Editorial: Dynamic exospheres of terrestrial bodies through the solar system

1 Background and motivation

The exospheres of terrestrial bodies across the solar system are not static, tenuous envelopes, but highly dynamic regions shaped by a complex interplay of surface processes, radiation, and plasma interactions. This topmost atmospheric layer forms a continuously evolving interface between the object’s surface or the underlying collisional atmosphere, the surrounding plasma environment, and space. The exosphere is generally collisionless, wherein particles travel long distances without any interaction, and is therefore highly sensitive to external forcing (Chamberlain and Hunten, 1987). For example, Mercury has an extremely tenuous and surface-bound exosphere composed of atoms like sodium and potassium released through processes such as photon-stimulated desorption, micrometeoroid impact vaporization, and solar wind sputtering (Killen et al., 2007). This creates a highly variable and asymmetric exosphere that responds rapidly to solar activity and the position of the planet along its orbit. In contrast, Venus, Earth, and Mars possess exospheres dominated by hydrogen and oxygen, originating from the photodissociation of water vapor, molecular oxygen, and carbon dioxide in their lower atmospheres (Johnson et al., 2008; Sánchez-Cano, 2025; Hunten, 1982). These exospheres are shaped by both thermal (Jeans escape) and non-thermal processes such as charge exchange and dissociative recombination, leading to significant atmospheric escape over geological time (Jakosky et al., 2018; Chaffin et al., 2024). The hydrogen corona of Mars also exhibits strong seasonal and solar cycle variability, as observed by the Hubble Space Telescope (HST) and Mars missions like the Mars Atmosphere Volatile EvolutionN (MAVEN), Mars Express (MEx), and Mars Global Surveyor (MGS), indicating a dynamic coupling between the lower atmosphere, upper atmosphere, and solar forcing (Bhattacharyya et al., 2017; Chaffin et al., 2014; Halekas, 2017; Heavens et al., 2018). The Moon also exhibits a transient exosphere, where species such as argon, helium, and hydrogen are released through radiogenic decay and solar wind interactions, forming a highly time-variable and surface-bound environment (Wilson et al., 2006; Wurz et al., 2007). Hence, the exosphere acts as a sensitive tracer of atmospheric loss, surface composition, and star–planet interactions, making it a key region for understanding long-term planetary evolution with implications that extend beyond our Solar System.

2 Contributions to this Research Topic

A total of 15 individual original research articles were contributed to the Research Topic entitled “Dynamic Exospheres of Terrestrial Bodies Through the Solar System”. In this section, we briefly summarize these articles by grouping them under common scientific themes.

2.1 Solar-driven variability of the geocorona

A subset of studies in this Research Topic focuses on the large-scale variability of Earth’s hydrogen exosphere driven by solar activity. Zoennchen et al. compared three-dimensional hydrogen distributions during solar minimum and solar maximum using TWINS Lyman- observations, demonstrating significant changes in the density and morphology of the geocorona over the solar cycle. Complementing this, Jung et al. estimated neutral densities near the subsolar region during solar maximum using XMM-Newton soft X-ray observations. These results demonstrate how variations in solar irradiance regulate the global structure and extent of the geocorona.

2.2 Ion-neutral coupling and space-weather consequences

Several papers focus on how exospheric neutrals influence plasma transport and magnetospheric dynamics on Earth. Lin et al. examined the variability of Earth’s ionospheric outflow in response to different exospheric hydrogen distributions, showing that assumptions about neutral hydrogen at Earth can influence polar wind and cusp outflow predictions. Cucho-Padin et al. investigated the role of the dynamic terrestrial exosphere in storm-time ring current decay, showing that the use of more realistic, time-dependent hydrogen density distributions can significantly modify charge-exchange losses and ring current energy during geomagnetic storms compared to simplified or static exospheric models.

Storm-time variability of the geocorona is further explored by Connor et al. and Zoennchen and Cucho-Padin et al. using complementary approaches. Connor et al. employed kinetic simulations to show how geomagnetic disturbances can alter exospheric hydrogen densities, while Zoennchen and Cucho-Padin used TWINS Lyman- observations to show that even weak geomagnetic activity can produce measurable changes in the outer exosphere.

Finally, Park and Connor analyzed outward fluxes of hydrogen energetic neutral atoms (H-ENAs) measured by IBEX-Lo at geocentric distances of 20–50 Earth radii, providing an important observational constraint on escaping or outward-moving neutral hydrogen populations. These studies collectively highlight the role of incorporating realistic, time-dependent exospheric densities into geospace models, particularly for understanding ion outflow, charge exchange, energetic neutral atom production, and ring current evolution during geomagnetic storms.

2.3 Comparative exospheres of terrestrial solar system bodies

Another set of contributions expands the focus beyond Earth to other terrestrial and airless bodies. Verkercke et al. developed a theoretical framework to predict the interannual variability of sulfur in Mercury’s exosphere and subsurface, connecting regolith diffusion and surface-exosphere exchange. Lierle et al. used resolved sodium D-line emissions to characterize dynamical processes in surface-bound exospheres, with relevance to Mercury, the Moon, and Europa. Shematovich and Bisikalo modeled non-thermal hydrogen populations in the extended corona of Mars, highlighting the role of solar wind charge exchange in producing suprathermal atoms that affect ultraviolet observations and escape. Bhattacharyya et al. presented a theoretical study of hydrogen Lyman- line shapes in the exospheres of terrestrial objects, examining how temperature, altitude, asymmetry, non-isothermality, and non-thermal atoms shape the emergent emission line profile. These studies illustrate how exospheres can be used as sensitive tracers of surface release, photochemistry, atmospheric escape, and solar wind interaction across diverse planetary environments.

2.4 Methods, diagnostics, and enabling models

The Research Topic also includes papers that advance the tools needed to interpret exospheric observations. Tenishev et al. reviewed analytical and numerical approaches for modeling exospheres, including applications to Mars, Venus, Enceladus, the Moon, and cometary environments, providing a methodological bridge between classical models and modern particle-based simulations. Rodríguez-Gómez presented solar spectral irradiance estimates from the CODET model for studies of Earth and Mars exospheres, emphasizing the importance of accurate solar inputs for interpreting planetary hydrogen emissions and their variability. Lammer et al. discussed ion cyclotron waves as a complementary diagnostic of extended exospheres, showing how pickup ions generated from neutral particles can produce wave signatures detectable by magnetometers and plasma instruments at Mercury, Venus, Mars, icy satellites, and comets. Finally, Ashworth et al. compared observed and forward-modeled Balmer- emissions, expanding the diagnostic framework beyond Lyman- and emphasizing the value of forward modeling to interpret faint hydrogen emissions. Collectively, these methodological studies reinforce a central message of the Research Topic: understanding dynamic exospheres requires coordinated progress in observations, kinetic modeling, radiative transfer, solar irradiance specification, and plasma-neutral interaction diagnostics.

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