Dust Streams From Jupiter--Our First In Situ Measurements of Jupiter's Environment!
In February, Eberhard Grün, Principal Investigator for the Dust Science Team, discussed their findings and analyses of dust stream emissions from the Jovian system in a noon-time presentation at JPL. Eleven of these periodic, narrow, collimated (in parallel paths) streams of dust particles, unlike any ever seen before, were detected by Ulysses in 1992. Since June 1994, seven streams have been seen by Galileo--starting at about the same distance (2 AU or about 300 million km) from Jupiter as Ulysses was when it first detected them. Carol Polanskey, Team Chief for the Dust Detector Subsystem (DDS), reports that the streams are getting more intense and lasting longer. (In July 1994, a reprogram of the Galileo DDS, enabled by analyses of data from the Ulysses dust detection experiment, fortuitously improved the DDS identification of particles in time to observe these intriguing streams.) The rate of events (as shown in the figure) escalated from less than one to nearly 1000 per day by the end of 1994. The most recent stream contained more than 2000 particles and continued for more than 3 weeks.
Dust in space? We're familiar with the dust particles suspended in Earth's atmosphere. Dust particles scatter light from the Sun, often producing a red-orange glow at sunset. To see interplanetary dust as Cassini saw it 300 years ago, Grün said that we must travel to places like Hawaii where there are very dark skies and low pollution levels. About one-half hour after the Sun goes down, one can look west, line up the Moon and Venus to establish the ecliptic plane, and observe the inverted cone-shaped "cloud" caused by sunlit interplanetary dust. Scientists believe that these particles are emitted under gas pressure from comets. Further away from the Sun, dust particles thought to be the result of collisions of asteroids and meteoroids have been detected by the Infrared Astronomy Satellite (IRAS). Since all bodies absorb heat from the Sun, these grains of dust can be viewed in the infrared (heat) portion of the electromagnetic spectrum. Interstellar (star) dust pervades the entire universe.
How do we know about dust in the outer solar system? The story is told by tiny particles, smaller than interplanetary dust, that are hitting the Dust Detector Subsystem (DDS) onboard Galileo. Particles passing through the sensitive area of the DDS (0.1 m
, about the size of a large colander), are ionized when they strike the back of the instrument. Polanskey explained that the speed of a dust particle can be determined from the difference between the pulse from collected ions and the pulse from collected electrons. The typical impact speed is 20 km/s (12.4 mi/s), a speed which would take the particle across the U.S. in about 3 minutes. Mass and charge can also be calculated. The particles, averaging 10
g, are typically less than or equal to 0.1 µm in radius--about one-tenth the size of the airborne dust particles seen when the Sun shines through your window at home. Direction can be found from the known time and orientation of the spinning spacecraft. The dust is affected by solar radiation pressure, solar gravity, and electromagnetic forces. Solar ultraviolet waves cause particles to be charged up to 5 volts. The spacecraft's extended journey to Jupiter permitted the dust collection studies to cover the range of 0.7 to 5 AU (about 1 million to 7.5 million km).
Why are dust particles being ejected from the Jovian system in streams of increasing intensity and over a longer span of time? Scientists are looking to Galileo to supply them with the data to answer this question. They theorize that small dust particles (>20 µm) are electromagnetically accelerated by Jupiter's magnetic field; those smaller than a few micrometers are accelerated to greater than escape velocity from Jupiter. (In fact, those streams are fast enough to be leaving the solar system!) But what is the source of this dust? The periodicity of the dust streams indicates a single source. According to one model, it is Io's volcanic activity; another model takes the position that the dust particles originate in Jupiter's gossamer ring. In 1994, members of the Dust Science Team considered the possibility of submicrometer-sized dust being produced by the impact of comet Shoemaker-Levy 9 on the gossamer ring particles, tidal disruption, or from the impact on Jupiter itself.
Armed with data gathered while Galileo is in orbit around Jupiter, scientists expect to find answers to the mystery of these surprising dust emissions. Presently, they are relying on the pattern of periodicity to predict that this June, after a smaller burst in May, Galileo's dust detector will be bombarded with the most intense stream ever, twice the size of the current stream, and will last for the longest period. Galileo will be 0.6 AU (about 0.9 million km) away from Jupiter at that time. We must wait and see. It won't be long now.
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