The Life of Titan

   Life is everywhere on Titan. It's not the microbes which space scientists vainly search for wherever there's liquid water, but pole to pole forests of gigantic trees, and luxuriant growths of aquatic vegetation of a scale not seen on Earth. The life chemistry is supported by the liquid methane seas, lakes and rivers, and a nearly Earth-pressure atmosphere of nitrogen with hydrogen as the breathing gas.
   Every image and every instrument return from the Huygens lander and the Cassini space probe reinforces the view and evidence of this life and none which seem credible contradict it. So far space scientists have remained oblivious to it, and instead are puzzled and misled by many of the findings of their own instruments. They come up with strange theories as to their meanings. If these very same findings had come from Mars, the same people would be jumping up and down saying "Mars is alive!", but coming from a "deep freeze" world where Earth life wouldn't survive 2 minutes, they simply can't conceive that such a different life to ours is existing and thriving.

   Titan, with its liquid methane seas, tides, Earth pressure atmosphere and similar polar tilt, is amazingly like the Earth in some ways, but not in ways that would make it a comfortable place for Earth people. Not only is there 1/7 gravity and a very long day (16 Earth days) and very long seasons (the year is 30 Earth years), but the temperature is 1/3 that of the Earth (94°K versus 290°K) and hence the life chemistry is vastly different. With seas of liquid methane, it seems likely that plants take in methane and turn it into carbon (coal? Tree trunks!) plus the obvious energized breathing gas, hydrogen, much as plants here (re)generate oxygen from carbon dioxide. Titanian vegetation seems to attain a scale that is probably best explained by the low gravity and (in the tropical seas) the need to flourish in strong tides. It has many similarities to Earth plants, but a possible major functional difference as well. I wish to speculate on the nature and biochemistry of Titanian life, but first let's have a look at the meat of the matter: what the Cassini probe and the Huygens lander have actually shown us!



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Excerpts of Emails with Ralph Lorenz


I hear the GCMS team says the spectrum indicates the "ice rocks" in the landed images "definitely" aren't made of ice. (I didn't know the GCMS looked externally... should that have been the DISR team?)

For a while I thought they were rocks too, but I figured out what they really were a couple of months ago, neither rocks nor ice. I confidently say you'll see lots of complex organic compounds when the spectrographic results are released - spectra of life.


Sagan
"extraordinary claims require extraordinary evidence"

PS: I've modified my description of Titan's tides a bit to include your figures: 100m bulge, 9%=9m bulge variance over orbit, and I mention this is versus 18cm variance for Earth. (Are there any estimates how much Titan might flex internally over an orbit? - this would have to be subtracted from the 9m tidal "head".)

>btw - no-one would have like to have found liquids on Titan more than me (well, perhaps Jonathan Lunine)
>Whatever people think they are seeing in the highly compressed DISR images, the evidence is just
>not good enough to say it is there, let alone to contradict the measurements from the other sensors.

"As I've said, we'd be happy to discover liquid.  We didn't. You can say
you see pink aliens, or liquid, or cities of gold in the images." - Ralph Lorenz

=============================


The Forests of Titan (Cassini)

(Jan 2023)

(Note: My original version of this 'book' was written before the discovery and extensive charting of forests on Titan's land areas by Cassini's SAR [Synthetic Aperture Radar] 2.4cm radar instrument, and also before the discovery of the dunes submerged in the tropical tidal seas. I now include sections devoted to these topics.)

   The Cassini SAR images of Titan's land areas, allowing for the difference between radar and visual images (eg, Google Earth), look like forests and unlike any other land surfaces found anywhere in the solar system besides Earth.
   And yet the space scientists couldn't see it, and eventually offered a rather bizarre explanation as to why the images were so lumpy rather than smooth.

   The trees are "Titanic" by Earth standards, having been measured by SAR readings as being up to 2 kilometers tall. This height estimate was originally released in an article claiming Titan had "underground caves", and that furthermore the radar returns were so ethereal as to indicate that the ground for the first two kilometers was mostly filled with air! The obvious explanation is that the radar was looking into a forest canopy.
   How could trees possibly be so tall? This is probably a factor in the disbelief. What limits the height of trees on Earth? It is said that leaves can only suck moisture up from the roots for about 120 meters. Earth's tallest trees, redwoods in California and eucalyptus in Tasmania, live in areas of light winds and still max out at about 140 meters. It is said that at the treetops it often looks like there's drought even while there's plenty of moisture further down.
   If we take 140 meters and then multiply by Titan's 1/7 of Earth's gravity and then factor in liquid methane being only 80% as dense as water, we get: 140 * 7 / .8 = 1,225 meters as the maximum height for trees on Titan. That would be if all else was equal, which is surely unlikely, but it shows that two kilometer tall trees is in the right ballpark. Height means much less on Titan than on Earth. Even the atmosphere extends much farther into space than ours, so that on the highest mountains the air is practicly no thinner than at sea level.

Titan's "Kissing Lakes" - from a Cassini radar sweep. (shown x2 scale, south up)

   Lest this be thought to be "off the wall", one amateur opinion based on wild imagination, I sent the above image to a professional cartographer who looked at satellite images every day by email, without telling him anything about it.  I said I would tell him the latitude and longitude after he evaluated it, and of course he assumed it was on Earth. He unhesitatingly pronounced this to be a "shallow lake" surrounded by "mixed forest" in "quite flat terrain", and he mentioned where runoff appeared to flow into and out of the lake. (Right & upper left) Quite flat terrain is typical of Titan.
   To get as much mileage as I could, I asked him "Why shallow?" and he replied "vegetation appears to be growing out of it. But it could be radar noise." Titan's prolific aquatic vegetation, as seen close up by the Huygens lander, is of a scale unknown here on Earth. And unlike with water, Cassini's SAR radar saw deep into liquid methane.

   He could see it was a radar image. He tried to guess which Earth satellite had taken the image, naming 2 or 3 possibilities from satellites he knew. And he also made a guess as to the ground scale. Only then did I tell him it was from the Cassini, and the world was Titan, not Earth, that the lakes were liquid methane and that the actual scale of 120 meters per pixel made the trees hundreds of meters tall. BTW: 73°N, 46°W. (Then he tried to recant and say it wasn't trees after all, but I told him I was sure he had it right.)

   There is radar swath after radar swath from the poles to the equator, at resolutions as fine as  a couple of hundred meters per pixel, all showing that Titan's entire land area is enveloped in verdant forest. They were recorded in various directions, some overlapping. Were the trees Earth sizes, the texture might be so fine as to be indecipherable.

In the full version of this image clip, Cassini's 2.4cm Synthetic Aperture Radar (SAR) radar records
in detail here as fine as about 225 meters per pixel, a typical swath of Titan landscape from a tropical
duney tide-flow sea through a forested region with gigantic trees in a region they named "Xanadu".

(Note that things looking like gigantic horizontal "branching" structures in forests
are generally the canopies of rows of trees rather than individual features.

Also: unlike water, Cassini's SAR radar can see deep into liquid methane,
so submerged features and vegetation merely look darker than those in air.)

Huygens Lander Instrument Readings Indicating Titanian Life

(Feb 5 2006)

In December 2005, results of the Huygens spectrographic findings were published in Nature. Having observed vegetation ubiquitous in the images, I was expecting to hear of "complex organic compounds" on the surface, without knowing just what these might be, and the reports didn't disappoint me. The synopsis was:

"The infrared reflectance spectrum measured for the surface is unlike any other in the Solar System; there is a red slope in the optical range that is consistent with an organic material such as tholins, and absorption from water ice is seen. However, a blue slope in the near-infrared suggests another, unknown constituent."

 

Handy Earthling eyes spectrum references, courtesy http://www.glenbrook.k12.il.us/gbssci/phys/Class/light/u12l2b.html

The "tholins" finding does indicate complex organics. Going into more detail:

"The four major characteristics of the surface spectrum are: (1) a relatively low albedo, peaking around 0.18 at 830 nm; (2) a red slope in the visible range; (3) a quasilinear decrease of the reflectivity by a factor of about two between 830 and 1,420 nm; and (4) a broad absorption, by ~30% of the local continuum, apparently centred near 1,540nm (although its behaviour beyond 1,600nm is poorly constrained) as seen in Fig. 15b. This spectrum is very unusual and has no known equivalent on any other object in the Solar System."

Item one says it's a dark color, reflecting at the most, 18% of sunlight, and that the brightest color is in a near infra-red wavelength. Item four says Titan's surface is unique and mysterious. Of specific interest:

"... a featureless blue slope is not matched by any combination of laboratory spectra of ices and complex organics, including various types of tholins. Depending on their composition and structural state (for example, abundance, extension and/or clustering of sp2 carbon bonds), organic materials in the near-infrared exhibit either distinct absorption bands (for example, bright yellow-orange tholins), or a feature-poor red slope (for medium to low-albedo organics), or a very dark and flat spectrum."

It was also said that the brighter "terrain" showed more redder than the darker, from which I very tentatively infer, along with the blue, that the leaves and-or soil are a purplish color, with the sea liquid more of a green. The final note on the blue slope was:

"Assessing the material responsible for the blue slope is a major challenge and also a prerequisite for a secure identification of the 1,540nm band."

Huygens Views of Titanian Life

Vague, murky shapes in the sea, from 25 Km up. How could so many bizarre things huddle together in one place? They appear to be giant aquatic plants! We see huge stems, branches, leaves, and perhaps even some roots. (The "bubbles" are methane frost or dew on the camera lens, as is the light colored band at the top. These are ubiquitous in the MRI (medium rez imager) but there are a few in the HRI (high rez, downward looking) imager as well) The tidal flow runs East - West through Huygens' channel. The lighter colored areas are likely mats of leaves, or surface reflections, but some patches are probably shallower 'water'.
Scale is about 3-1/2 Km left to right; South is towards the top; land is not far below the "bent arrow" feature and the bottom of the image, shallower 'water' perhaps explaining the lighter shade at the bottom. The colorized image attempts to show the real forms in the original image but contains much "artistic licence" as fine points of detail in the Huygens images are often distorted and unclear. The true colors are of course unknown.
 
 



Here we follow the plants down in altitude, obtaining different views of them. Again, various leaves and stems are in view. Following the arrows to a cluster of leaves, in 671 we finally get a vague glimpse of the stem holding that giant "leaf" (or whatever it is) up. The three dimensional nature of the scene - objects in the sea rather than just flat ground - seems to be demonstrated as the appearance changes with differing points and angles of view.
Also some other selected images of aquatic plants are shown: Bottom left, some shots of the "bent arrow". Right side, stems rising up from the sea floor, one at the lower right in 337-mri and more centrally in 388-mri splits into 3 veins at the base of a huge leaf the surface. The whitish "island" towards the right in 337 is probably also a leaf mat - we see it floating above a deep area rather than being shallow 'water' forming a shoreline.

Oodles of seaweed! Titan's tidal waterways seem as choked with giant plants as Earthly duck ponds or swamps are with small ones.


Whitecaps and sprays from the "Seas" chapter again. This time, let's look at the other features.
Those seem to be plant leaves or a stem that the flow hits to make that spray. Left:
Big leaves are seen sticking out of the liquid in the background.
Right: There are many stems in the water; some large light colored
ones curving around and some smaller dark, "jagged" ones with leaves.
Having re-examined these as I colorized them, I suspect the main features aren't sprays of liquid at all.
The edges as seen on the left (650) are so well defined it seems more like a leaf sticking out of the water.
Titan doesn't seem to need study by space scientists so much as by biologists and bio-chemists!

 Here is a scene mosaiced from Huygens' descent images. The insets show certain of the features as seen by the side looking imager towards the end of the descent. Noteworthy are the "bent arrow" (SLI-650), the "pipeline" (SLI-710), and the "seal faces" (SLI-698). Studying Titan gradually discloses that these are scenes of profuse aquatic vegetation in a shallow body of flowing liquid full of mud or sand bars, often in the form of undersea dunes. What appears to be a substantial island at first look turns out to be rafts of floating vegetation in a shallow area. Utterly contrary to the inital conclusion that Huygens saw no liquid, it would seem that it actually saw no land! The sea depth varies from the deep troughs to the crests of the dunes, and the intensity of the vegetation changes greatly with the depths. Huygens landed on perhaps the shallowest spot in the whole area, only a few centimeters deep on a small mud bar.
June 17, 2007:

Here's a great new (Nov. 2005) processed, radiometrically calibrated image of the landing view by the DISR team.

It seems to bring out some details of stems, shadows and patterns that were less clear in the raw images, and Huygens' light hitting the MRI image towards the lower right is well compensated for. In the animation, many small nearby stems seem to wave back and forth with the flow, especially at the bottom of the upper SLI image.

My colorized version on the right is intended to bring out details of plant stems and small leaves.
Are those bits in the foreground 'duckweed' floating on the 'water', tiny plants coming up, or large particles of silt? Animating the MRI images shows they don't drift around, so they aren't duckweed.
Notice that many of the shadows appear consistent with leaves above the surface rather than with rocks on the surface. Some farther back appear to be possibly reflections of the leaves in the liquid.

Here is yet another rendering of the view from on the ground. I attempted to improve the resolution over any single image by expanding several images and blending them together, then applying edge sharpening. At the bottom, the leaf(?) in front of the MRI & HRI imager windows moved a couple of times, and I used images from later on when it blocked less of the view.
 

        

As far as I know, this is the clearest view yet of Titan's surface. The visual improvement is great enough that I feel my colorized image is sufficient commentary about the nature of what we seem to be seeing on Titan!

In reviewing these higher resolution images, I suspect liquid methane may have splashed on the SLI imager window during the landing, such as one 'smear' along the top (mostly cropped off in this image), the blurry area at the bottom, and the left edge near the top. This last appears to run down in a streak more towards the middle, causing blur of the image in its path. Perhaps several of the vertical features are streaks, rivulets of methane on the glass, as certain vertical "features" appear to be superimposed on the other image features. Later on in the image sequence, a whitish blob appears in the lower left corner, perhaps having run down the window from above and accumulated on the 'window sill'.

The image was made by blending six SLI images at 3 x magnification and then using an "unsharp mask" sharpening filter, set to ~1.5 pixels radius, ~300%.
I think it is the heat wave 'rippling', coming (no doubt) from Huygens itself, that provides much of the fraction-of-a-pixel motion between frames that allows improved resolution by combining several images.

July 15 2006: This excellent image, PIA08115 (here reduced in size) from DISR, was posted almost the same day as my last one. It has generally better quality, the image artifacting has been filtered out, and problem areas have been well treated. The left edge has been cropped, removing the troublesome upper part, but unfortunately also some interesting bits lower down. The view of The Moon on the right gives a sense of the image scale: Things are closer and smaller than expected because Huygens' imagers are quite low to the ground compared to a standing person, and maybe Titan's closer horizon plays a part as well.
Fine MRI and HRI views are shown. There are some interesting features that are probably small plants about at the margin between the SLI and MRI view, though their exact forms remain elusive. Near the bottom of the MRI area is a very peculiar form that proves you can't go fishing anywhere without dredging up an old shoe! The HRI section is inevitably badly out of focus: one can't look clearly at one's feet through a telescope - here we needed instead more of a microscope, or perhaps a magnifying glass!
The color is unfortunately taken from data that captures only 6 pixels through the entire SLI view, so it can only give a vague impression of overall color and not distingush different colored objects.

Some of what I was calling "mud on the glass" at the upper side of the far shore in my image, towards the left, could possibly be big seaweed leaves draped on top of other things, hence the appearance of features crossing over other features without completely obscurring them could be real.



          
Huygens Images MRI & HRI 723, 823, 948 & 1191 (AKA 167)
Note that the items in the view are in liquid and are only an inch or 2 or 3 away, and that the cameras are looking down at a steep angle.
Here we see something has pretty much blocked the view of both these imagers immediately after the landing.
My guess is that it's a leaf. (There are other possibilities. The featurelessness is a bit puzzling, although it must be way out of focus.)
A while later it has moved part way out of the MRI (top), exposing an intriguing scene.
My guess is that it's mostly a mossy textured undergrowth, and that Huygens landed on this cushion.
(...after the penetrometer punched through the "sod" & roots.)
Still later it has moved a bit farther, opening the view of the MRI further, and of the HRI a bit.
It is also now catching Huygens' spotlight and is overexposed, darkening the background.
Not long before contact was lost (over an hour), it had moved still further, but also caught more of the spotlight.


IIRC, the spectroscopes resolved just a few pixels across the entire image view, much larger than the individual fine image features here, so to a great extent we have merely the average of the colors. It's probably better than monochrome, but not by much. One can't discern by color, for example, whether the larger roundish objects at the bottom of the SLI view are similar color to the things that are definitely attached to stems. A greatly different color might indicate they are unrelated, for example they might be clams rather than leaves. The left one especially seems clam shaped.



Grass in an inch of water. Note the reflections that show a few stems are above water, and the shadow of the leaf in the upper corner, and compare these with the shadows in the Huygens surface images above this one.
 


This image shows how difficult interpretation of image features on a living world can be in monochrome. Can you discern which areas are duckweed and which are white foam? Can you tell which blades of grass are vertical and which are horizontal?

(Color image "answer" below.)

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Color image "answer" to monochrome image.

Animal Life

There are promising signs of possible animal life, but nothing that seems reliable beyond serious doubt. There are a couple of things that look perhaps like starfish in the after landing view. One appears to be sitting on a large leaf and weighing it down. Something drifts or swims by while Huygens is on the ground, appearing in just one of the MRI frames, but it is pretty small and formless and could be almost anything.

It only stands to reason that anything but Titanic (huge) animals would have been too distant for Huygens to make out during most of the descent, and alas there are no pictures between 1/2 kilometer elevation and the ground. At some earlier points I thought I could see a the head of a seal, a giant newt and a huge squid in the descent images, but upon further examination they don't seem very convincing. The "seal head" is probably some vegetable apparition similar to the floats put up by kelp only much, much larger. (Image SLI 698. Many large roundish "kelp float balls" are also visible in HRI images.)


  What looked to me rather like a "newt" appears to be a geological formation of some sort (plus the usual sprinkling of seaweed) or may itself be a mass of seaweed.


This "squid" is more believeable in size but at 100+ meters still way bigger than any Earth creature. At least some of its "tentacles" look like seaweed. It certainly appears to cast a shadow that indicates it's well off the bottom.

Unfortunately for seeing any land life, Huygens descended over the 'water' area, and the last views of land (besides small tide flats or mud or silt bars) [it turns out they're dune crests] are from almost three kilometers up:
 

Almost at the right end of the "land", we see a big fat stem of a giant plant, rising from the sea and climbing onto the shore. Such stems creeping onto the shore appear here and there in the images, mostly distant views, and also in the closest Cassini images from space. The plants seem to spread like horizontal vines, stems with branches growing out of them. Or, my Earth viewpoint may be backwards, and these are plant roots on the land, supplying plants in the liquid. A third possibility is that the function of these growths is to collect and metabolize hydrocarbon haze particles drifting down from the upper atmosphere. Whatever, this whole landscape has neither rivers nor roads nor linear fissures, but giant plant growths creeping between the sea and the land! The near point of land looks especially 'bushy' or at least convoluted. The long skinny "islands" offshore also seem to be stems of plants. Other broader islands could be leaves or mats of leaves, or mud/silt bars. The area of strange sea plants shown in photos above is partly in view at the far left, including the "bent arrow". Behind the island are seen the linear features dubbed "cat scratches" when fields of them, lined up, were seen by Cassini radar, and which may be huge plant stems, aligned with the tidal flows. The closest one looks like it is connected to the large root or vine system towards the left side of the island.
(Panorama by René Pascal: http://www.beugungsbild.de/huygens/huygens.html)
A point of indirect evidence in favour of the existence of animal life is something that looks like a large leaf with two spots and some lines on it, seen just above the "shoreline" just right of center. It reminds me of tropical plants whose leaves or flowers mimic very specific flying insect parts, in order that that insect will land on it and try to "mate" with it, thereby (iirc) pollenizing the plant. I can't think of a reason the "leaf" (or is it a flower?) should be so decorated unless there are pollinators to trick.

My opinion at the moment, barring spotting something more definite, is that Titan probably has animal life just because it has plant life that looks quite far evolved, but anything one might consider to be direct evidence is highly suspect. Is it possible for a world to have evolved all that interesting plant life and yet have no amimal life so far?
 
 



Cassini Views

Having glimpsed the nature of the landscape with Huygens, we can now turn to some more distant views from Cassini and make sense of them, which we wouldn't have been able to do without Huygens. That individual plants appear to be visible from space as far as Cassini must stay from Titan is remarkable to say the least. Color photography from Cassini's VIMS camera shows the colors of the objects within or covering much of the liquid are different from the land surface color.
 
 




ColorCassini VIMSview showing Titan's colorful seascape in "false" infrared colors. (Makes my colorizations look conservative!) It appears to reinforce the Huygens plant views.
Best guesses: The whitish areas are land areas, above liquid. The dotty blue areas on the land are roots or vines. Compare the 'vines' and gaps between islands with the Huygens panorama above. The one at the top may be holding the ground in place with its roots and preventing the tide from washing it away. Irregular dotty linear features in the sea would seem to be aquatic leaves and stems. The scale is mind boggleing.

 

Patterns of floating duckweed .
in spite of the differences,
compare with the Cassini images.

Clippings of Cassini Titan images from space (Top & Left or Middle). The second image was evidently taken from just 1200 Km above Titan's surface which possibly makes the rez 7 meters/pixel - most are from much greater distances. At the finest visible levels, the features vaguely correspond, but exact matches are not seen. This is true in any digital images, but more so here. We are probably seeing finer detail than it might seem when comparing the two images. Factors causing differences from image to image of the same area include (a) atmospheric distortion, "twinkling", which may shift the position of a feature as well as make it more or less visible, (b) the time of day of the image, because the shadows of the vines, especially on land, will move and change in form and intensity, (c) the tidal flow because it may cause aquatic plants to shift in position and orientation, and (d) the actual sea level with changing tide, though I'm still looking for a definitive view of this. For comparison, the image of Enceladus terrain from 5200 Km at 30 meters/pixel (right or below) shows the quality of the optics.
On the northwest island especially (2nd image) as well as elsewhere, the outlines of huge "vine" plants or root systems of sea plants seem to be visible coming out of the sea and spreading across the land, similarly to those on the island Huygens viewed. The dotty patches in the sea also seem to be plant parts - even individual branches of leaves seem visible here and there, though accumulations of duckweed on Earth ponds can have a similar appearance and they are actually composed of many small independent leaves (see duckweed image above). The shorelines seem indistinct probably partly because of the gradually shallowing liquid and beaches, and partly because in that shallower liquid, more plant leaves cover the surface. All those plant leaves on the sea surface probably confuse radar findings, searches for glints of light off open liquid, and spectral readings expecting liquid methane. Note also the streaks of the tidal flows, silt probably creating shallows in the tidal lee of the islands. Titan's tropics may be vast swampy wetlands.
The top image only vaguely corresponds with the view from Huygens, whose landing area is, unfortunately, just to the left of the second image. (More recent imaging has located the actual landing spot within Cassini space views.)
 

This color image  of the South- and North- Saturn-facing Seas was
taken by Cassini's Visual and Infra-red Mapping Spectrometer.
(The equator runs between the two seas, South is left, and about the image bottom directly faces Saturn.)

The image appears to show 'reddish' colored aquatic plant growth obscurring much of the sea
surface, blunting the "spectral reflections" that were expected for a liquid surface and resulting
in the plainly erroneous conclusion that the bodies that appear to be liquid must be dry, which
conclusion also contradicts the T14 radio occultation & Bistatic Scattering experiment in which
"periodic areas with a dialectric constant (1.6) consistent with liquid hydrocarbons" [these would
be deeper areas] were detected with "chaotic terrain" [vegetation sticking out of the liquid at
the dune crests] between them, in an equatorial region which is where the duney tidal seas lie.

Character of Titanian Plant Life

On a clear day with the sun overhead, sunlight is delivering 100 watts per square foot to the Earth's surface. Saturn and it satellites receive only one watt. Titan under its rather obscurring atmosphere receives considerably less solar energy, currently estimated at around 1/10th watt per square foot. This is still 350 times brighter than Earth under our full moon. In our north on an overcast winter's day there is also only 1 watt. Even on Earth, there are deeper sea plants and perhaps shade-loving plants that thrive with as little light as Titan gets or less. So energy-wise, plants are possible but likely to be slower-growing. On the other hand, the limiting factor to plant growth on Earth is not sunlight but the availability of that vital trace gas, carbon dioxide (CO2). If extra CO2 is introduced in a greenhouse, plants grow faster.
There is probably a trace gas or multiple trace gasses plants need for photosynthesis on Titan, too, so if that is somewhat more abundant than our CO2, it does not necessarily follow that Titanian plant growth is hundreds of times slower than Earth's. Indeed, one may suspect from their size that they must grow rapidly indeed, or else live a very long time. Plants could also filter out various trace organic aerial compounds for their carbon, and combine the carbon with the hydrogen to produce... you guessed it, CH4, methane. One report suggests that Titanian plants could gain indirect solar energy by converting the energetic particles formed by ultraviolet induced methane breakdown in the upper atmosphere (where the sunlight is full strength), and thence drifting down to the surface, back into methane, and that this cycle could actually provide more energy than direct photosynthesis itself.

Two factors limit the size of trees on Earth: gravity and wind. Titan has only 1/8 of our gravity but the air is thicker, so how tall plants and tress can grow on Titan is likely to be limited by their resistance to being uprooted or broken by the wind, which in turn is related to the highest normal wind speeds. Huygens measured 15 Km/hour surface winds, but we have little idea whether more severe weather is to be expected frequently, rarely or never. But Titan does not currently seem to have trees as such. Instead, various types of gigantic vines, or the root systems of various aquatic plants, grow out of the sea and spread over the land. They all seem to originate or end in the sea. Perhaps the land is currently too dry to support plant life. Earth was very dry in the Permian period and the early Triassic period on Earth.

Other aquatic plants prefer to stay in the methane, growing leaves up their long stems or growing up to the surface and forming giant "water lilly" mats. Whatever else they are, some Titan plants are Titanic! (What a well-named planet!) It is possible Cassini has been seeing beds of them from space, and it seems small wonder that the shallow, very transparent seas with mats of leaves on the surface don't provide very distinct shorelines to Cassini's imagers.

I should mention some forms of Titanian plant life I've noted but haven't touched on elsewhere.

One is "Titanian Kelp". On Earth, beds of kelp grow in shallower ocean areas with strong waves. These annuals consist of anchor roots, a very long stem, a float bulb, and limp leaves. The float bulb drags the leaves up to the surface where there is the most light. Titanian kelp is probably similar, but the float bulbs and their waving leaves seen in the descent images are gigantic, instead of bulbs just a few inches across. Features in SLI 698 (below left) may be a bulb and some leaves. The possible "bulb" looks like it has eyes, and double eyelike features are seen in a number of other places as well.

Another is, for want of a better word, "horsetails". There are what appear to possibly be striped stems at various sizes from huge in the descent images to small at the bottom center of the SLI after landing images. This and other vaguer ones on the ground are too close to the camera and out of focus, and may be ordinary dark stems with leaves at intervals (they may be "horse tales" instead!), but the ones in the descent images certainly have much the appearance of horsetails.

Finally, features dubbed "cat scratches" in Cassini RADAR images seem to be giant horizontal floating stems, aligned with the tidal flows. Two match up between the RADAR and distant Huygens views. One in the Huygens views appears to connect with the huge root or stem crossing the land area and once dubbed the "landing strip". What may be a smaller specimen of one of these is seen floating on the sea in image SLI 710. (Note the "horsetail" stripes on the stem.)

Character of Titanian Animal Life

July 2006: I've just looked at PIA08112 and other recently released COLOR images from the Huygens probe. PIA08112 is a bear to download and try to view, but it is six of the best views ever done of Titan looking downwards as Huygens descended. It appears the spectrographic data was incorporated into the imager data to provide actual if low-rez color, but the caption is unenlightening. I can't say with any sort of assurance, but there are some patches, especially at the right end of the "oval", that are most reminiscent of "bio-diverse" beds of mussels and clams with seaweed and snails, attached to rock outcroppings. (but if so, they are gigantic!) Mussels and clams are of course animals, with freeswimming larvae, so this (along with the idea that some of the roundish objects in the after landing view may be clams) is another bit of - very tentative - evidence of animal life on Titan. Or it could all be seaweed or even boulders, giant gravel bars. This excerpt is from 8112's third view (middle left):

Excerpt from PIA08112-c. Huygens' landing site is towards the lower left. Near the shores, one seems to get some idea of the depth of the liquid, and there is the sense that liquid is flowing, pouring over the rock, from left to right, Eastwards. This is in fact the correct direction for the tidal flow at the time of the landing.