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

Paolo Attacked Puzzles… 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 I’ll Discuss a Few More:

Doublet Craters: History of Topic “Martian doublet craters,” V.R. Oberbeck & M. Aoyagi, J. Geophys. Res., 77, (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.

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

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)

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, km diam., and found 3 likely pairs with separations exceeding random expectations  2.3% doublets, less than on Earth and Venus Venus to 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  )

NEA Binaries are too Close to Make Doublets Separation can be larger for oblique impacts Separation of craters can be zero if pair are un- favorably aligned, even if widely separated Tidal forces can affect separation 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? Perihelion (AU) Walsh (2009) Plot shows that typical separation of satellites and binaries is about 4 times the radius of the primary. Only 1 out of the sample of 35 is separated widely enough (~15 times primary radius) to produce a double crater. ~15% of NEAs have satellites or are binaries so <0.5% of craters made by NEAs should be visibly double.

Space Weathering is Fast… Or is it? (It is a Puzzle!) “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 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. Walsh et al. (2008) Binzel et al. (2010)

2008 TC3: Linking an Asteroid to a Bolide to a Meteorite! 2008 TC3 was the 1 st 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. 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 ! Almahata Sitta fragment on the ground in Sudan (P. Jenniskens) TC3 atmospheric train (M. Mahir) TC3 asteroid moving (W. Boschin, TNG) Catalina Sky Survey TC3 Reflectance Spectrum: Wm. Herschel Telescope (Fitzsimmons, Hsieh, Duddy & Ramsay) TC3 Lightcurve (Clay Center Observatory)

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 2/3 rd ureilite; 1/3 rd 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!

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

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

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

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.

Extra slides

Example: Rosetta and (21) Lutetia Rosetta flies by 100 km Lutetia in July Arguments abound about meteorite 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 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)! Barucci et al. (2005) Vernazza et al. (2009) Lutetia/meteorite spectral comparisons

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” “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)