The Solar System at Ultraviolet Wavelengths 663
FIGURE 1 Pioneer VenusOrbiter Cloud
Photopolarimeter (OCPP) image of Venus at
3650 A. ̊observed Venus in the extreme ultraviolet wavelength range
(550–1250A) during its flyby. It detected emissions due to ̊
helium, ionized oxygen, atomic hydrogen, and an atomic
hydrogen–atomic oxygen blend. In 1994, an extreme ultra-
violet spectrograph (EUVS) was launched aboard a sound-
ing rocket to observe the Venusian atmosphere from 825 to
110 A. The EUVS identified several species, including N I, ̊
N II, N 2 , H I, O I, and O II. The results of the EUVS mea-
surements are consistent with earlier observations byIUE,
Pioneer Venus,Venera 11and 12 , and theGalileoEUV spec-
trometer. TheEUVEprovided the first full EUV (70–760A) ̊
spectrum of Venus in 1998 and made brightness measure-
ments on the He I (584A) and O II (539 ̊ A) lines. The FUV ̊
spectrum of Venus is dominated by the CO Fourth-positive
band system, as well as by neutral oxygen and carbon fea-
tures, and has been measured byHUT(820–1840A) in 1995 ̊
and byCassiniUVIS.
IUEspectra of the Venus day side and night side ob-
tained while Venus was near elongation displayed SO 2 ab-
sorptions at 2080–2180A, which when combined with the ̊
column densitiesreported by thePioneer Venusorbiter
and with ground-based observations, are a measure of the
SO 2 mixing ratiowith altitude and its variation at the top
of the cloud deck. This provides information on its varia-
tion in spatial distribution and permits models to be con-
structed of the planet’s atmospheric dynamics. Observations
of the Venus night side withPioneer Venusorbiter andIUE
detected the Venus nightglow, which is caused by the emis-
sion bands of nitric oxide (NO). Because of the short life-
time of NO on the night side, this finding implies the rapid
day side–night side transport of material in the Venus at-
mosphere. Observations of the Venus day side have led to
the discovery that the dayglow emission is carbon monox-
ide fluorescence, probably due to fluorescent scattering of
solar Lyman-alpha radiation.3.3 Mars
The atmosphere of Mars, like that of Venus, is dominated
by carbon dioxide, and also consists of small amounts of N 2 ,
H 2 O, and their photochemical products. Mars’ atmosphere
is much less dense than Venus’s atmosphere and is relatively
transparent at most wavelengths. Therefore, ultraviolet to
infrared observations of Mars reveal information about both
its atmosphere and its surface. The observations of Mars by
the UVS instruments onMariner 6and 7 were the first to
reveal the ultraviolet dayglow of that planet; later observa-
tions byMariner 9confirmed and extended these results.
NUV spectra (Fig. 2a) revealed the presence of CO (a-X)
Cameron bands, CO+(B-X), CO+ 2 (A-X), and CO+ 2 (B-X)
features. FUV spectra displayed oxygen features at 1304 and
1356 A, neutral carbon emission features at 1561 and 1657 ̊