If you have a chance to view Jupiter through a telescope or binoculars this summer, don't forget to view the planet's four large moons discovered by Galileo in 1610. Scientists are most interested in these Galilean satellites.

Good binoculars will show the Galilean moons. They appear as bright "stars" on either side of Jupiter. From innermost to outermost they are Io, Europa, Ganymede and Callisto. As these moons orbit around Jupiter, they will shift positions from night to night.

It's impossible to identify the moons without looking at a reference chart. However, Sky & Telescope magazine and Astronomy magazine print the chart each month. We will also print a chart next month in the Post-Gazette's Monday morning Stargazing feature.

Galileo would be amazed at what we have learned about the four large moons he discovered 400 years ago. Io is the most volcanically active body in the solar system. Its surface is covered by sulfur in different colorful forms. Jupiter's immense gravity causes tides in the solid surface 300 feet high on Io, generating enough heat to give rise to the volcanic activity.

Europa's surface is mostly water ice, and there is evidence that it may be covering an ocean of water or slushy ice. Icy oceans may also lie deep beneath the crusts of Callisto and Ganymede. Europa intrigues astrobiologists because of its potential for having a "habitable zone." Life forms have been found thriving near subterranean volcanoes on Earth and in other extreme locations. That may be analogues to what may exist on Europa.

Ganymede is the largest moon in the solar system, larger than the planet Mercury and almost as big as Mars. It's the only moon known to have its own internally generated magnetic field. Callisto's surface is heavily cratered and ancient, providing scientists a record of events from the early history of the solar system.

-- By Dan Malerbo, Buhl Planetarium and Observatory