R9.2

Asteroid Families
David Nesvorny
William F. Bottke
David Vokrouhlicky
Alessandro Morbidelli
Robert Jedicke
Definition
„ Asteroid family is a group of asteroid fragments
with similar orbits and spectra that was produced
by a collisional breakup of a large parent body
„ First three families identified by Hirayama (1918)
using a catalog of 790 asteroid orbits
Identification
„ Identification involves three steps:
1. Orbit computation (MPC, Lowell Obs. Catalogs)
Identification
„ Identification involves three steps:
1. Orbit computation (MPC, Lowell Obs. Catalogs)
2. Proper elements: more constant over time than
osculating elements (AstDys, Milani and Knezevic, 1994)
~200,000 proper elements (a,e,i)
Identification
„ Identification involves three steps:
1. Orbit computation (MPC, Lowell Obs. Catalogs)
2. Proper elements: more constant over time than
osculating elements (AstDys, Milani and Knezevic, 1994)
~200,000 proper elements (a,e,i)
3. Automated search for groups in 3-D space of
proper elements (HCM, Wavelets, Zappala et al. 1995)
Identification
Identification
Identification
More than 50 statistically robust ‘dynamical’
families identified to date
Identification
More than 50 statistically robust ‘dynamical’
families identified to date
Clear correlation with reflectance spectra:
members of a dynamical family have similar
taxonomic class (Cellino et al., 2002, Ast. III)
Identification
More than 50 statistically robust ‘dynamical’
families identified to date
Clear correlation with reflectance spectra:
members of a dynamical family have similar
taxonomic class (Cellino et al., 2002, Ast. III)
Steep Size-Frequency Distribution (SFD) of
large members (also a signature of impactgenerated debris) (Zappala et al., 2002, Ast. III)
Basic properties
Steep SFD
Small family members more numerous
than small background asteroids???
Eunomia family’s
‘raw’ SFD
Observationally
incomplete for H>13
Basic properties
SFD of asteroid families is more shallow than
that of the background for H>15, a signature of
collisional evolution (Morbidelli et al., 2003; Bottke et al., 2005)
Debiased SFDs
H<13
Basic properties
SFD of asteroid families is more shallow than
that of the background for H>15, a signature of
collisional evolution (Morbidelli et al., 2003; Bottke et al., 2005)
Debiased SFDs
H>15
Ages of Asteroid Families
Ages of asteroid families can be determined by
detailed modeling of orbital evolution
Young families (<50 My): backward integration
to show convergence of orbits
Example: Karin cluster (5.8+/-0.2 My old family)
Age of the Karin Cluster
Age of the Karin Cluster
Age of the Karin Cluster
Gravity only
With Yarkovsky
Ages of Old Families
Old families (>50 My): a fit to orbital distribution
of a family using detailed modeling of Yarkovsky
effect (Bottke et al., 2001; Vokrouhlicky et al., 2005)
Total displacement da =
(da)V from ejection speed + (da)Y from Yarko
Model fits for V and age to produce observed da
(see review talk by M. Broz, tomorrow 15:00)
Ages of Asteroid Families
3 known very young families:
Iannini family (<5My)
Karin cluster (5.8+/-0.2 My)
Veritas family (8.3+/-0.5 My)
Ages for ~20 old families:
Agnia: 100+/-30 My (youngest)
Maria: 3.0+/-1.0 Gy (oldest)
Ejection speeds <100 m/s (<20 m/s for Karin)
Color-Age Correlation
„ Colors from Sloan Digital Sky Survey (SDSS)
Moving Object Catalog (Ivezic et al., 2001)
„ Five bands: 0.36, 0.48, 0.63, 0.77, 0.91 μm
„ ~50,000 MB asteroids:
mainly 13<H<16
Color-Age Correlation
„ Slope becomes redder over time. A signature of space weathering?
Karin-Koronis Color Comparison
Spectral slope
Koronis
Karin
Jedicke et al. (2004)
Hydrodynamic Modeling
„ Application to asteroid families is useful to learn about
physics of large-scale collisions, interior properties of
asteroids, etc. (Michel et al., 2001-2004)
„ SPH & N-body (Benz and Asphaug, 1995; Richardson et al., 2000);
shock propagation, fracture, gravitational accumulation
„ We modeled Karin cluster formation
„ Simulations of impacts into monolithic parent bodies
„ Surprisingly good fits to the observed orbital and size
distributions (Nesvorny et al., 2005)
Hydrodynamic Modeling
„ Observed Family
„ Model Family
V=15m/s
V=15m/s
Hydrodynamic Modeling
Observed and Model Size-Frequency Distributions
D=33 km parent body
5.75-km impactor
7 km/s impact speed
45 deg impact angle
Observed
Model
Hydrodynamic Modeling
Physical location of largest fragments in impacted asteroid
Hydrodynamic Modeling
Side view
Top view
„ Fragments are coherent but (heavily) fractured bodies
Implications for Satellite Formation
>10% for lightcurve
Asteroid Satellites
2% for AO and HST
Interrelations
Asteroid families are sources of near-Earth
asteroids (NEAs), meteorites, micro-meteorites,
and interplanetary dust
Determining linkages between these
populations and their parent bodies is a critical
goal of asteroid and meteoritic studies
For NEAs: presentations in session 7
For meteorites: Pieters, et al., and other talks/posters in this session
For interplanetary dust: Mann et al., and session 3
Interrelations
„ Late Miocene event:
a peak in terrestrial
accretion rate of cosmic
dust (Farley et al., 2005)
„ Veritas family:
semimajor axis ~3.17 AU
~150 km parent asteroid
8.3+/-0.5 My age
„ source of strongest
IRAS dust band
Conclusions
„ Properly analyzed, asteroid families help us to
understand the collisional history of the main
belt and dynamical processes acting on small
bodies in planetary systems
„ We also learn about physics of large-scale
collisions, interior and surface properties of
asteroids, space weathering, etc.
„ Asteroid families are important source regions
of NEAs, meteorites and interplanetary dust
Part 6: Interrelations
„ Asteroid dust bands: discovered by IRAS in 1984
as small-scale features in broad infrared emission
of the zodiacal cloud (Low et al., 1984)
„ Originally thought to be produced in Eos, Themis and
Koronis families (large and old) (Dermott et al., 1984)
„ Interestingly, the Veritas family (inclination ~ 9.3 deg)
provides a better fit to IRAS observations than
the Eos family (inclination ~ 10.3 deg)
Taken together, these results suggest that the
asteroid dust bands are tracers of recent asteroid
collisions (Non-equilibrium production; Sykes and Greenberg, 1986)