Understanding the New Solar System by Editors of Scientific American

Author:Editors of Scientific American [SCIENTIFIC AMERICAN]
Language: eng
Format: epub
Tags: SCI000000
ISBN: 9780759527478
Publisher: Grand Central Publishing
Published: 2002-12-02T00:00:00+00:00

One of Galileo’s major discoveries was made during its very first orbital encounter—with Ganymede, Jupiter’s largest moon. About half an hour before the spacecraft reached its closest approach, the radio-noise instrument, designed to record ambient electrical fields, began to go haywire. The relatively quiet background radio signals seen throughout most of the Jovian system changed abruptly to a complex, active radio spectrum. For 45 minutes the activity remained intense, and then it ceased as suddenly as it had begun. When the radio noise commenced, the magnetometer readings shot up fivefold.

Plasma researchers had seen signatures of this sort before, when spacecraft carrying similar instruments entered and exited magnetospheres at Earth, Jupiter, Saturn, Uranus and Neptune. Two subsequent Ganymede flybys confirmed their suspicions: the moon is magnetized, generating a dipole field similar to those of these planets. No other satellite has such a field. Earth’s moon and Mars may have had fields in the past, but currently they exhibit only limited patches of magnetic variation that represent magnetized rocks on the surface. Like a set of nested Russian dolls, Ganymede has a magnetosphere contained within Jupiter’s huge magnetic domain, which in turn is embedded in the sun’s.

Tracking of the spacecraft signal allowed researchers to probe Ganymede’s gravity field and therefore its internal structure. They concluded that it probably has a dense core about 1,500 kilometers in radius with a surrounding icy mantle 700 kilometers deep. Geochemical models suggest that the core consists of a sphere of iron or iron sulfide enveloped in rock. The inner metallic core could produce the dipolar magnetic field.

Yet theorists are not sure quite how. Although scientists compare planetary magnetic fields to bar magnets, the analogy can be misleading. Solid iron at the center of a planet or large moon would be too hot to retain a permanent magnetic field. Instead a magnetic field is thought to involve a convecting, conductive liquid. Models of Ganymede indicate that its interior can easily become hot enough to melt iron or iron sulfide. But the same models show that convection will cease as the core gradually cools; the conditions required for convection should last only a billion years or so.

The answer may lie in the orbital resonance of the inner three Galilean satellites. Io goes around Jupiter precisely four times for each time Europa completes two circuits and Ganymede one. Like pushing a child’s swing in time with its natural pendulum period, this congruence allows small forces to accumulate into large outcomes—in this case, distorting the orbits from their default circular shape into more oblong ellipses. The effect on the moons is profound. Because the distance between them and Jupiter is continuously changing, the influence of Jupiter’s gravity waxes and wanes, stretching the moons by an ever varying amount. The process, known as tidal heating, drives the volcanism on Io and keeps Europa’s putative ocean from freezing.

Researchers used to think that tidal heating was of little consequence for Ganymede, the outermost of these three moons. But now they realize that the orbits may have shifted over time.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.