The Solar System at Ultraviolet Wavelengths 671
FIGURE 8 HSTSTIS images of Ganymede’s UV
aurora. The images represent neutral oxygen at 1356
A and reveal brightening in the polar regions that is ̊
variable over the fourHSTorbits during which the
imaging took place. (Figure reproduced with
permission from AAS Publications.)may be due to bright frost deposition or to the emplacement
of a relatively dark UV material. At this point, the source of
the dramatic, short-timescale changes in UV brightness on
Triton remains unknown.
FIGURE 9 Titan’s FUV spectrum as measured by theCassini
UVIS. The spectrum includes an overall continuum due to
reflected solar light, in addition to emission features due to
electron dissociation and excitation of molecular and atomic
nitrogen. (Figure courtesy of D. Shemansky.)
4. Observations of Solid Surfaces
Many solid-state materials that make up the surfaces of so-
lar system objects exhibit spectral absorption features, and
thus it is possible to identify or constrain the abundance of
solid components on the surfaces of these objects. This is
accomplished by comparing the spectral geometric albedo
of the object with the reflection spectrum of the solid-state
materials as measured in the laboratory. The following dis-
cussion focuses on ultraviolet observations of solid surfaces
throughout the solar system. Light is reflected from partic-
ulate surfaces by volume scattering and surface scattering.
At longer visible and near-IR wavelengths, volume scat-
tering dominates in most materials. At shorter NUV and
FUV wavelengths, surface scattering dominates. Absorp-
tions in the NUV are generally due to charge-transfer and
result in rather broad absorption features, compared with
absorption features in the near-IR, usually due to weaker
electronic transitions. Many non-ice materials are absorb-
ing in the NUV, including organics, sulfur compounds, and
many refractory materials. There is usually an absorption
edge between 3000 and 3700A. In contrast, ices such as ̊
H 2 O and CO 2 are not very absorbing at visible wavelengths
and at NUV wavelengths. (Water and CO 2 ice exhibit strong
absorption features near 1650A.) Atmosphereless bodies in ̊
the solar system experience weathering, whereby their sur-
faces are affected by bombardment of micrometeorites as
well as charged particles, either from the solar wind in the
case of the Moon and asteroids or from the magnetosphere