3.5-cm radar investigation of Mars and Mercury : planetological implications
<p>Bistatic radar observations have been made using the combined VLA/Goldstone radar instrument at X-Band wavelength (near 8500 MHz). This thesis contains a description of the instrument, observations, data reduction, and implications of some of the measurements. While the instrument has b...
| Summary: | <p>Bistatic radar observations have been made using the
combined VLA/Goldstone radar instrument at X-Band wavelength (near 8500
MHz). This thesis contains a description of the instrument, observations, data
reduction, and implications of some of the measurements. While the instrument
has been used to probe many objects, discussion will be restricted to the data sets
for Mercury and Mars. This technique has provided the first unambiguous radar
cross section maps of both planets, with surface resolutions as good as 150 km for
Mercury, and 100 km for Mars. The analysis of the radar cross section maps has
provided a rich harvest of new information about the surface and near-surface of
both planets.</p>
<p>Mars was observed twice during the opposition of 1988, and 3 times during the
opposition of 1992/93. During the 1988 observations, the subearth latitude was
∼ -24°, providing a good view into the south polar regions. The Martian season
at the time was mid southern summer (L_s ∼ 295°), and thus the seasonal CO_2
ice cap had sublimated away, exposing the residual south polar ice cap (RSPIC).
The RSPIC was the area with the highest cross section on the planet in 1988,
with a peak normalized cross section of 0.716. This is incredibly high, especially
considering that it was at an incidence angle of ∼ 66° at the time. The RSPIC
also exhibited the odd characteristic that throughout much of its extent, more echo
energy was received in the same sense circular (SS) polarization as that transmitted
than in the opposite sense (OS), a so-called polarization inversion. This is a
characteristic which has also been observed on the Galilean satellites, and on a
portion of Greenland, and may be true for all cold, clear icy regions. This seems
to be a result of the radar wave penetrating into a relatively lossless medium containing
many volume scatterers. In the case of the RSPIC, the lossless medium is
ice, whether CO_2 or H_2O, and the scattering centers are most probably cracks and
voids in the ice. Simulations indicate that the radar wave penetrates down to 10's
of meters into the ice layer, implying that at the time the RSPIC was very clean,
i.e., less than ∼ 1% volume fraction of contaminating dust, to that depth.</p>
<p>During the 1992/93 observations, the subearth latitude was ∼ +6° to ∼ +9°,
providing a tolerable view into the north polar regions. The Martian season at the
time was early northern spring (L_s ∼20°), and thus much of the seasonal CO_2 cap
was present, which covered the residual north polar ice cap (RNPIC). No regions
with enhanced cross section were found in the north polar regions, in stark contrast
to the south. Fits to a sensible backscatter function provide an indication of slight
cross section enhancements near the Chasma. Borealis, but the reliability of the fits
remains in question due to the restricted incidence angle range of the data. There
are at least 3 possible reasons for the fact that the north polar regions show no
cross section enhancements: 1 - there is some fundamental difference in the structure
and/or composition (amount of dust contaminant) of the two residual caps,
2 - the seasonal CO_2 cap which was present during the north polar experiments
absorbed enough of the incoming radar energy to obscure the RNPIC, and 3 - the
north polar regions were imaged with slightly poorer geometry. Some combination
of the three is most likely.</p>
<p>Many other regions with anomalous cross sections were found on the surface of
Mars. The large volcanic provinces of Tharsis and Elysium have very high cross
sections associated with them. These are most probably a result of the extremely
rough surfaces of the large volcanoes and their associated flows. One of the most
intriguing features in the Mars data set is a region which extends west from Tharsis
for over 2000 km, which displays no cross section distinguishable from the noise
in either polarization, which we have termed "Stealth." The surface and near surface
(to depths of meters) must be composed of very underdense material, with
an absence of volume scatterers (rocks). The proximity of Stealth to Arsia and
Pavonis Montes suggests that it may be comprised of pyroclastic materials which
were blown westward after eruptions from these two large shield volcanoes.</p>
<p>Mercury was observed twice during the conjunction in August of 1991, once
during the conjunction in November of 1992, and twice during the conjunction in
February of 1994. The data in 1992 were compromised by transmitter problems
and will thus only be very briefly discussed. The 1994 data will be only briefly
mentioned, as well, since the data reduction has not been fully completed, and
thus all results are very preliminary. During the 1991 observations, the subearth
latitude was ∼ + 10°, providing nearly as good a view of the north polar regions
of Mercury as is obtainable via earth based remote sensing. The feature with the
highest SS cross section (.079) in either of the radar images was near the nominal
polar position. This feature also exhibits a polarization inversion throughout much
of its extent, similar to the RSPIC on Mars. This feature is probably the signature
of water ice deposits in permanently shadowed regions near the pole, which
explains the reduced cross section when compared to the Martian RSPIC. The ice
may be covered by a thin layer of dust, which would protect the ice from erosion
from energetic sources as well as contributing to the reduced cross section. Other
regions with anomalous cross sections exist on the surface, most notably five large
quasi-circular regions, which we refer to as "basins." It is clear from the 1992
data that the Caloris basin has no such cross section enhancement in its interior,
and so our "basins" are different from Caloris in some manner. During the 1994
observations, the subearth latitude was ∼ -10°, providing nearly as good a view
of the south polar regions of Mercury as is obtainable from earth. Preliminary
results indicate that there is a region of enhanced cross section near the south
pole, similar to that near the north pole.</p> |
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