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

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SP-1231 96 II.3.2 <strong>The</strong> Orig<strong>in</strong> of SNC Meteorites Clearly, SNC meteorites are numerically rare, so perhaps a better way to assess <strong>the</strong>ir relative significance is to compare <strong>the</strong> total mass of those specimens observed to fall with <strong>the</strong> total mass of all o<strong>the</strong>r meteorite falls. This must be done with falls, because <strong>the</strong> sample set of those returned by collection trips and found by members of <strong>the</strong> public suffers horribly from bias. For <strong>in</strong>stance, certa<strong>in</strong> types of samples are more resistant to wea<strong>the</strong>r<strong>in</strong>g and so after land<strong>in</strong>g on Earth tend to be more resilient to erosion than o<strong>the</strong>rs. Thus over about a thousand years, <strong>the</strong>y appear to be more common than <strong>the</strong>y really are. Ano<strong>the</strong>r bias<strong>in</strong>g effect is that some samples look more like meteorites than o<strong>the</strong>rs (or perhaps it is more appropriate to say <strong>the</strong>y look less like ord<strong>in</strong>ary terrestrial rocks, enhanc<strong>in</strong>g <strong>the</strong>ir probability of collection)! Consider<strong>in</strong>g only <strong>the</strong> observed falls, it transpires that SNC meteorites constitute, by mass, some 0.25% of all meteorites. S<strong>in</strong>ce <strong>the</strong> total annual flux of extraterrestrial material to <strong>the</strong> Earth is estimated to be 40 000 t (much of it <strong>in</strong> <strong>the</strong> <strong>for</strong>m of micrometeorites, as will be discussed later), <strong>the</strong>n statistically, some 100 t of SNC meteorites arrive at <strong>the</strong> Earth each year. So, while SNC meteorites may be relatively rare with<strong>in</strong> <strong>the</strong> collections, <strong>the</strong>y may be actually quite commonplace. <strong>The</strong> fundamental question is, <strong>the</strong>n, where do <strong>the</strong> SNC meteorites orig<strong>in</strong>ate? How can we know if <strong>the</strong>y come from Mars? Two th<strong>in</strong>gs made <strong>the</strong>m stand out to researchers <strong>in</strong> <strong>the</strong> 1960s and 1970s. Firstly, <strong>the</strong>y were obviously igneous <strong>in</strong> orig<strong>in</strong>, be<strong>in</strong>g <strong>the</strong> crystallisation products of volcanic activity. This, <strong>in</strong> itself, is not unusual. <strong>The</strong>re are o<strong>the</strong>r classes of meteorite with this characteristic, represent<strong>in</strong>g <strong>the</strong> end-products of melt<strong>in</strong>g events that took place on asteroids early <strong>in</strong> <strong>the</strong> history of <strong>the</strong> <strong>Solar</strong> <strong>System</strong> (>4.5 Gyr ago). However, what made SNC meteorites different was that <strong>the</strong> distribution of <strong>the</strong> rare-earth elements was more like that of terrestrial basalts than melted meteorites. In o<strong>the</strong>r words, <strong>the</strong>y had planetary ra<strong>the</strong>r than asteroidal characteristics. <strong>The</strong> second phenomenon was that <strong>the</strong>ir <strong>for</strong>mation ages were substantially younger (i.e. 1.3 Gyr) than <strong>the</strong> age of <strong>the</strong> <strong>Solar</strong> <strong>System</strong> (e.g. Gale et al., 1975; Jagoutz & Wänke, 1986). <strong>The</strong> only reasonable way that this could happen was if <strong>the</strong> samples were <strong>for</strong>med on a geologically active planetary body, ra<strong>the</strong>r than an asteroid. Wasson & We<strong>the</strong>rill (1979) are generally acknowledged as first suggest<strong>in</strong>g <strong>in</strong> pr<strong>in</strong>t that Mars was <strong>the</strong> most likely possibility. But <strong>the</strong>re was a problem: at that time no lunar meteorites had been found and <strong>the</strong> dynamics of transport<strong>in</strong>g samples from Mars (which at its nearest po<strong>in</strong>t to Earth is 77 million km distant) are much more difficult than <strong>for</strong> <strong>the</strong> Moon (0.4 million km). This conundrum was resolved early <strong>in</strong> <strong>the</strong> 1980s when <strong>the</strong> first lunar meteorite was identified. (In fact, this was a most uncontroversial sample; all scientists agreed that it had to be from <strong>the</strong> Moon.) (L<strong>in</strong>dstrom, 1989). At about <strong>the</strong> same time, a recently retrieved SNC meteorite, EET A79001 (from Antarctica; hereafter E79), was analysed and found to conta<strong>in</strong> gases that were <strong>in</strong> most respects identical to those analysed at <strong>the</strong> martian surface by Vik<strong>in</strong>g. In <strong>the</strong> first <strong>in</strong>stance, it was recognised that <strong>the</strong> noble gas isotope ratios of gas trapped <strong>in</strong> what was believed to be shock-produced glass (possibly a product of <strong>the</strong> impact that ejected E79 from its parent) lay on a mix<strong>in</strong>g l<strong>in</strong>e between that of Mars and <strong>the</strong> Earth (Bogard & Johnson, 1983). It looked very much as though E79 conta<strong>in</strong>ed martian gases contam<strong>in</strong>ated by species absorbed from <strong>the</strong> terrestrial atmosphere. This concept was extended and confirmed by a consideration of nitrogen. Nitrogen <strong>in</strong> <strong>the</strong> martian atmosphere has a very dist<strong>in</strong>ctive isotopic composition, highly enriched <strong>in</strong> 15 N (McElroy et al., 1976). 15 N-enriched nitrogen was also found <strong>in</strong> E79 (Becker & Pep<strong>in</strong>, 1984). But noble gases are trace components, and nitrogen is a m<strong>in</strong>or contributor to <strong>the</strong> martian atmosphere. If <strong>the</strong> SNCs came from Mars, <strong>the</strong>n <strong>the</strong> major species of <strong>the</strong> martian atmosphere would have to be <strong>the</strong>re <strong>in</strong> <strong>the</strong> relevant amounts. <strong>The</strong> confirm<strong>in</strong>g evidence was duly found by Carr et al. (1985) and all <strong>the</strong> data (Vik<strong>in</strong>g and meteorite results) were showed to plot on a trend across more than 10 orders of magnitude. It is now accepted that by study<strong>in</strong>g E79 it is possible to ref<strong>in</strong>e <strong>the</strong> measurements made by Vik<strong>in</strong>g and <strong>the</strong>reby provide new constra<strong>in</strong>ts on <strong>the</strong> martian atmosphere. For example, <strong>the</strong> 12 C/ 13 C ratio of <strong>the</strong>
pr<strong>in</strong>cipal atmospheric component, CO 2, now quoted <strong>for</strong> Mars is, <strong>in</strong> fact, <strong>the</strong> meteorite measurement, not from <strong>the</strong> Vik<strong>in</strong>g data. Once one sample from Mars was identified it was possible to confirm that o<strong>the</strong>rs <strong>in</strong> <strong>the</strong> collection and returned from Antarctica were also martian. Because martian gases are not readily apparent <strong>in</strong> all of <strong>the</strong> samples, <strong>the</strong>ir presence or absence could not be taken as diagnostic. Indeed, some of <strong>the</strong> meteorites do not have any residual martian atmosphere. A far more useful and generalised criterion is a sample’s oxygen stable isotopic composition (a measurement requir<strong>in</strong>g only a few milligrammes of sample). Indeed, this is how A84 was confirmed as a martian sample – <strong>in</strong> most o<strong>the</strong>r respects, it is different to <strong>the</strong> o<strong>the</strong>rs of <strong>the</strong> family. Most notably it is extremely old (4.5 Gyr) – as old as <strong>the</strong> planet. Until recently, old age would almost certa<strong>in</strong>ly have consigned it to be<strong>in</strong>g of asteroidal orig<strong>in</strong>. Indeed, it is ironic that A84 languished <strong>in</strong> <strong>the</strong> Antarctic collection at Houston, unrecognised and believed to be diogenite (Mittlefehldt, 1994). As already mentioned, oxygen isotope systematics are <strong>the</strong> vital criterion <strong>for</strong> recognis<strong>in</strong>g meteorite families. Oxygen is 45wt‰ of any silicate-rich meteorite, so any characteristic discovered among <strong>the</strong> oxygen isotopes can be considered as diagnostic. Fortunately, oxygen has three isotopes: 16 O (most abundant), 17 O (1/200th of total) and 18 O (1/500th). <strong>The</strong> exact relative proportions of 17 O and 18 O can be measured very precisely and quoted accord<strong>in</strong>g to <strong>the</strong> general <strong>for</strong>mula <strong>for</strong> δ 17 O and δ 18 O (‰, per mil). As <strong>the</strong> difference between 16 O and 17 O is 1 mass unit and between 16 O and 18 O is 2 mass units, any samples from a homogenised parent body affected only by geological events must plot on a l<strong>in</strong>e of slope 1/2 (actually 0.52). All <strong>the</strong> rocks on Earth, its water etc def<strong>in</strong>e a well-accepted reference l<strong>in</strong>e called <strong>the</strong> terrestrial fractionation l<strong>in</strong>e. Robert Clayton and his colleagues at <strong>the</strong> University of Chicago (e.g. Clayton & Mayeda, 1983) found that dist<strong>in</strong>ct meteorite groups, or groups that were related, had separate l<strong>in</strong>es on an oxygen isotope diagram, because <strong>the</strong> solar nebula was not adequately mixed be<strong>for</strong>e solid silicate bodies separated out. On this basis, Mars might be expected to have a dist<strong>in</strong>guish<strong>in</strong>g l<strong>in</strong>e. Indeed it does, although it is predictable that <strong>for</strong> a terrestrial type-planet, <strong>for</strong>med <strong>in</strong> <strong>the</strong> <strong>in</strong>ner <strong>Solar</strong> <strong>System</strong>, <strong>the</strong> difference between Mars and Earth is small. <strong>The</strong> important parameter <strong>in</strong> def<strong>in</strong><strong>in</strong>g Mars (from martian meteorites) is <strong>the</strong> difference between <strong>the</strong> oxygen isotope composition of SNC meteorites and Earth, designated ∆ 17 O Mars. <strong>The</strong> ∆ 17 O Mars has now been measured to <strong>the</strong> martian meteorites/II.3 Fig. II.3.3/1. Carbonate globules <strong>in</strong> ALH 84001. A th<strong>in</strong>-section view, cover<strong>in</strong>g about 0.5 mm. <strong>The</strong> ‘nanofossil’ structures are found <strong>in</strong> <strong>the</strong> rounded brownish and clear globules. <strong>The</strong> globules are made up of <strong>the</strong> brownish iron carbonate (siderite) and clear magnesium carbonate (magnesite). <strong>The</strong> dark rims conta<strong>in</strong> iron oxide and sulphide materials. (A. Treiman/Lunar & Planetary Institute) II.3.3 <strong>The</strong> Martian Meteorites 97
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
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pyrolysis, or similar techniques, f
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Ignasi Casanova, Institute
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I AN EXOBIOLOGICAL VIEW OF THE SOLA
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SP-1231 18 surface pressure of CO 2
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SP-1231 20 10 bar befor</st
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SP-1231 22 kilometres in</s
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SP-1231 24 Laboratory Investigation
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I.3 Limits of Life
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temperatures of 95-106ºC. This sug
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(mainly found <str
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cannot exclude fin
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Responses to Cosmic Radiation <stro
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acid to identify the</stron
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Horneck, G. (1993). Responses of Ba
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SP-1231 Fig. I.4.2.2/1. Size distri
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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
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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
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Page 57 and 58: SP-1231 60 References regard to non
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Page 61 and 62: SP-1231 64 ities in</strong
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Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
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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.
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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
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Page 141 and 142: SP-1231 146 soils. In this approach
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SP-1231 148 discrimin</stro
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SP-1231 150 determin</stron
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SP-1231 Fig. II.5.10.2/2. Enantiome
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SP-1231 154 References achiral colu
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II.6 Team III: The
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in width and heigh
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Searchin</
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minerals or oxidat
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A second group of rocks, however, m
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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