Jupiter Uncloaks: Most Moons Ever Found at Once

Jupiter’s outer, irregular satellite system has long confounded predictions of what a moon should be. Fascinated particularly by the probable water-ice oceans on one of the six inner Jovian moons–Europa–moon-hunters and astrobiologists alike have begun employing some novel search strategies for even more exotic ones than Europa. Previously the scientists who classify known natural satellites have had 100 candidates to probe in our solar system. But with the latest observing tools, now the number of moons is climbing rapidly. Jupiter alone remains the primary parent body to an astonishing 39 moons thanks to a recent University of Hawaii sky survey.

In May of this year, a team headed by Hawaii astronomers Scott Sheppard and David Jewitt confirmed the largest number of satellites ever discovered at one time–eleven. The eleven, including one previously found but long-lost to view since 1975, now make up part of the outer irregular Jovian satellite system. Until just the last 400 years, only the Earth had an observable natural satellite. But with Galileo’s first crude telescope, astronomers looked with initial success to the Giant Planet, what might have first seemed to them as just a bright star. Jupiter first uncloaked its four biggest moons, now called the Galilean system or inner Jovian satellites. Since Galileo, nearly ten times more Jovian moons total have been discovered.

One-Hour Photo

To find their tiny moons, Sheppard and Jewitt peered through a telescope larger than Galileo himself. The mammoth 7 foot lens (88 inch, 2.2 m) sits majestically atop the remote 13,800 foot Mauna Kea volcano. Prospective moon-hunters wanting to use the Mauna Kea telescopes climb high enough to require a day to acclimatize at Hale Pohaku for one night before starting a new observing run. Compared to that first Galilean telescope that probed Jupiter, the tools are not only enormous but also precise. Even while the world’s largest observing domes of the nearby main Keck facility stand eight stories tall and weigh 300 tons, they operate with nanometer precision and move to instrument settings measured in units 10,000 times smaller than a human hair.

Orbits of 11 New Jovian Moons

Credit: University Hawaii Institute for Astronomy

All 11 moons were discovered using the so-called "8K" camera at Mauna Kea and are unusual additions to the neighboring astronomical zoo. The photographic signature or permanent record of the night sky is analyzed after the night’s observation run to pinpoint candidate moons conveniently using computers. This astronomer’s dream, the 8k CCD camera, was built at Hawaii’s Institute for Astronomy by Gerry Luppino and is one of the world’s largest astronomical cameras.

When first found from a series of 3 photographic panels or plates (typically taken about a half-hour apart), the visible objects that move against the relatively stationary star and galaxy background make the initial candidate list. But since asteroids between the Earth and Jupiter can also move against the galactic background, Sheppard and Jewitt could only be sure of their discoveries by honing in on the characteristic slower speeds that might demark an orbitting object.

Small, fast and dark, their sizes vary between 3 and 8 km (2-5 miles) across. They’re also a long ways from Jupiter itself. Their orbits are large, eccentric and inclined steeply to Jupiter’s equator. The farthest 9 of them orbit a path nearly 300 times bigger than Jupiter itself, compared to the more modest Earth-Moon ratio of about 60:1 and a lunar orbit that more closely mimics the innermost Jovian satellite Io, at 380,000 km. In total the farthest new moon discovered orbits at a collosal distance more than 40 million kilometers away from Jupiter, but makes the orbital round trip once every one to two years.

How Jupiter Became the King?

What is completely unknown about such irregular satellites is how they come to exist at all. When Jupiter was young, it is thought, many asteroids (or dynamical clusters) orbitted the Sun. As Jupiter condensed, its gravity began to bend the paths or even capture some of these stray asteroids. The best evidence for such a capture hypothesis is that many of these new satellites actually orbit in a direction opposite to the rotation of Jupiter, or otherwise follow what is known as retrograde orbits. Nine of the new Jovian moons are retrograde and join with the previous 5 known examples to yield 14 total.

But while the capture theory can explain the backwards orbits, that finding alone is only half the story of how actually to hold on to them once caught. The problem arises in slowing down the moon to a stable orbit. Following a large solar orbit requires lots of speed and energy, while going against the flow of Jupiter–if captured–is likely the only way to dissipate all that escape energy. At least for Jupiter in its present state, capture is almost impossible. As Jewitt noted: "The origin of the dissipation that lead to the capture of Jupiter’s irregular satellites is unknown. In fact, at the present time there is no plausible source of dissipation so that capturing satellites is presently almost impossible."

Only if Jupiter’s atmosphere extended well out into the capture zone, could friction start to slow down the moons and keep them from flying off or back into solar orbit. A much bloated Jupiter, and its atmospheric drag, could spawn such unusual tiny moons: captured into a backwards orbit, spanning vast relative distances but held just tightly enough to keep them as part of the 39 or more such moons. The best evidence for this theory appears to be the distinct families of moons that orbit at the same inclination to Jupiter’s equator as a kind of dynamic cluster that might have broken on capture. Indeed the Giant Planet has many characteristics of a mini-solar-system unto itself.

Europa Orbiter
NASA hopes to launch a Europa Orbiter mission in 2008, with the primary goal of determining if there indeed is a global, subsurface ocean.
Credit: NASA

What’s Next

Most critical to astrobiologists studying Jupiter’s moons, the eccentricity or oval shaped orbits of Jupiter’s moons are pumped or oscillated by tidal forces as they orbit. This input of Jupiter’s gravitational energy heats up the inner moons particularly without relying only on the Sun’s radiant heat, and thus gives an interesting way to provide one of the three ingredients for life–an energy source–even if far from the Sun.

What remains to be found among the Giant Planets like Jupiter and Saturn are some candidates that combine all three ingredients for primitive life: energy, liquid water and some atmosphere. Only Saturn’s moon, Titan, has an appreciable atmosphere, and only Jupiter’s Europa or Ganymede have any indications of water ice. But uniquely powerful tidal forces around the Giant Planets do offer some promising, non-radiant and non-volcanic heat sources.

Collaborators on the eleven Jovian moons and its related sky survey included Scott Sheppard, David Jewitt, Yan Fernandez and Gene Magnier

Related Web Pages

Small animated image show (270 kB) showing approximately 1 hour of observations on Dec. 5
Animation of how to distinguish mainbelt asteroids from moons
Evidence of bacteria on Jupter’s moon?-Astrobiology Magazine