Transcript Few rock ages, no impact melt ages prior to 3.9 Ga

(With thanks to my collaborators,
David Grinspoon and
Barbara Cohen)
What is Known about the Late Heavy
Bombardment and How it Might
Constrain Astrobiology
Clark R. Chapman
Southwest Research Institute Boulder CO
Astrobiology and Astrophysical &
Planetary Sciences Joint Colloquium
Univ. Colorado at Boulder 1 December 2003
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
(Cumulative) Crater Density
rate between Imbrium (3.85 Ga) and
final basin, Orientale (3.82 Ga)
 Few rock ages, no impact melt ages
prior to 3.9 Ga (prob. Nectaris age)
LHB
Implies: short, 50-100 Myr bombardment, but minimal basin formation
between crustal formation and LHB
Debate over “Cataclysm”
A Misconception
“Tail-end” of
accretion
 “Stonewall” effect
(Hartmann 1975, 2003)
destroys and
pulverizes rocks
prior to saturation
 Grinspoon’s (1989)
Post-crust,
pre-spike lull
defines LHB
two-dimensional
models concur
vs.
It Happened!
 No impact melts prior to
3.9 Ga (Ryder 1990, 2002)
 Lunar crust not pene-
trated or pulverized (but
constrains only topheavy size distributions)
 No enrichment in
meteoritic/projectile
material (not robust:
projectile material preferentially ejected)
(Mostly) uncontroversial sharp
decline in bombardment rate
from 3.90 Ga to 3.83 Ga
Further confusion on LHB decay:
>Basin formation decayed in 50 Myr
>Rocks degassed over 200 Myr
?
>Impact melts decayed over 1000 Myr
Time
[Chapman, Cohen & Grinspoon, 2003]
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. before 3.90 - 3.92 [4.1?] 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 the Nectaris
Basin, 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 was (or was not) a “lull”
in basin formation (and thus a cataclysm).
 Weak contraints (listed before):
Lunar crust intact
 Minimal meteoritic contamination

Conundrum Concerning Impact Melts:
Do they Reflect Impact Flux?
 No impact melts have been found securely older than Nectaris (3.92
Ga) although 2/3rds of known basins occurred stratigraphically
before Nectaris (Wilhelms, 1987). Where are their impact melts?

Cohen et al. (2000) found tiny melt clasts from 4.0 Ga extending all
the way to 2.4 Ga (only 2 of 7 melt-producing “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.
LHB

Numerous early basins yield no melts; yet more recent, inefficient
melt-production by small craters does yield 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.)
(Vesta?)
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.)
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 by 30-100 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

 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
 However, if melt preferentially veneers surface, as
much of it does, and older veneers are covered up,
then the 2-D model may approximate the 3-D reality.
Size Distributions: Values of
Differential Power-Law Index b
Crater Production Function: Areal
and Volumetric Implications
  .:
::.



  .:
::.
b= -4: equal mass
:..
:
Standard Function from
Neukum & Ivanov (1994)
b= -3: equal area,
saturation equilibrium
 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)
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.
Conclusions about Lunar
Evidence for LHB
 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.
Non-Lunar Evidence for LHB
 Cratered uplands on Mars/Mercury
(and even Galilean satellites!)
inferred to be due to 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.
Remnant Planetesimals:
Comets, Asteroids, Trojans, etc.
Accretion of planets from
planetesimals necessarily
results in diverse groups of
circumstellar and
circumplanetary small
bodies, subject to temporary
confinement among
dynamical resonances
We are here!
Asteroid belt
NEOs
Sun
Trojans
Proposed Dynamical
Origins for LHB
Size Distributions
*Accretional
*Collisional
*Tidal disruption
 Outer solar system planetesimals from late-forming Uranus/Neptune
(Wetherill 1975)
 Break-up of large asteroid (but a big enough asteroid is difficult to destroy)
 Extended tail-end of accretion; remnants from terrestrial planets
region (Morbidelli 2001)
 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)
 Late-stage post Moon-formation Earth/Moon-specific LHB (Ryder 1990)
More generally: any dynamical readjustment of the
planets in a planetary system that “shakes up” (e.g. by
changing positions of resonances) remnant small-body
populations…could occur late, even very late.
Qualitative Features of LHBs
(divide by 3 if Nectaris is 4.1 Ga)
K-T
 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
 Total LHB: ~100 basins, 1000s of
What does it take to
sterilize planet Earth???
K-T events. The 100 Myr bombardment would devastate life.
Why Giant Impacts are
Especially Lethal
 Environmental changes are nearly
atmosphere
surface/ocean
crust
instantaneous! (Most lethal, global
effects occur in a couple of hours to
a month or so.)

mantle
Impacts dominate or
destroy the atmosphere, dramatically
affect the surface
and oceans, but
their effects may not
fully involve the
crust and rarely the
upper mantle.
Very short compared with the lifetime
of an individual; most competing massextinction theories invoke changes
over 1000s to millions of years.
 Independent, compound global
effects (firestorm, ozone layer
destroyed, tsunami, earthquake,
oceans poisoned, “impact winter”
followed by global warming, etc.)
LHB Issues for Solar System
Astrobiology
 Lunar evidence on an LHB is less well
understood than commonly believed.
It must be re-evaluated: it is our baseline!
 How widespread in the solar system was
this lunar LHB?

 Which small-body reservoirs/dynamical
readjustments were responsible?
 Were other reservoirs/causes responsible
How was early
evolving life on Mars
or Europa affected? 
How will Earth’s
complex life be
affected in the future?
for earlier bombardments, or for the
cratered terrains and basins on other
planets/satellites/asteroids?
The future: the Earth is likely to suffer
another basin-forming impact (not soon!);
what else could be in our future?
LHB Issues for Extra-Solar
System Astrobiology
 It is plausible that similar, or even much more extreme, LHBs
(or VLHBs) would affect planets in other systems.


What planetary system configurations are most likely to result in smallbody reservoirs and unstable dynamics that would cause LHBs?
Are LHB/VLHB reservoirs astronomically observable (directly or indirectly)?
 What range of bombardment traits foster life (exchanging
materials, spurring evolutionary change)?
 How frequent would giant impacts have
to be to perpetually frustrate the origin or
evolutionary progression of life?
 How big an LHB surely sterilizes a planet?

What about “attic” storage and reseeding?
 How do LHBs compete with other cosmic
dangers to life in different stellar/galactic
environments?
Conclusions
 Are Late Heavy Bombardments
plausible during the early histories of
planets? YES!
 Could LHBs profoundly change
planetary environments and the origin
and evolution of life? YES!
 Was there a “cataclysm” in the
Earth/Moon system around 3.9 Ga? The
bombardment rate was high, but was it a
spike? We still DON’T KNOW!