Gisbert Peter

The organic-rich surface of comet 67P/Churyumov-Gerasimenko as seen by VIRTIS/Rosetta

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.

Exposed water ice on the nucleus of comet 67P/Churyumov–Gerasimenko

Although water vapour is the main species observed in the coma of comet 67P/Churyumov–Gerasimenko and water is the major constituent of cometary nuclei, limited evidence for exposed water-ice regions on the surface of the nucleus has been found so far. The absence of large regions of exposed water ice seems a common finding on the surfaces of many of the comets observed so far. The nucleus of 67P/Churyumov–Gerasimenko appears to be fairly uniformly coated with dark, dehydrated, refractory and organic-rich material. Here we report the identification at infrared wavelengths of water ice on two debris falls in the Imhotep region of the nucleus. The ice has been exposed on the walls of elevated structures and at the base of the walls. A quantitative derivation of the abundance of ice in these regions indicates the presence of millimetre-sized pure water-ice grains, considerably larger than in all previous observations. Although micrometre-sized water-ice grains are the usual result of vapour recondensation in ice-free layers, the occurrence of millimetre-sized grains of pure ice as observed in the Imhotep debris falls is best explained by grain growth by vapour diffusion in ice-rich layers, or by sintering. As a consequence of these processes, the nucleus can develop an extended and complex coating in which the outer dehydrated crust is superimposed on layers enriched in water ice. The stratigraphy observed on 67P/Churyumov–Gerasimenko is therefore the result of evolutionary processes affecting the uppermost metres of the nucleus and does not necessarily require a global layering to have occurred at the time of the comet’s formation.

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko

Rosetta observes sublimating surface ices Comets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchione et al. detected the spectral signature of solid CO2 (dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2 sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasier et al. studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day. Science, this issue p. 1563, p. 1566; see also p. 1536 Solid carbon dioxide on the surface of a comet nucleus has been found by the Rosetta spacecraft. Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice–rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO2 ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO2 ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO2 ice and the depth of the eroded surface layer. We interpret the presence of CO2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

Mercury radiometer and thermal infrared spectrometer--a novel thermal imaging spectrometer for the exploration of Mercury

The MERTIS instrument is a state of the art imaging spectrometer in the TIR range onboard ESAs Bepi Colombo mission to the planet Mercury. MERTIS has four science goals: the study of Mercurys surface composition, identification of rock-forming minerals, mapping of the surface mineralogy, and the study of surface temperature variations and of the thermal inertia. The instrument is designed to achieve a signal-to-noise-ratio above 100 in the 7-14 µm range with a spectral channel width of 90 nm and a nominal spatial ground resolution of 500 m within the complex thermal and radiation environment of Mercury.

VIRTIS imaging spectrometer for the ESA/Venus Express mission

The selection of the Venus Express mission by ESA in 2002 was the occasion to propose the VIRTIS imaging spectrometer for the payload of this mission to Venus. After the discovery of the infrared windows in the near infrared from ground based observations in the 80ies, it was realized that the surface of Venus is accessible to infrared observation on the night side of Venus. Imaging spectroscopy in the visible and near infrared is therefore a powerful tool to study the Venus atmosphere down to its deepest levels. VIRTIS, the imaging spectrometer of the Rosetta mission (Coradini et al, 1998), as the second generation instrument of this kind after the Phobos/ISM (Bibring et al, 1989), Galileo/NIMS (Carlson et al, 1990) Mars Express/OMEGA (Bibring et al, 2004) and Cassini/VIMS (Brown et al, 2000), is perfectly fitted for extensive observations of the infrared and visible spectral images of Venus, with its unique combination of mapping capabilities at low spectral resolution (VIRTIS-M channel) and high spectral resolution slit spectroscopy (VIRTIS-H channel).

CHEOPS: a space telescope for ultra-high precision photometry of exoplanet transits

The CHaracterising ExOPlanet Satellite (CHEOPS) is a joint ESA-Switzerland space mission (expected to launch in 2017) dedicated to search for exoplanet transits by means of ultra-high precision photometry. CHEOPS will provide accurate radii for planets down to Earth size. Targets will mainly come from radial velocity surveys. The CHEOPS instrument is an optical space telescope of 30 cm clear aperture with a single focal plane CCD detector. The tube assembly is passively cooled and thermally controlled to support high precision, low noise photometry. The telescope feeds a re-imaging optic, which supports the straylight suppression concept to achieve the required Signal to Noise.

MERTIS: reflective baffle design and manufacturing

Optical instruments for remote sensing applications frequently require measures for reducing the amount of external, unwanted stray light in the optical instrument path. The reflective planet baffle design and manufacturing process for the thermal infrared imaging spectrometer MERTIS onboard of ESAs cornerstone mission BepiColombo to Mercury is presented. The baffle has to reflect the unwanted solar flux and scattered IR radiation, and minimize the heat load on the instrument. Based on optical stray light simulations and analyses of different baffle concepts the Stavroudis principle showed the best performance and the smallest number of internal reflections. The setup makes use of the optical properties of specific conic sections of revolution. These are the oblate spheroid, generated by rotating an ellipse about its minor axis, and the hyperboloid of one sheet, obtained by the rotation of a hyperbola around its conjugate axis. Due to the demanding requirements regarding surface quality, low mass and high mechanical stability, electroforming fabrication was selected for the baffle. During manufacturing, a layer of high strength nickel alloy is electrodeposited onto a diamond turned aluminum mandrel. The mandrel is subsequently chemically dissolved. Not only the baffle, but also the baffle support structure and other mating components are electroformed. Finally, the baffle and support structure are assembled and joined by an inert gas soldering process. After the optimum baffle geometry and surface roughness has been realized, the remaining total heat flux on the baffle is only dependent on the selection of the appropriate, high reflective coating.

Deep space instrument design for thermal infrared imaging with MERTIS

MERTIS is a miniaturized thermal infrared imaging spectrometer onboard of ESAs cornerstone mission BepiColombo to Mercury. It shall provide measurements in the spectral range from 7-14 μm with a spatial resolution of maximal 300 m and 80 spectral channels in combination with radiometric measurements in the spectral range from 7-40 μm. The instrument concept therefore integrates two detector systems sharing a common optical path consisting of mirror entrance optics and reflective Offner spectrometer. Uncooled micro-bolometer and thermopile radiometer technology are implemented for lowest power consumption. Subsequent viewing of different targets including on-board calibration sources will provide the desired performance. Special attention is spent on the fully passive thermal design in the harsh environment around Mercury. The article will provide an overview of the 3 kg - instrument design and highlight the concept of the subsystems and technologies used. The status of the development process will be reported.

MERTIS-thermal infrared imaging of Mercury: advances in mid-IR remote sensing technology for planetary exploration

MERTIS (MErcury Radiometer and Thermal infrared Imaging Spectrometer) is part of ESAs BepiColombo Mercury Planetary Orbiter mission to the innermost planet of the Solar system. MERTIS is designed to identify rock-forming minerals, to map the surface composition, and to study the surface temperature variations with an uncooled microbolometer detector in the hot environment of Mercury. MERTIS is an advanced IR instrument combining a pushbroom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics and in-fight calibration components for a wavelength range of 7-14 and 7-40 μm, respectively. First results of the ongoing MESSENGER project at Mercury have shown a more complex geology and higher variability of features than previously thought. The MESSENGER studies have demonstrated the need to gain global high-resolution mid-IR spectral and temperature data to achieve a better understanding of the planetary genesis. The MERTIS measurements will acquire this currently missing data set. This article gives a summary of the instrument requirements and its design. We are reporting on the actual instrument development progress, and the status of system and subsystem qualification efforts.

Thermal infrared imaging of Mercury - MERTIS - a new remote sensing technology

MERTIS (MERcury Thermal infrared Imaging Spectrometer) is an advanced infrared remote sensing instrument that is part of the ESA mission BepiColombo to the planet Mercury. The scientific goals of MERTIS science are surface composition analyses, identification of rock-forming minerals, mapping of the surface mineralogy, and studies of surface temperature variations. MERTIS combines a push-broom IR grating spectrometer (TIS) with a radiometer (TIR), which operate in the wavelength region of 7-14 μm (TIS) and 7-40 μm (TIR), respectively. The instrument represents a modular concept of the sensor head, electronic units and power/calibration systems. The integrated instrument approach allows the subsystems TIS and TIR to share the same optics, instrument electronics and in-fight calibration components. The instrument is designed to achieve a signal-to-noise ratio above 100 in the 7-14 μm wavelength range with a spectral channel width of 90 nm. The TIS optical design combines a three mirror anastigmat (TMA) with a modified Offner spectrometer. The spatial resolution will be about 500 m globally and better than 500 m for 5-10% of the Mercurys surface. With an uncooled microbolometer detector, the instrument can be operated in the hot environment of Mercury without the need for a cryogenic cooling system. We are reporting on the measurement requirements, the status of the instrument development, and ongoing qualification efforts.