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Pioneer and Voyager spacecraft passing by Jupiter measured radiation leaving Jupiter's cloud tops, but we can only theorize about the nature of radiation within the atmosphere. In contrast, the net flux radiometer (NFR) in the Galileo Probe will directly sample the local radiation flows within and below the Jovian cloud layers.
As the Probe descends through various atmospheric layers, there will be observable changes in the net radiation flux. These changes will reveal the driving forces for atmospheric motions: If more radiation enters a layer than leaves it, that layer is radiatively heated, and other layers are radiatively cooled. The temperature differences that tend to arise from the radiative heating and cooling produce buoyancy differences and, ultimately, atmospheric motions. Identification of such layers, and the magnitude of the heat deposited or lost, will comprise the fundamental measurements of the NFR.
A second major objective of the NFR is to help identify components of the Jovian atmosphere. The vertical profile of net radiation flux will show dips in regions where the atmosphere absorbs radiation relatively strongly. Furthermore, the NFR will measure dips (which are increases in opacity) with several spectral bands. The magnitude of such dips can be correlated with the temperature and pressure measurements of the Probe's atmospheric structure experiment and with the particle backscatter measurements of the nephelometer. Together, these three data types will provide significant constraints on the nature of the atmospheric material causing each region of greater absorption.
The NFR uses a rotating optical head in which detectors view a 40-degree cone of atmosphere through a diamond window. The viewing cone is centered at a 45-degree angle as the most representative angle for estimating the integrated energy from an entire hemisphere. A horizontal rotation axis allows both upward and downward hemispheres to be viewed by the same optics and detectors. During the descent into a continuously hotter and denser atmosphere, the NFR will rapidly alternate between looking upward and downward. Measuring the difference in radiation intensity between these two views will determine the magnitude and direction of the net flow (net flux) of radiative energy.
Behind its single diamond window, the NFR has six lithium, tantalate, pyroelectric detectors viewing through filters extending from the visible to infrared wavelengths. One filter is used to measure the deposition of solar energy, while a second is used to measure the integrated infrared energy flux. Three additional spectral regions were chosen to help identify various atmospheric species.
It is well established that molecular hydrogen is the major source of gaseous opacity in Jupiter's atmosphere. However, hydrogen has "windows" or "holes" in its spectrum through which the atmosphere would lose tremendous amounts of radiation to space were it not for the minor constituents of methane, ammonia, and water vapor that fill the holes in the hydrogen spectrum. By measuring the net flux as a function of altitude in the hydrogen windows, it is possible to estimate the abundances of the trace gases. This provides a crude backup to some of the mass spectrometer measurements. More directly valuable are the measurements of opacity contributions by particulates within the atmosphere. The location of cloud layers by their effects on infrared and visible opacity also provides a partial check on the cloud particle observations of the nephelometer.
Because the net flux during descent becomes very small compared to the total energy flux, there are severe requirements on optical symmetry between upward and downward views (the classic problem of subtracting two large numbers applies here). To ensure that detector illumination and window characteristics don't change between upward and downward views, the entire optical system is rotated as a unit to obtain the two views.
In addition to the net flux measurement, the NFR also measures, every other minute, the upward atmospheric flux and the flux from an onboard blackbody. The onboard radiometric calibration system is used to monitor system performance during descent. In the net flux mode, the NFR looks up and down twice per second. In the calibration mode, the NFR flips between two internal radiation sources: a blackbody at ambient temperature and a hot blackbody at a controlled temperature of 410 K (280 deg F). In the upflux mode, the NFR flips between viewing downward and viewing the heated target.
In each mode, the net flux signal is integrated for 5.5 seconds and sampled every 6 seconds. In each two-minute NFR cycle, there are 20 six-second instrument cycles, of which 17 are devoted to net flux measurements, and one each to upflux measurement, blackbody measurement, and analog zero check. This same data format is used throughout the descent, providing a vertical resolution of about 1.2 kilometers (0.7 miles) while the Probe is descending rapidly, and gradually finer resolution as the descent rate slows. At a level of pressure 10 times that at Earth's sea level, the vertical resolution will be about 0.2 kilometers (0.12 miles).
The NFR has a mass of 3 kilograms (6.6 pounds) and will use an average of 10 watts during descent. It was built by Martin-Marietta Denver Aerospace. The principal investigator is L. Sromovsky at the University of Wisconsin (Madison). Other team members are H. Revercomb (also at Madison), J. Pollack at Ames, P. Silvaggion and J. Hayden at Martin-Marietta in Denver, and M. Tomasko at the University of Arizona (Tucson).
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