# GLOSSARY

This glossary defines various terms that I use in this dissertation, and also gives some basic equations and values of constants.

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.