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Emily LakdawallaOctober 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!

resupplying for #DPS17 pic.twitter.com/xUy3WBmvlm

— James Tuttle Keane (@jtuttlekeane) October 13, 2017

Off to another week of science sketching! #DPS17 pic.twitter.com/WQ8xKhuCYT

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

The road back to Pluto begins here. #DPS17 workshop for Pluto/Kuiper Belt follow on missions. pic.twitter.com/ILSMGdBJOi

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

The Kuiper Belt is replete with diverse objects, each worthy of future exploration (hopefully much sooner than the era of Discovery) #DPS17 pic.twitter.com/vf2Tbkf8SY

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Jason Hofgartner—Triton had active plumes, why doesn’t Pluto? Maybe we visited in the wrong season. #DPS17 pic.twitter.com/kFVrpG7mkl

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Peter Buhler—pits on Pluto’s convective cells grow by sublimation as they move towards cell margins at ~7 cm/yr #DPS17 pic.twitter.com/tPJnoTqHUR

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Dale Cruikshank—solid state greenhouse might operate in Pluto’s icy/tholiny regolith, possibly causing blow outs and mobilizing it #DPS17 pic.twitter.com/5VFFu2jzPm

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Orkan Umurhan—Extensive evidence for pervasive paleo-glaciation on Pluto. Still working out exactly how glaciers flow and erode Pluto #DPS17 pic.twitter.com/AvhcSB1QSC

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Tanguy Bertrand—Sputnik Planitia is shaped by feedbacks between climate, dynamics, and ice over a variety of timescales #DPS17 pic.twitter.com/dhypp1qIK4

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Alissa Earle—Methane is widely distributed in unusual ways across Pluto. Methane on peaks and southern slopes of Al-Idrisi montes #DPS17 pic.twitter.com/qZV76i4lnV

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

Andy Ingersoll—Juno is revealing the interior of Jupiter. Amongst the many discoveries: the great red spot has a root! #DPS17 pic.twitter.com/cEMSUAaNTf

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

See my @NatureAstronomy cover art over at the #DPS17 art show—complete with uncropped Titan! pic.twitter.com/BqCdBSOV4I

— James Tuttle Keane (@jtuttlekeane) October 16, 2017

I caught @jtuttlekeane in action at #dps17 pic.twitter.com/h72Cumomhu

— julie_rathbun (@LokiVolcano) October 16, 2017

Will Grundy—Eris and Makemake might be “Sputnik Worlds” with global N2 glaciers. Convection would help keep them bright. #DPS17 pic.twitter.com/k2YVrFlTQp

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Mike Bland—Subduction of Europa’s icy crust might happen if the conditions are just right (otherwise the crust folds) #DPS17 pic.twitter.com/ZhcdrkIHJ9

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Melissa McGrath—there are now multiple independent detections of Europa’s Pwyll plume. Hints of it in Galileo radio occultation data #DPS17 pic.twitter.com/4F8ntejord

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Jamie Molaro @spacejammie —scintering of ice grains rapidly transforms Europa’s regolith from fluffy to crunchy #DPS17 pic.twitter.com/dqhGT8XrP3

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Fachreddin Tabataba-Vakili—there’s an apparently fixed, pentagonal arrangement of cyclones at Jupiter’s south pole—a vortex crystal #DPS17 pic.twitter.com/nWh47jS55K

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Some behind the scenes info: I usually sketch talks in pen, and color them in after. #DPS17 pic.twitter.com/XOFGuNJ7DP

— James Tuttle Keane (@jtuttlekeane) October 17, 2017

Sue Smrekar @SueSmrekar — VOX: a New Frontiers 4 proposal to return to Venus. Radar, IR spectrometer, gravity, and atmo probe! #DPS17 pic.twitter.com/kMmdSRj2xD

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

Erich Karkoschka—new Titan albedo map accounting for atmo scattering. They’re amazing: no stitching artifacts & 5x better resolution! #DPS17 pic.twitter.com/jwNLHJOxyS

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

Yohai Kaspi—Jupiter’s odd zonal harmonics tell the story of winds at depth. Jupiters winds go down ~3% of its radius #DPS17 pic.twitter.com/dcRTfzRcgG

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

Sabine Stanley @PlanetSabine —Uranus/Neptune’s multipolar dynamos arise from thin conducting layers. Thick layers yield dipoles #DPS17 pic.twitter.com/41bdIBwX1k

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

Simone Marchi—Organics in asteroids hitting Ceres would be later thermally destroyed—but might excavate volatiles from within Ceres #DPS17 pic.twitter.com/TSsZs6Nu3T

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

I’ve got a lot of coloring to do. Beer helps. #DPS17 pic.twitter.com/NwQ9MzLd40

— James Tuttle Keane (@jtuttlekeane) October 18, 2017

Bill McKinnon—Ceres is more oblate than it should be. Maybe a fossil figure from a paleorotation, or bottom-loading from convection #DPS17 pic.twitter.com/eSl3kqn2Z7

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

William A. Smith—Racial battle fatigue: chronic micro/macro aggressions wear people down, endangering their well-being and heath #DPS17 pic.twitter.com/dDVCMHeNEw

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

William A. Smith—‘04 NYT study showed employers were more likely to hire white men w/ criminal records than black men w/ clean slates #DPS17 pic.twitter.com/fIEaW0A0I9

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

“Absurdly ludicrously brilliant” Sarah Hörst @PlanetDr —Titan’s atmo is a veritable wonderland of crazy chemistry. We need to go back #DPS17 pic.twitter.com/XnrzUz0TSa

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

Julie Rathbun @LokiVolcano —Loki is back to its usual, periodically erupting self (cause of periodicity unknown, though) #DPS17 pic.twitter.com/iQ93s8FJfe

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

Katherine de Kleer—Io’s largest volcanic eruptions happen on its trailing hemisphere. Also detecting resurfacing of Loki s/ LBT AO #DPS17 pic.twitter.com/2ONT3M4sZR

— James Tuttle Keane (@jtuttlekeane) October 19, 2017

Bruno Sicardy—particles orbiting Haumea will naturally migrate out to just past 3:1 resonance—forming the observed thin/dense ring #DPS17 pic.twitter.com/LytElvPGvX

— James Tuttle Keane (@jtuttlekeane) October 20, 2017

Amanda Zangari @AmandaZangari —an amazing occultation of New Horizons follow on target, MU69. Is it a potato, close/contact binary? #DPS17 pic.twitter.com/hi12BV8lNd

— James Tuttle Keane (@jtuttlekeane) October 20, 2017

Pablo Santos-Sanz—new occultation of Haumea has revised its density—now inconsistent w/ hydrostatic homogeneous body. Differentiated? #DPS17 pic.twitter.com/I6PHKbnobm

— James Tuttle Keane (@jtuttlekeane) October 20, 2017

Zeeve Rogoszinski—secular resonance during early planet migration may give Uranus a large (but not large enough) obliquity #DPS17 pic.twitter.com/1sl3Fijcph

— James Tuttle Keane (@jtuttlekeane) October 20, 2017

Zeeve Rogoszinski—secular resonance during early planet migration may give Uranus a large (but not large enough) obliquity #DPS17 pic.twitter.com/1sl3Fijcph

— James Tuttle Keane (@jtuttlekeane) October 20, 2017

Andrew Hesselbrock—slowly rotating planets with rings (like ancient Mars) may form “boomerang moons” that fall back onto the planet. #DPS17 pic.twitter.com/aWhh0EJ3KO

— James Tuttle Keane (@jtuttlekeane) October 21, 2017

Read more: pretty pictures, GRAIL, the Moon, explaining science, art, conference report

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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