Jupiter is the largest planet in the solar system, and claims one of its most enigmatic satellites. In 1979, heavy volcanic activity was discovered on Io - the first such instance confirmed outside of planet Earth. The gases emitted during Io's volcanic eruptions are the foundation for research into a variety of space phenomena occurring around Jupiter, and Nikon products are proving most vital to the study.
We asked Dr. Shoichi Okano, head of the project and Professor at the Planetary Plasma and Atmospheric Research Center (PPARC), Graduate School of Science, Tohoku University, for details regarding the research.
Well, the purposes of the observations we perform here are to attempt to shed light on the nature of space phenomena occurring on all of the planets in our solar system, and to gain a clearer understanding of the evolution and current state of the planets. At PPARC, we are conducting remote observation of various planets using optical methods and radio techniques. I work in the planetary spectroscopy section and my primary responsibility is to carry out observations using optical instruments.
The solar system comprises nine planets, Earth being the third. Its size and atmospheric characteristics are considered "average". The planets vary greatly in these respects - from Jupiter, which is far, far larger than Earth, to Mercury, which is extremely small and barely even has an atmosphere. What drives me to continue researching is the belief that the observation of physical phenomena on each of the planets may one day lead to a greater understanding of the entire solar system. Currently my time and energy are focused on a study of Jupiter.
In terms of size and power, Jupiter is unrivalled in our solar system. Its diameter is 11 times that of the earth, and its gravitational pull 2.4 times greater. As the Earth, Jupiter has a magnetic field, and the strength of the magnet is an almost incomprehensible 20,000 times that of our planet. What's more, the monstrous planet performs a complete rotation in only ten hours - more than twice as quickly as Earth. It's easy to see how Jupiter got its name, from the supreme deity in Greco-Roman mythology.
Of course, there is more to Jupiter than its immense size and force. It has 61 moons in constant orbit around it - including one called "Io", one of the four Galilean moons and the one physically closest to the big planet. Io, about the same size as the moon that orbits Earth, was found to have numerous volcanoes - volcanic activity was confirmed during the U.S.'s Voyager mission of the late 1970s. Other than Earth, Io is the only body in the solar system on which the existence of volcanoes has been proven.
Attempts to view other planets from the earth are complicated by the characteristic sway of the earth's atmosphere, called "seeing" - these factors tend to make the view somewhat blurry. It's comparable to trying to view pebble stones on a river bed as the water flows over them. However, thanks to rapid development in Adaptive Optics to negate the effects of seeing, we have succeeded in actually locating volcanoes on Io, though the smoke from their eruptions remains beyond viewability.
Volcanic activity was initiated by fluctuations in Jupiter's powerful gravitational pull. It's the same tidal effect our moon has on the earth. Just as our moon's movement helps dictate the ebb and flow of our seas, the solid surface of Io underwent violent, repeated expansion and contraction, causing friction which heated the satellite, resulting in volcanic activity.
Currently Io is the only satellite that we know has volcanoes. It is most likely due to the fact that Io is the closest of the four Galilean moons to Jupiter. The balance between Jupiter's gravitational forces and the speed at which Io orbits the planet keep the satellite from being drawn closer to Jupiter. Should its orbiting speed decrease and Io move closer, the planet's gravitational pull would break Io into pieces.
Io's volcano emits enormous quantities of gas higher than 100km above its surface. As the gas is ionized, either due to illumination by solar ultraviolet radiation or by collision with surrounding plasma, it becomes what is known as "Jupiter plasma". Jupiter is surrounded by an extremely strong magnetic field and rotates at very high speeds. Once the gas has been ionized, it is captured by the magnetic field and begins a high-speed orbit of the planet along the line of the field.
Io emits huge volumes of gas during its 42-hour full orbit of Jupiter. Total mass of emitted gas can reach as much as 1,000 kilograms per second. Once the gas is ionized, the ions and electrons are trapped by the magnetic field of Jupiter and they accumulate along the orbit of Io. This results in, among other things, a large, doughnut-shaped mass of ions - a "plasma torus" - that is glowing and visible from the earth using a telescope. The majority of the plasma in the planet's magnetosphere comes from the volcanoes of Io.
During our observation we pay close attention to sodium atoms and sulfur ions, as they are relatively easy to see. Sodium atoms are distributed tens of millions of kilometers away from Jupiter. We refer to it as a "sodium nebula" as that's the form it takes when distributed. We believe that some sodium ions, however, are caught in the planet's magnetic field, and again they become neutralized, reverting to atoms with the velocity to escape the gravitational pull of Jupiter. This explains why sodium atoms spread so far from Jupiter. As shown in the figure, neutral sodium atoms glow as they are illuminated by solar radiation.
The figure has been scaled based on the radius of Jupiter (RJ, 71,000km). As you can see, the sodium atoms are sent as far as 450RJ (32 million kilometers) from the planet. To break free from Jupiter's magnetosphere and travel such distances, the atoms would have to be moving at least 60 kilometers per second - over five times the speed required in the case of Earth. The generation of this amazing speed is one of the key targets of our research.
We can observe plasma torus by tracking sulfur ions - these ions emit light when they collide with electrons, making them viewable via optical instruments.
Influenced by the magnetic force of Jupiter, the plasma torus orbits the planet at great speeds, syncing itself with the planet's rotation cycle. As we observe a plasma torus at a fixed point, we see the sulfur ions either come closer to or go farther away from the earth. Considering the principles of the Doppler effect as they apply here, the wavelength of light when the ions move closer to Earth is shorter than that of the light when the ions are moving farther away. We can estimate the difference in wavelength using the rotational speed of Jupiter.
However, there is a small discrepancy between the estimated difference in wavelengths and the actual difference. The reason for this is the delay between the ionization of the ions emitted from Io's volcanoes and the point in time where the ions' movement matches the speed of the magnetic field. In other words, carefully observing this delay will teach us a great deal about the ionization process, which leads us to another one of Jupiter's many phenomena - the aurora.
Yes, they do. In addition to the auroras witnessed at the poles, which resemble headbands, three auroras have been observed at the foot of the magnetic field line passing Jupiter's satellites. We call these "footprint auroras". The auroras occur in different locations because of the origin of the magnetic field line, and because the satellites themselves are separated.
No, actually they take the shape of a line in the direction of Jupiter's rotation. The three places where footprints have been observed are at the bottom of the magnetic field line passing the satellites Io, Europa and Ganymede, all Galilean moons. Io's footprints can be explained by the volcanic gas it emits, which appears to interact with the magnetic field line passing by it. This does not explain why footprints were observed at the other two locations, though. Neither of the two satellites has volcanoes, so why would we see auroras there? This is another mystery we hope to unlock. Since Jupiter has so many moons, we may be able to observe other footprints in the future.
They are difficult to view with visible light from the earth, because auroras on Jupiter glow mainly in ultraviolet light, and such short-wavelength light is absorbed by the earth's atmosphere.
In 2001, when the space craft Cassini passed the dark side of Jupiter on its way to Saturn, an aurora was observed only with visible light. We cannot visually confirm auroras on the side of the planet illuminated by the sun from Earth even if there are actually auroras there. I thought the only way this might become possible was if we were to achieve sufficiently high wavelength resolution to suppress unwanted illumination by the Sun. To my dismay, even using a 188cm-diameter reflecting telescope and a high-resolution spectroscope at the National Astrophysical Observatory in Okayama, I could not confirm the occurrence of auroras. It appears that they don't glow so brightly on the illuminated side of Jupiter. This is yet another perplexity that I am hoping to solve.
Yes, auroras have also been seen on Saturn, Uranus and Neptune. Keys to the occurrence of auroras are a planet's magnetic field and atmosphere. The intensity of the magnetic field depends on the planet's components, size and rotation speed. These characteristics of Mars and Venus are not conducive to the occurrence of auroras.
Primarily at our Iitate observatory in Fukushima prefecture, during the period when Jupiter becomes visible in the night sky. It takes Jupiter 11 of our years to make one complete orbit around the sun, whereas Earth orbits once each year. Every year, when Earth is as close to Jupiter as it can get, observation is also performed on the Hawaiian island of Maui. The altitude of the observation site there is 3,000 meters, making it more stable than our facility in Iitate. The improved view allows us to collect more data. When observing sodium atoms from Jupiter, we use a telescope with an ultra-wide field of view, and we employ the Fabry-Perot interferometer in the observation of sulfur ions.
Sulfur ions in a plasma torus glow at a specific wavelength when they collide with electrons. In order to isolate that light, high-resolution equipment is necessary. Invented about 100 years ago by Charles Fabry and Alfred Perot, the interferometer's design enables it to offer consistent brightness when wavelength resolution is increased, unlike the spectrometer, which is generally used to measure light.
Using the light entering the figure, from the left as an example, we can see that some of it passes through both mirrors A and B and the lens, arriving at the focal plane. A closer look reveals that some light passes through mirror A, then is reflected off of mirror B, bounces again off mirror A and finally passes through mirror B. Not all of it, though - some of the light bounces once again off mirrors B and A respectively, before finally passing through mirror B. Altogether, there are four lights, all having traveled different distances. This difference is responsible for the concentric circular pattern on the focal plane. The wavelength to be isolated depends on the distance between A and B, and on the angle at which the light hits the mirror.
A mirror which has a high-reflectivity coating applied to one side is called a "half mirror". When two such mirrors are placed in parallel with the coated sides facing each other, some light will pass through both mirrors. Most light, however, will be reflected repeatedly off of both mirrors. Obviously the distance that the light travels before reaching the lens differs depending on how many times it repeated reflection between the mirrors. Since light is in essence a wave, the waves will be stronger if they are in phase. If they aren't, however, it is possible that they will cancel each other out. When observing light exiting the Fabry-Perot interferometer through a lens, it appears in the form of concentric circles. As this pattern is determined by the wavelength of the light, the distance between the mirrors, and the angle of light incident on the mirror, it is possible to isolate light of a specific wavelength.
The entrance of a spectrometer is merely a narrow slit, so it can only detect a small amount of light. On the other hand, the Fabry-Perot interferometer uses the surfaces of the mirrors to collect a great deal more light. Brightness and resolution are high. The wavelength of light to be isolated can also be changed, by adjusting the angle of the interferometer.
The Fabry-Perot interferometer requires high-precision glass elements with total uniformity in terms of flatness and thickness. Placing them parallel to each other at an exact distance is very difficult. Though the interferometer we are currently using was manufactured in the U.K., we asked Nikon and
Nikon Engineering (in Japanese language only) to produce the whole optical system as they are highly advanced in terms of experience and technological capabilities. An interferometer optical system for wide-field of view planet observation is made up of semiwide-field of view and ultrawide-field of view equipment.
In addition to my research, I have been using Nikon products for some time now. When I was in high school, I bought Nikon's 6.5cm-diameter refracting telescope. However, one of the fixing screws was a little bent, and I couldn't attach the eyepiece. I wrote to Nikon to explain the situation, and not long after I received a nice letter and a new screw from them. From that moment forward I've trusted Nikon products, and have continued to use them for many years.
Getting back to the optical equipment......the one Nikon developed is also being used for observation of our moon. It is Earth's lone satellite and as small as Jupiter is large. It is sometimes referred to as a naked star with no atmosphere. However, when atoms are expelled from the surface, that simple action creates a thin atmosphere - observed, of course, with the ultrawide field of view equipment produced by Nikon.
I actually didn't begin until 1999, so it's only been four or five years......but it has long been a dream of mine. When I was a child, Mars came very close to the earth. This was back in 1953. With the small telescope I had at the time, the red planet looked like nothing more than a red bean. Even so, it made quite an impression on me. Still, when it came time to choose a major, I went with practicable physics instead of astronomy because I thought physics might provide me with a broader choice of jobs or career paths.
But my strong desire to study nature and the sky never waned, so I switched to geophysics. I was involved in researching the upper atmosphere after graduating from college, from 1970 to 1995. From '95 to '99 I researched auroras at the National Institute of Polar Research. Now I'm here - right where I've always wanted to be, doing exactly what I've always wanted to do.
It will be good to finally be able to perform close-up observation of Jupiter. However, the powerful rays of the planet's radiation belt will greatly shorten the effective life of the observational equipment when it gets close to the planet, so I think it will take a very, very long time to learn everything there is to know. It is vital that we do all we can to further our knowledge and understanding of the great planet.
My main goals right now are to figure out why sodium travels away from Jupiter and how it goes so far, and exactly how gas emitted from the volcanoes of Io becomes Jupiter plasma. The two are linked and I can't help but think that finding the answers will reveal to us a great deal about the seemingly infinite space phenomena of our solar system.
For the several billion years since the birth of the planets in our solar system, they have been illuminated by the sun and undergone slow, gradual change. When an astronaut first set foot on our moon, he left a footprint. That means that dust had been accumulating on the moon's surface all that time. I believe that by studying two extremes - a planet like Jupiter enveloped in a dense, enormous atmosphere, and our moon, where it seems virtually nothing happens - I can learn about "weather" in space.
No one knew of the existence of satellites until Galileo. I hope that the value of the research we're doing will also be realized several hundred years from now, and that people understand the knowledge belongs to each and every one of us.
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