Yeah -- the dark streak appears to be a shadow cast by a ridge of some type, as the sun is coming from the bottom of the image. In addition, the extreme blow-up, while rife with compression artifacts, does show that the uppermost of the small white spots is also casting a shadow in the proper direction. It's the only one of the white spots that shows a shadow above it, although that might be more because the reflections are saturating the pixels so much that the shadows from the larger spots are being wiped out.

I'm tempted to think that the fragmentation of the main, central-peak-like white spot along its edges is real, though the details are rather wiped out by the jpeg artifacts.

Two tongue-in-cheek things that occur to me, looking at the zoomed-in image:

1) The secondary white spots look like a long quonset-style building, with smaller outbuildings arranged around it...

2) The main spot looks like the saucer section of a Constitution-class starship, with the longer piece representing the engineering hull. Not much left of the nacelles, just a few small pieces, so they must have blown apart upon impact...



JUST KIDDING! But, hey, with the jpeg artifacts, you can almost convince yourself that you're seeing a regular structure in the high-albedo parts of the image, just as if they were artificial. And obviously, the scale is all wrong for these things to be anything but natural formations. Gonna be really, really interesting to see these features at higher resolution.

-the other Doug

I don't know, those look more like doming than subsidence to me. In other words, the features you point to look like positive-relief features, not negative-relief as you would expect from subsidence.

-the other Doug

Thanks for the new image post, Gladstoner. It shows off very well what I've been seeing -- a lot of units emplaced over large craters and basins.

On this new image, there is a large basin that has been overlain by a field of what appears to be impact-emplaced ejecta from another basin. I have circled the basin in green in my annotated image, and traced the edges of the overlying unit in blue. (Very roughly, obviously.) The overlying unit is noticeably raised above the unit underlying, and appears to develop cracks on the far left extent (in this image) of its emplacement.

-the other Doug

Exactly what I said. I'm pretty sure we're right, Julius.

-the other Doug

The best one I've seen is this Mariner 10 attempt at a true-color image from its Venus flby. It was produced by Ricardo Nunes from Mariner's clear and blue filters.

Here's the URL for this and other Mariner 10 images by him:

Mariner 10 - 1973/75 - "Mission to Mercury and Venus"



-the other Doug

Looks like the upper overhanging ledge is a bed of conglomerate rock. Those "large grains" look like stream-rounded pebbles, and they are eroding out of the conglomerate matrix onto the thin ledge and onto the ground below. (We've seen several other examples of conglomerate rock here at Gale; specifically, while heading to Yellowknife Bay from Bradbury Landing.)

I'd say this suggests that the unit in question was deposited fluvially. It doesn't prove it, but it is suggestive.

-the other Doug

Well, think about it. If you're prospecting for the best places to melt through the ice down into the Great Ocean, you need to characterize the surface on a global scale. You can't run your ice-penetrating radar and sounding radar globally, so you have to have good enough photo coverage to match visual characterizations to the deep-structure information you get slices of from those lower-resolution, more limited coverage instruments. Then you can apply those matches to figure out all of the good potential ocean entry points, where the ice crust is the thinnest.

I would be really surprised if there aren't good visual cues in the high-resolution images of the surface that correlate to the thickness of the crust beneath. It might take some analysis, and the cues might be subtle. Bit I bet we'll find them.

Now, this all makes sense if you're using the next mission to plan your assault on the Great Ocean with a melting probe. If you're planning on bringing your melting probe with you on this next flight, well -- good luck finding a good, thin-crust spot to land it on within your mission timing constraints.

-the other Doug

And if you can't see Le Bourget well, we can get the locals to drive out and light it with their automobile headlights...

-the other Doug

Oh, yes -- I love to speculate. I sure didn't mean to quash speculation! I was encouraging it, I thought, by bringing up reasons other than just long-string impacts for the formation of crater chains.

I guess we've been pushing the "let's wait for better pictures" line on Dawn's approach to Ceres so much, it's a little hard to stop. We are now seeing sufficient detail to start some serious speculating. So, I take it back -- let's not wait for better pictures, let's see what we can see in the current ones.

In that spirit, I want to share a general impression from the pictures to date, though it's something I've noted before. And it fits in with a lot of the other observations.

It looks to me like Ceres has been through several epochs of what I'm beginning to dub, in my head, as "splash resurfacing." I keep thinking I'm seeing a lot of overlapping units, all of which are cratered to some extent, but all of which also appear to cover over much older cratered terrains. The smoothest areas seem to be where two splash-emplaced units overlap, where one was relatively young when the second was emplaced. There seem to be quite observable units, classifiable by superposition.

I'm getting a sense of a body that, upon impact, splashes rather than creating the type of ejecta we're used to seeing on Earth, the Moon, Mars, etc. We're talking impacts large enough to form what would be basins on larger, rockier worlds. On Ceres, such events seem to have rebounded and relaxed, perhaps to the point of being impossible to recognize after a gigayear or two, so you don't see "basins," but that's the kind of impact I'm thinking about.

Those impacts look to have created sheets of material that fly around Ceres for a while and then emplace themselves, perhaps in patterns and locations far enough removed from the impact to make it difficult to work resurfacing events back to their impacts.

Some of the squirrely arcuate ridges and gorges may be the result of multiple splash sheets created by a given impact interacting with each other before they fell back down onto the surface. After all, Ceres is a small body, so ejecta can fly around that little world a few times before finally re-impacting the surface.

I'm wondering what exactly happens when impact heating from a Cerean "basin-forming event" is very rapidly infused into gigatons of relatively warm ice...

Remember that tectonic cracking of a crust can express itself as chains of endogenous collapse pits. There is at least one major rift in the pictures above that expresses itself as semi-arcuate with straight, steep sides at one end and devolving into chains of craters as it becomes narrower and less distinct at the other end.

Let's wait for closer imagery before we jump to conclusions, eh?

Yep -- that's Adam Steltzner, of MSL landing fame, up there energetically (as always) describing JPL's ideas on building a Europa lander, complete with a melting mole to try and access open water or at least deep ice. Interesting to look at the notes on the board, the flip chart, and the PowerPoint page hung on the board at the far right of the picture.

-the other Doug

While there does appear to be a bright albedo feature at the visible rotational pole, it's good to recall that this pole has also been pointing at Sol for, what, decades? Instead of being a cold trap that causes ice deposition, this would be the spot of highest insolation (such as it is, out in Pluto space) on this dwarf planet.

Maybe dark ices absorb more solar heating than higher-albedo ices at this sol-pointing pole and are selectively driven off, to re-accumulate on the shadowed side. Leaving the higher-albedo ices behind, and thus showing what looks like a "polar ice cap," even though it's a net deflational, not depositional, feature.

I don't insist on this interpretation, but it fits the observed facts...

-the other Doug

Now, that looks like an ejecta sheet to me. Why? Because, if you look closely, there are buried craters underneath this "tilled" terrain, ghost-like impressions of craters that seem to have formed a rugged surface like that which borders on the ejecta sheet, but has been filled in and covered by the ejecta. The contrast of fresh, well-formed craters to the ejecta sheet is obviously different from similarly sharp craters on the adjoining non-sheet-covered surface. The ejecta itself is grooved (the "tilling" effect) along the direction of flow, which seems to pile up in the side of the large crater that has been noted as looking as if the floor has a landslide in it.

Again, there are similar features on the near side of the Moon, including the Fra Mauro formation, which appear very similar, and which are ejecta sheets emplaced by basin-forming impacts. The similarities are so compelling to me that this is a no-brainer, I think.

-the other Doug

It looks more like a massive ejecta sheet to me, and we do see those commonly on our own Moon. They are most often seen, on the Moon, as ejecta from large basin impacts. As a comparison, take a look at the Fra Mauro formation on the Moon's near side.

In fact, this resembles that kind of structure quite a bit.

-the other Doug

I have also read that the Moon is at saturation when it comes to craters at the sizes of 5 meters and below. That means that continuing cratering doesn't add more craters at any greater rate than it erases old craters such that the crater population becomes relatively stable. I would guess that's true for Mercury, also.

-the other Doug

In the previous image, and also in the animations showing this region, I'm tempted to say that it's not ejecta that's so bright. This "spot" is resolved to the point that, when not saturating the ccd's, it looks like a pyramidal structure (in shape, not at all suggesting it was artificially constructed) that is definitely lighter in color than the surrounding terrain. It doesn't exactly follow the curves of the crater wall it seems to overly, either.

I'm wondering if this could be a constructional landform -- looks a little like a volcano-like structure in this image, at any rate.

Boy, it'll be nice to get better pictures of these areas. Patience... I must learn patience...

-the other Doug

Interesting that this jet comes from a surface in shadow. It also occurs to me that some of the first jets observed came from the shadowed parts of the neck region.

Is there possibly a mechanism working here that would cause jets to break through more often when the surface is shadowed? Or is it more likely that the new jet actually started while that region was sunlit and we only saw it after it went into shadow?

As far as the temperature of Philae goes, if the surface upon which it rests is extremely cold, wouldn't that tend to encourage any heat it develops within the lander to follow the second law and basically flow to the cold surface?

It would very much depend on how much of Philae's structure is in direct contact with the surface, and the thermal characteristics of both that portion of Philae's structure in contact as well as the thermal characteristics of the surface. But I know that if you set a metal box on a very cold surface, heat it draws in from above will flow into that surface and keep the temperature in the metal box colder than if it box was not in contact with that cold surface. Would heat be depleted faster from Philae into the comet's surface than it would radiate into vacuum?

-the other Doug

Absolutely gorgeous! No one sets the scene quite the way you do, Damia. It's appreciated!

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

Interesting -- y'all remember those things Curiosity came across early on, that looked like popped mud bubbles that had frozen into stone?

Looks to me like this formation shows one of those bubbles as an erosional remnant in the uppermost layers of what has most effectively resisted erosion, circled in red in the attached detail.

I also see a lot of other little circular-to-spherical features in the remnant formation, here. Some might be the remains of small impacts, but others might well be the final remains of other of these "popped bubble" formations.

-the other Doug

Well... if dust is observably being sputtered off of Mars, as these observations suggest, that sort of explains where the red dust covering parts of Phobos and Deimos came from. I had always thought that dust plumes from impacts, even over billions of years, didn't seem like they would provide enough material to cause the pigmentation on the moons we see today. If these solar-wind-generated sputtered air-and-dust plumes have been happening for millennia, and if some fraction of the plumes are accelerated by the solar wind interactions out to the distance of the moons, we then have a process for the material transfer that makes more sense, and explains what we see on the moons.

Also, this shows rather strongly how solar wind interactions with the upper atmosphere could well have sputtered off a relatively thick Martian atmosphere over billions of years, doesn't it? Consider that a lot more gas molecules would get accelerated to escape velocity by such interactions than dust particles, and that we can see how many dust particles have been boosted (enough to account for the red coloration of the moons), and you get a good gut-level appreciation of the long-term effectiveness of the solar wind's sputtering capabilities.

-the other Doug

The pyramidal forms of many of the ventifact rocks we see on Mars speak to changes in wind patterns over the millennia. If prevailing winds have always been from a single direction, you would only see wind erosion strongly affecting the sides of the rocks facing into the prevailing winds. The pyramidal shapes strongly suggest exposure to long-term prevailing wind patterns from a variety of different directions. And yes, that means that, given a long enough exposure to winds in a given area, you would tend to see erosion on all sides of a given rock. Hence the pyramidal shapes.

-the other Doug

My understanding of shatter cones is that you can see a single shatter cone in a given rock face quite readily. At least, shatter cones on lunar rocks often manifest as single conical fracture zones in a larger rock face. House Rock, at North Ray Crater at the Apollo 16 Descartes landing site, displayed a single shatter cone along a 10 by 40 meter rock face, and it was unquestionably a shatter cone. So, no, just because there was once enough force pushed through a rock to cause a shatter cone doesn't mean you'd expect to see shatter cones all over the rock face, or on all sides of a given, smallish rock.

Also, with its obvious history of impact churning, the Martian surface should be expected to display evidence of the powerful forces released by hypersonic impacts. It would be unusual, and begging an explanation, if we didn't see shatter cones in some of these rocks. The question should not be "Why are we seeing a shatter cone in the occasional rock?" It should be "Why haven't we seen more shatter cones in these rocks?" The answer to this question is likely that wind erosion has subsequently altered the faces of most rocks to such a degree that telltale signs of shatter cones have been obliterated on many rocks where they might otherwise have been observed.

-the other Doug

But... if it was resent, that implies that the flash system is back up and running, correct? Otherwise, how would it be there to be resent?

-the other Doug