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elakdawalla
QUOTE (SpaceListener @ Jan 31 2008, 07:54 AM) *
Any comments with respect of the origin of rays? smile.gif

The rays really, really, really look like extensional features, little graben, radiating out from some upwarp at the center of the crater. After staring at the image for a while I am leaning toward the interpretation that the crater and the fractures have nothing to do with each other, that the crater simply happened to form on top of the approximate center of the preexisting fractures. The other possibility is that the fractures show you lines of preexisting weakness in the crust in the center of Caloris basin, so that when the impactor hit it triggered the deformation at the surface. Regardless of which idea you subscribe to, though, the general plan and structure of the fractures results from preexisting structures in Caloris and the position of the crater on top of them is coincidental.

--Emily
pgrindrod
the 'spider' feature looks quite a lot like some kind of giant radiating dike swarm. the troughs (graben) form above the subsurface dikes, which radiate from some central magma chamber. there are some on earth, probably hundreds on venus (which iíve done a bit of work on), and possibly some on mars too (alba patera for example).

itís a shame that in this case, the central part of the radial fracture pattern is obscured by that impact crater. what are the chances eh? i agree emily, the crater position looks coincidental to me.

pete
tedstryk
This is probably just superficial, but it really looks like what sometimes happen when you throw a rock onto a frozen ice surface that hasn't fully frozen underneath but is frozen enough that the rock doesn't break through to the water. Perhaps it happened while their was still liquid magma under the hardened surface of Caloris.
DFortes
I don't know if anyone else has mentioned it, but the spidery thing looks similar to features on Enceladus, where impact craters appear to have modified the stress field in the crust so as to 'focus' fracture sets.
Anyway, that's my tuppence-worth.
Julius
QUOTE (tedstryk @ Jan 31 2008, 06:59 PM) *
This is probably just superficial, but it really looks like what sometimes happen when you throw a rock onto a frozen ice surface that hasn't fully frozen underneath but is frozen enough that the rock doesn't break through to the water. Perhaps it happened while their was still liquid magma under the hardened surface of Caloris.




I believe laser altimeter data will help shed further light onto the nature of the' spider'.
vikingmars
smile.gif Here are the results of my own Messenger imaging color processings : a lot of science to be expected from this wonderful mission !
Enjoy smile.gif
JJR2
After all the whining I did earlier, I'm just now watching recordings of the MESSENGER press conference (I may have missed parts of the forum disscussion, too---I apologize in advance).
Looking at the Caloris "spider" feature, ( http://messenger.jhuapl.edu/gallery/scienc.../Prockter06.jpg ) it looks like some of the surrounding small craters are concentrated into radial lanes, especially in the upper left quadrant of the image. Why would impact ejecta, if that's what did this, do that?
Next, to grossly oversimplify: assuming Mercury has a large, active, at least partly molten core, and considering the Solar tidal forces acting on it, and so on, why does the apparent volcanism here seem to more closely resemble the Moon than, say, Io?
JRehling
[...]
belleraphon1
All... love the visuals..... and I think the "spider" will be just one of many surface suprises...... and those color mosaic... what splendor.....

What really strikes me is the magnetic field. Only three solid planetary bodies in Sol harbor intrinsic magnetic fields. Mercury, Ganymede, and Earth. Current theory holds that intrinsic magnetic fields require rotating liquid iron cores. Which implies hot interiors that have not cooled down. Two of these worlds, Mercury and Ganymede, almost the same size, but of VERY different surface composition (rocky versus icy), have scarred, rifted, older exteriors. Worlds that are young at heart..... but old on the outside (kinda like ME).

And the vageries of the exospheric atmosphere...... worlds are so much more than what we can sense, visually.
I really want to dive under that silicate skin and rove the liquid waves of iron..... what are your secrets? why are you there?

Craig
nprev
Poetic, Craig; well said! smile.gif

Don't forget also that Mars used to have one, and it quit (why?) Messenger's study of Mercury's magnetic field may well be the most enlightening part of the entire mission in terms of comparative planetology.

EDIT: I just discovered that our resident Poet Laureate has had a second poem about the first encounter published by the Messenger team: http://messenger.jhuapl.edu:80/poem2.html

I never cease to be amazed by both the range and depth of talent & knowledge displayed by UMSF members. Enjoy! smile.gif
belleraphon1
Thanks nprev! smile.gif

But Stu is the true jewel..... beautiful, sir!

Yes, it appears Mars had an intrinsic magnetic field early on..... and some of the remnant frgaments are fairly strong!

As for Venus... hmmmmm..... the lack of a field cannot be soley due to slow planetary rotation, Mercury's rotation is long as well..... Venus I would also love to dive into...... does she have one thick lithosperic plate over a stagnant mantle and core?
Or are there other complications................ ???

All just so fascinating..... I was born when planets were basically just dots in the sky, litttle known.... and now I can virtually partipate in roving worlds....

Craig
Gladstoner
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Juramike
QUOTE (Gladstoner @ Feb 1 2008, 03:28 AM) *
The spider grabens seem to radiate from a dark albedo feature that is offset from the crater.


Nice illustration and observation!
Phil Stooke
The spider fracture pattern looks to me like what I would expect from a dome-like uplift - either related to isostatic compensation in the old basin, or later intrusions.

But I'm particularly interested in this feature on the SW edge of Caloris:

Click to view attachment

a diffuse halo nothing like crater rays or ejecta. The multispectral images show it has a non-impact-like composition, as well. And it has an irregular depression in the middle. To me, it looks like a volcanic plume deposit of some kind.

Phil
nprev
Nice find, Phil! That is indeed intriguing. Additionally, the thing looks like it's sitting on some sort of darker flow itself, or at least there seems to be a sharp boundary (a scarp?) to the right of the feature.

Can't wait for the orbital phase...bet we find a lot of things like this!
Gladstoner
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Gladstoner
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Bill Harris
QUOTE
The spider fracture pattern looks to me like what I would expect from a dome-like uplift - either related to isostatic compensation in the old basin, or later intrusions.

Bingo. The radial graben are clearly extensional features. The impacting asteroid excavated the basin, and the mobile, plastic mantle overlying the fluid iron core made an isostatic adjustment. I'll suspect that altimetry will show that Caloris isn't a basin...

It'll be interesting to see what the Skinakas Basin has to offer, as well as the Weird Terrain at the antipode of Caloris, etc.

--Bill
Nirgal
Based on all the superb color reproductions posted here by tedstryk, ugordan, Vikingmars and others
I tried my own (hand-)colorized interpretation of one of the Mercury recent fly-over-limb images...
:-)


Stu
Browsing the Planetary Photojournal yesterday, looking for a pic to use in a talk, I found a Magellan image that looked naggingly familiar... then it came to me...

Click to view attachment

Venus image info: http://photojournal.jpl.nasa.gov/catalog/PIA00150

Not suggesting anything, just pointing out a similarity, that's all... Does make you think tho, especially with the speculation here about Caloris actually being Mercury's Tharsis... wink.gif

Edit: Also interesting: Navka
pgrindrod
stu, i think it could be a very valid similarity! iíve hunted through some of my own images and found a couple more features on venus that look similar.

Click to view attachment Click to view attachment

there are about 160 on venus, and over half of these have a raised central summit region, showing that uplift might be playing a part in their formation. in fact, you can have some fun inflating a balloon in a box full of flour (sand would probably be better) to show how uplift can cause radial fractures.

Click to view attachment

but back to mercuryÖi donít know how differently magma bodies (if they are the cause) behave inside mercury, but i wasnít expecting this kind of feature (i was more hopeful for titan, but that doesnít seem to have borne out). very exciting stuff though. the topography of the area will be very interesting i imagine.

pete
Juramike
QUOTE (pgrindrod @ Feb 4 2008, 05:02 AM) *
(i was more hopeful for titan, but that doesn't seem to have borne out).



Actually, I've been wondering if Sotra Facula on Titan might be a Nova - something like Ganesa Macula that bulged up, cracked in a goofy way, then oozed out a ways from the center. The T25 RADAR Swath image (PIA09182) shows bright "cracks" (cliff faces?), but doesnt' quite show the RADAR darker fracture depths you'd expect on a Nova. (Could these be infilled due to Titans erosion and dune sands and harder to see at this RADAR resoultion?)

If it indeed is a Titan version of a Nova, it's fracture pattern seems more random when compared to Venus and Mercury's regular star-crack pattern. Perhaps the goofy fracture pattern on Titan is due to it's different crustal materials?

Click to view attachment

(Alternatively, the radar bright features might be the steep edges of cryovolcanic flow boundaries.)

((This feature is right at the edge of the T28 Swath and there might be subtle hopeful hints of associated darker cracks, but it's of lower resolution.))

-Mike
tasp
I suspect a similarity to the "Tiger Scratches" on Iapetus. In that example however, the compressional effect is offset along the flight vector of the impactor into the Iapetan equivalent of mantle. We do not see a 'spider' pattern, in that case, just the released compressional effects of the impacting body under and away from the rim.

Hate to harp about this, but we really do need a good close up set of observations of the Tiger Scratches. Neglecting everything else on Iapetus, the Tiger Scratches themselves, in view of this new Mercury finding, strongly merit additional study.


Amazing feature on Mercury, and one we can learn more about by studyoing additional examples. It is an intersting probe of deep material properties.


ngunn
A query - would subsidence produce a similar pattern as doming upward? Both involve extension of the surface. When molten metal cools from the outside in you tend to get dimple formation and tensional cracking in the middle. I'm thinking large 'ball' of molten rock following the Caloris impact, again cooling and shrinking from the outside inward. The last bit to solidify can't fill the available volume, hence subsidence and tensional cracking.
Gladstoner
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tedstryk
QUOTE (Gladstoner @ Feb 4 2008, 09:03 PM) *
but I believe subsidence or contraction would produce concentric fractures
Click to view attachment


Just guessing of course, but I wouldn't rule out subsidence and contraction here.
Juramike
QUOTE (Gladstoner @ Feb 4 2008, 04:03 PM) *
contraction would produce concentric fractures
Click to view attachment


Exactly like a cooling pumpkin pie.

When contracting, the central part is pulling equally away on all the rest the mass, so the cracks are concentric.

http://www1.istockphoto.com/file_thumbview...pumpkin_pie.jpg
(no cracks, but it shows the slumping pattern is concentric)

-Mike
Phil Stooke
The problem with ngunn's suggestion is that impacts don't produce vast pools of molten rock. A bit but not the amount he's suggesting. And Mercury isn't hot enough to change that very much. There are apparent melt ponds in the ejecta blankets of some of the bigger basins in these new images - that's where most of it goes. Later volcanism is almost certainly needed rather than impact melt.

Phil
edstrick
Looking at the crack pattern, they are arramged in what averages out as a circular pattern of subsidence features, extending under the "bottom" edge of the impact crater superimposed on the basin. The inner edge of the subsidence ring is most circular. I get a feeling, quite possibly bogus, that there's another impact crater entirely buried under the volcanic fill that's localizing the subsidence pattern around it's rim.

The subsidence features in the "top" part of the ring<image coordinates here> are "wavy", not straight line segments like normal graben and vary in width in a pattern also unlike classic graben seen most everywhere.

I have a feeling we're looking at the result of partial and irregular magma withdrawal from under a thick, perhaps nearly solidified basin filling flow.
PhilCo126
Not to denigrate the story, but the 'shrinkage' hypothesis has been around since Mariner 10 following discovery of the scarps. See Murray & Burgess, Flight to Mercury (1977, Columbia University Press). This book is an excellent context primer for Messenger, if you can find it.

This book is still available as "Old New Stock" via Amazon.com smile.gif
tedstryk
This discussion is about the contraction/shrinkage of basin floors, not global contraction.
ngunn
Note the radial shrinkage pattern in this metal ingot (not the lid wink.gif ) here:
Click to view attachment
edstrick
Mariner 10 saw a very interesting pattern of cracks on the smooth fill in the imaged portion of the Caloris basin floor.. There were also some ridges, but they were different from the general planetwide system of scarps. I have great anticipation for Messenger views of the floor of Caloris with optimum sun-angles for topographic views.
MarcF
The same kind of pattern is barely visible at the western side of the Caloris floor on the Messenger images. Should be indeed quite impressive with low sun illumination (I'm also thinking about the spider).
Marc.
peter59
New image after one week hiatus.
Sullivan crater.
Gsnorgathon
So - we've got two large craters right next to each other on the terminator. They both look to me to be about the same size, yet one's got a flat floor and the other's a double-ring.
volcanopele
Here is a quickie mosaic:

elakdawalla
QUOTE (Gsnorgathon @ Feb 6 2008, 02:12 PM) *
So - we've got two large craters right next to each other on the terminator. They both look to me to be about the same size, yet one's got a flat floor and the other's a double-ring.

Good observation, that strikes me as odd too. I wonder if the double-ring one is just a crater within a crater. It'd be interesting to clip out a bunch of Mercurian craters and rank them by size and see where the various morphological transitions (bowl -> central peak -> peak ring -> multiring) occur.

--Emily
Gsnorgathon
That's a good point about the crater-in-crater possibility. I've seen more than a few that remind me of pit craters on Ganymede; in fact, there's a fairly large one at the bottom right of volcanopele's nifty mosaic. There's also that big crater (more or less) right in the middle of the spider. Given all that, the possibility of a pseudo double-ring crater doesn't seem all that low.

Still, it'd be pretty cool if it turned out to be a bona fide double-ring, and that there were some compositional / structural / whatever reasons for two similar sized craters with such different morphologies.
Gladstoner
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dvandorn
It occurs to me that the conditions during (and, especially, directly after) the most recent general resurfacing of Mercury could preserve crustal elasticity longer than you might expect. The earliest that resurfacing could have happened would have been right at the end of the LHB, I would think -- the current surface overlays what looks like lunar highland terrain, just covered in thick lava frosting... rolleyes.gif

IIRC, the Sun was a little hotter three and a half billion years ago than it is now. And Mercury lies so close to the Sun that even a robust outward heat flow would be reversed into an inward flow on Mercury's Sun-facing side.

During the long nights (assuming that Mercury's days weren't all that much different at the end of the LHB than they are now), you'd have outward heat flow and a general cooling of the crust. But the long days could have reversed the heatflow, adding a good percentage of the heat lost during the night back into the crust.

Under such circumstances, the crust could have remained fairly elastic for a rather long period of time, I would think. I'd also think that this might result in a greater degree of crustal differentiation than you see on the Moon or even Mars -- might that explain some of the very dark craters? Pockets of materials that were effectively sorted out of the crust to a certain depth, and then exhumed?

-the other Doug
Phil Stooke
I'm not sure about the rest of your argument, Doug, but the early sun was cooler, not hotter.

Phil
dvandorn
I guess I was thinking of the T-Tauri stage, which is supposed to be a pretty intensely hot period, sun-wise. But I suppose that happened earlier on, and is still a controversial theory...? rolleyes.gif

Seriously, thanks. I actually don't recall where I picked that supposed bit of information. Just goes to show that your brain can lie to you -- don't trust it!

-the other Doug
Juramike
QUOTE (Gladstoner @ Feb 6 2008, 09:58 PM) *
I assume a ring structure results from a stronger rebound, but how do varying properties of rock affect rebounding, not to mention the crater diameter?


Check out this freely available article showing impacts into solids of differing viscosity (this must've been fun to do!)
Fink, et al. Proc. Lunar Planet. Sci 12B (1981) 1649-1666. "Impact cratering experiments in Bingham materials and the morphology of craters on Mars and Ganymede".
Article freely available here.

(Lotsa cool diagrams and crater pictures - crater morphology (rings, rim heights, central peaks, ejecta splats) is dependent both on the velocity of the impactor and the viscosity of the impactee)

-Mike
Bill Harris
QUOTE (dvandorn @ Feb 6 2008, 10:07 PM) *
It occurs to me that the conditions during (and, especially, directly after) the most recent general resurfacing of Mercury could preserve crustal elasticity longer than you might expect. The earliest that resurfacing could have happened would have been right at the end of the LHB, I would think...
<snip>
-the other Doug

Remember, early in it's development, Mercury supposedly collided with a planetesimal which stripped off most of it's silicate crust and left a core and mantle with a thin crust. This could have created a surface with atypical mechanical properties. I think that the (relatively) common double-ringed craters are due toi the material and not chance double-impacts.

In a way, the "mantle crust" condition on Mercury may be closer to the Earth's surface than any other rocky planet. With Earth's mantle convection (plate tectonics), the majority ocean basins are basaltic (AKA "sima") and the continental crust is granitic (AKA "sial").

--Bill
ngunn
NOVICE HAT ON
Can somebody here help with a couple of basic facts (or estimates) about Mercury's crust?
1/ What is its temperature, below the layer affected by diurnal variations?
2/ How deep are those variations thought to penetrate?
Can't really get my head round what might be going on without some idea on those points.
JRehling
[...]
bgarlick
If the large metal core of Mercury implies that it collided with another body in its past (and lost its upper mantle and crust), and our moon implies that a Mars sized object hit Earth and splashed off the moon, is it out of the ream of possibility that Mercury is the remnants of the impactor that hit Earth?

Did a 'Mars sized' planet strike Earth, splashing off the material that formed the Moon and then the remants (mostly the core of the original impactor) somehow settle into a tight orbit around the sun and is now known as Mercury?
JRehling
[...]
ngunn
QUOTE (JRehling @ Feb 7 2008, 06:25 PM) *
at the equator, the crust should be about 125C (surprisingly mild, eh?)


Unscientifically guessing from my own experience in caves, I suppose the variability is very slight more than 20 or so meters down.


Thanks for that JR. Surprisingly mild indeed.
On the second figure maybe Earth comparisons are misleading though, because of the big difference in day length.
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