1. Recent Developments in PLANETARY CRATERING Recent Developments in PLANETARY CRATERING Clark R. Chapman Southwest Research Institute Boulder, Colorado, USA THOMAS A. MUTCH LECTURE Brown University, Providence, R.I., 17 December 2001

2. Slow accretion… hypervelocity collisions… lunar and planetary cratering. Why planetary surfaces are cratered

3. Two Seminal Books…

4. The History of Research in Planetary Cratering “Few...would contend that men have become progressively more intelligent over the past centuries…[yet we believe that we] will never repeat those errors of observation and interpretation made in previous generations… [We’re] so involved in ‘original’ research that we have no opportunity to review the history of thought in our own fields and to become familiar with the origins of ideas recorded in this history.” Tim Mutch, first paragraph in Geology of the Moon Princeton Univ. Press, 1970 Mutch’s book, written before the Apollo 11 mission, is rich with insights that can still guide our studies of lunar cratering today.

5. Interplanetary Correlation of Geologic Time Mutch traces the modern stratigraphic view of lunar studies to the epochal works of Gene Shoemaker and his colleagues. In the early 1960s, they devised a lunar stratigraphy and established an approach to linking it with the stratigraphic histories of other planets using radiometric ages (when possible) and asteroid/comet/meteoroid cratering rates. A young Eugene Shoemaker on the rim of Meteor Crater S-L 9

6. Toward a Planetary Cratering Paradigm, 1950 - 1990 Meteor Crater, other terrestrial and lunar craters mainly due to impact Late Heavy Bombardment (terminal cataclysm at 3.9 Ga) recognized on Moon; applied to Earth, Mars, Mercury, Jovian system Subsequent constant cratering on all planets “Steep” power-law size distribution due to asteroids Minimal contribution by secondaries, endogenic craters Geological record on most bodies (incl. Callisto, Ganymede, Mercury, Mars) is ancient; most satellites, all asteroids are geologically “dead”

7. Cratering: General Issues Observations. Sizes, spatial densities and clustering, morphologies, geological relationships, leading/trailing side asymmetries, far-field effects of ejecta Origins. Comets, asteroids, circum- Jovian objects (e.g. S-L 9), secondaries, endogenic features (e.g. collapse pits) Chronology. Early intense bombardment (?), modern impact rates: date units, determine resurfacing rates

8. An Evolving 21st Century Perspective: Outline of Lecture The Late Heavy Bombardment isn’t what it seems…and may have an exotic origin…or origins Cratering on Galilean satellites is dominated by secondary cratering, confused by endogenic features and active erasure processes, and minimally cratered by small comets Asteroid surfaces (at least Eros) are wholly unlike the lunar regolith; dominated by boulders, with almost NO small craters!

9. Late Heavy Bombardment… or “terminal cataclysm” Proposed in 1973 by Tera et al. who noted a peak in radiometric ages of lunar samples ~4.0 - 3.8 Ga Sharply declining basin-formation rate between Imbrium (3.85 Ga) and final basin, Orientale (3.82 Ga) Few rock ages, and no impact melt ages prior to 3.92 Ga (Nectaris age) Implies: short, 50-200 Myr bombard- ment, but minimal basin formation between crustal formation and LHB Crater Density 4.5 4.0 3.5 3.0 2.5 ? ?

10. Debate over “Cataclysm” “Stonewall” effect (Hartmann, 1975) destroys and pulverizes rocks prior to saturation Grinspoon’s (1989) 2-dimensional models concur No impact melts prior to Nectaris (Ryder) Lunar crust not pene- trated or pulverized (but constrains only top-heavy size distributions) No enrichment in meteoritic/projectile material (not robust) A Misconception...It Happened!

11. Same LHB for Asteroids, Moon? [Data summarized by Bogard (1995)] Moon The LHB, as defined by basin ages, is a narrow range (shown by pink box). Predominant lunar rock ages range from 3.7 to 4.1 Ga. (Impact melts are restricted to <3.92 Ga.) HED meteorite ages range from 3.4 to 4.3 Ga. So rock ages correlate poorly with basin ages. And asteroid bombard- ment extended 300 Myr after end of lunar rock resettings. HED Parent Body

12. Evidence for Commencement of LHB from Impact Melts Ryder (1990): Impact melts produced more efficiently than rock ages are reset; should record history of basin impacts. But no impact melts have been found older than the Nectaris Basin (3.92 Ga)! Therefore, there was a dearth of basin formation before Nectaris, followed by a cataclysm. However, 2/3rds of known basins occurred stratigraphically before Nectaris (Wilhelms, 1987)…so where are their impact melts? Cohen et al. (2000) find melt clasts from 3.9 Ga extending all the way to 2.8 Ga. (only 2 of 7 melt- producing “events” occurred during the LHB). Conclusion: Melts strongly biased to recent events

13. LHB Conclusions Sharp decline in lunar basin formation from 3.85 Ga (Imbrium) to 3.82 Ga (Orientale, the very last one) strongly constrains dynamics of LHB source bodies Until the processes that cause sampling bias for impact melts are understood (3-D models), their absence from ancient times provides minimal constraint on pre-Nectaris bombardment rate Hence, whether LHB was a “cataclysm” or just an inflection in a declining flux remains unknown Mismatch in lunar/asteroidal age histograms means (a) different LHBs or (b) different sampling biases

14. Jovian Cratering: the Big Questions What are absolute cratering ages of surfaces? Did the LHB include the Jupiter system? Did Ganymede “die” 4 Gyr ago, or did activity persist? Are places on Europa only millions of years old? What is the size distribution of impactors? What kinds of projectiles made the craters? Ancient (even current) asteroidal bombardment? Kuiper Belt/Oort Cloud comets (incl. disrupted S-L9’s)? Secondary craters from rare large ones (or endogenic)? Does a paucity of craters at some size imply resurfacing, relaxation, or reduced production?

15. Cratering: Voyager Perspective Io -- “No impact craters >1 km” Europa -- “Craters are not plentiful” Ganymede -- Intense early bombardment assumed; dark terrains 3.8-4 Ga, bright terrains 3 - 3.8 Ga. Palimpsests recognized. Callisto -- Saturated cratering (including basins) >4 Ga. Paucity of craters >50 km diameter: “viscous relaxation”?

16. Large Impact Craters on Europa Indicate Thickness of Ice Crust Largest impacts make concentric ring structures (puncture ice crust) Pwyll is transi- tional (has crater form but flatter) Craters <20 km diam. are bowl- shaped, formed within multi-km thick crust Pwyll

17. Cratering Age of Europa’s Surface (Big Craters) (Zahnle 2001) 1 km comets impact Jupiter every 200 yr 1km comets impact Europa every 3.3 Myr (making 20 km craters; Pwyll may be the latest one) Given ~12 to 20 craters >20 km, average age of Europa’s surface is about 50 Myr (uncertainty is a factor of several) Parts of Conamara Chaos appear to encroach on Pwyll ray; certainly localities have ages < 1 Myr. Europa is active now!

18. Power-Law Size Distributions (Proportion of Big Ones to Small Ones) Dominated by Big Ones (Eros craters <100m diam.) Large Lunar Craters Saturation Equilibrium (Small lunar-regolith craters) Small Ones Dominate (Gaspra/Lunar Production Fcn.)

19. The Relative Plot (R Plot) Shows spatial densities of craters as function of size relative to saturation

20. Europa: From Voyager’s Pits to Youthfulness of “Wedges” Voyager “craters” are manifestation of pits/spots/domes chaos 1 km craters under- saturated 1000 X

21. Small Craters in Pwyll Ray Conamara Chaos in extended ray system of Pwyll Chaos Small, clustered craters on plate 20 km

22. Near-field Secondary Craters around Tyre Projectile punctured ice crust. Secondaries asymmetrically distributed. (Measured by B. Bierhaus)

23. Clusters of Distant Secondaries on Europa Dots show x,y positions of hundreds of secondary craters in clusters, far from any primary crater. Probably, <20% of small craters are primaries. (Measured by Beau Bierhaus)

24. Crater Clusters on Ganymede Clusters on uniform unit probably indicate secondary cratering. Hi-res G28 s0552445452

25. Unit Ages/Production Function: Implications from Small Craters If small craters are primaries and we know or can infer production function, then we can derive relative (and absolute) ages for units But inferences about production function are shaky and most small craters are secondaries A conservative estimate (Bierhaus 2001) is that ~80% of small craters are clustered Reasonable estimates of ejecta volumes and impact velocities from known craters >10 km in diameter imply that all of Europa’s small craters could be secondaries Combined with indications of few small craters on J3 & J4, this suggests paucity of small comets

26. Deformation of Larger Craters and Landforms on Ganymede Galileo Regio

27. Callisto: Degradation Found at Medium-High Resolution Larger craters “wasting away” -- perhaps sublimation of volatiles, leaving dark-colored lag What process redistributes lag to form flat, undulating regions? Are small craters few due to blanketing or few small comets?

28. “Pits” on Callisto, with Mono- modal Size Distribution R R

29. Emergent Themes from Galileo Studies of Galilean Satellite Cratering Surfaces are young, active. Even on Callisto, where large, ancient features are preserved, sublimation rapidly degrades and erases smaller-scale features Tectonic processes (perhaps associated with sub- crustal oceans) have greatly modified surface topography in recent epochs on Europa & Ganymede On youthful Europa, where the secondary crater fields of the few large primaries are most easily studied, they dominate over primaries Endogenic “pits” confuse issues on Europa, Callisto There is a dearth of small comets compared with extrapolations of large-comet size distribution

30. Ida Spacecraft Imaging of Craters on Asteroids, Small Satellites Phobos and Deimos Ida Mathilde Gaspra Eros

31. Gaspra: First Asteroid Imaged by a Spacecraft

32. Ida Looks Much Like the Moon

33. NEAR Science Team Visits the Spacecraft before Launch

34. Mathilde and Its Huge Craters C-type asteroids may be very different places, at all scales, compared with S-type asteroids like Ida and Eros

35. Largest Craters on Mathilde and Eros The scars of three previous impacts can be seen on the planetary disk. Mathilde’s shape is dominated by its giant craters. Himeros and Psyche are large compared with the radius of Eros, but its elongated shape is not primarily due to impact.

36. “Ponds” from Low-Altitude Flyover

37. NEAR-Shoemaker’s Landing Spot on Eros  How typical is the edge of Himeros of Eros?  How typical is Eros of other asteroids? Inset shows Himeros Estimated positions of last images end within a 50 meter diameter crater

38. Eros is Covered with Rocks

39. Final Landing Mosaic

40. Closest Image of Eros

41. R Plot: Eros Craters & Boulders

42. Eros is NOT Like the Moon! The Moon has craters. Eros has rocks. “It’s a relatively flat plain with a lot of craters of the five to fifty foot variety… Thousands of little one and two foot craters.” Neil Armstrong, as quoted in “Geology of the Moon” by Tim Mutch

43. Yarkovsky Effect Depletes Small Projectiles! Fewer centimeter-to-meter scale projectiles means fewer meter-to-tens-of-meter sized craters on Eros and other asteroids. Fewer small projectiles means that ejected/exhumed boulders are shattered rarely, so there are more of them. Few small craters, Many small boulders

44. Comparisons of Craters on Spacecraft-Imaged Asteroids GASPRA Big craters absent (except “facets”?); small craters undersaturated. Young and/or made of strong metal. IDA Saturated with craters (or nearly so). Lunar-like megaregolith, ~2 Gyr age, possible “rubble pile” dominated by two large pieces. MATHILDE Ida-like but supersaturated by giant craters. Low-density/voids may cause compression, minimal shock transmission, anomalous ejecta velocities. EROS Ida-like but shattered shard, only source of data at hi-res scales. Amazing! Could Ida be like this at hi-res, too?

45. Some Final Thoughts on Planetary Cratering... Not all insights are from the last 10 years… “…there is good reason for thinking that meteoritic bombardment was concentrated during an early period in the Moon’s development and that the formation of large basins was also restricted to that period.” -- Tim Mutch, before Apollo 11 Surfaces once thought to be ancient and dead are often found to be young, maybe even currently active The idea of a unique size-distribution of projectiles, impacting everywhere in the solar system at a constant rate over time…is an incredible over-simplification. Small body populations have all kinds of compositions, subject to collisions, weathering, and various chaotic dynamical forces. There is MUCH still to be learned!

46. Ida Craters Saturated < 1 km Diameter

47. Eros shows no spatial color variability, unlike Ida Even small rocks are usually the same color as the rest of Eros Possibilities: Coated with electro- statically levitated dust? Maturely space- weathered while in near- Earth orbit? In many ways, Eros resembles Ida…but not in color heterogeneity Probable composition: ordinary chondrite (favoring L/LL but not yet secure)

48. Tool for Counting (Big) Craters and (Small) Boulders Measurement tool developed by J. Joseph and P. Thomas Sparse craters are measured in whole image Boulders more- than-well sampled in one-quarter of image This image from Low Altitude Flyover (10/00)

49. Densities of Craters and Boulders on Eros vs. Size

50. “Ponds” and “Beaches”? “Ponds” are flat, level, and are sharply bounded “Beaches” (not always seen) surround some ponds and are relatively lacking in either craters or boulders Although stratigraphically younger, ponds may have more small craters than typical terrains, suggesting that boulders may armor crater production How are they formed? Electrostatic levitation, seismic shaking? If mass- wasting, why don’t lunar ponds exist? “Ponds” are flat, level, and are sharply bounded “Beaches” (not always seen) surround some ponds and are relatively lacking in either craters or boulders Although stratigraphically younger, ponds may have more small craters than typical terrains, suggesting that boulders may armor crater production How are they formed? Electrostatic levitation, seismic shaking? If mass- wasting, why don’t lunar ponds exist?

51. Fifth Last Image (largest boulders are 3 meters across)

52. Why are there so few Small Craters? A Logic Tree SMALL PROJECTILES EXIST SMALL PROJECTILES RARE But Don’t Craters Made but Make Craters Erased Somewhat Down by Fewer Orders of Mag. * Armored by Boulders * Surface is Impervious (?) * Surface is like Putty (?) * Covered by Ejecta or Dust (would cover boulders, too) * Eroded (by what?) * Seismic Shaking * Yarkovsky Effect plus Some of These * Yarkovsky Effect in the Main Belt (Bell, 2001)

53. Eros/Moon Comparisons Ejecta is very widespread on Eros, much lost to space, few generations of churning Lunar ejecta is repeatedly churned in situ, becomes very mature Rocks (ejecta blocks from far-away large impacts and exhumed from below) remain in place, cover the surface of Eros Lunar rocks are fragmented and eroded; surface is covered by craters Flat, pond-like deposits (of fines) common in depressions -- few rocks or craters Electrostatically levitated dust on Moon does not form ponds, at least not commonly The Surface of Eros is NOT like the Lunar Regolith!