Below One Earth: The Detection, Formation, and Properties of Subterrestrial Worlds

The Solar System includes two planets—Mercury and Mars—significantly less massive than Earth, and all evidence indicates that planets of similar size orbit many stars. In fact, one of the first exoplanets to be discovered is a lunar-mass planet around a millisecond pulsar. Novel classes of exoplanets have inspired new ideas about planet formation and evolution, and these “sub-Earths” should be no exception: they include planets with masses between Mars and Venus for which there are no Solar System analogs. Advances in astronomical instrumentation and recent space missions have opened the sub-Earth frontier for exploration: the Kepler mission has discovered dozens of confirmed or candidate sub-Earths transiting their host stars. It can detect Mars-size planets around its smallest stellar targets, as well as exomoons of comparable size. Although the application of the Doppler method is currently limited by instrument stability, future spectrographs may detect equivalent planets orbiting close to nearby bright stars. Future space-based microlensing missions should be able to probe the sub-Earth population on much wider orbits. A census of sub-Earths will complete the reconnaissance of the exoplanet mass spectrum and test predictions of planet formation models, including whether low-mass M dwarf stars preferentially host the smallest planets. The properties of sub-Earths may reflect their low gravity, diverse origins, and environment, but they will be elusive: Observations of eclipsing systems by the James Webb Space Telescope may give us our first clues to the properties of these small worlds.     

Planetary Magnetic Fields and Solar Forcing: Implications for Atmospheric Evolution

The solar wind and the solar XUV/EUV radiation constitute a permanent forcing of the upper atmosphere of the planets in our solar system, thereby affecting the habitability and chances for life to emerge on a planet. The forcing is essentially inversely proportional to the square of the distance to the Sun and, therefore, is most important for the innermost planets in our solar system—the Earth-like planets. The effect of these two forcing terms is to ionize, heat, chemically modify, and slowly erode the upper atmosphere throughout the lifetime of a planet. The closer to the Sun, the more efficient are these process. Atmospheric erosion is due to thermal and non-thermal escape. Gravity constitutes the major protection mechanism for thermal escape, while the non-thermal escape caused by the ionizing X-rays and EUV radiation and the solar wind require other means of protection. Ionospheric plasma energization and ion pickup represent two categories of non-thermal escape processes that may bring matter up to high velocities, well beyond escape velocity. These energization processes have now been studied by a number of plasma instruments orbiting Earth, Mars, and Venus for decades. Plasma measurement results therefore constitute the most useful empirical data basis for the subject under discussion. This does not imply that ionospheric plasma energization and ion pickup are the main processes for the atmospheric escape, but they remain processes that can be most easily tested against empirical data. Shielding the upper atmosphere of a planet against solar XUV, EUV, and solar wind forcing requires strong gravity and a strong intrinsic dipole magnetic field. For instance, the strong dipole magnetic field of the Earth provides a “magnetic umbrella”, fending of the solar wind at a distance of 10 Earth radii. Conversely, the lack of a strong intrinsic magnetic field at Mars and Venus means that the solar wind has more direct access to their topside atmosphere, the reason that Mars and Venus, planets lacking strong intrinsic magnetic fields, have so much less water than the Earth? Climatologic and atmospheric loss process over evolutionary timescales of planetary atmospheres can only be understood if one considers the fact that the radiation and plasma environment of the Sun has changed substantially with time. Standard stellar evolutionary models indicate that the Sun after its arrival at the Zero-Age Main Sequence (ZAMS) 4.5 Gyr ago had a total luminosity of ≈70% of the present Sun. This should have led to a much cooler Earth in the past, while geological and fossil evidence indicate otherwise. In addition, observations by various satellites and studies of solar proxies (Sun-like stars with different age) indicate that the young Sun was rotating more than 10 times its present rate and had correspondingly strong dynamo-driven high-energy emissions which resulted in strong X-ray and extreme ultraviolet (XUV) emissions, up to several 100 times stronger than the present Sun. Further, evidence of a much denser early solar wind and the mass loss rate of the young Sun can be determined from collision of ionized stellar winds of the solar proxies, with the partially ionized gas in the interstellar medium. Empirical correlations of stellar mass loss rates with X-ray surface flux values allows one to estimate the solar wind mass flux at earlier times, when the solar wind may have been more than 1000 times more massive. The main conclusions drawn on basis of the Sun-in-time-, and a time-dependent model of plasma energization/escape is that:

1. Solar forcing is effective in removing volatiles, primarily water, from planets,
2. planets orbiting close to the early Sun were subject to a heavy loss of water, the effect being most profound for Venus and Mars, and
3. a persistent planetary magnetic field, like the Earth’s dipole field, provides a shield against solar wind scavenging.

     

The Planets and Titan Observed by ISO

Infrared spectroscopic observations of planets and Saturn's satellite Titan with the Infrared Space Observatory led to many significant discoveries that improved our understanding on the formation, physics and chemistry of these objects. The prime results achieved by ISO are: (1) a new and consistent determination of the D/H ratios on the giant planets and Titan; (2) the first precise measurement of the ¹⁵N/¹⁴N ratio in Jupiter, a valuable indicator of the protosolar nitrogen isotopic ratio; (3) the first detection of an external oxygen flux for all giant planets and Titan; (4) the first detection of some stratospheric hydrocarbons (CH₃, C₂H₄, CH₃C₂H, C₄H₂, C₆H₆); (5) the first detection of tropospheric water in Saturn; (6) the tentative detection of carbonate minerals on Mars; (7) the first thermal lightcurve of Pluto. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands, and the United Kingdom), and with the participation of ISAS and NASA.     

Atmospheric Escape and Evolution of Terrestrial Planets and Satellites

The origin and evolution of Venus’, Earth’s, Mars’ and Titan’s atmospheres are discussed from the time when the active young Sun arrived at the Zero-Age-Main-Sequence. We show that the high EUV flux of the young Sun, depending on the thermospheric composition, the amount of IR-coolers and the mass and size of the planet, could have been responsible that hydrostatic equilibrium was not always maintained and hydrodynamic flow and expansion of the upper atmosphere resulting in adiabatic cooling of the exobase temperature could develop. Furthermore, thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets and Saturn’s large satellite Titan efficiently.     

Isotopic Fractionation by Gravitational Escape

Present natural data bases for abundances of the isotopic compositions of noble gases, carbon and nitrogen inventories can be found in the Sun, the solar wind, meteorites and the planetary atmospheres and crustal reservoirs. Mass distributions in the various volatile reservoirs provide boundary conditions which must be satisfied in modelling the history of the present atmospheres. Such boundary conditions are constraints posed by comparison of isotopic ratios in primordial volatile sources with the isotopic pattern which was found on the planets and their satellites. Observations from space missions and Earth-based spectroscopic telescope observations of Venus, Mars and Saturn's major satellite Titan show that the atmospheric evolution of these planetary bodies to their present states was affected by processes capable of fractionating their elements and isotopes. The isotope ratios of D/H in the atmospheres of Venus and Mars indicate evidence for their planetary water inventories. Venus' H₂O content may have been at least 0.3% of a terrestrial ocean. Analysis of the D/H ratio on Mars imply that a global H₂O ocean with a depth of ≤ 30 m was lost since the end of hydrodynamic escape. Calculations of the time evolution of the ¹⁵N/¹⁴N isotope anomalies in the atmospheres of Mars and Titan show that the Martian atmosphere was at least ≥ 20 times denser than at present and that the mass of Titan's early atmosphere was about 30 times greater than its present value. A detailed study of gravitational fractionation of isotopes in planetary atmospheres furthermore indicates a much higher solar wind mass flux of the early Sun during the first half billion years. This revised version was published online in August 2006 with corrections to the Cover Date.     

Water and Brines on Mars: Current Evidence and Implications for MSL

Liquid water is a basic ingredient for life as we know it. Therefore, in order to understand the habitability of other planets we must first understand the behavior of water on them. Mars is the most Earth-like planet in the solar system and it has large reservoirs of H₂O. Here, we review the current evidence for pure liquid water and brines on Mars, and discuss their implications for future and current missions such as the Mars Science Laboratory. Neither liquid water nor liquid brines are currently stable on the surface of Mars, but they could be present temporarily in a few areas of the planet. Pure liquid water is unlikely to be present, even temporarily, on the surface of Mars because evaporation into the extremely dry atmosphere would inhibit the formation of the liquid phase, where the temperature and pressure are high enough so that water would neither freeze nor boil. The exception to this is that monolayers of liquid water, referred to as undercooled liquid interfacial water, could exist on most of the Martian surface. In a few places liquid brines could exist temporarily on the surface because they could form at cryogenic temperatures, near ice or frost deposits where sublimation could be inhibited by the presence of nearly saturated air. Both liquid water and liquid brines might exist in the shallow subsurface because even a thin layer of soil forms an effective barrier against sublimation allowing pure liquid water to form sporadically in a few places, or liquid brines to form over longer periods of time in large portions of the planet. At greater depths, ice deposits could melt where the soil conductivity is low enough to blanket the deeper subsurface effectively. This could cause the formation of aquifers if the deeper soil is sufficiently permeable and an impermeable layer exists below the source of water. The fact that liquid brines and groundwater are likely to exist on Mars has important implications for geochemistry, glaciology, mineralogy, weathering and the habitability of Mars.     

A Comparative Study of the Influence of the Active Young Sun on the Early Atmospheres of Earth, Venus, and Mars

Because the solar radiation and particle environment plays a major role in all atmospheric processes such as ionization, dissociation, heating of the upper atmospheres, and thermal and non-thermal atmospheric loss processes, the long-time evolution of planetary atmospheres and their water inventories can only be understood within the context of the evolving Sun. We compare the effect of solar induced X-ray and EUV (XUV) heating on the upper atmospheres of Earth, Venus and Mars since the time when the Sun arrived at the Zero-Age-Main-Sequence (ZAMS) about 4.6 Gyr ago. We apply a diffusive-gravitational equilibrium and thermal balance model for studying heating of the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by IR-radiating molecules like CO₂, NO, OH, etc. Our model simulations result in extended thermospheres for early Earth, Venus and Mars. The exospheric temperatures obtained for all the three planets during this time period lead to diffusion-limited hydrodynamic escape of atomic hydrogen and high Jeans’ escape rates for heavier species like H₂, He, C, N, O, etc. The duration of this blow-off phase for atomic hydrogen depends essentially on the mixing ratios of CO₂, N₂ and H₂O in the atmospheres and could last from ∼100 to several hundred million years. Furthermore, we study the efficiency of various non-thermal atmospheric loss processes on Venus and Mars and investigate the possible protecting effect of the early martian magnetosphere against solar wind induced ion pick up erosion. We find that the early martian magnetic field could decrease the ion-related non-thermal escape rates by a great amount. It is possible that non-magnetized early Mars could have lost its whole atmosphere due to the combined effect of its extended upper atmosphere and a dense solar wind plasma flow of the young Sun during about 200 Myr after the Sun arrived at the ZAMS. Depending on the solar wind parameters, our model simulations for early Venus show that ion pick up by strong solar wind from a non-magnetized planet could erode up to an equivalent amount of ∼250 bar of O⁺ ions during the first several hundred million years. This accumulated loss corresponds to an equivalent mass of ∼1 terrestrial ocean (TO (1 TO ∼1.39×10²⁴ g or expressed as partial pressure, about 265 bar, which corresponds to ∼2900 m average depth)). Finally, we discuss and compare our findings with the results of preceding studies.     

Solar Variability and Climate Impact on Terrestrial Planets

Some possible factors of climate changes and of long term climate evolution are discussed with regard of the three terrestrial planets, Earth, Venus and Mars. Two positive feedback mechanisms involving liquid water, i.e., the albedo mechanism and the greenhouse effect of water vapour, are described. These feedback mechanisms respond to small external forcings, such as resulting from solar or astronomical constants variability, which might thus result in large influences on climatic changes on Earth. On Venus, reactions of the atmosphere with surface minerals play an important role in the climate system, but the involved time scales are much larger. On Mars, climate is changing through variations of the polar axis inclination over time scales of ～10⁵–10⁶ years. Growing evidence also exists that a major climatic change happened on Mars some 3.5 to 3.8 Gigayears ago, leading to the disappearance of liquid water on the planet surface by eliminating most of the CO₂ atmosphere greenhouse power. This change might be due to a large surge of the solar wind, or to atmospheric erosion by large bodies impacts. Indeed, except for their thermospheric temperature response, there is currently little evidence for an effect of long-term solar variability on the climate of Venus and Mars. This fact is possibly due to the absence of liquid water on these terrestrial planets.     

Outgassing History and Escape of the Martian Atmosphere and Water Inventory

The evolution and escape of the martian atmosphere and the planet’s water inventory can be separated into an early and late evolutionary epoch. The first epoch started from the planet’s origin and lasted ～500 Myr. Because of the high EUV flux of the young Sun and Mars’ low gravity it was accompanied by hydrodynamic blow-off of hydrogen and strong thermal escape rates of dragged heavier species such as O and C atoms. After the main part of the protoatmosphere was lost, impact-related volatiles and mantle outgassing may have resulted in accumulation of a secondary CO₂ atmosphere of a few tens to a few hundred mbar around ～4–4.3 Gyr ago. The evolution of the atmospheric surface pressure and water inventory of such a secondary atmosphere during the second epoch which lasted from the end of the Noachian until today was most likely determined by a complex interplay of various nonthermal atmospheric escape processes, impacts, carbonate precipitation, and serpentinization during the Hesperian and Amazonian epochs which led to the present day surface pressure.     

Feedstocks of the Terrestrial Planets

The processes of planet formation in our Solar System resulted in a final product of a small number of discreet planets and planetesimals characterized by clear compositional distinctions. A key advance on this subject was provided when nucleosynthetic isotopic variability was discovered between different meteorite groups and the terrestrial planets. This information has now been coupled with theoretical models of planetesimal growth and giant planet migration to better understand the nature of the materials accumulated into the terrestrial planets. First order conclusions include that carbonaceous chondrites appear to contribute a much smaller mass fraction to the terrestrial planets than previously suspected, that gas-driven giant planet migration could have pushed volatile-rich material into the inner Solar System, and that planetesimal formation was occurring on a sufficiently rapid time scale that global melting of asteroid-sized objects was instigated by radioactive decay of ²⁶Al. The isotopic evidence highlights the important role of enstatite chondrites, or something with their mix of nucleosynthetic components, as feedstock for the terrestrial planets. A common degree of depletion of moderately volatile elements in the terrestrial planets points to a mechanism that can effectively separate volatile and refractory elements over a spatial scale the size of the whole inner Solar System. The large variability in iron to silicon ratios between both different meteorite groups and between the terrestrial planets suggests that mechanisms that can segregate iron metal from silicate should be given greater importance in future investigations. Such processes likely include both density separation of small grains in the nebula, but also preferential impact erosion of either the mantle or core from differentiated planets/planetesimals. The latter highlights the important role for giant impacts and collisional erosion during the late stages of planet formation.     

Infrared Spectroscopy of Solar-System Planets

Most of our knowledge regarding planetary atmospheric composition and structure has been achieved by remote sensing spectroscopy. Planetary spectra strongly differ from one planet to another. CO₂ signatures dominate on Mars, and even more on Venus (where the thermal component is detectable down to 1 μm on the dark side). Spectroscopic monitoring of Venus, Earth and Mars allows us to map temperature fields, wind fields, clouds, aerosols, surface mineralogy (in the case of the Earth and Mars), and to study the planets’ seasonal cycles. Spectra of giant planets are dominated by H₂, CH₄ and other hydrocarbons, NH₃, PH₃ and traces of other minor compounds like CO, H₂O and CO₂. Measurements of the atmospheric composition of giant planets have been used to constrain their formation scenario.     

Planetary Radiation Budgets

The energy state of a planet depends fundamentally on its radiation budget. Measurements made from space over past decades have led to significant revisions of ground-based estimates, both of the reflected fraction (the Bond albedo) of solar radiative flux and of the emitted thermal infrared radiation flux, for the Earth as well as for the other planets. After a brief survey of methods and difficulties in accurately determining planetary radiation budgets, we note contradictions in existing tabulations of global parameters, in particular Bond albedo. For the Earth, such contradictions are unjustified, considering that global and annual means as well as the seasonal cycle of Earth Radiation Budget components have now been determined with high accuracy. The Earth's Bond albedo is close to 0.3. Net storage of energy in the Earth-ocean system is close to zero, with a well-established annual cycle of amplitude close to ±12 Wm⁻². Some contradictions remain for the other terrestrial planets. For the giant planets, modern reduced values of the Bond albedo imply reduced but still significant internal energy generation.     

The helium abundance on Jupiter

Methods of determining helium on Jupiter (and the Jovian planets) are critically surveyed. Current information is consistent with solar abundance, He/H₂ = 0.11 by number. The available lines of evidence are the mean density, spectral-line broadening, and stellar occultations. Methods usable from spacecraft flying by are discussed. Observation of far-infrared emission has great promise, but we may have to await the development of entry probes for the greatest assurance.This is one of the publications by the Science Advisory Group.     

Thermospheric X-Ray and Euv Heating by the Young Sun on Early Venus and Mars

In view of the low H₂O abundance in the present Venusian and Martian atmospheres several observations by spacecraft and studies suggest that both planets should have lost most of their water over the early active period of the young Sun. During the first Gyr after the Sun arrived at the Zero- Age-Main-Sequence high X-ray and EUV fluxes between 10 and 100 times that of the present Sun were responsible for much higher temperatures in the thermosphere-exosphere environments on both planets. By applying a diffusive-gravitational equilibrium and thermal balance model for investigating radiation impact on the early thermospheres by photodissociation and ionization processes, due to exothermic chemical reactions and cooling by CO₂ IR emission in the 15μm band we found expanded thermospheres with exobase levels between about 200 km (present) and 2000 km (4.5 Gyr ago). The higher temperatures in the upper atmospheres of both planets could reach “blow-off” conditions for H atoms even at high CO₂ mixing ratios of 96%. Lower CO₂/N₂ mixing ratio or higher contents of H2O vapor in the early atmospheres could have had a dramatic impact from the loss of atmosphere and water on both planets. The duration of this phase of high thermal loss rates essentially depended on the mixing ratios of CO₂, N₂, and H₂O in the early atmospheres and could have lasted between about 150 and several hundred Myr.     

The ionospheres of the major planets

Physical and chemical processes which affect the equilibrium distribution of ionization in the atmospheres of Jupiter, Saturn, Uranus and Neptune are reviewed. Current models imply readily detectable ionospheres for all four planets and suggest that protons should represent the dominant positive ion. Attention is directed to the probable importance of dissociative ionization of H₂ as a source of H⁺. A number of potentially important loss mechanisms for H⁺ are discussed including a possible reaction of H⁺ with vibrationally excited H₂. Protons may be removed efficiently at lower altitudes by reaction with CH₄ and this process may offer a simple remote means for location of the turbopause.This is one of the publications by the Science Advisory Group.     

Cometary dynamics

The modern theory of cometary dynamics is based on Oort's hypothesis that the solar system is surrounded by a spherically symmetric cloud of 10¹¹ to 10¹² comets extending out to interstellar distances. Dynamical modeling and analysis of cometary motion have confirmed the ability of the Oort hypothesis to explain the observed distribution of energies for the long-period comet orbits. The motion of comets in the Oort cloud is controlled by perturbations from random passing stars, interstellar clouds, and the galactic gravitational field. Additionally, comets which enter the planetary region are perturbed by the major planets and by nongravitational forces resulting from jetting of volatiles on the surfaces of the cometary nuclei. The current Oort cloud is estimated to have a radius of 6 to 8 × 10⁴ AU, and to contain some 2 × 10¹² comets with a total mass of 7 to 8 Earth masses. Evidence has begun to accumulate for the existence of a massive inner Oort cloud extending from just beyond the orbit of Neptune to 10⁴ AU or more, with a population up to 100 times that of the outer Oort cloud. This inner cloud may serve as a reservoir to replenish the outer cloud as comets are stripped away by the various perturbers, and may also provide a more efficient source for the short-period comets. Recent suggestions of an unseen solar companion star or a tenth planet orbiting in the inner cloud and causing periodic comet showers on the Earth are likely unfounded. The formation site of the comets in the Oort cloud was likely the extended nebula accretion disc reaching from about 15 to 500 AU from the forming protosun. Comets which escape from the Oort cloud contribute to the flux of interstellar comets, though capture of interstellar comets by the solar system is extremely unlikely. The existence of Oort clouds around other main sequence stars has been suggested by the detection by the IRAS spacecraft of cool dust shells around about 10% of nearby stars.     

The Milky Way 3-Helium Abundance

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources impacts stellar evolution, chemical evolution, and cosmology. The abundance of ³He is derived from measurements of the hyperfine transition of ³He⁺ which has a rest wavelength of 3.46 cm (8.665 GHz). As with all the light elements, the present interstellar ³He abundance results from a combination of Big Bang Nucleosynthesis (BBNS) and stellar nucleosynthesis. We are measuring the ³He abundance in Milky Way H ii regions and planetary nebulae (PNe). The source sample is currently comprised of 60 H ii regions and 12 PNe. H ii regions are examples of zero-age objects that are young relative to the age of the Galaxy. Therefore their abundances chronicle the results of billions of years of Galactic chemical evolution. PNe probe material that has been ejected from low-mass (M≤ 2M _⊙) to intermediate-mass (M∼2–5M _⊙) stars to be further processed by future stellar generations. Because the Milky Way ISM is optically thin at centimeter wavelengths, our source sample probes a larger volume of the Galactic disk than does any other light element tracer of Galactic chemical evolution. The sources in our sample possess a wide range of physical properties (including object type, size, temperature, excitation, etc.). The ³He abundances we derive have led to what has been called “The ³He Problem”.     

The evolution of massive stars

The evolution of stars with masses between 15 M ₀ and 100M ₀ is considered. Stars in this mass range lose a considerable fraction of their matter during their evolution.The treatment of convection, semi-convection and the influence of mass loss by stellar winds at different evolutionary phases are analysed as well as the adopted opacities.Evolutionary sequences computed by various groups are examined and compared with observations, and the advanced evolution of a 15M ₀ and a 25M ₀ star from zero-age main sequence (ZAMS) through iron collapse is discussed.The effect of centrifugal forces on stellar wind mass loss and the influence of rotation on evolutionary models is examined. As a consequence of the outflow of matter deeper layers show up and when the mass loss rates are large enough layers with changed composition, due to interior nuclear reactions, appear on the surface.The evolution of massive close binaries as well during the phase of mass loss by stellar wind as during the mass exchange and mass loss phase due to Roche lobe overflow is treated in detail, and the value of the parameters governing mass and angular momentum losses are discussed.The problem of the Wolf-Rayet stars, their origin and the possibilities of their production either as single stars or as massive binaries is examined.Finally, the origin of X-ray binaries is discussed and the scenario for the formation of these objects (starting from massive ZAMS close binaries, through Wolf-Rayet binaries leading to OB-stars with a compact companion after a supernova explosion) is reviewed and completed, including stellar wind mass loss.     

Formation and evolution of low-mass Algols

We discuss the origin, evolution and fate of low-mass Algols (LMA) that have components with initial masses less than 2.5 M₀. The semi-major axes of orbits of pre-LMA do not exceed 20–25 R₀. The rate of formation of Algol-type stars is ～ 0.01/year. Magnetic stellar winds may be the factor that determines the evolution of LMA. Most LMA end their lives as double helium degenerate dwarfs with M₁/M₂ ～ 0.88 (like L870-2). Some of them even merge through angular momentum loss caused by gravitational waves.     

Irregular Satellites in the Context of Planet Formation

All four giant planets in the solar system possess irregular satellites, characterized by large, highly eccentric and/or highly inclined orbits. These bodies were likely captured from heliocentric orbit, probably in association with planet formation itself. Enabled by the use of large-format digital imagers on ground-based telescopes, new observational work has dramatically increased the known populations of irregular satellites, with 74 discoveries in the last few years. A new perspective on the irregular satellite systems is beginning to emerge.We find that the number of irregular satellites measured to a given diameter is approximately constant from planet to planet. This is surprising, given the radically different formation scenarios envisioned for the gas giants Jupiter and Saturn compared to the (much less massive and compositionally distinct) ice giants Uranus and Neptune. We discuss the new results on the irregular satellites and show how these objects might be used to discriminate amongst models of giant planet formation.