1

SOLAR ACTIVITY AND LIFE (*)

M. Messerotti 1 and J. Chela-Flores 2, 3 (1) INAF-Trieste Astronomical Observatory, Loc. Basovizza n. 302, 34012 Trieste, Italy (2 The Abdus Salam ICTP,Strada Costiera 11, 34014 Trieste, Italy, and (3) Instituto de Estudios Avanzados, IDEA, Caracas 1015A, Venezuela

Abstract. Recent claims advocate a downward revision of the solar oxygen abundance (Socas- Navarro and Norton, 2007). This is a reflection of what may be called a ‘solar crisis’, whereby we mean that previous consensus in our understanding of our nearest star was unfounded. The implications for solar physics and chemistry are obvious and much research in the near future will give us a much clearer understanding of the Sun. We wish to point out that our recent efforts have been motivated by the implication that this uncertainty in the physicochemical aspects of the Sun have for the origin of life on Earth (Messerotti, 2004). Within the COST 724 Action,we have discussed various general aspects on the new science of astrobiology (Messerotti and Chela-Flores, 2007a, b; Chela-Flores and Messerotti, 2007, and Chela-Flores et al., 2007). The present robust programs of various space agencies reinforce our hope for a better understanding of the bases of astrobiology. Currently, there are four significant missions: SOHO, Trace, Stereo, and Hinode. With a more realistic model of the Sun, more reliable discussions of all the factors influencing the origin of life on Earth will be possible.

1. Introduction

During the early stages of the study of the origin of life 1-2 not enough attention was paid to the question of the correlation of chemical evolution on Earth and the all-important evolution of the still- to-be understood early Sun 3. Today, due to the advent of a significant fleet of space missions and the possibility of performing experiments in the International Space Station (ISS), a meaningful study of the factors that may have led to an early onset of life on Earth begins to be possible. We can understand general trends of the influence of space climate and weather on the evolution and distribution of life. As suggested in the previous section solar climate during the first Gyr of the Earth was radically different. The earliest relevant factor was excessive solar-flare energetic particle emission, a phenomenon that has been recorded in meteorites. These extraterrestrial samples provide information on events that took place during this early period after the collapse of the solar nebula disk. Gas-rich meteorites have yielded evidence for a more active Sun. A considerable number of young stars with remnants of accretion disks show energetic winds that emerge from the stars themselves. Similar ejections are still currently observed from our Sun. For this reason it is believed that some of the early Solar System material must keep the record of such emissions 4. Information on the energetic emission of the Sun during this period can be inferred from data on X ray and UV emission (larger than 10 eV) from pre-main-sequence stars 5. We may conclude that during pre-main- sequence period, solar climate and weather presented an insurmountable barrier for the origin of life anywhere in the solar system. In the Hadean, conditions may still have been somewhat favorable, especially with the broad set of UV defense mechanisms that are conceivable. The high UV flux of the early Sun would, in principle, cause destruction of prebiotic organic compounds due to the presence of an anoxic atmosphere without the present-day ozone layer 6-7. Some possible UV defense mechanisms have been proposed in the past, such as atmospheric absorbers and prebiotic organic compounds, Oceanic UV defense mechanisms increase the suite of possible planetary habitable zones. Nature may have provided UV defense mechanisms in the scenarios for an origin of life in hydrothermal vents 8, benthic regions 9 and in deep subsurface environments 10. 11 Most attempts to deal with this problem have involved atmospheric absorbers such as SO2 , and organic hazes, where it has been shown that UV absorption by steady-state amounts of high-altitude organic solids produced from methane photolysis may have shielded ammonia sufficiently to maintain surface temperatures above freezing12. Even in the absence of atmospheric shielding it is possible to 2

conceive that sufficient UV absorbers were present in the ocean to allow for the accumulation of organic material 13, such as organic polymers from electric discharges and HCN (hydrogen cyanide) polymerizations, solubilized elemental sulfur, and inorganics such as Cl, Br, Mg2+, and SH, or Fe2+. There would have been a complete UV defense mechanism if there was a frozen ocean 14, since these authors have shown that bolide impacts between about 3.6 and 4.0 Gyr BP could have episodically melted an ice-covered early ocean. Thaw-freeze cycles associated with bolide impacts could have been important for the initiation of abiotic reactions that gave rise to the first living organisms. Oil slicks or large amounts of organic foams provide other examples of possible UV defense mechanism15. In addition, both biological and physical aspects have motivated other UV defense mechanisms. Cyanobacteria are represented by some of the oldest fossils from the Archean, although the date of such fossils remains a controversial subject 16-17. There seems to be little evolutionary change in these prokaryotic microorganisms. But we are certain that the atmosphere of the Archean Earth was anoxygenic. We expect that the early microbes must have used various means of avoidance of radiation damage. Some of these are attenuation by the water column of their aquatic habitats by the presence of some UVR absorbing substance. It is known that water itself does not protect life. Indeed, UVR is known to penetrate the water column up to at least 50 meters. But if the water contains iron, or nitrogenous salts, UVR is efficiently screened (cf., ref. 18 for references to the literature). On the other hand, UV absorbing pigments may provide an alternative UV screening strategy. One well-known example is scytonemin 19. A physically motivated UV defense mechanism has been suggested in the past 20.

THE RELEVANCE OF SPACE WEATHER IN ASTROBIOLOGY

The incidence of non-ionizing UVR on the early surface of Earth and Mars to a large extent can be inferred from observations. Ionizing radiation, mainly due to nuclear and atomic reactions is relevant: X-rays are emitted spanning the whole spectrum of hard X-rays to soft X-rays (0.01-10 nm); gamma rays are present too. The primary components affecting space climate are: galactic cosmic rays and solar cosmic radiation. To these we should add events that contribute to solar weather, such as solar particle radiation that consisted of the low-energy solar-wind particles, as well as more energetic solar burst events consisting of solar particles that arise from magnetically disturbed regions of the Sun. These events vary in frequency according to the eleven-year cycle. However, scenarios for an early onset of life that have been proposed in the past have to deal with space weather that was radically different in the early Sun. Knowledge of the prehistory of solar particle radiation can be approached with a combined effort from observations of present-day emissions, together with studies of energetic solar particles recorded in extraterrestrial materials, notably the Moon material that became available with the Apollo missions, as well as with the study of meteorites. Life emerged on Earth, during the Archean (3.8 - 2.5 billion years before the present, Gyr BP). We should first of all appreciate the magnitude of the ionizing radiation that may have been present at that time. According to some theoretical arguments the origin of life may be traced back to the most remote times of this eon (3.8 Gyr BP). Indeed, isotopic and geologic evidence suggest that photosynthesis may have been already viable by analysis of the biogeochemical parameter delta 13 C 21. Besides in the Archean the atmosphere was to a large extent anoxic. As a result the abundance of ozone would not have acted as a UV defense mechanism for the potential emergence of life. UVB (280–315 nm) radiation as well as UVC (190–280 nm) radiation could have penetrated to the Earth's surface with their associated biological consequences 22-23. If the distribution of life in the solar system took place by transfer of microorganisms between planets, or satellites, knowledge of solar weather becomes fundamental during the early stages of its evolution, to have some constraints on the possible transfer of microorganisms, as investigated extensively by Horneck and co-workers 24. The most radiation resistant organism known at present exhibits a remarkable capacity to resist the lethal effects of ionizing radiation. The specific microorganism is a non-spore forming extremophile found in a small family known as the Deinococcaceae. In fact, Deinococcus radiodurans is a Gram-positive, red-pigmented, non-motile bacterium. It is resistant to ionizing and UV radiation. Various groups have studied these microorganisms 25-27. Members of this bacteria taxon can grow under large doses of radiation [up to 50 grays (Gy) per hour]. They are also known to recover from acute doses of gamma-radiation greater than 10,000 Gy without loss of viability. Out of the several closely related species only the radio-resistance of D. radiodurans appears to be the result of an evolutionary process that selected for organisms that could tolerate massive DNA damage. Such 3

microorganisms demonstrate that life could have survived at earlier times when the Earth surface was more exposed to solar radiation. They are also examples of living organisms that could survive in the extreme environments that the magnetosphere of Jupiter provides to the surface of the icy Galilean satellites. This aspect of the microbiology of the Deinococcaceae is significant, since the Jovian system does provide on its icy moons environments where life could have emerged 28. We shall return to this topic in the Discussion.

THE INTERSTELLAR ENVIRONMENT AND THE ORIGIN OF LIFE

Solar systems originate out of interstellar dust, namely dust constituted mainly out of the fundamental elements of life such as C, N, O, S, P and a few others (Ehrenfreund and Charnley, 2000). Just before a star explodes into its supernova stage, all the elements that have originated in its interior out of thermonuclear reactions are expelled, thus contributing to the interstellar dust. Recent work has some implications on the question of Space Weather. Two sets of experiments have established that UV radiation plays a significant role in the synthesis of some of the precursors of the biomolecules, especially the amino acids 29-30. The expanding dust and gas form typical 'planetary nebulae at a later stage of stellar evolution. Stars whose mass is similar to that of the Sun remain at the red giant stage for a few hundred million years. The star itself collapses under its own gravity compressing its matter to a degenerate state, in which the laws of microscopic physics eventually stabilize the collapse. This is the stage of stellar evolution called a 'white dwarf '. Stellar evolution of stars more massive than the Sun is far more interesting: after the massive star has burnt out its nuclear fuel a catastrophic explosion follows in which an enormous amount of energy and matter is released. These 'supernovae' explosions are the source of enrichment of the chemical composition of the interstellar medium. This chemical phenomenon, in turn, provides new raw material for subsequent generations of star formation, which leads to the production of planets. Our solar system may have been triggered over 5 Gyr BP by the shock wave of a supernova explosion. Indeed, there is some evidence for the presence of silicon carbide (carborundum, SiC) grains in the Murchison meteorite, where isotopic ratios demonstrate that they are matter from a type II supernova 31. Around 4.6 Gyr BP on the nascent planet, the organic compounds may have arisen in the following manner: . At the end of accretion organic compounds would have been incorporated or delivered by small bodies, both comets and meteorites . Planetary processes, such as those that may have occurred close to hydrothermal vents, may have synthesized organic compounds.

THE ROLE OF COMETS IN THE FORMATION OF THE BIOSPHERE

Comets develop gaseous envelopes when they are in the close vicinity of the Sun. In fact, as the comet approaches the Sun, the higher temperature of interplanetary space induces the cometary ice to begin sublimating. Gas leaves the comet carrying some of the dust particles. On the other hand, the Earth’s biosphere is that part of this planet where life can survive; it extends from a few kilometres into the atmosphere to the deep-sea vents of the ocean, as well as into the crust of the Earth itself. It is generally agreed that in the evolution of the Solar System, comets deliver part if not all of the volatiles to the early biosphere. The Delsemme model based on the assumption of the cometary origin of the biosphere argues that an intense bombardment of comets has brought to the Earth most of the volatile gases present in our atmosphere and most of the carbon extant in the carbonate sediments, as well as in the organic biomolecules 32. With the measuring ability that was available throughout the 20th century molecules have been identified by their spectra in a wide range of wavelengths, and even by in situ mass spectrometry during spacecraft flybys of comet P/Halley. These measurements provided composition of parent volatiles and dust, properties of the nuclei and physical parameters of the coma. Besides the Halley comet already mentioned, two other comets also likely to have originated in the Oort cloud (as the Halley comet) were particularly useful objects for remote sensing measurements: . Comet Hale-Bopp was discovered in 1995, 21 months before its approach to pre-perihelion, which allowed for careful preparation. Its closest approach to Earth was at 1.3 astronomical units (AU). Its gas and dust output were much greater than those for Comet P/Halley. 4

. Comet Hyakutake was discovered in 1996. Its closest approach to Earth was 0.1 AU the same year33. These comets led to the detection of HCN, methane, ethane, carbon monoxide, water, as well as a variety of biogenic compounds.

THE ORIGIN OF THE ENVIRONMENT FAVORABLE FOR LIFE

The earliest geologic period (the lower Archean) may be considered virtually as a 'heavy bombardment period'. During that time various small bodies, including comets, collided frequently with the early planet carrying precursors of the biomolecules that eventually started off the evolutionary process on Earth and its oceans. The mechanism of cometary bombardment appears not to be restricted to our solar system. In addition, comets may be the source of other volatiles in the biosphere, as well as the biochemical elements that were precursors of the biomolecules. Collisions of comets, therefore, are thought to have played a significant role in the formation of the hydrosphere and atmosphere of habitable planets, such as the Earth. The source of comets is the Oort cloud and Kuiper belt. These two components of the outer solar system seem to be common for other solar systems. Noble gases are released from the interior of the planet at ocean-spreading centres (i.e., the mid-oceanic ridges) when new crust is formed, as well as from volcanoes and hot springs. Examples are argon (Ar), xenon (Xe) and krypton (Kr), which are inert and accumulate in the atmosphere. On the other hand, noble gases are present in the Earth’s atmosphere with mixing ratios ranging from almost 1 percent in the case of argon to about 1 part per billion (ppb) for xenon. The heavy noble gases are primitive, since they were captured from the solar nebula when the Earth formed 4.5 Gyr BP. The high value of the ratio Xe / Kr in meteorites argue in favour of bombardment of the early Earth by comets, rather than the meteorites themselves. Besides, the relative abundance of the isotopes in atmospheric xenon is different from those in meteorites. For these reasons comets are probably better candidates for supplying the heavy noble gases than meteorites 34-35. Additional indirect evidence for cometary contributions to the volatiles of the Earth biosphere come from observations of the noble gas content in Jupiter’s atmosphere by the Galileo mission. Comets that contributed to Jupiter’s mass were formed at temperatures lower than those predicted by present models of giant- planet Additional indirect evidence for cometary contributions to the volatiles of the Earth biosphere come from observations of the noble gas content in Jupiter’s atmosphere by the Galileo mission. Comets that contributed to Jupiter’s mass were formed at temperatures lower than those predicted by present models of giant-planet formation 36. This conclusion is based on the observed abundance of Ar, Kr and Xe. The noble gas abundances in Jupiter are significantly different from those in the Sun (and the solar nebula). The enrichment in these noble gases is the result of comets contributing a significant fraction to Jupiter’s inventory of elements heavier than hydrogen and helium. In order for comets to be able to trap sufficient noble gases to produce the observed abundances in Jupiter, they had to have formed at temperatures below 30 K. In big bang cosmology, after the ‘moment of decoupling of matter and radiation’, hydrogen atoms were formed and helium atoms arose from the combination of deuterium with itself. However, fast cosmic expansion did not allow a thermonuclear steady state being formed. This generated a fraction of deuterium. It is estimated that this cosmic ratio of D / H had an upper bound of some 30 parts per million (ppm). D cannot be created de novo. So the variable presence of D / H is a marker for understanding various aspects of the evolution of solar systems. Since D can react easily inside stars, it is not surprising that its abundance in interstellar matter be smaller than the original cosmic abundance. Yet, in Jupiter, for instance, the value is higher than in interstellar matter, reflecting its abundance in the protoplanetary nebula about 5 Gyr BP. (The Jupiter abundance is of the order of 20 ppm, closer to the expected cosmic abundance.) In the Earth's seawater this ratio is about 8 times the value of the solar nebula. The D / H ratio is known in three comets: Halley, Hyakutake and Hale- Bopp. This work on the D / H ratio suggests that cometary impacts have contributed significantly to the water in the Earth's oceans. However, because the D / H ratio observed so far in comets is twice the value for Earth’s oceans, it may be argued that comets cannot be the only source of ocean water.

SOLAR ACTIVITY AND THE CHEMISTRY OF THE ORIGIN OF LIFE

Meteorites are a possibility for delivering the precursors of the amino acids and nucleotides. This possibility has been amply demonstrated by the organic chemistry analysis of the Murchison meteorite37, as well as in other meteorites 38. There is a reasonable explanation for the source of the 5

biomolecule precursors that were found in Murchison and other meteorites. Models of interstellar grain mantles 39 support the view that organic compounds were synthesized in interstellar space, which in turn were to be delivered on planetary surfaces during the late accretion period. Experiments have shown that Space Weather has a significant role to play in this process, since UV radiation will drive the synthesis of photochemical processes 29. The oxygenation of the Earth atmosphere as well as the chemical and biological evolution of life was due to a large extent to solar UV radiation 41. Several processes were at play with intensities was proportional to the flux emitted by the Sun. We emphasize that the Standard Solar Model predicts that the solar flux of the young Sun was lower than the present one, whereas high-precision solar evolutionary models 42 provide a value significantly higher than the present one. The 'dim Sun' hypothesis should be ascertained by observation and experiment. This hypothesis is crucial since it plays a fundamental role in identifying (via reverse modeling) the evolution of the atmosphere and hydrosphere. The relevant factors to clarify are, firstly, those that may have catalyzed chemical evolution of life. Secondly, other relevant factors are those that had an opposite effect, namely, factors that may have biased biological evolution. Furthermore, observational evidence on solar-like stars at different evolutionary stages indicate that during the whole time span relevant to the appearance of life on Earth (4.5 to 3.5 Gyr BP), the early Sun might have experienced a highly active phase in its particle and radiation environment 43. In any case the focus of experimental work has been on the synthesis of the main three organic compounds, either by incorporation at the end of accretion, or by delivery of the precursors of the biomolecules that may have evolved on early planets that support life. The Stanley Miller experiment assumed that the prebiotic synthesis of biochemical was due to a process of chemical synthesis that took place in conditions that resembled the early Earth 44. Self- assembly into living cells included the following three classes of molecules: polymeric nucleic acids (information coding), polypeptides for the formation of proteins, and complex lipids for the formation of semi-permeable membranes. Experimentally the Miller assumptions are that chemical evolution occurred in an atmosphere of a mixture of methane, ammonia, hydrogen and water vapor. Solar energy was imitated by an electric discharge. It is well known that this scheme produces amino acids. Subsequent work along these lines led to the synthesis of nucleic acid polymers. It is possible that the primitive atmosphere did not have much free hydrogen or even hydrogen compounds, as assumed in the Miller experiment. Since the late 1970s it became clear that the early atmosphere was composed of carbon dioxide and nitrogen, rather than the reducing atmosphere assumed in the Miller experiment 45. But in a weakly reducing atmosphere, where little free hydrogen was present UV photons may interact with carbon dioxide, carbon monoxide and nitrogen with only a small amount of hydrogen. This process will produce hydrogen cyanide, HCN, which has been shown to lead to amino acids in prebiotic experiments imitating the primitive oceans 46. In fact, hydrogen cyanide polymers are heterogeneous solids varying in color from yellow to orange, from red to black may be among the organic macromolecules most readily formed within the solar system, for instance the black crust of comet Halley might consist largely of such polymers 47. The main sources of energy available for the processes that led to the first living cell are not only solar radiation, but also some alternatives to the Miller scenario were possible such as hydrothermal vents 48. Another possible source of energy was volcanic lightning 49. Also, energy arising from collisions of meteorites and comets with the surface of the early Earth is a possible source of energy to trigger the synthesis of the early biomolecules that would eventually constitute the first living cell 50. There is general agreement that oxygen was absent from the early atmosphere. After the appearance of the first cells, filamentous cyanobacteria living in the Archean in colonial mats (stromatolites) began to oxygenate the atmosphere. But solar UV radiation can break water molecules leading to oxygen and ozone, which will eventually protect the early stages of evolution from the high-energy UV radiation. Keeping in mind the relevance of amphiphilic lipids for the formation of semipermeable membranes, the chemical evolution that began in a primordial pond must take into account the formation of lipids that would eventually encapsulate the complex organic molecules both amino acids that would form proteins, as well as nucleotides that would form nucleic acids (RNA and DNA). Amongst the lipids phospholipids have been shown to form liposomes consisting of a single molecular bilayer surrounding an aqueous core. Such models of the primitive cell have remarkable properties such as the encapsulation of biomolecules 51. This approach illustrates the possible pathway to the first cell. The self-assembly of all the molecules would lead to a primitive protocellular structure. 6

RNA synthesis in a Miller type of experiment remains as a challenge. Yet some significant RNA analogs have been discussed with promising results 52. In spite of these difficulties and preliminary successes, the first living system of the early Earth has been conjectured to be RNA. Walter Gilbert referred to this possibility as an RNA world 53. The main motivation for supporting the RNA world is that not only has RNA along with DNA, the possibility of codifying genetic information, but some RNAs have catalytic properties 54. This property has been confirmed even in modern organisms by the fact that in ribosomes protein synthesis is catalyzed by RNA 55. It still remains as a challenge to get deeper insights into the transition from chemical to biological evolution. The role of Space Weather remains critical for the preservation of life from its earliest microbial stages. Two factors are of paramount importance, the stability of both Sun and the Earth magnetic field. By the analysis of the late stages of the evolution of the Sun, Space Weather is likely to play an increasingly more relevant role in understanding the preservation and eventual inhibition of life on Earth. Similar conditions are expected to occur in exoplanetary systems between the star and its orbiting terrestrial-like planets. Hence the application of the above analysis to exoplanetary environments should hold upon eventual modifications that would be forced upon us by the specific stellar and exoplanetary characteristics.

DEEPER INSIGHTS INTO THE ORIGIN OF LIFE FROM SPACE WEATHER

We have seen above that there are many outstanding questions in the frontier of astrobiology and astronomy, in spite of considerable progress since Oparin took his first steps 1. Fundamental factors such as space climate and space weather should set additional constrains on possible theories for the origin of life. They should also make further additions to our understanding of the early evolution of life. For instance, the Sun changed considerably in time its temperature and luminosity, key factors for astrobiology that require better understanding, so that we would be in a position to predict with a certain degree of confidence what were the conditions like during the first Gyr of the evolution of the Solar System. The violent eruptions of solar flares during its earliest stage (T-Tauri), soon gave way to a very broad range of physical conditions that have to be clarified by further research. There are several reasons to raise the question: Why do we need improved understanding, and predictions of solar activity? One reason arises from theoretical modeling of the earliest organisms. Predictions range broadly in their claims. Improved understanding of solar activity in the first billion years of the Earth would provide essential clues. Additional aspects of astrobiology, such as the question of the distribution of life in the Solar System, also depend on further research outside the frontier of astrobiology, namely by acquiring improved understanding of solar activity, such as the preliminary information that has been possible to retrieve from the ISS. Space climate and space weather are also fundamental for understanding the early evolution of life. The DNA repair mechanisms that extremophilic organisms evolved in response to a continuously changing space environment. The Sun changed considerably in time its temperature and luminosity, key factors for astrobiology that require better understanding, so that we would be in a position to predict with a certain degree of confidence what were the conditions like during the first Gyr of the evolution of the Solar System. The violent eruptions of solar flares during its earliest stage (T-Tauri), soon gave way to a very broad range of physical conditions that have to be clarified by further research. There are several reasons to raise the question: Why do we need improved understanding, and predictions of solar activity? One reason arises from theoretical modeling of the earliest organisms. Predictions range broadly in their claims. Improved understanding of solar activity in the first billion years of the Earth would provide essential clues. Additional aspects of astrobiology, such as the question of the distribution of life in the Solar System, also depend on further research outside the frontier of astrobiology, namely by acquiring improved understanding of solar activity, such as the preliminary information that has been possible to retrieve from the ISS. Much progress has been achieved since the 1990s missions for studying the Sun. Firstly, remarkable progress has been possible with the Solar and Heliospheric Observatory (SOHO), a joint NASA-ESA spacecraft. SOHO is concerned with the physical processes that form and heat the Sun's corona, maintain it and give rise to the expanding solar wind. A second mission launched in the last decade is Ulysses with measurements of the Sun from a polar orbit. It is also dedicated to interplanetary research especially with interplanetary-physics investigations, including close (1992) and distant (2004) Jupiter encounters. More recently, the Transition Region and Coronal Explorer (TRACE) is giving us information on the three-dimensional magnetic structures that emerge through 7

the photosphere, defining both the geometry and dynamics of the upper solar atmosphere. In this respect, the Solar Terrestrial Relations Observatory (STEREO) to be launched in February 2006 will study the nature of coronal mass ejections (CME), which in spite their significant effects on the Earth, their origin, evolution or extent in interplanetary space remains as a challenge. We need improved understanding so as to be able to make predictions on this contemporary phenomenon. Such knowledge will allow us to extrapolate into the past and investigate which constraints CME poses on the origin and evolution of life, and its possible distribution in the solar system by spreading through interplanetary space. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is also extending the information we have on the basic physics of particle acceleration and large energy release in solar flares. Solar-B with a coordinated set of optical, EUV and X-ray instruments, will investigate the interaction between the Sun's magnetic field and its corona, to yield an improved understanding of the driving force behind space weather. Together with NASA's Solar Dynamics Observatory (SDO). in the foreseeable future we will have a much better insight into space weather and its influence on Earth and near-Earth space by studying the solar atmosphere in many wavelengths simultaneously. But, with so much present and future work, we would like to add a small contribution on two topics that affect especially the interface between the solar sciences and astrobiology. It concerns the possible space weather conditions that may be favorable for the origin and evolution of life. Besides the early Earth, the most likely scenarios for early life are Mars and the Jovian moon Europa. Ulysses has demonstrated two relevant aspects in this context, namely knowledge of the heliosphere vulnerability to cosmic rays, and to the interaction of interstellar gas. A first reason for preserving Ulysses type of missions is related with the 30 year-old problem of the interaction of the solar system with a stream of neutral helium atoms. This cloud is flowing into the solar system, coming from the direction of the constellation Sagittarius. Because the atoms in the stream are uncharged, the heliosphere does not interact with them. The size and leakiness of the heliosphere are important topics. The Ulysses spacecraft GAS instrument were able to sample directly the flow. Currently, SOHO is monitoring the helium stream. Besides, the NASA's Extreme Ultraviolet Explorer (EUVE) and Advanced Composition Explorer (ACE) are collaborating in this endeavor. The stream's temperature, density and velocity are relevant parameters. All the data supports the view that the solar system is colliding with a vast interstellar cloud. The cloud's abundant hydrogen does not easily penetrate the heliosphere because hydrogen atoms in the cloud are ionized by interstellar ultraviolet radiation. Like cosmic rays the hydrogen atoms are charged and, thus, are prevented to enter the interior of the solar system. The numbers retrieved by Ulysses are important to understand the size and leakiness of the heliosphere. Once again, persevering with the solar missions like Ulysses (or Ulysses itself) should be useful, especially in the future when we should address an important question: whether in the past 2 - 3 Gyrs similar events may have produced space climate and weather harmful, or beneficial for the evolution of life in the solar system. There is another reason for persevering with Ulysses-type of missions. The focus of space weather from the point of view of astrobiology gives us a hint. Jupiter's moon Io is emitting volcanic particles at passing spacecraft. The dominant source of the jovian dust streams is Io's volcanoes 56. In September 2004 Io emitted dust particles whose impact rate was recorded by the Cosmic Dust Analyzer on board of Ulysses. The discovery of this phenomenon dates back to 1992 when, a stream of volcano dust hit Ulysses as it approached within 1 AU from Jupiter 57. Cassini's dust detector is more capable than the instrumentation on Ulysses when faced with a similar event 58. In addition to mass, speed, charge and trajectory, Cassini measured elemental composition finding sulfur, silicon, sodium and potassium, whose origin is volcanic. This raises a question that will deserve further attention in the future: There is much spectroscopic evidence for the presence of non-ice substances on the surface of Io's neighboring moon Europa 59. In particular, Galileo Near-Infrared Mapping Spectrometer (NIMS) evidence for the presence of sulfur compounds has been discussed in detail 60-61. Prior to the discovery of the dust stream, it had been suggested that the sulfur contamination of the icy surface of Europa was due to the implantation of S from the Jovian magnetosphere 62. In fact, volcanic material from Io's more than 100 volcanoes escapes and disperses through the jovian system painting the other satellite surfaces. However, since the sulfur on the Europan icy surface is patchy and not uniform, and based on combined spectral reflectance data from the Solid State Imaging (SSI) experiment, the NIMS and the Ultraviolet Spectrometer (UVS), it has been argued that that the non-water ice materials are endogenous in three diverse, but significant terrains 63. Effusive cryovolcanism is clearly one possible endogenous source of the non-water-ice constituents of the surface materials 64.

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2. Constraints of the ancient Sun on the origin of life on Earth

The rationalization of the lunar cratering record provides some guidance for estimating the possibility of life first arising on Earth. Distinct temporal possibilities for the earliest possible time for the first appearance of life are possible with additional inputs from closely related scientific areas. The lunar record may be supplemented with information retrieved from firstly, biogeochemistry (namely with data related to the fractionation of the stable isotopes of the biogenic elements). Secondly, associated with the earliest fossils of stromatolites our current understanding of micropaleontology leads to further possible constraints on the first appearance of life on Earth. However, various theories of the evolution of the early Sun will further constrain the origin of the earliest life on Earth.

DATING THE ORIGIN OF LIFE FROM MOON RECORDS, BIOMARKERS AND FOSSILS

Although the processes taking place during this period are not represented in the geological record, the current scenario of planetary origin gives us a means of inferring the activity that may have frustrated or encouraged emergent life. During the first 100 million years the flux of impactors would have set up the conditions for the separation of iron and silicate, giving rise to a metallic core. During this formation of the planetary embryo a major impact with another planet-size body would have given rise to the expulsion of a large amount of matter from the embryonic Earth and given rise to the Moon (Canup and Asphaug, 2001). The satellite cooled quickly, but did not form an atmosphere, possibly due to the smaller cross section than the Earth. Another significant effect of the Moon- forming impact was to blow away the original atmosphere that the embryonic Earth had captured from the solar nebula (Kasting and Catling, 2003). The planet was much more dynamic geologically and most of the records of large impacts were deleted, but the same geological activity was most likely responsible for partial out gassing of a secondary atmosphere, the exact nature of which can be inferred from the isotopic composition of the noble gases: It has been shown that comets are capable by themselves of providing noble gases in the correct proportions provided that the laboratory experiments duplicate the conditions for cometary formation (Owen and Bar-Nun, 1995). Besides the temperatures had descended to about 100 degrees Centigrade or below by about 4.4 Gyr BP (Schwartz and Chang, 2002). This scenario for planetary origin allows the possibility of an early origin and evolution of life on Earth. However, it should be remembered that the lunar record demonstrates that some difficulties may arise in this scenario since the Imbrium basin on the Moon was formed by a large impact as late as 3.8 Gyr BP. This implies the persistence of catastrophic impacts for life on Earth, since our planet has a larger effective cross section than our satellite (Sleep et al., 1989). The photosynthesis of prokaryotes includes the stromatolitic-forming cyanobacteria, formerly called blue-green algae. In this process a specific enzyme that leads in several steps to the synthesis of glucose captures carbon dioxide. But the carbon dioxide in the environment and nutrients contain the two stable isotopes of carbon 12C and 13C. The process of photosynthesis favours 12C over 13C. Geologic process partitions the stable isotopes in opposite ways; for instance limestone is depleted in 12C and enriched in 13C. The fossil records of organic matter that have been enriched in 12C can be traced back in sedimentary rocks right back to some of the earliest samples such as the 3,800 Myr-old metamorphosed sedimentary rocks from Isua, West Greenland. These geochemical analyses of the ancient rocks militate in favour of the presence of bacterial ecosystems in the period that we are discussing in this section, namely 3.8-3.9 Gyr BP (Schidlowski, 1988; Schidlowski et al., 1983). The question of the metamorphism to which the Isua samples have been subjected has raised some controversy in the past (Hayes et al., 1983). Stromatolites consist of laminated columns and domes, essentially layered rocks. Prokaryotic cells called cyanobacteria form them. In addition, they are users of chlorophyll-a to capture the light energy that will drive the photosynthetic process. These microorganisms are mat-building communities. At present they are ubiquitous, even in the Dry Valley lakes in Antarctica mat-building communities of cyanobacteria have been well documented (Parker et al, 1982). Right back into ancient times such mats covered some undermat formation of green sulphur and purple bacteria. Such underlying microorganisms are (and were) anaerobes that can actually use the light that impinges on the mat above them by using bacteriochlorophylls that absorb wavelengths of light that pass through the mat above them (Schopf, 1999). Not only has the cyanobacterium spread worldwide, but it has also extraordinary temporal characteristics. Stromatolites have persevered practically without changes 9

for over 3 billion years. The exact date for the earliest stromatolitic fossils is at present under discussion (Brasier, et al, 2002; Schopf et al., 2002). They have been dated at around 3.5 Gyr BP (Schopf, 1993). Hence, the origin of life if the fossils are accepted, must be in the time interval discussed in this section, or even earlier considering the complexity of a cyanobacterium itself.

ISOTOPIC FRACTIONATION OF THE NOBLE GASES ON EARTH

A signature of the early Sun is provided by isotopic fractionation of the five stable noble gas elements, namely, He, Ne, Ar, Kr, and Xe. The early atmosphere arose from collisions during the accretion period, the so-called heavy bombardment of the surface of the Earth. Planetesimal impacts increase the surface temperature affecting the formation of either a proto-atmosphere or a proto-hydrosphere by degassing of volatiles (Matsui and Abe, 1986). This generated a 'steam atmosphere'. One of its consequences was a rapid hydrodynamic outflow of hydrogen, including some of its compounds such as methane, carrying along heavier gases in its trail (Hunten, 1993). The mechanism postulated is that of aerodynamic drag. The upward drag of noble gas atoms of similar dimension competes with an opposite force due to gravity. Hence, since the various isotopes of these gases have different masses the net result is the occurrence of a mass-dependent fractionation of the various noble gas isotopes. For even heavier atoms, the gravity effect can be stronger than the aerodynamic drag and such atoms would not show the remarkable fractionation typical of the noble gases. By looking at other main-sequence stars at equivalent early periods of their evolution, we became aware of an associated larger output of solar EUV radiation. With the early Sun such an ultraviolet excess radiation is a possible factor that can trigger the phenomenon of mass fractionation in the noble gases. The case of the 22Ne/20Ne ratio is an example, since its value is larger than in the Earth's mantle, or in the solar wind. The observed fractionation of the noble gases can be taken as a signature of two aspects of the early Sun: firstly, the presence of the postulated escape flux, and secondly (more relevant for the main topic of this paper), as evidence for the solar energy source that drives the outward flux of gases. The emergence of appropriate conditions for life on Earth has to wait until the decrease of solar radiation that characterizes the terrestrial accretion period. The beginning of such a favourable period begins once accretion has ended. The surface heat flux diminishes, leading to the steam atmosphere raining into a global ocean (Kasting 1993). This splitting of a primitive atmosphere into a hydrosphere and a secondary atmosphere leaves behind carbon and nitrogen compounds that will be ingredients for subsequent steps of chemical evolution and, eventually, the dawn of life.

DEPLETION OF VOLATILE ELEMENTS ON THE MOON

The Moon is depleted of volatile elements such as hydrogen, carbon, nitrogen and the noble gases, possibly due to the fact that the most widely accepted theory of its formation is the impact of the Earth by a Mars-sized body during the accretion period. Exceptionally though, volatiles are abundant in lunar soils. The lunar surface evolved during the heavy bombardment period, adding material with a different composition to the Sun, and not derived from the Sun. Ions from the solar wind are directly implanted into the lunar surface (Kerridge, 1975; Kerridge et al, 1991). This component was detected during the Apollo missions. The isotopic composition of the noble gases in lunar soils has been established as being subsequent to the formation of the Moon itself. But nitrogen has a special place in the research for the nature of the astrochemistry of the early solar system. Unlike some of the other biological elements (CHNOPS or carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur), in lunar soils it is estimated that between 1.5 and 3 Gyr there was an increment of some 50 % in the ratio 15N/14N. This result has been abundantly confirmed. By performing single grain analyses Wieler and co-workers have searched for evidence of a predominantly non-solar origin of nitrogen in the lunar regolith (Wieler et al, 1999). There have also been attempts to analyze trapped N in the lunar regolith (Hashizume, 2000). These works suggest that, on average, some 90% of the N in the grains has a non- solar source, contrary to the view that essentially all N in the lunar regolith has been trapped from the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30 per cent in the 15N/14N ratio. The origin of non-solar component is an open problem. Indeed, Ozima and co-workers propose that most of the N and some of the other volatile elements in lunar soils may actually have come from the Earth's atmosphere rather than the solar wind (Ozima et al, 2005). This hypothesis is valid provided the escape of atmospheric gases, and implantation into lunar soil grains, occurred at a time when the Earth had essentially no geomagnetic 10

field. This is a valuable approach since it could clearly be tested by examination of lunar far-side soils, which should lack the terrestrial component. This question is not just pertinent to the astrogeological aspects of the evolution of the Moon, but by giving us a solid grasp on the evolution of the early Earth atmosphere, those factors that influenced the conditions favourable to the onset of life on Earth will be clearer. Hopefully with the availability of new missions, such STEREO involving two spacecraft in heliocentric orbit to study coronal mass ejections (CMEs), further measurements of the isotopic N- abundances may contribute to sorting out the astrochemical signatures of the early solar system that are awaiting to be deciphered. Such knowledge of N, one of the most intriguing of the six CHNOPS elements, will be considerable progress in the study of the origin of life on Earth.

PREPARING THE SOLAR SYSTEM FOR THE EMERGENCE OF LIFE

Various processes may have contributed to an early onset of the phenomenon of life, solar activity being one of the most relevant. The more intense solar wind of the early Sun would have a dramatic effect on the possibilities of preparing the Solar System for the emergence of life. In fact, the shock wave of the encounter of the intense solar wind with the spreading accretion disk blows away the residual gas and fine dust still present in the disk. Some evidence for this assertion may be found in meteorites (Bertout et al, 1991). In spite of the fact that the processes taking place from that moment onwards are not represented in the terrestrial geologic record, the current scenario of planetary origin gives us a means of inferring the activity that may have frustrated, or encouraged, the emergence of life. During the first 100 million years the flux of impactors would have set up the conditions for the separation of iron and silicate, giving rise to a metallic core. During this formation of the planetary embryo a major impact with another planet-size body gave rise to the expulsion of a large amount of matter from the primitive Earth, giving rise to the Moon. Our satellite cooled quickly, but it did not form an atmosphere. This may have been due to the smaller lunar cross section compared to the Earth. The original atmosphere that the Earth had captured from the solar nebula must have been largely blown away by the intense solar wind of the T-Tauri phase of the solar evolution. The planet was much more dynamic geologically and most of the records of large impacts were deleted, but the same geological activity was most likely responsible for partial out gassing of a secondary atmosphere, the exact nature of which can be inferred from the isotopic composition of the noble gases. It has been shown that comets are capable by themselves of providing noble gases in the correct proportions. This remark has been confirmed by laboratory experiments duplicating the conditions for cometary formation (Owen and Bar-Nun, 1995). Temperatures had descended to about 100 degrees Centigrade after the end of accretion at 4.4 Gyr BP. This scenario for planetary origin allows, in principle, the possibility of an early origin and evolution of life on Earth, provided that the solar climate and solar weather were sufficiently clement. However, it should be remembered that the lunar record demonstrates that some difficulties may arise in this scenario, since the Imbrium basin on the Moon, for instance, was formed by a large impact as late as 3.8 - 3.9 Gyr BP (Hartmann et al, 2000). This was a real cataclysmic spike in the cratering record. This event is known as the Late Heavy Bombardment (LHB). This implies the persistence of catastrophic impacts for the emergence of life on Earth, since our planet has a larger effective cross section than our satellite (Sleep et al, 1989). Recent discussions of the origin and intensity of the late heavy bombardment is further supported by more recent work (Gomes et al., 2005) that suggests that the LHB was triggered by the rapid migration of the giant planets. This phenomenon produced major changes in the space weather conditions. But even more, it triggered a massive delivery of planetesimals into the inner Solar System. Those conditions were an impediment for the emergence of life. Alternatively, if life had emerged before the LHB, it would most likely have been annihilated and started again after the major perturbations of the LHB had faded out. The analogous problem of bombardment of terrestrial-like planets in extra-solar systems is the subject of further recent attention (Levison et al., 2003).

EFFECTS OF RADIATION

As astrobiology studies the origin, evolution, distribution and destiny of life in the universe, in the present section we shall discuss in turn the four stages at which solar and extra-solar physics have a frontier in common with astrobiology. The incidence of non-ionizing UVR on the early surface of Earth and Mars to a large extent can be inferred from observations. Ionizing radiation, mainly due to nuclear and atomic reactions is relevant: X-rays are emitted spanning the whole spectrum of hard X- 11

rays to soft X-rays (0.01-10 nm); gamma rays are present too. The primary components affecting space climate are: galactic cosmic rays and solar cosmic radiation. To these we should add events that contribute to solar weather, such as solar particle radiation consisting of the low-energy solar-wind particles, as well as more energetic solar burst events consisting of solar particles that arise from magnetically disturbed regions of the Sun. These events vary in frequency according to the eleven- year cycle. However, scenarios for an early onset of life that have been proposed in the past have to deal with space weather that was radically different in the early Sun. Knowledge of the prehistory of solar particle radiation can be approached with a combined effort from observations of present-day emissions, together with studies of energetic solar particles recorded in extraterrestrial materials, notably the Moon material that became available with the Apollo missions, as well as with the study of meteorites. First of all we consider the magnitude of the ionizing radiation that may have been present at the time when life emerged on Earth, during the Archean (3.8 - 2.5 Gyr BP). According to some theoretical arguments (Mojzsis et al, 1999), the origin of life may be traced back even earlier, during the Hadean (4.6 - 3.8 Gyr BP). Indeed, these authors argue that the simplest interpretation of carbon isotopic data may point to the presence of diverse photosynthesizing, methanogenic, and methylotrophic bacteria on Earth before 3,85 Gy BP. Isotopic and geologic evidence suggest that in the Archean the atmosphere was anoxic (Walker et al, 1983). As a result the abundance of ozone would not have acted as a UV defense mechanism for the potential emergence of life. UVB (280–315 nm) radiation as well as UVC (190–280 nm) radiation could have penetrated to the Earth's surface with their associated biological consequences (Margulis et al, 1976; Cockell, 1998). From the point of view of extrasolar radiation, gamma ray bursts are powerful explosions that are known to originate in distant galaxies, and a large percentage likely arises from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely directed beams of gamma rays that race off into space. The Swift mission, launched in November 2004, contributes to determine recent burst rates. Such data allows the evaluation of life's robustness during the Ordovician (510 - 438 million years ago). During this geologic period there was a mass extinction of a large number of species (440-450 million years ago). This was the second most devastating extinction in Earth history. Present evidence has led to the conjecture that the extinction was triggered by a gamma ray burst (Thomas et al., 2005). There is no direct evidence that such a burst activated the ancient extinction. The conjecture is based on atmospheric modelling. The main conclusion to be derived from these calculations is that gamma ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, UVR from the Sun could kill much of the life on land and near the surface of oceans and lakes, and disrupt the food chain. Solar radiation has had an effect on the distribution of life. We should consider the underlying presence of variable, and to some extent, incompletely known output of solar radiation during its first Gyr. Experiments have been performed in the recent past at the ISS. We should keep in mind that during the early life of the Sun, the UV flux was much higher than it is today. The relevant wavelength regions are the XUV and soft X rays. These wavelengths are absorbed at the top of the atmosphere. Research at the ISS has been supplemented with laboratory tests. Several problems related with early biological evolution have been discussed in the past under the simulation of the early solar radiation environment (Lammer et al, 2002). Work on space weather influence on biological systems include the implications for the biosphere of magnetic field reversals (Biernat, et al, 2002); the influence on biological systems of solar flares (Belisheva, et al, 2002), and some work on uracil dosimetry to estimate the possible preservation of the molecules of life (Bérces, et al, 2002). If the distribution of life in the solar system took place by transfer of microorganisms, knowledge of solar weather is needed for the early stages of its evolution, to have some constraints on the possible transfer of microorganisms, as investigated extensively by Horneck and co-workers (Horneck and Cockell, 2001 for references). Bacillus subtilis is a Gram-positive harmless bacterium. It is capable of producing endospores resistant to adverse environmental conditions such as heat and desiccation and is widely used for the production of enzymes and specialty chemicals. The inactivation of B. subtilis spores has been studied in the Earth's orbit under different simulated ozone-column abundances to provide quantitative estimates of the potential photobiological effects of such an early ozone-free atmosphere (Horneck and Cockell, 2001). These authors find that the spectral sensitivity of DNA increases sharply toward shorter wavelengths from the UVB to UVC region. They conclude that this is the primary reason for the observed high lethality of extraterrestrial UV radiation that could provide a barrier to the distribution of life in the solar system. However, it should be kept in mind that the 12

most radiation resistant organism known at present exhibits a remarkable capacity to resist the lethal effects of ionizing radiation. The specific microorganism is a non-spore forming extremophile found in a small family known as the Deinococcaceae. In fact, Deinococcus radiodurans (whose name comes from the Greek for "terrible berry that withstands radiation") is a Gram-positive, red-pigmented, non-motile bacterium. It is resistant to ionizing and UV radiation. Several authors have studied these (Battista, 1997, Daly et al. 2004 and Levin-Zaidman et al. 2003). Members of this Family can grow under chronic radiation [50 grays (Gy) per hour] or recover from acute doses of gamma radiation greater than 10,000 Gy without loss of viability. Survivors are often found in cultures exposed up to 20,000 Gy. Seven species make up this Family, but it is D. radiodurans, whose radio-resistance appears to be the result of an evolutionary process that selected for organisms that could tolerate massive DNA damage. For the sake of comparison, the bacterium E. coli is approximately 200 times less resistant to gamma radiation, whereas humans cannot tolerate radiation of up to 5 Gy. Independent of the various UV defense mechanisms discussed in Sec. 2, the surface of the Earth is largely protected from cosmic radiation by the atmosphere itself. The annual dose of cosmic radiation for Germany is 0.3 mGy/year at sea level and 25 mGy/year at an altitude of 15 Km (Baumstark-Khan and Facius, 2001). Besides, also for comparison, we know that the survival fraction for mammalian cells in radiotherapy becomes negligible for a dose of 500 Gy (Kassis and Adelstein, 2004). It appears that the capacity of extremophiles to withstand ionizing radiation is due to adaptation to desiccation, as both environmental challenges (lack of water and excessive radiation doses) lead to similar massive DNA repair mechanisms. In this context, cyanobacteria have extraordinary ability to withstand desiccation and then rapidly absorb water when it becomes available. For example, a cyanobacterial population in gypsum quickly regains its ability to photosynthesize after addition of water (Van Thielen and Garbary, 1999). Possibly the radiation resistance ability of D. radiodurans may be due to its genome. (It assumes an unusual toroidal morphology that may contribute to its radio-resistance.) Solar radiation has also been a factor in the destiny of life. Indeed, the question of solar radiation also has a frontier with the fourth aspect of astrobiology. In about 4-5 billion years the brightness of the Sun will increase and its radius will increase (Sackmann et al. 1993). The consequence of the Sun abandoning its present steady state will lead to a swelling of its outer atmosphere. At the same time while the radius is increasing helium atoms will be at such a temperature that fusion into beryllium and carbon will occur. This is known in nuclear physics as the triple alpha point. This process lasts a few seconds. The energy from this ‘helium flash’ will lead to a sequence of events that will largely increase the emission of solar wind, carrying away a large fraction of the solar mass. This stage is well known to us. Indeed, there are many known examples of the stellar mass that the increased solar wind will take away (this is the planetary nebula stage). These events will set definite constraints of the destiny of life in our own solar system. Our knowledge of other stars mapped on a Hertzsprung- Russell diagram gives us enough confidence with the later stages of the evolution of our own Sun as it leaves the Main Sequence. These phenomena set strong constraints to the destiny of life in the solar system. But some further work on the models of the sun is necessary before making definite predictions on the period following the departure from the Main Sequence.

3. CONSTRAINTS ON THE ORIGIN OF LIFE DUE TO THE ANCIENT SUN

Today, a large number of space missions aim at a better understanding of our nearest star. Indeed it is possible to aim at solving the problem of the origin of life in the light of understanding a second example of life in the Solar System. We shall argue below in favour of a close collaboration between the programs of the solar and planetary missions. We work within the scope of astrobiology (the study of the origin, evolution, distribution and destiny of life in the universe) and astronomy (including the study of planetary dust and the SpW). Both disciplines should search analogous objectives, as we shall endeavour to demonstrate with a few examples. RELATIONSHIP BETWEEN ASTROBIOLOGY AND SOLAR PHYSICS

Interplanetary space is far from empty. On the contrary, it is filled with high-energy particles and radiation, forming clouds of hot plasma emitted by the Sun. Such events are particularly relevant for the Earth, especially for telecommunications. This area of research is generally referred to as Space 13

Weather (SpW). Recently, the precise boundaries of this emerging field have discussed (Messerotti 2005). Essentially, what is relevant is that SpW is a relatively new term that is commonly misused to mean, instead, the set of activities aimed at modelling and predicting Space Weather. Many ambiguities exist about the meaning of SpW in terms of the set of phenomena it represents. For such reasons, analysis of solar activity for SpW applications has been considered. Special emphasis has been given to the preliminary steps towards building foundation ontology to clearly define the physical concepts that come into play when referring to solar SpW, as opposed to other sources of the particle emission filling up the interplanetary space. Other concepts that have been discussed are the relationships that lead to the definition of SpW and Space Climate as aspects of the more general problem of Space Meteorology (in analogy with Earth Meteorology). We wish to call attention to non-solar particles filling interplanetary space that are remarkably relevant for the growing number of space missions, both dedicated to the exploration of the Sun, as well as to the exploration of the outer Solar System. The exploration of Jupiter and Saturn included the Galileo Mission for Jovian exploration during the period 1995 till 2003. It also included the Cassini-Huygens Mission, currently in progress. Both efforts in space exploration have confirmed a discovery that was due to the exploration of the Sun. In fact, Ulysses during the period 1992-2004 (cf., below) discovered that even beyond 3 AU from the Jovian system there is a most remarkable jet of particles travelling in a direction opposite to the solar wind. The source of these jets is a combined effect of the Jovian magnetosphere fed by the highly volcanic activity of the Galilean satellite Io that is only 6 RJ from the planet (RJ denotes the Jupiter radius, 71,492 km).

ON THE ULYSSES, GALILEO AND CASSINI-HUYGENS MISSIONS

Ulysses is a solar probe, currently in use that was launched in October 1990. Its most significant achievement to date is to have made the first-ever measurements of the Sun from a polar orbit. However, on its orbit, it made a close flyby of Jupiter, making subsequently a second set of measurements in its next encounter with this planet. The spacecraft used a Jupiter swing-by in February 1992 to transfer to a heliocentric orbit with high heliocentric inclination. It was at this time when the seminal discovery of Jovian stream of particles was made. Knowledge about the Jovian system had grown from two sets of spacecraft, namely the Pioneers 10 and 11 in the years 1974–1975 and Voyagers 1 and 2 in the summer of 1979. The Pioneer spacecraft probed the radiation environment of Jupiter and studied the main characteristics of the Jovian environment. On December 7, 1995 Galileo began its prime mission: a two-year study of the Jovian system. Several successful flybys led to another exciting mission-the Galileo Millennium Mission, extending into 2001. Data was collected on Io and Europa, and studies made of the effects of radiation on a spacecraft close in to Jupiter. The Cassini spacecraft, on its way to Saturn, went past Jupiter in late 2000 and for a few weeks, both spacecraft observed Jupiter. Galileo has contributed to elucidating what is the source of the interplanetary sulfur jets (including other volcanic elements as well). Right from the beginning of the Galileo Mission the icy surface of Europa, and other icy Galilean satellites, were studied by spectroscopic means (Noll et al., 1995). Subsequent measurements with NIMS (Fanale et al. 1999; McCord et al. 1998) have provided some evidence of various chemical elements on the icy surfaces. Although the NIMS data allows various interpretations (a situation that hopefully will improve during future missions), we should discuss at present the implications of some of these possibilities, in preparation for the planning of what type of biogenic signatures should be searched for, when probing the Europa icy surface for signs of life. Such knowledge should, in turn, provide some insights on th mechanisms that were at play during the origin of life on Earth under the influence of the early Sun. We should recall in this context that on Earth there are chemical compounds that are associated with metabolism, or microbial decomposition. Mercaptan, for example, is one of the most intriguing interpretations suggested by the data that is now available. Geobiochemistry suggests that if future missions that are now planned for Europa are able probe in situ the icy surface the biogenicity of the surficial sulfur could be ascertained. Besides, miniaturized equipment that could test such biogenicity bioindicators is already available. When Galileo entered Jupiter's orbit further information was obtained of the radiation environment to which the Io and Europa are exposed. (We focus our attention on the nearest two satellites. Pioneer 10 flew by Jupiter in December 1973, the first space probe to do so, and discovered a large magnetic tail of Jupiter’s magnetosphere. What is more relevant form the point of view of our paper was the 14

discovery of micron sized dust particles (Humes et al. 1974). The impact detectors of Pioneers 10 and 11 spacecraft recorded 15 impacts of dust particles in the neighbourhood of Jupiter, but at that time it was suggested that the source of some of the dust particles was due to comets (Srama et al. 2004). As we have seen above, the Ulysses spacecraft flew by Jupiter in 1992 (Grun et al. 1993). The Jovian system was recognized as a source of discontinuous streams of sub-micron dust particles. Galileo detected streams within 2 AU from Jupiter as the mission began its measurements on arrival at the Jovian System (Grun et al. 1996). Not only Pioneer 10, but alsoVoyager had observed small dust in abundance at two places. Firstly, the Jovian ring at 1.8 RJ and its weak extension out to 3 RJ (Showalter et al. 1995). Secondly, Voyager also observed Io’s volcanic plumes that were found to reach heights of about 300 km. Electromagnetic interaction of the particles making up the dust streams was evident on two separate occasions. We should recall a primary objective of the Ulysses mission: It encompassed probing interplanetary dust. (A cosmic dust sensor was included in its 55-kilogram payload.) Subsequently, Ulysses passed over the rotational south pole of the Sun in mid-1994 at 2 AU, and over the north solar pole in 1995. We wish to underline how the unexpected discovery of Jovian dust by Ulysses promises to be significant for astrobiological issues, especially the origin of life on Earth during the early part of the evolution of the Sun. In fact, Europa promises to be an additional relevant abode for life. Sadly, we have to face the absence of definite well-funded further planetary exploration missions aimed at the icy satellites of Jupiter within the next few years. The characteristics of interstellar dust that is continually being added to the SpW main constituents, assumes a relevant position. It deserves to be included in a wider scope of SpW beyond the exploration of solar physical effects. Nevertheless, it is relevant to recall that there is no real absence of studies for space exploration: A new such study by the European Space Agency (ESA) has recently been concluded (Renard, P. et al. 2005): the “Europa Microprobe In Situ Explorer” (EMPIE). The Ulysses and Galileo data revealed dust particles when these spacecraft were outside the Jovian magnetosphere. The arrival direction showed significant correlations with the ambient interplanetary magnetic field (Grun et al. 1993, 1996). It was subsequently demonstrated that only particles in the 10 nm size range can couple strongly enough to the interplanetary magnetic field to show the effects observed by Ulysses (Zook et al. 1996). The corresponding impact speeds were deduced to be in excess of 200 km/s. and the size of the particles was found to be in the range of 10 nm. Similar size dust particles were detected by Galileo that originated from the inner Jovian system within several RJ from Jupiter (Grun et al. 1996, 1998). These streams are strongly interacting with the planet’s magnetic field (Graps et al. 2000). Cassini’s closest approach to Jupiter was at a distance of 137RJ, whereas at the same time (December 30, 2000), Galileo was at about 14 RJ. This occasion allowed simultaneous measurement of the Jovian dust streams. Dust trajectories exist which intersect both the Galileo and the Cassini- Huygens orbit. These measurements provided a direct estimate of the time-of-flight of the grains between about 14 and 137 RJ. The above remarks demonstrate the still-to-be understood complexity of the interaction between the dust arising from Io and the Jovian magnetosphere (Sarma et al. 2004). Further research is necessary to gain confidence on the distribution of the sulphur arising form Io, as well as elsewhere in interplanetary space.

SPACE WEATHER ISSUES RAISED BY THE DISCOVERY OF THE DUST STREAM

The dust instrument of Ulysses detected eight strong peaks after June 2003 that have been identified as Jupiter dust streams. A peak in late November 2002 is also from Jupiter, and is the most distant dust stream detected so far (3.3 AU from Jupiter). Secondary objectives of the mission have been to include interplanetary-physics investigations, measurement in the Jovian magnetosphere during the Jupiter encounter, the detection of cosmic -ray bursts, and a search for gravitational waves. The Cosmic Dust Analyser onboard of Ulysses was able to record mass spectra of impact events from dust stream particles. These particles are driven out from the Jovian magnetosphere. To extract the chemical composition of particles, comprehensive statistical analysis of the dataset was accomplished. The results strongly implied that the vast majority of the stream particles were originating from Io. This finding indicates that volcanic gases are the dominant source of particles that reach the Ionian exosphere. Biogeochemistry measurements are clearly the most adequate means of discussing the interpretation of the patchy surface of Europa that we have sketched in the present work. However, due to various decisions of the main space agencies, at present it is not possible to predict with certainty when funds will be available for landing even a very modest amount of 15

equipment on Europa’s surface. However, completely different funding priorities constrain the missions that are related with solar exploration. Solar exploration is understandably a higher priority than the search for extraterrestrial life. This assertion can be justified due to the many effects that the Sun can have with telecommunications and various aspects of the space weather research. We have tried to argue in the present paper that the common frontier of solar research with astrobiology militates in favor of exploiting to the full those solar missions that, like Ulysses, have to move their probes over very large polar orbits that require the probe itself to approach considerably to the orbit of the planets of the outer solar system. To what extent the icy patches of Europa’s surface are affected by the particle streams that are ejected by the Jupiter magnetosphere into interplanetary space is of considerable interest to basic science. One aspect of the search of life on Europa can be incorporated into the wider program of solar system exploration. The four miniprobes (EMPIE studies), or by lander of the JPL studies would eventually lead to a direct approach, but in the meantime we can begin to obtain valuable information that could clarify the intriguing source of the sulphur contamination of Europa. . To sum up, we have endeavoured to show that by extending the scope of SpW to non-solar interplanetary particles, fundamental questions on the frontier between astrobiology and solar physics can be discussed. We have focused on the question that concerns the emergence of life on the Earth that was allowed by the activity of the ancient Sun. We have attempted to show that better understanding of a second genesis of life in the solar system (in the Jovian satellite Europa) would add significant insights into the basic problem of astrobiology, namely the origin of life in the universe.

4. SOLAR ACTIVITY IMPRINTS ON THE LUNAR REGOLITH AND ON FOSSILS

Today, due to the advent of a significant fleet of space missions and the possibility of performing experiments in the International Space Station (ISS), a meaningful study of the factors that may have led to an early onset of life on Earth begins to be possible. Our review lies within the scope of astrobiology (the study of the origin, evolution, distribution and destiny of life in the universe) and astronomy (including Space Weather). Both disciplines should search analogous objectives, as we shall endeavour to illustrate in this short review with a few examples. Preliminary modelling of the Sun does not allow useful extrapolations into the distant past in order to study in detail the solar physics influences on the emergence and early evolution of life on Earth (Jerse, 2006).

SPACE WEATHER IMPRINT ON THE ORIGIN AND EVOLUTION OF LIFE ON EARTH

Interplanetary space is far from empty. On the contrary, it is filled with high-energy particles and radiation, forming clouds of hot plasma emitted by the Sun. Such events are particularly relevant for the Earth, especially for telecommunications. This area of research is generally referred to as Space Weather. We can understand general trends of the influence of space climate and weather on the evolution and distribution of life. An important factor for understanding fully the origin and evolution of life on Earth is the evolution of the Sun and our galactic neighborhood. We consider the constraints that present knowledge of our own star and its galactic environment imply for the emergence and evolution of life on Earth. This, in turn, will provide further insights into what possibilities there are for life to arise in any of the multiple solar systems that are known to date. Fortunately, the particles that have been emitted by the Sun in the past have left a record in geologic samples in small bodies of the solar system in the Hadean (4.6 - 3.8 billion years before the present, Gyr BP) and Achaean (3.8 - 2.5 Gyr BP). It is generally agreed that the latter period corresponds with the emergence of life, but we cannot exclude possible earlier dates for the onset of life on Earth. The difficulty encountered in the simultaneous study of astrobiology and Space Weather is not insurmountable. Fortunately, considerable information can be retrieved from observations of extraterrestrial samples, either meteorites, or lunar material. Similarly, it is possible that we could retrieve bioindicators of the imprint that our galactic environment may have left on the fossil record of life on Earth. We will consider the fossils that represent an imprint of anomalous conditions in our environment since the Proterozoic. We have studied with special attention the records that may give some information about the factors favorable for life. Such data may be retrieved from the Sun during a period when fossils of animals were not available, during or at the end of the Archean. Such imprints are available in the upper layer of the lunar surface, on its regolith. 16

THE IMPRINT OF SPACE WEATHER ON THE LUNAR REGOLITH

The expansion of the solar corona induces a flux of protons, electrons and nuclei of heavier elements (including the noble gases) that are accelerated by the high temperatures of the solar corona, or outer region of the Sun, to high velocities that allow them to escape from the Sun's gravitational field. The solar wind is so tenuous that at a distance of 1 AU during a relatively quiet period, the wind contains approximately five particles per cubic centimeter moving outward from the Sun at velocities of 3x105 to 1x106 ms-1; this creates a positive ion flux of just over 100 ions per square centimeter per second, each ion having an energy equal to at least 15 electron volts. The solar wind reaches the surface of the Moon modifying considerably its upper surface or ‘regolith’. We have considerable information on the lunar regolith thanks to the Apollo Missions. In the years 1969-1972 these missions retrieved so much material and made it available to many laboratories that influenced much of our preliminary understanding of the origin of life on the early Earth. By so doing the solar wind modifies its structure leaving a tell-tale hint of how it changes over geologic time, since the Moon is an inactive body being modified only by the impacts of external objects such as meteorites. Much more recently, the Genesis Mission was NASA’s first sample return mission sent to space. It was the fifth of NASA’s Discovery missions. Genesis was launched in the year 2001 with the intention to bring back samples from the Sun itself. Three years later, after crash-landing, the probe was retrieved in Utah. Genesis collected particles of the solar wind on wafers of gold, sapphire, silicon and diamond. The amount of stardust collected by Genesis was about 1020 ions, or equivalently, 0.5 milligrams. Preliminary studies indicate that contamination did not occur to a significant extent. Genesis is providing us with important information for getting a deeper understanding of the early solar system, and hence an additional opportunity beyond fossils for a closer approach to the mystery of the origin of life on Earth. The Moon is depleted of volatile elements such as hydrogen, carbon, nitrogen and the noble gases, possibly due to the fact that the most widely accepted theory of its formation is the impact of the Earth by a Mars-sized body during the accretion period. Exceptionally though, volatiles are abundant in lunar soils. The lunar surface evolved during the heavy bombardment period, adding material with a different composition to the Sun, and not derived from the Sun. Ions from the solar wind are directly implanted into the lunar surface (Kerridge et al, 1991; Wieler et al, 1999). This component was detected during the Apollo missions. The isotopic composition of the noble gases in lunar soils has been established as being subsequent to the formation of the Moon itself. In order to get further insights into the early solar system, evidence has been searched for a predominantly non-solar origin of nitrogen in the convenient source of information that is represented by the lunar regolith 5. This search suggests that, on average, some 90% of the N in the grains has a non-solar source, contrary to the view that essentially all N in the lunar regolith has been trapped from the solar wind, but this explanation has difficulties accounting for both the abundance of nitrogen and a variation of the order of 30 per cent in the 15N/14N ratio. The Moon regolith presents a very challenging geological phenomenon. It consists of a very large number of grains with a rich history regarding their exposure to the Sun. Two parameters are useful in the systematic study of the lunar regolith: firstly, its ‘maturity’ namely, the duration of solar wind exposure and, secondly the ‘antiquity’, namely, how long ago the exposure took place. For the maturity parameter a useful way to measure it is in terms of the abundance of an element from the solar wind that is efficiently retained. The element nitrogen is a good example. (Alternatively solar noble-gas elements can be used.) Both antiquity and maturity have been used to learn about the evolution of the early solar system, especially the ancient Sun, the knowledge of which is needed for a comprehensive understanding of the problem of the origin of life on Earth. The exposure age to galactic cosmic rays produce certain nuclides in amounts proportional to the time the sample spends at the topmost part of the surface (some 2 metres). The contract between the known low abundance of a certain nuclide and the one induced by cosmic rays produce an indicator of antiquity. The antiquity parameter has been discussed in detail (Kerridge, 1975).

SPACE WEATHER IMPRINTS ON THE EARLY EARTH

As suggested in the previous section, solar climate during the first Gyr of the Earth was radically different. The earliest relevant factor was excessive solar-flare energetic particle emission, a 17

phenomenon that has been recorded in meteorites. These extraterrestrial samples provide information on events that took place during this early period after the collapse of the solar nebula disk. Gas-rich meteorites have yielded evidence for a more active Sun. A considerable number of young stars with remnants of accretion disks show energetic winds that emerge from the stars themselves. Similar ejections are still currently observed from our Sun. For this reason it is believed that some of the early Solar System material must keep the record of such emissions (Goswami, 1991). Information on the energetic emission of the Sun during this period can be inferred from data on X ray and UV emission (larger than 10 eV) from pre-main-sequence stars 8. We may conclude that during pre-main-sequence period, solar climate and weather presented an insurmountable barrier for the origin of life anywhere in the solar system. In the Hadean, conditions may still have been somewhat favorable, especially with the broad set of UV defense mechanisms that are conceivable. The high UV flux of the early Sun would, in principle, cause destruction of prebiotic organic compounds due to the presence of an anoxic atmosphere without the present-day ozone layer (Canuto et al., 1982; 1983). UV defense mechanisms have been proposed, such as atmospheric absorbers and prebiotic organic compounds.

DOES THE GALACTIC ENVIRONMENT INFLUENCE OF THE EVOLUTION OF LIFE?

Gamma ray bursts (GRBs) are powerful explosions that produce a flux of radiation detectable across the observable Universe. These events possibly originate in distant galaxies, and a large percentage likely arises from explosions of stars over 15 times more massive than our Sun. A burst creates two oppositely directed beams of gamma rays that race off into space. If a GRB were to take place within the Milky Way we would have to consider the possibility of mass extinctions comparable to the other known sources, such as the meteoritic collision with the Earth (cf., the next section), or a singular abundance of sulfur in the atmosphere due to the causes that are reviewed below. Mass extinctions have eliminated a significant fraction of life on Earth. GRB, together with meteoritic collisions, or an atmosphere that has gone through a transition unfavorable to the Earth biota are three likely causes that need to be discussed together, as we have attempted to do in the present review. The Ordovician is the second oldest period of the Paleozoic Era, thought to have covered the span of time between 505 and 440 million years before the present (Myrs BP). The late Ordovician mass extinction took place at approximately 440 Myrs BP may be at least partly the result of a GRB. Due to expected depletion of the ozone layer arising from the incoming energetic flux, the solar ultraviolet radiation that is normally shielded would give rise to a severely modified ecosystem. It is known that all marine animals suffered mass mortalities during the Late Ordovician Mass mortalities at the close of the Cambrian and late in the Ordovician resulted in the unique aspects of the Ordovician fauna. The Swift mission, launched in November 2004, contributes to determine recent burst rates. During evolution of life certain events triggered large-scale extinctions. We consider one of the most remarkable possible candidates. The Late Ordovician extinction created new opportunities for benthic and planktonic marine fauna. Radiation during post-Ordovician glaciation led to many new taxa typical of the Silurian. GRBs within our Galaxy have been repeatedly suggested to be a possible threat to life on Earth (Thorsett, 1995; Scalo and Wheeler, 2002; Melott et al, 2004). Some effects similar to those due to a nearby supernova should be expected. GRBs are less frequent than supernovae, but their greater energy output results in a larger region of influence, and hence they may pose a greater threat. It is likely (Melott et al., 2004) that in the last billion years (Gyr), a GRB has occurred close enough to have dramatic effects on the stratospheric ozone, leading to detrimental effects on life through increases in solar ultraviolet (UV) radiation, which is strongly absorbed by ozone. A major question has been the timescale for atmospheric chemistry: most of the GRB influence comes in seconds or minutes as compared to months for the case of supernovae. There is no direct evidence that such a burst activated the ancient extinction. The conjecture is based on atmospheric modeling (Thomas et al., 2005). The main conclusion to be derived from these calculations is that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer. Recovery could take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun could kill much life on land and near the surface of oceans and lakes, and disrupt the food chain. Nevertheless it is important to recall, as we shall do in the next two sections based on the fossil record that there are two other competing theories for mass extinctions during earlier geologic periods, such as the suggested Ordovician mass extinction.

18

EXTINCTIONS BY ASTEROID IMPACTS, COMETS OR BIOGEOCHEMICAL ACTIVITY

Evidence for impact from the geologic boundary between the Cretaceous and Tertiary periods (the so called K/T boundary) are the Chicxulub crater in the Yucatan Peninsula in Mexico, and the element iridium (Ir) layer. In addition, impact ejecta such as pressure-shocked stone support the meteoritc impact hypothesis. Perhaps the better discussed evidence for the K-T impact some 65 Myrs BP was the Ir-abundance coinciding with the geologic evidence of mass extinction. In spite of the very abundance of Ir in well-studied meteorites, the Ir-rich deposit may alternatively be interpreted as volcanic ejecta. In other words, the possibility remains that the layer could instead have been produced by volcanic Ir-rich eruptions. The Permian period gave way to the Triassic at about 251 Myrs BP, At that time the Earth experienced its greatest mass extinction known to us. Ninety percent of all marine species, including the trilobites, disappeared, while on land pervasive extinctions opened the way for the rise of the dinosaurs. But despite the magnitude of mass extinction its cause is a source of controversy (Kerr, 2001). A new analysis of rock that marks the Permian-Triassic (P-T) extinction now suggests that it was caused by the hypervelocity impact of an asteroid or comet similar to the one thought to have led to the extinction of dinosaurs at the K-T boundary. There is some evidence for some catastrophic event that gave rise to the P-T extinction. Paleontologic evidence seems to suggest that a single event may have been responsible for the P-T transition. One such possibility shall be discussed in the next section. Noble gases such as helium and argon apparently were trapped in molecular cages of carbon (fullerenes). This hypothesis follows the extraction of the gases from rocks at the P-T transition (Becker et al., 2001). Analyses of these gases show that their isotopic compositions are analogous to those found in meteorites, and are not typical of the Earth-bound abundances. This is some evidence that a major impact may have delivered the noble gases to Earth at the time close to the period when the extinctions did take place. Indeed, this suggestion provides an indicator for a P-T impact that is analogous to the earlier theory of the impact at the K-T boundary, an event that we saw above to have been supported by the Ir-data. Fullerenes are also candidates for indicators of impact. Previous work by others showed that they are present in rock at the K-T boundary. Together these findings suggested that fullerenes are impact markers like iridium. That prompted Becker and her colleagues to look for the compounds in rock at the P-T boundary at the in South China, and in southwest Japan and reported the detection of fullerenes in boundary rock, but not in similar rock a few centimeters to meters above, or below the boundary. However, it should be kept in mind that fullerenes can be produced by, for instance, forest fires. In the case of the K-T mass extinction shocked quartz was detected, (i.e., distinctively veined crystals made only in the extreme pressures of large impacts). Shocked quartz has not been identified with the same certainty at the P-T transition, but the noble gas indicators may offer additional evidence. Fullerenes can trap gas atoms. When the gases trapped in fullerenes from P-T-boundary rocks was analyzed (Becker et al., 2001), it was found that the abundance of helium-3 was significantly enhanced above what it was immediately above or immediately below the boundary. The ratio of helium-3 to helium-4 was typical of meteorites. Besides the ratio of argon-40 to argon-36 in boundary fullerenes is likewise analogous to that of meteorites.

ATMOSPHERIC HYDROGEN SULFIDE AND THE P-T EXTINCTION

The warming caused by volcanoes through carbon dioxide emissions would not be large enough to cause mass extinctions by itself. That warming, however, could set off a series of events that led to mass extinction. During the Permian Triassic (P-T) extinction 95 percent of all species on Earth became extinct, compared to only 75 percent during the K-T when a large asteroid apparently caused the dinosaurs to disappear. Volcanic carbon dioxide would cause atmospheric warming that would, in turn, warm surface-ocean water. Normally, the deep ocean gets its oxygen from the atmosphere at the poles. Cold water there soaks up oxygen from the air and because cold water is dense, it sinks and slowly moves equator-ward, taking oxygen with it. The warmer the water, the less oxygen can dissolve and the slower the water sinks and moves toward the equator (Kump et al., 2005). The constant rain of organic debris produced by marine plants and animals, needs oxygen to decompose. With less oxygen, fewer organics are aerobically consumed. In the Permian, if the warming from the volcanic carbon dioxide decreased oceanic oxygen, especially if atmospheric oxygen levels were lower, the oceans would be depleted of oxygen. 19

Once the oxygen is gone, the oceans become the realm of bacteria that obtain their oxygen from sulfur oxide compounds. These bacteria strip oxygen from the compounds and produce hydrogen sulfide. Hydrogen sulfide kills aerobic organisms. Humans can smell hydrogen sulfide gas, the smell of rotten cabbage, in the parts per trillion range. In the deeps of the Black Sea today, hydrogen sulfide exists at about 200 parts per million. This is a toxic brew in which any aerobic, oxygen-needing organism would die. For the Black Sea, the hydrogen sulfide stays in the depths because our rich oxygen atmosphere mixes in the top layer of water and controls the diffusion of hydrogen sulfide upwards. At the end-Permian, as the levels of atmospheric oxygen fell and the levels of hydrogen sulfide and carbon dioxide rose, the upper levels of the oceans could have become rich in hydrogen sulfide catastrophically. This would kill most the oceanic plants and animals. The hydrogen sulfide dispersing in the atmosphere would kill most terrestrial life. Another aspect of this extinction is that hydrogen sulfide gas destroys the ozone layer. Once this process has started, methane produced in the ample swamps of this time period has little in the atmosphere to destroy it. The atmosphere becomes one of hydrogen sulfide, methane and ultra violet radiation. Biomarkers of photosynthetic sulfur bacteria in deep-sea sediments were recently reported in shallow water sediments of an age comparable to the P-T transition (Grice et al., 2005). These bacteria live in places where no oxygen exists, but there is some sunlight, as it may have happened at the end of the Permian. Confirming the evidence for these microorganisms would provide evidence for hydrogen sulfide to have been the cause of the mass extinctions.

CONCLUDING REMARKS

The main thesis that we have maintained in this work is that solar activity, space weather and astrobiology should be brought within a unified framework. This approach naturally leads us to the suggestion of exploiting instrumentation from somewhat dissimilar sciences (astronomy and astrobiology) with a unified objective. We have attempted a preliminary comprehensive discussion of how research in the conditions of the early Sun combine with observations in several disciplines to give us insights into the factors that lead to the emergence of life in a given solar system (biogeochemistry, lunar science, micropaleontology and chemical evolution). These considerations are necessary to approach the conditions that will allow life to emerge in a given solar system anywhere in the universe.

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