New Iapetian image series Options

[Rob Pinnegar]

The view of July is finally giving us a view of unexplored terrain, just prior to the close encounter of september 2007.
Really looking forward to that.

Yeah, that area was imaged in August 2004, but from so far away (10 million kilometres) that the only details visible were the basic outline of the Roncevaux Terra basin, and its central peak. So, although we *have* seen it, it's effectively unexplored.

The LPSC 2005 abstract by Denk et. al., "The first six months of Iapetus observations by the Cassini ISS camera", shows processed versions of these images. It's online and is easily located with a Google search. (Don't bother checking the raw images from August 2004 -- you can't see anything in those.) This gives a good idea of the size of that basin. It's about the same size as the huge one on the Saturn-facing side.

[TritonAntares]

Hi!

Yeah, that area was imaged in August 2004, but from so far away (10 million kilometres) that the only details visible
were the basic outline of the Roncevaux Terra basin, and its central peak.
So, although we *have* seen it, it's effectively unexplored.

I've looked through the images of this period and probably found some of those you meant.
Here a takeout:

Date: 2004-07-22
Distance: 3.489.136 km
Filters: CL1 and CL2
Difficult to say wether it's the western rim of the large southern bassin in Roncevaux Terra at the terminator.

This chart could be helpful:


The eastern rim of the bassin is nice in this pics from October 2004:


Bye.

[ugordan]

The lower left image is the same as the one I made a while ago, using stretched color, but toned down to better match the approximate true color. The contrast is stronger in my composite so darker details aren't very visible, though: Iapetus in color.

There's also the CICLOPS release showing the same image sequence in stretched color.

[TritonAntares]

Hi!

CASSINI took another few Iapetus images about 10 days ago,
here 3 of them:

Date: 2004-09-18, 20 and 21
Distance: 2.359.595 / 2.486.163 / 2.517.820 km
Filters: CL1 and CL2

Nice sunset out there...

Bye.

[nprev]

That recent small sequence is really striking to me; Iapetus looks a lot like a CD or DVD!

In fact...is this significant or coincidental? It looks like the dark area subtends a suspiciously well-defined angular area in this view (90 deg of the total spherical area of Iapetus?), which in turn would seem to argue for external deposition from some source. In any case, the geometry of the dark area is remarkable.

[tasp]

I suspect a thermo-reactive gas is introduced into the Iapetan environment at a fixed point in its' orbit about Saturn. Most likely when Iapetus traverses the Saturnian magnetotail.

The gas persists in the vicinity of Iapetus, and in equatorial to the midlatitudes the Iapetan surface temperatures are sufficient for the gas to 'tholinize' and coat the surface. The gas is used up (or dissipates into the void) in less than the ~80 days it takes for Iapetus to return to the place in it's orbit where the gas is replenished, thus, we do not see the darkening extending further around in longitude than we do.

That there is some 'ponding' of the gas in low areas seems apparent, there is some darkening outside of the main areal extent of Cassini Regio in the lower latitudes.

Additionally, the more directly perpendicularly sunward facing segments of crater bowls north and south of Cassini Regio also show the darkening, as expected, the local heating conditions being sufficient for the chemical staining reaction to occur.


The gas also appears to interact with Hyperion. Quantities of it appear to pond in the craters, and when the chaotic rotation of Hyperion causes the sun to shine perpendicularly into a given crater, the crater bowl reflects enough solar radiation onto the crater floor to make a local warm spot. The gas reacts, and we get dark crater bottoms all over Hyperion.

The same basic mechanism, a thermo-reactive gas being introduce into the Hyperionian and Iapetan environments, explains the dark areas of each moon.

[nprev]

That is both an interesting and highly creative hypothesis, Tasp. Skeptical questions must follow, though:

1. What is the nature of this thermoreactive gas? I presume that it is being scattered off of Saturn's upper atmosphere somehow.

2. Why do no other moons but Iapetus & Hyperion exhibit any apparent effects from this environmental condition? Probable minor compositional differences do not seem adequate. Additionally, this thermoreactive agent would presumably have a relatively high molecular weight, and therefore its transport throughout the region would not necessarily be confined to the outer limits of Saturn's magnetosphere in accordance with magnetic current flow patterns.

Sorry; I am not trying to put you on the spot at all...just trying to flesh out this most interesting thought of yours, which would have never occurred to me!

[tasp]

The gas comes from Titan's atmosphere. This implies only objects exterior to Titan's orbit will experience the darkening as there is no transport mechanism running from Titan inward.

The atmospheric gases of Titan contain carban and nitrogen and hydrogen. The thermosensitive chemical reaction (way out of my schooling here) might be a polymerization of those Titanian gases. I suspect the orange color of Titan and the rich brown of Iapetus are derived from similar compounds and elements.

I don't know if slow steady leakage of atmospheric gases from Titan is what is happening, or if big impact events on Titan liberate gases sporadically.

Maybe both processes have occured over the history of the solar system.

[TritonAntares]

Hi!

CASSINI took another few Iapetus far distance images 3 days ago,
here a takeout:

Date: 2006-11-05
Distance: 4.438.289 km
Filters: CL1 and CL2

View with Solar System Simulator:


By the end of this month we'll get a nice view from about 2 mio. km:


Finally a little change in the angle of view....

Bye.

[ugordan]

Here's the simulated pixel size Iapetus will appear on Nov 27. Obviously not that useful except for low phase observations.

[TritonAntares]

Hi once again,

CASSINI took another four Iapetus far distance images,
here a takeout:

Date: 2006-11-16
Distance: 3.128.691 km
Filters: CL1 and CL2

View with Solar System Simulator:


We're getting 'closer'...

Bye.

[TritonAntares]

Hi,

CASSINI took a series of 41 Iapetean far distance pics yesterday,
here a first takeout:

Date: 2006-11-27
Distance: 1.997.980 km
Filters: CL1 and CL2
There's an interesting dark feature east (right) of the 'Snowman'.

View with Solar System Simulator:


Part of Steve Albers map:

The mentioned dark structure contains at least one crater with a dark ground.

Bye.

[TritonAntares]

Hello again,

I always wondered about the dark patches in Iapetus' bright region Roncevaux Terra as seen on this map:

The left part is illuminated by saturnlight, the right part is made up of VOYAGER findings.
The mentioned dark feature east (right) of the 'Snowman' contains at least one crater with a dark ground.
But what about the dark markings on the bad resolved VOYAGER side?
Are these mostly shadowing effects like the craters in the northern hemisphere presume or real dark patches like the 'horseshoe' next to the border of the saturnlighted part or the black triangle and the spots around it?
Actually the triangle partly seems to overspread an underlying not so dark area...

Some ideas?

Bye.

[angel1801]

I would not know either. But we will know alot better after September 10, 2007 when Cassini will image this area with resolution as good as 200m per pixel!

[Phil Stooke]

The latest two Iapetus images, enlarged and merged.

Phil