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.

Comments

Lorentz transformation

Lorentz transformation and its inverse : Define an event to have spacetime coordinates ( t , x , y , z ) in system S and ( t ′, x ′, y ′, z ′) in a reference frame moving at a velocity v with respect to that frame, S ′. Then the Lorentz transformation specifies that these coordinates are related in the following way: {\displaystyle {\begin{aligned}t'&=\gamma \ (t-vx/c^{2})\\x'&=\gamma \ (x-vt)\\y'&=y\\z'&=z,\end{aligned}}} where {\displaystyle \gamma ={\frac {1}{\sqrt {1-{\frac {v^{2}}{c^{2}}}}}}} is the Lorentz factor and c is the speed of light in vacuum, and the velocity v of S ′, relative to S , is parallel to the x -axis. For simplicity, the y and z coordinates are unaffected; only the x and t coordinates are transformed. These Lorentz transformations form a one-param...

The Ptolemaic System

Explaining the nonuniform motions of the five planets was one of the main challenges facing the astronomers of antiquity. The Greeks developed many theories to account for retrograde motion and the loops that the planets trace out against the background stars. One of the most successful and enduring models was originated by Apollonius of Perga and by Hipparchus in the second century B.C. and expanded upon by Ptolemy, the last of the great Greek astronomers, during the second century A.D. sketches the basic concept, usually called the Ptolemaic system. Each planet is assumed to move in a small circle called an epicycle, whose center in turn moves in a larger circle, called a deferent, which is centered approximately on the Earth. Both the epicycle and deferent rotate in the same direction As viewed from Earth, the epicycle moves eastward along the deferent. Most of the time the eastward motion of the planet on its epicycle adds to th...

Copernicus and the Arrangement of the Planets

In the years after 1500, Copernicus came to realize that a heliocentric model has several advantages beyond providing a natural explanation of retrograde motion. In the Ptolemaic system, the arrangement of the planets—that is, which are close to the Earth and which are far away—was chosen in large part by guesswork. But using a heliocentric model, Copernicus could determine the arrangement of the planets without ambiguity. Copernicus realized that because Mercury and Venus are always observed fairly near the Sun in the sky, their orbits must be smaller than the Earth’s. Planets in such orbits are called inferior planets. The other visible planets—Mars, Jupiter, and Saturn—are sometimes seen on the side of the celestial sphere opposite the Sun, so these planets appear high above the horizon at midnight (when the Sun is far below the horizon). When this happens, the Earth must lie between the Sun and these planets. Copernicus therefore conclude...

PhySite

Search This Blog