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Posted by: David Palmer Mar 27 2015, 10:38 AM

It has been suggested that the highly anomalous bright spots on Ceres represent cryovolcanic or evaporative plumes, and one of the pieces of evidence presented for this model, has been the fact that they seem to project above the rim of the crater which hosts them. However, the plume model is highly implausible, for three main reasons:

1) A plume would spread out and be diffuse, and not be concentrated in one super-bright spot.....an example would be the plumes of Enceladus, which are not even visible with the sun to the observer's back (equivalent to the orientation of Dawn when it was photographing Ceres), but rather the plumes of Enceladus are only visible when back-lit. Any plume intense enough to produce the surface brightness of the feature on Ceres, would be expected to spread out over a vast area, similar to what we see with the volcanic plumes of Jupiter's Io (which ARE visible when "fore-lit," appearing as large umbrella or parabola-shaped features rising above the limb)

2) Any plume activity vigorous enough to be visually conspicuous would result in ice crystals settling down (as "snow") on the surface, at least locally, or even globally (as is the case with Enceladus), resulting in a very high surface albedo in at least the crater hosting the bright spots. And yet there is nothing of the sort there....in general, Ceres' surface is a relatively uniform grey, even directly adjacent to the bright spots.

3) We would expect a plume to be variable, whereas the bright spot (albeit completely unresolved) was seen by Hubble years ago.....which makes the case even more strongly, to the effect that the surrounding landscape should by now have a thick layer of snow and be highly reflective, if indeed there are active plumes.

As an alternative to the plume model, I would like to propose the following hypothesis: that the bright spots represent cryovolcanic spring mounds which, due to the very low surface gravity of Ceres, have grown to enormous heights....the water flows out of a fissure but quickly freezes, and then more flows out on top of that, and more on top of that....till we end up with a gigantic stalagmite-shaped structure of highly reflective ice, which may be hundreds of meters high, even perhaps exceeding a kilometer. This formative mechanism would be rather similar to that of the black and white smokers on the ocean floor of Earth where, due to the buoyancy of the water, we see an environment that simulates a very low gravity regime, and in which vertical chimneys of precipitated minerals form (which would be unstable in a high-gravity surface environment).

If the outflow is liquid (not high-speed ice particles as in the case of Enceladus), then we do not face any of difficulties presented by a plume.....all the water (very quickly turning to ice) would stay in the immediate region of the vent. And while it would freeze quickly, over time it would also sublime at a substantial rate, which likely accounts for the thin water vapor atmosphere detected by Herschel. But because of the low gravity and relatively high temperature (up to minus 35 Celsius), and the comparative lack of atmosphere, this water vapor is quickly lost to space, and so does not coat the surrounding surface, except perhaps the small amount that manages to reach the poles.

David Palmer

Posted by: nprev Mar 29 2015, 04:10 PM

Problem is, liquid water just cannot exist below an ambient pressure of about 6 mb. Based on the http://d32ogoqmya1dw8.cloudfront.net/images/research_education/equilibria/h2o_phase_diagram_-_color.v2.jpg I suspect that any liquid water emerging from a vent would instantly become a cloud of ice crystals.

Posted by: dudley Mar 29 2015, 08:25 PM

Found both posts in this new thread interesting and informative. Am left with two impressions. First-- that an icy plume is not likely to adequately explain the bright spot, and second-- that a large ice mound is also unlikely.
Given the seeming need to account for the bright spot within a crater that has apparently fallen dark, and other observations, it appears that some phenomenon entirely new to us may be at work.

Posted by: David Palmer Mar 30 2015, 01:25 AM

Actually, and contrary to popular belief, liquid water does NOT flash to vapor in a vacuum, it initially boils at the surface but then freezes over before very much volume is lost, due to the fact that the phase change is highly endothermic. In fact, it's now believed that liquid water has flowed on Vesta, even though it's never had an atmosphere (see http://www.iflscience.com/space/claims-vesta-once-had-water). And for a good laboratory demonstration of what happens to water in a vacuum, see https://www.youtube.com/watch?v=UG7nsZkVZc0
I conclude that, if liquid water were to flow from a vent or fissure on Ceres, that it would indeed form a spring mound. And it would also be easier for Ceres to have a subsurface ocean than is generally believed: if the water were saturated with salt and/or ammonia, the freezing point could be as low as minus 100 Celsius, which is not much warmer than the estimated average surface temperature of minus 106 Celsius. And even with a body as small as Ceres, we would expect the interior to be warmer than the exterior, due to radiogenic heating, release of heat due to gradual freezing of a subsurface ocean, and possibly serpentization (an exothermic reaction) if ferromagnesian silicates are in contact with liquid water.

David Palmer

Posted by: marsbug Mar 30 2015, 08:46 AM

Wouldn't any significant water ice on the surface outgass, and leave a detectable signature in ceres exosphere?

Posted by: David Palmer Mar 30 2015, 10:18 AM

Yes, and that's exactly what's been detected by the Herschel space telescope.

Posted by: marsbug Mar 30 2015, 02:39 PM

Sorry, I meant by Dawn, as I recall that Herschel came up empty for water on a later observation which makes me wonder if the source is not actually intermittent.... but then I remembered it cannot sample the exosphere (at least directly).

Posted by: katodomo Mar 30 2015, 05:25 PM

I seem to remember that when the plumes were discovered one calculation for a possible source was sublimation from about 0.6 km² of surface ice near perihelion only.

Are the spots small enough for that?

Posted by: dudley Mar 30 2015, 06:16 PM

NASA seemed to have downplayed the possibility of a mound at the location of the bright spot, in their March 2nd press briefing. They used the apparent absence of such a feature, among other reasons, to argue that a cryovolcano was an unlikely explanation for the bright spot.

Posted by: David Palmer Mar 30 2015, 10:15 PM

That was before they observed that the bright spot was still showing when it had moved close to the linb, such that the crater rim should have hidden it if it were a low-relief feature. So the only logical explanations would seem to be that it is a tall, narrow solid structure, or else a plume. But a plume seems to be ruled out by the considerations I listed at the start of this thread. So unless we hypothesize that it is an artificially-constructed highly-reflective glass tower, the only possibility would seem to be that it is a spring mound of ice that has grown to ridiculous heights due to the low gravity (but which is narrow enough to not be conspicuous as a mound in the center of the crater....nothing more than one pixel wide at the range at which it was being photographed). And while the artificial-structure hypothesis is of course a possibility, we need to consider all natural explanations before we go down that road.

Posted by: elakdawalla Mar 30 2015, 10:23 PM

Remember that the science team isn't monolithic. The person doing the downplaying March 3 was Carol Raymond, the deputy principal investigator. The person talking about the mounts at LPSC was Andreas Nathues, Team Lead for the Framing Camera. They were both talking about the same data, but have different interpretations.

Posted by: dudley Mar 30 2015, 11:45 PM

Under these circumstances, it's difficult to evaluate Dr. Nathues observations. He speaks about the probability of a mound at the site of the bright spot, but the images supporting this conclusion have apparently been withheld. A NASA spokesman has even maintained that the bright spot appearing to persist in an otherwise darkened crater is an illusion of some sort.

Posted by: TheAnt Mar 31 2015, 03:40 AM

@David Palmer: Correct, a "spring mound" is possible, (though getting its energy from heat below I found the word geyser to be more to the point in describing it.)
Anyhow if the water got a temperature near freezing, it would boil briefly when the latent heat bring a minor part of the water into a gaseous phase. While the rest would simply freeze out. The question still remain if the feature is made of ice or salts.

@katodomo: The white spots might be of the right size, in fact since they're not resolved (they might still be less than one pixel in size) so the question is not if its small enough but one additional source might be needed, and as posted in the approach thread - there's two areas that show water vapour.
That the water vapour that have been detected come from sublimation of ice is still my favourite hypothesis.

Posted by: dudley Mar 31 2015, 07:29 PM

I'm not really clear on why the proposed 'spring mound' would need to be tall and thin in shape. If it's 4 km across, and the crater is a couple of km deep, the mound could be three km tall, and still catch the Sun when the crater was dark, couldn't it? The strong brightening reported for the bright spot at about local noon seems to point to a fairly broad, flat feature. A tall, thin feature would present nearly minimum aspect to the Sun at this time, would it not?

Posted by: JohnVV Mar 31 2015, 08:26 PM

well we will know in a bit , once the spacecraft gets close again
http://i.stack.imgur.com/Tzh3C.jpg

Posted by: David Palmer Mar 31 2015, 08:50 PM

I was suggesting that it was tall compared to its base, RELATIVE to what we would see on the higher-gravity Earth or even Mars. Mt. Sharp in Gale Crater of Mars is considered a "high," relatively steep mountain with its summit 5.5 km above its base, but the base is about 80 km wide.....which are proportions similar to what we see with "stratovolcanoes" on Earth (totally different geologic origin from Mt. Sharp, of course)....the upshot being that we're used to mountains being much wider than they are high, so to see one that would be of about the same height as the width (similar to Wyoming's Devils Tower, made famous in "Close Encounters"), would strike us as being very dramatic indeed, and visually would appear quite "spindly." Whereas an Earthly "spring mound" would always be a low-relief feature due to the far higher gravity.

To be one pixel (or smaller) in diameter as seen from the photographed distance, while simultaneously projecting above the crater rim, we're probably looking at a feature no wider than it is tall. A change in brightness with the changing sun angle in conjunction with the changing angle of view from the spacecraft, may be related to the reflective properties and orientation of the ice crystals and/or salt crystals making up its surface (although we could also be dealing with mirror-like relfections off glass surfaces, if we want to consider the artificial-construct model.....but of course, that would be a hypothesis of last resort, and as Sagan said, "extraordinary claims require extraordinary evidence").

Posted by: Gladstoner Mar 31 2015, 10:21 PM

As we've heard, something noteworthy was spotted when the bright-spot area was near the limb (or at least that's been the impression). It's interesting that no images of this have yet been released -- including the missing frames in the gibbous-Ceres rotation sequence. Other unreleased images of the bright spot(s) at the limb likely include the half- and crescent-Ceres images, as well as any unscheduled imagery (which very well could have been taken if something extraordinary was spotted).

Plus, if there are geysers/spring mounds present, they could consist of a field of structures rather than a single edifice:

On Earth, fields of small cinder cones form when magma ascent becomes insufficient to form a single volcano. Compare Mauna Loa with the volcanic field around Flagstaff, Arizona. Any activity on Ceres could be meager, so I'd think a field of vents would be more likely.

Plus, visible plumes may not necessarily be the same thing as the bright spots as we've seen them. As the area moves close to the limb while Ceres is in a crescent phase, any otherwise faint plumes could become visible when back lit, even if the bright terrain becomes less conspicuous:



Again, all of this considers the possibility of venting activity. The truth could hold something completely different in store.

Posted by: ollopa Mar 31 2015, 11:05 PM

I'd hate this thread to get embarrassingly out of hand. I met one MPI person at LPSC who just shrugged: "Pixels". (a short) Time will tell.

Posted by: Gladstoner Mar 31 2015, 11:30 PM

Well, the "pixels" in question seem to be in still-unreleased images as indicated here:

http://www.nature.com/news/bright-spots-on-ceres-could-be-active-ice-1.17139?WT.mc_id=TWT_NatureNews

Quote:

New images from NASA’s Dawn spacecraft show the spots, known as ‘feature number 5’, at changing angles as the dwarf planet rotates in and out of sunlight. The pictures reveal the spots even when they appear near the edge of Ceres, when the sides of the impact crater would normally block the view of anything confined to the bottom. That something is visible at all in this circumstance suggests that the feature must rise relatively high above the surface.

“What is amazing is that you can see the feature while the rim is still in the line of sight,” said Andreas Nathues, a planetary scientist at the Max Planck Institute for Solar System Research in Göttingen, Germany. Nathues, who leads the team for one of the Dawn cameras, revealed the images on 17 March at the Lunar and Planetary Science Conference in The Woodlands, Texas.

In the meantime, we have to rely on the quote above, released images, and our imagination.

Posted by: David Palmer Apr 1 2015, 05:14 AM

Reply to Gladstoner: I doubt that an extended field of vents is consistent with the one-pixel (or less) size of the bright spot. Plus, such small vents would not account for the apparent protrusion of the top of the mysterious object above the crater rim. And as I explained in my original posting, a plume is not plausible, as it would have coated the surrounding landscape with snow (like what we see on Enceladus).

MOD: All-caps comment removed for violation of rule 2.6.

Posted by: Explorer1 Apr 1 2015, 05:47 AM

Because they are under no obligation to do so yet.
Maybe after Opnav 6 finds out the ground truth they'll be shown? Otherwise it would just be more speculation....

Posted by: Gladstoner Apr 1 2015, 08:31 AM

Ice coatings on Enceladus are deposited as the moon sweeps up material from the E ring, which was originally derived from the moon's plumes (or at least that's how I've understood it).

In Ceres' case, escaped plume material would be more likely to spread out completely along its much larger solar orbit. Plus, the ice particles would be more susceptible to sublimation being so much closer to the sun.

And at 4 km/pixel, there is still potentially quite a bit of real estate covered.

Again, this may all be for nothing when we learn the truth in the next weeks and months, but it's fun to play with the cards dealt on the table.

Posted by: David Palmer Apr 1 2015, 11:58 AM

If it were a private enterprise, that could be the case, but because it is taxpayer funded, it seems to me that they have an obligation to immediately release everything (and if they don't, it just fuels conspiracy-type theories, to the effect that they're trying to hide something).
With the Mars rover Curiosity, all raw photos are quickly released on a dedicated website, and I don't know why things should be any different with Dawn.

Posted by: David Palmer Apr 1 2015, 12:20 PM

Reply to Gladstoner: you make a good point, that ice is a lot less stable at Ceres as compared with Enceladus, and so it would be harder to coat the surface, and for any such coating to persist. However, that point could also be used as an argument against the hypothesis of a currently-active plume: if there were a plume that was intense enough to generate the bright spot, and the component ice crystals evaporated before the plume was able to spread out and coat the surface, then the water vapor emission signal from Ceres should be FAR stronger than what was picked up by Herschel, and it should also be continuous (not just the wisp of vapor detected at perihelion that we would expect from 0.6 square km of exposed ice).
I'm thinking that a sharp mountain-shaped spring mound would fit the observations best, but that the surface is likely to be rather dessicated and consist mostly of a crust of salt over a core of ice, where the salt is an evaporative remnant that would serve to protect the ice from further rapid evaporation (and bring the water vapor signal in line with the minimal amount detected by Herschel), but such a crust would also be nearly as reflective as snow.

Posted by: algorimancer Apr 1 2015, 01:42 PM

Unfortunately for us, that happens not to be the case. Certainly not "immediately"... they have a schedule when imagery will be moved into a public archive, and it is months as opposed to minutes.

For a broader perspective, I work (as a statistician) in medical research. Public release of raw data almost never happens in this field, only partially because of patient confidentiality issues. Even raw lab data is rarely released. The product is publications of summaries & interpretations of the data. Sadly, this is associated with a bias where results which displease someone tend not to be published, whether because the results conflict with what has been previously published (potentially embarrassing) or because no statistical or practical significance was found (difficult to find journals interested in publishing). From this perspective, I'm absolutely amazed and pleased with the imagery releases by the planetary science community -- which is not to say that I would not enjoy greater openness even more.

Posted by: dudley Apr 1 2015, 03:33 PM

It's been reported that thermal measurements of the bright spots reveal that they have the same temperature as their surroundings. This was considered surprising since light-colored areas should reflect more light, and, so, remain cooler than dark ones.
If there is continual freezing of water emerging from the interior, this would release a certain amount of heat. It seems a remarkable coincidence that this added heat would affect the bright spots' heat budget to just the right degree, so as to make it very closely match that of their darker surroundings.

Posted by: Explorer1 Apr 1 2015, 04:05 PM

http://www.planetary.org/blogs/guest-blogs/marc-rayman/20150401-dawn-journal-preparing-to-photograph-ceres.html

According to this, there is still some uncertainty in the rotation rate, so some uncertainty about whether the spots will show up on the (still thin) crescent). Consequence of the fast rotation.

Posted by: katodomo Apr 1 2015, 04:11 PM

Just as a clarification, Dawn's framing camera was effectively paid 50% from R&D funds of the German Ministry of Science and 50% from (non-taxpayer-related) own funds of the Max Planck Society. MPAe in fact had to push quite a bit through the local federal parliament reps to get the government's contribution. JPL contributed a small sum, basically buying rights to use the camera for navigation that way.

Posted by: fredk Apr 1 2015, 05:07 PM

MER, Cassini, and MSL are the exceptions, not the rule. You can read more about the reasons for this on this forum and elsewhere, but essentially we have scientists spending years (and sometimes decades) of their lives working on a mission and they deserve to be able to publish any discoveries made with the mission themselves. In some cases releasing images or data publicly might allow scientists not on the mission to publish the discoveries first.

With the three exceptions, it was deemed unlikely that many important discoveries would be made with the images themselves, so it was safe to release them in real time. When you're visiting a body for the first time, like Ceres or Comet 67P C-G, it's possible or even likely that the images will reveal important new discoveries, so their release is limited.

Posted by: nprev Apr 2 2015, 04:13 AM

MOD MODE: Thanks to the members who have correctly pointed out that image release policies can and will vary between missions,

I also must remind all to keep rules 1.9 and 2.6 firmly in mind at all times. Spurious accusations and/or conspiracist conjectures or implications will be removed immediately and members posting same may be suspended. This is the first and last warning.

Posted by: David Palmer Apr 2 2015, 05:35 AM

[quote name='dudley' date='Apr 1 2015, 08:33 AM' post='219247']
It's been reported that thermal measurements of the bright spots reveal that they have the same temperature as their surroundings. This was considered surprising since light-colored areas should reflect more light, and, so, remain cooler than dark ones.


Does anyone know what instrument on Dawn measures the surface temperature, and does it have the same resolution as the Framing Camera's visible-light photos? If so, I'm not sure we're in a position yet to tell if the temperature of the bright spot is the same as its surroundings, as we still haven't resolved the bright spot with the Framing Camera.

Posted by: Explorer1 Apr 2 2015, 05:47 AM

VIRS does those measurements. Home page here: http://dawn.artov.rm.cnr.it/vir/vir.html
No info on resolution (since that would depend on distance), but it is based on the same spectrometers Rosetta and Cassini carry.

Posted by: dudley Apr 2 2015, 04:28 PM

It was stated that the bright spots having the same temperature as their surroundings came as a surprise. This seems to indicate that there was enough confidence in the accuracy of the measurements to give rise to such a response. If a significant difference in temperature could have been missed because of a margin of error, wouldn't the results have been considered ambiguous, rather than surprising?

Posted by: katodomo Apr 2 2015, 04:28 PM

As it says on that site it's pretty much a VIRTIS-M sensor head. The -M means "Mapping", not "Medium Resolution" btw. ESA has the technical data for VIRTIS available online http://sci.esa.int/venus-express/33964-instruments/?fbodylongid=1447 (for VIRTIS on Venus Express, only the "Mapper Subsystem"). The infrared channel in all VIRTIS systems actually runs from 0.95-5 nm, not 1-5 nm.

Posted by: dudley Apr 2 2015, 04:41 PM

The resolution at the time of the discussed temperature measurement was given as 11 km per pixel. This was sufficient that measurements at a light-colored splash crater were markedly colder than the darker surroundings.

Posted by: David Palmer Apr 3 2015, 01:30 AM

Based on that info, it sounds like the resolution is too coarse (at this point) to be able to say anything definitive about the temperature of the bright spot, since the actual width of the bright spot (at one pixel or less in the Framing Camera) is at most 4 km. The "light-colored splash crater" is presumably much larger than that.

Posted by: dudley Apr 3 2015, 04:35 PM

The presence of a spring mound on the surface of Ceres appears to be contingent on a number of factors, each of which must occur in a particular way, before this could happen.
First there must be a good deal of water in Ceres' interior.
Then, the water must be kept liquid either by sufficient radiogenic heat and/or the inclusion of the necessary amount of minerals, probably salts.
Then, there must be sufficient pressure on the water, from overlying layers of material to force the water to quite near the surface.
Then, There must be a crater deep enough to reach the underground water.
Then, the coating of salt on the spring mound, left after the top layer of ice has sublimed away, must not act to retain enough heat to melt the underlying ice, which would presumably destroy the spring mound.
It has been established that halite, mineral rock salt, is essentially transparent to infrared radiation. It could act like a the glass of a greenhouse, allowing heat to build up beneath it.
When a number of contingencies must all cooperate to produce a phenomenon, the more of these there are, the less likely is the phenomenon to occur.

Posted by: David Palmer Apr 4 2015, 08:45 AM

Yes, a number of "contingencies" are required, but who could have anticipated all the "contingencies" involved in Enceladus' activity? And it seems to me that a spring mound on Ceres (NOT a currently-active plume) fits the observations best....nobody seems to have a better explanation. And as Sherlock Holmes supposedly said,
"Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth" (although we obviously need more information in this case before we are at that ideal position).

Posted by: dudley Apr 4 2015, 06:11 PM

Most improbable things do not happen, that is, of course, what makes them improbable. We know that Enceladus has cryovolcanic activity, so the current probability of this being the case is essentially 1. The same can not be said for Ceres. The observation that a number of improbabilities were overcome, in the case of Enceldus does not establish the likelihood of cryovolvanism on Ceres, which is a special case, unto itself. It lacks the tidal friction of Enceladus. It hasn't the volume to make the retention of sufficient radiogenic heat likely. Without either of these, subsurface liquid water appears unlikely.

If one posits that the bright spots are very ancient, extinct cryovolcanoes, from an era when the emergence of liquid water from the interior was more probable, other questions emerge.
First-- Should not the supposed salty overcoating have become pitted and soiled by meteoric, and micro-meteoric impacts? With a minimum albedo of .4, this does not appear to be the case. Naturally disrupted, soiled salt deposits on Earth are observed to have significantly lower albedos than this.

Second-- It's thought likely that relief features on Ceres are prone to subsidence, due to the long-term ductility of the icy underlying layers. This appears to be the case in the uniquely large impact basin that has been observed. It shows relatively faint traces of itself, apparently having largely subsided.
Shall a very old spring mound have not subsided in this same way? Shall it still stand above the rim of the crater in which it lies? It seems less than likely.

Posted by: Gerald Apr 4 2015, 11:12 PM

Up to this point, I'd say that's within reach, http://www.unmannedspaceflight.com/index.php?s=&showtopic=7789&view=findpost&p=218832.
But then it would be necessary to show, that water can stay liquid while being transported through kilometers of ice.
Might be fast, short-term (explosive) eruptions could be possible, maybe driven by steam from water suddenly exposed to vacuum after being released through fractures by some tectonics (e.g. due to thermal stress, or due to phase transitions between modifications of water ice), freezing down to snow or frost, together with salty dust. The question is, why is the bright spot so local? If it's the result of an eruption, shouldn't the bright material be more dispersed in low gravity?
Maybe it's a weathering-resistant remnant of an old (cryo?)volcano (http://en.wikipedia.org/wiki/Volcanic_pipe / magma cone analog).
There are certainly many other options, like sinkholes due to comet-like sublimation activity, or impacts, exposing fresh material.
I'm sure Dawn will narrow down the options within the next few months. One or a few a-priori maybe unlikely or unconsidered options may emerge as likely with new data.

Posted by: dudley Apr 5 2015, 03:39 PM

Yes, the bright spots already seem oddly confined in area, even without proper optical resolution. I'm wondering just how small they will turn out to be. If they're still unresolved when resolution reaches about 2 km per pixel, or better, things will start to get very interesting, albedo-wise. Then we may have to reconsider one of those unconsidered options.

Posted by: David Palmer Apr 6 2015, 12:01 AM

The problem there is that such a feature would represent a depression in the terrain, whereas it SEEMS that the object in question projects above the crater rim when seen on the limb of Ceres.

Posted by: ngunn Apr 6 2015, 08:57 PM

The curvature of Ceres is enough to 'elevate' the centre of a crater to the point that it remains visible at the limb, provided the crater walls are not too high. My rough calculation yields 2.5km for the rim height required to obscure that point in this case.

Posted by: dudley Apr 6 2015, 09:46 PM

An interesting point. The surface on which the crater lies curves away from us in much less distance than it would on Earth, or even the Moon. One would have thought that Dr. Nathues and others advocating the necessity of a mound had considered this, and still found it necessary. Perhaps they have some means of estimating the height of the the crater walls, such as from the length of their shadows, and believe them to exceed 2.5 km.

Posted by: elakdawalla Apr 6 2015, 10:18 PM

Oh, that's a very good point that I had forgotten to consider and is obvious in retrospect We don't have enough data yet to know the actual shape of the crater floor or the height of the rim with respect to it. We'll have to wait and see the stereo data.

Posted by: David Palmer Apr 7 2015, 10:00 AM

On the basis of that consideration, it sounds like the spring mound I was arguing for, will likely not need to be nearly as high. However it seems we can still rule out anything but a positive-relief feature, as a crater or sinkhole or other negative-relief feature (exposing clean subsurface ice) would disappear when seen edge-on near the limb of Ceres.

Posted by: dudley Apr 10 2015, 06:19 PM

According to the published schedule, Dawn should be making images of Ceres today. If past experience is any indication, we may hope to see these by the 15th or 16th. Planned images, taken on the 14th should probably be released by the 19th or 20th. Given current uncertainties about Ceres' period of rotation, the bright spots may, or may not be visible in these images.

Posted by: Habukaz Apr 10 2015, 06:26 PM

The images http://eyes.nasa.gov/dsn/dsn.html will not show the brightest spots. The next OpNav might.

Edit: I see your post might be interpreted that way.

Posted by: Gerald Apr 10 2015, 07:54 PM

In http://www.unmannedspaceflight.com/index.php?s=&showtopic=7960&view=findpost&p=219430 I've posted reprojections of some of the RC2 sequence in order to show the bright spot at almost the same position.
Comparing the visibility of the bright spot relative to the surrounding crater with other craters / crater rims / craters in craters when near the terminator, I can't perceive hints to a particularly high peak related to the bright spot.
I'm curious what we'll learn from the new sequence.

Posted by: JohnVV Apr 11 2015, 10:36 PM

-- better here
-----------------------
if the rotation of Ceres is close to correct then in the next few days we should start to see the bright spot
( i am using the "dawn_ceres_v02.tpc")
the times are in UT in the screenshots
upper left on the limb
http://imgbox.com/FFKCxZI7
visible on the top
http://imgbox.com/Zge2i801 http://imgbox.com/JYkDMtsZ

Posted by: David Palmer Apr 12 2015, 12:41 PM

Since the bright spots are unresolved in the pictures so far released, and thus appear to be less than one pixel wide, it seems unlikely that any associated topographic peak (or mound) would show up in such photos or projections based on them.

Posted by: JohnVV Apr 12 2015, 06:55 PM

to make matters worse
the gif that was released is a 8 bit indexed normalized 5% to 95 % image
almost 100% useless for most things

good for the angle triangulation done earlier ( the rim of approx. 2.5 KM )
but that is about it

now if the 12 bit raw data ( basically the pds img ) was around ??????
(or is the CCD 16 bit ?)

Posted by: Habukaz Apr 13 2015, 02:26 PM

One theory (prominent?) among the researchers is that the brightest spot is located in a depression, not elevated over the surrounding terrain:

(Chris Russell)

http://www.bbc.com/news/science-environment-32290122

Posted by: dudley Apr 13 2015, 08:18 PM

The newly released images use the VIR data to compare the temperatures of two different bright areas on Ceres. Area 5 is the much discussed double bright spot. Area 1 is another, smaller appearing and somewhat less visually brilliant bright spot.

Area 1 seems to be cooler than its surroundings, appearing as dark and prominent in the thermal infrared, just as might be expected of an area that is light colored in the visual range. Area 5 essentially disappears in the thermal infrared range. No temperature difference from its surroundings is apparent.
Since Area 1 is smaller than Area 5 it is presumably even less well resolved optically, less distinguishable from its surroundings than is Area 5. Even so, it was possible to find a lower temperature in Area 1 than the area around it. It seems that it should have been possible to do the same for Area 5.
Since Area 5 does not appear to be cooler than its surroundings, there is a question we must ask. What is there about this light colored area that prevents it, in common with other light colored surfaces, from reflecting enough solar energy to lose a greater amount of heat than a darker surface?

Posted by: Gerald Apr 14 2015, 05:03 PM

... so there is no evidence for a peak from the images.
At other locations there is evidence. To make it more feasible, here a montage made from three reprojected single images:

Compare the distances of last visibility of a presumed peak/rim to the terminator.
The distance of the bright spot to the terminator in frame06 is about the same as the rim in frame08. The bright spot is already in the shadow under otherwise similar conditions where the rim is still visible.