538 PART 4^ |^ THE SOLAR SYSTEM
planet formation in the solar nebula suggest that Uranus and
Neptune formed closer to the sun, in the neighborhood of
Jupiter and Saturn. Gravitational interactions among the Jovian
planets could have gradually moved Uranus and Neptune out-
ward to their present locations. One such model has Neptune
forming closer to the sun than Uranus and the two planets later
switching places as they moved out. As you will learn later in this
chapter, the migration of giant planets could have triggered the
late heavy bombardment episode. Th ese are interesting hypoth-
eses, and they illustrate how uncertain the histories of the outer
planets really are.
Th e highly inclined axis of Uranus may have originated late
in its formation when it was struck by a planetesimal that would
have to have been as large as Earth. Th at impact also may have
disturbed the interior of the world and caused it to lose much of
its heat. Uranus now radiates less than 10 percent more heat than
it receives from the sun. Th ere is now just enough heat fl owing
outward to drive circulation in its slushy mantle and generate its
magnetic fi eld. A more recent study shows that tidal interactions
with Saturn could have altered Uranus’s axis of rotation as it was
pushed outward. In this case, an older catastrophic hypothesis is
being challenged by a newer evolutionary hypothesis (How Do
We Know? 19-1).
Impacts have been important in the history of the moons
and rings of Uranus. All of the moons are cratered, and some
show signs of large impacts. Meteorites and the nuclei of comets
striking the moons may create debris that becomes trapped
among the orbits of the smaller moons to produce the narrow
rings. Th e ring particles observed now could not have lasted
since the formation of the planet, so they must be replenished
with fresh material now and then. When the fi nal history of
Uranus is written, it will surely include some dramatic impact
events.SCIENTIFIC ARGUMENT
How do astronomers know what the interior of Uranus is like?
This argument must combine observation with theory. Obviously,
you can’t see inside the planet, so planetary scientists are limited
to a few basic observations that they must connect with models
in a chain of inference to describe the interior. First, the size of
Uranus can be found from its angular diameter and distance, and
you can fi nd its mass by observing the orbital radii and orbital
periods of its moons. Its mass divided by its volume equals its
density, which implies that it must contain a certain proportion
of dense material such as ice and rock, more than there is inside
Saturn. Add to that the chemical composition obtained from spec-
tra, and you have the data needed to build a mathematical model
of the interior. Such models predict a core of heavy elements and
a mantle of ices mixed with heavier material of rocky composition.
Going from observable properties to unobservable properties is
the heart of astronomical research. Now use this approach to build
a new argument: What observational properties of the rings of
Uranus show that small moons must orbit among the rings?the top of the cliff and dropped a rock over the edge, it would
fall for 10 minutes before hitting the bottom. Nevertheless, cra-
ter counts indicate that the cliff and the ovoids are old. Miranda
is no longer geologically active, but you can read hints of its
active past on its disturbed surface.
If the ovoids were caused by convection in Miranda’s icy
mantle, then heat rising from its interior must have been the
dominant factor. Miranda is so small that its heat of formation
must have been lost quickly, and there is no reason to expect it
to have been strongly heated by radioactive decay. Tidal heating
could have occurred if Miranda’s orbit was once made slightly
eccentric by a resonance with other moons. Your knowledge
of tidal heating in other moons can help you understand
Miranda.
A History of Uranus
Th e challenge of comparative astronomy is to tell the story of a
world, and Uranus may present the biggest challenge of all the
objects in our solar system. Not only is it so distant that it is dif-
fi cult to study, but it is also peculiar in many ways.
Uranus formed from the solar nebula, as did the other Jovian
planets, but calculations show that Uranus and Neptune could
not have grown to their present size in the slow-moving orbits
they now occupy so far from the sun. Computer models of
■ Figure 24-12
Miranda, the smallest of the fi ve major moons of Uranus, is only 480 km
(290 mi) in diameter, but its surface shows signs of activity. This photomo-
saic of Voyager 2 images reveals that it is marked by great oval systems of
grooves. The smallest features detected are about 1.5 km (1 mi) in diameter.
(U.S. Geological Survey, Flagstaff, Arizona)
Visual