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What are the Astrobiological Constraints
from What is Known about
the Late Heavy Bombardment?
Clark R. Chapman
Southwest Research Institute Boulder CO
NAI General Meeting 2003
Tempe, Arizona
12 February 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
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
Debate over “Cataclysm”
A Misconception
“Tail-end” of
accretion
 “Stonewall” effect
(Hartmann 1975)
destroys and
pulverizes rocks
prior to saturation
 Grinspoon’s
Post-crust,
pre-spike lull
defines LHB
(1989) twodimensional
models concur
vs.
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
meteoritic/projectile
material (not robust)
(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, 2002]
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
 Outer solar system planetesimals from late-forming Uranus/Neptune
(Wetherill 1975)
 Break-up of large asteroid (but big enough asteroids 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 & 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
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. Life would be devastated at the end of the 100 Myr.
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 LHB is less well
understood than commonly believed.
It must be re-evaluated: it is our baseline!
 How widespread was this lunar LHB?

 Which small-body reservoirs/dynamical
readjustments were responsible?
 Were other reservoirs/causes
How would early
evolving life on
Mars or Europa
have been affected?
Earth’s complex life
in the future?
responsible for earlier bombardments,
or for the cratered terrains and basins
on other planets/satellites/asteroids?
 The future: 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 bombardments 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?
 How do LHBs compete with other cosmic
dangers to life in different stellar/galactic
environments?