January’s Jupiter challenge

by Dave Blane
Features on Ganymede imaged with an 11-inch telescope.

Jupiter reaches opposition in January, and so rises at around sunset, and will be at its best throughout the month when it will be visible the entire night. At -2.6 magnitude it presents a large 46 arc second disc and is a fine target for binocular and telescopic observers.

Observing challenge!
Is it possible that users of large telescopes (300mm +) may be able to detect albedo surface features on Jupiter’s moons? This is a formidable task because even Ganymede, the largest of the Galilean moons, can only appear as large as 1.8 arc seconds in diameter while the smallest Galilean moon, Europa, never appears larger than 1 arc second in diameter.

Under favourable conditions (such as will occur in January when Jupiter reaches opposition) a few features have reportedly been seen. Studies have shown that the eye’s ability to detect subtle contrast differences on a target object can be improved by reducing the difference in brightness between the object and the background against which it is observed.

Consequently, the best time to try to detect surface markings on Jupiter’s moons is when they are transiting the planet.

This sounds like an interesting project for someone using, say, the 26-inch refractor in Johannesburg and, as encouragement, an amateur with an 11-inch telescope and a video camera has managed to image these albedo surface features on Ganymede (see accompanying image).

If anyone attempts this challenge, please post your results (positive or negative) to the Shallow Sky Section.