Late Heavy Bombardment: Evidence From Cratering Histories of the Moon, Planets, Satellites, and Asteroids Clark R. Chapman Southwest Research Institute, Boulder CO 39 th AAS/DPS Meeting Orlando, Florida “Exploring the Lunar Late Heavy Bombardment” #46.01, 10:30 a.m., 11 October 2007 Also see: Chapman, Cohen, Grinspoon (2007), Icarus 189,

Lunar Impact Basins The Moon, like most solid-surfaced bodies in the solar system, 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 approximately dated, from argued geological associations with dated lunar rocks. At least 2/3rds of all basins are pre-Nectarian. Basins are common on Mars, Mercury, Ganymede, Callisto, Iapetus, Rhea, Tethys, Vesta, and other bodies. But (except possibly Vesta), we have no absolute radiometric ages for these basins. While they may all have been created during the same epoch, interpretations about when they formed come only from indirect dynamical and geophysical arguments.

Late Heavy Bombardment… or “Terminal Cataclysm” Proposed in 1973 by Tera et al. who noted a peak in radiometric ages of lunar samples between ~4.0 and ~3.8 Ga Wilhelms (1987) documents a sharply declining basin-formation rate between Nectaris (~3.92 Ga) and final basin, Orientale (~3.82 Ga) There are few rock ages, and virtually no impact melt ages, prior to 3.92 Ga (probable Nectaris age) (Ryder, 1990) Basins produce copious melts (~10% of involved materials) Small craters produce small amounts of melt 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 (Cohen, last talk) Implies: short, Myr bombardment, with minimal earlier basin formation between crustal formation and this LHB After Wilhelms (1987) ? LHB (Cumulative) Crater Density

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. Zahnle et al. (2007) Strom et al. (2006)

What Happened Before Nectaris? (i.e. before [4.1?] Ga) Fragmentary geology remains from earlier times. 50% of Wilhelms’ “definite” basins pre-date Nectaris (and 70% of all “definite”+“probable”+“possible” ones). Surprisingly, almost no impact melts pre-date Nectaris Basin. How could the earlier basins not produce melts? Perhaps those melts are somehow “hidden” from being collected! (Even though some pre-Nectarian rocks exist.) During the long period from crustal solidification until the oldest known basins, there was (or was not) a “lull” in basin formation (and thus a cataclysm). Weak contraints: Lunar crust intact (constrains top-heavy size- distribution) Minimal meteoritic contamination (perhaps projectile material preferentially lost)

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 that Vesta wasn’t cratered even more: its basaltic crust has remained largely intact. Smaller bodies have comparatively large craters, too. But Stickney (10 km diam.) is not a “basin”.

Lunar, HED Rock Resetting Ages [Data summarized by Bogard (1995)] Moon 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 ages correlate poorly with basin ages. HED meteorite ages range from 3.2 to 4.3 Ga. So bombardment in the asteroid belt extended ~300 Myr after end of lunar rock degassings… or there are selection biases. HED Parent Body Body (Vesta?) 4.4 Time 3.3 Age span for small lunar melts (Cohen et al., 2000)

Saturation by km craters would have pulverized/destroyed 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]). Comminution by a couple generations of large-crater saturation is NOT like modern churning of uppermost meters of regolith [next slide]. Grinspoon’s (1989) mathematical model seemed to verify the stonewall effect. But it is a 2-D model; he converts 100% of crater floor to melt while the real percent (volumetrically) is much less. If melt preferentially veneers surface and older veneers are covered up, then the 2-D model could approximate the 3-D reality. A New Look at the “Stonewall”: Is the LHB a “Misconception”?

Size Distributions: Values of Differential Power-Law Index b

Crater Production Function: Areal and Volumetric Implications Crater size distribution is not a simple power-law Areal saturation is dominated by craters 100 meters to 2 km diameter (surficial regolith) craters 30 km to 100 km diameter (which penetrate down kilometers) Volumetric processing is dominated by largest craters/basins “Steep” size distribution for <1 km craters churns/comminutes upper few meters of lunar soil (particle sizes <100 microns)  .: :: .    .: :: .  :   .. :  .: :: .  Standard Function from Neukum & Ivanov (1994) b= - 4: equal mass b= - 3: equal area, saturation equilibrium

We Need to Model the 3-D Emplacement/Collection of Melts Model needs: (building on work by L. Haskin and students) %-tage melt production as function of diameter 3-D mapping of emplacement of melts and other ejecta time-history of megaregolith excavation, deposition, and “churning”, varying the impactor size-distribution gardening/impact destruction near surface over last ~3.5 Gyr analysis of collection/selection criteria and biases Some qualitative sampling biases are clear: if each new basin distributes its melts uniformly throughout the volume of the megaregolith, and churns earlier melts uniformly, then impact melts collected at the surface should sample the basin formation history in an unbiased fashion. If each new basin distributes melts in a surface veneer, and older melts are covered by ejecta blankets, then surface sampling will be dominated by most recent basin.

South Pole-Aitken, Orientale: What are their Absolute Ages? 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 is able to date samples that are unambiguously associated 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.

Basin Ages 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 (Stöffler & Ryder, 2001, Space Science Reviews: critical re- evaluation of isotopic ages of lunar geologic units.)

Production of Late Basins? 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 mag. 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 have ~1 to 10 Earth masses! To avoid a ridiculously massive solar nebula in the terrestrial planet region, there must be 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?)

Basin Degradation due to Viscous Relaxation (Baldwin, 2006) Lunar viscosity 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 Ga Imbrium Ga Crisium Ga Nectaris Ga Humorum Ga Werner–Airy Ga Baldwin’s Crater Degradation Classes

“Recent” Basins on Terrestrial Bodies Caloris on Mercury Argyre on Mars Orientale on the Moon The single Martian meteorite with a resetting age ~4.0 Ga

Martian Basins and Chronology Mars has prominent basins (Argyre, Hellas, etc.) Frey et al. (2007), who have been studying Quasi- Circular Depressions (QCDs) from MOLA, find numerous old basin-sized features There is a tendency for the Mars community to regard these features as being very old, pre-Noachian, perhaps ~4.3 Ga (using the widely adopted 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 a single 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.

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 (Morbidelli, next talk)

Basins on Galilean Satellites Valhalla on Callisto Gilgamesh on Ganymede

Odysseus on Tethys

IapetusIapetusIapetus

Cratering in the Jovian System (R. Strom) Approx. SFD for Ganymede (Galileo)

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

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

The Neukum Model The size-distribution (SFD) of craters on the lunar highlands is the same as on the maria. The SFD of asteroids that struck the Moon has been the same from 4.3 Ga through the LHB period to the present day. Satellites of Saturn and Jupiter have the same SFD as do craters on the Moon (with a “shift”). Therefore, outer solar system (OSS) cratering was mainly by asteroids, not comets. The “shift”? The OSS SFD may match the lunar SFD, but only by shifting it, in the sense that the asteroids strike at low, planetocentric velocities! To Neukum, the LHB is mainly an unremarkable stage in a generally monotonically declining flux beginning when the lunar crust solidified; on many bodies (Mars, Iapetus) we can see back to 4.3 to 4.4 Ga according to Neukum. ? ? Saturn’s moons are cratered by asteroids in Saturnicentric orbits???

It is not just the interpretations that differ: the data disagree! Neukum says lunar/Martian production function shape was the same during LHB and present; Strom says it changed dramatically Neukum Strom We need to understand why these differences persist!

Outer Solar System Basins Crater size distribution differences between terrestrial planets and OSS satellites suggest – consistent with dynamical simulations – that OSS cratering is primarily by comets/KBOs. But, there are caveats: Basin SFD could reflect viscous relaxation of features Planetocentric impactors could be important (Pop. 2) Comet/KBO size distribution is poorly known We have zero direct knowledge of the absolute geological chronology for OSS satellites. If there was no OSS LHB, the observable basins could have formed as soon as crusts solidified. For heavily cratered terrains, we don’t know how many generations of super-saturation have happened. If the cratering rate has remained high (perhaps augmented by cratering by planetocentric bodies), then OSS basins could be unexpectedly young… consistent with a cumulative history of bombardment that might have destroyed small moons, creating even more planetocentric bombardment. Studies of OSS cratering statistics provide information on relative stratigraphic chronology only!

Conclusions Basin formation has been ubiquitous in the solar system On the Moon, many basins formed just before an abrupt halt 3.8 Ga Plausibly a cataclysm 4.0 – 3.8 Ga But bombardment flux is uncertain between 4.3 and 4.0 Ga Earth must have undergone same LHB Plausibly, Mars, Mercury, and Venus had same LHB (asteroidal) But there is no direct age data Mercury could have had later vulcanoid bombardment Observable Martian geological record formed since 4.0 Ga Outer solar system bombardment was by different, non-asteroidal impactor population/s Presumably comets, possibly at similar epoch as LHB (e.g. Nice model) Saturation, planetocentric cratering adds complexity Virtually no chronological constraints

Qualitative Features of LHBs (divide by 3 if Nectaris is 4.1 Ga) On Earth, 1 “Chicxulub” (K-T boundary event, 100 million MT) every 10,000 years. Each kills virtually every complex lifeform, most fossilizable species go extinct, radiation of many new species One basin-forming event (10 billion MT!) every 500,000 years. Each erodes atmosphere, transforms ecosphere, boils oceans The 100 Myr bom- bardment would devastate life. Total LHB: ~100 basins, 1000s of K-T events. The 100 Myr bom- bardment would devastate life. What does it take to sterilize planet Earth??? K-T