Cassini VIMS: Visual and Infrared Mapping Spectrometer - CERTIFIED

The Visible and Infrared Mapping Spectrometer (VIMS) was designed to generate spectral maps of the Saturn system at visible and near-infrared wavelengths. It was also frequently used to record the occultation of stars by Saturn and its moons, as well as by Saturn's rings. VIMS was assembled at NASA's Jet Propulsion Laboratory (JPL) for the Cassini Mission to Saturn. Its components were supplied by JPL, the Agenzia Spaziale Italiana in Italy and France's Centre Nationale de la Recherche Scientifique.

Scientific objectives of VIMS include the following:

• Determine the distribution of the gases and aerosols in Saturn's atmosphere and their time variability
• Study Saturn's atmospheric dynamics, including the characterization of its ammonia clouds
• Map the radial, azimuthal and vertical distribution of the material in Saturn's rings
• Discern the mineralogical composition and surface properties of the particles comprising Saturn's rings
• Constrain and identify the mineralogical properties of Saturn's icy satellites, including the non-icy contaminants found across the system
• Map the composition and geology of Titan's surface
• Determine the distribution of the gases and aerosols in the atmosphere of Titan and their variability with time

VIMS Detectors:

• VIS: Visible-spectrum spectrometer
• IR: Infrared spectrometer
• Often the two spectrometers were operated together, and data was merged into a single data cube spanning the full wavelength range. During data processing, each of the main detector channels can be separated out.

The design of the VIMS instrument was largely based on the Near-Infrared Mapping Spectrometer (NIMS) which flew on the Galileo mission to Jupiter. Both the VIMS and NIMS benefited from the long history of imaging spectrometers that preceded them. VIMS incorporated two channels to cover its full wavelength range. The Italian-built VIMS Visual Channel (VIS) recorded light at wavelengths between 0.35 and 1.05 µm in 96 individual spectral channels, with a spectral resolution of 7.3 nm. The VIMS Infrared Spectral Region Component (IR) which was built by JPL, picked up in the near infrared at 0.85 µm and extended the coverage of the VIMS instrument out to 5.1 microns with 256 spectral channels, at a spectral resolution of 16.6 nm. Separate telescopes collected light for each spectrometer, and each spectrometer operated in a variety of observational modes. However, they were usually operated together in compatible modes to generate a single data cube spanning the full wavelength range available to both portions of the instrument.

In addition to its standard imaging mode, with data-cube sizes up to 64x64 pixels, VIMS was able to operate in continuous LINE-mode (used for close Titan flybys and some radial ring scans) and in POINT-mode (for stellar occultations). The instrument also contained an off-axis solar port which was used to obtain spectra of the Sun for calibration purposes and to monitor solar occultations by Titan, Saturn and the rings.

The engineering details of the VIMS instrument and the science objectives it was built to address are described in further detail in the Space Science Reviews paper by Brown et al. (2004).

Read these Important Notes about searching for VIMS images of rings, or for close encounters with moons using the OPUS and the Image Atlas search tools listed below.

• The OPUS search engine allows for an interactive search of the VIMS data in the PDS archive based on a wide variety of search parameters, including observation geometry and target.
• PDS Imaging Atlas is another image-search tool with VIMS data, where parameter search and thumbnail evaluation is possible.
• The Master Schedule is a time-ordered listing of observations by all instruments. This may help find data based on particular events, however the OPUS tool and Image Atlas linked above will be much simpler to use in most circumstances.
• The Event Calendar is an interactive event-finding tool that can be used to search for data associated with particular events.
• Catalogs of VIMS Stellar and Solar Occultations Observations are available. They include information on the latitudes and ranges of Titan and Saturn occultations, as well as the radial coverage and opening angle of ring occultations

Cassini VIMS Data Portal at the Université de Nantes, organized by flyby of various moons.

Cassini VIMS Online Data Volumes at the PDS

• These Data Volumes provide raw cube data and documentation.
• The Data Volumes include VIMS QUBE EDRs and VIMS Spectral Cubes.
• Additional information can be found at the PDS Imaging Node. (see Volume 3-94)

VIMS Rings Occultation Data

• The Occultation Data is a set of peer-reviewed data sets located at the PDS Ring-Moon Systems Node
• The data contains reduced and calibrated optical depth profiles for Saturn's rings derived from VIMS stellar occultation observations at the Rings Node.
• The data includes radial profiles at 1 and 10 km resolution derived from more than 60 VIMS occultations.

Occultation Geometry (In preparation at the rings node)
Plots showing the occultation geometry for each observation of Saturn's rings will be available (in progress at the rings node).

The Cassini VIMS archive of Titan: From browse products to global infrared color maps, Icarus Vol 319 pp121-132, S. Le Mouélic et al. (2019) [DOI:10.1016/j.icarus.2018.09.017]

ISIS 3

• The ISIS 3 is a software package can be used to process VIMS data.
• Users should use these Step-by-Step Instructions for cartographic and science data if they wish to create their own products. (Note: this applies only to imaging and LINE-mode data, and not to occultation data, though the format of the raw data is the same in all cases)

VIMS-IR Pixel Timing

• VIMS was used in modes that were not a part of the initial instrument design. The VIMS infrared channel used an articulating secondary mirror to produce a raster scan of the scene being observed.
• Users who wish to construct geometry for individual pixels within the scan must deal with characteristics of the VIMS internal clock.
• The VIMS-IR Pixel Timing linked above is a white paper which has been generated and the resulting refinement has been included in the ISIS 3 software.

The VIMS instrument underwent shifts in the wavelength calibration of the spectrometer since the mission launch. Large shifts occurred during the Jupiter fly-by in 2000, followed by a period of stability until Saturn orbit insertion. Small shifts occurred throughout the orbital tour until the end of mission in September 2017. The wavelength shifts require a time-dependent radiometric calibration technique to be deployed to preserve radiometric accuracy. The final report covers unresolved issues, quantifies the time-dependent wavelength shift, and described a compensatory scheme that provides an accurate calibration for both specific intensity and intermediate frequency for the VIMS measurements made throughout the Cassini Mission.

VIMS Wavelength and Radiometric Calibration Final Report

• The Final Report includes the data to the end of the mission in September 2017 and derived the calibration extending back to before the Jupiter fly-by in 2000
• The Calibration Data Files for the final radiometric calibration are in a gzipped-tar file.

The VIMS team has archived operational details for each observation, including command and mode data. These archives are listed below.

• These files require knowledge of mission planning to be useful to the user.
  IOI Files: Instrument command files used to acquire data for specific requests 126 MB TXT file
• Requests: Observation details (e.g. primary bv, secondary bv) for each observation - 5.6 MB CSV file
• SFOFs: Short forms generated for each observation - 350 MB TXT file