Galileo has performed seven close fly-bys of the moon Io. In all orbits but the 24th, there is a mysterious spike in the star scanner’s radiation measurements peaking within a few radii of the moon. An example can be seen in this figure:
Star Scanner Data for the I25A Orbit. The count rate is the star scanner’s native unit uncorrected for several small instrument biases. C/A is the time of Closest Approach to Io. MEC is the time of the predicted Magnetic Equator Crossing. RJ = Jovian Radii = 71,492 km.
When the star scanner is looking straight at a star, it reports its brightness. When it is looking away from a star, it reports total noise which is a combination of dim stars, nebula, etc in the background and radiation-induced noise. The pattern of dim lights is known and can be subtracted off the total noise figure giving just a measure of the radiation.
The spike in radiation counts, if caused by an unexpected light source, seems to be at least as bright as the star Alpha Centauri, to possibly 120 times that. There is a roughly exponential decline over the course of a few minutes and over a few thousand km as Galileo moves away from Io. This could be a real measurement but it is also consistent with how the star scanner returns to normal after being “blinded” by very bright light.
There are several possible causes of these spikes listed here with the latter two having the strongest support:
1) High energy particles spilling off Io It was discovered early during the Galileo mission that there are high energy particles over the polar regions of Io . Essentially, Io is a contact point in a huge stream of electricity flowing between Jupiter’s poles along its magnetic field lines. On the 24th orbit, Galileo made its only equatorial pass of Io and saw no spike - this would seem to be compatible with the idea of polar “electron beams”. However, on other orbits where the spike was observed by the star scanner, Galileo EPD measurements of high energy electrons showed no evidence of this spike .
2) Stray light The idea here is that light from Io is reflecting around inside the star scanner’s baffle or otherwise leaking into the star scanner core. This is not likely due to:
Ø This only happens at Io even though Europa has higher albedo and Galileo has flown by at similar distances to this and other Galilean moons.
Ø The star scanner is powered down for a portion of each revolution when there is a possibility of stray light entering the device. Several in-house studies have said that this is not happening. The power-down technique has been used successfully for every other moon and planet that Galileo has passed.
Ø The star scanner, independent of radiation measurements, reports star intensity. Nothing unusual was seen there.
3) Sunlight reflected off of dust and gas Io’s volcanoes are very active. There are many visible light images that show blue plumes extending hundreds of kilometers above the surface of Io. A diffuse light source filling the star scanner slit could be much dimmer than a pinprick of a star and yet provide the same amount of light response. The star scanner is most sensitive to blue and near UV light.
4) Spacecraft glow This has not been observed at Jupiter but it seems quite plausible that, in the vicinity of Io with its thin atmosphere of SO2 and O2, some sort of a chemical reaction is occurring on or near the spacecraft surfaces. This effect has been observed with the Space Shuttle and other low earth orbiting spacecraft although those reactions often seem to involve nitrogen. Galileo carries a solid state camera which has not reported any evidence of this glow while immersed in it. However, the spike is limited to regions near Io and, unfortunately, this is when the camera would have been pointed to the bright surface below and thus washed out any diffuse glow. It is interesting to note that Galileo’s Plasma Subsystem detected particles as high as 500 km above the surface of Io. Even when the plumes are in darkness, a blue fluorescence of the plume itself has been noted by Galileo’s camera looking from a distance.