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NIMS - Near-Infrared Mapping Spectrometer

MISSION OBJECTIVES

• Map the surface minerals on the Galilean satellites at a spatial resolution of 5 to 30 Km.

• Identify the phases and mixtures of the surface minerals on the Galilean satellites.

• Correlate mineral distributions with SSI images.

• Determine the Jovian atmosphere cloud structure over a wide range of phase angles.

SUMMARY

NIMS has two major objectives. The first objective is to look at the surfaces of the satellites of Jupiter to see what they're made of. The second objective is to study the atmosphere of Jupiter to determine such things as the characteristics of the Jovian cloud layers, the variations over time and space of the constituents of the atmosphere, and the temperature versus altitude profile.

For the satellites, the geological structures will be mapped to determine their mineral distributions. Resolutions of 25 kilometers per NIMS pixel or better are planned for the Galilean satellites Europa, Ganymede, and Callisto. NIMS will make distant observations of Jupiter's volcanic moon Io, at resolutions of 120 to 600 kilometers, to determine the moon's surface composition and to measure temperatures of the hot spots. NIMS will monitor Io's volcanic activity in every Galileo orbit. In addition, spectral analyses will be done for some of the smaller satellites and the planet's ring.

Since NIMS measures infrared radiation from the atmosphere of Jupiter, it will contribute to compositional studies, the nature of clouds, motions, and energy balances. NIMS will be able to monitor ammonia, water vapor, phosphine, methane, and germane and to look for previously undetected molecules. Phosphine, which is formed in the deep interior (more than 1000 kilometers deep below the clouds at temperatures near 1000 kelvin) and is rapidly destroyed at observable altitudes, is a tracer of huge upwellings of gas from deep inside the planet. NIMS will map the abundance of phosphine over a wide range of latitudes and longitudes. The goal is to understand the major deep-seated circulation patterns that power the "near-surface" meteorology (planet-girdling cloudy zones, drier belts, and localized cyclonic storm systems such as the Great Red Spot).

The NIMS instrument is sensitive from 0.7 to 5.2 micrometers, overlapping the wavelength range of SSI. The telescope associated with NIMS is all reflective (uses mirrors and no lenses) with an aperture of 229 millimeters. The spectrometer of NIMS uses a grating to disperse the light collected by the telescope. This method is often used by instrument makers rather than use of the familiar prism. The dispersed spectrum of light is focused on detectors of indium antimonide and silicon.

The NIMS weighs 18 kilograms and uses 12 watts of power on average.

NIMS DESCRIPTION

• The NIMS consists of a 22.8-cm diameter (f/3.5), 80-cm focal length, Ritchev- Chretien telescope with a spatial scanning secondary mirror and diffraction grating spectrometer.

• The InSb detectors are cooled to cryogenic temperatures (64 K) via a single-stage passive radiative cooler.

• The entrance slit of the telescope is focused onto the detectors via a 40-cm focal length (f/3.5) Dall-Kirkham collimator backed by a wide-angle (20-cm focal length, f/1.8) flat-field camera.

DESIGN DETAILS

• The spectrometer employs a dual-blazed plane diffraction grating with 39 lines/mm (30% of the area blazed for 1.9 \065m and 70% for 3.8 \065m).

• The 15 InSb detectors are used with the first order of the grating for wavelengths greater than 1.9 \065m and the 2 Si detectors are used with the second order of the grating for shorter wavelengths.

• The spectral range of the NIMS is 0.7 to 5.2 \065m.

• All mirrors and the grating are made from fused-silica.

• All 17 detectors have an active area of 0.2 x 0.2 mm and quantum efficiencies > 70%.

• The NIMS has an angular field of view of 10 mrad x 0.5 mrad (20 pixels, cross-cone direction) and an angular resolution of 0.5 mrad x 0.5 mrad (one pixel).

• The scanning mirror and grating are driven by direct-current torque motors.

• A tuning fork chopper (63 Hz resonance frequency) modulates the detected radiation with an approximate 50% duty cycle.

INSTRUMENT PARAMETERS

• Instrument Mass: 18 Kg
• Power Consumption:
  • Instrument: 12.0 W
• Microprocessor: 1802 type
• ROM/RAM: 3 Kb/3 Kb
• Fields of View: 10 x 0.5 mrad
• Size:
  • Optics 83 x 37 x 39 cm
  • Electronics 20 x 25 x 13 cm
• Thermal Range:
  • Operating (GLL 3-210)
    • Electronics: -20 to 35 Deg. C
    • Optics: -163 to -118 Deg. C
    • Focal Plane Array: -203 to -173 Deg. C
  • Non-operating (GLL3-210)
    • Electronics: -20 to 35 Deg. C
    • Optics: -163 to 50 Deg. C
    • Focal Plane Array: -203 to 50 Deg. C
• Instrument Modes:
  • Off
  • POR
  • On
  • Safe
  • Full Map
  • Full Spec.
  • Long Map
  • Long Spec.
  • Short Map
  • Short Spec.
  • Fixed Map
  • Band Edge Map
  • Band Edge Spec.
  • Stop & Slide Map
  • Stop & Slide Spec.