共查询到20条相似文献,搜索用时 31 毫秒
1.
2.
The properties of magnetohydrodynamic waves and instabilities of laboratory and space plasmas are determined by the overall
magnetic confinement geometry and by the detailed distributions of the density, pressure, magnetic field, and background velocity
of the plasma. Consequently, measurement of the spectrum of MHD waves (MHD spectroscopy) gives direct information on the internal
state of the plasma, provided a theoretical model is available to solve the forward as well as the inverse spectral problems.
This terminology entails a program, viz. to improve the accuracy of our knowledge of plasmas, both in the laboratory and in
space. Here, helioseismology (which could be considered as one of the forms of MHD spectroscopy) may serve as a luminous example.
The required study of magnetohydrodynamic waves and instabilities of both laboratory and space plasmas has been conducted
for many years starting from the assumption of static equilibrium. Recently, there is a outburst of interest for plasma states
where this assumption is violated. In fusion research, this interest is due to the importance of neutral beam heating and
pumped divertor action for the extraction of heat and exhaust needed in future tokamak reactors. Both result in rotation of
the plasma with speeds that do not permit the assumption of static equilibrium anymore. In astrophysics, observations in the
full range of electromagnetic radiation has revealed the primary importance of plasma flows in such diverse situations as
coronal flux tubes, stellar winds, rotating accretion disks, and jets emitted from radio galaxies. These flows have speeds
which substantially influence the background stationary equilibrium state, if such a state exists at all. Consequently, it
is important to study both the stationary states of magnetized plasmas with flow and the waves and instabilities they exhibit.
We will present new results along these lines, extending from the discovery of gaps in the continuous spectrum and low-frequency
Alfvén waves driven by rotation to the nonlinear flow patterns that occur when the background speed traverses the full range
from sub-slow to super-fast.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
3.
R. L. Moore 《Space Science Reviews》1981,28(4):387-421
The empirical properties of the various dynamic phenomena are reviewed and interrelated with emphasis on recent observational results. The topics covered are:
- Introduction
- Aperiodic Phenomena
- Externally Driven Phenomena
- Umbral Flares
- Inverse Evershed Flow
- Internally Driven Phenomena
- Penumbra
- Penumbral Grains
- Evershed Flow
- Umbra
- Umbral Dots
- Inhomogeneity of the Umbral Magnetic Field
- Umbral Turbulence
- Oscillations and Waves
- Chromosphere
- Umbra: Oscillations and Flashes
- Penumbra: Running Waves and Dark Puffs
- Photosphere
- Overview
4.
Magnetohydrodynamic (MHD) theory has been used in space physics for more than forty years, yet many important questions about space plasmas remain unanswered. We still do not understand how the solar wind is accelerated, how mass, momentum and energy are transported into the magnetosphere and what mechanisms initiate substorms. Questions have been raised from the beginning of the space era whether MHD theory can describe correctly space plasmas that are collisionless and rarely in thermal equilibrium. Ideal MHD fluids do not induce electromotive force, hence they lose the capability to interact electromagnetically. No currents and magnetic fields are generated, rendering ideal MHD theory not very useful for space plasmas. Observations from the plasma sheet are used as examples to show how collisionless plasmas behave. Interpreting these observations using MHD and ideal MHD concepts can lead to misleading conclusions. Notably, the bursty bulk flows (BBF) with large mean velocities left(〈 v 〉≥400 km s right) that have been interpreted previously as E×B flows are shown to involve much more complicated physics. The sources of these nonvanishing 〈 v 〉 events, while still not known, are intimately related to mechanisms that create large phase space gradients that include beams and acceleration of ions to MeV energies. The distributions of these nonvanishing 〈 v 〉 events are associated with large amplitude variations of the magnetic field at frequencies up to and exceeding the local Larmor frequency where MHD theory is not valid. Understanding collisionless plasma dynamics such as substorms in the plasma sheet requires the self-consistency that only kinetic theory can provide. Kinetic modeling is still undergoing continual development with many studies limited to one and two dimensions, but there is urgent need to improve these models as more and more data show kinetic physics is fundamentally important. Only then will we be able to make progress and obtain a correct picture of how collisionless plasmas work in space. 相似文献
5.
O. Alexandrova C. H. K. Chen L. Sorriso-Valvo T. S. Horbury S. D. Bale 《Space Science Reviews》2013,178(2-4):101-139
Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: (i) weakness of collisional dissipation and (ii) presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field, with the Kolmogorov scaling $k_{\perp}^{-5/3}$ for the perpendicular cascade and $k_{\|}^{-2}$ for the parallel one. Solar wind turbulence is compressible in nature: density fluctuations at MHD scales have the Kolmogorov spectrum. Velocity fluctuations do not follow magnetic field ones: their spectrum is a power-law with a ?3/2 spectral index. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuation spectrum has a local flattening. Around ion scales, magnetic spectra are variable and ion instabilities occur as a function of the local plasma parameters. Between ion and electron scales, a small scale turbulent cascade seems to be established. It is characterized by a well defined power-law spectrum in magnetic and density fluctuations with a spectral index close to ?2.8. Approaching electron scales, the fluctuations are no more self-similar: an exponential cut-off is usually observed (for time intervals without quasi-parallel whistlers) indicating an onset of dissipation. The small scale inertial range between ion and electron scales and the electron dissipation range can be together described by $\sim k_{\perp}^{-\alpha}\exp(-k_{\perp}\ell_{d})$ , with α?8/3 and the dissipation scale ? d close to the electron Larmor radius ? d ?ρ e . The nature of this small scale cascade and a possible dissipation mechanism are still under debate. 相似文献
6.
In previous publications (Keppens et al.: 2002, Astrophys. J. 569, L121; Goedbloed et al.: 2004a, Phys. Plasmas
11, 28), we have demonstrated that stationary rotation of magnetized plasma about a compact central object permits an enormous
number of different MHD instabilities, with the well-known magneto-rotational instability (Velikhov, E. P.: 1959, Soviet Phys.–JETP Lett. 36, 995; Chandrasekhar, S.: 1960, Proc. Natl. Acad. Sci. U.S.A. 46, 253; Balbus, S. A. and Hawley, J. F.: 1991, Astrophys. J. 376, 214) as just one of them. We here concentrate on the new instabilities found that are driven by transonic transitions of
the poloidal flow. A particularly promising class of instabilities, from the point of view of MHD turbulence in accretion
disks, is the class of trans-slow Alfv’en continuum modes, that occur when the poloidal flow exceeds a critical value of the slow magnetosonic speed. When this happens, virtually
every magnetic/flow surface of the disk becomes unstable with respect to highly localized modes of the continuous spectrum.
The mode structures rotate, in turn, about the rotating disk. These structures lock and become explosively unstable when the
mass of the central object is increased beyond a certain critical value. Their growth rates then become huge, of the order
of the Alfv’en transit time. These instabilities appear to have all requisite properties to facilitate accretion flows across
magnetic surfaces and jet formation. 相似文献
7.
Harrison H. Schmitt 《Space Science Reviews》1975,18(3):259-279
The geology of the decade of Apollo and Luna probably will become one of the fundamental turning points in the history of all science. For the first time, the scientists of the Earth have been presented with the opportunity to interpret their home planet through the direct investigations of another. Mankind can be proud and take heart in this fact. The interpretive evolution of the Moon can be divided now into seven major stages beginning sometime near the end of the formation of the solar system. These stages and their approximate durations in time are as follows:
- The Beginning — 4.6 billion years ago.
- The Melted Shell — 4.6–4.4 billion years ago.
- The Cratered Highlands — 4.4–4.1 billion years ago.
- The Large Basins — 4.1–3.9 billion years ago.
- The Light-colored Plains — 3.9–3.8 billion years ago.
- The Basaltic Maria — 3.8–3.0 (?) billion years ago.
- The Quiet Crust — 3.0 (?) billion years ago to the present.
8.
Charles H. Aldrich 《Space Science Reviews》1985,42(1-2):131-144
As problems we are interested in become more complex, we often find our simulations stretching the limits of available computer resources. For example, an interesting problem is simulation of dissipation processes in sub-critical collisionless shocks. To simulate this system our simulation box must contain the shock and its upstream and downstream regions over the entire length of a run. If the shock moves with any appreciable speed the box must then be considerably larger than the shock thickness making it hard to resolve the shock front itself with a reasonable number of grid points. A solution to this problem is to run the simulation in the frame of reference of the shock. Particles are injected upstream of the shock and leave the simulation box downstream. With the shock stationary in the simulation box, we only need to contain enough of the up and downstream regions for the fields, etc., to settle down and separate the shock from the box boundaries. In this tutorial we consider some basic algorithms used in a practical particle injection code, such as the two dimensional WAVE code used at Los Alamos. We will try to present these ideas in a simple format general enough to be easily included in any particle code. Topics covered are:
- Smoothly Injecting Particles.
- Generating the Distribution Functions.
- Time Dependent Injection Density.
- Boundary Conditions on Fields and Particles.
9.
J. R. Barcus 《Space Science Reviews》1972,13(2):295-312
Analysis of recent observations (from balloons, spacecraft, and surface observatories) demonstrate regional, shell, and nearpoint conjugacy at L ~ 7 during precipitative events which were characterized by local acceleration as well as release of gradient-drifted electrons injected during substorms. A number of new features of magnetospheric dynamics relating to substorm development and sudden-commencement effects, have been brought to light which, though poorly understood at present, may prove of considerable importance and are worthy of further investigation.
- During the initial period of instability in substorm evolution, preceding the slower magnetotail convective injection, precipitation of waves of electrons in rapid polewards motion exhibit L-shell conjugacy near midnight.
- Transient, large scale expansions of the magnetospheric electron population accompanied by temporally imbedded substorms display large scale regional conjugacy and are simultaneously observed as similarly transient intensity dropouts at balloon altitudes.
- Precipitation from gradient-drifting electrons in the dayside magnetosphere exhibits near point-conjugacy, at least down to the order of 50 km and quite probably less.
- Analysis of the approach to and attainment of spectral equilibrium in the precipitation observed from drifting electrons may provide information about either, or both, the source spectrum at injection and the process of local release.
- The specific precipitation effect sometimes observed at the time of an SC remains a rather puzzling feature, although it seems clear now that the acceleration and/or release process responsible is of a highly local nature and works selectively at small pitch angles well within the magnetospheric boundary. Coupling of the interplanetary shock with the magnetosphere must be an important aspect, but the details are not clear as yet.
- On at least one occasion, a large part (perhaps all) of the magnetospheric electron population varied in a nearly synchronous manner in response to solar wind induced distortions during the variable compressive phase of a sudden commencement geomagnetic storm.
10.
T. K. Breus 《Space Science Reviews》1982,32(3):361-376
The planned missions to Comet Halley, which will arrive at the nearest space of the Sun in 1986, have recently revived interest in studying solar wind interaction with comets. Several unsolved problems exist and the most urgent of them are as follows:
- The character of the solar wind interaction with comets: bow shocks and contact surface formation near comets; similarities and differences of solar- wind interaction with comets and with Venus. The differences are probably associated with a great extension of neutral atmospheres of comets (due to a practical lack of cometary gravitation) and the ‘loading’ of the solar wind flux by cometary ions during the interaction.
- The anomalous ionization in cometary heads.
- The problem of the anamalously high accelerations of ions in the plasma tails of comets.
- The variability of plasma structures observed in cometary tails.
11.
V. I. Moroz 《Space Science Reviews》1981,29(1):3-127
The investigations of Venus take a special position in planetary researches. It was just the atmosphere of Venus where first measurements in situ were carried out by means of the equipment delivered by a space probe (Venera 4, 1967). Venus appeared to be the first neighbor planet whose surface had been seen by us in the direct nearness made possible by means of the phototelevision device (Venera 9 and Venera 10, 1975). The reasons for the high interest in this planet are very simple. This planet is like the Earth by its mass, size and amount of energy obtained from the Sun and at the same time it differs sharply by the character of its atmosphere and climate. We hope that the investigations of Venus will lead us to define more precisely the idea of complex physical and physical-chemical processes which rule the evolution of planetary atmospheres. We hope to learn to forecast this evolution and maybe, in the far future, to control it. The last expeditions to Venus carried out in 1978 — American (Pioneer-Venus) and Soviet (Venera 11 and 12) — brought much news and it is interesting to sum up the results just now. The contents of this review are:
- The planet Venus — basic astronomical data.
- Chemical composition.
- Temperature, pressure, density (from 0 to 100 km).
- Clouds.
- Thermal regime and greenhouse effect.
- Dynamics.
- Chemical processes.
- Upper atmosphere.
- Origin and evolution.
- Problems for future studies
12.
Katia Ferrière 《Space Science Reviews》2006,122(1-4):247-253
We present a theoretical overview of low-frequency waves and instabilities in collisionless, multi-component plasmas with
gyrotropic (
) thermal pressure. We show that the complete dispersion relation can be obtained in the framework of a mixed magnetohydrodynamic
(MHD)-kinetic formalism, which uses the MHD mass, momentum, and induction equations, together with the kinetically corrected
version of the double-adiabatic equations of state. The complete dispersion relation contains not only the three standard
modes (fast, slow, and Alfvén) from double-adiabatic MHD, but also the mirror mode from kinetic theory. We examine the stability
properties of these four modes, firstly in the case of a uniform medium, and secondly in the case of a stratified and rotating
medium. We also discuss the connections with the quasi-interchange modes (interchange and translation) often referred to in
the context of magnetospheric physics. 相似文献
13.
D. Lal 《Space Science Reviews》1972,14(1):3-102
Recent examinations of extraterrestrial materials exposed to cosmic rays for different intervals of time during the geological history of the solar system have generated a wealth of new information on the history of cosmic radiation. This information relates to the temporal variations in
- the flux and energy spectrum of low energy (solar) protons of ? 10 MeV kinetic energy;
- the flux and energy spectrum of (solar) heavy nuclei of Z > 20 of kinetic energy, 0.5–10 MeV/n;
- the integrated flux of protons and heavier nuclei of ? 0.5 GeV kinetic energy, and
- the flux and energy spectrum of nuclei of Z > 20 of medium energy — 100–2000 MeV/n kinetic energy.
14.
The containment lifetime of the cosmic radiation is a crucial parameter in the investigation of the cosmic-ray origin and plays an important role in the dynamics of the Galaxy. The separation of the cosmic-ray Be isotopes achieved by two satellite experiments is considered in this paper, and from the measured isotopic ratio between the radioactive 10Be (half-life = 1.5 × 106 yr) and the stable 9Be, it is deduced that the cosmic rays propagate through matter with an average density of 0.24 ± 0.07 atoms cm-3, lower than the traditionally quoted average density in the galactic disk of 1 atom cm-3. This paper reviews the implications of this result for the cosmic-ray age mainly in the context of two models of confinement and propagation: the homogeneous model, normally identified with confinement to the galactic gaseous disk, and a diffusion model in which the cosmic rays extend into a galactic halo. The propagation calculations use:
- a newly deduced cosmic-ray pathlength distribution.
- a self-consistent model of solar modulation.
- an up-to-date set of fragmentation cross sections.
15.
Certain aspects of the Sun and resulting geomagnetic disturbances can be studied better on the source surface, an imaginary spherical surface of 3.5 solar radii, than on the photospheric surface. This paper presents evidence that the Sun exhibits one of the most fundamental aspects of activities most clearly during the late-declining phase of the sunspot cycle. It is the period when 27-day average values of the solar wind speed and of geomagnetic disturbances tend to be highest during the sunspot cycle. Important findings of this study on the late-declining phase of the sunspot cycle are the following:
- By introducing a new coordinate system, modifying the Carrington coordinates, it is shown that various solar activity phenomena, solar flares, the brightest coronal regions, and also the lowest solar wind speed region, tend to concentrate in two quadrants, one around 90° in longitude in the northern hemisphere (NE) and the other around 270° in longitude in the southern hemisphere (SW). For this reason, the new coordinate system is referred to as the NESW coordinate system.
- It is shown that the above results are closely related to the fact that the neutral line exhibits a single wave (sinusoidal or rectangular) in both the Carrington coordinates and the NESW coordinate system during the late-declining phase. The shift of the neutral line configuration during successive solar rotations during the late-declining phase causes longitudinal scatter of the location of solar flares with respect to the neutral line in a statistical study. The NESW coordinate system is designed to suppress the shift, so that the single wave location is fixed and thus a ‘nest’ of solar flares emerges in the NE and SW quadrants.
- It is also shown that the single wave is the source of the double peak of the solar wind speed and two series of recurrent geomagnetic disturbances in each solar rotation, making the 27-day average solar wind and geomagnetic disturbances highest during the sunspot cycle. The double peak is a basic feature during the late-declining phase, but is obscured by several complexities which we identified in this paper; see item 8.
- The single wave of the neutral line configuration can be approximated by three dipole fields, one which can be represented by a central dipole (parallel or anti-parallel to the rotation axis) and two hypothetical dipoles on the photosphere. This configuration is referred to as the triple dipole model.
- The location of the two hypothetical photospheric dipoles coincide with the two active regions (solar flares, the brightest coronal region) and also the lowest solar wind speed region in the NESW coordinate system; the lowest solar wind regions are the cause of the valleys of the double peak of the solar wind speed.
- The two hypothetical dipole fields actually do exist at the location of the two active regions in a coarse magnetic map (5 × 5°). The two dipoles follow the Hale–Nicholson polarity law. Thus, they are real physical entities.
- The apparent meridional rotation of the dipolar field on the source surface during the sunspot cycle results from combined changes of both the central dipole field and of the two photospheric dipoles, although the central dipole remains axially parallel or anti-parallel. Thus, the Sun has a general field that can be represented by an axially aligned dipole located at the center of the Sun throughout the sunspot cycle, except for the sunspot maximum period when the polarization reversal occurs.
- The complexity of recurrent geomagnetic disturbances can also be understood by having the NESW coordinate system for various solar phenomena and the relative location of the earth with respect to the solar equatorial plane.
- As the intensity of the two dipoles decreases toward the end of the sunspot cycle, the amplitude of the single wave decreases, and the neutral line tends to align with the heliographic equator.
- The neutral line shows a double wave structure during certain epochs of the sunspot cycle. In such a situation, it can be considered that two NESW coordinate systems are present in one Carrington coordinate, resulting in four active regions.
- The so-called classical “sector boundary” arises when the peaks (top and bottom) of the single wave reached 90° in latitude in both hemispheres.
- In summary: A study of the late-declining period of the sunspot cycle is very important compared with the sunspot maximum period. In the late-declining period, the Sun shows its activities in the simplest form. It is suggested that some of the basic features of solar activities and recurrent geomagnetic disturbances that have been studied by many researchers in the past can be synthesized in a simplest way by introducing the NESW coordinate system and the triple dipole model. There is a possibility that the basic results we learned during the late phase of the sunspot cycle can be applicable to the rest of the sunspot cycle.
16.
Daniel Gómez 《Space Science Reviews》2006,122(1-4):231-238
17.
The Science Advisory Group 《Space Science Reviews》1973,14(3-4):347-362
The requirements of systematic exploration of the outer solar system have been intensively studied by a Science Advisory Group (SAG) of consulting scientists for the National Aeronautics and Space Administration (NASA). Comets and Asteroids were excluded from this study, as a separate group is planning missions to these bodies. This paper and accompanying articles on specific related scientific subjects written by members of the SAG, summarize the findings and recommendations of this group. These recommendations should not be interpreted as official NASA policy. Following some general introductory remarks, a brief sketch is given of the development and current status of scientific missions to the inner planets by the U.S. and the U.S.S.R. With this perspective, the development of the U.S. program for investigation of the outer solar system is described. The scientific focus of outer solar system exploration has been studied in detail. The relationship of the outer planetary bodies to one another and to the inner planets, as parts in a unified solar system evolved from a primitive solar nebula, is emphasized. Deductions from outer solar system investigations regarding the conditions of the solar nebula at the time of planetary formation have been considered. Investigations have been proposed that are relevant to studies of the atmospheric structure and dynamics, internal structure of the planets, satellite composition and morphology, and planetary and interplanetary fields and energetic particles. The mission type and sequence required to conduct a systematic exploration of the outer solar system has been developed. Technological rationales for the suggested missions are discussed in general terms. The existing NASA program for outer solar system exploration is comprised of four missions:
- Pioneer 10 fly-by mission to Jupiter and beyond, currently underway, with launch on 3 March 1972;
- Pioneer G, intended for a similar mission with planned launch 2–22 April 1973; and
- Two Mariner Jupiter/Saturn fly-bys in 1977, with experiment selection scheduled for late 1972 and detailed engineering design during 1972–74.
- 1976 Pioneer Jupiter/Out-of-Ecliptic (One Mission)
- 1979 Mariner Jupiter/Uranus Fly-bys (Two Missions)
- 1979 Pioneer Entry Probe to Saturn 1980 Pioneer Entry Probe to Uranus via Saturn Fly-by (Three Missions)
- 1981/1982 Mariner Jupiter Orbiter (Two Missions).
18.
A. Pedersen R. Grard K. Knott D. Jones A. Gonpalone U. Fahleson 《Space Science Reviews》1978,22(4):333-346
Quasi-static electric fields have been measured with two spherical probes supported by cable booms providing a baseline of 42 m for the measurement. The performance of the experiment is outlined to demonstrate that electric fields can be measured with accuracies of ±0.7 mV m-1 and ±1.0 mV m-1 in the dawn-dusk and satellite-sun directions respectively. These uncertainties can be considerably reduced under favourable plasma conditions. Examples of typical observations are described.
- The average electric field is always characterized by an irregular structure with time scales 0.5–5 min and with amplitudes of a few mV m-1.
- During substorms dawn-dusk electric fields up to 20–30 mV m-1 have been observed over intervals of 30–60 s.
- Oscillating electric fields with peak-to-peak amplitudes up to 10 mV m-1 and periods of 3–10 min have been observed following magnetospheric disturbances.
19.
We investigated the effect of mass accretion on the secondary components in close binomy systems (M total ≤ 2.5 M ⊙ M 2,0 ≤ 0.75 M ⊙) exchanging mass in the case A. The evolution of the low-mass close binary systems (M total ≤ 2.5 M ⊙) exchanging the mass in the case A depends on the three main factors: -the initial mass ratio (q 0 = M 2,0/M 1,0), which determines the rate of mass transfer between components; -the inital mass of the secondary component (M 2,0) and -the effectiveness of the heating of the photosphere of the secondary component, by infalling matter. The second factor allows to divide all systems into two essentially different groups:
- systems in which the secondary component is a star with a radiative envelope, or with a thin convection zone in the uppermost layers;
- and systems in which secondary component has a thick convective envelope or is fully convective.
20.
Roger E. Summons Pierre Albrecht Gene McDonald J. Michael Moldowan 《Space Science Reviews》2008,135(1-4):133-159
Life, as we know it, is based on carbon chemistry operating in an aqueous environment. Living organisms process chemicals, make copies of themselves, are autonomous and evolve in concert with the environment. All these characteristics are driven by, and operate through, carbon chemistry. The carbon chemistry of living systems is an exact branch of science and we have detailed knowledge of the basic metabolic and reproductive machinery of living organisms. We can recognise the residual biochemicals long after life has expired and otherwise lost most life-defining features. Carbon chemistry provides a tool for identifying extant and extinct life on Earth and, potentially, throughout the Universe. In recognizing that certain distinctive compounds isolable from living systems had related fossil derivatives, organic geochemists coined the term biological marker compound or biomarker (e.g. Eglinton et al. in Science 145:263–264, 1964) to describe them. In this terminology, biomarkers are metabolites or biochemicals by which we can identify particular kinds of living organisms as well as the molecular fossil derivatives by which we identify defunct counterparts. The terms biomarker and molecular biosignature are synonymous. A defining characteristic of terrestrial life is its metabolic versatility and adaptability and it is reasonable to expect that this is universal. Different physiologies operate for carbon acquisition, the garnering of energy and the storage and processing of information. As well as having a range of metabolisms, organisms build biomass suited to specific physical environments, habitats and their ecological imperatives. This overall ‘metabolic diversity’ manifests itself in an enormous variety of accompanying product molecules (i.e. natural products). The whole field of organic chemistry grew from their study and now provides tools to link metabolism (i.e. physiology) to the occurrence of biomarkers specific to, and diagnostic for, particular kinds of metabolism. Another characteristic of living things, also likely to be pervasive, is that an enormous diversity of large molecules are built from a relatively small subset of universal precursors. These include the four bases of DNA, 20 amino acids of proteins and two kinds of lipid building blocks. Third, life exploits the specificity inherent in the spatial, that is, the three-dimensional qualities of organic chemicals (stereochemistry). These characteristics then lead to some readily identifiable and measurable generic attributes that would be diagnostic as biosignatures. Measurable attributes of molecular biosignatures include:
- Enantiomeric excess
- Diastereoisomeric preference
- Structural isomer preference
- Repeating constitutional sub-units or atomic ratios
- Systematic isotopic ordering at molecular and intramolecular levels
- Uneven distribution patterns or clusters (e.g. C-number, concentration, δ 13C) of structurally related compounds.