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The Question

(Submitted August 21, 1996)

I understand Jupiter is not dense enough to walk around on, and that it has a deadly radioactivity about it. That doesn't seem reasonable to me, because it's too far from the Sun. How do you account for this? Is the radioactivity stable, or does it fluctuate? And what is its Source? Just curious. Thanks a lot. I NEVER heretofore have had any interest in Astronomy--that is, until I had a couple of sons to talk with. Now Mom has fun also.

The Answer

It is true that you cannot walk on Jupiter. This is because the atmosphere is very deep, perhaps comprising the whole planet (which would mean there is no solid surface to walk on!). It is composed mainly of hydrogen and helium, with small amounts of methane, ammonia, water vapor and other compounds. At great depths within Jupiter, the pressure is so great that the hydrogen becomes metallic.

This metallic hydrogen core generates a huge magnetic field around Jupiter. It's like the magnetic field of Earth, but Jupiter's field is 10 times stronger. This magnetic field traps high energy particles --- the particles come from the Sun, from outside the Solar system, and from the volcanoes on Io, one of Jupiter's many moons. The trapped particles then creates the radiation belt. This is similar to the Van Allen belts around Earth, but can be up to 10,000 times more intense. These levels of radiation near Io are so high that a human would absorb a lethal dose in just minutes.

It's not quite correct to call this "radioactivity", which is different from radiation. Radiation can refer to a lot of things: sunlight, broadcast signal from a radio station, microwaves in your microwave oven, ... Some are dangerous, some are not. Radioactivity refers to the properties of certain atoms and subatomic particles that spontaneously decay and emit particular types of radiation (gamma-rays, alpha particles etc.) that are often dangerous and sometimes deadly.

We hope you and your sons will continue to enjoy astronomy!

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