Recent Issues in Planetary Cratering

1. Recent Issues in Planetary Cratering Clark R. Chapman Southwest Research Institute, Boulder CO Lunar and Planetary Institute Houston, Texas 26 October 2007

2. Outline • What causes the absence of small craters on Eros and Itokawa? • Frequency of “Tunguskas” and defining the small end of the size distribution for dangerous impactors. • Inconsistency in statistics for binary NEAs and doublet craters. • The role of secondary craters: lessons from Europa and Mars. • What is the cratering evidence for the Late Heavy Bombardment? • Maybe some philosophical musings…

3. What causes the absence of small craters on Eros and Itokawa?

4. Eros and Itokawa Itokawa Eros Itokawa Eros

5. SFD for Craters and Boulders on Eros: Absence of Small Craters

6. Eros is NOT Like the Moon! Eros has rocks. The Moon has craters.

7. Why are there so few Small Craters on Eros and Itokawa? (But O’Brien [DPS, 2007] cannot make it work)

8. Frequency of “Tunguskas” and defining the small end of the size distribution for dangerous impactors.

9. Meteoroid/Asteroid Numbers and Magnitudes; Impact Frequencies and Energies (Harris, 2007) • Meteoroids (upper left) are the “tail” of the distribution of larger, danger-ous objects, pro-duced by colli-sions, cratering, and cometary disintegration. • Meteors and bolides are the visible mani-festation of the infrequent, rarely witnessed events that pose a real danger. • Recent revisions by Alan Harris suggest that Tunguskas occur every few thousand years. Maybe the 1908 devastation was done by a much smaller (more frequent) impact (cf. Boslough 2007).

10. Death Threat from Impacts, by Asteroid Diameter and Location of Impact • Statistical mortality rate once Spaceguard Survey is complete in a few years (NEO Science Definition Team [SDT], 2003; tsunami data corrected by Chapman) • Current rate (many hundreds/ year) will be down to a couple hundred per year, mainly by removing threat of “Global” impactors > 2 km diameter • Dominant threat will remain for “Tunguskas,” for which there is a several-% chance this century that one will strike and kill hundreds or thousands of people. • Thus Tunguskas and their smaller cousins will dominate public interest in the impact hazard. (For nominal case) Global Worldwide Deaths (Annual) Land Tsunami Asteroid Diameter (km) How the mortality will diminish from the three kinds of impacts as the Spaceguard telescopic searches continue

11. What is the Smallest NEO that is Dangerous? LSST Model of 30 m NEA 1998 KY26 (radar) This will be a vital issue for decision-makers • Metallic objects, of whatever size, are little impeded by the atmosphere; they are responsible for most craters <1 km diameter but are only ~3% of NEOs. • The 2003 SDT report considered ~50 m diameter to be the smallest truly dangerous non-metallic impactor. • Some published analyses suggest that the threshold is lower, near 30 – 40 m. • Should society be unconcerned about deflection/ evacuation if a 25 m body threatens? They impact ~10 times as often as 50 m impactors. • Officials will have to make such decisions once the new surveys begin discovering thousands of small NEOs and some appear (within uncertainties) likely to impact. • We need research (physical nature of small NEOs, propagation of shock wave from high altitude, possibility of igniting flammable materials on the ground, etc.)

12. Numbers of Small NEOs Now Known and to be Discovered by Spaceguard 2 Incremental numbers: 0.5 mag. Intervals centered on listed mag. and size. Data courtesy A. Harris (June 2007) • The discovery rate for 10 m NEAs may go up 2000 times! • By the end of SG2, we will know nearly half of Tunguska-class NEAs. • We will then be tracking 2 million 30 m objects; any threatening one will demand attention, even if impact damage might be minimal. • Think of the implications for asteroid/meteoroid/meteorite research: a quarter-million known objects 5 m in size! H Diam. (m) Known Now SG1 (goal) SG2 (goal) No. % of Tot. No. % of Tot. No. % of Tot. 17.75 1000 234 59 280 83 333 98 22.02 140 162 3.5 450 9 4000 83 24.26 50 147 0.09 1200 0.6 80000 40 25.36 30 85 0.01 640 0.08 2 million 20 27.75 10 17 1e(-6) 200 1e(-5) 400000 2 29.26 5 6 3e(-8) 30 3e(-7) 200000 0.2

13. Questions about Small Impacts • How dangerous are meteorite falls? (Few people have been hurt or killed so far, but the population density has increased dramatically.) • Will military establishments (e.g. the U.S. Air Force Space Command) release useful data (including lightcurves and energy estimates) on bolides observed by their “assets”? • What are realistic “meta-error bars” on our “knowledge” of the frequency of impacts by Tunguska-class and smaller objects? • There are order-of-magnitude differences among acoustic, satellite, & lunar impact flux estimates • What do social scientists believe will be the reaction to discovery of an actual 5-to-30 m body predicted to strike the Earth? • Should a person run toward a 30 m impact to study it or enjoy it (like one of my colleagues says he would do) or run away from it because it is dangerous? • Think about the recent impact in Peru!

14. Final Thoughts on Small Impacts • We must understand the threshold between the surely harmless and the possibly harmful…research on this vital question is urgently needed, before the Spaceguard-2 discoveries start overwhelming us. • Large meteoroids and bolides may do little or no damage, but their brilliant impacts into the Earth’s atmosphere will be the aspect of the impact hazard that will be manifest to the public, reported by the news media, and to which officials must respond. Peruvian meteorite and crater

15. Inconsistency in statistics for binary NEAs and doublet craters.

16. Binary NEAs and Doublet Craters • You would think that the recent discovery that 20% - 40% of Near-Earth asteroids are binaries would finally explain the long-standing mystery about the frequency of doublet craters on the terrestrial planets. • You would be wrong! Virtually all NEA binaries are so closely spaced that they would make single craters, not doublets.

17. Rubble Piles, Monoliths, Asteroidal Satellites, Cometary Fluff-balls The Latest Astronomical Results on Near-Earth Asteroid Physical Properties • Approximately 20% of observed NEAs are double bodies or have satellites; another 20% appear to be contact binaries (results from Arecibo and Goldstone delay-doppler images). • But among dozens discovered, only one has wide enough separation to form a doublet crater. • Spin data indicate that NEAs >200 m diameter are mostly rubble piles (including the contact binaries), whereas NEAs <200 m in size are monoliths. • Some asteroids (including dormant comets) could be very weak and fragile. Holsapple

18. History of Doublet Crater Research • Title: Martian doublet craters. Authors: Oberbeck, V.R., Aoyagi, M. Publication: J. Geophys. Res., Vol. 77, p. 2419 - 2432 Publication Date: 00/1972 • 1978: Alex Woronow debated Oberbeck about whether O & A correctly modeled spatial randomness. Result was inconclusive. Mars may well have an overabundance of paired craters, but nobody could be sure. • Topic resurrected in 1991 by Melosh and Stansberry who argued that 3 doublets on Earth must have been formed by impact of binary asteroids (this is before any asteroid satellites had been discovered). • Research in 1990s by Melosh, Bottke, Cook, and others re-examined Martian doublets and extended the analysis of doublets to Venus. • Since the mid-1990s, thinking about doublet craters has been in the context of Dactyl and SL-9.

19. 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

20. 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)

21. Observed Frequencies of Double Craters on Earth • 3 pairs of 28 craters >20 km • Ries/Steinham would not be recognized on many other bodies: on Earth, spatial density of craters is very low, so even this distant pair of craters of very dissimilar sizes stands out • Ries/Steinham also have identical dated ages • Kara/Ust Kara had been considered to be a pair, but appear to have very different ages • Problem: statistics of small numbers Clearwater Lakes

22. Observed Frequencies of Doublets on Other Planets • 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 Earth and Venus • Venus • Cook, Melosh & Bottke (2003) found 2.2% of 10-150 km diameter craters were doublets, but that “splotches” (due to smaller impactors unable to make it through the Venus atmosphere) imply ~14% doublets on Venus • Moon, Mercury, satellites • I’ve found no definitive studies • Doublets have been found, however

23. Geometry of NEA Binaries and Opportunity 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? • Separation 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

24. Opportunities: Outer Solar System and Elsewhere There’s a fine PhD thesis here! • Classic doublet found in Cassini image of Tethys [right, thanks to Paul Schenk] • Binary TNOs (hence binary comets?) are widely separated, increasing the chances for finding doublet craters. • Some satellite surfaces are very youthful with small crater densities (Europa, Enceladus, Miranda, Triton), so possibilities for confusion with random pairing are reduced. (But confusion with secondaries and sesquinaries may be heightened in planetary systems with many moons and rings.)

25. The role of secondary craters: lessons from Europa and Mars.

26. Crater Production Function • Shoemaker first proposed steep branch as secondaries • Neukum (and most others eventually) considered it an attribute of primaries • Evidence from Europa and Mars now suggests Shoemaker was right after all • Another question: Big, secondaries from basins? (Wilhelms) “Secondary Branch” T.P. Highlands

27. Secondary Craters on Europa (and the Moon) (Bierhaus et al., Nature, 2005) • From studies of spatial clustering and size distributions of ~25,000 craters on Europa, Bierhaus concludes that >95% of them (consistent with all of them) are secondaries! • Simple extrapolation to the Moon (if craters in ice behave as in rock) shows that secondaries could account for all small craters on the “steep branch” of the size-frequency relation! Near-field secondaries Far-field secondaries

28. Secondaries Dominate Mars(McEwen et al. 2005) “The Rayed Crater Zunil and Interpretations of Small Impact Craters on Mars” (Alfred S. McEwen, Brandon S. Preblich, Elizabeth P. Turtle, et al.,2005) • Zunil produced enough secondaries to account for 1 Myr of Neukum production function • Zunil may have made a billion craters >10m diam

29. Gaspra Craters: What Are the Real Counts? • Chapman sees 20 craters >500 m diameter • Neukum sees 5 craters >500 m diam. • The power-law indices differ by more than 1 • Much hinges on Gaspra: Neukum cites Gaspra as the best proof that the “steep branch” is primaries, not secondaries. • We must get this right!

30. Crater Size Distributions These are made by ..... these! (But apparently NOT!) • Differential slope (craters 100m-1km): -4.8 • Differential slope (NEOs 5m-50m): -3.5 • What about the main belt and Gaspra? • Saturation equilibrium crater densities R=0.1-0.2 on Ida indicate differential slope -3.5 • Subdued craters on Gaspra also R=0.1-0.2 → -3.5 • Fresh craters superimposed on subdued craters have slope -4.3: must be “interplanetary secon-daries” from a recent event (Bottke & Chapman LPSC 2006) Gaspra Shoemaker 1965 Neukum Bottke 2006

31. Most Craters < few km diameter are Secondaries! Implications? • Small-crater production is very episodic and dominated by one or a few impact events • Because of shallow slope (~-2) of production function of source craters (5-100 km diam.), most secondaries are produced by occasional 100 km cratering events, or by largest cratering events during exposure duration of counting surface • Secondaries are very spatially clustered • On Europa mosaic E17LIBLIN01, spatial densities of small, distant secondaries vary by factors exceeding 50! (This is typical.) • Cannot assume that secondary cratering “averages out”. Inferred cratering ages could be wrong by two orders of magnitude! • Problem for dating small units from craters <5 km diameter, serious problem for ages based on craters <1 km diameter

32. What is the cratering evidence for the Late Heavy Bombardment?

33. The LHB: Lunar Impact Basins See: Chapman, Cohen, Grinspoon (2007), Icarus 189, 233-245. Basins are also common on Mars, Mercury, Ganymede, Callisto, Iapetus, Rhea, Tethys, Vesta, etc. But (except possibly Vesta), we have no absolute radiometric ages for these basins. The only reasons for believing that they were created during the same epoch, come from indirect dynamical and geophysical arguments. • The Moon is covered with multi-ringed impact basins. • Paul Spudis’ map (lower left) shows only the most prominent ones. • Wilhelms, Spudis, and Wood believe that there are at least 45 basins, many highly degraded. • Nectaris and younger frontside basins have been dated, from argued geological associations with dated lunar rocks. • At least 2/3rds of all basins are pre-Nectarian.

34. Late Heavy Bombardment… or “Terminal Cataclysm” After Wilhelms (1987) • Proposed in 1973 by Tera et al. to explain peak in radiometric ages of lunar samples ~4.0 -- ~3.8 Ga • Wilhelms (1987) shows sharp decline in basin-formation rate between Nectaris (~3.92 Ga) and final basin, Orientale (~3.82 Ga) • There are few rock ages, and virtually noimpact melt ages, prior to 3.92 Ga (probable Nectaris age) (Ryder, 1990) • Basins produce copious melts (~10% of involved materials) • Small craters produce few melts because efficiency of melt-production increases with crater size and basin-forming projectiles volumetrically dominate shallow SFD • So impact melts should be a robust marker of the history of basin formation ? (Cumulative) Crater Density Implies: short, 50-100 Myr bombardment, with minimal earlier basin formation between crustal formation and this LHB LHB

35. Schematic Representations of Lunar Cratering History Alternatives • Can various steady declining flux models have a high enough rate at 3.9 Ga without being too massive early on? (Destroy the crust, contaminate it…or require unrealistically massive projectile population.) • From cratering/age-dating perspective, we can’t observe the history before 4.0 Ga [grey areas] Strom et al. (2006) Zahnle et al. (2007)

36. What Happened Before Nectaris? (i.e. before 3.90 - 3.92 [4.1?] Ga) • Fragmentary geology remains from earlier times. • 50% of Wilhelms’ “definite” basins pre-date Nectaris (as do 70% of all “definite”+“probable”+“possible” ones). • Almost no impact melts pre-date Nectaris Basin. How could the earlier basins not produce melts? Perhaps they are somehow “hidden” from being collected! (Even though some pre-Nectarian rocks exist.) • Weak contraints: • Lunar crust intact (constrains top-heavy size-distribution) • Minimal meteoritic contamination (perhaps projectile material preferentially lost)

37. Large Crater (Basin) on Vesta • “Crater”, seen edge-on with prominent central peak in HST image, is ~460 km in diameter, nearly as large as Vesta itself: it is a basin! • It is plausible, but unproven, that this basin was the source of the numerous “Vestoid” asteroids and thus for the HED achondritic meteorites. • Ar-Ar ages for eucrites range from 4.3 to 3.2 Ga, an interval much longer than the lunar LHB, but centered on the same epoch. • However, the “fresh”, fairly un-space-weathered spectra of Vesta and Vestoids suggest that the basin formed recently. • Yet a long-standing mystery is why Vesta wasn’t cratered even more: its basaltic crust has somehow remained largely intact. Smaller bodies have comparatively large craters, too. But Stickney (10 km diam.) is not a “basin”.

38. Lunar, HED Rock Resetting Ages The LHB, as defined by basin ages, is a narrow range (100 Myr LHB shown by pink box). Predominant lunar rock ages range from 3.6 to 4.2 Ga. (Impact melts are restricted to <4.0 Ga.) So rock/melt ages correlate poorly with basin ages…implying selection biases. HED ages span 3.2 to 4.3 Ga. So bombardment in the asteroid belt extend-ed ~300 Myr after end of lunar rock resettings… or there are selection biases. [Data summarized by Bogard (1995)] Moon HED Parent Body(Vesta?) Age span for small lunar melts (Cohen et al., 2000) extends to 2 Ga, long after last basin Time 3.3 4.4

39. A New Look at the “Stonewall”: Is the LHB a “Misconception”? • Saturation by 30-100 km craters would have pulverized early melt-rocks (Hartmann, 1975, 2003), creating artificial rock-age spike. • But “it is patently not the case” that all rocks would have been reset or “pulverized to fine powder” (Hartmann et al., 2000 [presumably one of his co-authors]). • Grinspoon’s (1989) mathematical model seemed to verify the stonewall effect. • But it is a 2-D model, simulating only a pure veneer. • Areal saturation is dominated by • craters 100 meters to 2 km diameter (steep SFD repeatedly churns/comminutes upper few meters of surficial regolith) • craters 30 to 100 km diameter plus basins (which sporadically penetrate down kilometers, creating a coarse, poorly churned megaregolith )

40. South Pole-Aitken, Orientale:What are their Absolute Ages? AND we need modeling studies of megaregolith evolution and of sampling biases! • South-Pole Aitken is relatively old and very large: is its age 4.3 or 4.0 Ga? • Orientale is the youngest basin. But is its age 3.72 or 3.84? • If the “Vision” lunar program can date samples that are unambiguously as-sociated with these basins, then we can determine the duration of the LHB. • The crater Cantor is in between the two basins and close to both. We could sample near it.

41. Basin Ages (Stöffler & Ryder, 2001, Space Science Reviews: critical re-evaluation of isotopic ages of lunar geologic units.) • Numerous un-certainties remain in the association of dated samples to specific basins • Bottke et al. (2007) explore extremes: Nectaris as old as 4.12 Ga, Imbrium as young as 3.72 Ga

42. Dynamical/Collisional Models Try to Produce Late Basins Modern view is that outer planets must have migrated 100s of Myr after solar system formation, with sweeping resonances stirring up main-belt asteroids cataclysmically, and perhaps with Uranus/Neptune stirring up KBOs. It is difficult to avoid a late terminal cataclysm! • Planetesimals left over after terrestrial planet formation have a main-belt-like size distribution. • They are dynamically depleted (just like modern-day Near-Earth Asteroids) and collisionally evolve. • The lunar impact flux declines by 4 orders of magnitude by the time visible lunar basins formed. • In order to form the 4 largest, most reliably dated basins during broadest allowed time interval for LHB, initial planetesimal population must be ~1 to 10 Earth masses! • To avoid a ridiculously massive solar nebula in the terrestrial planet region, there must have been a late cataclysm to produce even a few basins around 3.9 Ga. (Bottke et al., 2007, Icarus: Can planetesimals left over from planetary formation form basins as late as 4.1 to 3.7 or 3.8 Ga?)

43. Main-Belt Asteroids Caused the LHB (Strom et al., 2005) • Shape of main-belt asteroid SFD matches lunar highland craters • Shape of NEA SFD matches lunar maria craters • Size-selective processes bring NEAs from main belt to Earth/Moon • A solely gravitational process bringing main-belt asteroids into Earth-crossing orbits could produce highland SFD (e.g. resonance sweeping) • BUT, main-belt SFD may not be unique…could reflect a collisionally evolved population anywhere in the solar system • The “Nice Model” could produce a comet shower followed by an asteroid shower

44. Basin Degradation due to Viscous Relaxation (Baldwin, 2006) Baldwin’s Crater Degradation Classes • Lunar viscosity 1025 poises at 4.3 Ga, increased factor of 4 by time Orientale formed • Viscosity would have had to increase an (unphysical) factor of ~40 if all basins were formed during a short LHB • Assumption: degradation is by viscosity only; not by erosion, filling, crater overlap, etc. Basin Class Calc. Age Orientale 2 3.8 Ga Imbrium 3 3.84 Ga Crisium 4 3.91 Ga Nectaris 7 4.1 Ga Humorum 9 4.23 Ga Werner–Airy 10 4.3 Ga

45. “Recent” Basins on Terrestrial Bodies Radar image by the soon-to-be shut down Arecibo The single Martian meteorite with a resetting age ~4.0 Ga Caloris on Mercury Orientale on the Moon Argyre on Mars

46. Martian Basins and Chronology • Mars has prominent basins (Argyre, Hellas, etc.) • Frey et al. (2007) have studied Quasi-Circular Depressions (QCDs) from MOLA, find many old basin-sized features. • The Mars community regards these features as being very old, pre-Noachian, perhaps ~4.3 Ga (using cratering chronologies of Neukum and Hartmann, both of whom are skeptical of a cataclysmic LHB). • But the chronology for ancient Mars is unknown, except for the resetting age of ALH84001, a rock of unknown provenance. • If an LHB spike occurred on the Moon after 4.0 Ga, then it probably occurred on Mars (and Mercury, and throughout the inner solar system) as well, and all of Frey’s QCDs could be younger than 4.0 Ga. • The pre-Noachian and Noachian could be very compressed in time, perhaps with a burst of impact-produced heating and watery climate.

47. Basins on Outer S.S. Satellites Odysseus on Tethys Iapetus Valhalla on Callisto Gilgamesh on Ganymede

48. Cratering in the Jovian System (R. Strom) The disparity between the Moon and Callisto could be reconciled by the amazing “Neukum shift” (coming, several slides later) Approx. SFD for Ganymede (Galileo)

49. Saturn Satellite Cratering (in a Solar System Context) Closed symbols = Hyperion Open symbols = Phoebe Cassini counts by P. Thomas (J. Richardson, DPS 2007) From Chapman & McKinnon (1986)

50. Saturnian Impactor Population Asteroidal impactors for inner solar system (Bottke et al. 2005, O’Brien et al. 2005) Phoebe Saturnian system impactor population (Jim Richardson, DPS 2007)