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Through a 12-inch telescope the change in brightness from the leading side to the trailing side is clearly visible. That is what Cassini noted in his small telescope, that the satellite virtually disappeared as it orbited Saturn.

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This light side / dark side appearance has been a mystery, unsolved yet, but becoming closer. Still, it might be a century. . .until our species knows . . .or our species may somehow stumble, like we have recently, and not get up. . .

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This moon's name is sometimes spelled Japetus, though more rarely nowadays.

Giovanni Cassini discovered Iapetus in 1671. Cassini discovered the major gap in Saturn's ring system that bears his name (Cassini's division, the Cassini division, the Cassini gap) which is product of (mostly)Titan's orbital resonance with the particles in Saturn's huge ring system.

Cassini noted that the moon (he named Iapetus) was brighter on one side of Saturn than the other -- a fact known now for 336 years, but still without clear explanation -- and likely not to be credibly explained until a probe made for the purpose is sent to land there and examine the light surface and the dark surface.

That isn't likely to occur soon unless this last scheduled flyby reveals something of surpassing interest. With so many terrestrial problems related to Earth's exponentially growing population -- our species may not be able to muster the wherewithal to seriously expand our knowledge of the vicinity of the gas giant planets, and certainly not soon. That said, I hope the high resolution images of the equatorial range of mountains scheduled for ths flyby reveal something of surpassing interest, as well as images gathered elsewhere in the borderlands between the light and dark material.

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Check this image: http://ciclops.org//media/me/2007/3730_8554_1.jpg
I'll try to get this link fixed.

Images: Nasa.

THE CASSINI FLYBY:

http://ciclops.org/view.php?id=3730

I recommend interested readers go to the site and read the desciptions of what is planned to resolve the 300-year old dark side/light side mystery that Iapetus presents.

None of us, here, except perhaps the very youngest of us, will live long enough to see another probe sent to Saturn and Iapetus to resolve the light/dark mystery unless something of extreme interest is detected in these nearest approach images that can pique our species attention enough to force us to build and send another probe. Can any of you think of anything we could image on Iapetus that could do that?

Hopefully the team of those who chose the targets for this flyby opportunity will have chosen images that show us answers to that old mystery and the new mystery of the equatorial mountain range, and be definitive. Otherwise I think these are good solid choices to further fleshout our overall knowledge and coverage of the bright-sided face of Iapetus as well as the dark side.

As I write this it is history. The plan began 33 hours ahead of closest approach.

http://ciclops.org/view.php?id=3730

Planned image coverage for Cassini's Iapetus Flyby of September 10, 2007 Is presented in this image from http://ciclops.org

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Note: Image products created in Celestia. Iapetus map by Steve Albers.

The images recorded in 2005 suggest that the dark material was "recently deposited" from an outside source, and these higher resolution images may help reinforce or deny the theory. The singular lack of fresh "white" craters in the dark material argues the recent formation case strongly. No one can as yet rule out Titan as a source of the material coating the orbitally leading surface, though the dynamics are not apparent.

ANOTHER MYSTERY

Another mystery is the equatorial ridge of very large size (13,000 meter-tall mountains above the reference ellipsoid, and some places 20,000 meters tall above the plane) within the dark region of Iapetus, roughly bisecting this dark "Cassini Regio." (Well, better than roughly bisecting.. .it is more like as good as you can imagine in nature.)

These are images taken along the equator.  But in some future, the Triple Range area, which we will need to see, hopefully soon.[less than a hundred years.]

The 2005 images indicate an ancient surface with five impact scars larger than 350 km across. The straight ridge along the moon's equator is something not seen before, but a number of hypotheses have arisen to explan it, none that yet convince me.

One of the most interesting (to me) images planned is the single frame to be acquired covering the large impact basin in northeastern Cassini Regio, to correspond with the earlier CIRS polarization observation, in Saturn-shine. Of particular interest in this planned image is a crater located at 6° North, 36° West. Earlier observations revealed a landslide deposit covers most of the floor of this crater. (The collapse of a portion of the the south wall of this 11 km tall scarp within what appears to be a smaller flat-bottomed crater is intriguing -- so I am hopeful of learning more from this image -- though I would have very much liked a very high resolution image of the top and bottom of the landslide area.)I am still waiting to see the image!

This does not look to me like a crater in a crater.  It looks like an old ice mine. That has now collapsed.

Examine this image:

http://ciclops.org/media/ir/2005/711_1413_1.jpg

I suggest Iapetus is a Saturnian "capture", and that Iapetus is something that drifted in from beyond Neptune, from out in the Kuiper Belt. and eventually was captured by Saturn. The high inclination of the orbit to Saturn's equatorial plane suggest Iapetus is a later add-on to the system -- yet still ancient -- meaning it was captured early is the history of the solar system. I will be long gone before that theory could be proven.

I do not subscribe to one of the popular current Iapetus-equatorial-mountain-range-theories which involves the radioactive decay of Al 26, a phenomenon that happens early in the solar system formation stage, and seems quite reasonable. Since Iapetus is the only object in the solar system to exhibit such a structure -- what happened to the others that likely could also have shared this phenomenon? (Ganymede, Callsito, even perhaps Europa -- though deep enough in Jupiter's gravity well to undergo tidal heating. Callisto? how much of it was melted, and is it still harboring a fluid core?) The idea of 10% "rock" by mass for Iapetus would perhaps provide enough heating to entirely liquify or fluidize the moons interior, but the posit of and the moon's highspeed rotation surface flexure thrusting these ranges above the surface seems a little hard accept. I hope imaging acquired during this flyby helps to negate the current ideas of how the equatorial range formed -- or provide something to more closely confirm one of the ideas.

Later, under much more extensive processing, these "dark things" in what looks like white water ice craters may yield the reddish-brown color of the dark side of Iapetus! Only common powdered coal (Black and wet) could appear any darker. The albedo is in the .03 to.05 range, while the light side is in the .6 to .7 range.

Below is the most disturbing image I have seen so far of the exhibited released images! It turns some of my thinking about the reddish stuff on its head! But I may be okay after reflection upon it. Anyone else see the disturbing part of the images?

Here is some NASA text for this image:

"Dark material splatters the walls and floors of craters in the surreal, frozen wastelands of Iapetus. This image shows terrain in the transition region between the moon’s dark leading hemisphere and its bright trailing hemisphere. The view was acquired during Cassini's only close flyby of the two-toned Saturn moon.

The image was taken on Sept. 10, 2007, with the Cassini spacecraft narrow-angle camera at a distance of approximately 6,030 kilometers (3,750 miles) from Iapetus. Image scale is 36 meters (118 feet) per pixel."

Image: NASA, JPL, CICLOPS

Imagine walking this terrain! Perhaps your great great grand children will.

Please realize this: It is far easier to engineer a two-hour walk in a space suit on the icy surface of Iapetus -- than to engineer a two-hour walk anywhere on Venus.

Why? Well, you can take a little heat source with you and walk around on Iapetus. Even build a habitat there like on the Moon (Earth's "Moon") but likely more homey since Iapetus seems to have plenty of water to melt -- and maybe the brownish reddish stuff can be used to make food. In fact, there are lots of places in the solar system that are better abodes than the Moon!

But Venus is actually "impossible" for our kind of life form to build abodes upon or deep within.

Problem with Venus: Too hot. No cheap way to get rid of the heat. Where would you put it? The vacuum bottle of space? Radiate it into space? That is the only way you can get rid of the absorbed solar energy on Venus. Imagine taking one of the outer solar system Ice moons and gently snowballing Venus, melting all that water ice and creating a pool of liquid water miles deep on Venus. Would that cool Venus?

I may geek it out later, the math that is, say for an Iapetus-sized body or a big body like Callisto brought to Venus. But unless you can long-term turn the Sun down, I can't imagine even a huge volume of water brought to Venus having a lifetime of 500,000 years, and those would be hot muggy years. Only way to cool Venus down would be to move it a long way out from the Sun, into the Mars orbital or asteroid belt distance, and then strive for radiative equilibrium and add a water moon. I think a few hundred thousand years would see some significant cooling? Well we would have to geek the figures to see when you could walk on New Venus!

Yes, it is easier to for your walk on Iapetus. Or the Moon.

As Venus is, Earth will become. Remember that.

Image NASA, JPL, CICLOPS

ALL IMAGES: NASA
Here are the ancient equatorial mountains of Iapetus.
Note: The white areas near the top of the mountains, on their flanks and edges of their crests appear to be "white areas of ice" not covered by the reddish-brown dark side materials, adjacent on these gigantic peaks, as though these are steep areas not covered by the impact or fall of the dark material.

More details near the terminator:

Image: NASA, CICLOPS; raw image!

I will likely modify these comments over the next few days weeks and months, as I get to examine the images that will undoubtedly make history.

I encourage you to visit the noted website.

Imagine, more than a billion kilometers away. . .