To Galileo Science Instruments Contents PageThe nephelometer aboard the Galileo Probe will measure the physical structure of Jupiter's clouds, including the location of cloud layers and the size, concentration, and shape of cloud particles. The spherical or nonspherical shape of the particles indicates their state--solid (ice) or liquid.
Instruments of a similar design flew on the four Pioneer Venus Probes in 1978, and are also being developed for future probes of Saturn and Titan.
The objective is to measure Jovian cloud particles ranging in size from 0.2 to 20 micrometers (8 millionths of an inch to 1/80,000th of an inch) at 10 to 1000 kPa (0.1 to 10 bars of atmospheric pressure). Densities greater than three particles per cubic centimeter (316 particles per cubic foot) will be measured at altitude intervals less than 1 kilometer (0.6 miles).
As the Probe falls through the atmosphere, it is expected to pass through two and maybe three major cloud layers. The highest layer, occurring at a pressure level of 20 to 50 kPa (0.2 to 0.5 bars), is composed primarily of ammonia. A haze of moderately small particles overlies this layer. Deeper into the atmosphere, where the pressure is about 100 to 200 kPa (1 to 2 bars), may lie a layer composed of ammonium hydrosulfide particles. Below these two, at a pressure level of about 200 to 500 kPa (2 to 5 bars), lies a third layer composed primarily of water particles. The cloud particles may be icy crystals in the higher, colder clouds and liquids or melting solids in the lower, warmer cloud layers.
An understanding of the physical properties of the cloud particles is essential to understanding the planet's energy balance. Jupiter gives off twice as much heat as it receives from the Sun, despite the fact that it is nearly 45 light minutes (half a billion miles) from our star. In fact, Jupiter nearly became a star itself. Instead, it is a giant ball of hydrogen and helium, with complex weather systems driven by its rotation and internal heat. It is important to learn about Jupiter's energy balance to understand its basic processes.
Information from the nephelometer, atmospheric structure instrument, and mass spectrometer will provide profiles of the atmosphere's composition, particulate properties, pressure, and temperature as a function of altitude. Exact information on the Probe's rate of descent will be required to compile these profiles.
By comparing these data to various models of the atmosphere and adding it to the information gathered by the Orbiter's remote-sensing instruments, the characteristics of cloud particles at the Probe's entry site can be well defined.
The instrument consists of a forward scatter unit, a backward scatter unit, and electronics. Immediately after the Probe jettisons its parachute, the nephelometer will extend a 13-cm (5-inch) arm. The forward scatter-sensing mirrors are located on the tip of this arm. As the spinning Probe drops through the clouds, cloud particles pass through a sampling volume defined by these mirrors and the illumination provided by an infrared laser beam. Cloud particle sizes are determined by the intensity of scattered light intercepted by the four mirrors positioned to detect light scattered at angles 5.8, 16, 40, and 70 deg from the main beam. A heater unit on the mirror assembly aids in preventing condensation or frost formation during the Probe's descent.
The backscatter unit measures light scattering at an angle of 178 deg from the main beam. It is mounted behind the forward scatter unit and just clears the outer surface of the Probe's skin. It also uses an infrared laser beam to illuminate the particles.
The electronics unit is pressure tight, while both the forward and backward scatter units are vented so that the internal pressure of the units essentially follows the pressure inside the Probe. The instrument mass is 4.4 kilograms (9.7 lbs.), and it operates on an average 13.5 watts.
Principal investigator for the nephelometer experiment is Dr. Boris Ragent of NASA/Ames Research Center. There are also four coinvestigators. The instrument was built by the Martin-Marietta Corporation of Denver, Colorado.
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