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.
Dmitry V. Bisikalo, Pavel V. Kaygorodov, and Valery I. Shematovich
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.
Maxim Litvak and Anton Sanin
The Moon and Mars are the most explored planetary bodies in the solar system. For the more than 60 years of the space era, dozens of science robotic missions have explored the Moon and Mars. The primary scientific goal for many of these missions was declared to be a search for surface or ground water/water ice and gaining an understanding of its distribution and origin. Today, for the Moon, the focus of scientific exploration has moved to the lunar polar regions and permanently shadowed regions (PSRs). PSRs do not receive any direct sunlight and are frozen at very low temperatures (< 120 K), acting as cold traps. They are considered to be a storehouse that preserves records of the solar system’s evolution by trapping water ice and potentially other volatile deposits brought by comets and asteroids over billions of years. For Mars, the water/water ice search was part of an attempt to find traces of ancient extraterrestrial life and possibly to understand how life appeared on Earth. Current Mars is cold and dry, but its high latitudes and some equatorial regions are enriched with surface and subsurface water ice. Scientists argue that oceans could have existed on ancient Mars if it was warm and wet and that different life forms could have originated similar to Earth’s. If this is the case, then biomarkers could be preserved in the Martian ground ice depositions. Another popular idea that ties water ice permafrost on the Moon and Mars is related to the expected future human expansion to deep space. The Moon and Mars are widely considered to be the first destinations for future manned space-colony missions or even space-colony missions. In this scenario, the long-term presence and survival of astronauts on the lunar or Martian surface strongly depend on in situ resource utilization (ISRU). Water ice is at the top of the ISRU list because it could be used as water for astronauts’ needs. Its constituents, oxygen and hydrogen, could be used for breathing and for rocket fuel production, respectively. The Moon is the closest body to Earth and discussion about presence of water ice on the Moon has both scientific and practical interest, especially for planning manned space missions. The focus further in space is on how subsurface water ice is distributed on Mars. A related topic is the debates about whether ancient Mars was wet and warm or if, for most of its history, the Martian surface was covered with glaciers. Finally, there are fundamental questions that should be answered by upcoming Mars and Moon missions.