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Solar energetic events can have considerable effects on planetary ionospheres. However, the erratic nature of these solar energetic events make observations difficult. Here we show a mutual radio occultation observation, which serendipitously occurred just 10 minutes after a large solar flare impacted Mars. This resulted in the largest lower ionospheric layer ever recorded, where it was 278% its typical size. We used in-situ soft x-ray irradiance measurements to show a threefold increase in flux. This infers a different relation of soft X-ray to this layer’s density than previously thought, with variations depending on the amount of spectrum ‘hardening’ leading to the increase of ionisation from secondaries. The response of Mars’ ionosphere to major solar energetic events has been known, but such observations are challenging. Here, the authors show mutual radio occultation observations revealing that a solar flare in May 2024 enhanced the lowest ionospheric layer of Mars by 278% of its typical size.

The surfaces of the Martian moons, Phobos and Deimos may offer a stable environment for long-term operation of platforms. We present a broad assessment of potential scientific investigations, as well as strategic and operational opportunities offered by long-term operation of an instrumented lander. Studies using observations of Mars’ moons, and the detailed new findings expected from the JAXA Martian Moons eXploration (MMX) mission, International Mars Sample Return (MSR) Campaign and other upcoming Mars missions, provide a driver for feasibility and trade studies for follow-on missions that would build on the knowledge gain from those missions. We discuss the scientific questions and operational objectives that may be pertinent for landed platforms on the martian moons, including (1) monitoring and scientific investigations of Mars’ surface and atmosphere, (2) scientific investigations of the martian moons, (3) monitoring and scientific investigations of the space environment, (4) data relay for Mars surface assets or interplanetary missions and 5) use in a Mars navigation/positioning system. We present results from visibility calculations performed using the SPICE observation geometry system for space science missions, and a Phobos shape model. We compute as a function of location on Phobos, visibility quantities that are most relevant to science and operational objectives. These include visibility from Phobos of the Sun, Earth, Mars surface and atmosphere, Deimos, and Jupiter. We also consider occultation events by the Mars atmosphere of Earth and Deimos that may provide opportunities for radio science. Calculations are performed for a study period spanning one Mars year in a hypothetical future operational scenario (1 Jan 2030–18 Nov 2031). We combine visibility metrics to identify locations on Phobos most suitable for long-term operation of a platform. We find the Mars-facing side of Phobos, and limited areas on the leading and trailing sides, satisfy the most requirements defined for Mars and Phobos science, space environment monitoring, and data relay/navigation. We demonstrate that compliance with requirements related to visibility of Mars and its atmosphere are not mutually exclusive with those that are better satisfied on Phobos’ anti-Mars side, such as those aided by maximizing their cumulative view factor to the ecliptic plane (i.e. visibility to the Sun, Earth or outer solar system). Finally, our methodology allows to assess the extent to which combined visibility metrics can meet mission requirements. The process we describe can be used to support landing site identification and selection on planets, moons and small bodies.

Reports of methane detection in the Martian atmosphere have been intensely debated. The presence of methane could enhance habitability and may even be a signature of life. However, no detection has been confirmed with independent measurements. Here, we report a firm detection of 15.5 ± 2.5 ppb by volume of methane in the Martian atmosphere above Gale Crater on 16 June 2013, by the Planetary Fourier Spectrometer onboard Mars Express, one day after the in situ observation of a methane spike by the Curiosity rover. Methane was not detected in other orbital passages. The detection uses improved observational geometry, as well as more sophisticated data treatment and analysis, and constitutes a contemporaneous, independent detection of methane. We perform ensemble simulations of the Martian atmosphere, using stochastic gas release scenarios to identify a potential source region east of Gale Crater. Our independent geological analysis also points to a source in this region, where faults of Aeolis Mensae may extend into proposed shallow ice of the Medusae Fossae Formation and episodically release gas trapped below or within the ice. Our identification of a probable release location will provide focus for future investigations into the origin of methane on Mars.A methane spike 15.5 ± 2.5 parts per billion by volume was detected in the Martian atmosphere above Gale Crater on 16 June 2013 by Mars Express, independently confirming the debated in situ observation by the Curiosity rover a day earlier.

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

Mars Express has opened a new chapter in the exploration of Phobos, thanks to its elliptical orbit that allows for regular flybys, sometimes as close as 50 km. Initially designed for studying Mars’ surface and atmosphere, its instruments have provided a wealth of data, including precise geophysical bulk parameters, insights into Phobos’ interior, high-resolution images, remote sensing data of its surface, and observations of interactions between Phobos and the space environment. This resurgence of interest in the Martian moons revolves around the fundamental question of how they formed. Despite the abundance of data gathered by Mars Express, Phobos’ surface composition remains uncertain with many unresolved questions. This ambiguity prevents a definitive confirmation or refutation of the asteroid capture scenario, which likens Phobos to a D-type asteroid. This scenario contradicts the current orbits of the moons and poses other dynamical challenges. However, an alternative hypothesis suggests that Phobos and Deimos formed through the accretion of a rocky debris disk in Mars’ orbit. Their refined shapes, low mass, and bulk density support this idea. The non-detection of MARSIS echoes implies a porous interior, aligning with the low bulk density, indicative of high porosity. The next phase of Phobos exploration will be led by JAXA’s Mars Moon eXplorer (MMX) mission, set to quasi-orbit Phobos, land on its surface, and return a sample to Earth in 2029.

The Martian mesosphere and thermosphere, the region above about 60 km, is not the primary target of the ExoMars 2016 mission but its Trace Gas Orbiter (TGO) can explore it and address many interesting issues, either in-situ during the aerobraking period or remotely during the regular mission. In the aerobraking phase TGO peeks into thermospheric densities and temperatures, in a broad range of latitudes and during a long continuous period. TGO carries two instruments designed for the detection of trace species, NOMAD and ACS, which will use the solar occultation technique. Their regular sounding at the terminator up to very high altitudes in many different molecular bands will represent the first time that an extensive and precise dataset of densities and hopefully temperatures are obtained at those altitudes and local times on Mars. But there are additional capabilities in TGO for studying the upper atmosphere of Mars, and we review them briefly. Our simulations suggest that airglow emissions from the UV to the IR might be observed outside the terminator. If eventually confirmed from orbit, they would supply new information about atmospheric dynamics and variability. However, their optimal exploitation requires a special spacecraft pointing, currently not considered in the regular operations but feasible in our opinion. We discuss the synergy between the TGO instruments, specially the wide spectral range achieved by combining them. We also encourage coordinated operations with other Mars-observing missions capable of supplying simultaneous measurements of its upper atmosphere.

Still delivering ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Håkan Svedham et al . open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life. Observations of infrared emission from CO 2 , O 2 and NO established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus, but were unable to identify the altitude of the emission. But it is reported here that day-side CO 2 emission extends from 90–120 km altitude, with a peak at ∼115 km. Night-side O 2 emission peaks at 96 km and is visible over the range 95–100 km. The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90–120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft 1 and ground-based 2 , 3 , 4 observations of infrared emission from CO 2 , O 2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission 1 owing to a lack of data and of an adequate observing geometry 5 , 6 . Here we report measurements of day-side CO 2 non-local thermodynamic equilibrium emission at 4.3 µm, extending from 90 to 120 km altitude, and of night-side O 2 emission extending from 95 to 100 km. The CO 2 emission peak occurs at ∼115 km and varies with solar zenith angle over a range of ∼10 km. This confirms previous modelling 7 , and permits the beginning of a systematic study of the variability of the emission. The O 2 peak emission happens at 96 km ± 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted 8 .

We present observations of both the (0–0) and (0–1) bands at 1.27 and 1.58 μm of the O2(a1Δg − X3Σg−) nightglow made with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument aboard Venus Express. The observations were conducted in both nadir and limb viewing modes, the latter constituting the first systematic investigation into the vertical distribution of the volume emission rate of the infrared oxygen nightglow in Venus' upper atmosphere. Limb measurements from 42 orbits covering the latitude range 7°S to 77°N are analyzed. The peak altitude of the volume emission rate occurs typically between 95 and 100 km, with a mean of 97.4 ± 2.5 km. The vertical profile is broader near the equator, with a full width at half maximum of 11 km, a factor 2 larger than at middle latitudes. A double peak is frequently observed, with the lower and upper peaks occurring near 96–98 km and 103–105 km, respectively. On average, the nightglow appears brightest in the vicinity of the antisolar point. This conclusion is consistent with past ground‐based observations and nadir measurements by VIRTIS. We mapped the global mean O2 nightglow intensity from VIRTIS data collected during 880 orbits. Patchy features of the nightglow intensity observed in nadir view are correlated with the thermal brightness at 4.23–4.28 μm. The observed positive correlation is consistent with downwelling (upwelling) of oxygen atoms accompanying compressional heating (expansion cooling) or with modulation by gravity waves. Finally, from simultaneous measurements of the 1.27 and 1.58 μm bands, we have estimated the ratio of the transition probabilities A00/A01 to be 63 ± 8.

Mars Express was conceived and built by ESA as a successor of the unsuccessful Russian Mars-96 mission. It was planned from the onset as an orbiter and lander mission to be able to carry out long-term, remote sensing and in-situ scientific investigations of the planet Mars and its environment. As an exceptionally successful workhorse and a backbone of the Agency’s Science Programme in operation at Mars since end December 2003, Mars Express has proven to be a highly productive mission returning excellent scientific value for the investments made by ESA and its Member States. This paper is intended as the introduction to the series of papers that make this special collection. It briefly reviews the history of the mission, its science goals, its uniqueness while establishing its complementarity with other Mars missions in a collaborative context. It also lists the teams and operational aspects and innovations that made this mission a success. Then the paper highlights Mars Express’s scientific achievements throughout its 20-year lifetime. Mars Express results and discoveries continue playing an essential role in understanding the geological, atmospheric and climate evolution of the Red Planet and determining its potential past habitability. To conclude, a preview of the science and other topics covered by this collection is given. Mars Express, a pioneering mission for Europe at Mars, is currently continuing on its long scientific journey around the Red Planet.

We report and analyze here observations of strong infrared emissions from the limb of the Venus upper atmosphere during daytime, taken by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard Venus Express. We focus on the measurements taken during the first 4 months of nominal operations. The emissions observed at 4.3 μm and at 2.7 μm are attributed to CO2 fluorescence of solar radiation and are detected up to about 160 km and 130 km, respectively, while the CO fluorescence at 4.7 μm is observed up to about 120 km. The emissions are detected in both the channels of VIRTIS, at different spatial and spectral resolutions (resolving powers about 1800 and 400), for the periapsis and the apoapsis of the Venus Express orbit. From these data sets we built up 2‐D maps of the emissions as well as vertical profiles, which are then studied in order to characterize their variations with geophysical parameters, like solar illumination and emission altitude. Several analyses are performed in order to understand the VIRTIS behavior, to determine systematic effects in the data, and to propose appropriate corrections. We also present comparisons with a theoretical nonlocal thermodynamic equilibrium (non‐LTE) model of the Venus upper atmosphere. The agreement is very encouraging, in general, and the main variability observed in the data, with solar zenith angle and altitude, can be understood with the model. We conclude that the present data set opens brilliant perspectives for deriving densities and rotational temperatures in the upper mesosphere and lower thermosphere of Venus.