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Dawn approaches Ceres, From opnav images to first orbit
Habukaz
post Feb 13 2015, 08:58 PM
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Resurfacing or not, you'll notice that the visible cratering is seriously uneven. Take a look at Gerald's thumbnail, for instance:



Notice how much farther up the apparent cratering goes on the right side than on the left. Keep your eye fixed at that area of Ceres' disk in the animation, and you can see how the "crater line" (inspired by the tree line..) goes drastically further down when Ceres rotates towards the right in the animation. Something appears to be different between at least parts of lower and upper Ceres, and illumination cannot explain all of it, as far as I can see (which is not to say that Ceres' upper hemisphere isn't also heavily cratered).


(in the same animation, it also looks like there's a massive chasm trailing to the left of the "twin crater" structure in the lower hemisphere - wonder if that's real..)


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algorimancer
post Feb 13 2015, 09:31 PM
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Exactly the point. There is a distinct line between the heavily cratered region at the bottom and the smoother terrain to the north -- seems to be a drop in the surface level (not steep enough to call a cliff) between the "southern highlands" and the "equatorial plains". True, we may be missing a heavily cratered region about the north pole due to orientation and lighting. A partial explanation may be that the topography is smoothed by viscous relaxation about the equatorial region, but the polar regions are cold enough to preserve the rougher topography -- but why the abrupt transition from one to the other?
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dvandorn
post Feb 13 2015, 11:11 PM
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Statistically, if other cratered bodies are anything to go by, there should not be a dichotomy in terms of what portions of Ceres were hit by large impactors and what portions were not. I think one thing that is fooling us right now is that the lower half of the disk we see is at a lower sun angle, and thus shows more relief. The northern half may be nearly as heavily cratered as the southern, just washed out due to more overhead lighting in these images.

Also, a lot of the "smoother" terrain, even in the southern portion of the images, looks a little to me like basin floors, which may well end up smoothing themselves out more than ringwalls thrown up by large impacts.

It's tremendously exciting to know the resolution will improve an awful lot over the next week or two!

-the other Doug


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Sherbert
post Feb 14 2015, 12:23 AM
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Fantastic work Gerald!

One has to say Ceres, at this point, looks a lot more like an Asteroid than a "Dwarf Planet".
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Gerald
post Feb 14 2015, 01:04 AM
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QUOTE (algorimancer @ Feb 13 2015, 08:52 PM) *
Gerald, this is some really impressive processing you're doing. I hope you write-up your methodology some day, I would like to have an educated basis for doing something similar (though how many firsts like this will we have?)...

Thanks for all your encouragements! smile.gif

In this case I've averaged over all raw registered images, call this mean m. Call a raw registered image r. Calculate 3 * (r - 0.75 m).
The parameters 3 and 0.75 are specific to this sequence. I've not yet found a simulated model working better than the mean.
You may play with these parameters to enhance details on different regions of the surface.

When working on the reflectance, and on pinning down global parameters, I came across this approach. I've needed the average of all linearized images to compare it with simulated reflectance models. So the above processing has been serendipitous near a presumed way to a topographic map / DEM.
Currently I'm working on estimators for the reflectance functions for small areas on the surface, kind of data reduction, intended to be usable to infere inclination and albedo data. The surface seems to be very rough after a first analysis, such that I expect a reflectance down to pixel scale not easily covered by the Phong model. This needs some theoretical preparation. I'll try to explain the role of the estimators later, together with maps of some of them, provided the approach works.
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jgoldader
post Feb 14 2015, 01:46 AM
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There could be phase/shadowing effects. Patience, all will be revealed soon. smile.gif
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Paolo
post Feb 14 2015, 08:23 AM
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a new paper in open access that will probably be soon outdated (still, some interesting ideas...)
Short-term variability on the surface of (1) Ceres
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Habukaz
post Feb 14 2015, 10:25 AM
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QUOTE (dvandorn @ Feb 14 2015, 12:11 AM) *
Statistically, if other cratered bodies are anything to go by, there should not be a dichotomy in terms of what portions of Ceres were hit by large impactors and what portions were not. I think one thing that is fooling us right now is that the lower half of the disk we see is at a lower sun angle, and thus shows more relief. The northern half may be nearly as heavily cratered as the southern, just washed out due to more overhead lighting in these images.

Also, a lot of the "smoother" terrain, even in the southern portion of the images, looks a little to me like basin floors, which may well end up smoothing themselves out more than ringwalls thrown up by large impacts.


I don't think either that such a dichotomy would make much sense (although e.g. Enceladus does not seem that terribly far away from having one (in terms of being cratered); and it does have one when comparing the poles specifically).

But I don't see how the sun angle could explain how the apparent cratering changes with latitude and longitude both, like it does in the images. There really does appear to be some sort of transition(s) going on near the equator, indeed perhaps into basins. It might be that there are just small strips of terrain that have a slightly different topography (and that the north and south are mainly the same), but I feel confident that either the 12 Feb or 19 Feb images will vindicate my view that there is something (potentially quite trivial) going on there. cool.gif

Indeed, there is just 5 days until the 221 pixel global images of 19 Februrary are taken; that date should be marked as a milestone for the exploration of the solar system. laugh.gif


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0101Morpheus
post Feb 15 2015, 04:47 AM
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QUOTE (Sherbert @ Feb 13 2015, 07:23 PM) *
Fantastic work Gerald!

One has to say Ceres, at this point, looks a lot more like an Asteroid than a "Dwarf Planet".


What are you expecting a dwarf planet to look like? Ceres is the first dwarf planet that has been encountered by humanity since the definition was termed by the IAU.
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dvandorn
post Feb 15 2015, 06:23 AM
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What I guess I would have expected of a dwarf planet to look like would be, um, well -- a little more well-rounded. The overall impression I get from the opnavs is of a lumpy body that is out-of-round in significant places.

It doesn't give the same impression of smooth roundness that, say, Enceladus does (as a body of very similar size). I suppose I was expecting something a little more like that. Instead, it seems to have corners and gouges and big uplifted sections and long connected depressions. Complex and fascinating, but less overall rounded than I expected.


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Habukaz
post Feb 15 2015, 10:55 AM
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I think Ceres comes across as pretty spherical in the raw frames. I suspect that image processing is exaggerating and/or creating unevenness.


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nprev
post Feb 15 2015, 12:12 PM
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Pretty sure we're looking at significant compositional (as well as situational) differences between Ceres & Enceladus here that could explain this. Not only is the latter subject to some degree of tidally-induced internal heating, it's also predominantly icy.

By contrast, Ceres probably doesn't have any sort of significant internal energy source left & it's much rockier. Big impacts therefore tend to persist a LOT longer, so, yeah, I think it's gonna be quite lumpy-looking.


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JTN
post Feb 15 2015, 12:38 PM
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QUOTE (dvandorn @ Feb 15 2015, 06:23 AM) *
It doesn't give the same impression of smooth roundness that, say, Enceladus does (as a body of very similar size).

Ceres has about twice the radius of Enceladus (see diagram). I think it's probably Enceladus that's anomalous in its size class for being so round.
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Sherbert
post Feb 15 2015, 02:07 PM
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As a layman, a Planet, whether "Dwarf" or otherwise, should be spherical and have surface terrain elevations small in comparison to the radius of the body. In other words, appear from such a distance, to be fairly smooth. It is clear that some serious chunks have been taken out of Ceres. It is not of sufficient size to sustain a basically flat terrain within it's orbital environment. I realise this is a definition requiring nearer object observation than available from Earth, but to categorise an object in space knowing next to nothing of its physical appearance seems presumptuous at best. That argument is for another place.

My comment was more a general descriptive observation of what we could see up until now, as opposed to more problematic and possibly, ill advised, speculation as to what the various features might be.
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0101Morpheus
post Feb 15 2015, 03:09 PM
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QUOTE (JTN @ Feb 15 2015, 07:38 AM) *
Ceres has about twice the radius of Enceladus (see diagram). I think it's probably Enceladus that's anomalous in its size class for being so round.


I was going to post something like that but you beat me. Good job.

Most objects about Ceres size are heavily cratered and without any neighbors Ceres can't resurface without a giant impact. And if there isn't a trick of the light and Ceres dichotomy is real, than that just reminds me of Mars, whose hemispheres are significantly from each other for reasons not yet well understood. And I think we can all agree that Mars is unambiguously a planet.
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