Mercury Flyby 1 Options

[tedstryk]

but I believe subsidence or contraction would produce concentric fractures
[attachment=13431:Strom02b.jpg]

Just guessing of course, but I wouldn't rule out subsidence and contraction here.

[Juramike]

contraction would produce concentric fractures
[attachment=13431:Strom02b.jpg]

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.

[Guest_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

[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 ) here:

[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:

Attached thumbnail(s)

 

[elakdawalla]

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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