This chapter will discuss the global distribution of effective temperature
on each of the icy satellites. Again, the main purpose is to present the
data, and interpretation will largely be reserved for subsequent chapters,
especially Chapter 7.
Determining Surface Thermal Conditions from Observed Effective Temperatures
Below I list factors affecting the observed effective temperature that
should be kept in mind when translating observed TE's
into actual surface temperatures, and when comparing Voyager 1 and Voyager
2 temperature data.
Local Temperature Contrasts
As the previous chapter demonstrated, it is probably not meaningful to
ascribe a single kinetic temperature to a region of a satellite, as the
slopes of the IRIS spectra probably indicate the ubiquitous presence of
local temperature variations with magnitudes of several tens of degrees.
What I do in this chapter is map the regional variations in effective temperature,
TE,
determined by the least-squares fitting of the spectra, which is an indication
of the total amount of thermal radiation emitted by a region.
Beaming
Planetary surfaces are known to emit thermal radiation in an anisotropic
manner, so that the effective temperature of a given region varies with
viewing geometry. This effect is known as `beaming'. It should be taken
into account when mapping surface temperatures, especially with the Voyager
IRIS data set in which different regions are observed with very different
viewing geometries. Appendix A therefore describes the beaming observed
on the Galilean satellites and compares it to that on the Moon, the only
airless body for which there is comparable disk-resolved thermal emission
data. The conclusion is that although crude measurements of the beaming
on Ganymede and Callisto are obtainable from the IRIS data, and are comparable
to anisotropies observed on the Moon, the data are insufficient to allow
correction of the observed TE's for beaming effects.
There are likely to be discrepancies of a few oK in TE
measurements of the same region over the range of viewing geometries found
in the IRIS data, higher phase angle observations being colder.
Emissivity
The effective temperature TE is related to the actual
surface temperature T of an isothermal surface by the emissivity
where R is the radiant flux integrated over all wavelengths
and \ep\ is the Stefan-Boltzmann radiation constant. Therefore
For the undoubtedly non-isothermal surfaces of the Galilean satellites
this equation is too simplistic, as there is no single T. However,
it should be kept in mind that if the \ep\ value of around 0.94 determined
by the least squares fitting in Chapter 4 is a real emissivity, mean
surface temperatures will be 1.6% higher than TE, and a surface
with TE = 130o K will have a mean temperature closer
to 132o K. This emissivity effect is in addition to the viewing
geometry effects just discussed.
Distance from the Sun
Because of the eccentricity of Jupiter's orbit, its distance from the sun
changed between the two Voyager encounters. At the Voyager 1 encounter
on 5 March 1979 the solar distance was 5.29 A.U., and at the Voyager 2
encounter on 9 July 1979 the distance was 5.33 A.U. (from the American
Ephemeris, 1979). The mean solar distance is 5.20 A.U. This change also
has an effect on surface temperatures. Equilibrium temperature TEQ
at
solar distance D A.U., with solar incidence angle i
and bolometric
albedo A is determined by equating radiated thermal and absorbed
solar radiation:
where FS1 is the solar constant at 1 A.U. So, other things
being equal, TEQ is proportional to D-0.5. The same
relationship with solar distance is essentially true for nonequilibrium
temperatures resulting from a surface with non-zero thermal inertia. Therefore,
to be compared with Voyager 1 temperatures, Voyager 2 temperatures should
be increased by a factor of $\sqrt{(5.33/5.29)}$, or 1.004. A temperature
of 130.0o K seen by Voyager 2 is equivalent to a temperature
of 130.5o K seen by Voyager 1, or a temperature of 131.6o
K when Jupiter is at its mean solar distance. The correction between the
two encounters is small enough compared to other effects that I have not
applied it, in the interests of simplicity.
Global Lightcurves
As mentioned in the Introduction, all the icy Galilean satellites have
significant orbital lightcurves, Europa and Ganymede being 34% and 15%
brighter, respectively, on the leading hemisphere, and Callisto being 13%
brighter on the trailing hemisphere (all values for the V filter). For
constant surface thermal inertia, this will introduce temperature variations
that may show up as discrepancies in the Voyager 1 and 2 effective temperatures
for a given latitude and local time. It is interesting to determine the
approximate magnitude of these temperature variations.
Equilibrium temperature varies as (1-A)0.25 (Equation 9,
above). Albedo variations therefore have more effect on temperature on
bright surfaces. If the dark-hemisphere bolometric albedos of Europa, Ganymede,
and Callisto are, say 0.55, 0.30, and 0.13 (c.f. Squyres and Veverka, 1981;
Buratti and Veverka, 1983), and the bright-hemisphere albedos are thus
0.737, 0.345, and 0.147, then dark-hemisphere equilibrium temperatures
will be warmer by 14.3%, 1.7%, and 0.5% respectively. These values are
from the hemispherically averaged lightcuves so local variations will be
somewhat larger. The effect on Europa, with its high albedo and large lightcurve,
is considerable, while on Callisto there is almost no temperature variation
due to the lightcurve. If the Ganymede lightcurve is due largely to the
larger amount of dark cratered terrain on the trailing hemisphere (as suggested
by Johnson et al (1983)), lightcurve effects will not be apparent
if temperatures are compared only between similar terrains.
Distribution of Effective Temperatures
Figures 13 and 14 are
isotherm plots of the effective temperature as a function of latitude and
time of day on the three icy satellites. Similarly, Figures
16 and 17 show equatorial TE
profiles versus time of day. On Fig. 16, showing
Ganymede, terrain types are distinguished, and spectra containing more
than 25% bright crater material are excluded. As there is a large overlap
in the latitude / time of day coverage of Ganymede from the two spacecraft,
there is a separate plot for each encounter in both
Figs. 13 and 16. On Callisto there is almost
no overlap, and data from both encounters are shown on the same plots.
For Europa, of course, there is only Voyager 2 data and only one plot is
needed in each figure.
The isotherms on Figs. 13 and 14
are hand-drawn using the values of TE at each of the
spectrum locations shown. For Europa and Ganymede the isotherms are almost
exact (i.e. almost all spectra lie between the appropriate isotherms).
On Callisto there is more scatter in effective temperatures and more compromises
were necessary in drawing the isotherms: see the Callisto discussion below.
I summarize the diurnal equatorial temperature distributions in Table
III. The maximum observed effective temperature is given, along with its
local time in degrees. The temperatures at sunset, midnight, and sunrise
(local time = 270o, 360o, and 90o) are
also shown. I give more detail on the circumstances of these observations
below.
Table III. Equatorial Effective Temperature Distributions
The `S/C' column gives the spacecraft (Voyager 1 or 2). Local time of temperature
maximum is in degrees. `--' indicates a lack of coverage. Neither spacecraft
precisely measured or located the maximum temperature on Callisto due to
incomplete coverage. Therefore the temperatures and local times of the
warmest spectra from each Callisto encounter are given.
Effective Temperature, oK
Satellite
S/C
Max.
(Time)
Sunset
Midnight
Sunrise
Europa
V.2
-
(-)
91
-
-
Ganymede
V.1
140
(210)
107
94
-
Ganymede
V.2
147
(190)
105
94
-
Callisto
V.1
156
(190)
<89
80
-
Callisto
V.2
158
(180)
-
-
75
Ganymede
The Voyager 2 temperature distribution, being simpler than Voyager 1, will
be described first.
The daytime temperature distribution in the Voyager 2 Ganymede data
shows strong albedo control, dark cratered terrain being warmer than bright
grooved terrain with the same illumination geometry. There is also a conspicous
cold spot associated with the crater Osiris and its very bright ray system,
up to 15oK colder than its surroundings. The albedo control
is seen best in Fig. 15, where isotherms are overlayed
on an actual image. It can also be seen on the diurnal profile in Fig.
16, though the full extent of the light/dark terrain temperature contrast
is not visible because none of the low-latitude warm spectra contain `pure'
light terrain. The terrain difference appears to decrease towards the terminator.
The subsolar effective temperature, on dark terrain observed at 33o
solar phase angle, is 147oK. The Voyager 2 nighttime temperature
distribution is very smooth, about 105oK at sunset on the equator
(on bright grooved terrain) and slowly cooling through the night, reaching
about 94oK at midnight. The region observed at night by Voyager
2, visible in far-encounter images, is mostly bright grooved terrain: no
nighttime field of view has more than 50% dark terrain.
Figs. 13 and 16 show
that Voyager 1 equatorial temperatures are comparable to those from Voyager
2 in the late afternoon (local times 225o--270o),
but at earlier local times they are cooler than Voyager 2. In fact it seems
that the diurnal temperature profile is more or less flat at local times
between 200o and 225o, whereas there is an appreciable
drop in surface temperature over this range of local times in the Voyager
2 data. The reason for this difference is not obvious, though it is possibly
due to beaming effects, as the Voyager 1 data was taken at higher phase
angles and the near-subsolar spectra are closer to the limb, where thermal
emission anisotropies might be more pronounced. Alternatively, there may
be a higher surface thermal inertia on the Voyager 1 hemisphere, which
would reduce the maximum diurnal temperature and shift it to a later local
time.
The maximum Voyager 1 effective temperature is 140oK at
a local time of 210o and a solar phase angle of 50o.
The equatorial temperature at sunset is about 107oK (on a mixture
of terrains) and the equatorial midnight temperature is 94oK,
on mostly dark terrain, though the nighttime temperature distribution is
not smooth, as discussed below.
Less albedo control is visible in the Voyager 1 than the Voyager 2
data, except that the warmest temperatures occur south of the equator in
a region that is mostly dark terrain. Albedo contrasts between the grooved
and dark terrains appear to be less marked than on the Voyager 2 hemisphere
(see, for instance, the cylindrical projection global mosaics of Johnson
et
al, 1983: the very dark Perrine Regio, seen by Voyager 1, was too close
to the limb to be seen reliably by IRIS). The subdued albedo contrasts
on this hemisphere would result in smoother isotherms.
There are two anomalously cold regions in the Voyager 1 data, additional
to the depressed early-afternoon temperatures already mentioned. The first
is north of the equator in the early afternoon (local time 200o),
outlined by the downward-dipping 130oK isotherm in Fig.
13. It includes dark terrain and thus is probably not albedo-related.
It was seen in several spectra and is probably real, though all these spectra
are rather close to the limb and it is possible that beaming effects are
responsible for the low thermal emission. If this is a true region of depressed
surface temperatures, it may be due to anomalously-high thermal inertia.
It is centered on 20o N, 10o W. The second cold spot
is in darkness, and is outlined by the 90oK isotherm near the
equator at a local time of around 330o in Fig.
13. This thermal anomaly is definitely real: it appears on several
spectra from independent scans with differing geometries. Again, it is
not due to albedo: the cold region includes more dark terrain than the
warmer areas to the north of it. If it is controlled by thermal properties
it is a low thermal inertia region, cooling more rapidly than its surroundings
during the night. It is centered on 0o N, 250o W.
Callisto
The abovementioned large scatter in effective temperatures on Callisto
is perhaps due to the wide range of viewing geometries (which will enhance
beaming effects) and the close spacing of the isotherms seen on Fig.
14 (which will increase the effect of pointing errors on TE).
Even so, few spectra on the Callisto plot have TE more
than 5oK different from the value obtained by interpolating
nearby isotherms. In the area of overlap of Voyager 1 and 2 coverage of
Callisto there are discrepancies of rarely more than 5oK, Voyager
1 spectra tending to be colder.
A much more complete temperature distribution is available for Callisto
than for Ganymede, because of the complementary nature of the two flybys.
The maximum effective temperature of 158oK occurs very close
to the subsolar point and daytime isotherms are quite symmetrical about
the noon meridian, except that the equatorial sunset temperature of about
89oK is 14oK warmer than the sunrise temperature
(75oK), with a slow cooling during the night. Suprisingly, Callisto
is colder than Ganymede at night despite its lower albedo and higher subsolar
temperature, and as a result the daytime temperature gradients across the
surface are much steeper than on Ganymede.
Chapter 7 shows that the late afternoon temperatures on Callisto seen
be Voyager 1 are too cold to fit with a thermophysical model that matches
the rest of the surface. This may indicate anomalous thermal properties
in this region (around 0--30o N, 330o W), or possibly
unusual beaming effects, as this region was observed at large emission
angles.
Europa
The small amount of data available is still sufficient to place interesting
constraints on Europa's temperature distribution. The sunset temperature
of around 91oK is comparable to that on the much darker Callisto,
and what can be seen of the daytime temperature gradient is much shallower
than Callisto's (i.e. afternoon cooling as a function of local time is
less rapid than on Callisto). There is measurable cooling during the early
part of the night. The range of longitudes included in this data is 90--160o
W,
in a bright portion of Europa's hemispherically-averaged lightcurve (Morrison
and Morrison, 1977).
