PREDICTING MARTIAN AND VENUSIAN METEOR SHOWER ACTIVITY

Based on the number of planet-approaching cometary orbits at Mars and Venus relative to the Earth, there should be ample opportunities for observing meteor activity at those two planets. The ratio of planet-approaching Jupiter family comets (JFCs) at Mars, Earth, and Venus is 4:2:1 indicating that JFC-related outbursts would be more frequent at Mars than the Earth. The relative numbers of planet-approaching Halley-type comets (HTCs) implies that the respective levels of annual meteor activity at those three planets are similar. We identify several instances where near-comet outbursts (Jenniskens, P.: 1995, Astron. Astrophys. 295, 206–235) may occur. A possible double outburst of this type at Venus related to 45P/Honda-Mrkos-Padjusakova may be observable by the ESA Venus Express spacecraft in the summer of 2006. Similarly, the Japanese Planet-C Venus orbiter may observe an outburst related to 27P/Crommelin’s perihelion passage in July 2011. Several additional opportunities exist to observe such outbursts at Mars from 2019 to 2026 associated with comets 38P/Stephan-Oterma, 13P/Olbers and 114P/Wiseman-Skiff.     

First measurement of helium on Mars: implications for the problem of radiogenic gases on the terrestrial planets

108 +/- 11 photons of the martian He 584-angstroms airglow detected by the Extreme Ultraviolet Explorer satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh. Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He+ by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(5), 3 x 10(4), and 7 x 10(3) cm-2 sec-1, respectively. The lifetime of helium on Mars is 5 x 10(4) years. The He outgassing rate, coupled with the 40Ar atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances of K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta (0)(t(0)/t)1/2 with delta (0) = 0/1, 0.04, and 0.0125 Byr-1 for Earth, Venus, and Mars, respectively. After a R2 correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassium is depleted by a factor of 2 in Venus and Mars. Mass ratios of helium and argon are close to 5 x 10(-9) and 2 x 10(-8) g/g in the interiors of all three planets. The implications of these results are discussed.     

Characterization of terrestrial exoplanets based on the phase curves and albedos of Mercury, Venus and Mars

The empirically derived phase curves of terrestrial planets strongly distinguish between airless Mercury, cloud-covered Venus, and the intermediate case of Mars. The function for Mercury is steeply peaked near phase angle zero due to powerful backscattering from its surface, while that for Venus has 100 times less contrast and exhibits a brightness excess near 170° due to Mie scattering from droplets in the atmosphere. The phase curve of Mars falls between those of Mercury and Venus, and there are variations in luminosity due to the planet’s rotation, seasons, and atmospheric states. The phase function and geometric albedo of the Earth are estimated from published albedos values. The curves for Mercury, Venus and Mars are compared to that of the Earth as well as theoretical phase functions for giant planets. The parameters of these different phase functions can be used to characterize exoplanets.     

Effects of impacts on the atmospheric evolution: Comparison between Mars, Earth, and Venus

Classified as a terrestrial planet, Venus, Mars, and Earth are similar in several aspects such as bulk composition and density. Their atmospheres on the other hand have significant differences. Venus has the densest atmosphere, composed of CO₂ mainly, with atmospheric pressure at the planet's surface 92 times that of the Earth, while Mars has the thinnest atmosphere, composed also essentially of CO₂, with only several millibars of atmospheric surface pressure. In the past, both Mars and Venus could have possessed Earth-like climate permitting the presence of surface liquid water reservoirs. Impacts by asteroids and comets could have played a significant role in the evolution of the early atmospheres of the Earth, Mars, and Venus, not only by causing atmospheric erosion but also by delivering material and volatiles to the planets. Here we investigate the atmospheric loss and the delivery of volatiles for the three terrestrial planets using a parameterized model that takes into account the impact simulation results and the flux of impactors given in the literature. We show that the dimensions of the planets, the initial atmospheric surface pressures and the volatiles contents of the impactors are of high importance for the impact delivery and erosion, and that they might be responsible for the differences in the atmospheric evolution of Mars, Earth and Venus.     

2000—2049年行星天象（一）

本文给出 2 0 0 0 - 2 0 4 9年行星天象的一部分 ,包括水星、金星的东、西大距 ,留及上、下合日 ;金星最亮和最接近地球 ;外行星冲日、最接近地球、合日、留 ;地球和外行星过远近日点计1 5个表     

Noble gases in planetary atmospheres: Implications for the origin and evolution of atmospheres

The radiogenic and primordial noble gas content of the atmospheres of Venus, Earth, and Mars are compared with one another and with the noble gas content of other extraterrestial samples, especially meteorites. The fourfold depletion of ⁴⁰Ar for Venus relative to the Earth is attributed to the outgassing rates and associated tectonics and volcanic styles for the two planets diverging significantly within the first billion or so years of their history, with the outgassing rate for Venus becoming much less than that for the Earth at subsequent times. This early divergence in the tectonic style of the two planets may be due to a corresponding early onset of the runaway greenhouse on Venus. The 16-fold depletion of ⁴⁰Ar for Mars relative to the Earth may be due to a combination of a mild K depletion for Mars, a smaller fraction of its interior being outgassed, and to an early reduction in its outgassing rate. Venus has lost virtually all of its primordial He and some of its radiogenic He. The escape flux of He may have been quite substantial in Venus' early history, but much diminished at later times, with this time variation being perhaps strongly influenced by massive losses of H₂ resulting from efficient H₂O loss processes.Key trends in the primordial noble gas content of terrestial planetary atmospheres include (1) a several orders of magnitude decrease in ²⁰Ne and ³⁶Ar from Venus to Earth to Mars; (2) a nearly constant ²⁰Ne/³⁶Ar ratio which is comparable to that found in the more primitive carbonaceous chondrites and which is two orders of magnitude smaller than the solar ratio; (3) a sizable fractionation of Ar, Kr, and Xe from their solar ratios, although the degree of fractionation, especially for ³⁶Ar/¹³²Xe, seems to decrease systematically from carbonaceous chondrites to Mars to Earth to Venus; and (4) large differences in Ne and Xe isotopic ratios among Earth, meteorites, and the Sun. Explaining trends (2), (2) and (4), and (1) pose the biggest problems for the solar-wind implantation, primitive atmosphere, and late veneer hypotheses, respectively. It is suggested that the grain-accretion hypothesis can explain all four trends, although the assumptions needed to achieve this agreement are far from proven. In particular, trends (1), (2), (3), and (4) are attributed to large pressure but small temperature differences in various regions of the inner solar system at the times of noble gas incorporation by host phases; similar proportions of the host phases that incorporated most of the He and Ne on the one hand (X) and Ar, Kr, and Xe on the other hand (Q); a decrease in the degree of fractionation with increasing noble-gas partial pressure; and the presence of interstellar carriers containing isotopically anomalous noble gases.Our analysis also suggests that primordial noble gases were incorporated throughout the interior of the outer terrestial planets, i.e., homogeneous accretion is favored over inhomogeneous accretion. In accord with meteorite data, we propose that carbonaceous materials were key hosts for the primordial noble gases incorporated into planets and that they provided a major source of the planets' CO₂ and N₂.     

Deep versus shallow origin of gravity anomalies, topography and volcanism on Earth, Venus and Mars

The relation between gravity anomalies, topography and volcanism can yield important insights about the internal dynamics of planets. From the power spectra of gravity and topography on Earth, Venus and Mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l=30 for Earth, 40 for Venus and 5 for Mars. To interpret the low-degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics. For these viscosity profiles we then compute gravity and topography kernels, which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth. With these kernels, we firstly compute an expected gravity-topography ratio. Good agreement with the observed ratio indicates that for Venus, in contrast to Earth and Mars, long-wavelength topography is largely dynamically supported from the sublithospheric mantle. Secondly, we combine an empirical power spectrum of density anomalies inferred from seismic tomography in Earth’s mantle with gravity kernels to model the gravity power spectrum. We find a good match between modeled and observed gravity power spectrum for all three planets, except for 2?l?4 on Venus. Density anomalies in the Venusian mantle for these low degrees thus appear to be very small. We combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the Martian and Venusian mantles. Gravity kernels for l?5 are very small on Venus below ≈800 km depth. Thus our inferences on Venusian mantle density are basically restricted to the upper 800 km. On Mars, gravity anomalies for 2?l?5 may originate from density anomalies anywhere within its mantle. For Mars as for Earth, inferred density anomalies are dominated by l=2 structure, but we cannot infer whether there are features in the lowermost mantle of Mars that correspond to Earth’s Large Low Shear Velocity Provinces (LLSVPs). We find that volcanism on Mars tends to occur primarily in regions above inferred low mantle density, but our model cannot distinguish whether or not there is a Martian analog for the finding that Earth’s Large Igneous Provinces mainly originate above the margins of LLSVPs.     

Atmospheric angular momentum variations of Earth, Mars and Venus at seasonal time scales

Atmospheric angular momentum variations of a planet are associated with the global atmospheric mass redistribution and the wind variability. The exchange of angular momentum between the fluid layers and the solid planet is the main cause for the variations of the planetary rotation at seasonal time scales. In the present study, we investigate the angular momentum variations of the Earth, Mars and Venus, using geodetic observations, output of state-of-the-art global circulation models as well as assimilated data. We discuss the similarities and differences in angular momentum variations, planetary rotation and angular momentum exchange for the three terrestrial planets. We show that the atmospheric angular momentum variations for Mars and Earth are mainly annual and semi-annual whereas they are expected to be “diurnal” on Venus. The wind terms have the largest contributions to the LOD changes of the Earth and Venus whereas the matter term is dominant on Mars due to the CO₂ sublimation/condensation. The corresponding LOD variations (ΔLOD) have similar amplitudes on Mars and Earth but are much larger on Venus, though more difficult to observe.     

The Martian Nonhydrostatic Components of Moment-of-Inertia

The author puts forward the proposal in this paper that all the terrestrial planets (Venus, the Earth, and Mars) as well as the Moon deviate from hydrostatic equilibrium to some degree. The Earth's level of deviation of these four celestial bodies is minimum, and that of Mars is maximum. Moreover, the author estimates Martian nonhydrostatic components of the principal moments-of-inertia using five models for the interior of Mars. Comparison with other terrestrial planets shows that setting the range of mean moment-of-inertia ratio, I/MR², in 0.345 ~ 0.355for Mars is reasonable. This revised version was published online in July 2006 with corrections to the Cover Date.     

Magnetism and thermal evolution of the terrestrial planets

Of the terrestrial planets, Earth and probably Mercury possess substantial intrinsic magnetic fields generated by core dynamos, while Venus and Mars apparently lack such fields. Thermal histories are calculated for these planets and are found to admit several possible present states, including those which suggest simple explanations for the observations; whule the cores of Earth and Mercury are continuing to freeze, the cores of Venus and Mars may still be completely liquid. The models assume whole mantle convection, which is parameterized by a simple Nusselt-Rayleigh number relation and dictates the rate at which heat escapes from the core. It is found that completely fluid cores, devoid of intrinsic heat sources, are not likely to sustain thermal convection for the age of the solar system but cool to a subadiabatic, conductive state that can not maintain a dynamo. Planets which nucleate an inner core continue to sustain a dynamo because of the gravitational energy release and chemically driven convection that accompany inner core growth. The absence of a significant inner core can arise in Venus because of its slightly higher temperature and lower central pressure relative to Earth, while a Martian core avoids the onset of freezing if the abundance of sulfur in the core is ?15% by mass. All of the models presented assume that (I) core dynamos are driven by thermal and/or chemical convection; (ii) radiogenic heat production is confined to the mantle; (iii) mantle and core cool from initially hot states which are at the solidus and superliquidus, respectively; and (iv) any inner core excludes the light alloying material (sulfur or oxygen) which then mixes uniformly upward through the outer core. The models include realistic pressure and composition-dependent freezing curves for the core, and material parameters are chosen so that the correct present-day values of heat outflow, upper mantle temperature and viscosity, and inner core radius are obtained for the earth. It is found that Venus and Mars may have once had dynamos maintained by thermal convection alone. Earth may have had a completely fluid core and a dynamo maintained by thermal convection for the first 2 to 3 by, but an inner core nucleates and the dynamo energetics are subsequently dominated by gravitational energy release. Complete freezing of the Mercurian core is prohibited if it contains even a small amount of sulfur, and a dynamo can be maintained by chemical convection in a thin, fluid shell.     

Windblown sand on Venus: Preliminary results of laboratory simulations

Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.     

Dynamics and circulation regimes of terrestrial planets

By the study of simple analogues, either in the form of simplified numerical models or laboratory experiments, considerable insights may be gained as to the likely roles of planetary size, rotation, thermal stratification and other factors in determining the principal length scales, styles of global circulation and dominant waves and instability processes active in the respective climate systems of Earth, Mars, Venus and Titan. In this review, we explore aspects of these analogues and demonstrate the importance of a number of key dimensionless parameters, most notably thermal Rossby and Rhines numbers and a measure of the dominant frictional or radiative timescale, in defining the type of circulation regime to be expected in a prototype planetary atmosphere subject to axisymmetric driving. These considerations help to place Mars, Venus, Titan and Earth into an appropriate context, and may also lay the foundations for predicting and understanding the climate and circulation regimes of (as yet undiscovered) Earth-like extra-solar planets. However, as recent discoveries of ‘super-Earth’ planets around some nearby stars are beginning to reveal, the parameter space determined from axisymmetrically forced prototype atmospheres may be incomplete and other factors, such as the possibility of tidally locked rotation and tidal forcing, may also need to be taken into account for some classes of extra-solar planet.     

Rummaging through Earth's Attic for Remains of Ancient Life

We explore the likelihood that early remains of Earth, Mars, and Venus have been preserved on the Moon in high enough concentrations to motivate a search mission. During the Late Heavy Bombardment, the inner planets experienced frequent large impacts. Material ejected by these impacts near the escape velocity would have had the potential to land and be preserved on the surface of the Moon. Such ejecta could yield information on the geochemical and biological state of early Earth, Mars, and Venus. To determine whether the Moon has preserved enough ejecta to justify a search mission, we calculate the amount of terran material incident on the Moon over its history by considering the distribution of ejecta launched from the Earth by large impacts. In addition, we make analogous estimates for Mars and Venus. We find, for a well-mixed regolith, that the median surface abundance of terran material is roughly 7 ppm, corresponding to a mass of approximately 20,000 kg of terran material over a 10×10-square-km area. Over the same area, the amount of material transferred from Venus is 1-30 kg and material from Mars as much as 180 kg. Given that the amount of terran material is substantial, we estimate the fraction of this material surviving impact with intact geochemical and biological tracers.     

On the classification of comet plasma tails

The investigation of plasma tails of comets is an important part of comet research. Different classifications of plasma tails of comets are proposed. Plasma acceleration in the tails is investigated in sufficient detail. Several cometary forms are explained. Plasma tails of Mars and Venus were observed during the first studies of these planets. They are associated with the capture of ionized atoms and exosphere molecules by the solar wind magnetized plasma flow. Distinct plasma tails of Mars and Venus are caused by the mass loading of the solar wind with heavy ions. It was shown that the transverse dimension of the tails of Mars, Venus, and comets can be quite accurately determined by production rate of the obstacle to the solar wind flow. While plasma tails of Mars and Venus are investigated by in situ measurements from spacecraft, observations of comet tails from the Earth make it possible to see the entire object under study and to monitor changes in its structure. A certain similarity of cometary and planetary tails can be explained by the nonmagnetic nature of both types of bodies. Thus, it is reasonable to suppose that investigations of plasma tails of comets can supplement the information obtained by in situ methods of the study of the planets. In this paper, plasma tails of comets, presumably analogous to the plasma tails of Mars and Venus, have been identified on modern photographs of comets (more than 1500 photographs viewed). Only quasi-steady laminar tails are considered. They are divided into two types: double structures and outflows. The paper attempts to define the 3D structure of double structures and to determine certain characteristics of outflows.     

The Origin and Evolution of the Atmospheres of Venus, Earth and Mars

The chemical compositions of the primordial atmospheres of Venus, Earth and Mars have long been a topic of debate between the experts. Some believe that the original atmospheres were a product of outgassed volatiles from the newly accreted terrestrial planets and that these atmospheres consisted primarily of carbon dioxide, nitrogen, water vapor and residual hydrogen and helium (e.g., Lewis and Prinn, <it>Planets and their Atmospheres,</it> Academic Press, Orlando, FL, 1984, pp. 62–63, 81–84, 228–231, 383). Still others think the earliest atmospheres were composed of the gas components of the solar nebula from which the solar system formed (i.e., hydrogen, helium, methane, ammonia and water). I consider the latter to be the correct scenario. Presented herein is a proposed mechanism by which the original atmospheres of Venus, Earth and Mars were transformed to atmospheres rich in carbon dioxide and nitrogen. An explanation is proposed for why water is so common on the surface of Earth and so scarce on the surfaces of Venus and Mars. Also presented are the effects the “great impact” (single cataclysmic event that was responsible for producing the Earth–Moon system) had upon the early atmosphere of Earth. The origin, structure and composition of the impacting object are determined through deductive analyses.     

Magnetic fields of mars and venus: Solar wind interactions

Recent U.S.S.R. studies of the magnetic field and solar wind flow in the vicinity of Mars and Venus confirm earlier U.S.A. reports of a bow shock wave developed as the solar wind interacts with these planets. Mars 2 and 3 magnetometer experiments report the existence of an intrinsic planetary magnetic field, sufficiently strong to form a magnetopause, deflecting the solar wind around the planet and its ionosphere. This is in contrast to the case for Venus, where it is assumed to be the ionosphere and processes therein which are responsible for the solar wind deflection. An empirical relationship appears to exist between planetary dipole magnetic moments and their angular momentum for Moon, Mars, Venus, Earth and Jupiter. Implications for the magnetic fields of Mercury and Saturn are discussed.Paper presented at the Lunar Science Institute Conference on Geophysical and Geochemical Exploration of the Moon and Planets, January 10–12, 1973     

Climatic change on the terrestrial planets

In this paper, we review the observational data on climatic change for the terrestrial planets, discuss the basic factors that influence climate, and examine the manner in which these factors may have been responsible for some of the known changes. Emphasis is placed on trying to understand the similarities and differences in both the basic factors and their climatic impacts on Venus, the Earth, and Mars. Climatic changes have occurred on the Earth over a broad spectrum of time scales that range from the elevated temperatures of Pre-Cambrian times (～10⁹ years ago), through the alternating glacial and interglacial epochs of the last few million years, to the small but significant decadal and centurial variations of the recent past. Evidence for climatic change on Mars is given by certain channel features, which suggest an early to intermediate aged epoch of warmer and wetter climate, and by layered polar deposits, which imply more recent periodic climate variations. No evidence for climatic change on Venus exists as yet, but comparison of its present climate state with that of outer terrestrial planets offers important clues on some of the mechanisms affecting climate. The important determinants of climate for a terrestrial planet include the Sun's output, astronomical perturbations of its orbital and axial characteristics, the gaseous and particulate content of its atmosphere, its land surface, volatile reservoirs, and its interior. All these factors appear to have played major roles in causing climatic changes on the terrestrial planets. Despite a lower solar luminosity in the past, the Earth and Mars have had warmer periods in their early history. In both cases, a more reducing atmosphere may have been the responsible agent through an enhanced greenhouse effect. In this paper, we present detailed calculations of the effect of atmospheric pressure and composition on the temperature state of Mars. We find that the higher temperature period is easier to explain with a reducing atmosphere than with the current fully oxidizing one. Both the very high surface temperature and massive atmosphere of Venus may be the result of the solar flux being a factor of two higher at its orbit than at the Earth's orbit. This difference may have led to a runaway greenhouse effect on Venus, i.e., the emplacement of volatiles entirely in the atmosphere rather than mostly in surface reservoirs. But if Venus formed with relatively little or no water, it may have always had an oxidizing atmosphere. In this case, a lower solar luminosity would have led to a moderate surface temperature in Venus' early history. Quasi-periodic variations in orbital eccentricity and axial obliquity may have contributed to the alternation between Pleistocene glacial and interglacial periods in the case of the Earth and to the formation of the layered polar deposits in the case of Mars. In this paper, we postulate that two mechanisms, acting jointly, account for the creation of the laminated terrain of Mars: dust particles serve as nucleation centers for the condensation of water vapor and carbon dioxide. The combined dust-H₂O-CO₂ particle is much larger and so has a much higher terminal velocity than either a dust-H₂O or a plain dust particle. As a result, dust and water ice are preferentially deposited in the polar regions. In addition, we postulate that the obliquity variations are key drivers of the periodic layering because of their impact on both atmospheric pressure and polar surface temperature, which, in turn, influence the amounts of dust and water ice in the atmosphere. But eccentricity and precessional changes probably also play important roles in creating the polar layers. The drifting of continents on the Earth has caused substantial climatic changes on individual continents and may have helped to set the stage for the Pleistocene ice ages through a positioning of the continents near the poles. While continental drift apparently has not occurred on Mars, tectonic distortions of its lithosphere may, in some circumstances, cause an alteration in the mean value of that planet's obliquity, which would significantly impact its climate. Atmospheric aerosols can influemce climate through their radiative effects. In the case of the Earth, volcanic aerosols appear to have contributed to past climatic changes, while consideration needs to be given to the future impact of man-generated aerosols. In the case of Mars, the atmospheric temperature structure and thereby atmospheric dynamics are greatly altered by suspended dust particles. The sulfuric acid clouds of Venus play a major role in its heat balance. Cometary impacts may have added substantial quantities of water vapor and sulfur gases to Venus' atmosphere and thus have indirectly affected its cloud properties. Calculations presented in this paper indicate substantial changes in surface temperature accompany these compositional changes.     

Dynamics of Asteroid 3200 Phaethon Under Overlap of Different Resonances

The paper is focused on studying the motion of asteroid 3200 Phaethon which approached the Earth in December 2017. We consider the dynamics of asteroid 3200 Phaethon, reveal its encounters with planets, mean motion and secular resonances, and estimate the predictability time and the causes of chaoticity. A peculiar feature in the dynamics of the object is that it passes through the unstable orbital resonance 3/7 with Venus and exhibits a gamut of apsidal-nodal resonances with Mercury, Venus, Earth, Mars, and Jupiter, as well as a large number of close encounters with terrestrial planets. These properties result in a chaotic character of the motion beyond a time interval between the years 1780 and 2350.

     

Origin and evolution of the atmospheres of early Venus,Earth and Mars

We review the origin and evolution of the atmospheres of Earth, Venus and Mars from the time when their accreting bodies were released from the protoplanetary disk a few million years after the origin of the Sun. If the accreting planetary cores reached masses \(\ge 0.5 M_\mathrm{Earth}\) before the gas in the disk disappeared, primordial atmospheres consisting mainly of H\(_2\) form around the young planetary body, contrary to late-stage planet formation, where terrestrial planets accrete material after the nebula phase of the disk. The differences between these two scenarios are explored by investigating non-radiogenic atmospheric noble gas isotope anomalies observed on the three terrestrial planets. The role of the young Sun’s more efficient EUV radiation and of the plasma environment into the escape of early atmospheres is also addressed. We discuss the catastrophic outgassing of volatiles and the formation and cooling of steam atmospheres after the solidification of magma oceans and we describe the geochemical evidence for additional delivery of volatile-rich chondritic materials during the main stages of terrestrial planet formation. The evolution scenario of early Earth is then compared with the atmospheric evolution of planets where no active plate tectonics emerged like on Venus and Mars. We look at the diversity between early Earth, Venus and Mars, which is found to be related to their differing geochemical, geodynamical and geophysical conditions, including plate tectonics, crust and mantle oxidation processes and their involvement in degassing processes of secondary \(\hbox {N}_2\) atmospheres. The buildup of atmospheric \(\hbox {N}_2\), \(\hbox {O}_2\), and the role of greenhouse gases such as \(\hbox {CO}_2\) and \(\hbox {CH}_4\) to counter the Faint Young Sun Paradox (FYSP), when the earliest life forms on Earth originated until the Great Oxidation Event \(\approx \) 2.3 Gyr ago, are addressed. This review concludes with a discussion on the implications of understanding Earth’s geophysical and related atmospheric evolution in relation to the discovery of potential habitable terrestrial exoplanets.     

The effect of radiative transfer on shear-flow instability in the atmospheres on mars and venus

The results of two theoretical investigations concerning the destabilizing effects of radiative transfer on stably stratified shear flows are applied to the CO₂ atmospheres Mars and Venus. It is found that radiatively modified critical Richardson numbers remain below plausible atmospheric values throughout the stratospheres of both planets. Above certain altitudes, however, in the upper stratospheres of these planets (≈50 km on Mars and ≈100 km on Venus), critical Richardson numbers begin to increase significantly above the nonradiating critical value. This trend continues until, in the lower thermosphere, critical Richardson numbers eventually surpass atmospheric values. This effect could lead to observably greater turbulent mixing in the upper atmospheres of Mars and Venus than might be expected from terrestrial observation and from nonradiating theoretical calculations.