Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

We collect observational evidence that supports the scheme of mass transfer on the nucleus of comet 67P/Churyumov–Gerasimenko. The obliquity of the rotation axis of 67P causes strong seasonal variations. During perihelion the southern hemisphere is four times more active than the north. Northern territories are widely covered by granular material that indicates back fall originating from the active south. Decimetre sized chunks contain water ice and their trajectories are influenced by an antisolar force instigated by sublimation. OSIRIS observations suggest that up to 20 per cent of the particles directly return to the nucleus surface taking several hours of traveltime. The back fall covered northern areas are active if illuminated but produce mainly water vapour. The decimetre chunks from the nucleus surface are too small to contain more volatile compounds such as CO₂ or CO. This causes a north–south dichotomy of the composition measurements in the coma. Active particles are trapped in the gravitational minimum of Hapi during northern winter. They are ‘shock frozen’ and only re-activated when the comet approaches the sun after its aphelion passage. The insolation of the big cavity is enhanced by self-heating, i.e. reflection and IR radiation from the walls. This, together with the pristinity of the active back fall, explains the early observed activity of the Hapi region. Sobek may be a role model for the consolidated bottom of Hapi. Mass transfer in the case of 67P strongly influences the evolution of the nucleus and the interpretation of coma measurements.

International audienceThe SPICAM experiment onboard Mars Express has accumulated during the last decade a wealth of observations that has permitted a detailed characterization of the atmospheric composition and activity from the near-surface up to above the exosphere. The SPICAM climatology is one of the longest assembled to date by an instrument in orbit around Mars, offering the opportunity to study the fate of major volatile species in the Martian atmosphere over a multi-(Mars)year timeframe. With his dual ultraviolet (UV)-near Infrared channels, SPICAM observes spectral ranges encompassing signatures created by a variety atmospheric gases, from major (CO2) to trace species (H2O, O3). Here, we present a synthesis of the observations collected for water vapor, ozone, clouds and dust, carbon dioxide, exospheric hydrogen and airglows. The assembled climatology covers the MY 27 to MY 31 period. However, the monitoring of UV-derived species was interrupted at the end of 2014 (MY30) due to failure of the UV channel. A SO2 detection attempt was undertaken, but proved unsuccessful from regional to global scales (with upper limit greater than already published ones). One particular conclusion that stands out from this overview work concerns the way the Martian atmosphere organizes an efficient mass transfer between the lower and the upper atmospheric reservoirs. This highway to space, as we name it, is best illustrated by water and hydrogen, both species having been monitored by SPICAM in their respective atmospheric reservoir. Coupling between the two appear to occur on seasonal timescales, much shorter than theoretical predictions

The Earth's hydrogen exosphere Lyman‐α radiation was mapped with the Solar Wind Anisotropies/Solar and Heliospheric Observatory (SWAN/SOHO) instrument in January 1996, 1997, and 1998 (low solar activity). The use of a hydrogen absorption cell allowed to disentangle the interplanetary emission from the geocoronal one and to assign the absorbed signal almost entirely to the geocorona. The geocorona was found to extend at least up to 100 Earth radii (R_E) with an intensity of 5 Rayleigh, an unprecedented distance well exceeding the recent results of Lyman Alpha Imaging Camera (LAICA) imager (∼50 R_E), and encompassing the orbit of the Moon (∼60 R_E). We developed a numerical kinetic model of the hydrogen atoms distribution in the exosphere, which includes the solar Lyman‐α radiation pressure and the ionization. The radiation pressure compresses the H exosphere on the dayside, producing a bulge of H density between 3 and 20 R_E, which fits observed intensities very well. The SWAN Lyman‐α distribution of intensity was compared both to LAICA (2015) and to Orbiting Geophysical Observatory number 5 (1968) measurements. Integrated H densities of SWAN at a tangent distance of 7 R_E are larger than LAICA/Orbiting Geophysical Observatory number 5 by factors 1.1–2.5, while we should expect a stronger effect of the radiation pressure at solar max. We discuss the possible role of H atoms in satellite orbits to explain this apparent contradiction. An onion‐peeling technique is used to retrieve hydrogen number density in the exosphere for the three SWAN observations. They show an excess of density versus models at large distances, which is likely due to nonthermal atoms (not in the model).

Monitoring CO₂ from space is essential to characterize the spatiotemporal distribution of this major greenhouse gas and quantify its sources and sinks. The mixing ratio of CO₂ to dry air can be derived from the CO₂∕O₂ column ratio. The O₂ column is usually derived from its absorption signature on the solar reflected spectra over the O₂ A band (e.g. Orbiting Carbon Observatory-2 (OCO-2), Thermal And Near infrared Sensor for carbon Observation (TANSO)/Greenhouse Gases Observing Satellite (GOSAT), TanSat). As a result of atmospheric scattering, the atmospheric path length varies with the aerosols' load, their vertical distribution, and their optical properties. The spectral distance between the O₂ A band (0.76 µm) and the CO₂ absorption band (1.6 µm) results in significant uncertainties due to the varying spectral properties of the aerosols over the globe. There is another O₂ absorption band at 1.27 µm with weaker lines than in the A band. As the wavelength is much closer to the CO₂ and CH₄ bands, there is less uncertainty when using it as a proxy of the atmospheric path length to the CO₂ and CH₄ bands. This O₂ band is used by the Total Carbon Column Observing Network (TCCON) implemented for the validation of space-based greenhouse gas (GHG) observations. However, this absorption band is contaminated by the spontaneous emission of the excited molecule O₂ ^*, which is produced by the photo-dissociation of O₃ molecules in the stratosphere and mesosphere. From a satellite looking nadir, this emission has a similar shape to the absorption signal that is used. In the frame of the CNES (Centre National d'Études Spatiales – the French National Centre for Space Studies) MicroCarb project, scientific studies have been performed in 2016–2018 to explore the problems associated with this O₂ ^* airglow contamination and methods to correct it. A theoretical synthetic spectrum of the emission was derived from an approach based on A₂₁ Einstein coefficient information contained in the line-by-line high-resolution transmission molecular absorption (HITRAN) 2016 database. The shape of our synthetic spectrum is validated when compared to O₂ ^* airglow spectra observed by the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY)/Envisat in limb viewing. We have designed an inversion scheme of SCIAMACHY limb-viewing spectra, allowing to determine the vertical distribution of the volume emission rate (VER) of the O₂ ^* airglow. The VER profiles and corresponding integrated nadir intensities were both compared to a model of the emission based on the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model. The airglow intensities depend mostly on the solar zenith angle (both in model and data), and the model underestimates the observed emission by ∼15 %. This is confirmed with SCIAMACHY nadir-viewing measurements over the oceans: in such conditions, we have disentangled and retrieved the nadir O₂ ^* emission in spite of the moderate spectral resolving power (∼860) and found that the nadir SCIAMACHY intensities are mostly dictated by solar zenith angle (SZA) and are larger than the model intensities by a factor of ∼1.13. At a fixed SZA, the model airglow intensities show very little horizontal structure, in spite of ozone variations. It is shown that with the MicroCarb spectral resolution power (25 000) and signal-to-noise ratio (SNR), the contribution of the O₂ ^* emission at 1.27 µm to the observed spectral radiance in nadir viewing may be disentangled from the lower atmosphere/ground absorption signature with a great accuracy. Indeed, simulations with 4ARCTIC radiative transfer inversion tool have shown that the CO₂ mixing ratio may be retrieved with the accuracy required for quantifying the CO₂ natural sources and sinks (pressure-level error ≤1 hPa; $X_{{\mathrm{CO}}_{\mathrm{2}}}$ accuracy better than 0.4 ppmv) with the O₂ 1.27 µm band only as the air proxy (without the A band). As a result of these studies (at an intermediate phase), it was decided to include this band (B4) in the MicroCarb design, while keeping the O₂ A band for reference (B1). Our approach is consistent with the approach of Sun et al. (2018), who also analysed the potential of the O₂ 1.27 µm band and concluded favourably for GHG monitoring from space. We advocate for the inclusion of this O₂ band on other GHG monitoring future space missions, such as GOSAT-3 and EU/European Space Agency (ESA) CO₂-M missions, for a better GHG retrieval.