Solar system exploration. Lab rivalry spices up solar system exploration.
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The early evolution of the inner solar system: a meteoritic perspective.
Formation of the solar system may have been triggered by a stellar wind. From then on, the solar system would have followed a conventional evolutionary path, including the formation of a disk and bipolar jets. The now extinct short-lived radionuclides beryllium-10 and, possibly, manganese-53 that were present in meteorites probably resulted from energetic particle irradiation within the solar system. Calcium-aluminum-rich inclusions (the oldest known solar system solids) and chondrules could have been produced by the bipolar jets, but it is more likely that they formed during localized events in the asteroid belt. The chondritic meteorites formed within the temperature range (100 to 400 kelvin) inferred for the midplane of classical T Tauri disks at 2 to 3 astronomical units from their central stars. However, these meteorites may retain a chemical memory of earlier times when midplane temperatures were much higher. Dissipation of the solar nebula occurred within a few million years of solar system formation, whereas differentiation of asteroidal-sized bodies occurred within 5 to 15 million years. The terrestrial planets took approximately 100 million years to form. Consequently, they would have accreted already differentiated bodies, and their final assembly was not completed until after the solar nebula had dispersed. This implies that water-bearing asteroids and/or icy planetesimals that formed near Jupiter are the likely sources of Earth's water.
Journal Article↗
The roles of humans and robots in exploring the solar system.
Historically, advocates of solar system exploration have disagreed over whether program goals could be entirely satisfied by robotic missions. Scientists tend to argue that robotic exploration is most cost-effective. However, the human space program has a great deal of support in the general public, thereby enabling the scientific element of exploration to be larger than it might be as a stand-alone activity. A comprehensive strategy of exploration needs a strong robotic component complementing and supporting human missions. Robots are needed for precursor missions, for crew support on planetary surfaces, and for probing dangerous environments. Robotic field assistants can provide mobility, access to scientific sites, data acquisition, visualization of the environment, precision operations, sample acquisition and analysis, and expertise to human explorers. As long as space exploration depends on public funds, space exploration must include an appropriate mix of human and robotic activity.
Humans↗
Solid organic matter in the atmosphere and on the surface of outer Solar System bodies.
Many bodies in the outer Solar System display the presence of low albedo materials. These materials, evident on the surface of asteroids, comets, Kuiper Belt objects and their intermediate evolutionary step, Centaurs, are related to macromolecular carbon bearing materials such as polycyclic aromatic hydrocarbons and organic materials such as methanol and related light hydrocarbons, embedded in a dark, refractory, photoprocessed matrix. Many planetary rings and satellites around the outer gaseous planets display such component materials. One example, Saturn's largest satellite, Titan, whose atmosphere is comprised of around 90% molecular nitrogen N2 and less than 10% methane CH4, displays this kind of low reflectivity material in its atmospheric haze. These materials were first recorded during the Voyager 1 and 2 flybys of Titan and showed up as an optically thick pinkish orange haze layer. These materials are broadly classified into a chemical group whose laboratory analogs are termed "tholins", after the Greek word for "muddy". Their analogs are produced in the laboratory via the irradiation of gas mixtures and ice mixtures by radiation simulating Solar ultraviolet (UV) photons or keV charged particles simulating particles trapped in Saturn's magnetosphere. Fair analogs of Titan tholin are produced by bombarding a 9:1 mixture of N2:CH4 with charged particles and its match to observations of both the spectrum and scattering properties of the Titan haze is very good over a wide range of wavelengths. In this paper, we describe the historical background of laboratory research on this kind of organic matter and how our laboratory investigations of Titan tholin compare. We comment on the probable existence of polycyclic aromatic hydrocarbons in the Titan Haze and how biological and nonbiological racemic amino acids produced from the acid hydrolysis of Titan tholins make these complex organic compounds prime candidates in the evolution of terrestrial life and extraterrestrial life in our own Solar System and beyond. Finally, we also compare the spectrum and scattering properties of our resulting tholin mixtures with those observed on Centaur 5145 Pholus and the dark hemisphere of Saturn's satellite Iapetus in order to demonstrate the widespread distribution of similar organics throughout the Solar System.
Ammonia↗
Extreme oxygen isotope ratios in the early Solar System.
The origins of the building blocks of the Solar System can be studied using the isotopic composition of early planetary and meteoritic material. Oxygen isotopes in planetary materials show variations at the per cent level that are not related to the mass of the isotopes; rather, they result from the mixture of components having different nucleosynthetic or chemical origins. Isotopic variations reaching orders of magnitude in minute meteoritic grains are usually attributed to stellar nucleosynthesis before the birth of the Solar System, whereby different grains were contributed by different stars. Here we report the discovery of abundant silica-rich grains embedded in meteoritic organic matter, having the most extreme 18O/16O and 17O/16O ratios observed (both approximately 10(-1)) together with a solar silicon isotopic composition. Both O and Si isotopes indicate a single nucleosynthetic process. These compositions can be accounted for by one of two processes: a single exotic evolved star seeding the young Solar System, or irradiation of the circumsolar gas by high energy particles accelerated during an active phase of the young Sun. We favour the latter interpretation, because the observed compositions are usually not expected from nucleosynthetic processes in evolved stars, whereas they are predicted by the selective trapping of irradiation products.
Journal Article↗
Widespread magma oceans on asteroidal bodies in the early Solar System.
Immediately following the formation of the Solar System, small planetary bodies accreted, some of which melted to produce igneous rocks. Over a longer timescale (15-33 Myr), the inner planets grew by incorporation of these smaller objects through collisions. Processes operating on such asteroids strongly influenced the final composition of these planets, including Earth. Currently there is little agreement about the nature of asteroidal igneous activity: proposals range from small-scale melting, to near total fusion and the formation of deep magma oceans. Here we report a study of oxygen isotopes in two basaltic meteorite suites, the HEDs (howardites, eucrites and diogenites, which are thought to sample the asteroid 4 Vesta) and the angrites (from an unidentified asteroidal source). Our results demonstrate that these meteorite suites formed during early, global-scale melting (> or = 50 per cent) events. We show that magma oceans were present on all the differentiated Solar System bodies so far sampled. Magma oceans produced compositionally layered planetesimals; the modification of such bodies before incorporation into larger objects can explain some anomalous planetary features, such as Earth's high Mg/Si ratio.
Journal Article↗
Criteria for the emergence and evolution of life in the solar system.
During the past years we have explored most of the bodies of the solar system by means of the Apollo, Venera, Viking, Voyager, and other space missions. We are now in a better position to be able to compare the conditions of other planets and satellites with those of the Earth in order to determine what is unique about our planet which permitted the emergence and evolution of life on it. On the basis of this and other available scientific information we have arrived at the conclusion that there are at least some twentyfive specific conditions or requirements which have to be fulfilled in order for life as we know it to appear and evolve in a planetary system such as ours. Most of these necessary conditions or requirements are mutually interdependent, but in order to discuss their role in depth they have been divided into five major general areas which are discussed in some detail herein. Planetary criteria, which relate to the physical properties of the planet as it is formed and as it becomes a differentiated cosmic body and potential abode of life. The mass, orbital characteristics and energetic relationships with the central star as well as the discrete separation of gas, liquid and solid phases of the planet are of utmost importance. Chemical criteria, which are concerned with the composition, availability of effective energy sources, and chemical constraints (solvent, pH range, redox potential) of the environment(s) where reactions take place for the prebiological formation of biochemical compounds. Protobiological criteria, which relate to the prebiologically synthesized oligomeric and polymeric biomolecules, how they interact cooperatively to form protobiological structures and functions (replication, catalysis, information transfer, etc.) and self-assemble to give rise to a living system. Evolutionary criteria, which are concerned with the processes responsible for the increase in complexity of organisms by genomic multiplication, symbiotic integration and cellular differentiation, as well as with the negentropic ability of organisms to continuously recycle all the volatile biogenic elements. Altogether these processes made possible the development and evolution of life from the simplest prokaryotic cell ancestor to a cognitive and manipulative multicellular organism (man). In order to extend this inquiry to other systems beyond our solar system a fifth set of requirements based on astronomical observations is also discussed, namely, the Stellar criteria, which relate to the elemental composition mass, lifetime, and other features of Main Sequence stars which may be surrounded by planetary systems similar to our own. Finally, a brief review is made on the probability of the existence of extraterrestrial life as well as of civilizations capable of interstellar communication in our Galaxy.
Biological Evolution↗
From organic chemistry in small bodies of the solar system to low temperature chemistry in the universe. Preface.
A COSPAR two days Symposium has been dedicated to "Prebiotic chemistry in Space" at the COSPAR Plenary Meeting, (Hamburg, Germany, July 1994). This Symposium was jointly organized by Commissions B (Space studies of the Earth-Moon system, planets and small bodies of the solar system) and F (Life sciences as related to space). Its goal was to review, from an interdisciplinary approach, our knowledge on organic and prebiotic chemistry in small bodies of the Solar System, and on low temperature chemistry and exobiology. The Symposium was sponsored by COSPAR and the IAU (session 1), ESA, NASA, and ISSOL (session 2).
Cosmic Dust↗
Time travel and chemical evolution: a look at the outer solar system.
Many space scientists think that the chemical conditions today on planets and moons of the outer solar system are similar to conditions on Earth soon after it formed. If so, we can learn much about the chemistry that led to life on this planet. We can also speculate about exotic habitats that might have given rise to other types of life. And if we are able to discern the chemical reactions now occurring in the outer solar system, we may be able to extrapolate these rules to other solar systems, and so define the habitable zones around other stars where the potential for life is high.
Chemical Phenomena↗
Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system.
Methane clathrate is expected to be an important carbon-containing ice in the outer solar system. We investigate the effect of electron irradiation by coronal discharge on several simple hydrocarbons enclathrated in or mixed with H2O or H2O+NH3 in simulation of the effects of the solar wind, planetary magnetospheric particles, and cosmic rays on surfaces containing these ices in the outer solar system and interstellar space. H2O+CH4 clathrate, H2O+C2H6, H2O+CH4+NH3, H2O+C2H6+NH3, and H2O+C2H2 are all initially white ices, and all produce yellowish to brownish organic products upon charged particles irradiation. Significant coloration occurs with doses of 10(9) erg cm-2, corresponding to short interplanetary irradiation times. Uranian magnetospheric electrons penetrate to approximately 1 mm depth and deposit this dose in 8, 30, 65, 200, and 500 years into the surfaces of Miranda, Ariel, Umbriel, Titania, and Oberon, respectively. Further irradiation of the laboratory ice surface results in a progressive darkening and a more subdued color. For a conversion efficiency to solids G approximately equal to 1 molecule keV-1, the upper limit for the time for total destruction of CH4 and other simple hydrocarbons in the upper 1 mm is 5 x 10(4) years (Miranda) to 3 x 10(6) years (Oberon). Remote detection of CH4 is possible only when its replenishment rate exceeds the destruction rate at the depth probed by spectroscopy. Reflection spectroscopy or irradiated H2O+CH4 frost is compared with the spectra of several outer solar system objects and to other relevant organic and inorganic materials. Ultraviolet-visible and infrared transmission spectroscopy of the postirradiation residues is presented. Persistence of color and of CH4 ice bands on Triton and Pluto suggests ongoing surface activity and/or atmospheric haze. Over 4 x 10(9) year time scales, > or = 10 m of satellite and cometary surface material is processed by cosmic rays to a radiation-hardened ice-tholin mixture devoid of CH4. Preaccretional chemistry, exogenous materials, and endogenous organic chemistry all contribute to the spectral properties of icy satellites which accreted simple CH(O) molecules. Radiation darkening traces the deposition of mobilized or impact-exposed carbon-bearing volatiles on these satellites. More exhaustive experiments are necessary to work out the detailed relationships between initial composition, exposure age, and color/albedo.
Ammonia↗
The role of chaotic resonances in the Solar System.
Our understanding of the Solar System has been revolutionized over the past decade by the finding that the orbits of the planets are inherently chaotic. In extreme cases, chaotic motions can change the relative positions of the planets around stars, and even eject a planet from a system. Moreover, the spin axis of a planet-Earth's spin axis regulates our seasons-may evolve chaotically, with adverse effects on the climates of otherwise biologically interesting planets. Some of the recently discovered extrasolar planetary systems contain multiple planets, and it is likely that some of these are chaotic as well.
Journal Article↗
Possible in situ formation of meteoritic nanodiamonds in the early Solar System.
Grains of dust that pre-date the Sun provide insights into their formation around other stars and into the early evolution of the Solar System. Nanodiamonds recovered from meteorites, which originate in asteroids, have been thought to be the most abundant type of presolar grain. If that is true, then nanodiamonds should be at least as abundant in comets, because they are thought to have formed further out in the early Solar System than the asteroid parent bodies, and because they should be more pristine. Here we report that nanodiamonds are absent or very depleted in fragile, carbon-rich interplanetary dust particles, some of which enter the atmosphere at speeds within the range of cometary meteors. One interpretation of the results is that some (perhaps most) nanodiamonds formed within the inner Solar System and are not presolar at all, consistent with the recent detection of nanodiamonds within the accretion discs of other young stars. An alternative explanation is that all meteoritic nanodiamonds are indeed presolar, but that their abundance decreases with heliocentric distance, in which case our understanding of large-scale transport and circulation within the early Solar System is incomplete.
Journal Article↗
Short-lived nuclides in hibonite grains from Murchison: evidence for solar system evolution.
Records of now-extinct short-lived nuclides in meteorites provide information about the formation and evolution of the solar system. We have found excess 10B that we attribute to the decay of short-lived 10Be (half-life 1.5 million years) in hibonite grains from the Murchison meteorite. The grains show no evidence of decay of two other short-lived nuclides-26Al (half-life 700,000 years) and 41Ca (half-life 100,000 years)-that may be present in early solar system solids. One plausible source of the observed 10Be is energetic particle irradiation of material in the solar nebula. An effective irradiation dose of approximately 2 x 10(18) protons per square centimeter with a kinetic energy of >/=10 megaelectronvolts per atomic mass unit can explain our measurements. The presence of 10Be, coupled with the absence of 41Ca and 26Al, may rule out energetic particle irradiation as the primary source of 41Ca and 26Al present in some early solar system solids and strengthens the case of a stellar source for 41Ca and 26Al.
Journal Article↗
Extinct (129)I in halite from a primitive meteorite: evidence for evaporite formation in the early solar system
Halite crystals from the Zag H3-6 chondrite contain essentially pure (monoisotopic) xenon-129 ((129)Xe) produced in the early history of the solar system by the decay of short-lived iodine-129 ((129)I) (half-life = 15.7 million years). Correlated release of (129)Xe and (128)Xe, produced artificially from (127)I by neutron irradiation, corresponds to an initial ((129)I/(127)I) ratio of (1.35 +/- 0.05) x 10(-4), close to the most primitive early solar system value. If the (129)Xe was produced by in situ decay, then the halite formed from an aqueous fluid within 2 million years of the oldest known solar system minerals.
Journal Article↗
The early solar system.
Life arose on an early Earth which was the product to the conditions present, and processes operating, during formation of the solar system. The formation and early state of the solar system are reviewed in order to better understand the nature of the early Earth, and to constrain the conditions present during the origin and early evolution of life on this planet.
Cold Temperature↗
The origin of chaos in the outer solar system
Classical analytic theories of the solar system indicate that it is stable, but numerical integrations suggest that it is chaotic. This disagreement is resolved by a new analytic theory. The theory shows that the chaos among the jovian planets results from the overlap of the components of a mean motion resonance among Jupiter, Saturn, and Uranus, and provides rough estimates of the Lyapunov time (10(7) years) and the dynamical lifetime of Uranus (10(18) years). The jovian planets must have entered the resonance after all the gas and most of the planetesimals in the protoplanetary disk were removed.
Journal Article↗
Niobium-zirconium chronometry and early solar system development.
Niobium-92 (92Nb) decays to zirconium-92 (92Zr) with a half-life of 36 million years and can be used to place constraints on the site of p-process nucleosynthesis and the timing of early solar system processes. Recent results have suggested that the initial 92Nb/93Nb of the solar system was high (>10(-3)). We report Nb-Zr internal isochrons for the ordinary chondrite Estacado (H6) and a clast of the mesosiderite Vaca Muerta, both of which define an initial 92Nb/93Nb ratio of approximately 10(-5). Therefore, the solar system appears to have started with a ratio of <3 x 10(-5), which implies that Earth's initial differentiation need not have been as protracted as recently suggested.
Journal Article↗
Band structure of the solar system: an objective test of the grouping of planets and satellites.
Alfvén in his early work on the origin of the solar system (1942-1946) noted a pronounced band structure in the gravitational potential distribution of secondary bodies, and suggested this feature to be directly related to the formation process. When the critical velocity phenomenon was later discovered, a close agreement was found between the planet-satellite bands on one hand, and the critical velocity limits of the major compound elements in the interstellar medium on the other, suggesting a specific emplacement mechanism for the dusty plasma which presumably constituted the solar nebula. Since the originally perceived band structure was outlined in a qualitative fashion, an attempt is made here to analyze the distribution by a statistical technique, testing the significance of clustering of the observational data in the bands. The results show that, with proper scaling of the parameters, such a band structure indeed appears, with features closely similar to those originally conceived. Some deviations are indicated by the cluster analysis, however; their significance is discussed in terms of process involved in the formation of the solar system.
Astronomical Phenomena↗