I am delighted to be asked to talk to you today. I am grateful to the Chancellor for his generous introduction, to the Committee for naming me Faculty Research Lecturer, and to the audience for coming to listen. And thanks are due as well to the many colleagues with whom I have worked closely and productively over the years, colleagues who were intimately involved in the intellectual adventures of which we will talk today. The list is long, because the project that I shall describe is grand in scope and was long in reaching completion and many contributed in important ways. I stand here as the representative of a superb research collaboration centered at UCLA and strongly supported at the Jet Propulsion Laboratory.

Introduction

My talk has two themes. One theme is announced by my title: magnetic fields. But at the same time I shall try to share with you the excitement of the research adventure that drives us to explore the frontiers of knowledge. Let’s start with magnetic fields.

Magnetic fields are ubiquitous in the universe. They control behavior of charged particles that fill most of space.  They appear to account for some of the structure observed in the universe. We earthlings live in a bubble in space, shielded by Earth’s magnetic field from direct interaction with most of the particulate matter, some potentially destructive, that fills the space around the Sun. We are affected by Earth’s magnetic field even though most are relatively unaware of its effects. Even in the scientific community, few think about planetary magnetic fields, their properties, their history, and their origin, the subjects of today’s talk.
 
We know a great deal about magnetic fields and how they affect magnetized materials and moving electrically-charged particles. But the way in which planetary and stellar magnetic fields are generated is, surprisingly, only partially understood. As scientists are attracted by puzzles, there are quite a few people working on the question of how planets produce magnetic fields.
 
In trying to understand magnetic fields in the solar system, we need first to identify them. Then we will consider how they are generated and what they have taught us about the history of the earth and other bodies. In the process, I will tell you a bit about the space research that my colleagues and I have been carrying out over several decades, about our latest discoveries, and about the new questions that these discoveries have raised.
 
Properties of Earth’s magnetic field

It is well known that compass needles point roughly north along the earth’s  magnetic field. The Chinese near 1000 AD are said to have discovered this remarkable phenomenon, and very soon it became central to navigation. As early as 1600 AD, William Gilbert, physician to Queen Elizabeth I, recognized that the tilt of Earth’s magnetic field varied systematically with latitude, pointing inward towards Earth in the northern hemisphere, outward towards the sky in the southern hemisphere, and along the surface near the equator. Gilbert proposed that Earth was like a magnetized sphere. (Note the change of direction relative to the spherical surface.)
 
In the next diagram, I show the magnetic field outside of a uniformly magnetized sphere. Here the lines show the direction along which a magnetized needle would orient itself at each point of space. We call them “magnetic field lines.” The pattern is called a dipole field, and I shall use that term to describe it.  It is the same pattern that would be produced if a very intense bar magnet were placed at the center of the sphere.
 
Now Gilbert got the general picture right, but the actual magnetic field of the earth is much more complicated than his picture suggests. There are localized regions of skewed fields that disorient compasses, swinging them to the east or west. Today the magnetic field has been measured over the surface of the earth, but that was not true in the 15th century. Indeed, as they journeyed across the sea, the sailors with Columbus feared for their safety when they found that the compass was drifting away from the direction of the north star. Columbus surreptitiously changed the markings on the instrument to calm his panicked men, thus violating Admiralty Law but possibly saving his mission.
 
The demands of navigation led to well financed expeditions that mapped the earth’s magnetic field. It soon became clear that the magnetic field of the earth is not only more complicated than a dipole field, but that it changes over years, shifting in direction and varying in magnitude. The first world magnetic chart dates from 1702 and was put together by Halley of comet fame. The field patterns were found to drift westward, very slowly but rapidly enough to matter to navigators. These measurements showing that Earth’s magnetic field can change significantly with time have provided critical insight for scientists. Today the magnitude of the field is decreasing by 0.05% per year.
 
Fashions come and go. Magnetism captured popular fancy near the end of the 18th century when the Austrian physician, F. A. Mesmer, became a celebrity by asserting that he could effect cures by stroking the ill with magnets.  Ultimately, he decided that the magnet was not necessary, that “animal magnetism” was at work. His cures became the fashion in Paris, but they failed to pass the rigorous review of a medical commission of which Benjamin Franklin was a member. Mesmer was denounced, left France, and died in Switzerland.
 
Well, let’s return to the less fanciful aspects of magnetic fields and consider whether we should think of the center of the earth as a slug of magnetized iron. Iron magnets were the only type of magnets known before 1821. In that year, when a Danish scientist, H. C.  Oersted, was running an electric current through a wire, he noticed that a nearby compass needle began to move. Soon after, A-M. Ampere, a French scientist, demonstrated that flowing electrical currents produce magnetic fields. So now there are two possibilities: a slug of magnetized iron or strong large-scale electrical currents flowing in the interior of the earth. How are we going to choose between these possibilities?

Part of the answer comes from studies of the temporal variation of Earth’s magnetic field. The changes can become large. It is a 20th century discovery that Earth’s dipole field has actually reversed its direction many times .  When the earth’s field reverses, a compass needle at the surface points south instead of north. This discovery dates to the 1920s  and some observations made by Motonari Matuyama, a professor at the Kyoto Imperial University in Japan. He was studying rocks formed from volcanic lava  Lava is rich in iron. The little iron magnets within the cooling fluid align themselves with the earth’s magnetic field. Once frozen into a solid phase, their alignment doesn’t change even if the field changes direction. In the volcanic rocks, Matuyama discovered that the iron in the young rocks was aligned along the direction of Earth’s magnetic field, but the iron in a second group of 1-2 million year old rocks was aligned in the opposite direction. When those rocks cooled, the earth’s magnetic field must have pointed south rather than north! He published a paper proposing that Earth’s magnetic field had reversed direction over geological time. In 1929, few paid much attention to this suggestion, and Matuyama stopped working on geomagnetism and instead turned to being a university president and acting in the No theater.

But eventually, particularly in the 1950s, extensive experimental research confirmed Matuyama’s proposal that the earth’s field flips back and forth between what we call normal (i.e., familiar and like it is today . . . an interesting definition of normality) and what we call reversed. How often does the field reverse? Measurements have shown that the field reverses its direction irregularly, but for the last 45 million years it has reversed on average about once every 200,000 years.

Also in the 1950s, ships exploring the oceans began to drag magnetometers (instruments that measure the strength and direction of the magnetic field), thus providing extensive measurements of the magnetic field variations along the ocean floor. Strange patterns emerged. There were stripes along which the magnetic field was larger than average, as it would be if small magnetic grains had been placed on the ocean bottom pointing north, thereby adding to the earth’s magnetic field, and stripes along which the magnetic field was smaller than average, as it would be if the magnetic grains pointed south. The stripes had different widths along the path of the ship, but on balance, northward and southward were equally probable.

How could one explain this phenomenon? The names associated with the explanation are F. J. Vine (a graduate student at Cambridge University) and P. M. Matthews, his research supervisor . Others before them had proposed that the ocean floor is constantly being renewed as molten volcanic matter pushes up along the mid-ocean ridges. Vine and Matthews noticed that if you start at the mid-ocean ridges and move outward to the east, the pattern of stripes looks like a mirror reflection of the pattern of stripes to the west. They suggested that as the hot material pushing up at the ridges begins to cool, the magnetic particles within it align themselves along the earth’s field, just as happens with volcanic rocks. Their direction is frozen in as the material solidifies. Matter immediately adjacent to the ridge on both sides is magnetized in the direction of Earth’s field at the time it cooled. When more melted matter pushes up, the solid matter previously present gets pushed to the side. If Earth’s field has reversed, the new melt solidifies with its magnetic particles pointing the other way. This explains the patterns on the ocean floor and gives strong support to the idea that the field has flipped many times. The ocean floor acts like the tape of our audio cassettes with magnetic patterns permanently imposed on the solidifying crust. As we can date the time of the magnetic reversals from studies of continental lavas, it also gives a way of figuring out how rapidly the sea floor is spreading, which in the early 60s was not yet known. It turns out that the crust moves a centimeter or so per year. Slow indeed, but the ocean has been around for a long time.

Field reversals have happened often over the millions of years for which we have evidence, and probably over many billions of years. This probably has not mattered to man, but it may have caused problems to some animal species. Magnetic fields are actually used by certain animals to provide orientation when the Sun is not visible. There are turtles who bury their eggs in the sand just landward of the water line at full moon, the time of the month’s highest high tides. The babies hatch at the next full moon and head for the sea.  But if the sky is clouded, they cannot see the water and don’t know which way to go. There is evidence that they use the magnetic field to help find the right direction for their first feeble steps. This was demonstrated in a lab experiment with  turtles whose paths at birth were changed when the field direction in the lab was reversed. A field reversal might cause problems if it occurred as quickly as it did in the lab experiment I have described. But natural reversals require many thousands of years, and one hopes that the turtles will adapt.

Earth’s magnetic field reversals may or may not be relevant to studies of the survival of species. But the fact that the magnetic field reverses is one of the key properties that needs to be accounted for when we explain why the earth and some other planets have magnetic fields.

Generating planetary magnetic fields

We will first consider the possibility that Earth’s field is produced by a large concentration of solid iron in the center.  This seems to be ruled out for two reasons. First, it is known that at high temperatures, iron loses its magnetic properties; the center of the earth is too hot for iron to act as a magnet. Second, we know that Earth’s field reverses direction. That is hard to do with giant slugs of iron.  And repeated flips of a giant iron magnet are inconceivable. But flips are easy to produce with currents. Currents easily flow in either direction through a wire or a metallic fluid. Just change the sense of current flow, and the magnetic field will flip. So we need to come up with a way of  making the earth’s magnetic field by generating currents inside the earth.

Later we will go more deeply into this matter, but first, let us consider where else magnetic fields are found in the solar system. We start with the Sun. It may seem odd, but the Sun was the second solar system body for which a magnetic field was detected. The story starts with observations made by Galileo in 1609. He detected dark spots on the surface of the Sun, and called them sunspots. He also didn’t know that the Chinese and others had reported sunspots 15 centuries earlier, but neither did his referees, so he was able to publish his observations. By following the displacement of the sunspots across the face of the Sun from day to day, Galileo was able to figure out how fast the Sun rotates about its axis. There were those who felt that by asserting that the Sun was not a perfect glowing orb but actually had a pock-marked surface Galileo was criticizing the perfection of God’s creation, a belief described by one of my colleagues   as “the Church’s doctrine of immaculate and stationary celestial bodies”. Galileo was always getting himself into trouble. But here, as in the case of planetary motion, he was right.

People have monitored sunspots nearly continuously from Galileo’s time onward. By 1843, a pattern was detected by H. Schwabe. The number of sunspots varies with a period of 11 years, which we refer to as the sunspot cycle. Sunspots have been found to be regions of intense magnetic fields. In these regions, the gases of the solar atmosphere are cooler than elsewhere on the solar surface. Cooler gases don’t glow white-hot, so they give a darkened appearance to parts of the surface. We can measure the direction of the magnetic field in sunspots and in the Sun as a whole body. The directions flip from one eleven year period to the next, just as Earth’s field flips. So flipping magnetic fields are not unique to Earth, and we believe that the Sun’s magnetic field, as well as the earth’s must be generated by currents flowing in the electrically-conducting rotating fluid of its interior.

The effect of Sun’s magnetic field can be identified throughout interplanetary space although it is only near the Sun, in the solar corona or the Sun’s crown, that there is visible evidence of magnetic structures. The solar corona contains hot gas that can be seen during totality of solar eclipses, which are rare events. During an eclipse, glowing light seems to outline the magnetic field in the region above the Sun’s surface. Total eclipses are sufficiently rare and sufficiently important scientifically that we are told that, “during the American Revolutionary War, hostilities were suspended in a part of the state of Maine for one day to permit the Reverend Prof. Williams and his collaborators from Harvard College to observe the eclipse of 1780.” He might have seen something like what is illustrated in the image we now examine. Note the glowing structures that extend from the Sun into space.  The intense glows are produced by electrons flowing along the magnetic field direction.

Some of the hot coronal gas has so much energy that it escapes from the Sun and flows out at supersonic speeds of about 240 miles per second or a million miles per hour. The image shows a bubble of hot gas moving out from the Sun. When the hot gas escapes from the Sun we call it the solar wind. The solar wind drags the Sun’s magnetic field out beyond the most distant planets. The magnetic field, blown out from the rotating Sun, makes a wound-up pattern much like that of a rotating lawn sprinkler.

At Earth, the solar wind is stopped by the planet’s magnetic field far above the surface so that our planet is enclosed in a protective magnetic cocoon called a magnetosphere that blocks direct access of both the solar wind and many of the potentially destructive extremely high-energy cosmic rays that stream towards us from the Sun. The magnetosphere is the protective “magnetic bubble: to which I referred at the beginning of this talk. Despite the magnetic shield, the solar wind does affect the earth’s immediate surroundings.

With R. L. McPherron, Ray Walker, and other colleagues, I have been trying to understand how the magnetic field of the solar wind affects the magnetosphere and the earth.  We study how the solar wind links to what are called magnetic storms or substorms, intermittent disturbances, of relatively short duration, that drive intense electrical currents towards the earth from space. These storms occur most often at the maximum of the sunspot cycle, producing effects that can be awesomely beautiful like the aurora, the ghostly lights of the polar night skies,  or economically  troublesome as when surging currents interfere with communications, damage sensitive satellite systems, or knock out power transformers and leave large parts of the country without power.

A recent magnetic storm coincided with a major pigeon race in France, and, as the article indicates, many homing pigeons which are thought to use the magnetic field for orientation, lost their lives when the magnetic field became disturbed in a magnetic storm.

What about the other planets? Well, some have magnetic fields. Some don’t. Jupiter’s field is about 13 times larger than Earth’s at the surface, and, like Earth’s field, is slightly tilted from its north-south axis. Saturn, Uranus and Neptune like Jupiter, have magnetic fields. Mercury which is much smaller than Earth has a planetary magnetic field with surface strength 1% of Earth’s, small but significant.

But not all of the planets have magnetic fields. The field at Venus was studied by Chris Russell of UCLA using the Pioneer Venus Orbiter. He discovered that Venus has no planetary magnetic field, or one so small (less than 0.01% of Earth’s) that it doesn’t matter. Venus is not much smaller than Earth. Why doesn’t it have a magnetic field? Mars has at most a very small field. Pluto has not yet been explored by any spacecraft, so we will have to wait to find out more about it.

With the fields of eight planets known (or almost known), can we find a pattern? We think that we can. In our description of this pattern, we have to assume that we understand the interior structure of the planets. We think that planets melt right through in the early stages of their development, allowing the heavy metallic elements to settle to the center under the pull of gravity. The planets then begins to cool. The outside layers cool most, forming solid surfaces in the case of the smaller planets. Within the deep interior there may be both solid and liquid heavy metals. These heavy metals of the deep interior are good conductors of electricity. The central core of the earth, for example, is solid but it is surrounded by a zone of melted iron-rich metal.

The boundary between the solid and the melted liquid is a source of heat. If a liquid metal is heated from the bottom, it will start flowing like a pot of water heated to a boil. In a rotating body like the earth, the moving fluid metal produces electrical currents and creates a magnetic field. The proposed ingredients for making a planetary magnetic field  are summarized in the figure, a list that makes it seem as if the problem has been solved. It would be nice to think so. The generation of planetary magnetic fields is an unsolved problem that has been with us for some time. There are claims that Einstein identified  the origin of Earth’s field as one of the five most important unsolved problems in physics. Why must I say that we haven’t solved the problem of how Earth’s magnetic field is generated? It is because the details are hard to work out mathematically and so it remains slightly uncertain that the analysis is heading in precisely the right direction.

In physics, explanations must not only sound reasonable, but also the numbers have to come out right. And it is only in the past few years that the biggest computers have been programmed to show what happens in a mathematical model of the earth, spinning around its axis once a day, heated from the center but cool on the outside. Does it generate enough current to explain Earth’s magnetic properties? P. Roberts of UCLA and G. Glatzmaier of Los Alamos National Laboratory have been able to show that computer calculations of such a mathematical model of the earth produces magnetic fields with the general characteristics of Earth’s magnetic field. They find that the field inside the earth gets twisted in odd ways, but the field near the surface looks generally familiar. A particularly exciting feature of the Glatzmaier-Roberts model is that it produces magnetic field reversals, which, as I mentioned earlier, is an essential feature of any correct model. So, it seems that we are almost there. But we are still testing the details to see if they really work, not only for Earth but for the Sun and for other planets as well.

Do the giant planets: Jupiter, Saturn, Uranus, and Neptune fit the picture? They remain partly liquid inside, though the current-carrying material isn’t quite the same as for Earth. They rotate. And they have magnetic fields.  Our understanding of these planets seems generally satisfactory though incomplete. We don’t yet understand the peculiar orientations of the magnetic fields of Uranus and Neptune.

The Sun has a layered interior with electrically-conducting fluid that is constantly being stirred up as heat rises from the interior. The magnetic fields that can be generated from this moving fluid layer could, in principle, explain the Sun’s magnetic field, though we have, as yet, no detailed analysis of the source of the 11 year cycle of reversals.

What about the smaller planets? Small planets cool off more rapidly than large ones. After about 4.5  billion years, which is the age of the solar system, a small planet might have a completely solid interior. This would seem to rule out the flow that we think is needed to produce a planet-wide magnetic field. Mars may be an example of such a frozen planet. It is hard to understand why Venus doesn’t have a magnetic field. It should not have cooled so much that its interior is solid. People have proposed explanations why Venus should have no field and have proposed explanations why  Mercury, though very small, should have one.  Still, physicists have not successfully  predicted in advance of the measurements which planets had  magnetic fields. That, you will agree, suggests that our understanding of planetary magnetism has a way to go.

When scientists don’t completely understand a physical process, they try to do more experiments.  For planetary scientists, the laboratory is the solar system. Ideally, we would look for more planets to see if by understanding their properties, we can learn more about how the fields are produced. Yes, you say, but there are no more planets. True. But there are moons. And they are close to planetary size and share many properties with the planets. So let’s add Earth’s moon to the list. Earth’s moon has no planetary magnetic field as we found from measurements on the Apollo 15 and 16 subsatellites which carried UCLA magnetometers. The Moon seems to be a case like Mars, so small that there is no molten material in its central core, or maybe there is no core at all. It does begin to seem as if small bodies don’t generate magnetic fields unless they happen to orbit as close to the Sun as Mercury which keeps them hot and preserves their melted interiors.

Now I come to the part where my UCLA colleagues and I have had the good  fortune to be able to add our own discoveries to the exploration of the solar system, and we found some truly surprising results. As you heard earlier, we have been working with colleagues from the Jet Propulsion Laboratory on the Galileo mission, a project that took almost two decades to reach fruition but which has been returning remarkable data from Jupiter since the end of 1995. The spacecraft carries a magnetometer not unlike those used to measure the strength and direction of the magnetic field on the sea floor. The instrument was designed and tested in our own laboratories and has been operating beautifully since Galileo’s launch in 1989.  Galileo has now made close passes by each of Jupiter’s four large moons, with 11 close encounters now complete, another one due next week and more coming over the next two years. The completed passes have given us a chance to find out about new worlds whose properties were only remotely anticipated.

Let me first introduce you to these strange and wonderful worlds. Here are some images of Jupiter (here seen in comparison with Earth to scale) and of the moons: Io, Europa, Ganymede, and Callisto. Lovers of antiquity will recognize these names as conquests of Jupiter (or Zeus) in the Greek myths. Io with its volcanoes is the best known. A sulfurous surface takes on a range of colors that makes it look like a partly burned pizza. And Io is volcanically active, the most active body in the solar system. Europa has ice rafts that seem to have moved in the fairly recent past. Though small, it seems to have a lot of water in its interior, and there are those who think that it may provide an environment that could support life. Ganymede is the largest moon in the solar system. Its pock-marked surface appears very austere. And Callisto, just slightly smaller than Ganymede, looks old and dead. The last image is one that I pulled off the Internet less than one week ago. At the top, the moons are shown with true relative sizes. Differences in their surface structure are apparent in the images at intermediate and high resolution in the two groups of images below the full disk images.

Prior to the Voyager missions of 1979, even the surface features of these bodies had not been resolved. For example, we did not know for sure if the moons of Jupiter had ever been heated hot enough to melt through. This means that we did not know if metals had sunk to their centers, and if the ice and rock that they contain were uniformly mixed or separated into layers, a process that we call differentiation.  Most people suspected that even if the moons melted early in the life of the solar system, they should have cooled rather quickly because they orbit a planet that is far from the Sun, and it was unlikely that their interiors were still hot enough to be even partially liquid. We didn’t know, but the Galileo magnetometer team realized that if the interiors were still melted, there might be magnetic fields. Evidence of  a rather large magnetic field would suggest that there was a current-carrying, liquid metal shell in the interior. So the presence or absence of a magnetic field would serve a bit like MRI imaging to give information about the interiors that we cannot see.

We would have been surprised if there were no surprises. We knew that Io had surprised people before. For example, volcanic activity had not been anticipated on a small moon like Io until, in a remarkably forward-looking paper, UC colleagues from Santa Barbara pointed out that Jupiter raises giant tides that move back and forth across Io and thus heat its interior. They suggested that active volcanoes could exist on Io. The volcanism was confirmed in the first images of that mysterious moon in the very week in which the prediction was published. Volcanoes contain molten lavas, and so it was clear that at least part of Io is molten, and possibly capable of generating flows and electrical currents that give rise to magnetic fields.   Earlier it had been noted that bursts of radio noise from Jupiter were produced by electrical currents flowing from Io, another oddity among the properties of the moons of planets. So we were prepared to look for signs of an internal magnetic field of Io. We had even guessed that that field would have its north pole pointing up.

What did we find? Let me explain. Jupiter, you will recall, has its own magnetic field, and as we moved in towards Jupiter, the magnetic field at the Galileo spacecraft became ever larger. This was expected because we know that Jupiter’s magnetic field becomes stronger as the distance between the spacecraft and the planet decreases. Then we approached Io and rather suddenly the strength of the magnetic field decreased by about 40%. The drop is just what we would have expected if two magnets were present, with Io’s magnetic field opposing Jupiter’s. We suggested that Io’s magnetic field at its surface was about 1/25th as strong as Earth’s at its surface. At about the same time, by interpreting the information from small changes in the radio signal that Galileo uses to communicate with us on Earth, another team including my ESS/IGPP colleague Jerry Schubert showed that Io is differentiated into a heavy metal core and a lighter outer layer. Put that together with the volcanoes as evidence for internal heating, and it seems probable that Io must have a magnetic field  and that its north pole does point up.

Why do I hedge on this statement saying a field is probable, not that there is a field? The problem is that Io’s volcanoes spew out lots of gaseous materials that complicate the analysis. With a telescope it is possible to see the clouds that envelope Io as it moves around its orbit. These clouds produce electrical currents outside of Io. We may have measured magnetic field changes produced by currents flowing in these clouds and not necessarily from the currents in a hot fluid core. So the question of Io’s magnetic field remains unsettled after only one close flyby with the Galileo spacecraft.

Ganymede was the next moon to be closely observed by the Galileo spacecraft. Ganymede’s orbit lies much outside of Io’s. Tidal forces weaken with distance and tidal heating is not important at Ganymede. This explains why Ganymede has no volcanoes and no other significant sign of being hot in its interior. It was thought possible that the interior had not ever separated into different layers, and even if it had separated, its interior was thought to have frozen solid over the lifetime of the solar system. So there was little reason to expect that it would have a magnetic field, but we were all wrong. At Ganymede, there is unambiguously an internal magnetic field that  opposes Jupiter’s. Galileo has now completed four close passes of Ganymede, approaching it from different directions, and our measurements confirm unambiguously that the magnetic field is produced by currents flowing within Ganymede.
 
Our findings support other measurements that reveal that Ganymede’s interior is highly structured.  So, as I indicated, the magnetic measurements are now being used to check out what’s going on inside of this Jovian moon. There are still some uncertainties in the details, but we lean toward the idea that Ganymede’s interior is like Io’s with a thick coating of ice on the exterior. This discovery must now be incorporated into theories of the formation and evolution of planets and moons.

The discovery of Ganymede’s magnetic field shows that something needs to be modified in the accepted description of the evolution of the solar system. There is a new puzzle to solve. We still don’t know just how Ganymede’s field is produced. Several different groups of people are working on the problem and some of them have ideas how Ganymede’s interior might have melted not only when it first formed, but again in times recent enough that it still has a molten center to produce its magnetic field. Others are trying to figure out ways of explaining the magnetic field without the need for re-melting the interior. In finding that Ganymede is magnetized, we have answered one question and raised many new ones. That may seem as if we don’t know what we are doing, but we regard it as scientific discovery at its most satisfying.

We are also excited about an additional discovery that Ganymede’s magnetic field carves out its own mini-magnetosphere that forms a magnetic cocoon within Jupiter’s magnetosphere just as Earth’s magnetic field makes a cocoon around it within the solar wind.
 
What about the other moons of Jupiter? Let me say that we are still confused after one flyby of Europa and three of Callisto. Europa appears to be differentiated into layers like Ganymede. Callisto is not.  Neither has a nicely ordered field, but they do affect the magnetic field in their neighborhoods; and several of us, particularly K. Khurana and I, are struggling to understand what is going on.
 
And finally, what additional measurements are needed to answer the question about Io’s magnetic field? Our only measurements near Io were made near its equator. We think that if we can collect data on other passes far from Io’s equator, ideally over its north or south pole, we can establish with certainty whether Io has an internal magnetic field. We will actually be getting those measurements if Galileo survives the punishing intense energetic particle bombardment of the inner magnetosphere of Jupiter and lasts until the year 2000. In December of 1999, Galileo will again fly by Io, this time over its south pole. We eagerly await those measurements which will clear up the uncertainty. Again the answer will leave us with new questions. If there is no magnetic field, we will know for sure that even though Io looks like Ganymede without its icy coat, the insides are very different. We will have to be sure that we understand why they are different and whether we can actually calculate the way in which these bodies produce their magnetic fields.

At the moment, we can summarize our knowledge of interiors and magnetic fields as shown in the last two figures. Though questions remain, we have contributed some bricks to building a scientific understanding of the bodies of the solar system.

Like discovery in geographical exploration, discovery in science involves identifying an objective, developing a strategy for reaching that objective, and providing evidence that your discovery is real. Sometimes the discovery can be exploited in ways that were not anticipated in advance. Many of these elements of research have parallels in the undertakings outside the sciences elsewhere on the campus. Our grand objective is to broaden human understanding of our origins, our condition, and our destiny, whether we talk about the evolution of the bodies of the solar system, the history of mankind, the pattern of the human genome, or the structure of language. In any area, there is a thrill in adding even a small piece to the collection of pieces that together begin to complete the big picture. It is a thrill that this talk gives me a chance to share.
 
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