Space missions, as EChO, or ground based experiments, as SPHERE, have been proposed to measure the atmospheric transmission, reflection and emission spectra. In particular, EChO is foreseen to probe exoplanetary atmospheres over a wavelength range from 0.4 to 16 micron by measuring the combined spectra of the star, its transmission through the planet atmosphere and the emission of the planet. The planet atmosphere characteristics and possible biosignatures will be inferred by studying such composite spectrum in order to identify the emission/absorption lines/bands from atmospheric molecules such as water (H2O), carbon monoxide (CO), methane (CH4), ammonia (NH3) etc. The interpretation of the future EChO observations depends upon the understanding of how the planet atmosphere affects the stellar spectrum and how this last affects the planet emission/absorption. In particular, it is important to know in detail the optical characteristics of gases in the typical physical conditions of the planetary atmospheres and how those characteristics could be affected by radiation induced phenomena such as photochemical and biological one. Insights in this direction can be achieved from laboratory studies of simulated planetary atmosphere of different pressure and temperature conditions under the effects of radiation sources, used as proxies of different bands of the stellar emission.
The Visible and Near Infrared (VNIR) is one of the modules of EChO, the Exoplanets Characterization Observatory
proposed to ESA for an M-class mission. EChO is aimed to observe planets while transiting by their suns. Then the
instrument has be designed to assure a high efficiency over the whole spectral range. In fact, it has to be able to observe
stars with an apparent magnitude Mv= 9÷12 and able to see contrasts of 10-4÷10-5 in order to reveal the characteristics of
the atmospheres of the exoplanets under investigation.
VNIR was originally designed for covering the spectral range from 0.4 to 1.0 μm [1] but now the design has been
reviewed and its spectral range has been extended up to 2.5 μm. It is a spectrometer in a cross-dispersed configuration
that, then, uses the combination of a diffraction grating and a prism to spread the light in different wavelengths and in a
useful number of orders of diffraction. Its resolving power is about 330 over the entire spectral range and its field of view
is approximately 2 arcsec.
The spectrometer is functionally split into two channels respectively working in the 0.4-1.0 μm and 1.0-2.5 μm
spectral ranges. Such a solution is imposed by the fact the light at low wavelengths has to be shared with the EChO Fine
Guiding System (FGS) devoted to the pointing of the stars under observation. The instrument works at 45K and its
weight is 6 kg.
This paper is devoted to the miniaturized Fourier Transform Spectrometer “MicroMIMA” (Micro Mars Infrared
MApper) design. The instrument has been designed for the spectral characterization and monitoring of the Martian
atmosphere, bound to investigate its composition, minor species abundances and evolution during time. The spectral
resolution of MicroMIMA is of 2 cm-1 (with the option to be extended up to 1 cm-1) that allows to recognize the spectral features of the main elements of interest in the atmosphere and in particular to assess methane abundance with ppb
resolution. The instrument configuration has been optimized in order to achieve the highest sensitivity in the 2 to 5 μm
spectral range, along with the reduction of noise, i.e. the Signal-to-Noise Ratio (SNR) has been used as figure of merit.
The optimization has been carried-out under the constraints of instrument mass, volume, power consumption and
spectral resolution. For the proposed optical layout evaluation of the theoretical SNR for different measurements was
performed accounting both for laboratory observations on Earth and acquisition of Martian atmosphere spectrum during
the mission. Moreover, the instrument trace gas detection capability was evaluated.
The Mars Infrared MApper (MIMA) is a FT-IR miniaturised spectrometer which is being developed for ESA ExoMars
Pasteur mission. The Martian Infrared MApper Fourier Spectrometer is designed to provide remote measurements of
mineralogy and atmosphere of the scene surrounding a Martian rover and guide it to key targets for detailed in situ
measurements by other rover experiments. Among the main scientific objectives of the MIMA instrument are to assist
the rover in rock/soils selection for further in-situ investigation and to identify rocks and soils on the Martian surface
which provide evidence of past/present biological activity. The instrument is also designed to measure the water vapour
abundance and vertical distribution and its diurnal and seasonal variation, dust opacity, optical properties, composition,
diurnal and seasonal variation. The instrument is a double pendulum interferometer providing spectra in the 2 - 25 μm
wavelength domain with a resolving power of 1000 at 2 μm and 80 at 25 μm. The radiometric performances are SNR >
40 in the near infrared and a NEDe = 0.002 in the thermal region. The instrument design is very compact, with a total
mass of 1kg and an average power consumption of 5 W.
The Mars Infrared MApper (MIMA) is a FT-IR miniaturised spectrometer which is being developed for ESA ExoMars
Pasteur mission. MIMA will be mounted on the rover mast and so it must be compact and light-weight. The scientific
goals and its thermo-mechanical design are presented in two companion papers [1] and [2]. In this work the optical
design will be reviewed and the results of the tests performed on some optical components will be presented. The design
has faced challenging constraints mainly linked to the requirement of keeping the performances good enough to fulfil the
scientific objectives of the mission, while, at the same time, it was imperative to keep the overall size and weigh within
the allocated resources. In addition the instrument must be able to operate in the very harsh environment of the Martian
surface and to withstand, without permanent damage, even harsher conditions as well as the severe dynamic loads
expected at landing on Mars. The chosen solution is a single channel double pendulum interferometer, covering the
spectral range between 2 and 25 micron, crucial for the scientific interpretation of the recorded spectra, with a resolution
variable between 10 and 5 cm-1. Since the spectral range is too wide to be covered by a single detector, it has been
decided to use two different detectors, mounted side by side, in a customised case. Such innovative solution has
obviously pros and cons and the optical design has been driven by the need to reduce the inconveniences, while
maintaining the advantages.
The Mars Infrared MApper (MIMA) is a FT-IR miniaturized spectrometer which is being developed for ESA ExoMars
Pasteur mission. MIMA will be mounted on the rover mast and so it must be compact and light-weight. The scientific
goals and its optical design are presented in two companion papers [1] [2]; the focus of this work is on the thermomechanical
design and testing. The instrument design faces challenging constraints both from the expected environment
and the allocated resources. The temperatures during operation are expected to be from -120 °C to +30 °C with the
presence of a low density but thermally effective atmosphere. Severe dynamic loads are foreseen during launch and
moreover at landing on Mars. The overall size is limited to an envelope of 140 mm x 140 mm x 120 mm and the mass to
less than 1 kg. The expected performances of this instrument should be comparable with those of much heavier ones
built in the past. An instrument compliant with these constraints has been conceived, introducing many innovative
solution with respect to the past experiences and making use of intensive modeling and testing to prove the survival to
the harsh environment.
Among the most challenging problems the mounting of the brittle KBr optics and the matching of its thermal expansion
coefficient with that of the supporting aluminium structure, in a temperature interval of more than 200 °C. Most of the
components have undergone thermovacuum tests in the low temperature range because none of them was expected to be
used in the -100 °C range.
Spatial details of surfaces acquired by means of imaging spectrometers and multiband cameras are degraded by many factors. The atmosphere placed between the instrument and the surface, optical aberrations and tracking errors are some sources. Due to these causes, the photons coming from the instantaneous field of view pertaining a certain pixel, are spread over a larger number of picture elements, causing a spatial filtering of the image. Natural surfaces are rarely composed of a single uniform material and, therefore, blurring causes also a mixing of spectra of mineralogic different units on the surface. The problem of image sharpening is then linked to that of spectral unmixing. In this work, we compare the use of different statistical techniques, as Principal Component Analysis, Linear Spectral Unmixing and Spectral Clustering for image sharpening purposes.
Francis Reininger, Angioletta Coradini, Fabrizio Capaccioni, M. Capria, Priscilla Cerroni, M. De Sanctis, G. Magni, Pierre Drossart, Maria Barucci, D. Bockelee-Morvan, Jean-Michel Combes, J. Crovisier, T. Encrenaz, Jean-Michel Reess, Alain Semery, Didier Tiphene, Gabriele Arnold, Uri Carsenty, Harald Michaelis, Stefano Mottola, Gerhard Neukum, G. Peters, Ulrich Schade, Fredric Taylor, Simon Calcutt, Tim Vellacott, P. Venters, R. Watkins, Giancarlo Bellucci, Vittorio Formisano, Francesco Angrilli, Gianandrea Bianchini, Bortolino Saggin, E. Bussoletti, L. Colangeli, Vito Mennella, S. Fonti, Jean-Pierre Bibring, Yves Langevin, B. Schmitt, M. Combi, U. Fink, Thomas McCord, Wing Ip, Robert Carlson, Donald Jennings
The visible infrared thermal imaging spectrometer (VIRTIS) is one of the principal payloads to be launched in 2003 on ESA's Rosetta spacecraft. Its primary scientific objective s are to map the surface of the comet Wirtanen, monitor its temperature, and identify the solids and gaseous species on the nucleus and in the coma. VIRTIS will also collet data on two asteroids, one of which has been identified as Mimistrobell. The data is collected remotely using a mapping spectrometer co-boresighted with a high spectral resolution spectrometer. The mapper consists of a Shafer telescope matched to an Offner grating spectrometer capable of gathering high spatial, medium spectral resolution image cubes in the 0.25 to 5 micrometers waveband. The high spectral resolution spectrometer uses an echelle grating and a cross dispersing prism to achieve resolving powers of 1200 to 300 in the 1.9 to 5 micrometers band. Both sub-systems are passively cooled to 130 K and use two Sterling cycle coolers to enable two HgCdTe detector arrays to operate at 70 K. The mapper also uses a silicon back-side illuminated detector array to cover the ultra-violet to near-infrared optical band.
VNIR is an imaging spectrometer working in the 350 divided by 1050 nm spectral range. It simultaneously acquires multiple images of the same region, each in a different narrow spectral band. The resulting images provide a spectrum for each point in the scene. The VNIR spectrometer together with an infrared channel form the instrument OMEGA (Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite') that will be used from a Russian orbiter to map the geochemical and mineralogical distribution of materials of the surface of Mars. The main purpose of VNIR is to extend the mapping capability towards short wavelengths, where a number of natural materials have their signatures, and to determine the location on the surface of spectral features mapped in the IR range. The instrument is composed of a lens objective and a concave holographic grating mirror based spectrometer. It can acquire images of 384 X N spatial pixels (N being the number of swaths, as the push broom technique is used) of 0.4 mrad each and 5 nm spectral resolution over 144 channels.
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