2017年10月24日 星期二

#DPS17: Wobbling the Moon and art by James Tuttle Keane

Emily Lakdawalla • October 24, 2017

#DPS17: Wobbling the Moon and art by James Tuttle Keane

James Tuttle Keane is increasingly famous (among planetary scientists anyway) for his remarkable illustrated notes from conferences. I'll post those at the end, but first I'll focus on Keane's own scientific work as illustrated by him and presented at last week's Division for Planetary Sciences meeting. Thanks to James for sharing his presentation with me, all 356 MB of it!

Keane's motivating question: What happens to the spin state of the Moon when something big whacks into it? It's an important question, because the spin of the Moon affects things like polar ice reservoirs, the magnetic field, and crater distributions.

Why care about the spin of the Moon?

James Tuttle Keane

Why care about the spin of the Moon?

Now, Keane isn't the first person to investigate the past spin of the Moon, not by a long shot. But most past workers have written down geophysical models that treat a new impact basin as a highly simplified shape and assume that the Moon settles down very quickly into its new spin state after an impact.

The moments of inertia of impact basins

James Tuttle Keane

The moments of inertia of impact basins

That was an appropriate thing to do, because we really didn't know enough about what was going on underneath impact basins to try more sophisticated models. Thanks to GRAIL, though, we have terrific new insight into what the Moon looks like beneath the surface. Especially for the beautiful, relatively youthful, multi-ringed Orientale basin, which GRAIL studied very closely before the mission ended.

So much for the setting of the work. Keane's research included sophisticated computer modeling of how an impact like Orientale would change the angular momentum of the Moon, and how that would change over time, and how those changes would produce changes in the spin of the Moon. He started with a new, detailed computer model of the Orientale impact produced by Brandon Johnson. It's an amazing simulation. I can watch (let's be truthful, I have watched) this simulation over and over and over again. It looks like a drop of water hitting a pond but all that swirling fluid began the simulation as solid rock. I haven't embedded it as an animation below because the full thing is 35 Megabytes, but it's totally worth it, if you have the bandwidth. Click to watch.

Model of the Orientale impact by Brandon Johnson, visualization by James Tuttle Keane
Model of the Orientale impact by Brandon Johnson, visualization by James Tuttle Keane

Click or tap to animate (warning: 35 MB)

Keane found that the hole punched in the Moon by the initial impact would represent such a huge mass deficit that the Moon would begin to tumble and quickly settle into a new spin state with Orientale centered at a spin pole. But the basin doesn't stay a huge hole in the ground forever. Because of isostasy (something I explain in detail here), solid material flows underground to push the center upward. And mare volcanism fills the basin with lava rock, which is denser than lunar crust. After some geologic quantity of time passes, the basin actually becomes a site of excessive mass. It's an unstable situation to have an excess mass at the pole of a spinning object. So the spin pole would shift again, but not in a straight line, because angular momentum is weird and counterintuitive. Instead, the location of the spin pole would spiral around toward its eventual stable position, winding up with Orientale and its excess mass located near the equator. (The "C2,0" stuff is a way to quantify the lunar gravity, relating to how mass is distributed in the lunar interior. The terminology is from something called spherical harmonics, but you don't need to understand that to understand "mass deficit" and "mass excess." This is a nice example of a figure that can be read and understood by people with multiple levels of knowledge to the best of each person's own capability.)

The evolving inertia tensor of an impact basin

James Tuttle Keane

The evolving inertia tensor of an impact basin

In summary:

Summary

James Tuttle Keane

Summary

Orientale could've begun its existence anywhere on the Moon; tumbled quickly after impact to move to a pole; and then reoriented to the equator over time. And Orientale was just the last, most recent of all the big lunar basins, the last episode of reorientation. The Moon has lots of big basins, all of which would've had histories like this. That's where Keane's work ends, for now, at least in terms of what he presented at the meeting, and geologists are left to consider what it means for their own work. I think it's pretty easy to conclude that no polar ice reservoirs would have survived all that reorientation; whatever reservoirs are at the poles today necessarily postdate basin-forming impacts.

I think Keane's work is a lovely example of the creativity required to be a good scientist. Discovering geologic history isn't just a matter of "reading the rocks," as it's often presented. We also have to go through thought experiments, imagine what could be, in order to develop ideas about what might have been. We'll never see what the worlds in our solar system looked like just a few tens or hundreds of millions of years after they formed; it's all in our imaginations. One path to success in science is to have a great imagination, developed through the practice of art.

Here's a small selection of Keane's pen-and-colored-pencil summaries of other people's work. I was relieved to learn on Twitter that he doesn't color these in real time, during the talks -- they're ink drawings, which he colors later as he goes over his notes. I say "relieved" because his ability to produce such work in 5 minutes made him seem superhuman!

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