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

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SP-1231 34 I.3.7 Survival of <strong>Life</strong><strong>for</strong>ms <strong>in</strong> Space In order to study <strong>the</strong> survival of resistant microbial <strong>for</strong>ms <strong>in</strong> <strong>the</strong> upper atmosphere and free space, microbial samples have been exposed <strong>in</strong> situ aboard balloons, rockets and spacecraft, such as Gem<strong>in</strong>i, Apollo, Spacelab, Long-Duration Exposure Facility (LDEF), Foton and Eureca, and <strong>the</strong>ir responses <strong>in</strong>vestigated after recovery (reviewed <strong>in</strong> Horneck, 1993). A priori, <strong>the</strong> environment <strong>in</strong> space seems to be very hostile to life. This is due to <strong>the</strong> high vacuum, an <strong>in</strong>tense radiation of galactic and solar orig<strong>in</strong>, and extreme temperatures. In <strong>the</strong> endeavour to disentangle <strong>the</strong> network of potential <strong>in</strong>teractions of <strong>the</strong> parameters of space, methods have been applied to separate each parameter and to <strong>in</strong>vestigate its impact on biological <strong>in</strong>tegrity, applied s<strong>in</strong>gly or <strong>in</strong> controlled comb<strong>in</strong>ations. Responses to Vacuum In free <strong>in</strong>terplanetary space, pressures down to 10 -14 Pa prevail. <strong>The</strong> pressure <strong>in</strong>creases <strong>in</strong> <strong>the</strong> vic<strong>in</strong>ity of a body because of outgass<strong>in</strong>g. In low Earth orbit, where most of <strong>the</strong> experiments were done, pressure reaches 10 -4 -10 -6 Pa. Experiments <strong>in</strong> space have demonstrated that certa<strong>in</strong> microorganisms survive exposure <strong>in</strong> space vacuum <strong>for</strong> extended periods of time, provided <strong>the</strong>y are shielded aga<strong>in</strong>st <strong>the</strong> <strong>in</strong>tense solar UV radiation. Most results are available from spores of <strong>the</strong> bacterium Bacillus subtilis. If shielded aga<strong>in</strong>st UV, spores survive at least 6 years <strong>in</strong> space (Horneck et al., 1994). <strong>The</strong> survival time is substantially <strong>in</strong>creased if <strong>the</strong> spores are exposed <strong>in</strong> multiple layers and/or <strong>in</strong> <strong>the</strong> presence of glucose as a protective. In <strong>the</strong> surviv<strong>in</strong>g spores, <strong>the</strong> genetic material is affected, as <strong>in</strong>dicated by an <strong>in</strong>creased mutation rate, delayed germ<strong>in</strong>ation, crossl<strong>in</strong>k<strong>in</strong>g of DNA and prote<strong>in</strong>, DNA strand break<strong>in</strong>g and <strong>the</strong> requirement of cellular repair processes to restore viability. This cellular damages is probably caused by <strong>the</strong> dehydration of <strong>the</strong> spores <strong>in</strong> vacuum. With <strong>in</strong>creas<strong>in</strong>g exposure time, free water is removed first, followed by <strong>the</strong> hydrated water and, f<strong>in</strong>ally, even chemically bound water and o<strong>the</strong>r volatile molecules may be removed. This has severe consequences on <strong>the</strong> stability and functionality of <strong>the</strong> membranes and macromolecules. Responses to <strong>Solar</strong> UV Radiation <strong>The</strong> spectrum of solar electromagnetic radiation spans several orders of magnitude, from short-wavelength X-rays to radio frequencies. At <strong>the</strong> distance of <strong>the</strong> Earth (1 AU), solar irradiance amounts to 1360 Wm -2 (<strong>the</strong> solar constant). Of this radiation, 45% is attributed to IR, 48% to visible and only 7% to <strong>the</strong> UV range. <strong>Solar</strong> UV radiation has been found to be <strong>the</strong> most damag<strong>in</strong>g factor of space as tested with dried preparations of viruses, bacterial and fungal spores (Horneck, 1992, 1993; Horneck et al., 1994). Action spectra of <strong>the</strong> solar photons at 160-320 nm <strong>for</strong> kill<strong>in</strong>g bacteriophage T1 or B. subtilis spores closely correlate with <strong>the</strong> absorption spectrum of DNA, <strong>in</strong>dicat<strong>in</strong>g DNA as <strong>the</strong> critical chromophore <strong>for</strong> lethality. <strong>The</strong> <strong>in</strong>cidence of <strong>the</strong> full spectrum of solar UV light (>170 nm) kills 99% of B. subtilis spores with<strong>in</strong> seconds. To have <strong>the</strong> same effect on Earth, <strong>the</strong> exposure to sunlight takes approximately 1000 times longer. This difference is attributed to <strong>the</strong> ozone layer that protects <strong>the</strong> biosphere from <strong>the</strong> most harmful fraction of solar UV radiation (170 nm), as well as to selected wavelengths. This is due to <strong>the</strong> generation of specific photoproducts <strong>in</strong> <strong>the</strong> DNA of microorganisms exposed to UV light <strong>in</strong> vacuum (Horneck, 1993).
Responses to Cosmic Radiation <strong>The</strong> radiation field of our <strong>Solar</strong> <strong>System</strong> is governed by components of galactic and solar orig<strong>in</strong>. <strong>The</strong> galactic cosmic radiation enter<strong>in</strong>g our <strong>Solar</strong> <strong>System</strong> is composed of protons (85%), electrons, alpha-particles (14%) and heavy ions (1%) of charge Z>2 (HZE particles). <strong>The</strong> solar particle radiation, emitted as <strong>the</strong> solar w<strong>in</strong>d and dur<strong>in</strong>g solar flares, comprises 90-95% protons, 5-10% alpha-particles and a relatively small number of heavier ions. In <strong>the</strong> vic<strong>in</strong>ity of <strong>the</strong> Earth, <strong>in</strong> <strong>the</strong> radiation belts, protons and electrons are trapped by <strong>the</strong> geomagnetic field. Among <strong>the</strong> ionis<strong>in</strong>g components of radiation <strong>in</strong> space, <strong>the</strong> heavy primaries (HZE particles) are <strong>the</strong> most effective species. To understand how cosmic particle radiation <strong>in</strong>teracts with biological systems, methods have been developed to localise precisely <strong>the</strong> trajectory of an HZE particle relative to <strong>the</strong> biological object and to correlate <strong>the</strong> physical data of <strong>the</strong> particle relative to <strong>the</strong> observed biological effects along its path. By use of visual track detectors sandwiched between layers of biological objects <strong>in</strong> a rest<strong>in</strong>g state – a concept known as <strong>the</strong> Biostack (Buecker & Horneck, 1975) – <strong>in</strong> a variety of test systems <strong>in</strong>clud<strong>in</strong>g viruses, bacterial spores, plant seeds or shrimp cysts, <strong>in</strong>juries such as somatic mutations <strong>in</strong> plant seeds, development disturbances and mal<strong>for</strong>mations <strong>in</strong> <strong>in</strong>sect and salt shrimp embryos, or <strong>in</strong>activation <strong>in</strong> bacterial spores were traced back to <strong>the</strong> traversal of a s<strong>in</strong>gle HZE particle (reviewed <strong>in</strong> Horneck, 1992). Such HZE particles of cosmic radiation are conjectured as sett<strong>in</strong>g <strong>the</strong> ultimate limit on <strong>the</strong> survival of spores <strong>in</strong> space because <strong>the</strong>y penetrate even heavy shield<strong>in</strong>g. <strong>The</strong> maximum time <strong>for</strong> a spore to escape a hit by an HZE particle (e.g. iron of LET >100 keV/m) has been estimated to be 10 5 -10 6 years. Responses to Temperature Extremes <strong>The</strong> temperature of a body <strong>in</strong> space – which is determ<strong>in</strong>ed by <strong>the</strong> absorption and emission of energy – depends on its position relative to <strong>the</strong> Sun and its surface type, size and mass. In Earth orbit, <strong>the</strong> energy sources are solar radiation (1360 Wm -2 ), Earth albedo (480 Wm -2 ) and terrestrial radiation (230 Wm -2 ). In Earth orbit, <strong>the</strong> temperature of a body can reach extreme values. Dur<strong>in</strong>g <strong>the</strong> major part of a hypo<strong>the</strong>tical journey through deep space, if shielded from solar <strong>the</strong>rmal radiation, microorganisms are confronted with <strong>the</strong> 4K cold empt<strong>in</strong>ess of space. Under <strong>the</strong>se very cold conditions, <strong>the</strong>rmodynamic and chemical reactions are nearly frozen. <strong>The</strong> photobiological response to solar UV radiation may <strong>the</strong>n be completely different from room-temperature conditions. Only <strong>the</strong> latter response (at room temperature) so far has been tested <strong>in</strong> space. Laboratory experiments under simulated <strong>in</strong>terstellar medium conditions po<strong>in</strong>t to a remarkably less damag<strong>in</strong>g effect of UV radiation at <strong>the</strong>se low temperatures. Treat<strong>in</strong>g B. subtilis spores with three simulated factors simultaneously (UV >110 nm, vacuum and low temperature of 10K), produces an unexpectedly high survival rate, even at very high UV fluxes. In this low temperature regime, <strong>the</strong> <strong>in</strong>activation cross-sections obta<strong>in</strong>ed are up to 2-3 orders of magnitude lower than at room temperature (Weber & Greenberg, 1985). <strong>The</strong> temperature profile of bacterial spore UV-sensitivity shows a maximum at 190K. From <strong>the</strong>se data, it has been estimated that, <strong>in</strong> <strong>the</strong> most general environment <strong>in</strong> space, spores may survive <strong>for</strong> hundreds of years (Weber & Greenberg, 1985). Chances and Limits of Interplanetary Transfer of <strong>Life</strong> <strong>The</strong> recent analyses of <strong>the</strong> martian meteorite ALH 84001, with its putative <strong>in</strong>dications of enclosed fossils (see Chapter II.3), has <strong>in</strong>itiated <strong>in</strong>creased <strong>in</strong>terest <strong>in</strong> <strong>the</strong> question of whe<strong>the</strong>r endolithic microorganisms can survive an <strong>in</strong>terplanetary journey. Although it will be difficult to prove that life can be transported through our <strong>Solar</strong> <strong>System</strong>, <strong>the</strong> chances <strong>for</strong> <strong>the</strong> different steps of <strong>the</strong> process to occur can be estimated. <strong>The</strong>se <strong>in</strong>clude (1) <strong>the</strong> escape process, i.e. <strong>the</strong> removal to space of biological material that has survived be<strong>in</strong>g lifted from <strong>the</strong> surface to high altitudes; (2) <strong>the</strong> <strong>in</strong>terim state <strong>in</strong> space, i.e. <strong>the</strong> survival of <strong>the</strong> biological material over timescales comparable with <strong>the</strong> <strong>in</strong>terplanetary passage (3) <strong>the</strong> entry process, i.e. <strong>the</strong> non-destructive deposition of <strong>the</strong> biological material on ano<strong>the</strong>r planet. Follow<strong>in</strong>g <strong>the</strong> <strong>in</strong>dentification of some limits of life under extreme conditions/I.3 35
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 <
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Page 9 and 10: The Exobio
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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
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Page 23 and 24: SP-1231 24 Laboratory Investigation
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Page 37 and 38: Horneck, G. (1993). Responses of Ba
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The depletion of c
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valley networks (Fig. II.2.5/2) are
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plate tectonic evidence has been ob
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Table II.2.7.4/1. Potential Mars <s
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II.3 The Martian M
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principal atmosphe
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Lines of evidence
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indicated by carbo
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quest for
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Anders, E. (1989). Prebiotic organi
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Urey, H.C. (1968). Origin</
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SP-1231 Fig. II.4.1.1/1. A Gilbert-
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SP-1231 Fig. II.4.1.3/1. Th
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SP-1231 114 Sinuou
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SP-1231 Fig. II.4.3/1. Ages of seve
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SP-1231 118 orbital characteristics
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SP-1231 120 II.4.5 The</str
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SP-1231 122 II.4.6 Rovers and Drill
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SP-1231 124 Site 2: Apolin<
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SP-1231 126 Site 4: Chasma area, ea
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SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 Fig. II.5.1/1. The<
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SP-1231 132 II.5.3 Isotopic Analysi
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SP-1231 134 chemical composition de
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SP-1231 Fig. II.5.7.1/1. Th
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SP-1231 138 optical microscope. Bot
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SP-1231 140 protons of discrete ene
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SP-1231 142 atures should be per<st
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SP-1231 Fig. II.5.7.7/1. Example of
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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 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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