516 PART 4^ |^ THE SOLAR SYSTEM
the rings arise from contamination in the ice, and some areas have
unusual compositions. Th e Cassini Division, for instance, contains
particles that are richer in rock than most of the ring. No one knows
how these diff erences in composition arise, but they must be related
to the way the rings are formed and replenished.
Like a beautiful fl ower, the rings of Saturn are controlled by
many diff erent natural processes. Observations from spacecraft
such as Voyager and Cassini will continue to reveal even more
about the rings. Such missions are expensive, of course, but they
are helping us understand what we are (How Do We
Know? 23-1).
The History of Saturn
Th e farther you journey from the sun, the more diffi cult it is to
understand the history of the planets. Any fully successful history
of Saturn should explain its low density, its peculiar magnetic
fi eld, and its beautiful rings. Planetary scientists can’t tell a com-
plete story yet, but you can understand a few of the principles
that aff ected the formation of Saturn and its rings.
Most of Saturn’s story parallels that of Jupiter. Saturn formed
in the outer solar nebula, where ice particles were stable. It grew
rapidly, becoming massive enough to capture hydrogen and
helium gas directly from the nebula. Th e heavier elements prob-
ably form a denser core, and the hydrogen forms a liquid mantle
containing liquid metallic hydrogen. Th e outward fl ow of heat
from the core drives convection currents in this mantle that,
coupled with the rapid rotation of the planet, produces its mag-
netic fi eld. Because Saturn is smaller than Jupiter, it has less liq-
uid metallic hydrogen, and its magnetic fi eld is weaker.
Th e rings of Saturn defi nitely are not primordial, meaning,
the material in them now has not been in its current form since
the formation of the planet. Saturn, like Jupiter, would have
been very hot when it formed, and that heat would have vapor-
ized and driven off any nearby small icy particles of leftover
material. Also, such a hot Saturn would have had a very dis-
tended atmosphere, which would have slowed ring particles by
friction and caused the particles to fall into the planet. Finally,
the processes that tend to destroy Jupiter’s ring particles also
apply to Saturn’s rings.
Planetary rings do not seem to be stable over 4.6 billion
years, so the ring material must have been produced more
recently. Saturn’s beautiful rings may have been created within
just the last 100 million years, an astronomically short time. One
suggestion is that a small moon or an icy planetesimal came
within Saturn’s Roche limit, and tides pulled it apart. At least
some of the resulting debris would have settled into the ring
plane. Another possibility is that a comet struck one of Saturn’s
moons. Because both comets and moons in the outer solar sys-
tem are icy, such a collision would produce icy debris. Bright
planetary rings such as Saturn’s may be temporary phenomena,
forming when violent events produce fresh ice debris and then
wasting away as the ice is gradually lost.Who Pays for Science?
Why shouldn’t you plan for a career as an
industrial paleontologist? Searching out
scientifi c knowledge can be expensive, and
that raises the question of funding. Some
science has direct applications, and industry
supports such research. For example, phar-
maceutical companies have large budgets for
scientifi c research leading to the creation of
new drugs. But some basic science is of no
immediate practical value. Who pays the bill?
A paleontologist is a scientist who studies
ancient life forms by examining fossils of
plant and animal remains, and such research
does not have commercial applications.
Except for the rare Hollywood producer about
to release a dinosaur movie, corporations
can’t make a profi t from the discovery
of a new dinosaur. The practical-minded
stockholders of a company will not approve
major investments in such research.
Consequently, digging up dinosaurs, like
astronomy, is poorly funded by industry.
It falls to government institutions and
private foundations to pay the bill for this
kind of research. The Keck Foundation
has built two giant telescopes with no
expectation of fi nancial return, and the
National Science Foundation has funded
thousands of astronomy research projects for
the benefi t of society.
The discovery of a new dinosaur or a new
galaxy is of no great fi nancial value, but such
scientifi c knowledge is not worthless. Its
value lies in what it tells us about the world
we live in. Such scientifi c research enriches
our lives by helping us understand what
we are. Ultimately, funding basic scientifi c
research is a public responsibility that society
must balance against other needs. There isn’t
anyone else to pick up the tab.23-1
Sending the Cassini spacecraft to Saturn costs each
U.S. citizen 56¢ per year over the life of the project.