Moons of Jupiter, Part 2


Tom Burns - Contributing Columnist



Jupiter has now become the dominant object in the sky. Around the middle of the night, look for it as an extremely bright “star” high in the south off of Leo’s front end.

Binoculars can show its four brightest, so-called “Galilean,” moons. Its brightest satellite, Ganymede, might occasionally be visible to the unaided eye if you block the bright glare of Jupiter with a note card — or so I argued last week.

In fact, Ganymede is the ninth largest object in the solar system. It is larger in diameter than Mercury and is bigger than all the known dwarf planets.

All four of the Galilean moons are large enough to be considered dwarf planets in their own right if they had formed in orbit around the sun instead of in orbit around Jupiter.

Ganymede is the largest, but the other satellites are significant in their own ways.

Io, innermost of the Galilean satellites, is just a little larger than Earth’s moon. It is so close to Jupiter that it is racked by radiation and gravitational and electromagnetic influences from the planet.

As a result, Io is the most geologically active object in the solar system. The interior of the satellite is continuously stretched and pulled by the planet’s gravitation, causing violently active volcanoes to spew liquid sulfur onto its surface and into its atmosphere. That’s what gives it a bright yellow/red color as seen from passing or orbiting spacecraft.

Io is caught up in Jupiter’s intense magnetic field. The planet’s magnetosphere, as it is called, carries along the lines of magnetism enormous quantities of radiation, which are quite similar to the Van Allen radiation belts around Earth. As a result, Io is constantly bathed in, what would be for a human, deadly radiation.

In its physical construction, Europa, the second satellite out from Jupiter, is typical of all the other satellites except Io. The Galileo orbiter discovered that their cross sections are much like the solar system’s inner planets, Earth included. At the center is a metallic core surrounded by a rocky mantle.

Here the similarity ends. Europa’s crust is composed of a thin layer of water ice. Europa’s icy surface is so smooth that it has been described as looking like a billiard ball.

Because Europa is so close to Jupiter, radiated heat from the planet probably causes a layer of slush or liquid water under the solid-ice crust.

Despite its odd abundance on Earth, liquid water is a rarity in the rest of the solar system. When we finally land a probe on Europa, we will almost certainly try to bore beneath the ice and test that water.

Why? Because you, and essentially all living animals, are a bag of water, as an old “Star Trek” episode once characterized us. We carried part of the water with us as we crept out of the sea. Of all the elements that are necessary for life, water seems to top the list.

Over the years astro-biologists have checked off planet after planet in their search for the conditions of life. Perhaps a billion years ago, Mars had the necessary water to harbor some sort of simple life form, but no liquid water remains. Only Europa holds the slimmest of chances.

At almost 3,000 miles in diameter, Callisto, the outermost satellite, is almost as big as the planet Mercury.

Unfortunately, its outermost status makes it a prime target for passing meteoroids. As they are sucked into Jupiter by the planet’s enormous gravity, those tiny hunks of rock sometimes hit the most conveniently placed target. The Voyager flybys showed Callisto to be the most cratered object in the solar system. It far surpasses Earth’s moon in that respect.

Jupiter and its moons look a bit like our solar system in miniature. The moons formed out of a disc of dust and gas that girdled the planet. That disc was very much like the ring of material around the early sun that eventually coalesced into planets.

In a medium-sized telescope, if you can only see three of the four Galilean moons or if one of the four is very close to the disk of Jupiter, you may be in for one of the greatest experiences of sky watching — a transit of a moon of Jupiter across the disk of the planet.

Scan the disk of the planet carefully. If the moon isn’t passing behind the planet (you have a 50-50 chance), you’ll see a small black speck, which is the shadow of the missing moon on the surface of the clouds. The moon is lost in the glare of the planet. Its shadow will move very slowly across the Jupiter’s disk.

If the moon is just about to touch the planet, you may see both the moon and its shadow briefly before the planet’s brightness drowns out the moon’s fainter glow.

If you were floating in the clouds of Jupiter right at the spot of the moon’s shadow, you would see the sun eclipsed by that moon. But a very small sun it would be. The sun is only one-fifth the size from Jupiter as it is from Earth.

Celebration of the Sun

Luckily for us, we don’t have to go to Jupiter to see an eclipse of the sun. A partial eclipse will be visible in Central Ohio on August 21. However, looking at the sun is dangerous to your vision, so you will need to have the right equipment and some understanding of what is happening before you look.

Thus, Perkins Observatory’s Celebration of the Sun programs take on special significance this year. On July 8, 15, and 22 starting at 4 p.m., the Perkins staff and volunteers from the Columbus Astronomical Society will talk about the eclipse, the science lore, and legend of our day star, observe the sun with solar-safe telescopes, weather permitting. Please call (740) 363-1257 for details or to reserve tickets.

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Tom Burns

Contributing Columnist

Tom Burns is the director of the Perkins Observatory in Delaware.

Tom Burns is the director of the Perkins Observatory in Delaware.