"At dawn on Ceres, feature number 5 appears bright. By dusk, it seems to fade"

From Nature article:



When would the feature have been visible at dawn? In the released images so far, only the evening terminator has been visible. Did they take more images after March 1 (OpNav 5)?

It's an odd statement in any case. You would expect any place to be colder at dawn than at dusk, so why would it be more active at dawn? The only explanation I can think of is that there is overnight frost there that burns off in the morning. Curiouser and curiouser.

After magnifying and a nonlinear stretch, I'm using this crop of image 5 of pia18920 more as an inspiration than as a remotely reliable interpretation to get an idea for a possible explanation of the dimming of the bright spot near dusk:
Click to view attachment
If I take the dark frame background as shadowed, there may be a peak immediately left of the bright spot, which either casts a shadow onto the bright spot near dusk, or hides some of the bright feature.
Such an approach may be able to avoid the assumption of diurnal changes of the surface.

That's just an idea for an alternative approach, but far from any evidence with this one image.

I'll also point out that variable reflection may indicate an asymmetry in the albedo of an ice-enriched central peak.

As a gedankenexperiment, let's imagine that the crater within which the Ceres White Spot ("feature number five") resides has a pronounced central peak. Even without any cryovolcanism, the central peak rebound will pull up the deepest levels of the impact target -- so let's say that, in the case of this crater, what was pushed up as a central peak shows where the impact punched through to a water ice layer, and as such is highly enriched in water ice as compared to the crater's rim and floor.

Now, as time goes along, the crater rim and floor will darken with ejecta from nearby impacts, and perhaps weren't as enriched in ice as the central peak. The central peak, however, as we have seen in lunar central peaks, has a tendency to cleave down as impacts happen onto it, leaving huge blocks of itself at the base of the peak.

Over time, the central peak will darken such that it's still brighter than the floor or walls of the crater, but not tremendously so. Then a decent-sized small impactor hits the central peak right near its top. All sides of the peak shed off accumulated dark covering debris, and if it's got a really high ice enrichment, it might leave nice, flat cleaved faces.

In other words, a mountain of ice with steep sides can get its darkening cover blasted off repeatedly over time, and each time it happens, the sides of the peak become highly reflective. But they don't necessarily reflect in all directions the same, with some slopes pointing more directly at a given observer than at another observer looking from a different direction. And, at least for a while after such an impact, the resulting sudden mass wasting that occurs (think avalanche or landslide) also piles layers of bright water ice at the foot of the peak.

So, at dawn, the sunlight strikes a (relatively) reflective ice surface and reflects the solar image brightly. As the day progresses, and the feature moves from left to right (as we've been looking at it), the reflection shifts from one bright facet of the peak to the next. But as we approach the right terminator, the light dims because the side of the peak we're looking at is rougher and darker, or just not "aimed" as directly back to the observation point.

This thought experiment derives a possible configuration of a crater that fits with what we know about crater formation, what (little) we know about the composition of Ceres, and what kinds of effects could cause variable brightness in reflections from potentially icy surfaces.

I surely don't insist this is the correct theory, but I think it fits all the facts we have at present. Personally, I'd prefer to see a variably-deposited snowfield surrounding a geysering central peak, but since that's what others are looking at, I figured I'd see if there are other ways to explain what we're seeing, and I think I found one. It's just not the one I want to see...

-the other Doug

Certainly some fascinating speculation on the thread so far. Rather unusual to have a mystery like this persist for this long after a first encounter, I think, but that's undoubtedly a consequence of Dawn's approach & orbital capture trajectory requirements.

The only conclusion I feel safe to make is very generic: Whatever it is, it's young, and I mean like thousands of years old maybe; I'd guess younger than that.

My rationale is that there seems to be only one of these regions on Ceres, and it's tiny. (Okay, two close together but REAL close, so almost certainly associated.) There are other geological provinces of sorts, but all of far greater extent. In turn, to me this argues for an exogenic origin since an endogenic process would likely not be confined to such a small area of this body.

In fact, and please correct me if I'm wrong, on every object we've examined with current active surface processes (volcanic or not) said processes are distributed across broad regions of the body. Iapetus' albedo process is global, Enceladus' geysers are regional but across a proportionally large area of the moon's surface, Titan's seas are polar.

Therefore, I'm putting my money on a VERY fresh pair of impact craters that have exposed underlying ice. That may well be sublimating when exposed to direct sunlight just like a comet's ices do, but that's the extent of activity.

Pure luck that this happened before Dawn arrived is all. If this model is correct, Ceres must periodically sport relatively dazzling bright spots until they are covered by either Sun-induced darkening of organics or dust from other impacts.

An endogenic strictly local process I could imagine would be a convective hot spot of the Hawaii type. But this would force a locked ice crust, since otherwise we would see a chain of peaks.
Another option would be occasional cryovolcanism to release excess decay heat, with consecutive darkening or sublimation, such that older spots fade.

Given the small scale of the bright spots, I immediately assumed that any visible plume would be too big to be caused by a sublimation feature, but then I remember that comets aren't very big, either..


That said, there's a very interesting detail to be found in the VIR LPSC talk:



combine that with this:



http://www.planetary.org/blogs/emily-lakda...n-at-ceres.html


An already fairly bright patch of surface that could potentially be very bright, but is not cold. Except from the VIR resolution being too low to properly measure the temperature of the bright spot (how likely is this?), are there other ways to explain these temperature measurements with a sublimation feature?

Thermal inertia and albedo may balance sometimes:
High porosity surface -> low thermal inertia -> fast warming in sunshine
Bright area = high albedo -> slow warming in sunshine

If the rule "the brighter, the more porous" holds in a properly adjusted way (think of fluffy bright frost versus dark massive rock), this might explain the temperature measurements.
But this would be rather fortuitous.

Was able to put together a quick 360 degree rotation video of Ceres using JohnVV's altimetry model and a hypsometric gradient in Maya.

https://www.youtube.com/watch?v=bLg40vE29q0

Click to view attachment

MOD NOTE: Full inline quote edited to point of discussion.

I agree completely - it is very unlikely that only one spot on the entire dwarf planet would have a volcano / geyser / oozing ice etc, where as craters in bigger craters are all over the place - we would expect a few young craters on a body of this size.

To help visualize the white spot fitting an impact crater I took a similar crater within a crater and used a moon simulator to watch it's brightness change as a function of phase angle - as you can see it is entirely consistent with the white spot picts seen to date.

MOON: (click to see animated GIF)
Click to view attachment


CERES:
Click to view attachment


Finally on another note: as to the central peak types of ideas for the white spot - I don't see any convincing central peaks on Ceres - they look like they are secondary crater caused with a couple that are questionable - but if 90+% clearly don't have them, it is highly unlikely any do.

I was just reading a letter in the current Nature advising against the use of rainbow color pallettes (http://www.nature.com/nature/journal/v519/n7543/full/519291d.html, probably pay-walled), which reinforces my notion that the height maps are best left as grey-scale. The letter referenced a blog discussing the topic (http://earthobservatory.nasa.gov/blogs/elegantfigures/2013/08/06/subtleties-of-color-part-2-of-6/), which suggests a compromise as a "color palette that combines a continuous increase in lightness with a shift in hue". I managed to replicate one of these, and it does indeed capture the nuances rather nicely, though still not as well as the grey-scale version.
Click to view attachment

Here's the R function I wrote to map proportions to smooth colors, replicating one of the scales in the Elegant Figures blog:

#Maps proportion (Prop) between 0 and 1 to smooth RGB gradient (from dark red to pale green)
# Returns RGB color corresponding to Prop
MapProportionToGradient1<-function(Prop)
{
RedCoef=c(0.1058718,2.6077408,-3.6915044,1.6695911)
GreenCoef=c(0.005883416,-0.186079535,1.357610332,-0.324070193)
BlueCoef=c(0.2063992,-0.1700811,-1.0998388,1.7351850)
RR=RedCoef[1]+RedCoef[2]*Prop+RedCoef[3]*Prop^2+RedCoef[4]*Prop^3
RR=RR
GG=GreenCoef[1]+GreenCoef[2]*Prop+GreenCoef[3]*Prop^2+GreenCoef[4]*Prop^3
GG=GG+.001
BB=BlueCoef[1]+BlueCoef[2]*Prop+BlueCoef[3]*Prop^2+BlueCoef[4]*Prop^3
c(RR,GG,BB)
}

Staring at these height maps, trying to reconcile the observed topography -- generally flat-floored craters, but overlaid on an underlying topography of ridges and valleys, and the floors of some valleys occupied by otherwise anomalously deep depressions...

Let's assume a thick ice crust (perhaps overlaying an ocean, but we can ignore that for now), upon which has rained probably billions of years of rocky material. Exposed ice sublimes away, leaving a rocky lag.

Dense rock on top of less-dense ice. Ice being subject to glacially slow flow (but Ceres has been here a long time). The denser regions of rocky lag will tend to subduct into the ice, dragging the surrounding lag-covered surface towards the subduction region, concentrating the denser rocky mass, and continuing the subduction. Meanwhile, cleaner ice wells-up between the subduction zones, further thinning the lag, reducing pressure, and allowing more ice to flow towards the surface. The net result (over hundreds of millions to billions of years) would be something like the observed ridge and valley network visible in the topography of Ceres.

EDIT: The sequence might look something like the following sequential profile, showing rocky lag in brown and ice in blue.
Click to view attachment

The geology of largish craters would be somewhat independent of the above process, each impact presumably exposing a region of cleanish ice, which is either partially sublimed away or otherwise covered with dust, and the underlying ice is left to gradually achieve an equilibrium flat-floored shape.

Light from left, near the terminator, examples and schemes:
Click to view attachment

From my previous link:



It would seem that at least some scientists find plenty of convincing central peaks.

As for the comparison with the lunar crater, I'd keep in mind that it is clearly resolved while the bright spot on Ceres isn't (it spans less than one pixel).

The dearth of rayed craters is also notable.

So I guess the only model we can actually build is this...

Good theory, impactors of varying density and composition giving different depression depths. I also got the impression of two or three very broad underlying bulges, analogous to large magma plumes, in this case warm water plumes, beneath the crust perhaps, possibly the effects of once liquid seas freezing and expanding, or heterogeneous composition of the material in the equatorial bulge. The amount of surface area all at pretty much the same maximum height seems to point to a very significant resurfacing event or events in the past, again the similarity to the Lunar Maria mentioned before.

So if there is or once was Cryovolcanism on Ceres, how many of the craters are actually Calderas? Given the likely relaxation properties of the crust and mantle, I guess that may be a tough one to put a figure on given the resolution of the images seen so far.

That is a well thought hypothesis, I had something similar in mind when I wondered if Ceres might turn out to look something like Miranda.
With large cracks created trough sublimation in the locations that have exposed ice by the larger impacts.
Now the images we got show a less dramatic surface, even so, I think your image with the rocky lag and ice might be a good start on understanding the upper part of Ceres crust.

Warm ice diapirs seem not unlikely -- definitely worth considering, as with other icy bodies.

I've been tempted do star doing crater count statistics, but this will be much more productive once higher resolution pics come back in a few weeks. Plus, I'd bet that there are some graduate students already hard at work on this.

I'm left really wishing that Dawn was equipped with an imaging radar -- that would answer SO many questions here. What are the proportions and distribution of rock and ice in the crust? How deep is the ice? Is there an ocean below? What detailed tectonic process are occurring at various regions about Ceres?

Given the choice, I'd have a nuclear powered radar imaging mission under construction now, targeted to Ceres and a selection of other asteroids.

Yes, the descoping that lost Dawn the magnetometer really hurts now. But if it was lose the magnetometer or lose the mission, the choice is easy. Still sucks though.

Radar just wasn't necessary with what we knew of these objects. If we knew now what we knew 10 years ago, of course we'd fly a different mission now.

If the white spot is confirmed an active vent, does that place Ceres ahead of Europa and Enceladus now? Ceres has a benign radiation environment and a potentially much shorter cruise with chemical rockets and/or direct destination ion propulsion. I think a Discovery mission at least with magnetometer and higher resolution cameras is feasible, with a separately funded lander.

from my post # 679
the EARLY shape file

the highpass image is the top 10% frequency of the image
and it is normalized 0-255

those that have not already extracted the data out of the cubesphere plate model
ceres_opnav5_512.txt.gz ( cubemapped 512x512 per face of the cube )

i have a isis3 demprep'ed cub
Ceres.DEM8ppd.cub.zip
32 bit float in meters of radii ( 0 to 360 long)
https://drive.google.com/file/d/0B6ZYAd08tZ...iew?usp=sharing



but remember this is NOT official so ......

How about another ion drive mission, carrying magnetometer plus radar (and of course an optical camera), with nuclear power. Spiral out, stop at Ceres and one or two other asteroids, then on to Europa. Perhaps survey a few Jupiter Trojans on final approach to Jupiter. Perhaps use a VASIMIR drive to cut travel time.

TBH I think it's public percception and funding that is more an object to such a follow on mission - Ceres will need to turn out to be the Europa of the inner solar system to get such a followup. Of course, it might do yet....

Interesting work with the DEM everyone. It made me think about the bright spots again. I remembered that there is a crater on Iapetus, Johun, that has a central ridge, rather than a central peak:

Click to view attachment

The DEM seems to suggest it, though again how real that is... It would explain why there are two bright spots: rather than having an ice/salt-rich central peak, the two ends of a central ridge would instead be ice/salt-rich.

The problem with that comparison is that Johun is decidedly oblong, so that the "central ridge" is still indeed central with respect to the crater walls. The images of the bright-spot crater show it to be much more circular; while the brighter of the two spots is central, the dimmer is decidedly closer to the wall. Not saying it couldn't be an offshoot of the main bright spot, but it doesn't appear to be the endpoint of a "central ridge".

We'll see soon.

No need to focus on the shape of that crater, though. Check out King crater on the Moon, circular with a very prominent two-pronged ridge of a central peak. It's the basic concept that is important here.

Phil

On the origin of Ceres, which have been mentioned earlier in a few posts:
The idea that Ceres might originated somewhere else than the current asteroid belt is connected with the Jupiter migration hypothesis.

"As the planets migrated, they stirred the contents of the solar system. Objects from as close to the Sun as Mercury, and as far out as Neptune, all collected in the main asteroid belt. Full text from the Harvard-Smithsonian Center for Astrophysics.

that "ridge" in the DEM is 100% pure synesthetic

it is a ARTIFACT from image processing
you can see this very easily in the mesh when compared to the image

Just speculating here while we wait for more detailed images: We obviously don't know the bright spot size with saturated pixels - but the last dim image looks unsaturated (doesn't max out the pixel intensity - though we have no way of knowing if the image was rebalanced - however since only one pixel was maxed out it doesn't really matter if it was rebalanced or not - all pixels (except possibly one) weren't saturated - though some could still have some bleed effects.). So I would say in this image it almost for sure is bigger than one pixel

In any event the smaller spot is clearly not saturated in the dimmest 2 images and only in adjacent 2 pixels in the middle image. The roughly 5x5 pixel size of the smaller white spot would indicate that the brighter spot is most likely even bigger (assuming a similar albedo). A roughly 9x9 pixel bigger white spot crater is consistent with observed images and suggests minimal adjacent pixel bleed over.

I have annotated a possible crater that roughly fits my hypothesis and consistent with the moon images I previously showed for comparative analog - it will be fun to see how off base this guess will end up being.

White_Spot_Annotated_Crater animated gif - click to animate.
Click to view attachment

The official word from the framing camera lead investigator, as quoted on the previous page, is that the brightest spot is not resolved (i.e. it is less than one pixel across in the images). How he came to that conclusion, I do not know. I haven't seen any comment on whether or not they think the dimmer spot is resolved.

Regardless of that, there are now less than 3 weeks left for the 49% illuminated 453 pixels OpNav 7 images - that's going to be good (though, I wonder how quickly images of the brightest spots at that resolution will be made available to us..).

There could be still-unreleased OpNav 4 and OpNav 5 images -- with different lighting circumstances -- that indicate that.

In anticipation of forthcoming higher-resolution images, I have summarized the various features that I’m eager to see up close. I'm sure I'm not alone....

1. Bright spot pair (obviously ):

Click to view attachment

2. Piazzi - Area of bright and dark mottling:

Click to view attachment

Click to view attachment

3. Large, flat crater with (possible) broad mounds:

Click to view attachment

4. Mound (?) in crater:

Click to view attachment

5. Resurfaced, hummocky region (with fractures?). This includes the large southern crater with a modified rim:

Click to view attachment

I hope to add many more items to the list....

Excellent summary of "the story so far" at Ceres. The bright spots are the clear winners of the popularity contest, though.

Dawn, Ceres images 2 to 14 of RC2 sequence pia18920.gif re-projected to simulate a stationary orbit above the bright spot:

-- edited see other thread
http://www.unmannedspaceflight.com/index.p...mp;#entry219452

Anyone know when the latest images will be released? i.e. OPNAV 6?
Or are they waiting until after OPNAV 7 before showing anything more?

EGU2015 Press Conference 2: "New results from NASA's Dawn spacecraft at Ceres" will start in 3 minutes.

http://client.cntv.at/egu2015/PC2

^ Interesting. I didn't watch all of it, but I extracted these pair of interesting images from VIR that show how one bright spot (spot 1) behaves differently from the brightest spot on Ceres (spot 5):





namely that the brightest spot disappears in the thermal data while the other one don't. The leftmost image for spot 1 is true colour (not specified for spot 5), while the rightmost is thermal data (for both).




I haven't seen any specific information having been provided beyond this tweet:



My own personal guess would be tomorrow or Wednesday for OpNav 6 and Friday, Monday or Tuesday for OpNav 7.

To summarize from the above stream, since i don't think it'll be available for later watching:

- Russell (Dawn mission PI) mostly laid out how Dawn was entering orbit, and how it will do RC3, timeline for Survey Orbit, HAMO, LAMO etc.
- Hoffmann (FC team Science/Operations) did some topographic comparison to Vesta and other bodies, and had a false color global map intended to show off albedo differentiation i think
- Tosi (VIR team member) presented VIR and the above two images pretty much.

- I think the three mostly recapped their slightly longer presentations from earlier today.
- Q&A session afterwards had questions regarding the spots (and, to Hoffmann, regarding statements from earlier Dawn presentations today) from Jonathan Amos, an AFP reporter who I think didn't give her name and two or so through the stream's chat function. Mostly answered with "not enough resolution yet, wait till RC3 is wrapped up".
- Press conference wrapped up after about 45 minutes due to lack of further questions.

I am able to still watch the Press Conference now (06:33 Pacific Daylight Time = 13:33 UTC) -- starting at the beginning --- at http://client.cntv.at/egu2015/PC2

I see the VIR images and the colour map have been released via the Photojournal now: http://photojournal.jpl.nasa.gov/keywords/dp

Yeah, they have now switched over to a regular stream - the stream during the conference itself used different software, and didn't allow you to scroll back or do any settings. They shut that one off at source the moment the press conference was declared over, with the streaming software immediately getting a 404 Error.

Pleasant surprise, given that the EGU website never said anything about streaming beyond live.

Hotspot not so hot?
http://phys.org/news/2015-04-dwarf-planet-...awns-giant.html


The "most famous" bright spot (officially named "spot 5") is represented in lower three images: it totally disappears in thermal wavelength.

"spot number 5" is not an official name. it's just a "placeholder" name used by the scientific team. Don't get used to it.

Makes one wonder which ones Spots 2 to 4 are, and whether "Spot" exclusively applies to the bright spots (or more generally to other albedo features too). And whether the one in the crater south of the equator is Spot 6.

If you look back through the Dawn threads you will find a map made from Hubble images with the spots numbered.

Phil

My guess for the reason for this is simply that Spot 5 is very small. It's probably cold, and thus probably emits little thermal radiation at VIR wavelengths, but it occupies such small fraction of a VIR pixel that the resulting reduced radiation from that pixel is lost in the noise. The other spot is much bigger so its signature is obvious, even though it's probably not as cold at Spot 5.

John

I think it might be interesting to note, though, that the 'dark' thermal signature of spot 1 is much bigger and more obvious than its signature in the visual spectrum; yet we see absolutely no signature of spot 5. Actual calculations could perhaps show that this is insignificant since even the "extended" signature could be too small to be detectable; but this stood out to me.

I'm not altogether certain that the bright spot in area 1 is much larger than that in area 5. The former is rather diffuse. Its hard to say exactly where it ends and the surroundings begin. In the visual range, it seems, at most, about half again as large as the area 5 spot. At the minimum, it may even be smaller than area 5.
In any case, the area 1 spot is dimmer, and the surroundings appear brighter than the area 5 spot. This should have minimized the temperature differences in area 1, it seems.
Given these considerations, it appears more likely that the area 5 bright spot should have shown some temperature difference in the thermal range.