Bolometric Albedo. (A). The ratio between the total reflected
sunlight (all wavelengths, all directions) and the total incident sunlight
on a planetary surface. The relevant albedo for thermal calculations.
Geometric Albedo. The ratio between the disk-integrated brightness
of a body at zero solar phase angle and the brightness of a normally-illuminated
A=1
Lambert disk of the same diameter.
Normal Reflectance. The ratio between the brightness of a normally
illuminated surface and the brightness of a normally-illuminated A=1
Lambert surface.
Opposition Surge. The sharp increase in the intensity of sunlight
reflected from a planetary surface at very small solar phase angles.
Local Time. Degrees of planetary rotation since local midnight,
e.g.for zero obliquity, sunset occurs at a local time of 270o.
Sometimes expressed using a `terrestrial' clock, in which case sunset occurs
at 6pm.
Incidence Angle. (i). The angle between the local surface
normal and the direction of the sun.
Emission Angle. (e). The angle between the local surface
normal and the direction of the observer.
Thermal Meridian. The line on a planetary surface for which
the directions of the sun, observer, and surface normal are all coplanar.
Radiance. The intensity of thermal emission (expressed as energy
per unit time, per unit solid angle, per unit area, per unit wavelength
or wavenumber).
Planck Curve, Blackbody Curve. ($B(\lambda,T)$). The thermal
radiance at wavelength $\lambda$ from a perfect radiator (a blackbody)
at temperature T, approximated by real planetary surfaces. The form
of the curve is
$$ B(\lambda,T) = {2 h c^2 \over \lambda^5} \left[\exp\left(h c \over
\lambda k T \right) - 1 \right] ^{-1} \eqno(35)$$
where $h$ is Planck's constant, c the speed of light, and k
is the Boltzmann constant.
Emissivity. (\ep). The efficiency of thermal radiation: the
ratio of the observed radiance to the radiance from a blackbody with the
same actual surface temperature. Emissivity can be specified as a function
of wavelength, or given as a wavelength-integrated value relating the total
thermal radiation to the expected blackbody radiation.
Equilibrium Temperature. The temperature at which the thermal
radiation from a planetary surface equals the absorbed solar radiation.
Brightness Temperature. (TB). The temperature
of a blackbody emitting the observed thermal radiance at the specified
wavelength.
Effective Temperature. (TE). The temperature
of a blackbody emitting the observed thermal radiance integrated over all
wavelengths.
Solar Constant. (FS1). The wavelength-integrated
energy flux from the sun (per unit area, per unit time) at 1 A.U\null.
The value used in this dissertation is 1.374 X 106 erg
cm-2 s-1 (Hanel et al., 1981).
Spectrum Slope. In this dissertation I use `spectrum slope'
to denote the increase in TB with increasing wavenumber
(decreasing wavelength), seen in all IRIS satellite spectra. I have adopted
the difference in TB between 500 cm-1 and
250 cm-1 as a standard measure of this slope.