) from the planet's center. Io is at about 5.9 R.
Jupiter, like Saturn and Uranus, is a ringed planet. The rings of both Saturn and Uranus were discovered from Earth-based telescopic observations, but Jupiter's rings are invisible to Earth-based instruments. In fact, their discovery hinged on a single photograph by Voyager 1, when the Sun's rays backlit the dust particles and made them visible to Voyager's sensitive cameras. "Particles" in Saturn's rings range from dust to the size of houses, but Jupiter's ring is believed to be composed entirely of dust particles.
The Galileo Orbiter will carry an instrument designed to measure dust stream motion in the vicinity of Jupiter. The dust detection instrument will be mounted on the spinning portion of the Orbiter.
Jupiter's rings are composed of three elements: the visible ring composed of micrometer-sized particles that extends about 7000 kilometers (4200 miles) broad and less than 30 kilometers (18 miles) deep; a faint disk extending from the inner edge of the visible ring all the way to the planet's atmosphere; and a halo that extends more broadly and has a depth of about 100,000 kilometers (60,000 miles).
What is the source of the particles in the rings? Current theory suggests that the micrometer-sized particles in the visible ring probably result from high-velocity impacts between small projectiles. Io's volcanoes probably supply the force needed to inject large quantities of fine particulates into Jupiter's plasma environment. If this theory is correct, there should be a "dust wedge" extending about 10-deg above and below the jovian equator out to about 700,000 kilometers (420,000 miles) (near the orbit of Europa). This is a distance of about 10 jovian radii (R) from the planet's center. Io is at about 5.9 R.
The dust detector will be able to measure the distribution, spatial extent, and orbital trajectories of the submicrometer-sized dust particles and to determine if there is indeed a dust wedge.
Jupiter's satellites are continually bombarded by meteoroids and dust particles--both interplanetary material and debris from within the jovian system. By measuring the dust flux near the satellites, much can be learned about the satellites' surface properties in relation to the dust flux. Spatial and temporal variations, as well as directional asymmetries in the dust flux, will give clues to the reasons for albedo (reflectance) variations of the satellites.
As meteoroids impact the surface of our moon, surface material flies up and is ejected from the impact crater. A small percentage of this ejecta receives enough velocity from the force of the impact to actually escape from the moon's gravity. Similar ejection processes probably occur on Jupiter's satellites, contributing to the dust environment around the planet. During close encounters with the satellites (less than 1000 kilometers (600 miles) from their surfaces), the dust detector will be able to detect such ejecta particles. From this measurement, scientists can estimate the total meteoritic influx on the satellites.
The dust detector will measure the electrical charge on the larger dust particles entering the instrument. By correlating these measurements with measurements of the flux of high-energy particles, one can study how dust grains become charged. Measurements of particle velocities will provide information on the frictional interaction with the plasma, while the measured particle mass distribution will give information on the source and transport mechanisms. If the dust particles carry an electrical charge, they may corotate with Jupiter's magnetic field. Electrostatic charges on dust particles are of particular interest. At Saturn, for example, electrostatic levitation is thought to be the cause of the mysterious radial spoke features in the rings, as fine dust particles are elevated above the ring plane by static electricity along the planet's magnetic field lines.
If large, low-density, "fluffy" aggregate particles become strongly charged, the electrostatic force can literally blow them apart, creating a swarm of micrometeoroids. A group in Heidelberg has observed these swarms in the Earth's magnetosphere with their HEOS experiment. A similar fragmentation is expected to occur when "fluffy" interplanetary particles enter Jupiter's magnetosphere.
The instrument is a modified version of the impact plasma micrometeoroid detector successfully flown on the HEOS-2 satellite. It consists of a multicoincidence detector and associated electronics, and a microprocessor to control the instrument's operation and process the data for telemetry to Earth.
Positively or negatively charged ions entering the sensor are first detected by the charge that they induce when they fly through the entrance grid. This charge signal will only be evaluated if the ion subsequently impacts the impact plasma detector. Dust particles--charged or uncharged--are detected by the plasma produced during the impact on the gold target of the sensor. After separation by an electrical field, the ions and electrons of the plasma are accumulated by charge-sensitive amplifiers, thus delivering two coinciding pulses of opposite polarity. The pulse height, or total charge, is a function of the particle mass times velocity. The rise time of the pulses depends only on the particle's speed. From both the pulse height and the rise times, the mass and impact speed of the dust particle can be derived. Redundancy in the instrument increases the accuracy of the measurements.
The instrument sensor weighs 2.3 kilograms (5 pounds) and requires 0.3 W. The electronics weigh
1.8 kilograms (4 pounds) and require 1.5 W. The sensitive area of the sensor is 1000 cm
(1 ft), and the unobscured field of view is 140 deg. The instrument will be able to detect particles with mass from 10
to 10
g and charges from 10
to 10
coulomb (positive) or 10
coulomb (negative). It will be able to detect as many as 100 impacts per second.
The principal investigator is Eberhard GrŸn of the Max Planck Institut fŸr Kernphysik (MPIK) in Heidelberg, Federal Republic of Germany. He is aided by an international team of six coinvestigators. The instrument was designed and built by the Space Electronics Group at MPIK with the help of outside contractors ARGE PEES, Wald Michelbach, and WFG Fischer GmbH, Stuttgart.
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