Approaching Iapetus - what makes it two-faced?

[vogon13]

I suspect the dark coating is very, very thin, and it renews itself. The color, as it turns out, is very similar to the dark crater floors of Hyperion.<br /><br />Iapetus is geologically dead, and has been so for a very long time.<br /><br />I'm hoping, perhaps, a recent impact crater on the equatorial ridge will be viewed at high resolution and might reveal the laminations that the incoming ring chunks made as they accumulated. Compositional variations in the ring materials should be visible in cross sectional views of the ridge. Depending on 'skidding effects' as they contacted the accreting ridge, perhaps some of the 'chunks' will still be evident. (this comes down to the material strength of an icy block being able to withstand the decel forces during emplacement. Obviously, 'splats' are going to pulverize the incoming materials, but a glancing contact on the top of the ridge, and a skidding slide out might leave recognizable pieces.<br /><br />Sure to be an exciting encounter.<br /><br /><br />Also, the detailed appearance of the transition between the dark and light zones on the 'Voyager Equatorial Peaks' may be very conclusive in understanding the dark material of Cassini Regio.<br /><br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[3488]

Hi vogon13.<br /><br />I agree too that Iapetus has been geologically dead since shortly after its birth. Yes the covering is extremely thin IMO too.<br /><br />I will pursue the idea of reducing Cassini's approach distance as already following my initial enquiry, that it would not interfere with the rest of the mission.<br /><br />Iapetus is dead, but is still of huge interest (to me anyway) none the less. <br /><br />What ever happens, the Iapetus encounter in September is going to be fascinating!!!!<br /><br />This sort of thing keeps me going, first to try & convince JPL to image Io on I33 & Amalthea on A34 with the Galileo mission (Unsuccessfully unfortunately, an opportunity squandered), assisted to get the DAWN mission to 1 Ceres & 4 Vesta reinstated (successfully) to get a dedicated Io mission off the ground & now to reduce the encounter distance between Cassini & Iapetus (pending)!!!<br /><br />Can't wait to see what my next idiot idea is going to be!!!<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[robnissen]

I'm sure I speak for most here in congratulating you for fighting the good fight. Keep up the good work.

 

[portercc]

Andrew, you are one fun guy to follow around the forums. How much closer than 932 mi would you hope to get? What exactly (size) would you think will be imaged at that distance?

 

[vogon13]

Keeping in mind, the Iapetus pass is near Cassini's apoapsis around Saturn and it will be moving rather more slowly than we are accustomed to near moons closer to Saturn.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[3488]

Hi portercc & vogon13.<br /><br />The fact that Cassini is near apoapsis, will make it even easier to reduce the encounter distance, as only a very minor burn would be required. I hope to get Cassini to within 240 KM / 150 miles of the surface of Iapetus.<br /><br />At that range, surface details of only three metres will be visible at closest approach. Even if nothing happens, the planned encounter will still deliver about 12 metres reso. We will still have much to talk about come this September.<br /><br />I will keep up the good fight.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[3488]

The Cassini spacecraft views terrain on the bright, trailing side of Iapetus in natural color. This side of Iapetus starkly contrasts with the much darker leading hemisphere, and some of the dark material seen here in association with craters near the terminator is an extension of the leading hemisphere terrain. <br /><br />This region was previously imaged by the spacecraft at a much finer resolution -- a spatial scale of less than 1 kilometer (0.6 miles) per pixel -- during a flyby at the close of 2004. This terrain was then on the moon's night side at the time, and Cassini imaged it using weak, reflected light from Saturn (see PIA06168). <br /><br />The present view looks toward Iapetus (1,468 kilometers, or 912 miles across) from about 24 degrees above the moon's equator. <br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[3488]

New thinking on Iapetus.<br /><br />See this link: Spaceflightnow.com.<br /><br />Dunno myself, I still prefer the collapsing rings theory.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[jaxtraw]

"Scientists calculate Iapetus originally rotated much faster--at least five hours, but less than 16 hours per revolution. The fast spin gave the moon an oblate shape that increased the surface area (in the same way the surface area of a round balloon stretches when the balloon is pressed into an oblate shape). By the time the rotation slowed down to a period of 16 hours, the outer shell of the moon had frozen. Furthermore, the surface area of the cold moon was now smaller. The excess surface material was too rigid to go back smoothly into the moon. Instead, it piled up in a chain of mountains at the equator."<br /><br />I've read this multiple times and can't make it make sense. Iapetus becomes oblate, which increases the surface area- fine. Why does the surface area suddenly get "smaller" in the next sentence, and what is this excess surface material of which they speak?<br /><br />Are they saying as it slowed down it became more spherical, and that piled up the mountain chain around the equator? I'm sorry for being thick here, but I don't get it at all.<br /><br />*raises hand to vote for collapsing rings theory*

 

[centsworth_II]

<font color="yellow">"Iapetus becomes oblate, which increases the surface area- fine. Why <br />does the surface area suddenly get "smaller" in the next sentence..."</font><br /><br />The surface area gets smaller when the spinning that maintains <br />the oblate shape slows and the shape gets more spherical.<br /><br />You are fine with the idea that when Iapetus becomes oblate the <br />surface area increases. So you should be fine with the reverse --<br />that when rotation slows and Iapetus goes from oblate to sphere,<br />the surface area decreases. <div class="Discussion_UserSignature"> </div>

 

[centsworth_II]

<font color="yellow">"...what is this excess surface material of which they speak?"</font><br /><br />For example: Take a sphere with a surface area of 100 square miles.<br />Squeeze it into an oblate shape with a surface area of 120 square miles.<br />If the material is liquid or plastic, it will flow out to fill out the extra area.<br />If it is solid, it will crack and split. In reverse, if the oblate shape reverts <br />to a sphere, the surface area will reduce. If the surface is fluid or plastic,<br />the material that originally was exposed in the excess 20 square miles<br />of surface in the oblate form, will flow back into the interior of the<br />spherical shape. If the surface is frozen and solid, the extra 20 square<br />miles will pile up along the line(s) of greatest pressure.<br /> <div class="Discussion_UserSignature"> </div>

 

[jaxtraw]

No, I'm not buying that at all <img src="/images/icons/smile.gif" /><br /><br />If the moon really had to change form that much, we should surely see a lot of faulting. Where is it? For instance, the oblate shape would have a lesser polar radius than the sphere. The poles would have to push "up"- the crust wouldn't fit at all so I'd expect to see some kind of radial faults from the poles. I'm certainly far from convinced that the result would be such a spectacularly regular mountain chain as we see.<br /><br />I'm voting "rings" for now <img src="/images/icons/smile.gif" />

 

[centsworth_II]

<font color="yellow">"...I'd expect to see some kind of radial faults from the poles."</font><br /><br />Interesting. I'd like to see the response to that from the equatorial<br />uplift proponents. Why would the excess surface material flow smoothly<br />over the rest of the surface and just build up at the equator? Maybe<br />they will say that the surface was plastic enough to allow flow as long<br />as there is at least one way out -- northward for material around the<br />south pole and southward for material around the north pole. The only <br />time the material built up was when the flow from the north met the flow<br />from the south and neither had a way out. <div class="Discussion_UserSignature"> </div>

 

[3488]

If Iapetus had shrunk, I would expect to see scarps, similar to those imaged by <br />Mariner 10 on Mercury. Asteroid 1 Ceres is also expected to show huge <br />faulting due to shrinkage when DAWN arrives.<br /><br />Iapetus does not show this. There is no evidence thus far of internal geological<br />activity, such as cryovolcanism. In September, when we get the very high resolution images,<br />that may change.<br /><br />The equatorial mountain ridge does appaer to me to be from a collapsed ring.<br /><br />When we get high resolution imagery of this feature ( I hope we get a high reso cross <br />section), we'll see. If we see cryovolcanic flows emanating from a summit groove, then <br />yes, the ridge is internally formed.<br /><br />If not & it looks like rubble, then it is a collapsed ring.<br /><br />Can't wait, not long now!!!!!!<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[jaxtraw]

I'm really excited about getting some proper images of 1 Ceres, it's a pity we'll have to wait so long to see a body so close to us-- these mission schedules always make me depressed about how old I'll be when they get there hehe.<br /><br />I think Ceres has been unfairly overlooked because it's "just an asteroid" (or at least it was until it became a dwarf planet <img src="/images/icons/smile.gif" /> ). I think it'll prove to be a fascinating little world once we can see more than a blur.

 

[CalliArcale]

I realize this is an Iapetus thread, not a Ceres thread, but just as a brief aside here's a link to Hubble's best pics of Ceres. It's slightly better than a blur. <img src="/images/icons/wink.gif" /><br /><br />Images of the Asteroid Ceres As It Rotates One Quarter <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

 

[jaxtraw]

Hey, I've been spoiled by NASA and ESA. I demand better resolution! <img src="/images/icons/smile.gif" /><br /><br />I think when we see it properly, we'll find Ceres is active too. That's my prediction!

 

[3488]

Hi jaxtraw, try this thread by rlb2.<br /><br />Fascinating stuff.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[vogon13]

Just heard we get to use the SAR at Iapetus in September !!<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[3488]

Thanks vogon13.<br /><br />Wow that's pretty cool.<br /><br />Very good news.<br /><br />We should have a good idea then as to the 'roughness' of the surface texture, thus<br />comparing SAR data to actual high resolution images, will reveal a tremendous amount.<br /><br />Where did you get that from??<br /><br />Is it known yet what the imaging track of both SAR & SSI will be?<br /><br />I could only find the Rhea encounter on Thursday 30th August 2007 & the close <br />Titan one on the next day, Friday 31st August 2007.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[brellis]

Snooping around UMSF's Iapetus 10 Sept flyby thread, a post from Rob Pinnegar<br /><br /><font color="yellow">The flyby geometry is really interesting. In early September, the apoapsis of Cassini's orbit will be just a little bit further out than Iapetus' distance from Saturn, but the semimajor axis of Cassini's orbit will of course be much smaller. This means that, when Cassini is furthest from Saturn, it will be a bit farther away from the planet than Iapetus, and will also be moving much more slowly than Iapetus.<br /><br />This allows Cassini to pass "in front" of Iapetus on the outbound leg, and then watch as Iapetus overtakes it. Then Cassini drops back towards Saturn as Iapetus moves away. This allows quite a variety of observing angles during the whole encounter.</font>/safety_wrapper> <div class="Discussion_UserSignature"> <p><font size="2" color="#ff0000"><em><strong>I'm a recovering optimist - things could be better.</strong></em></font> </p> </div>

 

[3488]

Thanks brellis.<br /><br />This is set to be fascinating.<br /><br />I found this on the CICLOPS site.<br /><br />I made several enquiries about the encounter distance being reduced. It looks like <br />we may have managed a reduction to 1,350 KM / 837 miles, not as much as I hoped for<br />but still a little less than the original. However this is not guaranteed.<br /><br />There is a mid range encoutner with Tethys on: Wednesday 29th August 2007.<br /><br />Also we have a Rhea encounter on Thursday 30th August 2007, then a Titan encounter on <br />Friday 31st August 2007, then the exciting Iapetus one on: Monday 10th September 2007.<br /><br />Cassini will be busy these two weeks.<br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[3488]

H20universe (Joel).<br /><br />Whats your take on this fascinating odd ball moon of Saturn?<br /><br />Not long now till the close encounter. <img src="/images/icons/laugh.gif" /> <br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>

 

[vogon13]

With the flyby coming up, thought I would extend my remarks on this fascinating 'space walnut'.<br /><br /><br />Best estimate for dating the Iapetan ring formation would be the late heavy bombardment period, ~3.7 billion years ago. Evidence is mostly satistical, as that is when it most likely would have happened. Appearance of feature does suggest immense age in any regard.<br /><br />My best suggestion for the source crater of the nascent ring materials remains the large elongated southern hemisphere crater on the SE edge of Cassini Regio.<br /><br />Why don't we see evidence of Cassini Regio type darkening all the way around Iapetus??<br /><br />Good question, Titan most likely was atmosphered during Iapetus' lengthy 'spin down' period, which I have seen estimated might have taken over 1 billion years, and therefore the darkening process should have been operative in that era, then why isn't Cassini Regio type darkening evident all the way around Iapetus ??<br /><br />Most likely, 'gardening' by smaller rocks and debris impacting Iapetus over the last 3+ billion years has drastically 'erased' an original Cassini Regio spanning 360 degrees of Iapetan longitude. That Iapetus is globally substantially less reflective than Tethys, Rhea, etc., might be indicative that the old global coverage has 'coated' the gardened surface materials, and interspersed in the newly exposed white materials would be dregs of the original dark coating.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[3488]

Thanks vogon13.<br /><br />Not long now, three weeks today till the 'big day'.<br /><br />I agree, that much of the surface we see today dates back to the Late Heavy Bombardment.<br /><br />You mentioned Titan. Is there not some theory that Titan's atmosphere was some <br />five times denser than it is now, or about 7.5 Bars???<br /><br />I hope very much we get to see images of Iapetus, showing down to about 5 metres resolution.<br /><br />At that scale, we will see if the darkening of Cassini Regio is two toned on even <br />individual ice boulders, low hills, etc (one half bright the other dark).<br /><br />That is interesting that you think that Cassini Regio might at one time had gone all the <br />way around.<br /><br />Gardening of the icy 'regolith'; will naturally be more intensive on the leading hemisphere,<br />so that makes perfect sense.<br /><br />The equatorial ridge does appear in places to have gaps in it, impacting events???<br /><br />If the ridge is not of internal origin (cryovolcanism) & is indeed a collapsed ring, then yes, <br />its makeup will be of low density & of low structural strength, due to the fact, that is is<br />mountainous rubble pile.<br /><br />Thanks for bumping this up.<br /><br />This subject is never boring. In three weeks time, we will have a lot to talk about. <img src="/images/icons/laugh.gif" /><br /><br />Andrew Brown. <div class="Discussion_UserSignature"> <p><font color="#000080">"I suddenly noticed an anomaly to the left of Io, just off the rim of that world. It was extremely large with respect to the overall size of Io and crescent shaped. It seemed unbelievable that something that big had not been visible before".</font> <em><strong><font color="#000000">Linda Morabito </font></strong><font color="#800000">on discovering that the Jupiter moon Io was volcanically active. Friday 9th March 1979.</font></em></p><p><font size="1" color="#000080">http://www.launchphotography.com/</font><br /><br /><font size="1" color="#000080">http://anthmartian.googlepages.com/thisislandearth</font></p><p><font size="1" color="#000080">http://web.me.com/meridianijournal</font></p> </div>