ANN ARBOR—Jupiter, named after the Roman king of the gods and the largest planet of the solar system, reigns all night long in Michigan’s August skies. It is the brightest object visible in the southeast portion of the sky around 10 p.m.

“Jupiter and its moons are now being investigated by an observatory from Earth named Galileo, which has been orbiting around Jupiter for almost two years,” said University of Michigan astronomer Richard Teske. “Among Galileo’s tasks are analysis of the planet’s cloudy atmosphere and close-up imaging of the surfaces of the most prominent of Jupiter’s 16 moons.”

Jupiter is brighter all year long than any star in the sky, according to Teske. “Besides the sun and moon, only Venus sometimes out-shines it. This month it is best to observe Jupiter before the 15th or after the 20th, because the full moon passes close to it on the 17th and overpowers its glory.”

Four of Jupiter’s 16 known moons are large enough to be planetary worlds of their own. Three are larger than Earth’s moon and one is larger than the planet Mercury. Teske says some astronomers remember their names—Io, Europa, Ganymede and Callisto—by repeating the phrase, “I Eat Green Carrots.” This month these four moons are barely visible to the naked eye under excellent observing conditions, but observers using binoculars will find it easy to see all four moons.

Io, Europa, Ganymede and Callisto are visible as tiny sparks arrayed in a ragged east-west line on either side of Jupiter. All four speed in separate orbits around the planet seeming to swing left and right of it every few days. For this reason, their configuration around Jupiter changes greatly from one night to the next. Some astronomy magazines publish diagrams showing the moons’ pattern alongside Jupiter for each evening of the month.

The Galileo spacecraft is making several scientific studies of Jupiter’s four largest moons as it orbits the planet. Each visit includes a close fly-by and a “gravity assist” maneuver. “The maneuver uses a moon’s gravitational pull to jog the spacecraft into a slightly different path to assure it will next fly close to another moon,” Teske says.

Planetary scientists carefully observe how each moon gravitationally deflects Galileo and use the resulting data to estimate how the moon’s interior is constructed. “For example, results from gravity data near the moon Europa, second outward from Jupiter, indicate there

Io, the closest large moon to Jupiter, shows intense volcanic activity and a strange surface chemistry that make it one of the most interesting planetary bodies yet explored. Over 200 large volcanoes exist on Io, many of them actively erupting. Earth, while much larger than Io, has only around 15 volcanoes of the same size. Unlike volcanic lavas on Earth, Io’s hot lavas are apparently mostly molten sulfur erupting from a super heated interior.

“Io’s interior is heated by Jupiter’s strong gravity stretching and compressing Io as it circles the planet,” Teske says. “In a similar way, you can heat a rubber band by stretching it rapidly and repeatedly. Io’s unusual volcanism has given planetary scientists a whole new way of thinking about how planets shed built-up heat from their interiors.”

Callisto, the most distant large moon from Jupiter, is a more typical solar system object, yet it too has something to teach about the history of planets. “Callisto’s surface is pock-marked everywhere with meteorite craters just as is Earth’s moon,” Teske explains. “This intense cratering is seen throughout the solar system. It is the signature of a final colossal event when the last pieces of space debris, which came together to form the sun’s children, were being quickly swept up by the planets and their moons.”

More information: 

• JupiterGalileo
• Io