dies must get their energy from buoyan- 
cy, derived either from internal heat or 
sunlight. This backward transfer of en- 
ergy from chaotic eddies to organized 
currents is also observed in the earth’s 
weather patterns, although it is a minor 
feature of the total energy cycle. Voyag- 
er measurements show that on Jupiter, 
however, this backward transfer is a 
major part of the energy cycle. Saturn’s 
eddy winds are more difficult to mea- 
sure, the eddies themselves being hard- 
er to see. Scientists hope that Saturn 
will yield its secrets for Voyager 2, 
which has a more sensitive imaging sys- 
tem and is being programmed to make 
these important observations. 
Besides making wind measurements 
Voyager also measured the atmospheric 
composition and total energy output of 
Saturn. Here another difference from 
Jupiter emerged, one that challenged 
scientists’ understanding of the forma- 
tion and evolution of these gas giants. 
Both Jupiter and Saturn radiate more 
power than they absorb as sunlight. 
The excesses come from the interiors of 
the planets, and theoretical models 
have been constructed to explain them. 
The models start with a hot ball of hy- 
drogen, helium, and other elements in 
the appropriate abundance ratios. (To 
arrive at Saturn’s present bulk density, 
one is constrained to start with mix- 
tures that resemble the sun’s.) Once the 
mass has cooled for 4.5 billion years — 
the solar system’s age measured from 
radioactive decay in meteorites — the 
energy output in the model can be com- 
pared with the measured output. For 
Jupiter, the computed value agrees 
with observation. For Saturn, however, 
the computed value is too low. 
Thus the planet generates more ener- 
gy than can be accounted for by the 
simple cooling and contraction of a ho- 
mogeneous mixture of solar composi- 
tion. Another energy source is needed. 
Fortunately such a source exists and 
had been predicted before Voyager. Sat- 
urn, being less massive, is cooling faster 
than Jupiter, and today its interior is 
colder than Jupiter’s. At some time, 
calculated to be about 2 billion years 
ago, Saturn’s internal temperatures 
dropped below a critical point where 
helium condenses at the surface of the 
planet’s fluid core of metallic hydrogen. 
As with a' terrestrial air mass that is 
cooled below the saturation point, drop- 
lets form and then rain. These helium 
raindrops fall great distances inside 
Saturn and release heat by rubbing 
against the hydrogen fluid as they fall. 
Averaged over 2 billion years, the ener- 
Two of Saturn 's large satellites, 
Tethys (above) and Dione, show up 
clearly against the blackness of 
space in this image made at a 
distance of eight million miles. 
gy released if half the helium were to 
condense would just about explain the 
excess heat unaccounted for by simpler 
models. Because Jupiter cools more 
slowly, the critical point at which heli- 
um condenses is only now being 
reached in the interior of that planet. 
Thus helium rain provides an extra 
energy source for Saturn but not for Ju- 
piter, in agreement with the observed 
energy outputs of the two planets. Ad- 
ditional verification comes from the ob- 
served abundances of helium in the two 
atmospheres. Voyager measured 6 per- 
cent helium molecules on Saturn, 10 
percent on Jupiter. The solar value 
would be 1 1 percent if the hydrogen 
were to combine into H 2 , as on the giant 
planets. The lower helium abundance 
in Saturn’s atmosphere implies a higher 
abundance in the core, as would be ex- 
pected if helium rain had been falling 
for 2 billion years. The raindrops form 
about halfway to the center of the plan- 
et, where the pressure is millions of at- 
mospheres. Yet the entire outer en- 
velope loses its helium uniformly 
because it is stirred by convection. 
Therefore, the depletion of helium in 
Saturn’s atmosphere reflects quite ac- 
curately the helium that has condensed 
at deeper levels throughout the history 
of the planet. 
Finding this explanation for Saturn’s 
extra internal heat is comforting to 
those who try to understand the solar 
system’s formation and early history. It 
is axiomatic that the planets formed si- 
multaneously 4.5 billion years ago. The 
initial composition of the material re- 
sembled that of the early sun, but was 
modified close to the sun by high tem- 
peratures. The gravitational energy of 
the formation event warmed all the 
planets, but only the most massive ob- 
jects have retained this energy to the 
present day. Jupiter and Saturn now 
seem to fit this picture. It is not an im- 
probable story. Similar events can be 
seen happening elsewhere in the galaxy 
today. The inference is that our solar 
system is not an unusual place. The 
conditions from which life evolved may 
have occurred on many of the billions 
of stars in our galaxy. □ 
50 
