Thermal infrared spectral band detection limits for unidentified surface materials.

Infrared emission spectra recorded by airborne or satellite spectrometers can be searched for spectral features to determine the composition of rocks on planetary surfaces. Surface materials are identified by detections of characteristic spectral bands. We show how to define whether to accept an observed spectral feature as a detection when the target material is unknown. We also use remotely sensed spectra measured by the Thermal Emission Spectrometer (TES) and the Spatially Enhanced Broadband Array Spectrograph System to illustrate the importance of instrument parameters and surface properties on band detection limits and how the variation in signal-to-noise ratio with wavelength affects the bands that are most detectable for a given instrument. The spectrometer's sampling interval, spectral resolution, signal-to-noise ratio as a function of wavelength, and the sample's surface properties influence whether the instrument can detect a spectral feature exhibited by a material. As an example, in the 6-13-mum wavelength region, massive carbonates exhibit two bands: a very strong, broad feature at ~6.5 mum and a less intense, sharper band at ~11.25 mum. Although the 6.5-mum band is stronger and broader in laboratory-measured spectra, the 11.25-mum band will cause a more detectable feature in TES spectra.

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity.

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-µm and 8-14-µm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 µm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-µm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-µm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-µm and 8-14-µm atmospheric windows.

Lunar eclipse: infrared images and an anomaly of possible internal origin.

Infrared images of the lunar eclipse of 13 April 1968 were obtained and compared with infrared images of the 19 December 1964 eclipse. A similarity of apparent strength and distribution of most thermal anomalies on the maria is evident from inspection of these images, indicating that these features are not ephemeral. One new linear thermal anomaly was discovered, which is thermally enhanced during the lunar afternoon. Its close relation to a lunar crustal fracture line and other features of probable internal origin suggests that this anomaly may be of internal origin.

Martian surface materials: effect of particle size on spectral behavior.

The presence of abundant limonite on Mars has long been the subject of controversy. Some advocates of abundant limonite also suggest that the albedo differences between Martian light and dark areas are caused by different sizes of particles in those areas. We show that the relative albedo is reversed from the blue to the red for samples of limonite with particles of different sizes. Observations of Mars reveal no blue-red albedo reversal between the light and dark areas. Consequently, the hypothesis of particle size control of albedo is incompatible with the presence of abundant limonite on Mars.

Infrared images of tycho on dark moon.

Infrared images of the thermal anomaly associated with the lunar crater Tycho were obtained during the lunar night after Tycho had ceased to be illuminated by the Suin for as long as 97 hours. In agreement with results of previous studies, these measurements show that the crater is warmer than its surroundings during the lunar night, and that the temperature of the thermal anomaly gradually decreases with time, being no longer detectable after new moon. This work provides strong evidence that the steeper crater walls facing the Sun before local sunset are warmer throughout the cooling phase, and that the Tycho anomaly is thus produced by solar, rather than internal, heat.