The Solar System

496 PART 4^ |^ THE SOLAR SYSTEM

Jupiter is mostly a liquid planet. It may have a small core of heavy elements not

much bigger than Earth. (NASA/JPL/University of Arizona)

Celestial Profi le 7: Jupiter

Motion:

Average distance from the sun 5.20 AU (7.78  108 km)

Eccentricity of orbit 0.048

Inclination of orbit to ecliptic 1.3°

Average orbital velocity 13.1 km/s

Orbital period 11.87 y

Period of rotation 9.92 h

Inclination of equator to orbit 3.1°

Characteristics:

Equatorial diameter 1.43  105 km (11.2 D⊕)

Mass 1.90  1027 kg (318 M⊕)

Average density 1.34 g/cm^3

Gravity at base of clouds 2.54 Earth gravities

Escape velocity 61 km/s (5.4 V⊕)

Temperature at cloud tops 130°C (200°F)

Albedo 0.51

Oblateness 0.064

Personality Point:

Jupiter is named for the Roman king of the gods, and it is the largest

planet in our solar system. It can be very bright in the night sky, and

its cloud belts and four largest moons can be seen through even a small

telescope. Its moons are visible even with a good pair of binoculars

mounted on a tripod or braced against a wall.

You can see that Jupiter is massive by watching its moons
race around it at high speed. Io is the innermost of the four
Galilean moons, and its orbit is just a bit larger than the orbit of
our moon around Earth. Io streaks around its orbit in less than
two days, whereas Earth’s moon takes a month. Jupiter has to be
a very massive world to hold on to such a rapidly moving moon
(■ Figure 23-2). In fact, you can use the radius of Io’s orbit and
its orbital period in Newton’s version of Kepler’s third law (see
Chapter 5) to calculate the mass of Jupiter, which is 1.9  1027 kg,
318 times Earth’s mass.
Learning the size and mass of Jupiter is relatively easy, but
you might wonder how astronomers know that it is made mostly
of hydrogen. Th e fi rst step is to divide mass by volume to fi nd
Jupiter’s average density, 1.34 g/cm^3. Of course, it is denser at the
center and less dense near the surface, but this average density
reveals that it can’t contain much rock. Rock has a density of 2.5
to 4 g/cm^3 , so Jupiter must contain material mostly of lower
density, such as hydrogen.
Spectra recorded from Earth and from spacecraft visiting
Jupiter show that the composition of Jupiter is much like that of
the sun—it is mostly hydrogen and helium. Th is fact was con-
fi rmed in 1995 when a probe from the Galileo spacecraft para-
chuted into the atmosphere and radioed its results back to Earth.
Jupiter is mostly hydrogen and helium, with traces of heavier
atoms that form molecules such as methane (CH 4 ), ammonia
(NH 3 ), and water (■ Table 23-1).
Just as astronomers can build mathematical models of the
interiors of stars, they can use the equations that describe gravity,
energy, and the compressibility of matter to build mathematical
models of the interior of Jupiter. Th ese models reveal that the
interior of the planet is mostly liquid hydrogen containing small
amounts of heavier elements. Th e pressure and temperature are
higher than the critical point for hydrogen, and that means
there is no diff erence between gaseous hydrogen and liquid
hydrogen. If you parachuted into Jupiter, you would fall through
the gaseous atmosphere and notice the density of the surround-
ing fl uid gradually increasing until you were in a liquid, but you
would never splash into a liquid surface.
Roughly a quarter of the way to the center, the pressure is
high enough to force the hydrogen to change into liquid metallic
hydrogen, which is a very good electrical conductor. Because
liquid metallic hydrogen has been very diffi cult to create and
study in the laboratory so far, its properties are poorly understood.
Th at is the reason why the models are uncertain about the depth
of the transition from normal to metallic liquid hydrogen.
Th e models are also uncertain about the presence of a heavy
element core in Jupiter. Th e planet contains about 30 Earth
masses of elements heavier than helium, but much of that may
be suspended in the convectively stirred liquid hydrogen.
Measurements by orbiting spacecraft indicate that no more than
10 Earth masses are included in a heavy element core. Some
astronomy books refer to this as a rocky core, but, if it exists, it