September - Jupiter and its Moons

Find Jupiter in the Sky

In January, 1610, Galileo pointed his telescope at Jupiter. That he could see the planet was no surprise—Jupiter is visible to the naked eye, so people had observed it for millennia. But accompanying Jupiter, he saw four "stars" which followed the planet across the sky from night to night, changing their positions relative to Jupiter and to each other. These were Jupiter's four largest moons—ultimately named Io, Europa, Ganymede, and Callisto—which we call the Galilean satellites in his honor. And that observation was revolutionary!

  • Galileo's MoonsExamples of Galileo's drawings, showing how Jupiter's four largest moons changed position over time.

This was the first time that any heavenly bodies were seen clearly orbiting something other than the Earth. It became much harder to believe that everything in the sky revolves around the Earth, as had been believed for centuries, and to deny the possibility that Earth could circle the Sun, bringing the Moon along with it.

You can see Jupiter and the four Galilean satellites much as Galileo saw them. Caltech astronomer Mike Brown says it's one of his favorite things to do. "I love the fact that you can look up in the sky and see them with a fairly crummy pair of binoculars," he said. "And you probably have a better view of them than Galileo did with his telescope.

"The main trick," he advised, "is to lean against a building to get your body and hands really steady. And once you do that, they're right there. They change every night, and it's an incredible sight!"

As the September IYA Discovery Guide suggests, you can find Jupiter after sunset this month above the southeastern-to-eastern horizon. It's the brightest star-like object you will see in that direction. Jupiter is so large that even though it is more than 613 million kilometers (380 million miles) from Earth this month (the distance varies depending on where Jupiter and Earth are in their orbits), it is the brightest light in the sky after the Sun, the Moon, and much-closer Venus.

We've gotten better views of Jupiter and its moons with more advanced telescopes, of course, both on the ground and in space. With their help, we've been able to see the colorful bands in Jupiter's atmosphere and the Great Red Spot, a storm more than twice the size of Earth, which may have persisted for 300 years or more. But for really great looks, we had to send our instruments out to Jupiter.


Up Close and Personal

Our first visits to Jupiter and its moons were courtesy of Pioneer 10 in 1973, Pioneer 11 in 1974, and Voyager 1 and 2 in 1979, each of which used the giant planet for a gravity assist to help it reach Saturn. Next came Ulysses in 1992, which swung by Jupiter to shift its trajectory into a polar orbit of the Sun. These flybys blazed a trail for the mission named for the astronomer whose 400th anniversary we mark this year. The Galileo spacecraft is the only one to date that has really spent time in the Jupiter system: 34 leisurely orbits over the course of nearly 8 years. It released a probe into the planet's atmosphere when it arrived in 1995, and became a probe, itself, when it plunged into Jupiter at the mission's end in 2003.

Rounding out Jupiter's visitors so far are Cassini, en route to Saturn at the end of 2000, and New Horizons, which got a boost in 2007 on its way to Pluto.

Among other fascinating insights into the king of the planets, these spacecraft revealed that Jupiter is encircled by rings and has thunderstorms a thousand times larger than those of Earth. But once again, the four Galilean satellites provided the most jaw-dropping surprises.

Rather than the icy moon one would expect so far from the Sun, Io turned out to be a world of hot volcanic eruptions that continuously recover its surface in sulfurous lava, making it look like a large extra-cheese pizza with black olives. In fact, it's the most volcanically active world in the Solar System.

Just as surprising in this frozen region, five times Earth's distance from the Sun, was evidence from Galileo's magnetometer that Europa, Ganymede, and Callisto harbor massive liquid oceans beneath their icy exteriors.

And Europa goes a giant step further. While the possible oceans of Ganymede and Callisto are thought to lie between ice shells as much as 170 km (over 100 miles) thick and floors of still more ice, Europa's icy crust is believed to be no more than a couple of dozen kilometers thick and to have geologically recent and possibly ongoing exposure to the surface. That's important, because it would give the ocean access to the organic material (possible building blocks for life) that is deposited by comets and to the oxygen that is released from the surface ice under the intense radiation that permeates the region.

The ocean's floor is believed to be Europa's rocky mantle, another source of nourishing chemicals, making Europa the Solar System's only world besides Earth to consist largely of a rocky interior covered in deep seawater. There may even be hydrothermal vents like those of Earth, where chemically enriched hot springs provide the basis for chemosynthesis, a process by which some organisms create organic compounds without the sunlight needed for photosynthesis. In short, Europa could be a home to life.

At last count, by the way, the four Galilean moons have turned out to be in the company of 59 smaller satellites.

Future missions

Two new missions are being readied to provide an even better look at Jupiter and its moons. Juno, scheduled for launch in 2011 and arrival at Jupiter in 2016, will be the first spacecraft in a pole-to-pole orbit around the planet. As Jupiter's rotation brings a new slice of the planet under each orbit, Juno will eventually cover the entire planet, probing its atmosphere and auroras, and mapping the gravity and magnetic fields.

NASA's Jupiter Europa Orbiter and the European Space Agency's Jupiter Ganymede Orbiter are scheduled for launch in 2020 and arrival at Jupiter in 2026, where they will spend at least three years exploring Jupiter and its moons, especially Europa and Ganymede. They may pave the way for a future lander and even a submersible probe that could determine with certainty whether Europa harbors living things—a discovery that would be as important to our understanding of life as Galileo Galilei's observations were to our understanding of the planets.

Pick up a pair of binoculars or a telescope this month and see these amazing worlds for yourself. "I grab people all the time at my house when it's a nice night and Jupiter's up and I say, hey, have you ever seen the Galilean satellites?" Mike Brown said. "I tell them, this is what Galileo saw. This is what changed the universe."














































Moon shadow on Jupiter.

The shadow of one of Jupiter's moons can be seen in this image of the giant planet.

Jupiter's swirling atmosphere.

Jupiter's swirling atmosphere and Great Red Spot (color-coded Voyager 1 image).

Io is covered in lava.

Io is covered in lava from its many, frequently erupting volcanoes.

Io stretched by the gravitational pull.

Illustration: Io's very elliptical orbit around Jupiter makes it experience variable tidal forces. The accompanying changing tidal friction generates enough heat deep inside Io that volcanoes erupt on its surface. A reduced version of this effect may keep Europa's subsurface oceans from freezing. (Credit: Torrence Johnson, JPL)

Europa's cracked surface.

Europa's surface is riddled with cracks and pits that suggest a sub-ice ocean in communication with the surface.

Ganymede, the Solar System's largest moon.

Ganymede, the Solar System's largest Moon, is bigger than planet Mercury and has its own magnetic field. (Credit: Galileo Project, DLR, JPL, NASA)

Callisto high density impacts.

Callisto retains one of the highest densities of impact craters in the Solar System, and is thus believed to have one of the oldest surfaces. (Credit: NASA):

Venus transit

Jupiter Takes a Beating

Two Hubble images stitched together show a "train" made up of bits of comet P/Shoemaker-Levy 9 taking aim at Jupiter in May 1994.