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The Bright Spots on Ceres
David Palmer
post Apr 2 2015, 05:35 AM
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[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.
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Explorer1
post Apr 2 2015, 05:47 AM
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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.
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dudley
post Apr 2 2015, 04:28 PM
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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?
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katodomo
post Apr 2 2015, 04:28 PM
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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 here (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.
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dudley
post Apr 2 2015, 04:41 PM
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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.
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David Palmer
post Apr 3 2015, 01:30 AM
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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.
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dudley
post Apr 3 2015, 04:35 PM
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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.
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David Palmer
post Apr 4 2015, 08:45 AM
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QUOTE (dudley @ Apr 3 2015, 08:35 AM) *
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.


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).
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dudley
post Apr 4 2015, 06:11 PM
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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.
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Gerald
post Apr 4 2015, 11:12 PM
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QUOTE (dudley @ Apr 3 2015, 05:35 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.
...

Up to this point, I'd say that's within reach, here a back-of-an-envelope calculation.
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 (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.
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dudley
post Apr 5 2015, 03:39 PM
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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.
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David Palmer
post Apr 6 2015, 12:01 AM
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QUOTE (Gerald @ Apr 4 2015, 04:12 PM) *
There are certainly many other options, like sinkholes due to comet-like sublimation activity, or impacts, exposing fresh material.



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.
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ngunn
post Apr 6 2015, 08:57 PM
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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.
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dudley
post Apr 6 2015, 09:46 PM
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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.
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elakdawalla
post Apr 6 2015, 10:18 PM
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QUOTE (ngunn @ Apr 6 2015, 12:57 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.

Oh, that's a very good point that I had forgotten to consider and is obvious in retrospect smile.gif 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.


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