Saturn’s Missing Helium

Like Jupiter, Saturn is thought to have a large rocky core. But unlike

Jupiter, data from Earth-based telescopes and spacecraft show

that the atmosphere of Saturn has a serious helium deficiency: Its

chemical composition is 96.3% hydrogen molecules, 3.3% helium,

and 0.4% other substances (by mass, 92% hydrogen, 6%

helium, and 2% other substances). This is a puzzle because Jupiter

and Saturn are thought to have formed in similar ways from the

gases of the solar nebula (see Section 8-4), and so both planets

(and the Sun) should have essentially the same abundances of hydrogen

and helium. So where did Saturn’s helium go?

The explanation may be simply that Saturn is smaller than

Jupiter, and as a result Saturn probably cooled more rapidly. (We

saw in Section 7-6 why a small world cools down faster than a

large one.) This cooling would have triggered a process analogous

to the way rain develops here on Earth. When the air is cool

enough, humidity in the Earth’s atmosphere condenses into raindrops

that fall to the ground. On Saturn, however, it is droplets

of liquid helium that condense within the planet’s cold, hydrogenrich

outer layers. In this scenario, helium is deficient in Saturn’s

upper atmosphere simply because it has fallen farther down into

the planet. By contrast, Jupiter’s helium has not rained out because

its upper atmosphere is warmer and the helium does not

form droplets.

In this scenario, Jupiter and Saturn both have about the same

overall chemical composition. But Saturn’s smaller mass, less than

a third that of Jupiter, means that there is less gravitational force

tending to compress its hydrogen and helium. This explains why

Saturn’s density is only about half that of Jupiter, and is in fact

the lowest of any planet in the solar system.