Transcript No Slide Title

An Inquiry about Evidence for
the Late Heavy Bombardment
Clark R. Chapman &
David H. Grinspoon
SwRI, Boulder CO
Barbara A. Cohen
HIGP, Univ. Hawaii, Honolulu
65th Meteoritical Society Meeting (2002)
UCLA, Los Angeles, CA Thurs. p.m., 25 July 2002
Late Heavy Bombardment…
or “terminal cataclysm”
After Wilhelms (1987)
 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 3.9 Ga (Nectaris age)
LHB
Implies: short, 50-100 Myr bombardment, but minimal basin formation
between crustal formation and LHB
Proposed Dynamical
Origins for LHB
 Outer solar system planetesimals from late-
forming Uranus/Neptune (Wetherill 1975)
 Break-up of large asteroid
 Extended tail-end of accretion; remnants from
terrestrial planets region
 Expulsion of a 5th terrestrial planet (Chambers
& Lissauer 2002; Levison 2002)
 OSS planetesimals and asteroids perturbed by
sudden expulsion of Uranus & Neptune from
between Jupiter & Saturn (Levison et al. 2001)
Relevance of Impact Melts
(Graham Ryder, 1990)
 Basin formation produces copious melts
(~10% of involved materials)
 Smaller craters contribute few melts
Melt formation efficiency increases with crater size
 Basins dominate involved materials because of
shallow size-distribution

 Impact melts are produced more efficiently
than rock ages are reset
Therefore, age-distribution of impact melts
should be robust evidence of basin
formation history (given unbiased sampling)
What Happened Before Nectaris
(i.e. prior to 3.90 to 3.92 Ga)?
 Fragmentary geology remains from earlier times.
 But 50% of Wilhelms’ “definite” basins pre-date Nectaris
(and 70% of all “definite”+“probable”+“possible” ones).
 Surprisingly, no impact melts pre-date Nectaris, so none
of the earlier basins formed melts… or 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 may (or may not) have
been a “lull” in basin formation (and thus a cataclysm).
 Weak contraints:
Lunar crust is “intact” (depends on size-distribution)
 Impactor “contamination” (projectile retention efficiency)

Debate over “Cataclysm”
A Misconception...
 “Stonewall” effect
(Hartmann, 1975)
destroys and
pulverizes rocks
prior to saturation
 Grinspoon’s (1989)
2-dimensional
models concur
It Happened!
 No impact melts prior
to Nectaris (Ryder,
1990)
 Lunar crust not pene-
trated or pulverized
(but constrains only
top-heavy size
distributions)
 No enrichment in
Time
meteoritic/projectile
material (not robust)
Conundrum concerning
Impact Melts
 No impact melts have been found older than the
Nectaris Basin (3.92 Ga) despite the fact that 2/3rds of
known basins occurred stratigraphically before Nectaris
(Wilhelms, 1987). Where are their impact melts?
 Cohen et al. (2000) found melt clasts from 3.9 Ga
extending all the way to 2.8 Ga (only 2 of 7 meltproducing “events” occurred back during the LHB).
Thus, many impact melts are found dating from more
recent times when we know that basins weren’t forming.
 Numerous early basins yield no melts; recent, in-
efficient melt-production by small craters yields melts!?
There is only one Conclusion: Collected impact
melts are strongly biased to recent events...
Lunar, HED Rock
Degassing Ages
The LHB, as defined by
basin ages, is a narrow
range (100 Myr LHB
shown by pink box).
[Data summarized by
Bogard (1995)]
Moon
HED
Parent
Body
Predominant lunar rock
ages range from 3.6 to
4.2 Ga. (Impact melts are
restricted to <3.92 Ga.)
So rock ages correlate
poorly with basin ages.
Time
3.3
4.4
(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.)
Non-Lunar Evidence for LHB
 Cratered uplands on Mars/Mercury
(and even Galilean satellites!)
inferred to be same LHB… but
absolute chronology is poorly
known or unknown.
 ALH84001 has a ~4 Ga resetting
age… but that is “statistics of one”.
 Peaks in resetting ages noted for
some types of meteorites (HEDs,
ordinary chondrites)… but age
distributions differ from lunar case.
Asteroidal vs.
Lunar LHB
 Kring & Cohen (2002)
summary of meteorite degassing ages
 Very “spread out”
compared with lunar LHB
 Somewhat “spread out”
compared with lunar rock
impact degassing ages
 Evidence is dissimilar!
Different impact histories, or
 Different selection biases

LHB
Lunar rock degassing ages
A New Look at the “Stonewall”

Saturation of megaregolith would have
pulverized/destroyed early rocks (Hartmann,
1975), 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)
 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 is much less
 However, if melt preferentially veneers surface,
as is generally expected to be true, then the 2-D
model may approximate the 3-D reality.
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.
LHB Conclusions
 If lunar basin formation sharply declined from 3.85 Ga
(Imbrium) to ~3.82 Ga (Orientale, the very last one), then
dynamics of LHB source bodies are strongly constrained.
 Until the processes that cause sampling bias for impact
melts are understood (3-D models), absence of melts
from ancient times provides a minimal constraint on the
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. We can’t
conclude anything about (a) until (b) is understood.