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.
Volcanoes created by Jupiter's incredible gravitational forces
First of all, please tell us what kind of work is done at PPARC.
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.
What is it that has you so interested in 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.
Hadn't the existence of volcanoes been observed from Earth until then ?
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.
Io is transformed by the astounding power of Jupiter. Is the same phenomenon occurring on any of Jupiter's other moons ?
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.
Does the volcanic activity on Io influence Jupiter in any way ?
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.
How can observation of volcanic gas and its distribution be performed from the earth ?
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.