The YORP Effect on Rubble PilesHow Centrifugal Reshaping of Small Aggregates

Kills the YORP Cycle

Thomas S. StatlerNational Science Foundation & Astrophysical Institute Ohio UniversityNational Science Foundation & Astrophysical Institute, Ohio University

Desireé Cotto-FigueroaAstrophysical Institute Ohio UniversityAstrophysical Institute, Ohio University

Derek C. RichardsonUniversity of MarylandUniversity of Maryland

Paolo TangaObservatoire de la Cote d’AzurObservatoire de la Cote d Azur

The results and opinions expressed in this talk are those of the author and do not reflect the views of the National Science Foundation.

eroid Physical Characterization: Equilibrium Shapes

Centrifugal FGravity This can

hForceychange

Equilibrium Configuration

Cohesion Compressive Strength

Shear

Tensile Strength

Friction Shear Strength

Strength

diation Recoil Torque

(torque) = ×R dF

ancels identically for anyancels identically for any flection-symmetric object hen summed over surface,

veraged over spin & orbit.

R

dFCM Doesn’t cancel for

asymmetric objects. Net residual torque is the YORP Effect.

y Should We Care about YORP?

YORP timescale is roughly (Diameter in meters)2 years.• Millions of years for km-sized objectsMillions of years for km sized objects• Decades for few-meter-sized objects

Orbit evolution is driven by radiation recoil forces (YarkovsyOrbit evolution is driven by radiation recoil forces (Yarkovsyffect) that depend on spin rate and obliquity. Both are altered

by YORP.

YORP-driven spin-up MAY be responsible for mass shedding, binary formation, and “top” shapes like 1999 KW4, 1999 RQ36.

S i h i i i i ll f l di i fSpin characterization is a potentially powerful diagnostic of material properties, complementary to spectroscopic haracterization.

RP Evolution of Rigid Asteroids

“YORP curves” give averagedYORP curves give averaged torque components at various obliquities.

Exact curves are a characteristic of asteroid shape & surface thermal properties& surface thermal properties.

Rigid-body shape is constant; hence objects evolve along j gdeterministic “YORP cycle”.

RP Evolution of Rigid Asteroids

Note that YORP does not inexorably spin objects up fasterspin objects up faster and faster!

Only one exampleOnly one example of various kinds of YORP cycles

RP is Extremely sitive to Small ueface Changes

pin

torq

usc

aled

sps

orqu

e Prediction: Small spin-driven shape changes may result in random-walk in

spin rate rather than YORP cycle

liqui

ty to spin rate rather than YORP cycle.

Statler 2009, Icarus 202 502

me object, 10 d b ld ca

led

obl

dom bouldertions

sc

ORP Evolution of Aggregate Asteroids

RST direct simulation of self-consistent spin evolution under the fluence of YORP and spin-driven shape change.

YORP h iYORP changes spinSpin changes shapeShape changes YORP

ombine pkdgrav N-body and TACO thermophysical codes.

st objects: idealized aggregates of identical gravitating spheres j gg g g g prandom packing in rotating equilibrium (Tanga et al. 2009 ApJL6, L197).

ravity-dominated regime: diameters of a few km, time scales of a w Myr.

ll h i d l t l tmaller, cohesive models to come later.

regate Evolution: Limiting YORP

Rigid body evolutionRigid body evolution (YORP cycle)

Aggregate evolution

Rigid body evolution (YORP cycle)

Aggregate evolution

P li i l

regate Evolution: Self-ting/Stochastic YORP

Aggregate evolution

Rigid body evolution (YORP cycle)

Myr

5 MyrAggregate evolution

Rigid body evolution (YORP cycle)

P li i l

regate Evolution: hastic YORP

Aggregate evolutionMyr

Rigid body evolution (YORP cycle)

6 MRigid body evolution (YORP cycle)

6 Myr

( y )

Aggregate evolution

P li i l

Monoliths and rubble piles behave completely differently in their

onsequences and Implications

Monoliths and rubble piles behave completely differently in their ORP-driven spin evolution.

Detailed spin characterization of asteroid families may thereforeDetailed spin characterization of asteroid families may therefore ovide a statistical indicator of material properties and structure.

o Example: Yarkovsky-driven orbit spreading of collisionalf ili diff f YORP l d lkfamilies differs for YORP cycle vs. random walk.o Bill Bottke/OSIRIS-REx team’s preliminary results exciting!

Simple scenario for YORP-driven binary formation needs to beSimple scenario for YORP driven binary formation needs to be assessed. (Stay tuned!)

Understanding spin evolution of sub-km-sized aggregates will g p gg gquire more sophisticated simulations that include

o Cohesiono Frictiono Frictiono etc.