Recent Developments inPLANETARY CRATERINGRecent Developments inPLANETARY CRATERING

Clark R. ChapmanClark R. Chapman

Southwest Research Institute Boulder, Colorado, USA

THOMAS A. MUTCH LECTURE

Brown University, Providence, R.I. , 17 December 2001

Slow accretion… hypervelocity collisions… lunar and planetary cratering.

Slow accretion… hypervelocity collisions… lunar and planetary cratering.

Why planetary surfaces are cratered

Two Seminal Books…

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.

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

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”

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

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!

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

Cra

ter

Den

sity

4.5 4.0 3.5 3.0 2.5

??

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!

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.

HEDParentBody

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 eventsConclusion: Melts strongly biased to recent events

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

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?

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”?

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

PwyllPwyll

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!

Power-Law Size Distributions(Proportion of Big Ones to Small Ones)

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Larg

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Smal

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The Relative Plot (R Plot)

Shows spatial densities of craters as function of size relative to saturation

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

Small Craters in Pwyll Ray

Conamara Chaos in extended ray system of Pwyll

Chaos

Small, clustered craters on plate

20 km

Near-field Secondary Craters around Tyre

Projectile punctured ice crust. Secondaries asymmetrically distributed. (Measured by B. Bierhaus)

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)

Crater Clusters on Ganymede

Clusters on uniform unit probably indicate secondary cratering.

Hi-res G28 s0552445452Hi-res G28 s0552445452

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

Deformation of Larger Craters and Landforms on Ganymede

Galileo Regio

Galileo Regio

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?

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

RR

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

Ida Ida

Spacecraft Imaging of Craters on Asteroids, Small Satellites

Phobos and Deimos

IdaIda

MathildeMathilde

GaspraGaspra

ErosEros

Gaspra: First Asteroid Imaged by a Spacecraft

Ida Looks Much Like the Moon

NEAR Science Team Visits the Spacecraft before Launch

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

Largest Craters onMathilde and Eros

The scars of three previous impacts can be seen on the planetary disk.

Mathilde’s shape is dominated Mathilde’s shape is dominated by its giant craters.by its giant craters.

Himeros and Psyche are large Himeros and Psyche are large compared with the compared with the radiusradius of of Eros, but its elongated shape is Eros, but its elongated shape is notnot primarily due to impact. primarily due to impact.

“Ponds” from Low-Altitude Flyover

NEAR-Shoemaker’s Landing Spot on Eros

How typical is the edge of Himeros of Eros?How typical is the edge of Himeros of Eros?

How typical is Eros of other asteroids?How typical is Eros of other asteroids?

Inset shows Himeros

Estimated positions of last images end within a 50 meter diameter crater

Eros is Covered with Rocks

Final Landing Mosaic

Closest Image of Eros

R Plot: Eros Craters & Boulders

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

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

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?

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!

Ida Craters Saturated < 1 km Diameter

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)

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)

Densities of Craters and Boulders on Eros vs. Size

“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?

Fifth Last Image (largest boulders are 3 meters across)

Why are there so few Small Craters? A Logic Tree

SMALL PROJECTILES EXISTSMALL PROJECTILES EXIST SMALL PROJECTILES RARESMALL PROJECTILES RARE

But Don’t Craters Made butMake Craters Erased

Somewhat Down byFewer Orders of Mag.

* Armored byBoulders

* Surface is

Impervious (?)

* Surface is like Putty

(?)

* Covered byEjecta or Dust(would coverboulders, too)

* Eroded (by what?)

* Seismic Shaking

* YarkovskyEffect plusSome of These

* YarkovskyEffect in theMain Belt (Bell, 2001)

Eros/MoonComparisons

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!