Also see: Chapman, Cohen, Grinspoon (2007),
Icarus 189, 233-245.
Late Heavy Bombardment: Evidence From
Cratering Histories of the Moon, Planets,
Satellites, and Asteroids
Clark R. Chapman
Southwest Research Institute, Boulder CO
39th AAS/DPS Meeting
Orlando, Florida
“Exploring the Lunar Late Heavy Bombardment”
#46.01, 10:30 a.m., 11 October 2007
Lunar Impact Basins
 The Moon, like most
Basins are common on Mars, Mercury,
Ganymede, Callisto, Iapetus, Rhea, Tethys,
Vesta, and other bodies. But (except possibly
Vesta), we have no absolute radiometric ages
for these basins. While they may all have
been created during the same epoch,
interpretations about when they formed come
only from indirect dynamical and geophysical
arguments.




solid-surfaced bodies in
the solar system, is
covered with multiringed impact basins.
Paul Spudis’ map (lower
left) shows only the most
prominent ones.
Wilhelms, Spudis, and
Wood believe that there
are at least 45 basins,
many highly degraded.
Nectaris and younger
frontside basins have
been approximately
dated, from argued
geological associations
with dated lunar rocks.
At least 2/3rds of all
basins are pre-Nectarian.
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
between ~4.0 and ~3.8 Ga
 Wilhelms (1987) documents a sharply declining
basin-formation rate between Nectaris (~3.92
Ga) and final basin, Orientale (~3.82 Ga)
 There are few rock ages, and virtually no impact
melt ages, prior to 3.92 Ga (probable Nectaris
age) (Ryder, 1990)
Basins produce copious melts (~10% of involved
materials)
 Small craters produce small amounts of melt
because efficiency of melt-production increases
with crater size and basin-forming projectiles
volumetrically dominate shallow SFD
 So impact melts should be a robust marker of the
history of basin formation (Cohen, last talk)
(Cumulative) Crater Density

LHB
Implies: short, 50-100 Myr bombardment,
with minimal earlier basin formation
between crustal formation and this LHB
Schematic Representations of Lunar
Cratering History Alternatives


Can various steady declining flux models have a
high enough rate at 3.9 Ga without being too
massive early on? (Destroy the crust,
contaminate it…or require unrealistically massive
projectile population.)
From cratering/age-dating perspective, we can’t
observe the history before 4.0 Ga.
Zahnle et al. (2007)
Strom et al. (2006)
What Happened Before Nectaris?
(i.e. before 3.90 - 3.92 [4.1?] Ga)
 Fragmentary geology remains from earlier times.
 50% of Wilhelms’ “definite” basins pre-date Nectaris (and
70% of all “definite”+“probable”+“possible” ones).
 Surprisingly, almost no impact melts pre-date Nectaris
Basin. How could the earlier basins not produce melts?
Perhaps 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:
Lunar crust intact (constrains top-heavy
distribution)
 Minimal meteoritic contamination (perhaps
projectile material preferentially lost)

size-
Large Crater (Basin) on Vesta
 “Crater”, seen edge-on with
prominent central peak in HST
image, is ~460 km in diameter,
nearly as large as Vesta itself: it
is a basin!
 It is plausible, but unproven, that
Smaller bodies have comparatively large craters,
too. But Stickney (10 km diam.) is not a “basin”.
this basin was the source of the
numerous “Vestoid” asteroids
and thus for the HED achondritic
meteorites.
 Ar-Ar ages for eucrites range from
4.3 to 3.2 Ga, an interval much
longer than the lunar LHB, but
centered on the same epoch.
 However, the “fresh”, fairly unspace-weathered spectra of Vesta
and Vestoids suggest that the
basin formed recently.
 Yet a long-standing mystery is
that Vesta wasn’t cratered even
more: its basaltic crust has
remained largely intact.
Lunar, HED Rock
Resetting 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
Predominant lunar rock
ages range from 3.6 to
4.2 Ga. (Impact melts are
restricted to <4.0 Ga.)
HED
Parent
Body
So rock ages correlate
poorly with basin ages.
(Vesta?)
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…
or there are selection
biases.
Age span
for small
lunar melts
(Cohen et
al., 2000)
Time
3.3
4.4
A New Look at the “Stonewall”:
Is the LHB a “Misconception”?
 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
[next slide].

 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.
If melt preferentially veneers surface and older veneers are
covered up, then the 2-D model could 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.
South Pole-Aitken, Orientale:
What are their Absolute Ages?




South-Pole Aitken is relatively old and very large: is its age 4.3 or 4.0 Ga?
Orientale is the youngest basin. But is its age 3.72 or 3.84?
If the “Vision” lunar program is able to date samples that are unambiguously associated
with these basins, then we can determine the duration of the LHB.
The crater Cantor is in between the two basins and close to both. We could sample near it.
Basin Ages
(Stöffler & Ryder, 2001, Space
Science Reviews: critical reevaluation of isotopic ages of lunar
geologic units.)
 Numerous un-
certainties
remain in the
association of
dated samples
to specific
basins
 Bottke et al.
(2007) explore
extremes:
Nectaris as old
as 4.12 Ga,
Imbrium as
young as 3.72
Ga
Production of Late Basins?
(Bottke et al., 2007, Icarus: Can
planetesimals left over from planetary
formation form basins as late as 4.1
to 3.7 or 3.8 Ga?)
 Planetesimals left over after
terrestrial planet formation have a
main-belt-like size distribution.
 They are dynamically depleted (just
like modern-day Near-Earth
Asteroids) and collisionally evolve.
 The lunar impact flux declines by 4
orders of mag. by the time visible
lunar basins formed.
 In order to form the 4 largest, most
reliably dated basins during
broadest allowed time interval for
LHB, initial planetesimal population
must have ~1 to 10 Earth masses!
 To avoid a ridiculously massive
solar nebula in the terrestrial planet
region, there must be a late
cataclysm to produce even a few
basins around 3.9 Ga.
Basin Degradation due to Viscous
Relaxation (Baldwin, 2006)
Baldwin’s Crater Degradation Classes
 Lunar viscosity 1025 poises at
4.3 Ga, increased factor of 4
by time Orientale formed
 Viscosity would have had to
increase an (unphysical) factor
of ~40 if all basins were
formed during a short LHB
 Assumption: degradation is by
viscosity only; not by erosion,
filling, crater overlap, etc.
Basin
Class Calc. Age
Orientale
2
3.8 Ga
Imbrium
3
3.84 Ga
Crisium
4
3.91 Ga
Nectaris
7
4.1 Ga
Humorum
9
4.23 Ga
Werner–Airy 10
4.3 Ga
“Recent” Basins on
Terrestrial Bodies
The single Martian meteorite with a
resetting age ~4.0 Ga
Caloris on Mercury
Orientale on the Moon
Argyre on Mars
Martian Basins and Chronology
 Mars has prominent basins (Argyre, Hellas, etc.)
 Frey et al. (2007), who have been studying Quasi-




Circular Depressions (QCDs) from MOLA, find
numerous old basin-sized features
There is a tendency for the Mars community to regard
these features as being very old, pre-Noachian,
perhaps ~4.3 Ga (using the widely adopted cratering
chronologies of Neukum and Hartmann, both of whom
are skeptical of a cataclysmic LHB).
But the chronology for ancient Mars is unknown,
except for the resetting age of a single rock of
unknown provenance.
If an LHB spike occurred on the Moon after 4.0 Ga, then
it probably occurred on Mars (and Mercury, and
throughout the inner solar system) as well, and all of
Frey’s QCDs could be younger than 4.0 Ga.
The pre-Noachian and Noachian could be very
compressed in time, perhaps with a burst of impactproduced heating and watery climate.
Main-Belt Asteroids Caused
the LHB (Strom et al., 2005)

Shape of main-belt asteroid
SFD matches lunar highland
craters

Shape of NEA SFD matches
lunar maria craters

Size-selective processes
bring NEAs from main belt
to Earth/Moon

A solely gravitational
process bringing main-belt
asteroids into Earthcrossing orbits could
produce highland SFD (e.g.
resonance sweeping)

BUT, main-belt SFD may not
be unique…could reflect a
collisionally evolved
population anywhere in the
solar system

The “Nice Model” could
produce a comet shower
followed by an asteroid
shower (Morbidelli, next talk)
Basins on Galilean Satellites
Valhalla on Callisto
Gilgamesh on Ganymede
Odysseus on Tethys
Iapetus
Iapetus
Cratering in the Jovian System
(R. Strom)
Approx. SFD
for Ganymede
(Galileo)
Saturn Satellite Cratering (in a
Solar System Context)
Closed symbols = Hyperion
Open symbols = Phoebe
Cassini counts by P. Thomas
(J. Richardson, this meeting)
From Chapman & McKinnon (1986)
Saturnian Impactor Population
Asteroidal impactors for inner solar system
(Bottke et al. 2005, O’Brien et al. 2005)
Phoebe
Saturnian system impactor population
(Jim Richardson, this meeting)
The Neukum Model
 The size-distribution (SFD) of craters on the
lunar highlands is the same as on the maria.
 The SFD of asteroids that struck the Moon has
been the same from 4.3 Ga through the LHB
period to the present day.
 Satellites of Saturn and Jupiter have the same
SFD as do craters on the Moon (with a “shift”).
 Therefore, outer solar system (OSS) cratering
was mainly by asteroids, not comets.
 The “shift”? The OSS SFD may match the lunar
?
SFD, but only by shifting it, in the sense that the
asteroids strike at low, planetocentric velocities!
 To Neukum, the LHB is mainly an unremarkable
Saturn’s moons
are cratered by
asteroids in
Saturnicentric
orbits???
stage in a generally monotonically declining flux
beginning when the lunar crust solidified; on
many bodies (Mars, Iapetus) we can see back to
4.3 to 4.4 Ga according to Neukum.
It is not just the interpretations
that differ: the data disagree!
 Neukum says lunar/Martian production function shape was the
same during LHB and present; Strom says it changed dramatically
We need to understand why these
differences persist!
Neukum
Strom
Outer Solar System Basins
 Crater size distribution differences between terrestrial
planets and OSS satellites suggest – consistent with
dynamical simulations – that OSS cratering is
primarily by comets/KBOs. But, there are caveats:



Basin SFD could reflect viscous relaxation of features
Planetocentric impactors could be important (Pop. 2)
Comet/KBO size distribution is poorly known
 We have zero direct knowledge of the absolute
geological chronology for OSS satellites.
If there was no OSS LHB, the observable basins could
have formed as soon as crusts solidified.
 For heavily cratered terrains, we don’t know how many
generations of super-saturation have happened.
 If the cratering rate has remained high (perhaps
augmented by cratering by planetocentric bodies),
then OSS basins could be unexpectedly young…
consistent with a cumulative history of bombardment
that might have destroyed small moons, creating even
more planetocentric bombardment.

 Studies of OSS cratering statistics provide
information on relative stratigraphic chronology only!
Conclusions
 Basin formation has been ubiquitous in
the solar system
 On the Moon, many basins formed just
before an abrupt halt 3.8 Ga



Plausibly a cataclysm 4.0 – 3.8 Ga
But bombardment flux is uncertain
between 4.3 and 4.0 Ga
Earth must have undergone same LHB
 Plausibly, Mars, Mercury, and Venus
had same LHB (asteroidal)



But there is no direct age data
Mercury could have had later vulcanoid
bombardment
Observable Martian geological record
formed since 4.0 Ga
 Outer solar system bombardment was
by different, non-asteroidal impactor
population/s



Presumably comets, possibly at similar
epoch as LHB (e.g. Nice model)
Saturation, planetocentric cratering
adds complexity
Virtually no chronological constraints
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.