A Very Short History of Reflectance Spectroscopy for Solar System Exploration: An Old Man's Recollection

Astronomers began the use of multi-wavelength studies of Solar System objects in the solar spectrum region, first by eyeballing the objects’ colors, such as for Mars, and quantitatively when the photographic process was developed in the last half of the 19th century. After the time of Galileo, astronomers came to emphasize gas and plasma studies, while solar system studies included dirt and rocks as subjects. I am more the dirt and rocks type and I will emphasize this history. I arrived on the scene in the 1960s when electronic detectors first began to trickle out to civilian scientists. These allowed far more accurate quantitative measurements, as well as measurements at infrared wavelengths. We, who had access to these new detectors, began using the astronomers’ telescopes to observe solar system objects almost at random during the nights we could get precious telescope time. My own early emphasis was to demonstrate that there are spectral expressions of rock-forming minerals (basalts) on the Moon. This seems primitive by today’s standards, but it is where we were in the1960s. Solar System studies grew from there, first with telescopes and then using spacecraft to carry our spectrometers to the objects. Spacecraft history developed slowly due to the dominance of geologists and picture taking over geochemists and the less visual spectroscopy. The delay of the launch of the Galileo spacecraft, due to the Challenger disaster, the first deep space mission to carry a visual-infrared imaging spectrometer, was a further setback. But we (and our graduate students) eventually succeeded at this. Today, we still find the telescope is an important tool but, nearly every spacecraft to Solar System object carries imaging spectrometers operating in the UV-visual-IR spectral region, where reflected solar radiation and emitted thermal radiation is detectable.


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