OFJ Field Journal from Dave Atkinson - 11/22/95

What is the Doppler Wind Experiment?

The Doppler Wind Experiment is really quite straightforward in concept, if not in application. If you release a helium balloon on a windy day, you can get an idea of wind speeds by watching the drift of the balloon as it rises in the sky. In the same way, we can infer Jupiter's wind speeds by monitoring the motion of the probe as it descends on parachute deeper and deeper into atmosphere. Unlike a balloon drifting upwards in our atmosphere, however, we can't watch the probe with our eyes (or with the orbiter's "eyes"). We can, however, "see" its motion by using something called the Doppler Effect.

The Doppler Effect was first described by the Austrian physicist Christian Johann Doppler in 1842 while he was writing about the colors of binary stars (systems where there are two stars that orbit about each other). As a source of radiation, either sound or light, is moving towards or away from an observer, the apparent frequency of the radiation is shifted. If the source is approaching, then the shift is towards higher frequencies. If the source is receding the shift is towards lower frequencies.

Most people are familiar with the Doppler Effect from hearing the apparent change in pitch of a siren from a passing fire engine, or horn from a passing train. Astronomers depend on the Doppler Effect to measure the speeds of stars, galaxies, and quasars, looking at how the color of visible light from these objects gets shifted. This shift is often called a "redshift," since the Doppler Effect moves visible light towards the red end of the spectrum if an object is moving away from us (if something is moving towards us, the visible light is "blueshifted").

When the Galileo probe encounters winds, its speed will change. This will be reflected by an apparent change in the probe radio signal frequency due to the Doppler effect. By making accurate measurements of the probe signal frequency the probe motion can be determined and, assuming the probe motion follows the changing winds, the wind speed at the probe location throughout descent can be calculated. If the winds blow the probe towards the orbiter, the Doppler effect will cause the received frequency to increase. If the wind causes the probe to move away from the orbiter, the Doppler effect causes the received frequency to decrease.

In concept, this is not a new idea. Similar methods have been used to measure the winds on Venus. However, Doppler wind measurements at Venus are different in significant ways. Venus is a nearby planet, the same size as the Earth, that spins very slowly. In addition, the Venus probes were tracked from the Earth, not from another spacecraft. Each of these factors complicates the Jupiter wind measurements. First, Jupiter is very large and spins very fast. The probe, while descending on parachute, moves with the spinning planet. This spinning motion can cause a Doppler shift of as much as 10,000 Hertz, or Hz. If our knowledge of the probe location is in error by as little as 0.5 degrees in longitude (in the east-west direction), the Doppler shift due to the spinning planet can be several hundred Hz more or less than we expect. Keep in mind that the winds are expected to cause a Doppler Shift of somewhere between 10-90 Hz, so if there's even a small uncertainty in the probe location, the wind measurements could get swamped by errors from not knowing the accurate Probe position, causing major problems. If Jupiter was not as large, or did not spin as fast, the effect of small errors in the probe location would not be nearly as large.

Another difficulty arises from the geometry of the probe, orbiter, and spinning planet. While the probe is sending data to the orbiter, the orbiter is almost directly above the probe. Since we are measuring horizontal winds, the winds do not cause the probe to approach or recede from the orbiter very much. If we were directly "in front of" or directly "in back of" the orbiter (so the winds would be blowing the probe straight towards us or away from us) then we could measure the Doppler shift much more easily. Instead, we only see a small part of the shift: for example, a wind of 100 meters per second (216 mph) will cause the distance between the probe and orbiter to decrease by only about 5 meters per second, resulting in a Doppler frequency (due to the winds) of only about 25 Hz.

It should be kept in mind, of course, that what we are *really* measuring is not the winds, but the motion of the probe. We expect, however, that the probe motion will trace the winds fairly closely. But how good an assumption is this? Al Seiff, who is in charge of the Galileo Atmospheric Structure Instrument, showed that the time it takes for the probe to "catch up" to the winds is about equal to the probe descent speed divided by the acceleration of gravity. Up high in the atmosphere, where the air is thin, the probe descends very rapidly since the parachute does not provide much "slowing" in thin air. If the probe is dropping at 100 meters per second, and the acceleration of gravity is about 23 meters per second per second, the probe response time is about 4 seconds. And, in 4 seconds at 100 m/s, the probe will drop through 400 meters. As a result we should be able to see changes in the wind that occur across altitudes of about 1/2 kilometer high in the atmosphere. As the probe descends and the atmosphere gets thicker, the parachute works better and the probe speed of descent decreases. Towards the end of the mission the descent speed may be 25 meters per second, in which case the probe will respond to wind changes in about one second. At 25 meters per second the probe will descend through 25 meters of atmosphere in one second. Late in the mission we can therefore expect to see wind variations that occur over altitude ranges as small as about 25 to 50 meters.

Finally, a question that cannot be ignored - why do we care? Why is it important to make the wind measurements? The simplest answer is that the winds are a key element to a complete understanding of Jupiter's weather. Just as knowledge of the composition of the atmosphere, the atmospheric pressures and temperatures, the location, composition, and densities of the clouds, the existence and location of lightning, and the overall atmospheric energy structure (including where solar energy is absorbed) is necessary for us to completely understand Jupiter's atmosphere, we need to know the dynamics, or motions, of the atmosphere.

But above and beyond this somewhat generic answer, the variation in the winds with altitude will yield clues to the source of energy that drives the circulation of Jupiter's atmosphere. On Earth the energy that powers the winds is ultimately derived from the Sun. The Earth's atmosphere, land, and oceans receive more solar energy near the equator than at the poles. It is this difference in heating (along with the somewhat more subtle effects of differences in the ability of oceans and land masses to retain heat, the effect of the Earth's rotation, topography, etc.) that is ultimately responsible for the Earth's winds. But Jupiter is further away from the Sun; it only gets 1/25th as much solar energy as does the Earth. And in planets even further away from the Sun--Saturn, Uranus, and Neptune-- the winds are also seen to be very large at the cloudtops. We therefore suspect that the energy causing the winds might come from a different source.

If the winds on Jupiter depend on energy input from the Sun, we'd expect that the winds would vanish a short distance beneath the cloud tops. If a more exotic source of energy supports the wind, such as latent heat released at the bottom of the water clouds, or internal energy from the center of Jupiter, the wind profile would be expected to fall off much more slowly. If energy originating in the center of Jupiter is supporting the winds, wind speed could still be as high as 70 meters per second (about 150 miles per hour) at the end of the probe mission.

As a final comment in this altogether too long journal, consider again the Earth's weather. The winds provide a good example - even on familiar Earth, the mystery of the winds is not completely solved. If we could somehow place the Earth in a laboratory, and experiment with different properties of the Earth that might affect the winds, we might decide to try making Earth spin faster, or slower, or take away the surface or oceans, or move the Earth closer to the sun, or farther from the sun. In each case, we could examine how the winds change and begin to understand how different factors on Earth affect the winds. Unfortunately, although I have not tried, my guess is that NASA would hesitate at funding experiments to move the Earth closer to the sun, spin it up, or remove its oceans. But consider the other planets in our solar system - Venus, a heavily clouded earth- sized, slowly rotating planet closer to the sun. Mars, a (virtually) cloudless planet smaller than the Earth with a thin atmosphere, no oceans, and somewhat farther from the sun. Jupiter, Saturn, Uranus, and Neptune - planets that are much larger than Earth, much more distant from the sun, no solid surface, and spinning very rapidly. Titan, the largest moon of Saturn - very far from the sun, smaller than the Earth, slowly spinning, with a very cold atmosphere similar in composition to the Earth (primarily nitrogen) and, perhaps (but by no means certain) some small seas of liquid methane/ethane. By studying the weather on these other bodies, we can begin to decipher some of the mysteries of our own atmosphere. In a very real sense the planets are therefore our laboratory for studying the Earth.