The Solar System at Ultraviolet Wavelengths 667
FIGURE 4 HSTimage of Jupiter aurora. The
magnetic “footprint” of Io, marking the location
where magnetic field lines joining the moon and
Jupiter connect with the planet, is also seen as a
bright spot with a tail outside the main auroral oval.
(Image credit: J. Clarke, NASA.)a composite ofIUESaturn spectra from the 1978–1980 time
period. The mixing ratio of the acetylene is about 1× 10 −^7.
Although acetylene is a well-known strong absorber of ul-
traviolet radiation, it alone cannot explain the low UV spec-
tral geometric albedo of Saturn that has been reported by
IUE. Other ultraviolet-absorbing materials must be present.
Comparisons of laboratory spectra of C 2 H 2 ,PH 3 , AsH 3 ,
and GeH 4 with theIUEobservations show that the best-
fit model for Saturn’s atmospheric ultraviolet spectrum in-
cludes absorptions by C 2 H 2 ,H 2 O, CH 4 ,C 2 H 6 ,PH 3 , and
GeH 4. The distribution of PH 3 and GeH 4 decreases with
increasing altitude in these models, suggesting that ultravi-
olet photolysis is an important process occurring at higher
altitudes.
Pole-to-pole mapping studies of the hydrogen Lyman-
alpha emission across Saturn’s disk led to the discovery of
pronounced spatial asymmetries in the emission. Other ob-
servations of hydrogen do not find a variation in intensity
with rotational period as with Jupiter. There is no rotational
bulge in the Lyman-alpha emission as seen on Jupiter. This
is probably due to the fact that Saturn’s magnetic pole is
coincident with the rotational pole, whereas in Jupiter’s case
the poles are offset.
Like Jupiter, Saturn displays auroral activity. On both
planets this auroral activity also creates aerosols that are de-
tectable in the ultraviolet as dark-absorbing regions.HST
FOC ultraviolet observations discovered a dark oval en-
circling the north magnetic pole that is spatially coincident
with the aurora detected by theVoyagerUVS.Voyager 2ul-
traviolet PPS measurements also demonstrate a geograph-
ical correlation between the auroral zones of Jupiter and
Saturn with UV-dark polar regions. Additional ultraviolet
observations with theHSTFOC of Saturn’s northern ultra-
violet aurora and polar haze support the hypothesis that the
polar haze particles are composed of hydrocarbon aerosols
produced during H+ 2 auroral activity. More recentHSTUV
imaging of Saturn’s aurorae (Fig. 5) shows that they behave
differently from Jupiter’s aurorae, varying in brightness and
shifting in latitude.3.6 Uranus
Uranus presents a unique observational circumstance to
the inner solar system observer because of the fact that its
pole is inclined 89◦to the ecliptic and that at the present
position in its 84 year orbit about the Sun it presents its pole