Authors:
Rudy, Donald James
Abstract:
Observations of Mars at wavelengths of 2 and 6cm were made using the VLA in its A configuration. Two seasons were observed; late spring in the northern hemisphere (LS ~ 60°) and early summer in the southern summer (LS ~ 300°). The sub-earth latitudes were 25°N and 25°S, for each of these seasons respectively. So the geometry for viewing the polar region was optimal in each case. Whole-disk brightness temperatures were estimated to be 193.2 K ± 1.0 at 2 cm and 191.2 K ± 0.6 at 6 cm for the northern data set and 202.2 K ± l.0 at 2 cm and 1 (...)
Observations of Mars at wavelengths of 2 and 6cm were made using the VLA in its A configuration. Two seasons were observed; late spring in the northern hemisphere (LS ~ 60°) and early summer in the southern summer (LS ~ 300°). The sub-earth latitudes were 25°N and 25°S, for each of these seasons respectively. So the geometry for viewing the polar region was optimal in each case. Whole-disk brightness temperatures were estimated to be 193.2 K ± 1.0 at 2 cm and 191.2 K ± 0.6 at 6 cm for the northern data set and 202.2 K ± l.0 at 2 cm and 195.4 K ± 0.6 at 6 cm for the southern data set (formal errors only). Since measurements of the polarized flux were taken at the same time, whole-disk effective dielectric constants could be estimated and from these, estimates of sub-surface densities could be made. The results of these calculations at 2cm yielded whole-disk effective dielectric constants of 2.34 ± 0.05 and 2.02 ± 0.03 which imply sub-surface densities of 1.24 g cm-3 ± 0.06 and 1.02 g cm-3 ± 0.05 for the north and south, respectively. The same calculations at 6 cm yielded effective densities of 1.45 g cm-3 ± 0.10 and 1.31 g cm-3 ± 0.07 from effective dielectric constants of 2.70 ± 0.09 and 2.48 ± 0.06 for the north and south data sets, respectively. From the mapped data these parameters were also estimated as a function of latitude between latitudes of 15°S and 60°N for the north data set; and between latitudes of 30°N and 60°S for the south data set. A region in which the brightness temperature behaves in an anomalous manner was discovered in both data sets. This region lies between about 10°S and 40°S. Here the brightness temperatures at both wavelengths in both data sets appears lower, by 4 K to 8 K, than a nominal model would predict. In addition to the effective dielectric constant and sub-surface density the radio absorption length of the sub-surface was estimated. The radio absorption length for most of these latitudes was about 15 wavelengths with formal errors on the order of 5 or 10 wavelengths. This is true for both data sets. The estimation of the effective dielectric constant at most latitudes was between 2 and 3.5 with only slight differences between the two different wavelengths. The two data sets show the same relative trends, but are off by a scaling factor. These estimates of the dielectric constant lead to estimation of the sub-surface densities as a function of latitude. Most calculations of the sub-surface density yielded results between 1 and 2 g cm-3 with errors on the order of 0.5 g cm-3. These results seem to imply that the sub-surface is not much different than the surface as observed by the Viking and Mariner missions. In line with this, an examination of the correlation of the dielectric constant at each wavelength with the thermal inertia, determined by the Viking infrared measurements, shows a relatively strong correlation, at both wavelengths, for the North data set. The South data set, however, shows little to nocorrelation between the radio parameters and the thermal inertia. Since the South data set is primarily composed of latitudes which contain the anomalous region, it is not suprising that the South data set shows no correlation. In addition, the thermal-radiative model used to estimate the above parameters was used to estimate the variability of the whole-disk brightness temperature of Mars. This was done in an effort to establish a background for those astronomers wishing to use Mars as a calibration source. The parameters investigated for their effect on the whole-disk brightness temperature of Mars were: the sub-earth longitude, the sub-earth latitude, the sub-earth time of day, the dielectric constant, and the radio absorption length. A nominal model was first created which established the variation of the brightness temperature as a function of season and radio absorption length. A nominal value of 2.2 was used for the dielectric constant, and the sub-earth latitude was set at 0°N and the sub-earth longitude was set at 75°W. The sub-earth time of day was held at noon for this nominal model. This is equivalent to a 0° phase angle. The most important geometric factor was the sub-earth latitude. The error in estimating the whole-disk brightness temperature of Mars by using the wrong sub-earth latitude can be as large as 5 to 10%. The charts presented will be useful to estimate the whole-disk brightness temperature which the thermal model would predict. It is believed that the error in this estimate is less than or equal to 5 K.
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