The Near-Infrared Mapping Spectrometer (NIMS)
(From Issue 2, July 1981)
The Galileo Near-Infrared Mapping Spectrometer (NIMS) has two major objectives. The first objective is to look at the surfaces of the satellites of Jupiter to determine their chemical composition. The second objective is to study the atmosphere of Jupiter to determine such things as the characteristics of the Jovian cloud layers, the spatial and temporal variations in the constituents of the atmosphere, and the temperature versus altitude profiles in the region of the atmosphere between 100 kPa (1 bar) (the Earth's atmospheric pressure at sea level) and 500 kPa.
The first objective essentially is to answer the question, "What are the moons of Jupiter made of?" We already know, from ground-based observations, that there is sulfur dioxide (SO
) on the surface of Io and water frost covering most of the other three Galilean satellites. But what is the dirty stuff, for example, that is mixed with the ice on Callisto? The NIMS has the capability of recognizing and classifying silicates, carbonates, nitrates, and other compounds that are the most likely candidates for the dirt in the snowballs of Callisto. One of the results of the NIMS observations will be a map of each satellite with identified compositional units. Using such a "geological" map, one would be able to tell at a glance what kinds of surface materials are located in which areas of the moons. These data will then be compared, satellite to satellite, and even Jovian satellites to other moons and asteroids, in an attempt to gain additional insight into the processes governing the evolution of the solar system.
In meeting the second objective, the NIMS will provide data that will help answer fundamental questions about the Jovian atmosphere. One result from the NIMS experiment should be a definition of cloud thicknesses and identification of layers of clouds as a function of depth in the Jovian atmosphere. Also, ground-based telescope measurements and Voyager have observed a number of free molecular species in the atmosphere of Jupiter, including water (HO), ammonia (NH
), phosphine (PH), and the exotic germane (GeH
). Repeated studies by the NIMS will focus on the variations of the abundance of each of these species as a function of time of day and Jovian season. Variations in these species should give clues about the atmospheric chemistry and dynamics. A final NIMS atmospheric objective, which is useful when mapping a region with no high clouds, is to determine the thermal profiles in the 100-to-500 kPa range. The Galileo photopolarimeter will be able to ascertain thermal profiles in the atmosphere above 100 kPa. By examining relatively cloud-free regions, NIMS can extend the thermal information down to 500 kPa 5 bars).
The Galileo NIMS is a brand new instrument. Nothing like it has ever flown before in space. It was selected because the near-infrared region of the electromagnetic spectrum [a region that begins with the deepest red your eyes can see (0.7-micrometer wavelength) and then moves out of the visible to wavelengths of 5 micrometers] is powerfully diagnostic for gaseous species and solid-state reflection characteristics. For these reasons, the NIMS is great for determining what is in the atmosphere of Jupiter and on the surface of the Jovian moons.
The NIMS consists primarily of a telescope and a spectrometer. The telescope directs light into the spectrometer, which then acts like a prism and separates the individual "colors" (i.e., wavelengths) in the nearinfrared. An array of detectors measures the intensity of response at each individual wavelength, and these data then form the spectrum of the area under observation. From this spectrum, which is made up of all the responses throughout the infrared region, the quantity and composition of the material or gas being observed can be deduced. Actually, because of a vast data bank of information on characteristic spectral signatures that is readily available, it is reasonably straightforward to identify what chemical compounds have been observed. It is a more subtle and difficult process to extract from the spectra the answer to the question, "How much of each species was seen?"
The NIMS instrument wasg built at JPL. The principal investigator is Dr. Robert Carlson of JPL. He is supported on the NIMS team by 13 other scientists from the U.S., England, and France.
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