Aeronautics and Space Administration
Galileo: Probe Into Jupiter
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The Nephelometer carried by the Galileo Probe into Jupiter's atmosphere is designed to search for clouds and determine their properties. The instrument measures the scattering of light from the atmosphere near the Probe. You can simulate the basic idea of this experiment very easily in the classroom.
You need an aquarium tank or similar rectangular glass enclosure (a deep square or rectangular glass dish will do). Also you need a thin beam of light from a small flashlight or a slide projector with an opaque card that has a small central hole. The experiment works best in a darkened room.
Fill the glass tank with water and shine the beam of light into the tank. If you place a mirror near the center of the tank you should see a reflection of the light (see sketch). View the tank from several angles. What do you observe? When you look at the tank perpendicular to the beam of light, do you see much scattered light? What is its color? What about reflected light? The angle between the light beam and your line of sight is the phase angle. The perpendicular view is a phase angle of 90 degrees. Changes in intensity and color of scattered light with phase angle gives information about the particles scattering the light.
Drop a small quantity of milk into the water in the light path. Observe the reflection from the mirror. Also, view the tank from different phase angles. Compare the scattered light with the no-milk conditions. What is the color of the reflected light from the mirror? Does it change color as you change the distance between the mirror and the light source? What further changes occur as the milk dissipates into the water? If necessary add more milk to see the effects.
The particles of milk in water act similarly to the particles of clouds in the Jovian atmosphere. The Nephelometer passes a beam of light through the Jovian atmosphere to a mirror suspended on an arm extended outside the Probe. The reflected light is analyzed to determine the characteristics of the particles. Also light scattered from the cloud particles back to the instrument is analyzed. This is analogous to your seeing the whiteness of the milk/water mixture without seeing a direct reflection of the light through the mirror. Observe that the back-scattered light become whiter as more milk is added, while the reflected light changes color more and becomes fainter.
Recall the color of the reflected light beam and compare it to the faint color in the light scattered 90 degrees to the light source. If you think of the light source in this experiment as being the sun, does the experiment and the resulting colors remind you of colors you see in the sky during the day and around sunset?
If you were going to the atmosphere of a planet for the first time what would you want to know about it to decide if your stay on the planet would be pleasant or if a robotic probe could survive there? An example; consider what you want to know about weather before going on a trip. Check the newspaper weather section and the TV news weather reports. Make a list of the reported atmospheric characteristics. What do these tell you about atmospheric conditions on Earth? Compare with experiments on the Probe. Are there other experiments you'd like to do at Jupiter?
Contrast the strength and variation with latitude of winds on Jupiter and Earth. Until the Galileo Probe enters the atmosphere, we have to estimate winds on Jupiter by tracking cloud features. Even after the entry of the Probe, cloud tracking will provide the only way to study the variation of the winds with latitude, longitude, and time. You can estimate winds in Jupiter's atmosphere using the cylindrical projection mosaics of Voyager 2 images included on the insert to this Educational Brief. The top mosaic on each side shows the appearance of the planet between +45 and -45 degrees latitude on May 27, 1979; one half of Jupiter (180 degrees of longitude) is on one side of the insert and the other half is on the other side. The bottom mosaic on each side shows the appearance of the same region of Jupiter 30 hours later. Locate cloud features you can identify on both images. The bright repeating features near 5° N latitude are called ''plumes''. Are there any plumes near 5° S latitude? The dark spots near 15° N are called ''barges''. Locate the Great Red Spot. Make a table of latitude and longitude for several cloud features in each mosaic and calculate the change in position for each feature. Is movement mainly in latitude or longitude?
Convert the change in position to a distance in kilometers using 1246 kilometers per degree of latitude or longitude. Divide the distance traveled by each cloud feature by the time interval between the mosaics to find the velocity of the cloud features. How strong are the winds and where do the strongest winds occur? Does the strength and/or direction vary with latitude? Are the motions mainly north-south or east-west? What is the speed of the Great Red Spot? How much change do you see in the cloud features? The Galileo Probe is plunging through Jupiter's atmosphere at about 7°N latitude. What types of cloud features and wind speeds will the Probe encounter?
Compare your findings for Jupiter with Earth. Determine the strength, direction, and pattern of Earth's winds from weather maps in a newspaper. Locate a book on Earth's weather to see the wind patterns over our entire planet. Visit your local TV weather department or weather bureau.
By Jupiter; E. Burgess, Columbia University Press, 1982
For progress reports and results from the Galileo mission, go to these World Wide Web sites.
Galileo Probe Homepage at NASA-Ames:
The Galileo Homepage at JPL: