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The Martian ionosphere originates from the ionization of the planetary neutral atmosphere by solar radiation. This conductive layer is embedded within the thermosphere and exosphere of Mars where it forms a highly variable interaction region with the solar wind. The Martian ionosphere has been continuously observed by the three plasma instruments MaRS, MARSIS and ASPERA-3 on Mars Express for the last 20 years (>10 Martian years). Those long-term observations laid a solid foundation for what we know today about the Martian ionosphere, and provided numerous opportunities for collaboration and coordinated observations with other missions. This review describes the most significant achievements of Mars Express for the ionosphere, such as the dynamics and structures of both day and nightside, its variability and couplings with the lower atmosphere, as well as the improvement of atmospheric and ionosphere modelling. Mars Express has also provided a better characterization of the role of several external and internal drivers in controlling the ionosphere, such as the Martian crustal magnetic fields, solar activity, seasons, dust lifting from the surface, and even the direct interaction of the Martian ionosphere with the coma of an Oort-cloud comet (C/2013 A1, Siding Spring). [ABSTRACT FROM AUTHOR]

This work combines 17 years of Mars Express radio science (MaRS) observations with proxies for insolation and local/global atmospheric dust to investigate the combined and individual effects on the dayside ionosphere of Mars from the top down to the ionospheric base. The increase in insolation from orbital apocenter to pericenter in combination with Mars' dust cycle causes an average rise of the whole photochemically dominated region of the dayside ionosphere, ranging from 13 km at the ionospheric base up to 22 km above the main peak during conditions without a global dust storm. The declining phase of the 2018 global dust storm was observed by MaRS on the southern hemisphere and close to pericenter. The observed lifting effect on the whole photochemically dominated region of the ionosphere from the increased insolation and the high local and global atmospheric dust levels exceeds that seen by MaRS from similar seasons during years without a global dust storm. The average ionospheric peak altitude at the subsolar point rises for increasing levels of local atmospheric dust until a maximum elevation is reached. This maximum depends on the available insolation at the top of the planetary atmosphere. Further increases of the local atmospheric dust levels do not lead to a further rise of the average ionospheric peak altitude in the investigated data set. This indicates a limit for the warming/expansion of the lower neutral atmosphere and the consecutive lifting of the ionosphere based on the available insolation and explains why regional dust storms can cause a similar lifting of the ionospheric main peak region as global dust storms. • The whole photochemically dominated region of the dayside ionosphere is affected by the Mars orbital seasons. • Insolation as well as atmospheric dust levels contribute to the rise of the ionosphere from Mars apocenter to pericenter. • The available insolation provides an upper limit for the lifting of the ionosphere during dust events. [ABSTRACT FROM AUTHOR]

Highlights: [•] Development of a flexible 1D photoch. model for the Mars and Venus lower ionospheres. [•] Lower Mars ionosphere behavior (MaRS observations) generally reproduced with the MCD. [•] Best model/MaRS agreement with the MCD low solar flux/clear atmosphere/MY24 scenarios. [•] VenusGRAM (VIRA) is too limited for reproduction of VeRa lower ionosphere observations. [ABSTRACT FROM AUTHOR]

This work uses a subset of "quiet" MaRS ionospheric dayside observations (MaRS quiet , 2004–2017) and a 1-D photochemical model (IonA-2) to investigate the potential formation processes of the excess electron densities merged with the base of the main ionosphere (Mm). 42% of the investigated MaRS observations contain identified Mm, which occur in a large variety of shapes ranging from smoothly decreasing electron densities to peak structures below the base of M1. The Mm appear over the full range of accessible solar zenith angles (50° - 90°) and are found between approximately 70 and 110 km altitude. Their base is found on average deeper in the atmosphere than the base of the averaged undisturbed MaRS electron density profiles. This indicates a dependence of the Mm formation on energy sources that penetrate deep into the atmosphere. This is supported by a strong positive correlation with increasing solar activity when solar flares, coronal mass ejections, and enhanced short solar X-ray and Ly-α intensities are more common. No relationship is found between the Mm occurrence rate and the magnitude/inclination of the weak crustal crustal magnetic field in MaRS quiet. Investigations with the IonA-2 photochemical model for undisturbed and flare conditions show that the ionization of the local neutral atmosphere by solar X-ray radiation <2 nm provides a satisfying explanation for detected Mm features with smoothly decreasing electron densities below the M1 base in combination with moderate slopes of the lower Mm region α Mm and altitudes of the lower boundary h L , S. While sufficient ionization energy reaches the region of interest during flares, no Mm features with peaks below the M1 base occur in any of the model electron density profiles. This supports the conclusion that the subgroup of merged excess electron densities with peaks or intermediate features (Mi) below the M1 base must have an origin different from the sole variability of solar X-ray radiation during undisturbed and solar flare conditions. The size of the identified Mm makes an exclusive meteoric origin of the Mm peak structures unlikely. It is indicative from the IonA-2 model results that the general increase/decrease of solar X-ray <2 nm leads to a correlated response of the Mm region. The sporadic occurrence of the merged excess electron densities in the MaRS observations is therefore assumed to be a combination of observational (increased observation noise level compared to the available amount of X-ray radiation <2 nm, shift of the lower baseline by ionospheric deviations from radial symmetry) and environmental (e.g. variations in solar X-ray) factors. [ABSTRACT FROM AUTHOR]

The objective of this White Paper, submitted to ESA's Voyage 2050 call, is to get a more holistic knowledge of the dynamics of the Martian plasma system, from its surface up to the undisturbed solar wind outside of the induced magnetosphere. This can only be achieved with coordinated multi-point observations with high temporal resolution as they have the scientific potential to track the whole dynamics of the system (from small to large scales), and they constitute the next generation of the exploration of Mars analogous to what happened at Earth a few decades ago. This White Paper discusses the key science questions that are still open at Mars and how they could be addressed with coordinated multipoint missions. The main science questions are: (i) How does solar wind driving impact the dynamics of the magnetosphere and ionosphere? (ii) What is the structure and nature of the tail of Mars' magnetosphere at all scales? (iii) How does the lower atmosphere couple to the upper atmosphere? (iv) Why should we have a permanent in-situ Space Weather monitor at Mars? Each science question is devoted to a specific plasma region, and includes several specific scientific objectives to study in the coming decades. In addition, two mission concepts are also proposed based on coordinated multi-point science from a constellation of orbiting and ground-based platforms, which focus on understanding and solving the current science gaps. [ABSTRACT FROM AUTHOR]

The radio science experiment RSI on-board Rosetta determined the mass of the nucleus of comet 67P/Churyumov–Gerasimenko at the start of the prime mission from 2014 August to November (GM  = 666.2 ± 0.2 m3 s−2 or 9982 ± 3 × 1012 kg) and shortly before the end of the mission from 2016 July to September (GM  = 665.5 ± 0.1 m3 s−2 or 9971.5 ± 1.5 × 1012 kg). The mass-loss is ΔM  = 10.5 ± 3.4·109 kg, about 0.1 per cent of the nucleus mass. Almost 50 per cent of the mass-loss occurred during the 32 d before and 62 d after perihelion. The nucleus mass combined with the new very precise nucleus volume of 18.56 ± 0.02 km3 yields a bulk density of 537.8 ± 0.6 kg m−3. This low bulk density suggests that the nucleus is highly porous. The porosity is constrained by the observed bulk density, the density of ices, mostly water ice, and the density of compacted nucleus dust material. For a range of compacted dust material density from 2000 to 3500 kg m−3, the porosity varies between 65–79 per cent when the dust-to-ice mass ratio F nucleus for the nucleus body lies in the range 3 ≤  F nucleus ≤ 7. The nucleus is thus a highly porous very dusty body with very little ice. The total mass-loss ΔM puts hard constraints on the models of interpretation of the observations from other instruments on Rosetta. The loss from gas, based on ROSINA and MIRO observations, suggests that F coma, the dust-to-gas mass ratio of cometary matter in the coma beyond the Hill sphere of the nucleus is of the order of F coma ≤ 0.5 and therefore at least six times, and perhaps as much as 14 times, smaller than F nucleus. It seems that the lost gas mass was overestimated by the instruments. The lost gas mass shall be 5.25 × 109 kg for F coma = 1. For any other gas mass values > 5.25 × 109 kg F coma is < 1. Most of the lifted dust mass will not leave the cometary Hill sphere but falls back to the nucleus. For 3 ≤  F nucleus ≤ 7 and F coma = 0.5, the fallback mass is between 1.8 ·ΔM and 4.3 ·ΔM. [ABSTRACT FROM AUTHOR]

The influence of solar zenith angle (SZA) and solar irradiance has been well characterized for the V2 layer in the Venus ionosphere, but not the V1 layer, where previous efforts were limited by data scarcity and incomplete SZA coverage. Here we use more than 200 radio occultation profiles from Venus Express with good SZA coverage to characterize how the V1 peak altitude, peak density, and morphology respond to changes in SZA and solar activity. The V1 and V2 peak altitudes vary little with SZA, and both peak electron densities vary with SZA in an approximately Chapman-like manner. These results imply that the thermal structures of the atmosphere and ionosphere between ∼125 km and ∼140 km vary little with SZA. As solar activity increases, the ratio of the V1 to V2 peak density increases, and the V1 morphology changes more than the V2 morphology. These results are due to the soft X-ray flux increasing relative to the EUV flux as solar activity increases. We compare the behavior of the V1 layer to the analogous M1 layer at Mars, and find that their peak altitudes respond differently to changes in SZA and solar activity. The V1 peak density also increases more with solar activity than the M1 peak density. These distinct behaviors arise from differences in their underlying neutral atmospheres. [ABSTRACT FROM AUTHOR]