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On 4 May 2022, the seismometer on Mars observed the largest marsquake (S1222a) during its operation. One of the most specific features of S1222a is the long event duration lasting more than 8 hr, in addition to the clear appearance of body and surface waves. As demonstrated on Earth, by modeling a long‐lasting and scattered surface wave with the radiative transfer theory under the isotropic scattering condition, we estimated the scattering and intrinsic quality factors of Mars (Qs and Qi). This study especially focused on the frequency range between 0.05–0.09 Hz, where Qs and Qi have not been constrained yet. Our results revealed that Qi = 1,000–1,500 and Qs = 30–500. By summarizing the Martian Qi and Qs estimated so far and by comparing them with those of other celestial bodies, we found that, overall, the Martian scattering and absorption properties showed Earth‐like values. Plain Language Summary: Since February 2019, NASA's InSight (Interior Exploration using Seismic Investigations, Geodesy, and Heat Transport) has been conducting quasi‐continuous seismic observation for more than three years. The seismic data from Mars has contributed significantly to a better understanding of the interior structure and the seismicity of the red planet. On 4 May 2022 (1222 Martian days after landing), another key event occurred, called S1222a. The event showed the largest seismic moment release (magnitude 4.7) and extremely long duration (>8 hr) with intense seismic scattering. As demonstrated on Earth, the long‐lasting scattered waves are useful for retrieving information about the structural heterogeneity within a planet. In this study, by applying the radiative transfer theory—which considers the energy transportation from the seismic source to the observation point—to Mars, we evaluated the energy decay rate due to seismic scattering and energy absorption by a medium. By comparing our results with those of other solid bodies, we found that the Martian scattering and absorption features were closer to the terrestrial ones than to the lunar ones. Key Points: We modeled the scattering effect of the largest marsquake (S1222a) using radiative transfer theory on a spherical MarsThe inversion revealed that the intrinsic and scattering quality factors below 0.1 Hz are 1,000–1,500 and 30–500, respectivelyWe summarized the Martian quality factors derived so far and found that they are relatively Earth‐like rather than Moon‐like [ABSTRACT FROM AUTHOR]

The intense seismic scattering seen in Apollo lunar seismic data is one of the most characteristic features, making the seismic signals much different from those observed on the Earth. The scattering is considered to be attributed to subsurface heterogeneity. While the heterogeneous structure of the Moon reflects the past geological activities and evolution processes from the formation, the detailed description remains an open issue. Here, we present a new model of the subsurface heterogeneity within the upper lunar crust derived through a full 3D seismic wave propagation simulation. Our simulation successfully reproduced the Apollo seismic observations, leading to a significant update of the scattering properties of the Moon. The results showed that the scattering intensity of the Moon is about 10 times higher than that of the heterogeneous region on the Earth. The quantified scattering parameters could give us a constraint on the surface evolution process of the Moon and enable the comparative study for answering a fundamental question of why the seismological features are different on various planetary bodies. Plain Language Summary: In the past Apollo missions, several seismometers were installed on the nearside of the Moon and they brought us the first seismic records from an extraterrestrial body. The derived lunar seismic data surprised us because of their extremely long duration (1–2 hr) and spindle‐shaped form, which were barely observed on Earth. These characteristics, which are different from earthquakes, are thought to reflect the subsurface heterogeneity. However, the inhomogeneous structure within the lunar crust is poorly constrained. To improve our knowledge of wave propagation on an extraterrestrial body, this study evaluated the subsurface heterogeneity through 3D seismic wave propagation simulation. After running some simulations under various structure settings, we found that a certain set of parameters well reproduced the Apollo seismic data, resulting in a new heterogeneous structure model of the Moon. The evaluated parameters were compared with those measured on the Earth and Mars, and we found that the Moon is more heterogeneous than others by about 10 times. This kind of comparison makes it easier to interpret the observed seismic signals on each solid body. Also, it is useful to explain the differences in their surface evolution scenarios. We believe that our results contribute to further extending comparative planetology. Key Points: Through full 3D seismic wave propagation simulation, we quantitatively evaluated the lunar seismic scattering propertiesWe found that a 10‐km thick scattering layer with 10% velocity fluctuation well‐reproduced the Apollo seismic observationOur results show that the upper lunar crust is about 10 times more heterogeneous than that of the Earth and Mars [ABSTRACT FROM AUTHOR]

A method for retrieving temperature and composition from 150 to 350 km in Earth's thermosphere using total number density measurements made via extreme ultraviolet (EUV) solar occultations by the Project for OnBoard Autonomy 2/Large Yield Radiometer (PROBA2/LYRA) instrument is presented. Systematic and random uncertainties are calculated and found to be less than 5% for the temperature measurements and 5%–20% for the composition measurements. Regression coefficients relating both temperature and the [O]/[N2] abundance ratio with EUV irradiance at 150, 275, and 350 km are reported. Additionally, it is shown that the altitude where [O] equals [N2] decreases with increasing solar EUV irradiance, an effect attributed to thermal expansion. Temperatures from 2010 to 2017 are compared with estimates from the MSIS empirical model and show good agreement at the dawn terminator but LYRA is markedly cooler at the dusk terminator, with the MSIS‐LYRA temperature difference increasing with solar activity. Anthropogenic cooling can explain this discrepancy at periods of lower solar activity, but the divergence of temperature with increasing solar activity remains unexplained. LYRA measurements of the exospheric temperature sensitivity to EUV irradiance are compared with contemporaneous measurements made at Mars, showing that the exospheric temperature at Mars is approximately half as sensitive to EUV variability as that of Earth. Plain Language Summary: The Large Yield Radiometer (LYRA) instrument onboard the Project for OnBoard Autonomy 2 (PROBA2) satellite measures the structure of the upper atmosphere by tracking the absorption of sunlight as it passes through the atmosphere as the Sun sets or rises as viewed from the satellite. This paper presents a method for determining atmospheric temperature to a high degree of accuracy from 150 to 350 km from the PROBA2/LYRA measurements. Additionally, a method for determining the relative abundance of atomic Oxygen and molecular Nitrogen is presented. Both temperature and relative abundance are known to change with changing extreme ultraviolet (EUV) radiation from the Sun. The dependence of these values on solar EUV radiation is reported, and is shown to be different from predictions of the MSIS model, frequently used to estimate atmospheric temperature and composition. Additionally, these results for Earth are compared with similar measurements made at Mars and show that the Earth's atmosphere heats twice as efficiently from solar EUV radiation as the Mars atmosphere. Key Points: Temperature and composition from 150 to 350 km from 2010 to 2017 are presentedDusk temperatures are markedly cooler than MSIS predictionsMars exospheric temperature is half as sensitive to extreme ultraviolet variability as that of Earth [ABSTRACT FROM AUTHOR]

We test the hypothesis that their dominant driver of a planetary ambipolar electric field is the ionospheric electron pressure gradient (∇Pe). The ionospheres of Venus and Mars are mapped using Langmuir probe measurements from NASA's Pioneer Venus Orbiter (PVO) and Mars Atmosphere and Volatile EvolutioN (MAVEN) missions. We then determine the component of the ionospheric potential drop that can be explained by the electron pressure gradient drop along a simple draped field line. At Mars, this calculation is consistent with the mean potential drops measured statistically by MAVEN. However, at Venus, contrary to our current understanding, the thermal electron pressure gradient alone cannot explain Venus' strong ambipolar field. These results strongly motivate a return to Venus with a comprehensive plasmas and fields package, similar to that on MAVEN, to investigate the physics of atmospheric escape at Earth's closest analog. Plain Language Summary: Every planet with an atmosphere generates a weak electric field, called an "ambipolar field," which plays a critical role in the escape of the ionosphere. Until recently, these fields had never been measured due to their low strength. However, by measuring the subtle shifts in the energies of electrons generated in the ionosphere, the total potential drop associated with this field has recently been measured at both Venus and Mars. These measurements permit us to make the first investigation of the fundamental physics that underpins this field. Specifically, we test the long‐held hypothesis that ambipolar fields are primarily generated by the gradient of electron pressure along magnetic field lines that connect the ionosphere to space. We find the potential drop at Mars is consistent with theory, but Venus' field is 10 times stronger than expected. Key Points: We map the ionospheres of Venus and Mars to investigate whether ambipolar fields are generated by the thermal electron pressure gradientMars' ambipolar potential drop is consistent with what would be expected from existing theory (∼0.7 V peaking at 220 km)Venus' potential drop (10 V) far exceeds what can be explained by the mean electron pressure gradient (1 V), motivating further research [ABSTRACT FROM AUTHOR]

Geologic mapping has been used for over 200 years as a technique to synthesize a complicated surface into a more simplified product, identifying similar types of surface features, and placing them into a relative stratigraphy. Geomorphologic mapping has applied those principles to other terrestrial bodies throughout the solar system and has formed an important product set to understand these surfaces, plan future exploration, and conduct different scientific endeavors. We created a geomorphologic map of the New Horizons encounter hemisphere of Pluto's binary companion, Charon. Ten primary geomorphologic unit categories were identified, covering approximately 35% of Charon's surface, and we used lower resolution data to speculate about other regions of Charon. Over 1,000 linear features were mapped, nearly 90% of them are tectonic in nature, and we use these to provide evidence of Charon being active in its past. Additionally, we placed the mapped features into a chronostratigraphic sequence, and we present a possible surface history for the body. The northern terrain typified by large crustal blocks is the oldest, having fractured early in Charon's history, and potentially similar blocks were submerged in a cryoflow of which the now solid surface of Vulcan Planitia is the remnant today. Plain Language Summary: Planetary scientists will create geologic maps of a surface to perform different kinds of studies. Geologic maps identify features of different types and help to distill many different kinds of data into an easier to use format from which scientific investigations can be done. We have created such a map for Pluto's largest companion, Charon, imaged during the flyby of New Horizons in July 2015. This geologic map covers approximately 35% of the best‐imaged region of Charon, and we mapped 16 different geologic units, over 1,000 linear features, and a variety of albedo features. We developed a chronology that places these features in time‐order, and we provide potential interpretations of the different features that we mapped. We found that Charon has potentially one of the most convincing examples of possible large cryoflow(s) of any heretofore imaged body in the solar system. Key Points: We present a new geomorphologic map of New Horizons' encounter hemisphere of Charon, showing units, linear features, and albedo featuresWe introduce a chronostratigraphic system to place features and units in a historic sequence and contextWe interpret units and features in local, regional, and global context, pulling from knowledge of other solar system bodies [ABSTRACT FROM AUTHOR]

Abstract: On the moon and other airless bodies, ballistically emplaced ejecta transitions from a thinning, continuous inner deposit to become discontinuous beyond approximately one crater radius from the crater rim and can further break into discrete rays and secondary craters. In contrast, on Mars, ejecta often form continuous, distinct, and sometimes thick deposits that transition to a low ridge or escarpment that may be circular or lobate. The Martian ejecta type has been variously termed pancake, rampart, lobate, or layered, and in this work we refer to it as “abrupt termini” ejecta (ATE). Two main formation mechanisms have been proposed, one requiring interaction of the ejecta with the atmosphere and the other mobilization of near‐surface volatiles. ATE morphologies are also unambiguously seen on Ganymede, Europa, Dione, and Tethys, but they are not as common as on Mars. We have identified up to 38 craters on Charon that show signs of ATE, including possible distal ramparts and lobate margins. These ejecta show morphologic and morphometric similarities with other moons in the solar system, which are a subset of the properties observed on Mars. From comparison of these ejecta on Charon and other solar system bodies, we find the strongest support for subsurface volatile mobilization and ejecta fluidization as the main formation mechanism for the ATE, at least on airless, icy worlds. This conclusion comes from the bodies on which they are found, an apparent preference for certain terrains, and the observation that craters with ATE can be near to similarly sized craters that only have gradational ejecta. [ABSTRACT FROM AUTHOR]