Huygens' Descending View of Titan
Posted by Emily Lakdawalla
17-01-2005 10:00 CST
Scientists from the Huygens Descent Imager Spectral Radiometer (DISR) team have released their first mosaic of images captured during Huygens' descent.
The mosaic is composed of 30 images captured by the Medium Resolution Imager of Huygens' Descent Imager Spectral Radiometer while the probe was spinning and descending toward Titan. The larger the image "stamp" is, the higher Huygens' elevation when she took the image. The elevations range from 13 kilometers (8 miles) down to 8 kilometers (5 miles). The entire mosaic spans a region about 25 kilometers (16 miles) across. The actual landing site of the probe was in the black region at the center of the mosaic. The imaging team plans to release more mosaics soon showing the landing site in higher detail.
One thing you can learn upon examining this mosaic is that the wind was not blowing the probe very hard as she descended. If it had been, there would be no "black hole" at the center of the mosaic--Huygens' horizontal drift would have carried her across the point directly below her ("nadir," in navigational terms). As it is, her motion was a rather gentle one, moving generally from the bottom of the image mosaic toward the top.
You can see that there are several different kinds of terrain that were visible to the probe. At the lower left is a bright-colored landscape that is heavily dissected by drainage channels. The drainage channels are forced to detour around a range of hills near the shore (the topography of the hills is easy to spot in the Huygens Side-Looking Imager photos). The channels empty into a very dark colored region. Scientists have been referring to the boundary between the light and the dark regions as a "shoreline," but have so far resisted calling the dark region a "sea."
While the regions with the drainage channels and shoreline seem fairly easy to interpret, the regions near the top and right sides of this mosaic are much harder to understand. What the white-on-dark features represent is still anybody's guess.
It's been harder than expected for the Descent Imager Spectral Radiometer team to interpret their pictures, for two main reasons. One is the loss of one of the two communications channels from Huygens. The DISR instrument split its bandwidth between the two channels, so the loss of "Channel A" means that only half of the DISR images that were acquired were returned to Earth. The lost images are scattered randomly throughout the data set, so while images were acquired in triplets (one each from a side-looking, angled-down, and downward-pointing camera), the returned triplets are often missing one or two of the component frames.
The other difficulty for DISR arose from Titan's recalcitrant atmosphere. DISR contained a Sun Sensor that was designed to help the instrument identify which direction it was pointing relative to the Sun as the probe rotated. Unfortunately, early in the descent, the probe was swaying much more strongly than anticipated, and the Sun Sensor was unable to get a "lock" on the Sun's position. (The reason for the strong swaying is not yet known for sure, but one possible explanation is higher-than-expected horizontal wind shear.) And later in the descent, Titan's haze proved to be deeper than expected, so the Sun was too dim and diffuse for the Sun Sensor to detect it. As a result, DISR was less sure of its orientation while it was taking pictures than was expected.
Despite the communications trouble, DISR still acquired 600 images. The DISR team is now located at the European Space Operations Centre, and is working almost without sleep to process and calibrate the images to improve their quality, and then mosaic them together. Give them some time, and they will show you all that Huygens had to reveal of Titan's surface.