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Dawn approaches Ceres, From opnav images to first orbit
jgoldader
post Feb 25 2015, 06:53 PM
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It's interesting that the secondary bright spot is almost in line with the first one and the sun angle. Could it be sunlight scattering or reflecting off a plume from the bright spot? The plume would have to be pretty high, given the viewing geometry, but still...
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algorimancer
post Feb 25 2015, 07:47 PM
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As a couple of others have noted, there's some really interesting stuff going on with the grooved terrain to the south and east of the bright spot. Just a little more resolution would be really helpful -- hopefully the whole rotation sequence will be posted, so image stacking could tease-out the details.
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Fran Ontanaya
post Feb 25 2015, 07:51 PM
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There's also a curved ridge to the left-bottom of the bright spot crater.
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ngunn
post Feb 25 2015, 08:09 PM
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QUOTE (climber @ Feb 25 2015, 06:28 PM) *
Looks like it's located dead center of a "crater" which seams quite unlikely


Not necessarily unlikely if the large old impact and the small new one both played a part in making the bright spot: the former collecting a buried 'puddle' of relatively clean near-surface ice and the latter exposing it.
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Gladstoner
post Feb 25 2015, 08:10 PM
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Those grooves look like they could be secondary crater chains from an impact basin. Do they line up with any known or suspected basin?
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Gladstoner
post Feb 25 2015, 08:13 PM
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And those white spots. They are so.... nonrandom.
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fredk
post Feb 25 2015, 08:29 PM
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How so non-random?

No one has posted a resampled version of the third image. The released image appears to be trivially oversampled 2x. So by subsampling (without smoothing) by 1/2, you recover close to the original image. Then I've supersampled (Lanczos) 4x. Here's the result:
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Gladstoner
post Feb 25 2015, 08:38 PM
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By nonrandom, I mean two anomalously bright spots appearing in only one place (or few places) on an otherwise drab surface. Better yet, one spot appears to coincide with the central peak of an ancient crater. I would expect a number of similar features scattered around the surface.
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Phil Stooke
post Feb 25 2015, 09:03 PM
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With such a small number of spots, I wouldn't be thinking about randomness or otherwise just yet.

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elakdawalla
post Feb 25 2015, 09:11 PM
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I'm trying to understand the "uncropped" images they released. I'm don't think they mean "uncropped", since they clearly aren't showing the full 1024-pixel-square FOV. Maybe the originals were windowed? I thought that maybe they meant "unenlarged", but if you look closely at the photos you'll see that there is something wonky with them -- there are several lines of pixels (some horizontal, some vertical) that have been repeated. Either there's a glitch in the camera, or there are some missing lines that were interpolated, or the image has been very slightly enlarged with nearest-neighbor sampling. It's bizarre.


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ngunn
post Feb 25 2015, 09:18 PM
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I agree that the bright spots appear non-random. You need a (most likely random) recent event to break through the ubiquitous dark surface covering but whether this exposes dark or light subsurface must depend on more ancient events at the same location. We saw both light and dark materials exposed on Vesta by impacts and landslips on slopes. I think that Ceres, by providing more such data points, should enable the story of all this to told at last.
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elakdawalla
post Feb 25 2015, 09:23 PM
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Continuing with my previous post: the "uncropped" images have been enlarged from the original by about 2.5 percent with nearest neighbor sampling, resulting in four vertical and five horizontal duplicated lines of pixels in the images. Here's a before and after comparison of one of the two images after taking those extra lines of pixels out.


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volcanopele
post Feb 25 2015, 09:31 PM
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Maybe they come out of a set of images where they created a movie where they tried to make them all equal in size, since Dawn slowly approaches Ceres during RC2, rather than pulling them from the raw images?


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elakdawalla
post Feb 25 2015, 09:36 PM
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That sounds plausible. I hate that I have to keep explaining to people on Twitter that NASA has not blurred these photos; that they started out as lower resolution and have been enlarged.


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JRehling
post Feb 25 2015, 09:53 PM
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The bright spots make me wonder if there's something like thermal segregation going on. On Iapetus, roughly half the surface is bright and half dark, and thermal segregation makes the "rich get richer" (the bright gets brighter and the dark gets darker). Maybe on Ceres, there's thermal segregation but the bright is only a tiny fraction of the surface, evacuating whatever dust might cover it through thermal processes driven by solar heating.

There's an interesting comparison set, potentially, with Ceres, Callisto, and Iapetus, all of which have some mixture of dark and icy surface. Those three rank in terms of rotational period exactly the opposite of how they rank in proximity to the Sun. Iapetus has extremely long days, but a dim Sun 9 AU away. Callisto has a shorter day, but a considerably brighter Sun. Ceres continues the trends: A very quick day, and yet more solar heating.

On Iapetus, we have full-blown thermal segregation, with the cold half seemingly stealing whatever ice sputters from the warm half.

On Callisto, ice sputters away, but doesn't particularly gather anywhere. Though on Ganymede, icy gathers on the coldest slopes near the poles.

On Ceres, a few very bright areas (ice, one supposes) are staying bright, and there's a seemingly binary distinction being maintained: Ice or no ice. Is ice sputtering away from the darker ~99% of Ceres as it does from the darker 50% of Iapetus? It's the rareness of the bright patches that stands out. If Ceres is geologically inactive and impact craters of different ages are the only features, why would only a few of them have this rare property, versus Callisto where all sufficiently large/recent craters show ice under the dirty stuff?
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