Clark R. Chapman

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Invited Review Physical Properties of Small
Bodies from Atens to TNOs

• Clark R. Chapman
• Southwest Research Inst.
• Boulder, Colorado, USA

Asteroids Comets Meteors 2005 Buzios, Rio de
Janeiro, Brazil, 9 a.m., Monday, 8 August 2005
Gary Emerson
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Classes of Small Bodies
By Orbital Class

• Inner-Earth Objects (IEOs or Apoheles)
• NEAs (Atens, Apollos, Amors)
• Main-Belt Asteroids (incl. Hungarias, Cybeles,
  Hildas, etc.)
• Trojans (of Mars, Jupiter, Neptune)
• Centaurs, Scattered-Disk Objects
• KBOs (Plutinos, Cubewanos)
• Oort Cloud (inner)
• Comets (JFCs, longer period comets)
• Planetary satellites (irregular, regular)
• IDPs, Meteoroids, Meteorites
• Small bodies 10 m to 1000 km diam.
• Pluto, 2003 UB313, other large TNOs

By Size
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Kinds of Physical Properties Observables and
How Well?

• Types of Observations
• spectral reflectance emission (UV radio)
• temporal variations (lightcurves, outbursts)
• satellite orbits, perturbations on other bodies
• imaging
• Earth-based (optical/IR AO, radar)
• Fly-by/orbital/lander spacecraft
• in situ measurements/sample return future
• Degrees of Knowledge of Properties
• rough size (no albedo), vis./IR colors
• spin period, albedo, spectral type, oblong/sph.
• detailed shape, major minerals/ices, spots
• detailed lab data on samples parent unknown
• large-scale geology, spatial compos., mass
• Detailed obs./measurement by orbiter/lander

What is Learned
composition, regolith spin, shape, volatiles mass
(density) structure, geology cosmochemistry,
geophysics
Minimal info/most objects Maximum info/few
bodies
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Colors of Centaurs, KBOs, SDOs

• Bi-modal colors
• especially Centaurs
• esp. not Cubewanos
• Weak correlations with orbital elements,
  dynamical groups
• Comets do not match colors of sources (implies
  processing)

Hainaut Delsanti database
Hainaut Delsanti database
Doressoundiram et al 2005
Delsanti et al 2004
B-R
i
e
a
a
q
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Main-Belt Asteroid ColorsThenand Now

• Asteroid data 35 years ago like TNO data today
• Disputed clusters partly OK
• Trends with a,e,i convincing only after debiasing
  (1975)
• Matching colors/reflectance spectra to mineralogy
  only fair (space weathering, etc.)
• Today abundant statistics, hi-res spectra, good
  compos.
• Colors for tens of thousands
• Reflectance spectra 1000s
• Good correspondence of taxonomy with meteorites
• Relationship of NEAs to main-belt asteroids
  clear
• Families as catastrophic collision products of
  (usually) homogeneous parent bodies

Hapke (1971)
Chapman (1971)
Lessons Learned
Data from Gehrels (1970)
Burbine et al (2001)
Ivezic et al (2002)
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NEA Colors(Binzel et al. 2004)

• S/Q type colors
• Space-weathered (like M.B.) gt5 km
• Range from ord. chond. M.B. lt2 km
• Spread of fresh to matured surfaces
• Implies there may be small M.B. Qs
• NEA colors vs. M.B.
• Qs are NEAs only
• More extremes
• D-types (upper-rt)
  10-18 of NEOs could
  be extinct comets
• Diversity like M.B.
• Outer M.B. under-
  represented a bit
  (beyond low
  albedo bias)

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Size Distributions
Main Belt
Tedesco et al. 2005

• NEAs less wavy than large Main Belt ast.
• TNOs have shallow slope at lt20 km diam.
• Comets truncated 0.6-4 km (Meech et al. 2004)
• Separate SDs for different families/groups

TNOs
Bernstein et al. 2004
NEAs
NASA SDT 2003
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Detailed Earth-based Studies of Individual
Objects (examples)
5145 Pholus 4 Vesta
4179 Toutatis
Cruikshank et al. 1998
The period of rotation, shape, density, and
homogeneous surface color of the Centaur 5145
Pholus S.C. Tegler et al. (2005)
Vernazza et al. 2005
Kryszczynska et al. 1999
HST
Bogard Garrison 2003
Polarization
Mukai et al. 1997
Hudson et al. 2003
Brown et al. 2000
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Shapes of Comet Nuclei Asteroids
Gaspra
Kleopatra
Tempel 1
Wild 2
Mathilde
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Geophysical Properties

• Spins, shapes, satellites, masses, densities,
  strengths, interior structures
• Most remote-sensing of surfaces reveals little
  about interior properties
• Rapid spins monolithic structure do slow spins
  imply rubble piles?
• Impact experiments, numerical modelling, scaling
  analysis
• NEAR laser altimetry probes interior of Eros

NEAR Laser Altimeter Eros
Neumann Barnouin-Jha 2005
Holsapple 2005
Korycansky Asphaug 2005
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Spacecraft Orbiters, Landers, and (soon) Sample
Returns

• Many fly-bys of small bodies
• Significant reconnaissance
• Surprises no 2 bodies same
• NEAR Shoemaker orbital mission to Eros (
  landed!)
• Detailed remote-sensing
• Composition ord. chondrite
• Impact, landers, sample ret.
• Deep Impact experiment
• Contact with Itokawa soon
• Awaiting sample returns by Stardust Hayabusa
• Must extrapolate physical properties measured for
  few visited small bodies to vast, heterogeneous
  population

NEAR XRS data suggest Eros composition ordinary
chondrites
Lim et al. 2005
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Unexpected Small-Scale Geology of Eros

• Flat ponds and beaches
• Small craters absent dominant boulders

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Surface Geology of Tempel 1
Preliminary answers at 11 am today!

• Flat, smooth areas craters ridges bright
  spots
• What processes are at work? Over what duration
  of time?

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Dynamics Relationships to Physical Properties

• Dynamical processes cause physical properties
• Spins and axis orientations due to Yarkovsky
  Effect
• Tidal interactions with planets/sun cause
  distortions and disruptions/disintegrations
• Collisions and catastrophic disruptions create
  families, rubble pile structures, satellites
  (initial spins, sizes)
• Physical properties elucidate dynamics
• Colors help identify dynamical families
• Yarkovsky/YORP effects depend on albedo, shape,
  thermal inertia, spin, density, etc.
• Dynamical analysis can determine physical
  properties
• Mass (hence density)
• Spins (very rapid spins indicate monolith, not
  rubble pile)
• Non-gravitational forces imply features of comet
  nucleus
• Dynamical analysis helps us study physical
  processes
• Specific ages for families specify rates for
  processes like space-weathering
• How perihelia evolve and facilitate
  volatilization

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NEO Impact Hazard 99942 Apophis (2004
MN4)

• In astronomy, only solar flares and impacts have
  major practical effects
• 18000 chance that 320m asteroid impacts 4/13/36
  ( South Asia tsunami)
• Physical properties affect
• Whether it hits keyhole
• How Yarkovsky affects it
• How we could attach to it, couple energy to
  divert it
• How it responds to forces
• How it responds to tidal forces during 2029
  fly-by
• Consequences of impact

In the extremely unlikely event that it will hit,
ground-zero will be somewhere on the red line
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Themes and Issues

• How much are we astronomers fooled by the
  space-weathered, impacted optical surfaces?
• Can we really comprehend how processes work at
  near-zero gravity?
• Really what are the densities, porosities,
  granular structures, strengths?
• Are these splitting/vanishing comets dust
  bunnies?
• Are M-types metallic cores? (many evidently
  arent)
• Regolith-free bare rocks vs. talcum powder
• Biased view from what penetrates our atmosphere
• What are we missing?
• 2003 UB313 we werent looking for
  high-inclinations
• Hypotheticals vulcanoids, Lou A. Frank LAFOs
• Interstellar small bodies?
• Asteroid belts/Oort clouds around other stars

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Asteroids/ Comets Evolving Perspectives
Traditional View
ASTEROIDS Rocky, metallic, no active geology,
cratered, collisional fragments, some
differentiated by heating
COMETS Icy, under-dense, no active geology,
pristineuntil they come close to the Sun, become
very active, disintegrate
Emerging Continuum
ASTEROIDS Under-dense, rubble piles, many
volatile-rich (except at surfaces), some
non-impact geology, many satellites NEAs tidally
evolved
COMETS Active, fluffy, evolved bodies with
complex geology (impact non-impact), easily
split precursor KBOs have satellites, interior
oceans