668 Encyclopedia of the Solar System
FIGURE 5 HSTimages of Saturn’s varying
aurorae. (Image credit: J. Clarke, NASA.)to Earth. This unusual inclination, combined with its great
distance from Earth, makes it impossible to use an Earth-
based instrument to undertake pole-to-pole comparisons as
was done with Jupiter and Saturn. Uranus has a geometric
albedo at NUV wavelengths of about 0.5, more than twice
that of Jupiter and Saturn. This suggests that additional
absorbers are present in the jovian and Saturanian atmo-
spheres that are not present in the atmosphere of Uranus.
Both Uranus and Neptune possess hot thermospheres and
stratospheres that are substantially clear of hydrocarbons
and other heavy constituents, making the UV albedos higher
than for Jupiter and Saturn. A sharp increase in measured
reflectance intensity at wavelengths longward of 1500Ais ̊
indicative of acetylene (C 2 H 2 ) present in the atmosphere
of Uranus.
Voyager 2spacecraft observations of Uranus found a very
small internal heat source compared to the large internal
heat sources found at Jupiter and Saturn. This suggests that
there is very little atmospheric mixing driven by heating
and buoyancy in the Uranian atmosphere. Thus, ultravio-
let observations are able to sense a deeper region of the
atmosphere.
The ultraviolet emissions from Uranus’ atmosphere have
been measured byIUEand theVoyagerUVS. To increase
the signal-to-noise ratio,IUEobservers used principally
low-resolution observations and binned broad-wavelength
regions together to search for broadband absorbers at ul-
traviolet wavelengths. Analysis of theIUEobservations de-
tected acetylene absorptions, which were also detected on
Jupiter and Saturn. Based on these observations, the mixing
ratio of the acetylene is estimated to be 3× 10 −^8. Analysis
of theVoyagerUVS observations of H 2 band ultraviolet air-
glow emissions shows aurora at both magnetic poles, which
are offset from the rotational poles by∼ 60 ◦. The auroral
emissions on Uranus are very localized in magnetic longi-
tude and do not form complete auroral ovals as are seen on
Jupiter and Saturn.3.7 Neptune
Neptune is so distant that only broadband ultraviolet mea-
surements are possible from Earth orbit. The geometric
albedo of Neptune measured byIUEis 0.5, which, like
that of Uranus, is twice that of Jupiter and Saturn. Below
1500 A, Neptune’s albedo is reduced by the higher hydro- ̊
carbon abundance carried into its stratosphere by its more
vigorous vertical transport. Most of the important data for
Neptune at ultraviolet wavelengths have come from the
UVS onboard theVoyager 2spacecraft and fromHST.CH 4
and C 2 H 6 abundances inferred from theVoyagerUVS solar