How would YOU study the Giant Spider of Mercury?

I have a confession to make. I think planetary geology is the coolest stuff ever - it brings me back to my childhood, in the same way that dinosaurs do. But although I've at least got a better background to play in planetary tectonics than I do to talk about anything that used to be alive, I'm a long way from an expert.

One of the fun things about NASA on the web is that the new images are right there, available to the public. We can look and speculate all we like, though if we want to publish something serious, we would still have to go through the peer review process.

Take "The Spider," for instance.



Andrew has already blogged about it, and so has Greg Laden. (And I've been buried in a combination of grading and snow, and wouldn't have looked at it if Callan Bentley hadn't told me about it - thanks, Callan!)

NASA has an explanation of what they see (on this, and some other images). In the case of The Spider, they say it looks like the product of extension - like the spider's legs are grabens. Callan pointed out to me that the "legs" appeared to be curved away from a diagonal line (from lower left to upper right, Callan?), and wondered if there was a fault that altered the stress pattern when a meteorite hit.

Now, I don't have any answers about Mercury. I've never been there, and... well, I'm no fan of extreme temperatures, so I don't have any desire to go. So, without being able to slap a compass on the outcrop, how could I possibly know anything? Isn't it all just wild speculation?

Well, yeah, but you could argue that geology is mostly wild speculation, too, because we can't go back to the past and actually see what the world was like. But just as with geology, we can make models and think about what they predict and look at what little information we have. And we can reject models that don't explain what little data we have available.

In this case, we've got a crater, and we have these features stretching out from it. We've got some information about topography, because the image is illuminated from an angle. (If you're like me, you have to really work to see the image in 3D, though. I know the craters are supposed to be holes in the ground, so I guess that the light is coming from the right side of the image... but it sure looks like that crater is actually a welt.) So all those lines that extend away from the crater consist of two parallel scarps, with a lower-elevation area between them.

Our other biggest tool is probably cross-cutting relationships. The little craters cut across the "legs," so they're younger. A few of the "legs" seem to cut across others, so maybe we're seeing a sequence of events there, too.

And then we've got... well, experience in many cases, and intuition, to start with. If I were a modeler with a good grasp of continuum mechanics, my tool chest would be a lot bigger. I can still play with analog materials, though, if I want to figure out how I could get that pattern of fractures.

So, what do I see? Well, I agree that the area looks like it has stretched. And looks as if it has mostly stretched in a circle. (Blowing up a balloon expands the balloon's surface like this. Kind of.) But Callan's right, the fractures aren't entirely radial. That's where I wish I had a good command of stress and continuum mechanics - it almost looks to me as if the stretching is interacting with a regional stress field, maybe something that's pulling everything right and left. But stress interactions, especially around the tips of fractures, work in ways that are often counter-intuitive (at least to me), so it would be nice to be able to test my intuition with something a bit more quantitative.

Anyway... what do you see? (And does it really look like a spider? I think it looks more like a dandelion, just waiting to have its seeds blown off. Or maybe a lollipop with fuzz stuck to it. Or maybe a stick figure of Shiva. But the NASA guys called it a spider, so I guess it's The Spider.)