Sky & Telescope February 1983 [antikvár]

Infrared Views of the Giant Planets David A. Allen, Anglo-Australian Observatory J ; IT IS NO COINCIDENCE that our eyes are sensitive to the very wavelengths at which the greatest amount of solar energy penetrates to the Earth's surface. However, the restrictions iinposed by this limited range of response are evident when we study the natural world. Bees, for example, see intricate patterns, at ultraviolet wavelengths, in flowers that are featureless to us, and pit vipers use long-wavelength infrared radiation to detect their warmblooded prey. A considerable range of infrared wavelengths are transmitted by the Earth's atmosphere, carrying information about a large variety of celestial phenomena. If we could perceive this region of the spectrum, what would we see? This question has concerned infrared astronomers for many years. Unfortunately, photographic emulsions are not sensitive to the wavelength region of interest to them. Only in the last decade have photoelectric detectors of sufficient sensitivity been available to permit detailed astronomical use. But even now the application of array detectors, which can render direct views of the sky, is still in its infancy. AN ADVANCE IN OBSERVING Infrared pictures have been made by scanning a single detector across the image of an object — or, more easily, moving the object's image across the detector — and building up the scene one pixel (picture element) at a time. Applications of this technique with the 92-inch Wyoming Infrared Telescope were described in the January, 1980, issue, page 18. Recently, software developed by Jeremy Bailey at the Anglo-Australian Observatory has enabled the 3.9-meter Anglo-Austra-lian Telescope (AAT) to produce infrared images. The AAT moves in a raster pattem, scanning a rectangular region of the sky up to 255 by 256 pixels in size, at any chosen spacing. Simultaneously, an infrared photometer measures intensities at a rate of 50 to 100 milliseconds per pixel. Observing in the infrared region of the spectrum has been likened to trying to use an optical telescope with the observatory lights on. The telescope, the dome, the night sky, and even the observer are sources of infrared radiation many orders of magnitude stronger than the astronomical objects under study. Normally, an observation is made by recording the brightness of the object under study, then quickly measuring the intensity of an adjacent area of empty sky, a process that is repeated many times each second. Taking the difference between these two values produces a "chopped" AC-like signal that eliminates unwanted interference. The AAT's photometer, however, is designed to operate without the need for frequent background readings. The great advantage here is in cutting the observing time in half. This, combined with the photometer's high sensitivity, low-noise electronics, and the AAT's precision controlled motion, allows observation of faint objects with very low contrast to be made much more easily than was previously possible. SNAKE'S EYE VIEW OF THE GIANT PLANETS The giant planets are particularly interesting in the infrared because they exhibit strong atmospheric absorption features. When viewed at wavelengths below two microns they appear very much as they would through an optical telescope. In the 2.2-micron region, however, atmospheric methane strongly absorbs incident sunlight; the planets are dimmed, becoming darkest in regions where methane is most abundant. Similarly, at 3.8 microns methane again dominates. Images obtained at these two wavelengths are therefore similar, except that a higher signal-to-noise ratio can be attained at the shorter wavelength, because the sky background is weaker. In contrast, at 4.8 microns some portions of the planet's atmosphere may be quite transparent, allowing any internally generated heat (released by gravitational collapse, for example) to escape. of all the planets, the infrared view of Jupiter is the most impressive. Images have often been made at about 4.8 microns, but those obtained at 2.2 microns do not seem to have been published before. The three shown here illustrate the transition in the planet's appearance as we Images of Jupiter taken on August 3, 1982. The 1.6-micron image (top) most closely resembles the visual appearance of thé planet. In the 2.2-micron view (middle) the belt structure is most pronounced, and bright areas are regions depleted in methane. The 4.8-micron picture (bottom) shows how strikingly different Jupiter appears by the light of its own thermal radiation. North is at the top in all these images, and very little relative rotation has occurred. The System I central longitude is 10°. These, and subsequent illustrations, except where noted, are copyright 1982 Anglo-Australian Telescope Board.