This multispectral map of Australia and surrounding seas was obtained by the Galileo spacecraft's Near Infrared Mapping Spectrometer shortly after closest approach on Dec. 8, 1990 from an altitude of about 50,000 miles.
The image shows various ocean, land and atmospheric cloud features as they appear in three of the 408 infrared colors or wavelengths sensed by the instrument. The wavelength of 0.873 micron, represented as blue in the photo, shows regions of enhanced liquid water absorption, i.e., the Pacific and Indian oceans. The 0.984- micron band, represented as red, shows areas of enhanced ground reflection as on the Australian continent. This wavelength is also sensitive to the reflectivity of relatively thick clouds. The 0.939- micron wavelength, shown as green, is a strong water-vapor-absorbing band, and is used to accentuate clouds lying above the strongly absorbing lower atmosphere. When mixed with the red indicator of cloud reflection, the green produces a yellowish hue; this indicates thick clouds.
The distinctive purplish color off the northeast coast marks the unusually shallow waters of the Great Barrier Reef and the Coral Sea. Here the blue denoting water absorption combines with the red denoting reflection from coral and surface marine organisms to produce this unusual color.
The Near Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft is a combined mapping (imaging) and spectral instrument. It can sense 408 contiguous wavelengths from 0.7 micron (deep red) to 5.2 microns, and can construct a map or image by mechanical scanning. It can spectroscopically analyze atmospheres and surfaces and construct thermal and chemical maps. Image Credit: NASA/JPL