The history of exoplanetary atmospheres studies is strongly based on the observations and investigations of the gaseous envelopes of hot Jupiters—exoplanet gas giants that have masses comparable to the mass of Jupiter and orbital semi-major axes shorter than 0.1 AU. The first exoplanet around a solar-type star was a hot Jupiter discovered in 1995. Researchers found an object that had completely atypical parameters compared to planets known in the solar system. According to their estimates, the object might have a mass about a half of the Jovian mass and a very short orbital period (four days), which means that it has an orbit roughly corresponding to the orbit of Mercury. Later, many similar objects were discovered near different stars, and they acquired a common name—hot Jupiters. It is still unclear what the mechanism is for their origin, because generally accepted theories of planetary evolution predict the formation of giant planets only at large orbital distances, where they can accrete enough matter before the protoplanetary disc disappears. If this is true, before arriving at such low orbits, hot Jupiters might have a long migration path, caused by interactions with other massive planets and/or with the gaseous disc. In favor of this model is the discovery of many hot Jupiters in elliptical and highly inclined orbits, but on the other hand several observed hot Jupiters have circular orbits with low inclination. An alternative hypothesis is that the cores of future hot Jupiters are super-Earths that may later intercept matter from the protoplanetary disk falling on the star. The scientific interest in hot Jupiters has two aspects. The first is the peculiarity of these objects: they have no analogues in the solar system. The second is that, until recently, only for hot Jupiters was it possible to obtain observational characteristics of their atmospheres. Many of the known hot Jupiters are eclipsing their host stars, so, from their light curve and spectral data obtained during an eclipse, it became possible to obtain information about their shape and their atmospheric composition. Thus it is possible to conclude that hot Jupiters are a common type of exoplanet, having no analogues in the solar system. Many aspects of their evolution and internal structure remain unclear. Being very close to their host stars, hot Jupiters must interact with the stellar wind and stellar magnetic field, as well as with stellar flares and coronal mass ejections, allowing researchers to gather information about them. According to UV observations, at least a fraction of hot Jupiters have extended gaseous envelopes, extending far beyond of their upper atmospheres. The envelopes are observable with current astronomical instruments, so it is possible to develop their astrophysical models. The history of hot Jupiter atmosphere studies during the past 20 years and the current status of modern theories describing the extended envelopes of hot Jupiters are excellent examples of the progress in understanding planetary atmospheres formation and evolution both in the solar system and in the extrasolar planetary systems.
Exoplanets: Atmospheres of Hot Jupiters
Dmitry V. Bisikalo, Pavel V. Kaygorodov, and Valery I. Shematovich
Composition and Chemistry of the Neutral Atmosphere of Venus
Ann Carine Vandaele
The atmosphere of Venus is quite different from that of Earth: it is much hotter and denser. The temperature and pressure at the surface are 740 K and 92 atmospheres respectively. Its atmosphere is primarily composed of carbon dioxide (96.5%) and nitrogen (3.5%), the rest being trace gases such as carbon monoxide (CO), water vapor (H2O), halides (HF, HCl), sulfur-bearing species (SO2, SO, OCS, H2S), and noble gases. Sulfur compounds are extremely important in understanding the formation of the Venusian clouds which are believed to be composed of sulfuric acid (H2SO4) droplets. These clouds completely enshroud the planet in a series of layers, extending from 50 to 70 km altitude, and are composed of particles of different sizes and different H2SO4/H2O compositions. These act as a very effective separator between the atmospheres below and above the clouds, which show very distinctive characteristics.
Although the second most abundant element in the cosmos is helium, noble gases are also called rare gases. The reason is that they are not abundant on terrestrial planets like the Earth, which is characterized by orders of magnitude depletion of—particularly light—noble gases when compared to the cosmic element abundance pattern. Indeed, geochemical depletion and enrichment processes mean that noble gases are highly versatile tracers of planetary formation and evolution. When our solar system formed—or even before—small grains and first condensates incorporated small amounts of noble gases from the surrounding gas of solar composition, resulting in depletion of light He and Ne relative to heavy Ar, Kr, and Xe, leading to the “planetary type” abundance pattern. Further noble gas depletion occurred during flash heating of mm- to cm-sized objects (chondrules and calcium, aluminum-rich inclusions), and subsequently during heating—and occasionally differentiation—on small planetesimals, which were precursors of planets. Some of these objects are present today in the asteroid belt and are the source of many meteorites. Many primitive meteorites contain very small (micron to sub-micron size) rare grains that are older than our Solar System and condensed billions of years ago in in the atmospheres of different stars, for example, Red Giant stars. These grains are characterized by nucleosynthetic anomalies, in particular the noble gases, such as so-called s-process xenon. While planetesimals acquired a depleted noble gas component strongly fractionated in favor of heavy noble gases, the Sun and also gas giants like Jupiter attracted a much larger amount of gas from the protosolar nebula by gravitational capture. This resulted in a cosmic or “solar type” abundance pattern, containing the full complement of light noble gases. In contrast, terrestrial planets accreted from planetesimals with only minor contributions from the gaseous component of the protosolar nebula, which accounts for their high degree of depletion and essentially “planetary” elemental abundance pattern. The strong depletion in noble gases facilitates their application as noble gas geo- and cosmochronometers; chronological applications are based on being able to determine noble gas isotopes formed by radioactive decay processes, for example, 40Ar by 40K decay, 129Xe by 129I decay, or fission Xe from 238U or 244Pu decay. Particularly ingrowth of radiogenic xenon is only possible due to the depletion of primordial nuclides, which allows insight into the chronology of fractionation of lithophile parent nuclides and atmophile noble gas daughters. Applied to large-scale planetary reservoirs, this helps to elucidate the timing of mantle degassing and evolution of planetary atmospheres. Applied to individual rocks and minerals, it allows radioisotope chronology using short-lived (e.g., 129I–129Xe) or long-lived (e.g., 40K–40Ar) systems. The dominance of 40Ar in the terrestrial atmosphere allowed von Weizsäcker to conclude that most of the terrestrial atmosphere originated by degassing of the solid Earth, which is an ongoing process today at mid-ocean ridges, as indicated by outgassing of primordial helium from newly forming ocean crust. Mantle degassing was much more massive in the past, with most of the terrestrial atmosphere probably formed during the first few 100 million years of Earth’s history, in response to major evolutionary processes of accretion, terrestrial core formation, and the terminal accretion stage of a giant impact that formed our Moon. During accretion, solar noble gases were added to the mantle, presumably by solar wind irradiation of the small planetesimals and dust accreting to form the Earth. While the Moon-forming impact likely dissipated a major fraction of the primordial atmosphere, today’s atmosphere originated by addition of a late veneer of asteroidal and possibly cometary material combined with a decreasing rate of mantle degassing over time. As other atmophile elements behave similarly to noble gases, they also trace the origin of major volatiles on Earth, for example, water, nitrogen, and carbon.
The Planetary Boundary Layer of Mars
The planetary boundary layer of Mars is a crucial component of the Martian climate and meteorology, as well as a key driver of the surface-atmosphere exchanges on Mars. As such, it is explored by several landers and orbiters; high-resolution atmospheric modeling is used to interpret the measurements by those spacecrafts. The planetary boundary layer of Mars is particularly influenced by the strong radiative control of the Martian surface and, as a result, features a more extreme version of planetary boundary layer phenomena occurring on Earth. In daytime, the Martian planetary boundary layer is highly turbulent, mixing heat and momentum in the atmosphere up to about 10 kilometers from the surface. Daytime convective turbulence is organized as convective cells and vortices, the latter giving rise to numerous dust devils when dust is lifted and transported in the vortex. The nighttime planetary boundary layer is dominated by stable-layer turbulence, which is much less intense than in the daytime, and slope winds in regions characterized by uneven topography. Clouds and fogs are associated with the planetary boundary layer activity on Mars.
Hot Planetary Coronas
Valery I. Shematovich and Dmitry V. Bisikalo
The uppermost layers of a planetary atmosphere, where the density of neutral particles is vanishingly low, are commonly called exosphere or planetary corona. Since the atmosphere is not completely bound to the planet by the planetary gravitational field, light atoms, such as hydrogen and helium, with sufficiently large thermal velocities can escape from the upper atmosphere into interplanetary space. This process is commonly called Jeans escape and depends on the temperature of the ambient atmospheric gas at an altitude where the atmospheric gas is virtually collisionless. The heavier carbon, nitrogen, and oxygen atoms can populate the coronas and escape from the atmospheres of terrestrial planets only through nonthermal processes such as photo- and electron-impact energizing, charge exchange, atmospheric sputtering, and ion pickup. The observations reveal that the planetary coronae contain both a fraction of thermal neutral particles with a mean kinetic energy corresponding to the exospheric temperature and a fraction of hot neutral particles with mean kinetic energy much higher than that expected for the exospheric temperature. These suprathermal (hot) atoms and molecules are the direct manifestation of the nonthermal processes taking place in the atmospheres. These hot particles populate the hot coronas, take a major part in the atmospheric escape, produce nonthermal emissions, and react with the ambient atmospheric gas, triggering the hot atom chemistry.
Planetary Atmospheres: Chemistry and Composition
The observed composition of a planetary atmosphere is the product of planetary formation and evolution, including the chemical and physical processes shaping atmospheric abundances into the present day. In the solar system, the gas giant planets Jupiter, Saturn, Uranus, and Neptune possess massive molecular envelopes consisting mostly of H2 and He along with various minor amounts of heavy elements such as C, N, and O (present as CH4, NH3, and H2O, respectively) and numerous additional minor species. The terrestrial planets Venus, Earth, and Mars each possess a relatively thin atmospheric envelope surrounding a rocky surface. The atmospheres of Mars and Venus are characterized by abundant CO2 with a small amount of N2, whereas the atmosphere of the Earth is dominated by N2 and O2. Such differences provide clues to the divergent pathways of atmospheric evolution. Numerous closely coupled physical and chemical processes give rise to the abundances observed in the planetary atmospheres of the solar system. These processes include the maintenance of thermochemical equilibrium, reaction kinetics, atmospheric transport, photochemistry, condensation (including cloud formation) and vaporization, deposition and sublimation, diurnal and seasonal effects, greenhouse effects, surface–atmosphere reactions, volcanic activity, and (in the case of Earth) biogenic and anthropogenic sources. The present understanding of the chemical composition of planetary atmospheres is the result of over a century of observations, including ground-based, space-based, and in situ measurements of the major, minor, trace, and isotopic species found on each planet. These observations have been accompanied by experimental studies of planetary materials and the development of theoretical models to identify the key processes shaping atmospheric abundances observed today.