Latitudinally Binned Results

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4.3 Latitudinally Binned Results

The dielectric constant has been measured by both ground-based radar and the Viking hi-static radar experiment. Radar estimations of the dielectric constant are slightly higher than those calculated from the thermal emission. This is many times the case, and explanations for this discrepancy are usually not very rigorous and mostly unsatisfactory. One explanation for the difference is that, the radar estimation, like the dielectric constant estimated radiometrically, can be masked by the effects of surface roughness. However, due to the radar backscatter law, the radar results are more susceptible to sub-surface discontinuities such as rocks than are the radio thermal emission results. With this in mind a brief review of the results of other workers in the field is in order.

Pettengill et al. (1973) measured the radar cross-section per unit area and in- ferred the dielectric constants of the surface of Mars from ground based radar at a wavelength of 3.8cm. They performed these measurements for areas in the region between latitudes 14° S and 22° S and between longitudes 70° W and 110° W. They found that the dielectric constants so determined varied over this region from around 1. 7 to about 5. Unfortunately this region is nearly complementary to the region mea- sured during this observation run so a direct comparison is not possible. Downs et al. (1975, 1973) also did a radar measurement. Their measurements were made at a wavelength of 12.6cm and in a band circling the globe between the latitudes 14°

S and 22° S. They measured an average reflectivity of 0.07 with large variations (on the same order as the average). These variations partially correlate with variations in the thermal inertia. For normal incidence this reflectivity translates into a dielectric constant of slightly less than 3.0.

Harmon and Ostro (1981) performed a radar experiment which separated the

North Data Set 46

diffuse and specular components. The specular component is much more useful in determining the reflectivity than the diffuse component. They made several measure- ments of which only two are in the region covered during the North observing run.

For the region centered on 39.8°W, 24°N, they found a reflectivity of 0.13 and for the region centered on 330.2°W, 24°N, they found a reflectivity of 0.061. These translate into dielectric constants of 4.6 and 2.8, respectively. Simpson and Tyler (1981 ), using the Viking orbiter, performed a bi-static radar experiment at wavelengths of 13cm and 3.6cm and found that the dielectric constant in the very northern latitudes ( </>

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60°) was relatively low, between 2 and 3, and seemed to decrease with latitude. At lower latitudes ( </>

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20°) they estimate a dielectric constant between 3.0 and 3.2 at longi- tudes near 250° W. Historically, there has often been a discrepancy between electrical properties as measured by radar and those measured by radio thermal emission, see Golden (1979); Muhleman (1972); Hagfors and Moriello (1965) and others.

Although the radio absorption length is on the order of 15 wavelengths, the di- electric constant estimated from our measurements is the value in the region one or two wavelengths below the surface. This is because it is the emissivity that deter- mines the dielectric constant. And the emissivity, which is due to the interface of two different dielectric constants, is affected only by the first one or two wavelengths.

Radiometric emissivity and polarization measurements are sensitive to the near sur- face as are the radar experiments. Although both are sensitive to nearly the same region (at the same wavelength), the radar measurements are more sensitive to sur- face and sub-surface roughness (see Muhleman, 1972). The low-incidence, bistatic radar is sensitive to the surface roughness in a slightly different way, as well as hav- ing to contend with the shadowing problem. As a consequence, all of our estimates of dielectric constant, which are at or below 3, are not out of line with these other workers. A comparison of radar and thermal characteristics can be found in J akosky

47 Chapter

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North Data Set 48

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49 Chapter

4

and Muhleman (1981) and Jakosky and Christensen (1986). Both show a correlation between thermal inertia and the radar reflectivity, similar in nature to the correlation between thermal inertia and dielectric constant discussed here.

The brightness temperatures averaged over longitude for each wavelength are shown in Figure 4.2 and 4.3. The two extremes of the suite of models which were fitted to the data are also shown. The brightness temperatures were averaged into 5° latitude bins starting at 87.5° N. Note the strong correlation between the two wavelengths. Both wavelengths show a dip in brightness temperature at about 35°

N, and both wavelengths exhibit behavior not well predicted by the suite of models at latitudes between 15° S and 35° S. The strong curvature at southern latitudes is due to the effects of the beam shape function and the radio emissivity. The drop at the northern end is due to these effects and, more noticeably, the North Polar Cold Region. The gentleness of the edge of the North Polar Cold Region in both the data and the model at 2cm is mostly due to the beam shape function and at 6cm partly due to the beam shape function and partly due to actual temperature variations.

In the models the edge of the North Polar Cold Region is very sharp, but since these have been convolved with a gaussian of the same size as the CLEA1 beam, this edge has been smoothed. The slope of the smoothed edge of the model is pretty well consistent with the data in the 2cm case. This implies that, as seen at 2cm, the edge of the North Polar Cold Region is a shar:p boundary. However, in the 6cm case, the slope of this smoothed edge is steeper than the data. This implies that, as seen at 6cm, there is no sharp edge to the North Polar Cold Region. This may be due to a poor estimate of the thermal parameters above 60°N.

It should also be stated that no attempt was made to adjust the models so that the edge of the Polar Cap in the models coincided with the edge of North Polar Cold Region in the data. This means there is a slight offset between the edge of the North

North Data Set 50

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