Virtis : an imaging spectrometer for the rosetta mission

https://doi.org/10.1016/S0032-0633(98)00025-7Get rights and content

Abstract

The VIRTIS scientific and technical teams will take advantage of their previous experience in the design and development of spectrometers for space applications. In fact, the various groups contributing to the VIRTIS experiment, from Italy, France and Germany, have been deeply involved in the CASSINI mission, with the experiments VIMS and CIRS. The targets of the ROSETTA mission are the most primi- tive solar system bodies : comets and asteroids. ROSETTA will study in detail a comet nucleus, the prime target of the mission, and will fly by one or two asteroids. The small bodies of the solar system are of great interest for planetary science and their study is crucial to understand the solar system formation. In fact it is believed that comets and, to a lesser extent, asteroids underwent a moderate evolution so that they preserve some pristine solar system material. Comets and asteroids are in close relationship with the plan- etesimals, which formed from the solar nebula 4.6 billion years ago. The global characterisation of one comet nucleus and one or two asteroids will provide basic information on the origin of the solar system and on the interrelation between the solar system and the interstellar dust environment.

The ROSETTA mission is designed to obtain the above mentioned scientific goals by : (a) in situ analysis of comet material ; (b) long period of remote sensing of the comet. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will give relevant “ground-truth” for the remote sensing information and, in turn, the locally collected data will be interpreted in the appropriate scenario provided by remote sensing investigation. The scientific payload of ROSETTA includes a Visual InfraRed Spectral and Thermal Spectrometer (VIRTIS) among the instrument on board the spacecraft orbiting around the comet.

This instrument is fundamental to detect and study the evolution of specific fingerprints—such as the typical spectral bands of minerals and molecules—arising from surface components and from materials dispersed in the coma. Their identification is a primary goal of the ROSETTA mission as it will allow us to identify the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to Sun is important information that can be obtained by means of spectroscopic observation. The VIRTIS design and its detailed science goals are reported hereafter.

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

References (27)

  • J Benkhoff et al.

    Planet Space Sci.

    (1996)
  • M.T Capria et al.

    Planet. Space Sci.

    (1996)
  • T.D Jones et al.

    Icarus

    (1990)
  • L Jorda et al.

    Planet. Space Sci.

    (1995)
  • M Podolak et al.

    Planet. Space Sci.

    (1996)
  • R Schulz et al.

    Planet. Space Sci.

    (1996)
  • M.F AHearn et al.

    Icarus

    (1997)
  • A Bar-Nun et al.

    ESA Report

    (1993)
  • Bocklee-Morvan, D. and Crovisier, J. (1992) The formation and composition of the atmospheres of small solar system...
  • T.Y Brooke et al.

    Nature

    (1996)
  • Carusi, A., Kresak, L. and Valsecchi, G. B. (1995) Electronic atlas of evolution of short-period comets, on line :...
  • Clark, R. N. (1981) Water frost and ice : the near infrared spectral reflectance 0.65–2.5 m. J. Geophys Res. 86-B4,...
  • M Combes et al.

    Icarus

    (1986)
  • Cited by (68)

    • Applications in remote sensing—natural landscapes

      2020, Data Handling in Science and Technology
    • Multispectral surface emissivity from VIRTIS on Venus Express

      2020, Icarus
      Citation 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, Icarus
      Citation 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.

    View all citing articles on Scopus
    View full text