Magellan collected measurements of the passive thermal emission from the surface of Venus at the same wavelength (12.6 cm) and viewing geometry as the synthetic aperture radar (SAR) [Pettengill et al., 1992]. These data have a low spatial resolution (20-90 km) but provide a useful guide to the dielectric properties (and thus the density or metallic mineral content) of the surface. Maps of the horizontal-polarized emissivity, Eh, resampled to a 4.6-km grid size, are on the GxDR CD-ROMS. It should be kept in mind that these maps are assembled using a weighted average of measurements, so the specific value of Eh at a given point on the ground should not be taken as precise. The original footprint values can be read from the ARCDR CD-ROMS by using the MGMDQE program available over Internet from MIT (app. 4).
If a surface is perfectly smooth, the emissivity will be the complement
of the Fresnel reflection coefficient, R, for the same polarization
and incidence angle (
)
 | (5) |
and the dielectric constant, 
,
can be found (by iteration) from
 | (6) |
 | (7) |
 | (8) |
where the h and v subscripts refer to horizontal and vertical polarizations. It is important to note the difference between the viewing geometries of the radiometer (25-45o) and the altimeter (within 15o of nadir); the emissivity should not be treated as the complement of the nadir reflectivity.
Table 3 presents values of
for a range of incidence angles and Eh assuming a plane interface,
which may be used to estimate the dielectric constant of many
plains surfaces to a reasonable accuracy. As the surface becomes
rougher (either at tens of meters scale or at wavelength scale),
the emissivity tends to pull toward the average of the values
for the two polarizations above. If we assume that this average
represents the emissivity of a completely rough surface, then
a second estimate of the dielectric constant can be made which
bounds the possible range for that terrain unit. These rough-surface
dielectric values are also shown in table 3.
Such estimates are not necessarily exact, because the rough-surface
estimate is not rigorous and some areas may have a mixture of
soil and rock. For the majority of surfaces to be mapped, however,
these values are a useful guide to the properties of the terrain
[Campbell, 1994].
The emissivity and the smooth- and rough-surface estimates of
dielectric constant should be provided as additional information
for each major map unit; the program described in Appendix 2
calculates these values. For all the ancillary data values derived
from the GxDR files, no standard deviation should be quoted, as
these values are weighted averages of the original footprint values
and the degree of oversampling varies with location on the planet.
The minimum and maximum of each parameter over a sample region
may be of value, so these ranges are included in the anc_data
output.
Terrestrial basalts have dielectric constants that depend largely on the rock density (fig. 5), with powders having values of ~2 and solid rocks reaching values of 9 [Ulaby et al., 1988]. Lava flows, crater ejecta blankets, highland areas, and plains on Venus all exhibit variability in their dielectric properties, possibly indicating changes in density, iron/titanium content, chemical alteration, post-emplacement formation of soil, or a combination of these factors. In the absence of other types of stratigraphic markers, these changes should be explored as possible clues to the emplacement history of the various deposits.