Virtis : an imaging spectrometer for the rosetta mission
Introduction
Comets are a heterogeneous class of objects as they accreted in the region spanning from Jupiter to Neptune or beyond, where the thermodynamical conditions were greatly non-homogeneous at the time of the solar system formation. (Rickman and Huebner, 1990). Comets are believed to be the most primitive objects in the solar system, because they have spent their life in a cold environment and are unlikely to be thermally altered. Only collision mechanisms can be responsible for an extensive resurfacing and, in many cases, for a collisional evolution (Farinella and Davis 1996). When a comet enters the inner solar system a coma is formed by the sublimation of ices, followed by dust ejection and differentiation of the subsurface layers.
On the basis of present knowledge, a comet nucleus is an irregularly shaped object, containing heterogeneous mixture of ices and dust, with variable surface albedo, composition and thermal properties. The spatial and temporal irregularity of the nucleus activity leads to a non-uniform surface, with topographic features and roughness at different scales. The accretion of the nucleus by collisions between grains may occur at low temperatures and low relative velocities (order of 1–10 m⧹s). This mechanism can produce a highly porous and loose aggregate with no tendency to subsequent compression due to very small gravitational forces. On the other hand, the thermal evolution of the nucleus, in the inner solar system, determines the sublimitation of ices and their successive re-condensation in colder regions, causing reduction of the pore sizes and an increase of the material strength (Haruyama et al., 1993). The comet dust and gas production was found to come from a limited number of discrete sources, with the rest of the surface almost completely inactive (Keller et al., 1988). The surface of P⧹Halley is a clear example : it is heterogeneous and displays two different units. The first is dominated by brighter, possibly unaltered materials, which could be interpreted as ices. The second is characterised by darker, probably altered assemblies such as complex organic and secondary inorganic (Keller et al., 1988). The abundance of volatile elements suggests that the related ices and gases may be primordial. The nature of the solid compounds of the comets (silicates, oxides, salts, organics and ices) is still unknown. These chemical compounds can be identified by infrared spectroscopy using high spatial resolution imaging to map the heterogeneous parts of a nucleus and high spectral resolution spectroscopy to determine the composition unambiguously.
The visual and infrared spectrum of the comet coma is characterised by a number of components comprising both gas emission bands and a dust continuum (Bocklee-Morvan and Crovisier, 1992). Ground-based visual spectroscopy has detected various atomic, radical and ionic species formed through photo-dissociation by solar ultraviolet radiation of the so-called parent molecules which are sublimated from the nucleus and possibly from a halo of volatile grains. Previous IR comet observations have shown that various hydrocarbons show emission bands between 3 and 4 μm.
Section snippets
Scientific objectives
The ROSETTA mission is devoted to the detailed study of a comet nucleus and two main belt asteroids.
A Multispectral Imager—covering the range from the near UV (0.25 μm) to the near IR (5.0 μm) and having moderate to high spectral resolution and imaging capabilities—is an appropriate instrument for the determination of the comet global (size, shape, albedo, etc.), and local (mineralogical features, topography, roughness, dust and gas production rates, etc.) properties.
The main scientific
Scientific requirements and required activity in the different mission phases
A summary of the scientific requirements needed to define the characteristics of VIRTIS is given hereafter.
Technical description
VIRTIS instrument is part of the ROSETTA orbiter payload. It is a visible and infrared imaging spectrometer designed to fulfil the objectives of the VIRSTM model payload instrument and to take into account the mission scenario. In order to fully achieve these objectives, the VIRTIS instrument performances have to exceed the specifications of the baseline model payload instrument (Bar-Nun et al., 1993). The VIRTIS instrument combines a double capability : (1) high-resolution visible and infrared
Virtis observation strategy
The ROSETTA mission is characterised by different mission phases. In what follows we have summarised the scientific objectives in these phases. During the six cruise phases VIRTIS will be generally switched off, as in other experiments, with the exclusion of periodical testing of VIRTIS and calibration sessions including Sun calibrations through the Solar calibration units (before any main event).
Due to the high relative velocity in the Mars fly-by, observations of the atmosphere at Mars will
Concluding remarks
From the previous discussion it is apparent that the VIRTIS experiment will allow us to acquire a new knowledge of the minor bodies physics and chemistry as well a knowledge of the complex relationships between comets and asteroids. The Cassini inheritance has been particularly valuable in designing the VIRTIS instrument. We expect to achieve with the ROSETTA mission not only new scientific results but also to build up in Europe a more experienced and efficient planetology community.
Unknown BIBs
Crovisier et al., 1995a, Crovisier et al., 1995b, Singer, 1981
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Cited by (68)
Applications in remote sensing—natural landscapes
2020, Data Handling in Science and TechnologyMultispectral surface emissivity from VIRTIS on Venus Express
2020, IcarusCitation Excerpt :Errors in altimetry would however present as a specific deviation of emissivity in all three bands and we will use this to test the tessera regions for altimetry errors in future. VIRTIS on Venus Express is the flight spare of a hyperspectral imager for the Rosetta comet orbiter (Coradini et al., 1998) and was not optimized for observations of the nightside of Venus. The Venus Express mission was focused on atmosphere observations (Drossart et al., 2007).
Mineralogy of Marcia, the youngest large crater of Vesta: Character and distribution of pyroxenes and hydrated material
2015, IcarusCitation Excerpt :Images taken by the Dawn Framing Cameras (FC) (Sierks et al., 2012) are also used for context and morphological analysis. VIR measures spectra between 0.25 and 5.1 μm and is derived from VIRTIS-M aboard Rosetta and Venus Express (Coradini et al., 1998). This imaging spectrometer has two spectral channels: the VIS channel, working between 0.25 and 1.05 μm, and the IR, operating between 1.0 and 5.1 μm.