Cassini and Galileo spacecraft analyse volcanic ashes at Jupiter
9 January 2001
In December 2000, when NASA's Cassini spacecraft will pass by Jupiter, scientists from the Max Planck Institut für Kernphysik in Heidelberg, Germany, will analyse microscopic ash particles from volcanoes on the giant planet's satellite Io. It will be the first in-situ analysis of surface material from a planetary satellite of our solar system other than the Earth's moon.On 30 December 2000 NASA's Cassini spacecraft, on its journey to Saturn, will pass by the giant planet Jupiter. Another NASA spacecraft -Galileo- has been exploring Jupiter since 1995. From different vantage points both spacecraft will study Jupiter, its satellites and its huge magnetosphere in a joint observation campaign between October 2000 and March 2001. This is the first time that two spacecraft simultaneously investigate the same gas giant planet from a close distance.
Cassini and Galileo have highly sensitive instruments for the analysis of microscopic dust particles on board which were built by the Max-Planck-Institut für Kernphysik in Heidelberg, Germany. The Max-Planck scientists will study streams of ash particles, finer than cigarette smoke, originating from volcanic plumes on Jupiter's moon Io. Patterns in the streams as they pass one spacecraft first, then the other, could give information about the particles' movement. These streams can serve as a monitor of Io's plume activity. Identification of the particles' composition would be sampling Io's surface material.
Io is the only moon in the solar system with presently active volcanism, being volcanically more active than the Earth. "The volcanic ash particles released from Io are accelerated to speeds of 300 km/sec by Jupiter's magnetic field," said Prof. Eberhard Grün, leader of the joint Cassini/Galileo dust investigations. "By measuring the particles' flight time between the two spacecraft which are more than 20 times the Earth-moon distance apart we can directly measure their speed. The particles are 4000 times faster than formula 1 world championship winner Michael Schumacher in his racing car. Only because the particles are so small (a thousandth of the thickness of a human hair) can they gain such high speeds in Jupiter's magnetosphere," explains Grün.
"With our Galileo dust instrument we have seen a surprisingly large amount of dust while Galileo was 20 million kilometers (50 times the Earth-moon distance) away from Jupiter in September this year," explains Dr Harald Krüger, leader of the dust investigations with Galileo. "If matter gets dispersed in space you usually expect more material to be close to the source than far away, just like water released from a hose is dispersed. We measured exactly the opposite which tells us that the particles accumulate due to the interaction of Jupiter's magnetosphere and the interplanetary magnetic field, a fact which we do not understand yet."
Furthermore, during the past five years of the Galileo dust exploration in the Jupiter system, large variations in the detected amount of dust have been found which could be due to changes in the Jovian magnetosphere, or to variations in the dust production of Io's volcanoes, or both. "If we better understand the movement of the dust particles in the magnetosphere we can use our dust measurements to monitor Io's volcanic activity," said Eberhard Grün. "Our joint Cassini/Galileo measurements can be a major step forward towards understanding these effects."
Large amounts of dust were also recognized with the Cassini dust instrument even further away from Jupiter. "In September this year we have measured the first dust particles from Io while Cassini was still 45 million kilometers away from Jupiter," explains Ralf Srama, responsible for the Cassini dust instrument. "By analysing the dust grains millions of kilometers away from Io we can analyse the moon's surface composition. Our Cassini measurements have confirmed that the particle composition resembles that of Io's surface."
With the Galileo dust detector various types of dust grains have been found throughout the Jupiter system: the planet's moons are surrounded by faint clouds of dust particles ejected from the moons' surface by impacts of interplanetary meteoroids. The planet itself is surrounded by a dusty ring system, part of which can be seen on images taken with large Earth-based telescopes. Only with the continuous Galileo dust measurements going on for five years now could Io be identified as the source of the ash particles detected even in interplanetary space 300 million kilometers (750 times the Earth-moon distance) away from Jupiter. More is presently known about cosmic dust in Jupiter's system than about natural (i.e. non-artificial) dust in the Earth environment.
In interstellar space, stars and planets form out of clouds of gas and dust permeated by magnetic fields. Since Io's dust particles behave like ions in a magnetic field, the Jovian system can serve as a laboratory for studying the processes of star and planet formation. With their dust detector on board ESA's Ulysses spacecraft, the scientists from Heidelberg successfully identified and analysed interstellar dust sweeping through our solar system. Due to their high speeds, the Io particles eventually become interstellar dust.
Only with the highly sensitive dust detectors on board Cassini and Galileo which measure dust grains hitting a 1000 cm2 metal target, could these particles be detected. In the case of the Io particles the dust concentration is so low that only one grain can be found in a cube with about 3 meters on a side. Although these ash particles are very interesting scientifically, they do not cause a danger to spacecraft.
The Cassini and Galileo dust instruments were built by an international team of scientists and engineers under the leadership of Prof. Dr. Eberhard Gr|n at the Max-Planck-Institut für Kernphysik in Heidelberg, Germany, with financial support from the German national space agency (DLR).
The Cassini and Galileo programs are international cooperative efforts involving NASA, ESA and several European academic and industrial contributors. Galileo has been orbiting Jupiter since December 1995, and has currently completed five very successful years of exploring the Jupiter system. Cassini/Huygens will reach Saturn in 2004 and explore the second-largest planet of our solar system and its satellites during a four-year orbital mission. Cassini's Huygens probe was built by ESA and will descend into the atmosphere of Titan, the largest of the Saturnian moons. Galileo and Cassini are operated by the Jet Propulsion Laborary which is a division of Caltech, Pasadena, CA.
