made. Recent dust collection, both above <strong>the</strong> terrestrial atmosphere (Love & Brownlee, 1993) and <strong>in</strong> <strong>the</strong> Greenland and Antarctica ice sheet (Hammer & Maurette, 1996), show that <strong>the</strong> Earth captures <strong>in</strong>terplanetary dust as micrometeorites at a rate of about 50-100 t per day. About 99% of this mass is carried by ‘large’ micrometeorites <strong>in</strong> <strong>the</strong> 50-500 mm size range. This value is about 2000 times higher than <strong>the</strong> most reliable estimate of <strong>the</strong> meteorite flux (about 0.03 t per day) recently estimated by Bland et al. (1996). This amaz<strong>in</strong>g dom<strong>in</strong>ance of micrometeorites already suggests <strong>the</strong>ir possible role <strong>in</strong> deliver<strong>in</strong>g complex organics to <strong>the</strong> early Earth 4.2-3.9 Gyr ago (see Section I.2.1.2.) when <strong>the</strong> micrometeorite flux was enhanced by a factor of 1000 (Anders, 1989). <strong>The</strong> nature of <strong>the</strong> large micrometeorites, <strong>the</strong>ir comparison with meteorites and <strong>the</strong> basic features of <strong>the</strong>ir carbon chemistry have been recently <strong>in</strong>vestigated. <strong>The</strong> t<strong>in</strong>y micrometeorites collected <strong>in</strong> <strong>the</strong> stratosphere, Interplanetary Dust Particles (IDPs), represent less than 1% of <strong>the</strong> micrometeorites and will not be discussed here. M<strong>in</strong>eralogical and chemical studies (Kurat et al., 1994) were first aimed at compar<strong>in</strong>g micrometeorites to meteorites. <strong>The</strong>y af<strong>for</strong>ded two unexpected features: • micrometeorites are related to <strong>the</strong> relatively rare group of <strong>the</strong> most primitive meteorites (carbonaceous chondrites) and predom<strong>in</strong>ently to <strong>the</strong> CM chondrites, which represent only 2% of <strong>the</strong> meteorite falls. So <strong>the</strong> composition of large micrometeorites that constitute <strong>the</strong> most abundant extraterrestrial matter accreted today by <strong>the</strong> Earth is under-represented <strong>in</strong> <strong>the</strong> most abundant meteorites, <strong>the</strong> ord<strong>in</strong>ary chondrites and <strong>the</strong> differentiated meteorites. This reflects major differences <strong>in</strong> <strong>the</strong> parent bodies generat<strong>in</strong>g <strong>the</strong> meteorites and <strong>the</strong> micrometeorites; • although close to CM chondrites, micrometeorites show marked differences: i) <strong>the</strong> martian meteorites/II.3 II.3.6 Carbon Compounds <strong>in</strong> Micrometeorites 103
Page 1 and 2:
SP-1231 Exobiology
Page 3 and 4:
Cover Fossil coccoid bacteria, 1 µ
Page 5 and 6:
4 I.5 Potential Non-Martian Sites <
Page 7 and 8:
6 6.2 Imaging of F
Page 9 and 10:
The Exobio
Page 11 and 12:
pyrolysis, or similar techniques, f
Page 13 and 14:
Ignasi Casanova, Institute
Page 15 and 16:
I AN EXOBIOLOGICAL VIEW OF THE SOLA
Page 17 and 18:
SP-1231 18 surface pressure of CO 2
Page 19 and 20:
SP-1231 20 10 bar befor</st
Page 21 and 22:
SP-1231 22 kilometres in</s
Page 23 and 24:
SP-1231 24 Laboratory Investigation
Page 25 and 26:
I.3 Limits of Life
Page 27 and 28:
temperatures of 95-106ºC. This sug
Page 29 and 30:
(mainly found <str
Page 31 and 32:
cannot exclude fin
Page 33 and 34:
Responses to Cosmic Radiation <stro
Page 35 and 36:
acid to identify the</stron
Page 37 and 38:
Horneck, G. (1993). Responses of Ba
Page 39 and 40:
SP-1231 Fig. I.4.2.2/1. Size distri
Page 41 and 42:
SP-1231 Fig. I.4.2.2/4. Evaporite w
Page 43 and 44:
SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45 and 46:
SP-1231 48 A detailed chemical anal
Page 47 and 48: SP-1231 Fig. I.4.3.1.1/1. Scheme of
Page 49 and 50: SP-1231 Fig. I.4.3.1.2/1. A: Compar
Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
Page 53 and 54: SP-1231 56 The iso
Page 55 and 56: SP-1231 Fig. I.4.3.2.3/1. Cholester
Page 57 and 58: SP-1231 60 References regard to non
Page 59 and 60: SP-1231 62 Klein,
Page 61 and 62: SP-1231 64 ities in</strong
Page 63 and 64: SP-1231 Fig. I.5.1.1/1. A 34×42 km
Page 65 and 66: SP-1231 Fig. I.5.2.1/1. Titan’s a
Page 67 and 68: SP-1231 Fig. I.5.2.2/2. Modelled Ti
Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
Page 73 and 74: SP-1231 I.6.3 What to Searc
Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81 and 82: cover the range 4.
Page 83 and 84: The depletion of c
Page 85 and 86: valley networks (Fig. II.2.5/2) are
Page 87 and 88: plate tectonic evidence has been ob
Page 89 and 90: Table II.2.7.4/1. Potential Mars <s
Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
Page 97: indicated by carbo
Page 101 and 102: Anders, E. (1989). Prebiotic organi
Page 103 and 104: Urey, H.C. (1968). Origin</
Page 105 and 106: SP-1231 Fig. II.4.1.1/1. A Gilbert-
Page 107 and 108: SP-1231 Fig. II.4.1.3/1. Th
Page 109 and 110: SP-1231 114 Sinuou
Page 111 and 112: SP-1231 Fig. II.4.3/1. Ages of seve
Page 113 and 114: SP-1231 118 orbital characteristics
Page 115 and 116: SP-1231 120 II.4.5 The</str
Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
Page 119 and 120: SP-1231 124 Site 2: Apolin<
Page 121 and 122: SP-1231 126 Site 4: Chasma area, ea
Page 123 and 124: SP-1231 128 Sharp, M., Parkes, J.,
Page 125 and 126: SP-1231 Fig. II.5.1/1. The<
Page 127 and 128: SP-1231 132 II.5.3 Isotopic Analysi
Page 129 and 130: SP-1231 134 chemical composition de
Page 131 and 132: SP-1231 Fig. II.5.7.1/1. Th
Page 133 and 134: SP-1231 138 optical microscope. Bot
Page 135 and 136: SP-1231 140 protons of discrete ene
Page 137 and 138: SP-1231 142 atures should be per<st
Page 139 and 140: SP-1231 Fig. II.5.7.7/1. Example of
Page 141 and 142: SP-1231 146 soils. In this approach
Page 143 and 144: SP-1231 148 discrimin</stro
Page 145 and 146: SP-1231 150 determin</stron
Page 147 and 148: SP-1231 Fig. II.5.10.2/2. Enantiome
Page 149 and 150:
SP-1231 154 References achiral colu
Page 151 and 152:
II.6 Team III: The
Page 153 and 154:
in width and heigh
Page 155 and 156:
Searchin</
Page 157 and 158:
minerals or oxidat
Page 159 and 160:
A second group of rocks, however, m
Page 161 and 162:
SCIENTIFIC METHOD AND REQUIREMENTS
Page 163 and 164:
The sample materia
Page 165 and 166:
surprising. To ach
Page 167 and 168:
The main</
Page 169 and 170:
Consideration of a diamond wire-saw
Page 171 and 172:
Schoonen, M.A. & Barnes, H.L. (1991
Page 173 and 174:
SP-1231 180 II.7.2 The</str
Page 175 and 176:
SP-1231 Fig. II.7.4/1. Look
Page 177 and 178:
SP-1231 184 II.7.5 A Possible <stro
Page 179 and 180:
Team IV was asked to examin
show all

Exobiology in the Solar System & The Search for Life on Mars - ESA

Create successful ePaper yourself

Delete template?

Save as template?