What are the Astrobiological Constraints

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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
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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)

?
Implies short, 50-100 Myr bombard- ment, but
minimal basin formation between crustal
formation and LHB
LHB
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Debate over Cataclysm
A Misconception vs.
It Happened!
Tail-end of accretion

• Stonewall effect (Hartmann 1975) destroys and
  pulverizes rocks prior to saturation
• Grinspoons (1989) two-dimensional models concur

• 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)

Post-crust, pre-spike lull defines LHB
(Mostly) uncontroversial sharp decline in
bombardment rate from 3.90 Ga to 3.83 Ga
Further confusion on LHB decay
gtBasin formation decayed in 50 Myr gtRocks
degassed over 200 Myr gtImpact melts decayed
over 1000 Myr Chapman, Cohen Grinspoon,
2002
Flux
?
Time
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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.

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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
Jupiters orbit
We are here!
Trojans
Asteroid belt
NEOs
Sun
Comets OSS planetesimals
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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 populationscould occur late,
even very late.
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Qualitative Features of LHBs

• 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 K-T events.
  Life would be deva-stated at the end of the 100
  Myr.

K-T
What does it take to sterilize planet Earth???
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Why Giant Impacts are Especially Lethal

• Environmental changes are nearly instantaneous!
  (Most lethal, global effects occur in a couple of
  hours to a month or so.)
• Very short compared with the lifetime of an
  individual most competing mass-extinction
  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.)

atmosphere
surface/ocean
crust
mantle
Impacts dominate or destroy the atmos-phere,
dramatically affect the surface and oceans, but
their effects may not fully involve the crust and
rarely the upper mantle.
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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 responsible for
  earlier bombardments, or for the cratered
  terrains and basins on other planets/satellites/as
  teroids?
• The future Earth is likely to suffer another
  basin-forming impact (not soon!) what else could
  be in our future?

How would early evolving life on Mars or Europa
have been affected? Earths complex life in the
future?
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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 small-body 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?