Jupiter
The clouds
of Jupiter
Fred Taylor and Patrick Irwin examine the Galileo mission’s contribution
to our understanding of Jupiter’s colourful chemical cloak.
he highly organized and brightly
coloured cloud structure on the
nearest and largest gas giant planet
Jupiter has been explored by the
Galileo orbiter/probe project, which
completed its nominal mission in
December 1997. At least four and
possibly as many as six distinct
layers of haze or
cloud, of different
composition and at
different depths,
appear to contribute
to the external
appearance of the
planet at low and
mid-latitudes. A
model of the
properties of these
clouds has been
developed from the
various data and theoretical
constraints. Aspects of the global
and time variability of the cloud
structure, and its coupling with
dynamical systems like the Great
Red Spot, are also becoming clearer,
allowing speculation about their
nature and origins. Analyses of the
full four-year data set, some of
which is still to be acquired, will
add further details of the
meteorological behaviour of
Jupiter’s atmosphere.
T
June 1999 Vol 40
I
t has been known for a long time that the
visible face of Jupiter consists of clouds,
with no surface in sight. In the last few
decades it has become clear that there is no surface at all in the usual sense, just a very deep
atmosphere which becomes denser with depth,
eventually condenses, and deeper still, becomes
metallic. This point was controversial for a
while, mainly because the clouds have quasipermanent structure associated with them
which, for some, implied a shallow atmosphere
pheric structure, dynamics and meteorology.
One of the most curious aspects is the bright
and varied colours (figure 1). They suggest that
we are observing, not just one single type of
cloud, but several, probably with different
chemical compositions. The colours also hint at
complex chemistry in the clouds, perhaps even
with exobiological implications, although the
latter remains pure speculation at this stage.
The other remarkable feature is the morphology; Jovian clouds form broad bands which
retain their overall organization for centuries.
These bands are studded with giant oval eddies,
some of which are long-lived and tend to be solitary (the GRS being the most famous example),
while others are less persistent and sometimes
occur in strings. There are also remarkable,
chaotic-looking clouds, made up of strands
which seem disinclined to mix with their surroundings, although this is at least partly an illusion which occurs because of their grand scale.
Some of the dark regions within the belts
appear very bright in thermal emission, especially near wavelengths of 5 microns where the
gaseous components – primarily hydrogen,
helium, methane and ammonia – are free of
absorption bands. It follows that these regions,
known as “hot spots”, are relatively cloudfree: emission from levels as deep as 8 bars in
pressure, where the temperature is around
300 K, or close to that on the surface of the
Earth, reaches space from the clearest regions.
High-resolution spectroscopy in these hot
spots can sound levels warm enough for
species such as water and phosphine to be present as vapour in appreciable amounts.
Possibilities with Galileo
1 Jupiter at visible wavelengths, with the Earth
on the same scale for comparison (NASA).
overlying a solid surface. Many of the theories
of the origin of the Great Red Spot (GRS), for
example, required the atmosphere to be flowing over some rigid topographical feature.
Previous knowledge and key questions
A quick look at a photograph of Jupiter reveals
that the clouds are complex (figure 1). Longstanding questions include: How many layers
of cloud are we seeing? What are they composed of? Where do they typically lie in pressure, temperature, and altitude? How are they
produced and dissipated? What are the colours
due to? What are the giant white ovals, the
brown barges, the Great Red Spot? Eventually
we should like to understand every aspect of
the chemistry and microstructure of Jovian
cloud systems, and how they relate to atmos-
The Galileo mission started in December 1995
with the insertion of an entry probe, and has
continued with remote sensing from the
orbiter. Prior to studies of Jupiter by the
Galileo mission, we lacked even basic knowledge of the Jovian clouds and the related issues
of atmospheric composition and its variability,
the atmospheric general circulation, and the
dynamical nature of the various kinds of giant
eddies. Galileo offers considerable advantages
over earlier investigations. First of all, the
probe data set is unique, since it is the first
acquired inside any of the outer planet atmospheres. However, care is required in its interpretation since the probe sampled only one
point in what is obviously a very inhomogeneous system. Next, the orbiting spacecraft is
close enough to the planet that it can resolve
features such as hot spots not only with its
camera, but also with infrared remote sensing
instruments. This offers the prospect of relatively cloud-free remote sensing of the atmosphere to a considerable depth, perhaps down
to the levels where aqueous clouds are expected to lie, where the pressure is several bars. It
3.21
Jupiter
also offers some unique observing geometry,
including infrared coverage of the night side of
Jupiter which is not accessible to Earth-based
telescopes, and high-resolution feature tracking over a range of zenith and phase angles to
obtain new information on the scattering properties of the cloud particles.
The science payload of Galileo includes the
first imaging spectrometer to be used on a mission to the outer planets. This device, called the
Near Infrared Mapping Spectrometer, NIMS
(Carlson et al. 1992), has allowed imaging of
Jupiter at 408 wavelengths from 0.7 to 5.2 µm
(figure 2). Spectra at each point can be inverted to retrieve vertical profiles of cloud opacity
and species abundances on Jupiter, with spatial
mapping of cloud-top temperature and morphology within belts and zones, as well as
localized features such as the Great Red Spot.
The solid-state imaging system (Belton et al.
1996) has a series of filters in the visible and
near-infrared which has been used to probe the
properties of the highest cloud and haze layers
(Banfield et al. 1998).
The purpose of this paper is to synthesize the
latest results with earlier observations and theoretical models to give an updated overall picture of the Jovian cloud systems. As we shall
see, there seem to be at least four layers of different condensed materials contributing to the
visual appearance of Jupiter, all highly variable
across the face of the planet. It is not possible
at this stage to characterize such a complex
arrangement fully or in detail, but the nature of
Jupiter’s clouds to first order is much clearer
than it was before Galileo.
Theoretical basis
Theoretical arguments provide a strong underpinning of the overall picture, by defining our
expectations of the overall elemental composition of Jupiter and hence of the abundances of
cloud-forming materials. The basis of current
thinking is that when the solar system formed,
the core of Jupiter formed by the initial accretion of planetesimals was large and cool
enough to retain all the material in the protosolar cloud, including the lightest elements,
hydrogen and helium. Thus, it should have the
same mix of elements as the Sun, modified,
possibly substantially, by the later accrual of
planetesimals and smaller solid debris in the
form of meteorites and comets.
A ball of gas the size of Jupiter will release
heat from its interior, due to fractionation of
heavier elements in the interior and overall
contraction. The energy budget of the planet
has been measured by airborne and spaceborne
radiometers (Hanel et al. 1981). The internal
power turns out to be a large fraction, about
70%, of the heat radiation which Jupiter
receives from the Sun. The release of such large
amounts of internal heat makes Jupiter deeply
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2 Simultaneous images of Jupiter at four infrared wavelengths (left to right, top to bottom: 1.61, 2.17,
3.01, and 4.99 µm), obtained by the Near Infrared Mapping Spectrometer (NIMS) on Galileo’s second
orbit of the planet in 1996. The difference in the appearance of the clouds is due primarily to the
wavelength dependence of the absorption properties of methane and ammonia in the atmosphere, and
to the transition from primarily reflected solar energy at short wavelengths to Jovian thermal emission
at longer wavelengths.
convective. Air generally rises in the cloudy
zones and descends in the dark belts, as discussed further below. The temperature as a
function of radial distance from the centre of
Jupiter can be calculated, and the expected
composition in terms of stable compounds can
also be worked out from the known mixture of
elements.
To first order, one expects that the abundant
elements will be present mostly in their fully
hydrogenated form (nitrogen as NH3, carbon
as CH4, and so on). It is straightforward to
consider reactions between them (H2S tends to
combine with NH3 to form NH4SH, for example) and calculate where on the profile these
compounds will condense.
The most recent study of this kind (CameronSmith 1998) confirms that the highest substantial condensate cloud on Jupiter should be
crystals of frozen ammonia, overlying solid
particles of condensed ammonium hydrosulphide, with a primarily aqueous cloud deeper
still. Such results are interesting, but do not
provide a definite solution since, among other
limitations, the method makes no allowance
for atmospheric motions – which move cloudforming materials in all directions, most critically upwards – for non-equilibrium chemistry,
or for heterogeneous processes. Still, they pro-
vide a framework in which to understand
experimental Jovian cloud studies.
Results from Galileo
The Galileo mission is not over, and much of
the data from the 16 orbits which had been
completed at the time of writing has still to be
analysed. However, preliminary results from
the probe and from the orbiter were published
nearly two years ago, and the full set of probe
results, plus the first papers on fully mature
orbiter analyses, have recently appeared (Journal of Geophysical Research, 25 September
1998). Thus, it is a good time to consider how
Galileo has revised our basic picture of the Jovian clouds and what questions remain.
The cloud structure on Jupiter is complicated. The easiest way to consider it further is in
terms of five different aspects, initially considered separately. These are: (i) The nature of the
hazes near the tropopause and above, which
form the uppermost layer or layers. (ii) The
main cloud decks which lie at pressures of
around 0.7 and 1.5 bars respectively. These
probably consist of NH3 and NH4SH and are
the main opacity sources accounting for the
variable appearance of Jupiter in the visible
and infrared. (iii) The deep cloud layer, which
is near the depth limit of detectability for
June 1999 Vol 40
Jupiter
0.01
stratospheric haze
pressure (bars)
0.10
haze, r = 0.5 µm
NH3, r = 0.75 µm
NH4SH, r =
1.00
10.00
100
5 µm contribution function
150
200
250
300
temperature (K)
remote sensing, and which is predominantly
aqueous. (iv) The horizontal variability in all
of the above, and finally, as a special case of
(iv), (v) the cloud structure in compact features
like the giant eddies, especially the GRS.
Hazes
The term “haze” is used to describe particulate
layers which are extensive horizontally, and
often vertically as well. Haze layers are generally relatively featureless compared to clouds,
with more clear space between the droplets or
ice particles, and have less total optical depth.
Jupiter is hazy at heights well above the main
cloud decks. This is not unexpected since the
atmosphere contains a substantial amount of
methane, which at high levels will be subjected
to dissociation by solar ultraviolet radiation,
followed by recombination into higher hydrocarbons which condense into liquid droplets.
This type of process is thought to go on in the
upper atmospheres of all the giant planets, and
something similar, but involving more nitrogen
compounds, is probably responsible for the
deep haze on Titan which tends to obscure the
surface of the satellite and gives rise to its
orange coloration. The term “tholins” (meaning
muddy) is sometimes used to describe the composition of this range of hydrocarbon and other
condensates of indeterminate composition, and
of the material which is produced in the laboratory by simulating the atmospheric processes
with UV lamps or electrical discharges.
A preliminary analysis (Banfield et al. 1998)
of the Galileo SSI multispectral observations at
visible and near-infrared wavelengths suggests
that the haze on Jupiter is made up of two separate layers, one high in the stratosphere, identified with the methane-derived tholin haze
described above, and another in the upper troposphere. The visible optical depth of the latter
has been estimated by the SSI investigators to
be in the range from 2 to 6, as opposed to only
about 0.1 for the stratospheric haze. These
numbers are probably too high, because the
SSI analysis assumed perfectly conservative
June 1999 Vol 40
0.45 µm
–50 µm
350
400
3 Vertical
temperature profile
and cloud
locations on
Jupiter, as
retrieved from a
NIMS spectrum at
a single location in
the North
Equatorial Belt,
near where the
Galileo probe
entered some six
months earlier
(after Irwin et al.
1998).
scattering by the haze particles. The actual
value of the single scattering albedo is currently unknown, but is likely to be less than 100%,
in which case the optical depth estimates
would decrease.
It can be speculated that this lower haze is
made up of products of the photolysis of
ammonia rather than methane, since it lies in
the stably stratified region just above the top of
the tropospheric convection cells and just
above the thick ammonia cloud. The ammonia
gas required to produce it could penetrate into
the stratified region as a result of breakthrough
convection similar to that which injects wet
tropospheric air into the lower stratosphere on
the Earth.
It has been argued on chemical grounds that
the principal component of the processed and
recondensed ammonia would probably be
hydrazine (Strobel 1973). However, hydrazine
exhibits in the laboratory the strong spectral
signature near 3 µm which is also characteristic of ammonia itself. Galileo infrared spectra
indeed show this feature in the main ammonia
cloud and from the broadened shape of the
absorption feature, a large particle size is
inferred. However in the overlying upper tropospheric haze, where small particles are
required by the reflectance spectra, small
hydrazine or ammonia particles do not seem to
be suitable candidates for the primary component since would both contribute sharp, narrow absorption features in the 3 µm region
which are not seen.
A very recent study (Guillemin et al. 1999)
offers a possible explanation in that it suggests
that the main product of the combined photochemistry of NH3 and PH3 should be not N2H4
but P2H4. It remains to be shown whether a
composition of condensed P2H4 and higherorder products (“pholins”) is consistent with
the spectroscopic observations.
The visible clouds
The white cloud which forms many of the features observed on Jupiter is condensed ammo-
nia ice. We know this because it occurs at the
level (about 0.7 to 1.0 bar) where the
temperature is appropriate for condensation of
this species in thermodynamical models such as
those discussed above. It also has the right spectral properties (Encrenaz et al. 1996). The
ammonia cloud is optically thick in the cloudy
zones in the visible and near-infrared, but partially transparent at wavelengths of 5 µm and
longer. This is the cloud which is primarily
responsible for the visible cloud structure on
Jupiter in ground-based and spacecraft imaging
experiments, and SSI observations place the
features in a narrow height range near 750 mb.
The primary evidence that the ammonia
cloud is transparent at thermal infrared wavelengths is that the colour temperature of the
emission from the cloudy zones, as measured
by NIMS, is inconsistent with a source as cool
as the ammonia cloud (Drossart et al. 1998).
The 5 µm window spectra are typically biased
more towards the blue end of the spectrum
than the Planck function for the temperature of
the ammonia cloud would predict, even on the
night side of the planet. Most of the radiance
being measured by the spectrometer must
therefore have originated below the ammonia
cloud, and must propagate through it. In addition, an anticorrelation is observed by NIMS
between the brightness in the reflected continuum and in the absorption bands of ammonia,
that also suggests that the emission in the belts
is being attenuated by the ammonia ice cloud.
Histograms of the brightness of Jupiter at
5 µm on the night side (Cameron-Smith 1998)
show a narrow peak at 165 K (1.0 bar) and a
much broader one at ~240 K (3.5 bar), corresponding essentially to the zones and belts
respectively. Day–night differences at 5 µm
show that reflected solar as well as emitted thermal radiation contributes to the emission on the
day side. Spectral fits to the night side 5 µm flux
(Drossart et al. 1998) show that it is due to deep
thermal emission attenuated by the cold ammonia cloud and that the transmission of the latter
is about 0.002, and its reflectivity about 0.15 at
5 µm. It must lie at p < 0.79 bar or its own emission would become significant enough to
destroy the fit. The corresponding values at 4 to
4.5 µm are reflectivity ~0.04 and p < 0.69 bar.
The histograms show that these values apply
over a wide area on Jupiter. Cameron-Smith
(1998) has speculated as to how the cloud
opacities in the zones should come to have
these preferred values. He showed that the
limb darkening in the zones is too flat to correspond to a purely absorbing cloud layer; that
would give rise to a dependence of the emitted
intensity on the secant of the emission angle,
which is not observed. He concluded that the
cold (NH3) cloud cannot be a pure absorber
but could be strongly scattering or contain
“microholes” produced dynamically, perhaps
3.23
Jupiter
by the Jovian analogue of “Rayleigh–Benard”
convection cells on the Earth.
Tb = 165 K
Tb = 240 K
zone
belt
Tb = 275 K
Deep clouds
Deeper in the atmosphere, below the ammonia
cloud, thermochemical models lead us to expect
that an ammonium hydrosulphide (NH3.H2S or
NH4SH) cloud will condense at around
1.5 bars and 180 K, the precise level depending
on the deep-atmosphere abundances of NH3
and H2S among other factors. Retrievals of the
vertical run of cloud opacity which fits the
NIMS 5 µm spectra described above place the
lower cloud opacity at about the level the models predict for ammonium hydrosulphide (figure 3). This work also shows that the NH4SH
cloud has some large particles, and that it, and
not the ammonia ice cloud, is the main variable
opacity source on Jupiter at thermal infrared
wavelengths. This in turn suggests that the NH3
and NH4SH clouds tend to occur in the same
locations, since the features in visible and
infrared (5 µm) images are anticorrelated.
The water cloud predicted by the thermochemical models would, on the temperature
profile measured by the Galileo probe, lie at a
pressure of around 5 bar. This is approaching
the maximum depth sounded by NIMS, and
this is achieved only in hot spots where the
water cloud may be reduced or eliminated by
strong vertical motions. In one particularly
transparent region near the Great Red Spot,
however, the SSI team made a rare detection of
a very deep cloud roughly 1000 km across and
lying at pressures perhaps as great as 4 bar,
which they tentatively identify as a water cloud
since no other known species on Jupiter would
condense at this level.
Horizontal variability
Discussion of the Jovian clouds has always discriminated between the relatively cloudy zone
regions and the more cloud-free belts. As noted
above, the reason these exist is because of the
release of internal energy, which has to be
advected up to levels near the main (ammonia)
cloud-tops, where it can radiate to space. The
process forms planetary-scale convection cells,
which have radial symmetry as a consequence
of the rapid rotation of the planet. The reason
for the number and spacing of belts and zones,
which are sufficiently invariant that the most
prominent have received descriptive names
(e.g. North Equatorial Belt, South Temperate
Zone), is still a matter for conjecture, primarily in the form of detailed fluid dynamical modelling (e.g. Williams 1979).
The rising part of each cell consists of air carrying condensates such as water and ammonia
which condense at the appropriate temperature
and pressure for that species, forming the zone
clouds. The descending branch of each cell is
relatively depleted in volatiles and is less cloudy
3.24
temp.
(K)
press.
(bars)
110
0.05
‘tholin haze’
0.5/0.04
110
hot spot
0.5/0.04
0.5/0.04
0.18
‘pholin haze’
135
0.7
20/6
2.2/0.07
2.2/0.07
160
1.4
?/?
2.0/0.07
2.0/0.07
180
1.6
?/?
1.2/1.3
0.0/0.0
~275
~5?
?/?
?/?
0.0/0.0
ascending air
ammonia
cloud
hydrosulphide
cloud and haze
water cloud
descending air
4 A model of the Jovian cloud structure and properties based on the available data and simple
physical–chemical models. The approximate temperature and pressure levels where the principal
cloud layers occur are shown to the left, while the representative visible (0.7 µm) and infrared
(5.0 µm) optical depths are marked on or below the layers themselves. The quantity Tb shown at the
top is the typical value of the brightness temperature at 5 µm for each type of region.
as a result, forming the belts. To these two
kinds of region we must add a third: the hot
spots. These are regions where the local meteorological conditions are such that the column is
nearly completely free of cloud, at least down
to the level where water condenses, and in some
cases deeper than that. The detection of a deep
water cloud by the Galileo imaging team, while
the probe, which descended in a hot spot,
found no water cloud at all, implies horizontal
inhomogeneity in the deeper clouds also. This is
not surprising since the convection cells on
Jupiter must extend to a great depth (although
not necessarily as single cells; chains of cells
could also do the job and may be more stable).
Figure 3 shows a five-component model of
the cloud system with typical values of the
cloud opacity in each vertical component for
the three types of region: zone, belt, and hot
spot. However, as we have already seen, this is
still a gross over-simplification. The atmosphere is very inhomogeneous horizontally as
well as vertically, even within a given region.
This is illustrated by plotting the centre-to-limb
variation in radiance within a zone, for example, as shown in figure 5. This figure shows
how futile it is in this case to try to fit centreto-limb scans to a simple model of Jupiter,
although this was often tried in the past with
lower-resolution Earth-based data. It is better,
but more difficult, to use observations of the
same location from different points on the
orbit, thus varying the scattering angles while
viewing the same atmospheric column.
The SSI data detected no lateral structure in
either of the haze layers, except at high
latitudes, over the poles. However, in the NIMS
data a wide band of high haze is clearly seen
centred over the boundary between the Equatorial Zone and the North Equatorial Belt, about
0 to 9°N (figure 2b). While it is not surprising
that haze is thickest over the poles (where the
high-altitude air tends to be stagnant and precipitation of energetic particles occurs) and
over the equator (where the solar intensity and
therefore the production of tholins is greatest),
we must again invoke some unknown property
of the dynamics of the atmosphere, this time at
a relatively high level, to explain why this is not
centred over the equator.
The giant eddies
The most common giant eddies are the white
ovals, which are all regions of enhanced cloud.
The NH3 cloud is optically thick in these spots,
as presumably are the ammonium hydrosulphide and water clouds if we could see
them. The clouds themselves are generally several kilometres higher in altitude, and the SSI
data shows that the hazes, too, are elevated
over the GRS and the white ovals.
The best-studied and probably the most interesting dynamical feature on Jupiter, with the
possible exception of the basic belt-zone structure, is the Great Red Spot. Figure 6 shows a
schematic of a proposed structure for the GRS,
June 1999 Vol 40
Jupiter
radiance (µWcm–2 Sr–1 µm–1)
0.40
0.30
0.20
χ = 0.01
0.10
χ = 10
χ=3
χ=1
χ = 0.03
χ = 0.3
0.00
0
20
60
40
emission angle (degrees)
χ = 0.1
80
5 Jovian limb-darkening curves, showing the thermal emission from Equatorial Zone clouds in a
wavelength band from 4.6 to 5.2 µm. The broken curves are NIMS measurements obtained within a
few hours of each other at slightly different latitudes, all within the range 2 to 13°N. The solid lines
are theoretical limb-darkening curves for a simple cloud model, consisting of a single layer of cold,
grey cloud. The large deviation of the measurements from the models, and from each other, show how
inhomogeneous the Jovian atmosphere is, even within a region which appears nearly uniform in
visible images (Cameron-Smith 1998).
6 A conceptual model of the structure of the
Great Red Spot based on Galileo observations
and an analogy with terrestrial hurricanes.
which has some structural features analogous
to terrestrial hurricanes, in that a relatively narrow column of rising, moist air spreads out into
a canopy with spiral arms of cloud extending
above the surrounding cloud tops. In the highest resolution NIMS images at 5 µm, the gaps
between the cloud spirals can be seen to be
bright, due to emission from considerable
depths, indicating clear regions of atmosphere
beneath the overlying canopy. From the size of
the GRS and its unique colour, it seems likely
that the rising moist air which forms the Spot
originates from a considerable depth, deeper
than any of the cloud layers already discussed.
The condensed material in the cloud will then
be a mixture of ammonia, ammonium hydrosulphide, water and other materials possibly
including phosphorus. The last of these is mentioned particularly because, in addition to being
present in large amounts on Jupiter, as evidenced by the observed abundance of phosphine, phosphorus has the property of being
intrinsically red, and truly red condensate cloud
materials are not common in nature.
Dry, cloud-free air descends around the spot,
and the cloud cap is tilted, presumably as a
June 1999 Vol 40
7 The detailed morphology of the Great Red
Spot inferred from NIMS observations (Baines
1996), showing the spiral structure in this
anticyclonic feature. The plot covers 50° in
longitude, 30° in latitude, and approximately
20 km in vertical relief (Baines 1996).
result of interactions between the vortex and
the prevailing wind in which it is embedded.
Figure 7 shows a cloud-top height map
inferred from NIMS data, from which it can be
seen that the cloud forming the visible top of
the GRS is about 10 km higher at one side than
the other, a significant distance in terms of vertical structure (about half of one pressure scale
height) but small, of course, compared to the
mean radius of the GRS, which is somewhat
larger than that of the Earth.
Outstanding questions
Much progress on deciphering the Jovian
clouds has been made with Galileo, but some
of it is quite speculative still and many questions remain. Perhaps outstanding among
these, other than almost everything to do with
the atmospheric dynamics, is the fact that we
still have no definite explanation for the
colours. The white clouds are surely ammonia,
but the yellows and browns are harder to
account for since pure ammonium hydrosulphide is white. Of course, none of the clouds
is likely to be pure, and there are many
coloured compounds of sulphur, including
allotropes of the element itself, which could
account for the colours on Jupiter if they were
present due to photochemistry or other nonequilibrium processes. However, we are not at
present much further forward on this point
than we were a decade or more ago.
Another major question remaining is that of
the Jovian dynamical meteorology. We see, and
have begun to characterize, the massive global
variability of cloud features and aspects such as
their latitudinal and temporal variations. However, we have no comprehensive theory of the
general circulation on Jupiter, nor do we understand the origins and true natures of the giant
eddies and the hot spots, to name only the two
most conspicuous aspects of Jupiter’s weather.
The variability within the cloud structures
which we have attempted to categorize here in
an averaged sense is very large, some of it
appearing organized and some chaotic. The
nature, and perhaps the origins, of these fluctuations will become clearer as Galileo extends its
coverage in space and time and more of the data
is analysed. The Composite Infrared Spectrometer on Cassini will obtain coverage of Jupiter
during its fly-by in December 2000. Its spectral
resolution and range are both superior to NIMS,
being designed to study atmospheric chemistry
on Titan, and should help to further unlock the
secrets of Jupiter’s colourful cloud chemistry. ●
Fred Taylor and Patrick Irwin are at the Dept of
Atmospheric, Oceanic & Planetary Physics, Oxford
University. The authors acknowledge the contribution of their colleagues on the Galileo Near Infrared
Mapping Spectrometer Team, especially Principal
Investigator Dr R W Carlson of the Jet Propulsion
Laboratory, California Institute of Technology; Dr
A L Weir and Dr P Cameron-Smith, Oxford University; Dr K H Baines, JPL; Drs T Encrenaz, P
Drossart, and M Roos-Serote, of the Observatoire
de Paris, Meudon. They also thank all of the other
Galileo scientists and engineers, especially Project
Scientist Dr T V Johnson of JPL.
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Roos-Serote M et al. 1998 J. Geophys. Res. 103 E10, 23,023.
Cameron-Smith P J 1998 D.Phil. Thesis, University of Oxford.
Baines K H 1996 Paper presented at the American Astronomical
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