Organic matter in carbonaceous meteorites: past, present and future research

Carbonaceous meteorites are fragments of ancient asteroids that have remained relatively unprocessed since the formation of the Solar System. These carbon-rich objects provide a record of prebiotic chemical evolution and a window on the early Solar System. Many compound classes are present reflecting a rich organic chemical environment during the formation of the planets. Recent theories suggest that similar extraterrestrial organic mixtures may have acted as the starting materials for life on Earth.     

Accretion of the Earth

The origin of the Earth and its Moon has been the focus of an enormous body of research. In this paper I review some of the current models of terrestrial planet accretion, and discuss assumptions common to most works that may require re-examination. Density-wave interactions between growing planets and the gas nebula may help to explain the current near-circular orbits of the Earth and Venus, and may result in large-scale radial migration of proto-planetary embryos. Migration would weaken the link between the present locations of the planets and the original provenance of the material that formed them. Fragmentation can potentially lead to faster accretion and could also damp final planet orbital eccentricities. The Moon-forming impact is believed to be the final major event in the Earth's accretion. Successful simulations of lunar-forming impacts involve a differentiated impactor containing between 0.1 and 0.2 Earth masses, an impact angle near 45 degrees and an impact speed within 10 per cent of the Earth's escape velocity. All successful impacts-with or without pre-impact rotation-imply that the Moon formed primarily from material originating from the impactor rather than from the proto-Earth. This must ultimately be reconciled with compositional similarities between the Earth and the Moon.     

The astrophysics of crowded places

Today the Sun is in a relatively uncrowded place. The distance between it and the nearest other star is relatively large (about 200,000 times the Earth-Sun distance!). This is beneficial to life on Earth; a close encounter with another star is extremely unlikely. Such encounters would either remove the Earth from its orbit around the Sun or leave it on an eccentric orbit similar to a comet's. But the Sun was not formed in isolation. It was born within a more-crowded cluster of perhaps a few hundred stars. As the surrounding gas evaporated away, the cluster itself evaporated too, dispersing its stars into the Galaxy. Virtually all stars in the Galaxy share this history, and here I will describe the role of 'clusterness' in a star's life. Stars are often formed in larger stellar clusters (known as open and globular clusters), some of which are still around today. I will focus on stars in globular clusters and describe how the interactions between stars in these clusters may explain the zoo of stellar exotica which have recently been observed with instruments such as the Hubble Space Telescope and the X-ray telescopes XMM-Newton and Chandra. In recent years, myriad planets orbiting stars other than the Sun--the so-called 'extrasolar' planets--have been discovered. I will describe how a crowded environment will affect such planetary systems and may in fact explain some of their mysterious properties.     

Modelling the glacial isostatic adjustment of the UK region

The glacial isostatic adjustment of the UK region has been considered in a number of recent studies. We have revisited this problem in order to: (i) highlight some key issues with regard to limitations in the ice modelling approach adopted in these studies and (ii) consider the constraints provided from observations of crustal motion available via continuous global positioning system monitoring. With regard to the first aim, we have found that: (i) previous studies have significantly overestimated ice thicknesses in regions where trim line field constraints were adopted and (ii) the duration of the glaciation phase of the UK ice sheet is a critical aspect of the model and that discrepancies in this model component have led to inconsistent inferences of Earth model parameters. With regard to the second aim, we have found that predictions of horizontal velocities (relative to a chosen site) based on a UK ice model calibrated to fit the regional sea-level database capture the geometry of the signal well but only account for 10% of the magnitude (for a range of Earth models).     

Introduction. Progress in astronomy: from gravitational waves to space weather

This brief paper introduces and reviews the 'visions of the future' articles prepared by leading young scientists throughout the world for the first of two Christmas 2008 Triennial issues of Phil. Trans. R. Soc. A, devoted, respectively, to astronomy and Earth science. Contributions in astronomy include the very topical gamma-ray bursts, new ideas on stellar collapse and the unusual atmospheres of synchronized planets orbiting nearby stars.     

Differential resonance-impedance radio pickup with a pulsed bias voltage for plasma diagnostics

A differential radio-frequency pickup with a pulsed bias voltage when investigating the physical characteristics of laboratory and ionospheric plasma of the Earth and planets is considered. It is shown that the use of this pickup simplifies the interpretation of the high-frequency measured signal and gives reliable experimental data.Translated from Izmeritel'naya Tekhnika, No. 9, pp. 55–57, September, 1995.     

Atmospheric dynamics of tidally synchronized extrasolar planets

Tidally synchronized planets present a new opportunity for enriching our understanding of atmospheric dynamics on planets. Subject to an unusual forcing arrangement (steady irradiation on the same side of the planet throughout its orbit), the dynamics on these planets may be unlike that on any of the Solar System planets. Characterizing the flow pattern and temperature distribution on the extrasolar planets is necessary for reliable interpretation of data currently being collected, as well as for guiding future observations. In this paper, several fundamental concepts from atmospheric dynamics, likely to be central for characterization, are discussed. Theoretical issues that need to be addressed in the near future are also highlighted.     

Microgravity Fundamental Physics Program for the New Millennium

In the new millennium, NASA will probe deeper into space, will search for blue planets and life in the universe, will send humans and robots to other planets and beyond, and will bring back space knowledge to enhance quality of life on earth. For the microgravity fundamental physics program to be vital and relevant, it must contribute to this overall endeavor. NASA's Microgravity Fundamental Physics Program has been designed to serve these bigger goals at the onset of the new millennium.     

Planetary heat flow measurements

The year 2005 marks the 35th anniversary of the Apollo 13 mission, probably the most successful failure in the history of manned spaceflight. Naturally, Apollo 13's scientific payload is far less known than the spectacular accident and subsequent rescue of its crew. Among other instruments, it carried the first instrument designed to measure the flux of heat on a planetary body other than Earth. The year 2005 also should have marked the launch of the Japanese LUNAR-A mission, and ESA's Rosetta mission is slowly approaching comet Churyumov-Gerasimenko. Both missions carry penetrators to study the heat flow from their target bodies. What is so interesting about planetary heat flow? What can we learn from it and how do we measure it?Not only the Sun, but all planets in the Solar System are essentially heat engines. Various heat sources or heat reservoirs drive intrinsic and surface processes, causing 'dead balls of rock, ice or gas' to evolve dynamically over time, driving convection that powers tectonic processes and spawns magnetic fields. The heat flow constrains models of the thermal evolution of a planet and also its composition because it provides an upper limit for the bulk abundance of radioactive elements. On Earth, the global variation of heat flow also reflects the tectonic activity: heat flow increases towards the young ocean ridges, whereas it is rather low on the old continental shields. It is not surprising that surface heat flow measurements, or even estimates, where performed, contributed greatly to our understanding of what happens inside the planets. In this article, I will review the results and the methods used in past heat flow measurements and speculate on the targets and design of future experiments.     

Chemical methods for searching for evidence of extra-terrestrial life

This paper describes the chemical concepts used for the purpose of detecting life in extra-terrestrial situations. These methods, developed initially within the oil industry, have been used to determine when life began on Earth and for investigating the Moon and Mars via space missions. In the case of Mars, the Viking missions led to the realization that we had meteorites from Mars on Earth. The study of Martian meteorites in the laboratory provides tantalizing clues for life on Mars in both the ancient and recent past. Meteorite analyses led to the launch of the Beagle 2 spacecraft, which was designed to prove that life-detection results obtained on Earth were authentic and not confused by terrestrial contamination. Some suggestions are made for future work.     

Ozone and life on the Archaean Earth

The trace gas ozone, produced in the present-day stratosphere, acts as a screen for UV radiation between 195 and approximately 290 nm, depending on its column abundance. On the anoxic Archaean Earth, such an ozone screen would not have existed. Although the presence of other screens, such as an organic haze, might have ameliorated the UV radiation flux, even assuming the worst-case scenario (no UV screen), it can be shown that early land masses and the photic zone of the oceans could have been colonized, suggesting that: (i) high UV radiation would not have prevented the colonization of land and (ii) it is unlikely that the fossil record can be used to constrain estimates of the UV radiation environment of the early Earth (although geochemical approaches and the study of extrasolar planetary atmospheres are likely to provide empirical constraints on the early photobiological environment).     

Astrobiology, space and the future age of discovery

Astrobiology is the study of the origins, evolution, distribution and future of life in the Universe, and specifically seeks to understand the origin of life and to test the hypothesis that life exists elsewhere than on Earth. There is a general mathematics, physics and chemistry; that is, scientific laws that obtain on Earth also do so elsewhere. Is there a general biology? Is the Universe life-rich or is Earth an isolated island of biology? Exploration in the Age of Enlightenment required the collection of data in unexplored regions and the use of induction and empiricism to derive models and natural laws. The current search for extra-terrestrial life has a similar goal, but with a much greater amount of data and with computers to help with management, correlations, pattern recognition and analysis. There are 60 active space missions, many of them aiding in the search for life. There is not a universally accepted definition of life, but there are a series of characteristics that can aid in the identification of life elsewhere. The study of locations on Earth with similarities to early Mars and other space objects could provide a model that can be used in the search for extra-terrestrial life.     

Materials science issues in the Earth and planetary sciences

Earth is a dynamic planet. Solid state convection in the deep interior is coupled to the motion of about a dozen rigid plates at the surface. Earthquakes, volcanoes and mountains are located mainly at the boundaries between plates and reflect the relative motion between them. The associated deformation processes span a wide range of regimes from high temperature dislocation and diffusion accommodated creep to brittle fracture, friction, fragmentation and granular flow. There is a long history of collaboration between earth and materials scientists in modeling the relevant micromechanics and formulating appropriate constitutive relations. Materials analysis in the Earth and planetary sciences pose special challenges. Pressure and temperature conditions in the Earth's interior reach 360 GPa and 8000 K so that constitutive equations must often be extended to pressure and temperature regimes well beyond laboratory limits. Deformation occurs over a range of temporal and spatial scales difficult to simulate in the laboratory. The emergence of deformation structures spanning many spatial orders of magnitude has made Earth sciences the test bed for modern ideas of self-organization and scaling. Finally, deformation mechanisms in earth materials are extremely sensitive to environmental factors, especially water. This factor alone explains most differences between large-scale deformation structures observed on Earth and those on the other terrestrial planets. Current problems in the Earth sciences that require a better understanding of material behavior include the mechanics of the earthquake instability, the migration of magma in the crust, the source and dynamical significance of observed heterogeneity in the deep interior, and the generation of the magnetic field.     

Searching for a shadow biosphere on Earth as a test of the 'cosmic imperative'

Estimates for the number of communicating civilizations in the galaxy, based on the so-called Drake equation, are meaningless without a plausible estimate for the probability that life will emerge on an Earth-like planet. In the absence of a theory of the origin of life, that number can be anywhere from 0 to 1. Distinguished scientists have been known to argue that life on Earth is a freak accident, unique in the observable universe and, conversely, that life is almost bound to arise in the course of time, given Earth-like conditions. De Duve, adopting the latter position, coined the phrase that 'life is a cosmic imperative'. De Duve's position would be immediately verified if we were to discover a second sample of life that we could be sure arose from scratch independently of known life. Given the current absence of evidence for life beyond Earth, the best way to test the hypothesis of the cosmic imperative is to see whether terrestrial life began more than once. If it did, it is possible that descendants of a second genesis might be extant, forming a sort of 'shadow biosphere' existing alongside, or perhaps interpenetrating, the known biosphere. I outline a strategy to detect the existence of such a shadow biosphere.     

Methods for determining regularization for atmospheric retrieval problems

The atmosphere of Earth has already been investigated by several spaceborne instruments, and several further instruments will be launched, e.g., NASA's Earth Observing System Aura platform and the European Space Agency's Environmental Satellite. To stabilize the results in atmospheric retrievals, constraints are used in the iteration process. Therefore hard constraints (discretization of the retrieval grid) and soft constraints (regularization operators) are included in the retrieval. Tikhonov regularization is often used as a soft constraint. In this study, different types of Tikhonov operator were compared, and several new methods were developed to determine the optimal strength of the constraint operationally. The resulting regularization parameters were applied successfully to an ozone retrieval from simulated nadir sounding spectra like those expected to be measured by the Tropospheric Emission Spectrometer, which is part of the Aura platform. Retrievals were characterized by means of estimated error, averaging kernel, vertical resolution, and degrees of freedom.     

Cosmochemistry of thorium, samarium, and cesium in the substance of ordinary chondrites and problem of disposal of actinide and fission product concentrates in the Earth crust

The hydrothermal treatment of the substance of unequilibrated Krymka chondrite is studied. This substance is a sample of the oldest basalts of the Earth group planets and is considered as a natural analog of matrices for immobilization of Th, Sm, and Cs. It is shown that Th and Sm are mainly fixed in phosphates, whereas Cs becomes a scattered element. In equilibrated chondrites, Cs is found in feldspars. A sol-gel synthesis procedure was developed, and tests were performed for ferro(alumino)silicophosphate matrices for immobilization of actinides and fission products. The corrosion resistance of the matrices with respect to Cs leaching appeared to be higher by an order of magnitude than that of the best samples of borosilicate glass. Suggestions are made on optimizing the isolation from the biosphere of concentrates of long-lived high-level nuclear wastes.     

Experiments on metal-silicate plumes and core formation

Short-lived isotope systematics, mantle siderophile abundances and the power requirements of the geodynamo favour an early and high-temperature core-formation process, in which metals concentrate and partially equilibrate with silicates in a deep magma ocean before descending to the core. We report results of laboratory experiments on liquid metal dynamics in a two-layer stratified viscous fluid, using sucrose solutions to represent the magma ocean and the crystalline, more primitive mantle and liquid gallium to represent the core-forming metals. Single gallium drop experiments and experiments on Rayleigh-Taylor instabilities with gallium layers and gallium mixtures produce metal diapirs that entrain the less viscous upper layer fluid and produce trailing plume conduits in the high-viscosity lower layer. Calculations indicate that viscous dissipation in metal-silicate plumes in the early Earth would result in a large initial core superheat. Our experiments suggest that metal-silicate mantle plumes facilitate high-pressure metal-silicate interaction and may later evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on the Earth and possibly on other terrestrial planets.     

Rigorous global optimization of impulsive planet-to-planet transfers in the patched-conics approximation

The rigorous solution of a generic impulsive planet-to-planet transfer by means of a Taylor-model-based global optimizer is presented. Although a planet-to-planet transfer represents the simplest case of interplanetary transfer, its formulation and solution is a challenging task when the rigorous global optimum is sought. A customized ephemeris function is derived from JPL DE405 to allow the Taylor-model evaluation of planets’ positions and velocities. Furthermore, the validated solution of Lambert's problem is addressed for the rigorous computation of transfer fuel consumption. The optimization problem, which consists in finding the optimal launch and transfer time to minimize the required fuel mass, is complex due to the abundance of local minima and relatively high search-space dimension. Its rigorous solution by means of the Taylor-model-based global optimizer COSY-GO is presented considering Earth–Mars and Earth–Venus transfers as test cases.     

Life as a cosmic imperative?

The origin of life on Earth may be divided into two stages separated by the first appearance of replicable molecules, most probably of RNA. The first stage depended exclusively on chemistry. The second stage likewise involved chemistry, but with the additional participation of selection, a necessary concomitant of inevitable replication accidents. Consideration of these two processes suggests that the origin of life may have been close to obligatory under the physical-chemical conditions that prevailed at the site of its birth. Thus, an extrasolar planet in which those conditions were replicated appears as a probable site for the appearance of extra-terrestrial life.     

The Planetary and Space Simulation Facilities at DLR Cologne

Astrobiology strives to increase our knowledge on the origin, evolution and distribution of life, on Earth and beyond. In the past centuries, life has been found on Earth in environments with extreme conditions that were expected to be uninhabitable. Scientific investigations of the underlying metabolic mechanisms and strategies that lead to the high adaptability of these extremophile organisms increase our understanding of evolution and distribution of life on Earth. Life as we know it depends on the availability of liquid water. Exposure of organisms to defined and complex extreme environmental conditions, in particular those that limit the water availability, allows the investigation of the survival mechanisms as well as an estimation of the possibility of the distribution to and survivability on other celestial bodies of selected organisms. Space missions in low Earth orbit (LEO) provide access for experiments to complex environmental conditions not available on Earth, but studies on the molecular and cellular mechanisms of adaption to these hostile conditions and on the limits of life cannot be performed exclusively in space experiments. Experimental space is limited and allows only the investigation of selected endpoints. An additional intensive ground based program is required, with easy to access facilities capable to simulate space and planetary environments, in particular with focus on temperature, pressure, atmospheric composition and short wavelength solar ultraviolet radiation (UV). DLR Cologne operates a number of Planetary and Space Simulation facilities (PSI) where microorganisms from extreme terrestrial environments or known for their high adaptability are exposed for mechanistic studies. Space or planetary parameters are simulated individually or in combination in temperature controlled vacuum facilities equipped with a variety of defined and calibrated irradiation sources. The PSI support basic research and were recurrently used for pre-flight test programs for several astrobiological space missions. Parallel experiments on ground provided essential complementary data supporting the scientific interpretation of the data received from the space missions.