PDS_VERSION_ID = 3 LABEL_REVISION_NOTE = "2006-11-30, SE first draft 2006-12-15, 70 character update." RECORD_TYPE = STREAM RELEASE_ID = 0001 REVISION_ID = 0000 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = VEX INSTRUMENT_ID = VIRTIS OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "VISIBLE AND INFRARED THERMAL IMAGING SPECTROMETER" INSTRUMENT_DESC = " Instrument Overview =================== VIRTIS (Visible Infra Red Thermal Imaging Spectrometer) is an imaging spectrometer which allows Venus Express to map details of the Venus planet from the surface to the mesosphere. Therefore its scientific objectives cover a large field, not only in meteorology of the middle atmosphere, as expected from optical remote sensing instruments, but also in mineralogy, through the near infrared deep windows sounding down to the surface, and in aeronomy of the upper atmosphere, through non-LTE emissions of CO2. Main unresolved questions on the planet Venus concern the stability of the cloud layers : are they permanently reformed from volcanism or low level surface/atmosphere interaction, or are they transient, related to episodic volcanism ? The meteorology of Venus atmosphere, and in particular the superrotation which let the upper layers rotating in 4 days when the solid planet rotates in 243 days is also a mystery which is important not only for the case of Venus, but also for other planetary bodies (like Titan), and for hydrodynamics modeling. The surface of Venus is mostly known from the radar observations by Magellan; a systematic mapping in the infrared window at medium resolution (30 km) could bring new clues to the knowledge of the mineralogy of Venus, in particular the dichotomy between low/high altitudes region in radar reflectivity, if related to optical anomalies. VIRTIS is constituted of three data channels in one compact instrument. Two of these channels, one visible (0.25-1micrometers) and one infrared (1-5 micrometers) are committed to spectral mapping, and are housed in the Mapper (-M) optical subsytem. One channel is devoted solely to spectroscopy at high resolution, and is housed in the High resolution (-H) optical subsystem (2-5 micrometers). Both channels usually operate simultaneously, but can also work separately, depending on observing modes. They are boresighted and combined operations therefore provide a spectral image of 64 mrad from the two VIRTIS-M channels, associated with one (or several) spectrum from VIRTIS-H channel. Scientific Objectives =================== As a generalist instrument, VIRTIS has many scientific objectives, which are listed below. They may be summarized in what we call a 'tomography' of the Venus atmosphere, i.e. a mapping of the various layers of Venus from the surface itself up to the mesosphere. Such maps, obtained with a repetition rate related to the dynamics of the atmosphere will give access to a dynamical study of Venus atmosphere, to approach with unprecedented constraints the problem of the dynamic of the atmosphere of Venus (and in particular its superrotation), the composition of the deep atmosphere, and the interaction between volcanism and atmospheric composition, the cloud structure, with the specific question of the UV absorbers, and the dynamic of the mesosphere, with specific questions related to the general problem of atmospheric escape. The main scientific goals of VIRTIS at Venus are the following: - Study of the lower atmosphere composition (CO, OCS, SO2, H2O) from 1-2.5 micrometers night side atmospheric windows - Study of the cloud structure, composition and scattering properties - Cloud tracking in the UV and IR, for retrieval of the vertical field of wind velocities - Measurements of the temperature field - Lightning search - Mesospheric sounding - Search for variations related to surface/atmosphere interaction - Temperature mapping of the surface - Search for seismic wave activity (tentative) In addition, VIRTIS also provides true colour high definition images of Venus of great value for public outreach program. Observation Modes =================== The scientific objectives are very different between day and night side conditions. In particular, the deep atmospheric windows located at 0.9 micrometers, 1.1, 1.18, 1.74 and 2.3 micrometers are sensitive to radiation coming from layers from the surface (in the shortest wavelength) to about 30 km (to be compared to the cloud level at about 60 km altitude), only in night side observations, sensitive to the thermal emission from the atmosphere. The solar reflection by the clouds on the day side contribute to more than 10000 flux than thermal emission, and completely overwhelm the thermal emission of the deeper layers. VIRTIS-M: main purpose = spectral mapping visible and infrared at moderate spectral resolution Night side - Surface mapping (using measurements at 1.02, 1.1, and 1.18 micrometers) - O2 emission (at 1.27 micrometers) - Lower atmospheric sounding (H2O) - Lightning search Day side - UV cloud absorber signature / correlation with IR absorptions - O2 emission (at 1.27 micrometers), also observed at day - CO2 non LTE emissions (nadir and limb observations) VIRTIS-H: main purpose = high spectral resolution for minor constituent detections, and rotation/vibration structure resolved in spectral bands Night side: - 2.3 micrometers lower atmosphere sounding: OCS, CO, H2O measurement (30 km altitude) - thermal profile from CO2 4.3 micrometers profile inversion Day side: - Fluorescent emission at 4.3 micron ( nadir and limb) - Cloud IR absorptions Due to the high sensitivity of its infrared detectors, the observations with VIRTIS give a high signal to noise on the day side in a very short time; night side observations on the contrary need longer integration (of the order of 1s or more), still fully compatible with orbital operations. The main difficulty for reconstructing images of the planet is the dwell time, too short at pericenter to allow VIRTIS to build images. This remark has lead the VIRTIS team to require observations at different positions along the orbit, contrary to the Mars Express observation strategy. Instrument Design =================== The VIRTIS instrument combines a double capability: (1) high-resolution visible and infrared imaging in the 0.25-5 micrometers range at moderate spectral resolution (VIRTIS-M channel) and (2) high-resolution spectroscopy in the 2-5 micrometers range (VIRTIS-H channel). The two channels will observe the same target in combined modes to take full advantage of their complementarities. VIRTIS-M (named -M in the following) is characterized by a single optical head consisting of a Shafer telescope combined with an Offner imaging spectrometer and by two two-dimensional FPAs: the VIS (0.25-1 micrometers) and IR (1-5 micrometers). VIRTIS-H (-H) is a high-resolution infrared cross-dispersed spectrometer using a prism and a grating. The 2-5 micrometers spectrum is dispersed in 8 orders on a focal-plane detector array. Technical Description =================== The instrument is divided into 4 separate modules: the Optics Module -- which houses the two -M and -H optical heads and the Stirling cycle cryocoolers used to cool the IR detectors to 70 K; the two Proximity Electronics Modules (PEM) required to drive the two optical heads; the Main Electronics Module -- which contains the Data Handling and Support Unit, for the data storage and processing, the power supply and control electronics of the cryocoolers and the power supply for the overall instrument. Proximity Electronics Modules: Each optical head requires specific electronics to drive the CCD, the two IRFPAs, the covers, the thermal control; the PEMs are two small boxes interfaced directly to the S/C and placed in the vicinity of the Optics Module to minimize interference noise. Optics Module: The -M imaging spectrometer and the -H echelle spectrometer optical heads are located inside the Optics Module, which in turn is divided into two regions thermally insulated from each other by means of MultiLayer Insulation (MLI): the Cold Box and the Pallet. The Pallet is mechanically and thermally connected to the SpaceCraft; inside the Pallet are located the two Stirling cycle cryocoolers. The heat produced by the cryocoolers compressors (a maximum of 24 Win closed loop mode) is dissipated to the spacecraft. The Cold Box contains the two optical heads and its main function is to act as a thermal buffer between the Optical Heads, working at 130 K, and the external environment (the S/C temperature ranges from 250 to 320 K). The Cold Box is mechanically connected to the Pallet through 8 Titanium rods, whose number and size were selected to minimize conductive heat loads and to avoid distortion upon cooling from room temperature. The structural part of the cold box is a ledge which is supported by the 8 titanium rods; on the ledge the two optical heads are mechanically fixed. Thermal insulation of the Cold Box is improved by means of MLI, while thermal dissipation from the Cold Box is achieved by means of a two stage passive radiator: the first stage keep the Cold Box temperature in the range 120-140 K, while the second stage is split in two parts, one for each optical head, and allows to reach the required 130 K. Another important component of the instrument are the two covers; they provide a double function: protection against dust contamination, internal calibration by means of an internally reflecting surface finish. They use a step motor and their operation is controlled by the PEMs. VIRTIS-M: The VIRTIS-M optical head perfectly matches a Shafer telescope to an Offner grating spectrometer to disperse a line image across two FPAs. The Shafer telescope produces an anastigmatic image, while Coma is eliminated by putting the aperture stop near the center of curvature of the primary mirror and thus making the telescope monocentric. The result is a telescope system that relies only on spherical mirrors yet remains diffraction limited over an appreciable spectrum and field: at +/- 1.8 degrees the spot diameters are less than 6 microns in diameter, which is 7 times smaller than the slit width. The Offner grating spectrometer allows to cover the visible and IR ranges by the realization, on a single grating substrate, of two concentric separate regions having different groove densities: the central one, approximately covering 30% of the grating area is devoted to the visible spectrum, while the external region is used for the IR range. The IR region has a larger area as the reflected infrared solar irradiance is quite low and is not adequately compensated by the infrared emissions of the cold comet. The visible region of the grating is laminar with rectangular grooves profile, and the groove density is 268 grooves/mm. Moreover, to compensate for the low solar energy and low CCD quantum efficiency in the ultra-violet and near infrared regions, two different groove depths have been used to modify the spectral efficiency of the grating. The resulting efficiency improves the instrument's dynamic range by increasing the S/N at the extreme wavelengths and preventing saturation in the central wavelengths. Since the infrared channel does not require as high a resolution as the visible channel, the lower MTF caused by the visible zone's obscuration of the infrared pupil is acceptable; the groove density is 54 grooves/mm. In any case, the spot diagrams for all visible and infrared wavelengths at all field positions are within the dimension of a 40 microns pixel. For the infrared zones, a blazed groove profile is used that results in a peak efficiency at 5 micrometers to compensate for the low signal levels expected at this wavelength. VIRTIS-H: In -H the light is collected by an off-axis parabola and then collimated by another off-axis parabola before entering a cross-dispersing prism made of Lithium Fluoride. After exiting the prism the light is diffracted by a flat reflection grating which disperses in a direction perpendicular to the prism dispersion. The prism allows the low groove density grating, which is the echelle element of the spectrometer, to achieve very high spectral resolution by separating orders 6 through 13 across a twodimensional detector array: the spectral resolution varies in each order between 1200 and 3500. Since the -H is not an imaging channel, it is only required to achieve good optical performance at the zero field position. The focal length of the objective is set by the required IFOV and the number of pixels allowed for summing. While the telescope is F/1.6, the objective is F/1.67 and requires five pixels to be summed in the spatial direction to achieve about 1 mrad2 IFOV, corresponding to 3 pixels (3 x .58 mrad x .58 mrad). Main Electronics Module: The Main Electronics is physically separated from the Optics Module. It houses the Power supply for all the experiment, the cooler electronics, the Spacecraft interface electronics, for telemetry and telecommanding, the interfaces with the Optics Module subsystems, and the DHSU (Data Handling and Support Unit) which is the electronics for the data handling, processing and for the instrument control. The data processing and the data handling activities into the DHSU are performed using an on-line philosophy. The data are processed and transferred to the spacecraft in real time. The mass memory (SSR) of the spacecraft is used to store or buffer a large data volume. The Main Electronics contains no additional hardware component for data processing and compression. All data processing is performed by software. Instrument Operations =================== VIRTIS produces the following types of TM data: - TC verification reports; - H/K data reports; - Event reports; - Memory reports; - Science reports (science data). They are transmitted to the S/C DMS through the RTU I/F except the Science reports that are transmitted on the High Speed I/F. If this is not available (e.g. failure) the instrument can be commanded to start a degraded Science mode which does not use the High Speed link. In this case the Science reports are transferred via the RTU I/F like the other TMs. " INSTRUMENT_TYPE = "IMAGING SPECTROMETER" END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "PICCIONIETAL2007" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END