The shocking surface of Io.

The shocking surface of Io

Jupiter's moon lo, the only known volcanically active body in the solar system besides the Earth, may also give off sparks, two space scientists suggest.

Ions and electrons trapped on the lines of Earth's magnetic field have often affected the operations of artificial satellites orbiting the planet. These electric charges have caused communications problems and other difficulties with the satellites by triggering discharges from such components as circuit boards and solar panels. Even greater effects occur in the strong magnetic fields of the solar system's giant outer planets. While Pioneers 10 and 11 and Voy agers 1 and 2 were in the powerful field of Jupiter, for example, their electronic circuits produced some incorrect photo details and their computer memories suffered altered settings.

The unusual appearance of lo's surface, rich in sulfur and sulfurous compounds, has fascinated researchers for a decade. Humberto Campins, now at the University of Florida in Gainesville, and E. Philip Krider of the University of Arizona in Tucson have simulated the effects of Jupiter's magnetic field on Io by bombarding a cylindrical sample of sulfur 6 centimeters in diameter and 4 cm high with an electron beam in a vacuum chamber at SRI International in Menlo Park, Calif. They varied the beam's intensity, whose weakest level they describe in the Aug. 11 SCIENCE as "comparable to that expected on the surface of Io," about 10 billion particles per second.

The scientists used an electric-field antenna to monitor bursts of electromagnetic radiation from the sample, such as might accompany sparks or lightning. They conducted the tests in the dark so they could see any such discharges. The observed discharges varied from event to event: Sometimes they lit up the sulfur's entire surface (at times even after the beam was turned off); on other occasions they formed branching "dendritic" patterns resembling terrestrial lightning.

The project continues, but Campins and Krider report they already have "evidence that discharge phenomena do occur on natural sulfur under conditions similar to those expected on the surface of Io." On Io itself, the scientists say, the sparks would probably be most numerous and intense at night, because the lower temperatures would lower the sulfur's electrical conductivity and enable stronger charges to build up. Their likeliest location would presumably be near Io's equator, where the concentration of electrons coming in along Jupiter's magnetic field lines would be greatest.

The discharges would be far too weak to detect from Earth, either in photos or by their static-like radio noise, though they may be visible to the Galileo spacecraft, due for launch toward Jupiter this October and scheduled to go into orbit around the planet in 1995 to study it and its moons.

Campins and Krider suggest similarly produced sparks may occur on the surface of Earth's moon when the moon is going through the tail of Earth's magnetic field. Other candidates include some asteroids and inactive comet nuclei (in both cases during strong solar flares), and perhaps even the dust shells around young stars when the dust is exposed to outpourings of highly ionized gas from the star at the center.