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To date, there is nothing that could allow us to figure out if we will ever know whether the Earth is the only inhabited planet in the universe. However, astrobiology has gone a long way since the very first experiments by Miller in 1950 and since then, questions related to astrobiology are feeding the exploration of our solar system, and even beyond with ambitious programs for the observation of extrasolar planets (TPF, Darwin). The answer is maybe already at hand, by a rock analyzed with a Martian rover, or further away, under a few kilometers of ice in the Jupiter neighborhood. . . This chapter deals with the preliminary steps of the astrobiological exploration of the solar system, we are currently witnessing. We focused on the search of organic matter and liquid water, but none of them is a clear indicator of past or present life. We did not discuss the next issue about measurement(s) (chemical, geological, spectroscopic), which could allow us to claim that we have finally detected life on another planet. The scientific debate about it is still at its very first stages, and opens the most fascinating prospects. [ABSTRACT FROM AUTHOR]

The new era of exploration of Mars that was initiated by NASA in 1997 and followed by ESA in 2003, is revolutionizing our understanding of the present and past of the planet Mars. More and more data are available, but many of these observations appear to be contradictory. In such condition, there is no consensus on what really happened on Mars, its climate evolution, the presence of liquid water, and the possibility of life. We know that there have been multiple kinds of climates on Mars, because the planet environment strongly varied with the oscillations of the planet's orbital and rotational parameters as well as with the evolution of the content of its atmosphere. I personally would conclude that, although Mars appears to have enjoyed conditions suitable for sustained liquid water on its surface 3.7-4.2 billion years ago, it seems that afterward the Martian climates did not allow surface liquid water except during episodical events. For several billions years, Mars has probably not been very suitable for life as we know it, except maybe in the deep subsurface. However, one can expect that new discoveries will soon, prove or disprove this opinion, bring new insight to the problems, and probably question some of the "certain facts" presented in this chapter. [ABSTRACT FROM AUTHOR]

This book is an attempt to blend an extended and cross-disciplinary research paper about an initially neglected obscure cosmic dust contamination of the early Earth into a kind of cosmic detective investigation, which should be comprehensible by the space sciences community and undergraduate students in general. The major objectives were to: [ABSTRACT FROM AUTHOR]

Surprising EMMA. The main star of this book, besides the Moon, is an obscure contamination of the young Earth that was falling from the sky. It was mostly deposited by invisible tiny cosmic dust grains that can hardly be felt between two fingers. It was neglected for a long time because its composition and the variation of its mass flux with time were not tractable. Therefore, its effects could not be predicted. [ABSTRACT FROM AUTHOR]

This section includes a heteroclite set of questions and comments of colleagues, which have still to be fully answered. Progress in science occurs not only with a few spectacular achievements that are generally cleverly advertised by funding agencies and the prestigious journals, but also by incessant questioning. They remain in the shadow for a while because you have to wait for an answer that might never come. [ABSTRACT FROM AUTHOR]

A world of "hot" spots. Until the very recent reports of Yada et al. (2004, 2005) individual tiny interstellar dust grains (stardust) were only sought in meteorites and SMMs. This is a diffcult study, which is based on the search for isotopic anomalies in tiny submicron-sized grains with ion analyzers. [ABSTRACT FROM AUTHOR]

Any study of micrometeorites involves a variety of biases, which start right away during their collection, and which have not been suffciently publicized. This section deals with the astonishing folklore of these biases. We shall question whether major differences observed between Antarctic micrometeorites and stratospheric micrometeorites could reflect kinds of complementary biases between the two collections of micrometeorites. Astonishingly, some of them would converge to enrich the SMMs collection in the most fine-grained fluffy dust particles accreted by the Earth. They might be possibly the most primitive material accreted by the Earth. But they would not give a representative sampling of the bulk micrometeorite flux, which is best obtained with the new Concordia micrometeorites collected in central Antarctica. For a change, biases developing around a small metallic plate flying at ~200m/sec in the stratosphere turned out to be quite helpful! [ABSTRACT FROM AUTHOR]

In Sect. 5.2, we quoted the excitement about tiny micrometeorites collected in the stratosphere (SMMs) with a collector plate coated with an approximately 100 μm-thick silicone oil layer, which was fixed under the wing of a stratospheric aircraft of NASA. As pointed out by Jessberger et al. (2001), "due to contamination and collection limitations most of the SMMs have been limited to the 5 to 25μm diameter range". Messenger (2000) describes these SMMs as:"diverse assemblage of materials, which are commonly very finegrained (~0.1 μm), heterogeneous and rich in volatile elements and carbon relative to all meteorite classes". [ABSTRACT FROM AUTHOR]

Colleagues still have trouble understanding how the dominant relationship between CM-type chondrites and AMMs was established. They also wonder about the meaning of this relationship in terms of solar system history, and they generally expect that it has been markedly altered upon atmospheric entry, except for the smallest particles collected in the stratosphere. They are also confused when hearing that in spite of strong similarities there are also marked differences between these two varieties of solar system materials. Consequently, some of them believe that this relationship has not been convincingly established. This section mostly relies on arguments already presented by Kurat and Maurette (1997) in a book published in French, which have only been improved recently through the analysis of the Concordia Antarctic micrometeorites (Duprat et al., 2003 and 2004). [ABSTRACT FROM AUTHOR]

The constraint of grain sizes. The discussion presented in the last section suggests that Antarctic micrometeorites would dominantly originate from comets. However, they contain refractory inclusions made of refractory oxides (Fig. 46), abbreviated as CAIs (see Sect. 6.2), which can only be formed at the highest temperatures (T ≥ 1800 K) close to the early Sun. They are at least 10 times larger than the very small inclusions of refractory oxides (sizes of about 0.1-1 μm) observed in the most fine-grained SMMs belonging to the group of the chondritic-porous aggregates (see Zinner, 2004). In contrast, micrometeorite CAIs are smaller than those observed in CM-type HCCs (Gounelle et al., 2001), which are in turn about 10 times smaller than those observed in the "dry" carbonaceous chondrites (including the CV, CO and CK types), where they can reach a size of about 1 cm. [ABSTRACT FROM AUTHOR]

A French word, "errance" (the straightforward approximate English translation is wandering) well describes the functioning of the mind when you do not know where you are going. You feel dumb just knowing that you will probably end up somewhere else. Then, you bury this shameful weakness of your neuronal system in the deepest secrecy and start again hunting for good data. However, this is an essential step of scientific inquiry, which is illustrated here below just wandering about the meaning of lunar cratering rates and the origin of comets, which are intimately interwoven with the early history of the solar system and in particular with the phenomenon of debris disk around stars. [ABSTRACT FROM AUTHOR]

The application of the accretion formula to the Earth-Moon system does not require the identification of the parent bodies of micrometeorites (i.e., asteroids or comets). However, this identification is necessary to better extrapolate this formula to other terrestrial planets, such as Mars. One would also like to understand how the invariant composition of micrometeorites constrains the nature of their parent bodies. We report on a few arguments that have not been thoroughly explored yet, and which run against the conventional view that most micrometeorites originate from the collisions and/or erosion of asteroids. The only remaining alternative is that they dominantly originate from comets. [ABSTRACT FROM AUTHOR]

In this section, EMMA is first extrapolated to the Moon, to hopefully get new clues about a confusing problem that people have failed to figure out since 1970. It deals with the so-called meteoritic contamination of the lunar crust in siderophile elements such as iridium, which was previously attributed to the crater-forming impactors and not to micrometeorites. We next move to Mars to try to check whether EMMA can account for the high sulfur and nickel contents of Martian soils measured with instruments carried by the rovers Spirit and Opportunity in 2004. Before 1999, this obscure contamination looked at first glance to be of a limited interest. Consequently, it was neglected in earlier works. We discovered recently that the true reason for this neglect was probably that the description of this contamination on the Moon and Mars requires facing an astonishing diversity of very dificult problems in planetology, in which we became bogged down. But it was too late to quit. [ABSTRACT FROM AUTHOR]

The basic time frame of EMMA is the formation time interval of the Earth, Δ(Earth) ~ 100 Myr. This value has been estimated from both the 129I- 129Xe radioactive chronometer (Pepin and Phinney, 1975; Staudacher and Allègre 1982) and modern theories about the formation of the Earth initiated by Wetherill (1994) -for a recent summary see Canup and Agnor (2000). In this scenario, the composition of the Earth's atmosphere reported in Table 1 would give the average composition of about 1000 billions of billions of billions (e.g., ~1030) of juvenile IDPs captured by the Earth after the formation of the Moon, and representing a total mass of material of ~5.1024 g. Amazingly, this composition turns to be quite similar to that of an aliquot of about 500 AMMs with sizes of around 100.200 μm, amounting to a few milligrams of material, that we used to infer the composition of a "micrometeoritic" atmosphere. [ABSTRACT FROM AUTHOR]

Ozima and Podosek (2002) clearly state: "In summary, there is still no satisfactory theory on the origin of terrestrial noble gases". This view is also supported by Porcelli and Pepin (2000), who note: "there still is no consensus on how the terrestrial noble gases originated". Two of the major problems with the distribution of noble gases on the Earth concern: [ABSTRACT FROM AUTHOR]

On the Earth, an astonishing balance between the absorption and scattering of solar radiation by the early Earth produced the remarkable benign greenhouse effect favourable to the origin and evolution of life. Indeed, the first constraint on any scenario is that the early oceans were not boiling or freezing! It is generally considered that the temperature has to be sufficiently high to prevent freezing at a time when the solar luminosity was smaller than today. But it has to be kept sufficiently low by some mysterious feedback effect, in order to protect the Earth from a runaway greenhouse effect, which led to a surface temperature of about 450 °C on Venus. In fact, the long-lasting micrometeorite thermospheric volcanism effective after the Moon-forming impact, should have ruled the post-lunar greenhouse effect that was critical for the birth of life. Indeed, this impact eradicated at once all atmospheric ingredients of the pre-lunar greenhouse effect at a time when the young Earth was already almost fully outgassed. Subsequently, micrometeorites released simultaneously greenhouse gases for heating and smoke particles for cooling. These micrometeorite ashes resided temporarily within a kind of giant thermospheric cocoon, which might have functioned as a self-regulating IR heater during the period of low solar luminosity. Indeed, it was simultaneously heated up from the inside through the aerodynamical braking of micrometeorites. [ABSTRACT FROM AUTHOR]

About 80% of the Cap-Prudhomme AMMs collected near the margin of the Antarctic ice sheets showed anomalous low sulfur contents (~0.1%) with regard to the value of ~ 3% measured in CM chondrites, to which about 95% of these AMMs are related. We thought first, like everybody, that sulfur, which is quoted as the most volatile of the moderately volatile elements, would have been lost upon frictional heating upon atmospheric entry. But then, why is it that Ca, which is the most refractory element, would have also been lost? Moreover, there was no size effect in the loss of S and Ca going from 200 μm to 30μm size grains. This runs against frictional heating, which increases with particle sizes. Therefore Kurat deduced that sulfur, Ca and Ni could have been preferentially lost during "cryogenic-Cweathering" effective in the first top meter of blue ice fields near the sea shore, involving the preferential leaching of carbonates, sulphates and sulfides (Maurette et al. 1992). [ABSTRACT FROM AUTHOR]

In this section micrometeorites are further divided in two distinct families: [ABSTRACT FROM AUTHOR]

Maurette (1998a and 1998b) gives a more detailed discussion of previous works supporting the role of unmelted micrometeorites in prebiotic chemistry. Krueger and Kissel (1987) quoted thermodynamic computations suggesting that the μm-size C-rich grains that they discovered in the tail of Halley's comet with their time-of-flight mass spectrometer on board the Vega spacecraft, when added to a prebiotic soup of organics, could trigger the formation of nucleic acids. Anders (1989) relied on the characteristics of the tiny stratospheric micrometeorites with sizes of about 5-15 μm, which amount to less than about 1% of the micrometeorite mass flux, to argue that micrometeorites played a major role in the delivery of organics to the Earth. As first quoted by Ponchelet (1989), we proposed in 1989 that much larger micrometeorites, similar to Antarctic micrometeorites, might have been functioning as individual microscopic chemical reactors on the early Earth during their interactions with gases and waters (Maurette et al., 1990, 1991b, 1998a, 1998b). Subsequently, Chyba and Sagan (1992) ceased fully supporting the role of the direct impact of comets in the delivery of such organics, and started to refer that of micrometeorites. [ABSTRACT FROM AUTHOR]

Exobiology is a multidisciplinary science that deals with the origin and early evolution of life. For the birth of life to occur, the setting of its cosmic cradle has to be right. The first step is to make early seas and to buildup a benign greenhouse effect that prevents their freezing and/or boiling. Next, an efficient prebiotic chemistry has to operate in this water to synthesize the key macromolecules that could start dividing, thus giving the first sketch of life. This synthesis requires: energy; liquid water; salts like phosphate; organic molecules such as amino acids; mineral surfaces behaving like catalysts of their reaction; etc. [ABSTRACT FROM AUTHOR]

The early Earth's atmosphere was much more massive than today. Before the condensation of water and the subsequent dissolution of CO2that precipitated into carbonates, the partial pressures of H2O and CO2were about 270 bars and 60 bars, respectively. Today, planetary exploration has revealed that the atmospheres of 8 planets and 3 of their approximately 120 satellites have a structure rather similar to that deduced from the vertical temperature profile of the contemporary Earth's atmosphere, which shows that the temperature alternately decreases and increases with height (troposphere, stratosphere, mesosphere, and thermosphere). In particular, because the giant planets and Venus are all topped by a high-temperature low-density thermosphere (Dowling, 1999), we can safely assume that the early Earth had also a thermosphere. [ABSTRACT FROM AUTHOR]

The chemical composition of the Earth's atmosphere can be defined by the Ne/N2,H2O/N2and CO2/N2ratios deduced from the total amounts of these four volatiles in the atmosphere. The same volatiles are released by hydrous-carbonaceous micrometeorites upon frictional heating as to generate a micrometeoritic model atmosphere. Its composition is inferred from the weight % contents of Ne, N2,H2O and carbon that were measured in about 500 Antarctic micrometeorites from the 100-200 μm size fraction. The composition of the micrometeoritic atmosphere was thus found to be strikingly similar to that of our blue planet. [ABSTRACT FROM AUTHOR]

According to the definition of Ozima and Podosek (2002), the Earth's atmosphere refers to all volatiles in surface reservoirs, including air, water and sedimentary rocks such as carbonates in which early CO2 is now trapped. [ABSTRACT FROM AUTHOR]

Micrometeorites and meteorites.The mass flux (number of tons, per year, for the whole Earth) of meteorites and micrometeorites can be estimated, both before and after atmospheric entry, from the works of Haliday et al. (1989), Zolensky et al. (1992), Love and Brownlee (1993), Bland et al. (1996), Hammer and Maurette (1996), Gounelle et al. (1999) and Duprat et al. (2003, 2004). [ABSTRACT FROM AUTHOR]