Abstract

Related topics that may well come to the reader's mind but which I won't discuss here include:

* how life might originate, evolve or live on an airless world (we don't know how it started on Earth either)
* Whether the "sap" is a liquid or a gas and its composition (I presume there's something of that nature - it's probably liquid; living membranes could doubtless hold pressure in.)
* how it spread to the other worlds from the one it started on
* whether carbonaceous chondrite meteorites that contain organic materials have their origin in one or more of these spheres
* whether bits of airless world vegetation have made their way to Phoebe, Hyperion or other minor solar system bodies, accounting for possible complex organic tholins and the dark colors seen on them.
 

Surface Compositions

Reviewing the main topography of Iapetus, the bright arctic and antarctic areas appear to have more CO2 ice than the more temperate areas. The light terrain of the trailing hemisphere is no doubt H2O and other ices and hydrated minerals, and the dark color of the leading face is a thin, fluffy*1 low density layer of complex organic materials[1, 2, 7, also see 6], evidently on top of the underlying light colored solid surface. Sulfur isn't mentioned in reports I've seen, but is probably present.

Iapetus's trailing hemisphere is frequently bombarded from behind by ionized particle radiation from Saturn's magnetic field. This would sterilize the rear facing surface. Bombardment in border areas depends on local topography: elevations and angles. High latitude areas have winters with no heat and light for up to 15 Earth years. Thus, the global brightnesses seen appear to be an excellent match for sterilized or very frozen versus livable areas.

The front of Iapetus has this form of two layer surface in common with Ganymede and Callisto [3, also see 4, 5, 6]. The surface of Ganymede shows more lateral variation within local areas but no hemispheric variation as its own strong magnetic field to protects it from Jupiter's field. Callisto with its weak, fluctuating magnetic field, evidently has less tholins on its trailing hemisphere (the blue-green area at the right of the globe above is in the leading hemisphere).

That the two layer surfaces exist, with a very low thermal density layer on top of a much more solid one, were deduced for Ganymede and Callisto from diurnal temperature reading variations as early as 1973[5]. For Iapetus, a slightly lower thermal density yet was found by Cassini[7]. Whether the dark organic materials are just dust and grit or whether they're composed of larger distinct objects, some may think the new views of Iapetus are too distant to ascertain, notwithstanding that some large scale features are visible to close scrutiny.

But the Ganymede and Callisto thermal evidence "best fit" description of the surface indicates that sunlight penetrates through this upper layer[6], and this, in common with the low density, well describes vegetation and poorly describes complex organic dust or grit, however porous it may be (within reason).

How could airless vegetation that (doubtless) originated in the Jupiter system thrive on a tiny sphere in the Saturn system, which gets only 25% as much solar energy? Surprisingly, Iapetus' dark side daytime temperatures hit about 130 degrees K, hardly 10 K lower than Ganymede. The light and temperature difference is no more than for plants on Ganymede living in a somewhat shaded locale.

Finally, one might well ask, Why these worlds but not, eg, Rhea or Europa? The answer may well lie in the soils. On Europa the heavier materials have sunk deep inside leaving a glare ice crust. This is probably the case for any of the inner moons of Jupiter and Saturn, including the larger bodies: Rhea, Tethys and Dione. One supposes that the geologically quieter outer moons attained solid surfaces earlier, and hence have surface soil consisting of deposits from later meteors and comets. The tidal churning of the inner spheres left mostly solid ice surfaces instead of soil until a pretty late date, when there weren't many good meteors and comets falling any more. Io, probably Europa and perhaps Enceladus are still too active to permit a buildup of meteoric mineral soil depsoits to remain undisturbed on the surface.

Callisto Images of Vegetation

The reasoning to explain this strange terrain was dubious. It was that bright ice had "sublimated away" leaving the darker organic grit behind between the "icy hills". But where did the ice go to? Why were incredibly steep "hills" left behind? By what mechanism could the ice travel away in the absence of an atmosphere? Why would ice sublimate at a temperature well below that where it is in a state susceptible to sublimation? Why didn't it happen on Ganymede or Europa?

Regardless, I didn't disagree with the "hills" description at the time. My mind simply rejected the idea that vegetation could assume such gigantic proportions, hundreds of meters in height. But the limiting factor to the sizes of vegetation on Earth is gravity. After seeing what appears to be huge vegetation on Titan I began to realize that enormous sizes - by Earth standards - are to be expected on worlds of light gravity. Conversely, on a world of high gravity, trees would be stubby little things compared with ours.

Unprejudiced inspection of details within the images reveals the nature of the surface to be entirely different than what was supposed. Some of the "hills" look like clumps of towering trees. The illusion of "steep icy hills" is created by the bottom parts of the trees being darker than the upper portions, shorter trees on the north side of many clusters, and by tall "underbrush" or shorter grayish trees forming many of the south faces, giving a steep 'hillish' silhouette. But there are also single trees, and the shadows clearly show that the appearance of seemingly vertical forms is real. The "icy hills" interpretation ignores several important aspects of the images. (Thankfully these are perspective views and the images weren't taken at noon, or we'd be calling them 'icy craters'!)

Callisto - Clip from Galileo image PIA03455, 5.6 meters per pixel ground scale, magnified x 2.
 (South is to the right; the shadows fall eastward; the "local time" angle is around 5:00-5:30 PM.)

In the area above, the impression they must be trees and clusters of trees is strong (not to say bloody well obvious!), with forms steeper than vertical showing clearly at a number of points. The brighter portions of the trees can in places be seen to begin above the ground... often about at the top of the "underbrush". And, if long thin branches aren't very evident, their shadows are, especially at the top of the large shadow at the center of the left edge of the image. All over the surface, which to a casual look looks pretty featureless, are fine vertically oriented features that would appear to be faint outlines of vegetation. Evidently, though the underlying contours of craters are visible beneath, we are looking into a canopy of large bushes or trees, of more Earthlike heights and mostly with fairly similar albedos. There appear to be several species. There are many more "fine" details in the scene than I would care to describe, but I note a couple of interesting ones:

Callisto - A detail (x 3) from the previous image.
At least 3 of these "bright patches" appear to start above the ground, as if supported by trunks. The appearance is more like balls of cotton candy than of solid ice. The two center ones are perhaps 125 meters tall, about the height of the tallest Earth trees.
There's an interesting "spike" sticking up from the leftmost cluster, whose long, thin shadow can be seen behind. The fine shadows of other thinner branches are also seen in this area, which can by careful observation by and large be traced back to silhouettes of almost invisible branches in front of the main shadow area.
Could any imaginable lifeless terrain have these sorts of features?

Is there more evidence that those bright tree tops and other "terrain" is foliage and not solid? A further visual case for that follows. For images with such high brightness contrasts, long exposures are needed to see into the dark areas, but then the bright area pixels often become saturated to FF's and no detail can be seen. The illusion of a smooth, icy white surface is created. A further complication of the Galileo images is that when the pixels become oversaturated they "spill" white into nearby pixels and even more image area is lost than the originally saturated pixels, obscuring even the shape of the bright area, as well as the details within. However, at the left edge (here rotated to be the bottom so vertical faces up) of perhaps the highest resolution image the Galileo ever took, 5 meters per pixel, the texture of the features shows through somewhat under the "spilled paint". If they're not some sort of branches with leaves, they're doing a fine job of mimicking them. This is best seen in the top-center of the rightmost clump shown (also in x 3 inset), just under and right of the large diagonal "paint spill", where several quasilinear forms with bright tips can be seen:

Callisto - Clip from Galileo raw image 605145127, 5 meters/pixel, Magnified x 2.

The bright tips of the shoots or branches visually match the bright treetops of the top image and are probably what those tops are composed of, perhaps explaining why the bases of the trees are darker than the upper portions.

The bushy nature of the features also appears to show up as a fuzzy silhouette in front of many if not most shadows, for example in the crater below. (Also see the bottoms of shadows in the first image.) While there is doubtless some ICT compression artifacting, the thin "bushy" material appears in compression blocks that should be wholly black if the surface were solid, verifying that the bushy silhouette is real. Again the shadows cast on the far wall by the more solid features at the front confirm the features are actually vertical rather than icicles drooping down the side of the crater, and some sunlight gets through the bushes to give a fuzzy top zone to the shadow. The appearance is in good harmony with the low density, sunlight admitting layer deduced by the thermal investigations, and vegetation is also in keeping with the spectral findings of complex organic materials:

These are the best images we have. If you're not seeing what I'm writing about, you might as well stop reading. Perhaps this is at least enough that the next generation will want to return for a better look. For myself, I've seen that Earth is by no means the only world covered with life, even in our own solar system.

We'll now leave Callisto with this continental scale natural color image. If it was about twice as wide to the right as it is, it would show the area in the hi-rez images above, and it would be interesting to see if they could be correlated. The images composing it were taken years before the hi-rez ones, doubtlessly at a different time of day.

Clip from PIA00562

Iapetus Images Seeming to Show Vegetation Similar to the Callistan Vegetation

There are several smaller bright points that look akin to the bright Callistan tree clusters. They are more distant, so we wouldn't suspect they might be trees without having already seen similar shapes on Callisto. They are still too small to be sure, but the resemblance is there. It also appears possible that some of the bright tree clusters have spread to cover very large areas, which we didn't see in the Callisto images.

Close examination gives the impression (though by no means the certainty) that the bright areas are on top of the darker areas, not the other way around, which would make them very different from the bright terrain of the trailing hemisphere. Regrettably those brighter areas are seen as saturated white here, giving the impression of ice or snow which probably masks its real nature and texture as with the Callistan treetops. Some points that seem interesting are detailed by letters and insets below.

I don't know the scale of the image, but some of these things seem even bigger than what's on Callisto or Titan. Probably they could only attain such stupendous sizes on a miniature world like Iapetus with .025 G's gravity! They aren't massless, but there they are almost weightless in comparison to their size.

I was at first shocked by this image of the bright side of Iapetus. Overall it's bright, but there are dark areas scattered everywhere! Could this somehow disprove that dark areas are vegetation? Inspection of the full scale image reveals that the dark areas are all in valleys, "cracks", gorges and meteor crater bottoms. But how could there be anything living anywhere in the path of Saturn's magnetic field, which strikes impartially from straight behind? But it's not the field that kills, it's the ionized particles it hurls at the surface. And ions are electrically charged, so like lightning they are attracted to the nearest point: the high ground. Perhaps the magnetic field lines themselves are bent towards the ridges. It would seem that all the ions veer to strike the tops of the steep slopes, leaving the nearby deeper valleys radiation free. Thus it would seem that in favorable low-lying locations, vegetation can indeed grow. An in depth study of this phenomenon might well yield a point by point explanation of why every dark area lies exactly where it does, demonstrating in detail the correlation between ionic bombardment and the dark/bright terrain.

I hope Cassini may yet be able to get some closer shots of Iapetus sometime before it expires. Better resolution was achieved than I expected from that flyby distance, but probably still, the images are only good enough to convince the convinced and intrigue truly open minded scientists, and they won't move those who scoff at the idea of life on airless worlds and see no reason to bother examining the image details, much less to go back for a closer look. Much that is unclear at 20 or 30 meters per pixel would doubtless be readily evident at 2 or 3, or at 5 as on Callisto (or even at 10 or 20 in color).

On the other hand, even with life Iapetus is a pretty small world to turn plans upside down for. Where are some 20 and under meters per pixel shots of temperate and polar zone Titan?, to show those things that look so much like trees in the radar images that in a blind test (ie, without being told it wasn't Earth), a maps and surveys professional who sees Earth satellite imaging every day unhesitatingly described a Titan radar scene [PIA08740 - The Kissing Lakes] as "a shallow lake surrounded by mixed forest."

References
 

[1] The Astrophysical Journal, volume 622, part 2 (2005), pages L149-L152
B. J. Buratti, D. P. Cruikshank, R. H. Brown, R. N. Clark, J. M. Bauer, R. Jaumann, T. B. McCord, D. P. Simonelli, C. A. Hibbitts, G. B. Hansen, T. C. Owen, K. H. Baines, G. Bellucci, J.-P. Bibring, F. Capaccioni, P. Cerroni, A. Coradini, P. Drossart, V. Formisano, Y. Langevin, D. L. Matson, V. Mennella, R. M. Nelson, P. D. Nicholson, B. Sicardy, C. Sotin, T. L. Roush, K. Soderlund, and A. Muradyan
http://www.journals.uchicago.edu/cgi-bin/resolve?id=doi:10.1086/429800&erFrom=-1648140469803188713Guest

Abstract

  The Visual and Infrared Mapping Spectrometer (VIMS) instrument aboard the Cassini spacecraft obtained its first spectral map of the satellite Iapetus, in which new absorption bands are seen in the spectra of both the low-albedo hemisphere and the H2O ice-rich hemisphere. Carbon dioxide is identified in the low-albedo material, probably as a photochemically [photosynthetically?] produced molecule that is trapped in H2O ice or in some mineral or complex organic solid. [the last would stand to reason] Other absorption bands are unidentified. The spectrum of the low-albedo hemisphere is satisfactorily modeled with a combination of organic tholin, poly-HCN, and small amounts of H2O ice and Fe2O3. The high-albedo hemisphere is modeled with H2O ice slightly darkened with tholin.<snip>
 

[2] This week: Dr. Bonnie Buratti Looks Back at Iapetus
Sept. 27, 2007
Todd J. Barber, Cassini lead propulsion engineer
http://saturn.jpl.nasa.gov/news/insider/insider20070927.cfm

"I asked Bonnie about early thoughts regarding the chemistry of the dark material, and without hesitation she confirmed the presence of hydrocarbons, even polycyclic aromatic hydrocarbons (PAHs). These intriguing molecules are nothing less than the suspected building blocks of the molecules of life, so their incidence raises more than a few eyebrows among scientists attempting to understand the origins of life on early Earth. Seemingly impossibly, Iapetus just became a lot more interesting."
 

[3] Scientific American, February 2000, The Galileo Mission to Jupiter and Its Moons.
Torrence Johnson (Galileo team leader)

[Callisto]
"Intriguingly, near-infrared spectra show not only water ice and hydrated minerals, as expected, but also four unusual absorption features near a wavelength of four microns. One appears to be carbon dioxide trapped in the surface, perhaps as inclusions in icy particles or bubbles produced by radiation damage to the surface.[? There seems to be little or no radiation on Callisto.] Two other spectral features probably represent sulphur in the surface, which may originate in Io's volcanic eruptions. The fourth spectral feature is the strangest. Its wavelength corresponds to that absorbed by carbon-nitrogen bonds. In fact, laboratory spectra of complex organic molecules called "tholins" by the late Carl Sagan are similar. Tholins are thought to resemble organic material in the solar nebula; clouds of interstellar ice grains have comparable spectra. Taken together, the data provides the first direct evidence that icy satellites contain the carbon, nitrogen and sulfur compounds common in primitive meteorites and comets. These materials are also some of the most important for life."

[Not mentioned in this article is that Ganymede spectra had much in common with Callisto WRT these materials. The CO2 finding appears to be similar to that of Iapetus, where it has been inferred that the CO2 is probably a "photochemical" effect. That some of the spectra were unknown isn't mentioned here, but it is in other writings. BTW Io has sulfur, but why would it have been the only Jovian moon endowed with it?]
 

[4] BENTON C. CLARK - Sulfur: Fountainhead of life in the Universe?

[The article begins with this preamble:]

"Sulfur is ubiquitous in the Universe and essential to all life forms
that we know. It supports the chemoautotrophic way of life and the
photosynthetic. It may inhabit niches we cannot imagine, and the life zone
about a star may therefore be wider than now estimated."
 

[5] Morrison and Cruikshank (1973); Hansen (1973)
 

[6] http://www.boulder.swri.edu/~spencer/dissn/therm.html

[In his work, Spencer points out a "difficulty" with the excellent thermal fits of the earlier studys' two layer models, but then notes that the objection would be invalid if sunlight penetrates the top layer. Thus, remarkably he describes, sight unseen, the "best fit" surface to be a two layer model with the top layer being vegetation. (...or else, solid but super light density and largely transparent complex dark organic material, which the images in no way support. Perhaps the reader can think of even more ridiculous scenarios.)]
 

[7] CASSINI CIRS OBSERVATIONS OF IAPETUS’ THERMAL EMISSION.
J. R. Spencer1, J. C. Pearl2, M. Segura2 and the Cassini CIRS Team, 1Southwest Research Institute, Dept. Space Studies, 1050 Walnut St. Suite 400, Boulder CO 80304, spencer@boulder.swri.edu, 2NASA-Goddard Spaceflight Center, Mail Code 693, Greenbelt, MD 20771.
www.lpi.usra.edu/meetings/lpsc2005/pdf/2305.pdf

"Thermal Modeling: Combining the FP3 daytime temperature maps with the FP1 dusk observation, we can constrain the thermal inertia of Iapetus’ dark material. Thermal inertia is remarkably low even compared to other airless icy satellites. The diurnal temperature profiles of Europa, Ganymede and Callisto can be fit with thermal inertias near 5 x 104 erg cm-2 s-1/2 K-1, [3,4], but Fig. 3 shows that a lower thermal inertia of ~3 x 104 erg cm-2 s-1/2 K-1 is needed to match Iapetus’ daytime and evening temperatures. [I doubt this is correct for Europa; don't have time to look into it.] Phoebe also has a low thermal inertia (2.5 x 104 erg cm-2 s-1/2 K-1, [2]), which we tentatively ascribed to its rapid rotation rate (rotation period = 9 hours) compared to the icy Galilean satellites, so that the diurnal wave sampled the least-compacted upper layers of the surface. However, this explanation does not work for Iapetus (rotation period = 79 days), indicating that the dark terrain may have a more porous surface than even the superficially- similar Callisto." ["Porous"? how about "nebulous"?]
 

Footnotes

*1 - "Fluffy" is a term that has been used to describe Callisto's surface, and it appears again for Iapetus in the temperature graph shown ("i.e., very fluffy"). As with ref [6], these authors were compelled to describe the surfaces in terms descriptive of vegetation.

*2 - Evidently the Iapetus image came from "raw images" at http://saturn.jpl.nasa.gov . I haven't had a chance to check up on the image metrics, if they are available. I selected this image from a number of possible candidates for its interesting features.