Huygens Camera Tricks

After the January landing it took me until July to figure out the big mystery of what I was looking at: aquatic vegetation!

Space scientists never made the effort, and discarded the Huygens images, the "meat in the sandwich" so to speak to tie all the other data together, as meaningless. Anything one said to them about their content was cast off with "the viewing conditions were poor" and "the compression artifacting was awful." These defects don't change the amazing and alien scenes recorded. When I emailed with reasons (not only the images) that the landing area had to be liquid (very shallow (3 inches?) at the touchdown point) and not dry ground, and also predicted that an upcoming publication would indicate "the stuff of life" on Titan's surface, I got this response:

"You can say you see pink aliens, or liquid, or cities of gold in the images." - Ralph Lorenz, JPL

With a typical hand-held digital camera, one snaps a color picture of familiar-looking Earth scenes in familiar types of Earth lighting at relatively close range, and there is usually not much question of what is being shown in the images. The Huygens cameras sent us very coarse and mostly distant monochrome images of an unfamiliar new world at unusual (methane transparent) light wavelengths, and they were highly compressed (read "made blurry") for real-time transmission over a slow data link to the Cassini, which was whizzing by overhead at 21,000 MPH and would be in radio range just four hours. Huygens thus had more the night vision of a bat than the sharp eyes of an eagle.

Organization of Huygens's single CCD optical sensing chip. The views and spectrums were fed to it via fiber-optic cables from the lenses and instruments.
(For next time if anybody cares, I recommend four filters in quartet squares of pixels: broadband and three methane-transparent wavelengths, for deriving either color or monochrome images from. [Of course someone else may have an even better idea.] Hindsight is great - The landing itself clarified what would give the best results, and unexpectedly showed fascinating scenes in which color imaging would have been extremely valuable! Before the landing, there wasn't even any guarantee that there would be enough light to show anything at all, so Huygens brought its own lamp to at least show the approaching landing! This proved to be a waste of space and battery power, but who was to know?)

The view through the three imager lenses. The bottom edge of the HRI view looks almost straight down and the HRI has a fairly high magnification. The top edge of the SLI view looks just above the horizon at the sky and is the least magnified, and so it sees the widest area. The views of the three imagers overlapped slightly, and Huygens spun slowly as it descended with the intent of seeing in every direction with overlapping views. Half the images were lost owing to a software problem, but the plan was ingenious and some areas and features were seen several times during the descent, first through the HRI, then the MRI and finally through the SLI, all viewing from different angles as Huygens descended. Various fine "mosaics" - composite images - were created from the Huygens images.

While we all wish the images were better and in color, it is something of a miracle that on man's first attempt, Huygens traversed the vast void to Saturn successfully along with the Cassini, a velocities/doppler shift problem that would have lost all the Huygens data was discovered and corrected en route, it was aimed and released successfully, entered the atmosphere of one of the most mysterious and unknown worlds in our solar system intact, descended by parachute successfully, and landed intact, while successfully transmitting half its data during descent and after landing on Titan. We must count ourselves lucky we have any pictures from this distant world at all in the early 21st century, and we have them of an amazing shallow seascape! Consider that about 1/2 of all Mars landers have failed, and Mars is much better known than Titan was.

But these low-quality, monochrome images present us with something of a puzzle. The Huygens pictures were expected to be adequate to identify land from sea (evidently they aren't!) and to show us grosser surface geology. In order to get full value of the many fine-detail and exceptionally interesting features Titan offers us in so many images, they must be carefully scrutinized and, where possible, compared to other images showing the same features or scene from different altitudes and angles. (Note: I am going to refer to the liquid methane - seas, lakes, rivers - as "water" for simplicity. Any actual water on Titan is a form of rock.)

It's Very helpful to magnify the images by two for viewing and study, and to download the zip archive copies of the actual raw images (link on title page) and adjust the contrast and brightness yourself. (The supposedly "raw" images available at ESA and JPL are already "randomly" contrast enhanced, some to a grotesque extreme, probably until the brightest pixel hit "white" and the darkest one "black".)

There are certain small image features in the same place from image to image, so obviously they originate with Huygens itself. My best guess is condensation droplets of some liquid - dew - on the window covering the lenses, picked up very high up and then probably frozen on until near ground level. That would be bad enough, but the strong light contrast across a "droplet" causes the entire 16 x 16 pixel block in which it resides to fill with strong compression artifacts. The medium-rez imager is worst, but the other two lenses are by no means free of these. (I thought for a while these were meteor craters - and therefore the sea must be dry or frozen over because it had craters on it - which shows how necessary it is not to form an opinion based on a casual glance at one or two of the images.) In the MRI images, there's a lighter band across the top. This would appear to result from fog or frost on the imager window. It is easy to mistake this top area for a shoreline. Indeed, icing during entry into Titan's atmosphere was a listed concern of the mission, and evidently it did happen: Huygens needed windshield wipers or a window heater!

Compression Artifacts Versus Real Image Data.

Bad as some of the image compression artifacts are, the Huygens images, descending and landed, portray well the scenes they are viewing, and the detail over much of each image is more or less down to the pixel level. It is a mistake to simply attribute to image artifacting every feature not immediately identifiable, especially the broad features defining an image.

There are some good indicators to what's real and what isn't. First, compression was done independently to 16 x 16 pixels image blocks. In many images the boundaries between some of these blocks are quite visible. One of the first things to look for are "features" that partly or wholly fill one compression block and stop abruptly at its edge(s). These are most likely artifacts, while features, even similar ones, that continue into the next block(s) are almost surely true image data. Compression artifacts bear a characteristic stamp of appearance, which soon becomes familiar to the student of the images.

Often there is real image data crossing a badly artifacted block where the appearance in that block is different from surrounding blocks, but it can still be made out as being evidently a contiguous feature. Many such features can be made out in 307mri, above, with the artifacts modifying but not obscuring the real image features.

It may be helpful to enumerate the various components that together make up a Huygens image scene, especially when that scene shows the liquid methane sea.

* First and perhaps most conspicuous are the "dewdrops" and the image compression artifacts they create. One must, again, recognize that it's a "splattered" window Huygens views its world through. At the top of the MRI images, there is a hazy band that appears to be fogged. Since the "dewdrops" and the fog don't evaporate, they are probably frozen for most of the descent. In the after-landing images, the "dewdrops" are gone, perhaps rinsed off by a splash of the liquid on the surface. Instead, there is a bright "overexposed" area along the left edge of the SLI imager, perhaps (guessing) indicating some methane splashed along the edge of the window.

* The critical angle of total internal reflection of the methane should be considered. Above this angle towards the horizon, one is seeing brighter surface reflections off the liquid, while below it, one is looking into the darker, yet very clear, liquid. This is easily sorted by imager. The more vertical images of the hi-rez and medium-rez imagers are looking downward below the critical angle, which for methane is 52 degrees. As Huygens gets closer to the surface, the side looking imager starts seeing the sea surface more. While the critical angle of total internal reflection isn't very clear owing to waves and other surface disturbances, the SLI sea images invariably look darker in something of a zone near the bottom than in the rest of the image, and generally undersea features, where they are visible at all, aren't very evident above this zone.

* Effects of waves and other surface disturbances. The methane seems to be so clear that waves are often not very visible, and in a few low-level HRI images it can be hard to remember that Huygens is above the sea and not in it. But when looking downward into the sea, they can have a distorting effect on the scene. There is some evidence in low-level HRI images that we are seeing faint reflections or refractions of features seen elsewhere in the image, or even from off the edge of the image, in places where they shouldn't be. In most HRI and MRI images, a fat, dotty line meandering from one edge of the image to another is probably the crest of a wave. Distortions of underwater features are greatest at the wave crests, but owing to methane's lower refractive index, distortion is less than under similar Earth water. In 307 above, I believe I can make out several waves forming horizontal-ish lines across the image. But they are generally, as here, quite faint.

By viewing it enlarged, a very close look at image 698-SLI reveals a sheer of spray seen coming from what appears to be the stem of a plant as a wave hits it from behind. The reflection of the spray is also visible in the water, helping to confirm it's a real feature. The plant stem trails off to the left with what seem to be multi-color spotted leaves coming off each side on the water. The rightmost and closest leaf looks like a gigantic "seal head", and it is in fact the feature that first caught my attention and I said "Wow, whatever that is, it's just not dead geology!"

* Objects on the sea surface. These include, especially, plant leaves and stems, and possibly spore pods. They can be light or dark in shade compared to other image features, and some plants look much darker than others. The "spore pods" (and that's just a wild guess) look much like jellyfish, but the two I've noticed seem to have a stem going to them.

* In images looking into the sea: more plant leaves and stems. In several low-level HRI images, however, there's more excitement: animal life is evident. There are things that look like a squid, fish, and frog-like and newt-like creatures (or unusual fish). In certain places, we may see features on the bottom, while in other images the bottom is very plain, but there are plant shadows lying on it. And scattered throughout, features quite difficult of interpretation that could be plants, sea creatures, or geology. (The more I look, the more it seems to me the bottom itself is pretty smooth and featureless, and that any features are things that aren't the bottom.)

* In images of land, the most common things in view are again stems and leaves, of giant vines whose roots are in the sea, crawling across the land, again in dark and light colors. Bright "islands" just offshore appear to be such roots sticking out of the 'water'. On the day of the landing, the dark vines crawling across the land were mistaken for "rivers", and I personally also thought for a while the straighter, light-colored stems looked like "roads", with "buildings" that were actually leaves. (Oops!)

* If there is any animal life on the land, I haven't noticed it.

Interpreting Huygens Images