The Solar System at Ultraviolet Wavelengths 665
FIGURE2c Ozone at high latitudes on Mars as measured by
Mariner 9UVS. A high-latitude spectrum is shown ratioed to a
low-latitude spectrum, compared with a laboratory spectrum of
the Hartley band of O 3. (Figure reproduced with permission
from Elsevier.)
varied with season. Ozone densities were highest in winter
and lowest in summer, anticorrelated with atmospheric wa-
ter vapor content. The correlation between higher amounts
of ozone with a cold, clean, dry atmosphere led to the con-
clusion that ozone is formed through the combination of
atomic and molecular oxygen, both of which are more read-
ily present when less water is available. SubsequentHST
observations have studied the seasonal variation of atmo-
spheric ozone at low latitudes and have linked low latitude
ozone abundance variations across the martian perihelion–
aphelion cycle with the large annual water vapor variation
due to the eccentricity of Mars’ orbit.HSTobservations
have also been used to study atmospheric aerosol (dust and
cloud) opacities. These UV observations demonstrate the
critical function that is performed by the small amounts of
H 2 O in the martian atmosphere, which control the buildup
of CO and O 2 , and sustain the stability of CO 2.
3.4 Jupiter
Jupiter, the target of numerous Earth-based observations as
well as spacecraft flybys, is composed of 90% hydrogen and
10% helium, with small amounts of ammonia and methane.
The uppermost layers of the atmosphere are observable in
the ultraviolet wavelength range and display products of
photochemical processes.
The first FUV spectra of Jupiter were measured in
sounding rocket experiments in the late 1960s and early
1970s. These early measurements displayed H 2 Lyman and
Werner band emissions and hinted at the presence of ab-
sorption features due to C 2 H 2 ,C 2 H 4 and NH 3. Stellar
occultation observations by theVoyagerUVS were sig-
nificant in providing measurements of upper atmospheric
FIGURE3a FUV geometric albedos of Jupiter and Saturn as
measured byIUE. The albedos are derived from composite
spectra of the planets between 1978 and 1980. The upper and
lower curves for each planet correspond to assumptions of
uniform and cosine-limb-darkened disks, respectively. The
dashed lines represent data that are uncertain in magnitude
owing to the subtraction of scattered light and are regarded as
upper limits. (Figure reproduced with permission from AAS
Publications.)FIGURE3b The spectral geometric albedo of Jupiter as
measured byIUE. The smooth solid line is the best fit from a
model that assumes a layer of haze particles with
single-scattering albedo of 0.42 that overlie a cloud deck with
geometric albedo of 0.25. (Figure reproduced with permission
from Elsevier.)