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OCR for page 16
hree hundred years ago, Isaac
Newton used the motions of the
~ Galilean satellites (the four moons
of Jupiter discovered by Galileo) to
determine fupiter's mass. A century
later, William Herschel deduced that
fupiter's density was anomalously low.
In the 20th century it became clear that
Jupiter was composed primarily of the
lightest elements, hydrogen and heli-
um. Further studies of Jupiter, com-
bined with analyses of the spectrum of
light reflecting off the planet, gave rise
to the so-called solar composition
model of the giant planets. That is, as
far as their overall elemental composi-
tions are concerned, Jupiter and also
Saturn appear to be pieces of the Sun
cooled down to planetary temperatures.
Unfortunately, the solar composi-
tion model does not work for Uranus
and Neptune, which are twice as dense
A composite image of the four Galilean
satellites and fupiter's Great Red Spot.
as Saturn. Their densities indicate that
they formed from material that was
rich in water, ammonia, and methane
ices and more deficient in the light
gases than Jupiter, Saturn, or the Sun.
Since oxygen and carbon are the third
and fourth most abundant elements in
the Sun after hydrogen and helium,
the modified solar composition model
was proposed to explain the creation of
all of the planets. This model starts
with a young Sun surrounded by a disk
fuzz {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
of leftover material, a mix of elements
similar in overall composition to that
of itself. During an early active phase,
which many young stars undergo, the
Sun ejected a wind of high-speed elec-
trons, protons, and heavier particles
that swept the hydrogen, helium, and
other gases out of the disk. The mix-
ture that remains has a composition
similar to that of the Sun, except for
the missing gaseous component. Close
to the Sun, where it is hot, the ices too
are lost, and only the rocks and metals
remain. This interpretation fits our
solar system, with small rocky planets
in the inner solar system and the
gaseous giant planets further out.
In this theory, timing is critical.
The giant planets had to have formed
before the gases were swept out of the
solar system. Timing might explain
the compositional difference between
the ice giants, Uranus and Neptune,
and the gas giants, Jupiter and Saturn.
According to theory, giant planets
could form faster at the orbits of
Jupiter and Saturn where the density
of material was higher and collisions
more frequent. Perhaps Uranus and
Neptune were just starting to accumu-
late gases when the Sun blew the
lighter gases out of the solar system.
The time that it takes to produce a
fupiter-size object depends on the
method of formation, and here there
are two possabilities. The slow way is
to first form a rock-ice core about 10
times the mass of Earth the resulting
dense, solid object is able to attract gas
and grow in mass once it reaches this
size. The fast way assumes that Jupiter
formed much the way the Sun did the
gas in one region of the solar nebula
became sufficiently dense that its col-
lective gravity caused it to collapse in a
spherically symmetric manner. If creat-
ed this way, Jupiter would resemble an
object known as a brown dwarf a star
with insufficient mass to sustain
nuclear fusion reactions in its core.
Distinguishing between these hypothe-
ses required determining if the giant
planets have rock-ice cores. While the
evidence indicates that Saturn,
Neptune, and Uranus do indeed have
cores, the nature of Jupiter's deep interi-
or remains unknown.
Another mystery about Jupiter con-
cerns the distance from the Sun at
which it formed. An analysis of Galileo
spacecraft data shows that Jupiter has
greater amounts of certain heavy ele-
ments than does the Sun. One explana-
tion for this suggests that Jupiter formed
far out in the solar system, where such
elements were more prevalent, and then
migrated inward toward its present
orbit. Another possibility is that Jupiter
formed approximately where it is today
but was more likely to collect heavier
elements than lighter ones. The key to
resolving which if either of these ideas
is correct is to determine the relative
amounts of hydrogen and oxygen in
Jupiter's atmosphere.
Studies of Jupiter also have the
potential to significantly improve our
understanding of planetary magneto-
spheres and their interactions with the
solar wind. fupiter's magnetosphere is
sustained in a manner different from
Earth's it derives its energy from the
rotation of the planet itself. In addi-
tion, Jupiter has the strongest magne-
tosphere in the solar system. By
studying Jupiter's magnetosphere,
especially using spacecraft to see
regions unobservable from Earth, we
could learn answers to questions
about a diverse set of objects, ranging
from Earth to distant pulsars.
Answering these questions requires
measurements both inside and above
Juipiter's atmosphere. The Jupiter
Polar Orbiter with Probes is, in a
sense, two missions in one. A carrier
spacecraft equipped with three probes
is launched toward Jupiter. As the
spacecraft nears the planet the probes
are released and penetrate Jupiter's
thick atmosphere, taking measure-
ments and reporting back data on
Jupiter's interior. Following the com-
pletion of the probe mission, the carri-
er enters a low-altitude polar orbit
about Jupiter from which vantage
point it conducts additional studies
for a year or more.
The Jupiter Polar Orbiter with
Probes mission has five primary objec-
tives. The first is to determine if Jupiter
has a core. The second is to measure
the water abundance below the visible
clouds and, hence, determine the
OCR for page 17
Jupiler Polar Orbiler with Probes
oxygen/hydrogen ratio. Both of these
investigations address outstanding
questions about the formation of
Jupiter and, thereby, the solar system.
To address the third objective, the
spacecraft's probes will measure the
deep winds to a depth of 100 bars
while another instrument may be able
to give some information about the
winds to thousands of bars. (Depth on
Jupiter is measured by the atmospheric
pressure, not by distance; 1 bar is the
atmospheric pressure at sea level on
Earth.) The deep winds may be key to
the extreme stability of the weather
systems observed at cloud top.
The fourth objective is addressed by
virtue of the spacecraft's cloud-skim-
ming orbit, which will permit more pre-
cise measurements of the planet's mag-
netic field than previously possible.
Similarly, the polar nature of the orbit
Guiding Themes Addressed Important Planetary Science Questions Addressed
Profile
Jupiter Polar Orbiter with Probes
Mission Type: Orbiter with atmos-
pheric probes
Cost Class: Medium
Priority Measurements:
· Probe Jupiter's interior with gravity
and magnetic field measurements
from a polar orbit.
· Measure condensable gas abun-
dances, temperature, wind velocity,
and cloud opacity down to the
1 00-bar pressure level.
· Determine how internally produced
plasma is ejected from a rotation-
dominated magnetosphere.
Artist's concept of the fupiter Polar
Orbiter with Probes spacecraft illustrat-
ing how the three probes will enter dif-
ferent parts of the planet's atmosphere.
permits the mission's fifth objective-
repeated visits to the hitherto unex-
plored polar magnetosphere to be
addressed. Taken together, these latter
two investigations will allow researchers
to map Jupiter's magnetosphere much
more accurately, learn more about the
magnetic field's origins inside Jupiter,
study how these fields interact with
Jupiter's moons, and teach us much
about Jupiter's magnetic activity.
Representative terms from entire chapter:
polar orbiter
