While alike the other Jovian planets in many ways, Uranus has
a curious feature that separates
it from the other Jovian planets, and as such hinders our
understanding of the system. Jupiter, Saturn, and Neptune all have
heat flows comparable to that absorbed from the Sun. Uranus however,
has an almost negligible internal heat flow, with total emission
equaling 
times the absorbed insolation, whereas Jupiter,
Saturn, and Neptune have energy balances of 
,
, and 
respectively (Pearl et al.
1990). In fact although
it is a third closer to the Sun than Neptune, within error bars the
two have identical effective temperatures. The effective temperature,
which is the temperature a blackbody would have that emits the same
integrated flux can also be used as a measure of energy output.
Many processes have
been proposed to try and explain this feature. One model in
particular suggests that the internal heat flow can be disrupted by layering of
convection cells caused by internal stratification. It has been known
for many years that this takes place in the Antarctic (Neal et al.
1969). Stevenson has shown that significant stratification can
effectively stifle heat flow (1985). He likens it to a ``Salt
Model,'' comparing the process to dropping salt into water. Even
in water not entirely saturated it is possible to form residue on the
bottom with
a large enough salt crystal if the time required to fall is much
smaller than the time required to diffuse. Stratification results,
and instead of one large convective zone, many small zones are created
and stabilized if the gradient in mean molecular weight is greater
than the temperature gradient. All modeling done to date has
considered the envelope of the planet to be one convective,
but recently it's been suggested that this stratification may
be trapping the outward flux of Uranus. In effect only the
outer third of the interior may be involved in the convective transport.
The last major observations in the thermal infrared of Uranus were
conducted in 1985, showing evidence for unexplained
stratospheric heating (Orton et al. 1987). A year later,
the Voyager 2 IRIS experiment produced spectra
between 200 and 400 cm
. These measurements gave a mean
stratospheric temperature of 55 K, with a maximum of 57 K at the
equator, and minima of 51 K at 
. However these
measurements as well as those both Orton were taken nearly 10 years
ago, in a different Uranian season. Since then the latitude of the
sub-earth point has changed by 30
. Because the thermal emission
time constant of the atmosphere is comparable to orbital period of the
planet, temperature changes tend to lag a season behind. The previous
observation took place at a sub-earth point of 
, very near
the summer solstice for the southern hemisphere. Although the
difference in techniques prohibits side-by-side comparisons, three
decades of ground-based work suggests symmetry in reflectivity about
the equator (Pearl et al. 1990). While over a Uranian period the
polar regions receive greater insolation than the equator,
calculations by Wallace predict seasonal polar variations of 5 K. The
current sub-earth point of 
will allow these theories to be tested.
Current thermal evolution models predict a temperature and thus luminosity significantly higher than the current values for Uranus and Neptune. The establishment of an effective evolution model requires a solution to the heat flow problem. While ground-based and Voyager observations are useful for constraining the atmospheric conditions, the last radiative-convective modeling was done in 1986 before the Voyager encounter (Appleby 1986). It has been shown that to explain the stratospheric heating that nearly 15% of the solar energy absorbed is deposited in the stratosphere. Appleby's model suggests that haze in the atmosphere is responsible, while recent work by Marley has shown this cannot be the case (1995).