A chemical survey of exoplanets with ARIEL

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio

First 3D test particle model of Ganymede's ionosphere

International audienceWe present the first three-dimensional multi-species ionospheric model for Ganymede, based on a test particle Monte Carlo approach. Inputs include the electromagnetic field configuration around the moon from the magnetospheric models developed by Leclercq et al. (2016) and by Jia et al. (2009), and the number density, bulk velocity and temperature distributions of the neutral exosphere simulated by Leblanc et al. (2017). According to our simulations, O2+ is the most abundant ion species, followed by O+, H2+ and H2O+. For O+ and O2+, the majority of ions produced impact the moon's surface, while for the other species the majority escapes Ganymede's magnetosphere. For all ion species, the escape occurs either in the direction of corotation of the Jovian plasma or through the Alfvén wings.To validate our model, the output of our simulations, performed under the Galileo G2 flyby conditions, are compared to the observations. These include the electron density derived by the plasma wave instrument (PWS), the ion energy spectrogram measured by the plasma analyzer (PLS) and the associated plasma moments (Frank et al., 1997a).On the one hand, the electron density found by our model is consistently underestimated throughout the flyby, being at least one order of magnitude lower compared to observations. We argue that the prime reason for this discrepancy comes from the exospheric density, which may be underestimated. On the other hand, we find a remarkably good agreement between the modeled ion energy spectrogram and that recorded by PLS, providing a validation of the test particle model. Finally, we compare the modeled plasma moments along the G2 flyby with those analyzed by Frank et al. (1997a). The data seems to be more consistent with an ionosphere dominated by O2+ instead of H+ or O+, as suggested previously in the literature. This supports our finding that O2+ is the dominant ion species close to the surface

Diamagnetic cavity at comet 67P/Churyumov-Gerasimenko: plasma characteristics and dynamics

International audienceThe flux gate magnetometer under the Rosetta plasma consortium (RPC-MAG) onboard the Rosetta orbiter identified a large number of unmagnetized plasma regions around the expanding cometary ionosphere of the comet 67P/Churyumov-Gerasimenko. The coupling between the cometary plasma and neutral gas through ion-neutral and electron-neutral collisions leads to these "diamagnetic cavities" within which the solar wind magnetic field cannot penetrate. In the present work we will study the electron density measurements from the mutual impedance probe (RPC-MIP) to characterize the structure and dynamics of these unmagnetized inner cometary plasma regions. It is observed that these are particularly homogeneous, compared to the highly dynamical magnetized plasmas observed in adjacent magnetized regions. Moreover, during the crossings of multiple, successive diamagnetic regions over time scales of tens of minutes to hours, the plasma density is almost identical in the different unmagnetized regions, suggesting that these unmagnetized regions may be a single diamagnetic structure crossed several times by Rosetta. About 15% of the unmagnetized plasma regions are found to be characterized by dynamic plasma enhancements over the stable background neutral gas variation. Detailed analyses on the plasma characteristics of the diamagnetic cavities and the plasma enhancements within them will be presented in the paper

Diamagnetic cavity at comet 67P/Churyumov-Gerasimenko: plasma characteristics and dynamics

International audienceThe flux gate magnetometer under the Rosetta plasma consortium (RPC-MAG) onboard the Rosetta orbiter identified a large number of unmagnetized plasma regions around the expanding cometary ionosphere of the comet 67P/Churyumov-Gerasimenko. The coupling between the cometary plasma and neutral gas through ion-neutral and electron-neutral collisions leads to these "diamagnetic cavities" within which the solar wind magnetic field cannot penetrate. In the present work we will study the electron density measurements from the mutual impedance probe (RPC-MIP) to characterize the structure and dynamics of these unmagnetized inner cometary plasma regions. It is observed that these are particularly homogeneous, compared to the highly dynamical magnetized plasmas observed in adjacent magnetized regions. Moreover, during the crossings of multiple, successive diamagnetic regions over time scales of tens of minutes to hours, the plasma density is almost identical in the different unmagnetized regions, suggesting that these unmagnetized regions may be a single diamagnetic structure crossed several times by Rosetta. About 15% of the unmagnetized plasma regions are found to be characterized by dynamic plasma enhancements over the stable background neutral gas variation. Detailed analyses on the plasma characteristics of the diamagnetic cavities and the plasma enhancements within them will be presented in the paper

Planetary Atmospheres

International audienc

Planetary Atmospheres

International audienc

Planetary Atmospheres

International audienc