«Astronomical and astrobiological imprints on the fossil records. A review. “From Fossils to Astrobiology”, Ed. J. Seckbach, Cellular Origins, ...»
Chela-Flores, J. Jerse, G., Messerotti, M. And Tuniz, C. (2008)
Astronomical and astrobiological imprints on the fossil records. A
review. “From Fossils to Astrobiology”, Ed. J. Seckbach, Cellular
Origins, Life in Extreme Habitats and Astrobiology, Springer,
Dordrecht, The Netherlands, pp. 389-408.
ASTRONOMICAL AND ASTROBIOLOGICAL IMPRINTS ON THE FOSSIL
RECORDS. A REVIEW
J. CHELA-FLORES 1,2, G. JERSE 3, M. MESSEROTTI 4,3 AND C.
TUNIZ 1 The Abdus Salam ICTP, Strada Costiera 11, 34014 Trieste, Italia, Instituto de Estudios Avanzados, IDEA, Caracas 1015A, República Bolivariana de Venezuela; 3Department of Physics, University of Trieste, Via A. Valerio 2, 34127, Trieste, Italia; and 4INAF-Trieste Astronomical Observatory, Loc. Basovizza n. 302, 34012, Trieste, Italia.
1. The common frontier of astronomy and astrobiology Both astronomy and astrobiology share a common frontier. Vertiginous progress in instrumentation such as novel microanalytical tools to study extraterrestrial materials, including those collected in space return missions, and availability of long ice cores and other fossil archives providing detailed records of the past terrestrial environment, can give deeper insights into the origin and history of life on Earth. The early stage of the Sun and other space-palaeoclimate conditions are relevant to the emergence of life on Earth.
The record of Earth’s condition in the past is studied by different scientific communities involved in space palaeoclimate research. A set of data derives from historical observations of the solar surface. Other data are based on laboratory studies of matter derived from the surface of planets, the Moon, meteorites and comets (Pepin et al., 1981), which contain imprints due to past space weather conditions, such as implanted ions and radionuclides produced by nuclear reactions induced by high-energy cosmic rays. A final set of data derives from terrestrial archives, including tree rings, ice and marine sediment cores, corals, lake varves, manganese nodules and other crusts that grow slowly at the bottom of the ocean. All these systems contain a detailed record of proxies revealing Earth- and space-climate conditions in the past. Such information can be retrieved with advanced instrumentation, such as high sensitivity analyzers of stable and long-lived isotopes.
We shall focus our attention on space weather as a factor that is relevant for the origin and evolution of life on Earth. Then we will review possible changes in the evolution of life in general. Moreover, we will discuss possible clues contained in the fossil records of past life on Earth including some aspects of evolution, especially of humans, thatmay be due to space palaeoclimate. In general, the fossil record of the
ancient Sun and of space palaeoclimate will yield insights into how our ecosystem may have evolved.
2. The impact of space climate and weather on living systems During the early stages of the study of the origin of life (Oparin, 1953; Ponnamperuma and Chela-Flores, 1995) not enough attention was paid to the correlation between chemical evolution of Earth materials and variability of the early Sun (Messerotti, 2004) or remote events taking place in our galactic neighbourhood. Today, a meaningful study of the factors that may have led to an early onset of life on Earth begins to be possible due to the advent of a significant fleet of space missions and the possibility of performing experiments in the International Space Station (ISS). Our review lies within the scope of astrobiology (the study of the origin, evolution, distribution and destiny of life in the universe) and astronomy. Both disciplines should search analogous objectives, as we shall endeavour to illustrate with a few examples in this short review. Preliminary modelling of the Sun does not allow useful extrapolations into the distant past in order to study in detail the influence of solar physics on the emergence and early evolution of life on Earth (Jerse, 2006).
Electromagnetic and particle radiation that originate from the Sun, and from other space sources external to the solar system, are continuously impinging upon the Earth environment at different time scales and in a broad range of energies (Messerotti, 2004).
The long-term evolution of the physical state of the space environment is referred to as space climate, whereas the short-term evolution is defined as space weather (SpW). The interplay between the impinging energy carriers and the relevant impacts at the planetary level is determined by the complex physical couplings among the galactic, the solar and the terrestrial environments and the processes occurring therein. For instance, highenergy particles that originate from galactic sources, known as galactic cosmic rays (GCR), interact with the Earth atmosphere and generate showers of secondary particles such as muons and neutrons. It should be remarked that the flux of the GCR at the Earth depends on: a) the position of the Sun in the Galaxy, since during its revolution around the galactic centre our star crosses environments richer or poorer of GCR sources on a time scale of 225 million years; b) the activity level of the Sun. This contribution to the GCR flux is due to higher solar activity producing denser and faster solar wind. Hence, the particle flux is continuously accelerated by the star, which carries the solar magnetic field and fills up the interplanetary space by defining the region of space confined by the interstellar wind (heliosphere). When the solar wind is denser, it acts as a more efficient shield to the GCRs. Consequently, a lower GCR flux can reach the Earth; c) the strength of the Earth magnetic field, which acts as a further shield. Other materials reaching the Earth include meteorites, asteroids, comets and cosmic dust.
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 neighbourhood. 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
ASTRONOMICAL AND ASTROBIOLOGICAL IMPRINTS
been emitted by the Sun or other galactic sources 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, Ga BP) and Achaean (3.8 - 2.5 Ga). In small bodies the geologic data has not been lost by metamorphism, as it has happened on the Earth. It is generally agreed that the latter period corresponds to 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 SpW 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 on the factors favourable 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 Achaean. Such imprints are available in the upper layer of the lunar surface, on its regolith.
3. The possible role of space palaeoclimate in mass extinctions and planetary evolution As suggested in the previous section, solar climate during the first Ga 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 (Goswami, 1991).
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 represented by meteorites could have retained the record of such emissions.
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.
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 favourable, especially with the broad set of UV defence 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). Some possible UV defence mechanisms have been proposed in the past, such as atmospheric absorbers and prebiotic organic compounds (Margulis et al., 1976;
Sagan and Chyba, 1997; Cleaves and Miller, 1998).
Inversions of the Earth's geomagnetic dipole represent a well-established geochronological framework. The most recent of these inversions, referred to as the Matuyama–Brunhes (M–B) transition, has been dated to about 780 ka ago.
J. Chela-Flores, G. Jerse, M. Messerotti and C. Tuniz
During a geomagnetic reversal, the dipole field strength is believed to decrease by about an order of magnitude. During this time, galactic cosmic rays can more easily penetrate into the Earth's atmosphere and thus increase the production of cosmogenic isotopes, such as 10Be. Evidence has been presented for enhanced 10Be deposition in the ice at 3,160–3,170 m, interpreted as a result of the low dipole field strength during the Matuyama–Brunhes geomagnetic reversal. If correct, this provides a crucial tie point between ice and marine core records (Raisbeck et al., 2006).
4. Traces of space-climate events in the geologic record
The solar corona is the outermost region of the Sun’s atmosphere. Its expansion induces a flux of protons, electrons and nuclei of heavier elements (including the noble gases).
These interplanetary particles are accelerated by the high temperatures of the solar corona, to high velocities that allow them to escape from the Sun's gravitational field.
The wind contains approximately five particles per cubic centimetre 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 centimetre per second, each ion having an energy equal to at least 15 electron volts. The solar wind reaches the surface of the Moon modifying 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. These missions gave an opportunity for detailed studies of isotopic fractionation of the biogenic elements on the surface of the Moon. In general terms, the preliminary understanding that the Apollo Missions added to the work that was available at the time on meteorites was related to the fractionation of H, C, N and S on the lunar surface. In fact, the preliminary hint that was relevant for the origin of life was that the distribution range of 32S/34S appears to be narrower than the isotopic ratio of hydrogen, carbon or nitrogen. For this reason, it was suggested that the fractionation of S isotopes would be the most reliable parameter for estimating biological effects (Kaplan, 1975; Chela-Flores, 2007). Deviations of 32S/34S from meteoritic values discovered on the Moon by the Apollo missions can be understood by the fact that the solar wind modifies its structure leaving a tell-tale signal of how it changes over geologic time, since the Moon is an inactive body being modified only by the impacts of meteorites and asteroids.
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, USA. 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. The objective is to obtain precise measures of solar isotopic abundances. By measuring isotopic compositions of oxygen, nitrogen, and noble gases we would have data that will lead to better understanding of the isotopic variations in meteorites, comets, lunar samples, and planetary atmospheres. This will lead to a deeper understanding of the
ASTRONOMICAL AND ASTROBIOLOGICAL IMPRINTS
early Solar system, and hence an additional opportunity beyond fossils for a closer approach to the mystery of the origin of life on Earth by being able to assess properly potential biomarkers that may be suggested from the point of view of biogeochemistry.
There will be also attempts to use Accelerator Mass Spectrometry (Tuniz et al., 1998) to detect a long-lived radionuclide of solar wind origin, for example such as 10Be and 26Al (Jull and Burr, 2006).
The Moon is depleted of volatile elements such as hydrogen, carbon, nitrogen and the noble gases, consistent with 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.