Talk of the "International Workshop on Paolo Farinella (1953-2000): the Scientists, the man", Pisa, 14-16 June 2010

1. Puzzling Attributes of
Small Asteroids
Clark R. Chapman
Southwest Research Institute
Boulder, Colorado, USA
Pat Rawlings, SAIC
Pat Rawlings, SAIC
Session: “Planetary Science: Small Bodies,
“Planetary
Collisions, and Satellites I”
I”
International Workshop on Paolo Farinella
(1953-2000): The Scientist and the Man
(1953-2000):
11:50, 15 June 2010
University of Pisa, Italy

2. Paolo Attacked Puzzles…
I’ll Discuss a Few More:
 Double asteroids don’t
match double craters
 Space weathering is very
fast, yet very slow
 2008 TC3 was a 3 meter
jumble of meteorite types
 NEAs in microgravity

3. Doublet Craters: History of Topic
 “Martian doublet craters,” V.R. Oberbeck & M. Aoyagi, J. Geophys.
Res., 77, 2419 - 2432 (1972).
 1978: Woronow inconclusively debated Oberbeck about whether
spatial randomness was correctly modeled. Conclusion back then:
Mars may or may not have an over-abundance of paired craters.
 Topic resurrected in 1991 by Melosh & Stansberry who argued that 3
doublets on Earth must have been formed by impact of binary
asteroids (this was before any asteroid satellites had been discovered).
 Farinella & Chauvineau (1993): slow synchronized spinning binaries
would be at correct separation for doublet craters; binaries might later
separate or, more likely, coalesce into contact-binary configuration
(common in radar delay-Doppler images of NEAs).
 In 1990s, Melosh, Bottke, Cook, et al. re-examined Martian doublets and
extended the analysis of doublets to Venus.
 Dactyl was discovered and the tidally disrupted SL-9 comet impacted
Jupiter, so doublet/multiple craters were analyzed in that context.

4. Methods of Forming Doublets
 Random impacts (unavoidable)
 Very oblique impacts, ricochet
(Messier, Messier A)
 Endogenic crater formation
(volcanoes, collapse pits, etc.)
 Atmospheric break-up, explosion
(Henbury)
 Tidal break-up (Shoemaker-Levy 9)
 Spatially clustered secondaries
 Impact of binary asteroid or comet

5. How to Recognize Doublets
 The certain way
 Adjacent craters with same
measured ages (Earth only)
 Overlapping craters with
shared walls (septum)
 The very likely way
 Adjacent craters with similar
relative ages
 Other unusual similarities
indicating, e.g., same oblique
impact angle
 The statistical approach
 Find a greater abundance of
doublets than predicted by
chance (doesn’t say which
ones are the true doublets,
unless the characteristics are
very unusual)

6. Observed Frequencies of
Doublets on Several Planets
 Earth
 3 pairs among 28 craters > 20 km diameter;
statistically significant because of very sparse
crater densities on Earth and same ages
 Mars
 Melosh et al. (1996) studied 133 craters on
northern plains, 5-100 km diam., and found 3
likely pairs with separations exceeding random
expectations  2.3% doublets, less than on
Earth and Venus
 Venus
 Cook, Melosh & Bottke (2003) found 2.2% of 10
to 150 km diameter craters were doublets, but
that “splotches” (due to smaller impactors
unable to penetrate the Venus atmosphere)
imply ~14% doublets on Venus
 Moon, Mercury, planetary satellites
 I’ve found no definitive studies
 But doublets exist (Moon; Mercury )

7. NEA Binaries are too Close to
Make Doublets
Main Issue:
Impacting NEAs form craters 10 – 20
times their own diameter. Most NEA
pairs are so close that, even with
favorable geometry, they form a
single crater. How can there be so
many doublet craters?
Walsh
Plot shows that typical  Separation can be larger
(2009) separation of satellites and for oblique impacts
binaries is about 4 times the
 Separation of craters can
radius of the primary.
be zero if pair are un-
Only 1 out of the sample of 35 favorably aligned, even if
is separated widely enough widely separated
(~15 times primary radius) to  Tidal forces can affect
produce a double crater. separation
~15% of NEAs have satellites
or are binaries so <0.5% of
craters made by NEAs should
Perihelion (AU) be visibly double.

8. Space Weathering is Fast…
Or is it? (It is a Puzzle!)
Binzel et al. (2010)  “Space weathering” is the process that
transforms the spectral reflectance (colors and
albedo) of the surface of an airless body by
reddening and/or darkening it (mainly by solar
wind; also micrometeorite impacts).
 Vernazza et al. (2009) study dynamically very
Walsh et al. (2008)
Walsh et al. (2008) young family asteroids and find that most space
weathering color changes occur in ~1 million yrs.
 Following a suggestion of Nesvorny et al. (2005),
Binzel et al. (2010) find that frequent, distant tidal
encounters with Earth by NEAs produce color
changes (tidal rejuvenation of surfaces?). Few
NEAs (or MBAs) are Q’s. [Can YORP spin-up help?]
 Yet bright crater rays persist for 100s of m.y.
 Rays from Tycho crater on the Moon (~100 m.y.
old) dominate the full Moon
 Copernicus rays are still prominent after 800 m.y.
 Mercury is periodically bombarded by solar wind,
yet rays from large, infrequent craters are vivid.

9. Catalina Sky Survey
2008 TC3: Linking an Asteroid to
a Bolide to a Meteorite!
TC3 Reflectance Spectrum: Wm. Herschel
Telescope (Fitzsimmons, Hsieh, Duddy & Ramsay)
TC3 Lightcurve (Clay Center Observatory)
TC3 asteroid moving
(W. Boschin, TNG)
TC3 atmospheric train (M. Mahir)  2008 TC3 was the 1st NEA ever discovered (Catalina
Sky Survey, 7 Oct. 2008) that was then predicted, for
sure, to impact Earth. Telescopic observations were
made before impact: lightcurve, reflectance spectrum.
 19 h after discovery, impact occurred and was
recorded over Sudan; ~700 paired meteorites (named
Almahata Sitta) have been collected so far.
Almahata Sitta fragment on the
ground in Sudan (P. Jenniskens)  This first-ever event was not a fluke: we must expect
future (maybe annual) predictions of meteorite strikes,
from existing and proposed modest telescopes,
without waiting for “next generation” surveys.
 But this meteorite is S T R A N G E !

10. TC3 = Almahata Sitta = a Jumble!
 Paolo and others have shown how small asteroids and meteorites are
produced by collisional disruption of their “parent bodies,” drift into
resonances by Yarkovsky, pumped-up e’s then deliver them to Earth.
 Almahata Sitta was first thought to be an unusual ureilite.
 But the 3-meter wide F-type asteroid is only 2/3rd ureilite; 1/3rd consists
of 5 different E chondrite lithologies, 2 H chondrites, and anomalous
achondrites (e.g. Bischoff, Horstmann, et al. “LPSC 41” & “Meteoroids 2010” ).
 How did this conglomerate breccia come together in the asteroid belt?
What would the spectrum of its parent asteroid look like? What held it
together (spinning once every 97 sec!) on its way to Earth?
 Other processes, not yet understood, must be at work!

11. Non-Intuitive Processes on Small
Asteroids that May Yield Meteorites
 Classical/cartoon model: chips from
solid rocky asteroids.
 1990s model: meteoroids dislodged by
cratering events and catastrophic
disruptions on “rubble pile” asteroids,
drift by Yarkovsky Effect into orbital
resonances, and are thereby converted
into Earth-crossing orbits.
 Very recent alternative (or additional)
modes: landslides and equatorial
escape after spin-up of “rubble pile”
near-Earth asteroids by YORP… or
distortion/disruption by planetary tides
 Scheeres et al. (2010) propose that
NEAs behave in microgravity with the
non-intuitive physics that governs
microscopic dust aggregates

12. Once Upon a Time: Collisions Ruled…
Now it’s mainly Sunlight and Tides
Tidal Mass-  Interasteroidal collisions (both catastrophic
Shedding disruptions and frequent, small cratering events) were
invoked to explain everything that happened to
Following a sug-sug- asteroids after early accretion and thermal
gestion by Nes-
Nes- processing: size distribution, spin rates and axis tilts,
vorny et al., Bin-
al., Bin-
zel et al. (2010)
al. liberation and delivery of smaller asteroids and
show that tidal meteorite fragments into resonances, asteroid
encounters with satellite formation, regolith properties, etc.
Earth (perhaps
even very distant  Yarkovsky Effect (reintroduced for 3rd time in the 20th
ones) “freshen” century by D. Rubincam in 1980s) shown by Farinella,
the colors of the Vokrouhlicky, Bottke and others to cause meteoroids
space-weathered
space- from anywhere in inner half of main asteroid belt to
surfaces of NEAs.
NEAs. drift into resonances, which deliver them to Earth.
 YORP Effect (resurrected from mid-20th century by D.
Rubincam in 1998) shown to be the major process
shaping the axial tilts and spin rates of smaller
asteroids. [Radzievskii 1954: “A mechanism for the disintegration of
asteroids and meteorites.”]
 These two Yarkovsky Effects may dominate the
physical and dynamical behavior of smaller asteroids.

13. YORP Spin-Up, Binary Formation,
and Mass Shedding…and Tides…
Ostro et al. (2006)
Ostro et al. (2006)
Gravitational
slope on KW4-α
 Arecibo radar data on NEA 66391 (1999 KW4; Ostro et al.), and How do Small
analyses/modeling by Scheeres, Fahnestock, Walsh, Michel, Richardson, et Asteroids Behave
al. open a new paradigm for the evolution of small rubble piles: in Microgravity?
 Asymmetric solar radiation spins some of them up, so mass moves to
zero-G equatorial ridge, shedding mass, forming satellite/s, escape or
What happened
reimpact of satellites, and escape of meteoroids into interplanetary space. to Itokawa’s
dust? What are
 ~1/3 of NEAs are binaries, or have satellites or contact-binary shapes, porosities of
implying a common evolutionary track. An NEA may undergo generations NEA’s? Are we
NEA’s?
of satellite formation during its dynamical life in the inner solar system. entering a
 No modeling has yet been done on meteoroid production rates, but this microscopic
could be a major source of meteorites. CRE ages may reflect such surficial world writ large?
landslide processes rather than impact-churned regolith processes. Expect surprise!
K. Walsh, P. Michel & D. Richardson (2008)
K. Walsh, P. Michel & D. Richardson (2008)

14. Conclusions…
 Intuition from our one-Earth-
gravity environment fails us
for small solar system bodies
 They evolve in their physical
traits very quickly…faster than
we can understand
 We’ve known that we have
asteroid pieces (the meteorites)
for more than 2 centuries, yet we
still don’t understand asteroidal
parent bodies
 These are the kinds of puzzles
Paolo would still be researching,
were he still with us.

15. Example: Rosetta and (21) Lutetia
Lutetia/meteorite spectral comparisons
Lutetia/meteorite  Rosetta flies by 100 km Lutetia in July
 Arguments abound about meteorite
Barucci et al. (2005)
analog/s for this M(W)-type asteroid
 “M” is mnemonic for “metal” but
Rivkin (2000) showed that a subset of
M’s have a 3µm hydration band (‘Wet’)
 Also, I suggested (1970s) that M-like
spectra might be enstatite chondrites
 But Lutetia was selected as flyby
target because of arguments that it
may be a carbonaceous chondrite
Vernazza et al. (2009)  Relevant data include polarization,
visible and radar albedos, thermal IR
emission spectra, UV/visible/near-IR
reflectance spectra, mass+shape →
bulk density
 Truth table → “wet” enstatite chondrite
 Rosetta may yield ambiguous results:
We need a TC3-like-event for an M(W)!

16. Short-Term Warnings: Spaceguard Survey
does Better than We Thought!
 Was it a miracle that telescopes saw what was plausibly the largest NEA to
impact Earth in 2008? No! Capability to see “final plungers” was overlooked.
 Analyses in the 1990s of the “Spaceguard Survey” only considered
cataloging of near-Earth asteroids (NEAs); short-term warning was evaluated
only for rare comets.
 Thus it was thought that there was only a tiny chance that a dangerous
inbound 30-m NEA would be seen, let alone a 3-m “TC3”.
 Short-term hazard warning was evaluated (NASA SDT 2003) for the “next
generation” surveys, but not for small NEAs and meteorite recovery.
“Consider a 30–40-m office-building-sized object striking
at 100 times the speed of a jetliner…. Even with the
proposed augmented Spaceguard Survey, it is unlikely
that such a small object would be discovered in advance;
impact would occur without warning.” – C. Chapman,
EPSL (2004).
“a short lead time for an NEO is extremely unlikely –
we can expect either decades of warning or none at
all” – Morrison, Harris, Sommer, Chapman & Carusi
(“Asteroids III” 2002)