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The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ −1 ), and the broad absorption feature in the 2.9-to-3.6–micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument on board the Rosetta spacecraft has provided evidence of carbon-bearing compounds on the nucleus of the comet 67P/Churyumov-Gerasimenko. The very low reflectance of the nucleus (normal albedo of 0.060 ± 0.003 at 0.55 micrometers), the spectral slopes in visible and infrared ranges (5 to 25 and 1.5 to 5% kÅ(-1)), and the broad absorption feature in the 2.9-to-3.6-micrometer range present across the entire illuminated surface are compatible with opaque minerals associated with nonvolatile organic macromolecular materials: a complex mixture of various types of carbon-hydrogen and/or oxygen-hydrogen chemical groups, with little contribution of nitrogen-hydrogen groups. In active areas, the changes in spectral slope and absorption feature width may suggest small amounts of water-ice. However, no ice-rich patches are observed, indicating a generally dehydrated nature for the surface currently illuminated by the Sun.

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.

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 .

Nowadays, astronomers want to observe gaps in exozodiacal disks to confirm the presence of exoplanets, or even make actual images of these companions. Four hundred and fifty years ago, Jean-Dominique Cassini did a similar study on a closer object: Saturn. After joining the newly created Observatoire de Paris in 1671, he discovered 4 of Saturn's satellites (Iapetus, Rhea, Tethys and Dione), and also the gap in its rings. He made these discoveries observing through the best optics at the time, made in Italy by famous opticians like Giuseppe Campani or Eustachio Divini. But was he really able to observe this black line in Saturn's rings? That is what a team of optical scientists from Observatoire de Paris - LESIA with the help of Onera and Institut d'Optique tried to find out, analyzing the lenses used by Cassini, and still preserved in the collection of the observatory. The main difficulty was that even if the lenses have diameters between 84 and 239 mm, the focal lengths are between 6 and 50 m, more than the focal lengths of the primary mirrors of future ELTs. The analysis shows that the lenses have an exceptionally good quality, with a wavefront error of approximately 50 nm rms and 200 nm peak-to-valley, leading to Strehl ratios higher than 0.8. Taking into account the chromaticity of the glass, the wavefront quality and atmospheric turbulence, reconstructions of his observations tend to show that he was actually able to see the division named after him.

Meiotic recombination, via the formation of crossovers (COs), re-assorts parental alleles and dictate which traits are inherited together, or not. The recombination rate is not homogeneous across the genome, and uncovering the causes of these fluctuations can inform our understanding of DNA recombination and genome evolution. In this study we show that presence of sequence divergence (polymorphism) between homologous chromosomes increases the local recombination rate in Arabidopsis thaliana. This effect is observed genome-wide, turning cold regions of the genome hot. This relocation of COs towards polymorphic regions occurs at the expense of non-polymorphic ones, which become colder. This global overhaul of the CO landscape is dependent on the mismatch repair (MMR) machinery. This suggests that MMR has a pro-CO role, driving COs to regions of the genome where it could detect sequence differences between homologous chromosomes. Our study demonstrates that polymorphism can be a major recombination driver in A. thaliana.