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The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors., Comment: 78 pages, 14 figures, published

The ion escape of Mars' CO$_2$ atmosphere caused by its dissociation products C and O atoms is {simulated} from present time to $\sim 4.1$ billion years ago (Ga) by {numerical models of the upper atmosphere and its interaction with the solar wind}. The planetward-scattered pick-up ions are used for sputtering estimates of exospheric particles including $^{36}$Ar and $^{38}$Ar isotopes. Total ion escape, sputtering and photochemical escape rates are compared. For solar EUV fluxes $\geq$\,3 times that of today's Sun (earlier than $\sim 2.6$ Ga) ion escape becomes the dominant atmospheric non-thermal loss process until thermal escape takes over during the pre-Noachian eon (earlier than $\sim 4.0\,-\,4.1$ Ga). If we extrapolate the total escape of CO$_2$-related dissociation products back in time until $\sim$ 4.1 Ga we obtain a {maximum} theoretical equivalent to CO$_2$ partial pressure of more than {$\sim 0.4$ bar through non-thermal escape during quiet solar wind conditions}. {However, surface-atmosphere interaction and/or extreme solar events such as frequent CMEs could have increased this value even further. By including the surface as a sink, up to 0.9\,bar, or even up to 1.8\,bar in case of hidden carbonate reservoirs, could have been present at 4.1\,Ga} The fractionation of $^{36}$Ar/$^{38}$Ar isotopes through sputtering and volcanic outgassing from its initial chondritic value of 5.3, as measured in the 4.1 billion years old Mars meteorite ALH 84001, until the present day, however, can be reproduced for assumed CO$_2$ partial pressures of {$\sim0.01 -- 0.4$\,bar without, and $\sim0.4 -- 1.8$\,bar including surface sinks, and} depending on the cessation time of the Martian dynamo (assumed between 3.6\,--\,4.0 Ga) - if atmospheric sputtering of Ar started afterwards., Comment: 63 pages (including 8 pages of supplement), 21 figures and 20 tables (including 12 figures and 15 tables in the supplement), resubmitted to Icarus

With a Monte-Carlo model we investigate the escape of hot oxygen and carbon from the martian atmosphere for four points in time in its history corresponding to 1, 3, 10, and 20 times the present solar EUV flux. We study and discuss different sources of hot oxygen and carbon atoms in the thermosphere and their changing importance with the EUV flux. The increase of the production rates due to higher densities resulting from the higher EUV flux competes against the expansion of the thermosphere and corresponding increase in collisions. We find that the escape due to photodissociation increases with increasing EUV level. However, for the escape via some other reactions, e.g.~dissociative recombination of O$_2^+$, this is only true until the EUV level reaches 10 times the present EUV flux, and then the rates start to decrease. Furthermore, our results show that Mars could not have had a dense atmosphere at the end of the Noachian epoch, since such an atmosphere would not have been able to escape until today. In the pre-Noachian era, most of a magma ocean and volcanic activity related outgassed CO$_2$ atmosphere could have been lost thermally until the Noachian epoch, when non-thermal loss processes such as suprathermal atom escape became dominant. Thus, early Mars could have been hot and wet during the pre-Noachian era with surface CO$_2$ pressures larger than 1 bar during the first 300 Myr after the planet's origin.

The influence of the hydrogen hydrodynamic upper atmosphere escape, driven by the solar soft X-ray and extreme ultraviolet radiation (XUV) flux, on an expected magma ocean outgassed steam atmosphere of early Venus is studied. By assuming that the young Sun was either a weak or moderate active young G star, we estimated the water loss from a hydrogen dominated thermosphere due to the absorption of the solar XUV flux and the precipitation of solar wind produced energetic hydrogen atoms (ENAs). The production of ENAs and their interaction with the hydrodynamic extended upper atmosphere, including collision-related feedback processes, have been calculated by means of Monte Carlo models. ENAs that collide in the upper atmosphere deposit their energy and heat the surrounding gas mainly above the main XUV energy deposition layer. It is shown that precipitating ENAs modify the thermal structure of the upper atmosphere, but the enhancement of the thermal escape rates caused by these energetic hydrogen atoms is negligible. Our results also indicate that the majority of oxygen arising from dissociated H$_2$O molecules is left behind during the first 100 Myr. It is thus suggested that the main part of the remaining oxygen has been absorbed by crustal oxidation., Comment: 6 figures and 2 tables

We present a 3D study of the formation of refractory-rich exospheres around the rocky planets HD219134b and c. These exospheres are formed by surface particles that have been sputtered by the wind of the host star. The stellar wind properties are derived from magnetohydrodynamic simulations, which are driven by observationally-derived stellar magnetic field maps, and constrained by Ly-alpha observations of wind mass-loss rates, making this one of the most well constrained model of winds of low-mass stars. The proximity of the planets to their host star implies a high flux of incident stellar wind particles, thus the sputtering process is sufficiently effective to build up relatively dense, refractory-rich exospheres. The sputtering releases refractory elements from the entire dayside surfaces of the planets, with elements such as O and Mg creating an extended neutral exosphere with densities larger than 10/cm3, extending to several planetary radii. For planet b, the column density of OI along the line of sight reaches 10^{13}/cm2, with the highest values found ahead of its orbital motion. This asymmetry would create asymmetric transit profiles. To assess its observability, we use a ray tracing technique to compute the expected transit depth of the OI exosphere of planet b. We find that the transit depth in the OI 1302.2A line is 0.042%, which is a small increase relative to the continuum transit (0.036%). This implies that the sputtered exosphere of HD219134b is unlikely to be detectable with our current UV instruments., Comment: Accepted to MNRAS. This paper is the second of a pair. Paper I: Folsom et al "Characterisation of the HD219134 multi-planet system I: Observations of stellar magnetism, wind, and high-energy flux"

HD 219134 hosts several planets, with seven candidates reported, and the two shortest period planets are rocky (4-5 $M_{\oplus}$) and transit the star. Here we present contemporaneous multi-wavelength observations of the star HD 219134. We observed HD 219134 with the Narval spectropolarimeter at the Observatoire du Pic du Midi, and used Zeeman Doppler Imaging to characterise its large-scale stellar magnetic field. We found a weak poloidal magnetic field with an average unsigned strength of 2.5 G. From these data we confidently confirm the rotation period of 42 days, measure a stellar inclination of 77$\pm$8 degrees, and find evidence for differential rotation. The projected obliquity of the two transiting super-Earths is therefore between 0 and 20 degrees. We employed HST STIS observations of the Ly$\alpha$ line to derive a stellar wind mass-loss rate of half the solar value ($10^{-14} M_{\odot} {\rm yr}^{-1}$). We further collected photometric transit observations of the closest planet at near-UV wavelengths centred on the Mg II h&k lines with AstroSat. We found no detectable absorption, setting an upper limit on the transit depth of about 3%, which rules out the presence of a giant magnesium cloud larger than 9 planet radii. Finally, we estimated the high-energy flux distribution of HD 219134 as seen by planets b and c. These results present a detailed contemporaneous characterisation of HD 219134, and provide the ingredients necessary for accurately modelling the high-energy stellar flux, the stellar wind, and their impact on the two shortest-period planets, which will be presented in the second paper of this series., Comment: Accepted for publication in MNRAS. This paper is the first of a pair, the second is: Characterisation of the HD 219134 multi-planet system II. Stellar-wind sputtered exospheres in rocky planets b & c. 11 pages 5 figures

Planetary embryos form protoplanets via mutual collisions, which can lead to the development of magma oceans. During their solidification, large amounts of the mantles' volatile contents may be outgassed. The resulting H$_2$O/CO$_2$ dominated steam atmospheres may be lost efficiently via hydrodynamic escape due to the low gravity and the high stellar EUV luminosities. Protoplanets forming later from such degassed building blocks could therefore be drier than previously expected. We model the outgassing and subsequent hydrodynamic escape of steam atmospheres from such embryos. The efficient outflow of H drags along heavier species (O, CO$_2$, noble gases). The full range of possible EUV evolution tracks of a solar-mass star is taken into account to investigate the escape from Mars-sized embryos at different orbital distances. The envelopes are typically lost within a few to a few tens of Myr. Furthermore, we study the influence on protoplanetary evolution, exemplified by Venus. We investigate different early evolution scenarios and constrain realistic cases by comparing modeled noble gas isotope ratios with observations. Starting from solar values, consistent isotope ratios (Ne, Ar) can be found for different solar EUV histories, as well as assumptions about the initial atmosphere (either pure steam or a mixture with accreted H). Our results generally favor an early accretion scenario with a small amount of accreted H and a low-activity Sun, because in other cases too much CO$_2$ is lost during evolution, which is inconsistent with Venus' present atmosphere. Important issues are likely the time at which the initial steam atmosphere is outgassed and/or the amount of CO$_2$ which may still be delivered at later evolutionary stages. A late accretion scenario can only reproduce present isotope ratios for a highly active young Sun, but then very massive steam atmospheres would be required., Comment: 61 pages, 7 figures, 3 tables, accepted to Icarus

Recent years have seen increasing efforts to directly measure some aspects of the general relativistic gravitomagnetic interaction in several astronomical scenarios in the solar system. After briefly overviewing the concept of gravitomagnetism from a theoretical point of view, we review the performed or proposed attempts to detect the Lense-Thirring effect affecting the orbital motions of natural and artificial bodies in the gravitational fields of the Sun, Earth, Mars and Jupiter. In particular, we will focus on the evaluation of the impact of several sources of systematic uncertainties of dynamical origin to realistically elucidate the present and future perspectives in directly measuring such an elusive relativistic effect., Comment: LaTex, 51 pages, 14 figures, 22 tables. Invited review, to appear in Astrophysics and Space Science (ApSS). Some uncited references in the text now correctly quoted. One reference added. A footnote added

The orbital distance at which close-in exoplanets maintain their initial mass is investigated by modelling the maximum expected thermal and nonthermal mass loss rates over several Gyr. Depending on an exosphere formation time and the evolution of the stellar X-ray and EUV flux we expect that thermal evaporation at orbital distances less than 0.05 AU may be an efficient loss process for hydrogen-rich exoplanets with masses less than 0.25 MJup. Our results indicate that nonthermal mass loss induced by Coronal Mass Ejections of the host star can significantly erode weakly magnetized short periodic gas giants. The observed exoplanets Gliese 876d at 0.0208 AU with a mass of about 0.033 MJup and 55 Cnc e at 0.045 AU with a mass of about 0.038 MJup could be strongly eroded gas giants, while HD69830b, at 0.078 AU, HD160691d at 0.09 AU and HD69830c at 0.18 AU belonged most likely since their origin to the Neptune-mass domain. The consequences for the planetary population predicted in paper I (Wuchterl et al. 2006) for CoRoTs first field are: (1) for orbital distances less than 0.05 AU (orbital periods less than days) weakly magnetized or highly irradiated gas giants may loose a large fraction of their initial mass and completely loose their gas envelopes. (2) Observed planetary mass spectra at these periods that resemble the initial ones would indicate a major effect of magnetic field protection and so far unknown thermospheric cooling processes. (3) At distances larger than 0.05 AU the impact of loss processes is minor and the observed mass spectra should be close to the initial ones., Comment: submitted to Astron. Astrophys. Lett

As a consequence of gravitomagnetism, which is a fundamental weak-field prediction of general relativity and ubiquitous in gravitational phenomena, clocks show a difference in their proper periods when moving along identical orbits in opposite directions about a spinning mass. This time shift is induced by the rotation of the source and may be used to verify the existence of the terrestrial gravitomagnetic field by means of orbiting clocks. A possible mission scenario is outlined with emphasis given to some of the major difficulties which inevitably arise in connection with such a venture., Comment: 5 pages, 1 figure, LaTeX, to appear in Advances in Space Research

The main theoretical aspects of gravitomagnetism are reviewed. It is shown that the gravitomagnetic precession of a gyroscope is intimately connected with the special temporal structure around a rotating mass that is revealed by the gravitomagnetic clock effect. This remarkable effect, which involves the difference in the proper periods of a standard clock in prograde and retrograde circular geodesic orbits around a rotating mass, is discussed in detail. The implications of this effect for the notion of ``inertial dragging'' in the general theory of relativity are presented. The theory of the clock effect is developed within the PPN framework and the possibility of measuring it via spaceborne clocks is examined., Comment: 27 pages, LaTeX, submitted to Proc. Bad Honnef Meeting on: GYROS, CLOCKS, AND INTERFEROMETERS: TESTING GENERAL RELATIVITY IN SPACE (22 - 27 August 1999; Bad Honnef, Germany)

Based on the recent finding that the difference in proper time of two clocks in prograde and retrograde equatorial orbits about the Earth is of the order 10^{-7}s per revolution, the possibility of detecting the terrestrial gravitomagnetic field by means of clocks carried by satellites is discussed. A mission taking advantage of this influence of the rotating Earth on the proper time is outlined and the conceptual difficulties are briefly examined., Comment: Talk given at the 32nd COSPAR Scientific Assembly, held at Nagoya, Japan, 12-19 July 1998, 4 pages LATEX

The Mars bow shock location and shape have been determined by examining the Phobos spacecraft magnetometer data. Observations show that the position of the terminator bow shock varies with interplanetary magnetic field orientation in the same way as at Venus. The shock is farthest from Mars in the direction of the interplanetary electric field, consistent with the idea that mass loading plays an important role in the solar wind interaction with Mars. The shock cross section at the terminator plane is asymmetric and is controlled by the interplanetary magnetic field. The shock is farther from Mars during solar maximum. Thus the solar wind interaction with Mars appears to be Venuslike, with a magnetic moment too small to affect significantly the solar wind interaction.

Data obtained with the Magma magnetometer on the subsolar passes of the Phobos spacecraft during its 3 elliptic orbits reveals a turbulent bow shock with a strong foot consistent with the reflection of solar wind protons. The bow shock lies at a subsolar distance of 1.47 + or - .03 R(M). The circular orbit phase of the mission reveals a bow shock with a highly varying location. The median terminator crossing lies at 2.72 Mars radii. The location of the bow shock in the terminator plane is sensitive to neither the EUV flux nor to planetary longitude.