Star Talk - Jupiter, the King of Planets

Jupiter, the king of planets was named for the king of Roman gods. Jupiter is actually a title, or perhaps a term of endearment, meaning "Father Jove"...Jove is his proper name. It's appropriate that the planet is named for the greatest of the Roman gods, for it is by far the largest of the planets. If you took all the other planets and all of their moons, and all of the asteroids and comets, then doubled that mass, Jupiter would still be bigger. Our solar system is basically the Sun and Jupiter...everything else is crumbs and leftovers.

We know very little about the interior of Jupiter. We know the outer clouds are mostly hydrogen, helium, and ammonia ice crystals. We know the surface temperature is - 127 degrees C. and rises rapidly at depth. Only one probe, Pioneer, has penetrated the clouds, and it didn't last very long. As Pioneer got close, it encountered radiation levels more than 100 times greater than we expected. We have no idea why. Jupiter is assumed to have a core of molten iron. Above that is probably a vast sea of metallic liquid hydrogen. Then various layers of gases at increasingly cooler temperatures until we reach the surface cloudtops.

Jupiter spins at a very high rate. A day there lasts only 10 1/4 hours. Winds blow constantly at up to 28,000 mph. Vast storms with lightning far more powerful than we see here on Earth rage with incessant fury. It's the spin acting like a giant centrifuge that causes the clouds to form into the famous colored bands. It also causes the planet to bulge at the equator, which you can actually see in the telescope. Although it took form at the same time as the rest of the planets, Jupiter is so big that it's still collapsing and condensing. In doing so, it generates heat on its own due to the unbelievable pressure and density inside. Jupiter gives off about twice the amount of heat that it receives from the Sun. Could it eventually light off a nuclear furnace at its core and become a star? No, it would need to be 80-100 times more massive to do that. But it will get smaller, and denser, and eventually cooler. Just not in our lifetime.

Now, there is one feature that's contributed greatly to both science and commerce here on Earth. Anyone know? Those under 20 with excellent eyesight can actually see what I'm talking about. The moons. Jupiter has 4 big ones and at least 59 smaller ones, and here's what we've learned by studying them.

In 1610, Galileo first saw the 4 "Galilean" moons (as they are now called). At that time it was scientific belief, and more importantly, church doctrine, that the Earth was the center of God's creation and thus the center of all celestial motion. These moons were the first direct evidence of motion not centered on Earth. When he published his discovery, and its implication, priests were soon knocking at his door to verify it. One, after peering through Galileo's telescope at the very obvious moons, reportedly said, "I see nothing." Later that year, Galileo saw that Venus goes through phases, like the moon, further proof of a sun-centered system. This time, he was placed under house arrest for life and threatened with charges of heresy, which carried a death penalty. But it was too late, he had already proved Copernicus' model of the solar system was accurate. That's number 1.

Britain ruled the seas for several centuries, due mostly to their advanced skill in navigation. At that time, all navigation was celestial, and very much dependant on knowing the accurate time. Pendulum clocks don't mix well with pitching and rolling ships, but the English figured out that the moons of Jupiter eclipsed each other at precise, and predictable, intervals. An important duty of the Royal Astronomer was to plot these times and produce tables for navigators. By consulting the table, and observing when a specific event occurred, the ship's clock could be kept accurate to within a couple of minutes, no matter where on earth they were. That's number 2.

Because they used it to keep time, astronomers and navigators had noted for years that this "clock in the sky" would run faster for half the year, then lag behind for the other half, by as much as 8 minutes. It was regular enough that they could allow for it, but still it was an annoyance. In 1675, Danish astronomer, Ole Roemer, recognized that this phenomenon must be related to Earth, not Jupiter, as it followed our year exactly. He finally determined that the difference was due to the travel time of the light from Jupiter. It had farther to travel when we were on the far side of our orbit. By dividing the time error by the diameter of Earth's orbit, he was the first to calculate the speed of light. He missed it by about 20%, but his value stood for nearly 200 years before anyone improved on it. And that's number 3.

If those 3 milestones of discovery came from observing just the moons of Jupiter, it boggles the mind to think what may come as we look further beneath the clouds and explore the mysteries of the big planet itself.