Quoting the HASI team's article in the Dec. 8 "Nature" (which is open-access --
http://www.nature.com/nature/journal/v438/...nature04314.pdf ):
"The complex permittivity of the surface material is measured after impact with the PWA mutual impedance probe, at five frequencies. As a first estimation, the mean relative permittivity within the sensor range (radius 1m, depth 2m) is of the order of 2, in reasonable agreement with the measurements performed with the radar on board Cassini...
"The measured relative permittivity (of the order of 2) constrains the soil composition. No evidence for the presence of liquid phase on the surface was returned by the signal of the radar altimeter."
I seem, however, to recall seeing somewhere recently an article that actually DID express puzzlement at the high electrical conductivity of Titan's surface as measured by HASI. This could easily be a false memory on my part -- it's a very faint recollection -- but I'll hunt around a little.
The "Nature" article also says a bit more about Huygens' radar altimeter:
"In addition to providing altitude (Fig. 8), the Radar Altimeter measures the signal backscattered within the footprint of the beam, whose diameter is 0.14 times the altitude. This signal is strong and smooth with small variations over the ground track, indicating a surface with little relief. The atmosphere was scanned and return signal from droplets was searched for, but no significant signature of rain could be found." That beam is probably wide enough to explain why the deep gullies seen by DISR didn't show up in the radar altimetry. Note also, however, the sonar altimetry from the SSP package during the final part of Huygens' descent (
http://www.nature.com/nature/journal/v438/...nature04211.pdf ):
"The Acoustic Properties Instrument–Sonar (API-S) recorded the approach to the surface on final descent (Fig. 1). API-S is a pulse send–receive sonar, where the time of flight gives distance (and hence final descent speed). The probe vertical speed just before landing was determined as 4.60
+0.05m/sec. The peak width and signal strength are influenced by surface topography, probe position and acoustic reflectivity according to the usual radar equation for an extended target.
"As Huygens descended towards the surface the sensor footprint shrank, and a smaller area of terrain was illuminated. Owing to variation in probe tilt and wind drift during descent, the sensor illuminated different areas of ground for each pulse, with partial overlap. Initial derivation of surface acoustic reflectivity shows no significant variation as a function of altitude, implying that the landing site as seen by Huygens is typical of the local surroundings (the maximum area sampled by API-S, for the highest altitude of around 90 m, is approximately a circle of 40 m diameter).
"For all returns the peak widths are typically 30–50 milliseconds wide, showing no trends. This implies that the surface is topographically similar over all sampled beam footprints. However, this width is greater than would be expected for a purely flat surface, implying that some small-scale vertical topography is present.
"The final peak immediately before impact is at a height of 14.4 m at the time of pulse transmission, with a beam footprint of ~26 square meters (equivalent to a circle of ~2.9 m radius). This final peak is recorded by the SSP at higher time resolution than previous ones, giving more information on surface structure (Fig. 1 inset). The relatively broad shape of this peak indicates that the surface cannot be completely flat, or concave over the footprint. However, the flat top of the peak also requires that there be some local height variation over the surface sampled within the footprint. Rock size determined from the postlanding surface images will provide a good starting point for further collaborative analysis.When averaged to a lower time resolution, the width of the final peak is entirely comparable to the width of the higher-altitude peaks, implying that they are seeing very similar terrain.
"Together these data suggest a surface that is relatively flat but not completely smooth; such an interpretation is compatible with the Descent Imager and Spectral Radiometer (DISR) surface images, suggesting that perhaps the DISR images show a typical surface that probably surrounds the probe in all directions. The fact that slight horizontal and vertical topographic variation is seen over the footprints, rather than a completely flat plain, implies a certain level of complexity during the history of surface formation in the region of the landing site."