Transcript Cratering on Mercury Clark R. Chapman (SwRI), R.G. Strom, J.W. Head, C.I.

Cratering on Mercury
Clark R. Chapman (SwRI),
R.G. Strom, J.W. Head, C.I. Fassett, W.J. Merline,
S.C. Solomon, D.T. Blewett, T.R. Watters
Geological Society of America Annual Meeting,
Session P4: “1st Global View of the Geology of Mercury”
Portland, Oregon, 20 October 2009
Origins of Craters on the Moon & Mercury
 Primary impact cratering

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High-velocity comets (sun-grazers, Jup.-family, long-period)
Near-Earth, Aten, and Inter-Earth asteroids
Ancient, possibly depleted, impactor populations
(accretionary remnants, Late Heavy Bombardment, vulcanoids)
 Secondary cratering
 Endogenic craters
(<8 km diameter, + basin secondaries)
(volcanism, etc.)
Mercury’s Crater Populations
 Basins: dozens of multi-hundred km peak-ring and multi-ring basins
tentatively identified by Mariner 10 (lower bound due to 45% coverage and high sun)
 Highlands craters: like heavily cratered terrains on the Moon, but fewer craters
<40 km diameter (due to embayment by widespread “intercrater plains,” which
may simply be older “smooth plains”)
 Lighter cratering of younger “smooth plains.” 2 alternatives for plains:


Basin ejecta plains (like Cayley plains on the Moon)
Volcanic lava flows (preferred origin, based on analysis of 3 MESSENGER flybys)
 Secondary craters: chains and clusters of small craters (<8 km diameter)
associated with large craters and basins
Stratigraphy/Chronology
 Stratigraphy/relative age-dating
Cross-cutting relationships
 Spatial densities of primary craters
(absolute ages relative to cratering rate)

 Absolute chronology
On the Moon, crater densities calibrated
by dated samples with specific geologic
associations with counting surfaces
 On Mercury, it is difficult and indirect
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Classic approach: assume cratering rate
changed with time just as on the Moon and
that sources were the same as on the Moon
(with minor adjustments, e.g. for higher vel.)
Direct approach: use known impact rates of
asteroids/comets (only good to factor of 2
and only for recent epochs)
Lunar Absolute Chronology. South PoleAitken (oldest basin), Orientale (youngest basin)
Apollo/Luna samples
have dated some basins
and maria between 3.9
and 3.0 Ga.
 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 Ga?
Mercury’s Geological History
Determined from Crater Record
 First Goal: Determine the relative
stratigraphic history from superimposed
crater densities.
 Second Goal: Determine the absolute
geological chronology.
Approach
First, measure crater size-frequency distributions (SFDs) on various geological units.
Determine spatial densities of craters,
emphasizing larger craters, which are less
likely to be secondaries (temporally/spatially variable).
Interpret the relative stratigraphic ages in
terms of absolute ages by applying models
(e.g. lunar cratering chronology, modified
by differences in Moon/Mercury cratering
flux and other geophysical or dynamical
constraints).
Most visible lunar basins formed
during the latter part of the Late
Heavy Bombardment (LHB) or
“Cataclysm” (Strom et al. 2006)
Smooth Plains West of
Caloris: Craters, “Hills”
(Small Craters)
 ~ 770 craters, green
 ~ 190 positive relief features (PRFs), yellow
R-Plots of SFDs for Small Craters
on Four M1 Flyby Frames
This “R-Plot” is a differential size-frequency plot
divided by D-3 such that the vertical axis shows log
of “spatial density” (vs. log diameter).
 Statistics are poor at D>10 km, but
cratered terrain is oldest, with
order-of-magnitude more craters
than on floor of the Raditladi basin
 Slopes of SFDs for craters <10 km
vary regionally; perhaps due to
varying contributions of the very
steep SFD for secondaries (pink)
 Craters reach empirical saturation
densities at large diameters in
heavily cratered terrain and at
diameters < a few km in the
heavily cratered terrain and in a
region rich in secondary craters
 Note extreme youth of Raditladi
double-ring basin
Interpretation Framework: Impactors
(Strom et al., 2005)
Late LHB = Population 1 = Main-Belt Asteroids
 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
As LHB declines, cratering by modern NEAs
dominates = Population 2
Pop. 1
process bringing mainbelt asteroids into Earthcrossing orbits could
produce highland SFD
(e.g. resonance
sweeping)
 The “Nice Model” could
produce a comet shower
followed by an asteroid
shower yielding the LHB
Pop. 2
Interpretational Framework:
Cratering Components
Caloris Basin Cratering
Stratigraphy
 Caloris mountains on
rim (measured by Caleb
Fassett) show old, Pop.
1 signature
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Crater density much
higher than on plains
SFD shape resembles
Pop. 1 on highlands
of Moon and Mercury
 Hence interior plains
must have later
volcanic origin, cannot
be contemporaneous
impact melt (other evidence)
 Interior plains have low
density, flat Pop. 2dominated signature
…so they formed
mainly after the LHB
had ended
Caloris Exterior Plains ~25%
Younger than Interior Plains
Important result: If exterior plains are even younger than
the Caloris interior plains, then they are certainly volcanic
flows. Thus the interpretation of knobby texture of the Odin
Formation as Cayley-Plains-like Caloris ejecta is wrong.
Caloris Basin
“Twin” Young
Basins on Mercury
Raditladi Basin Seen on M1 Flyby
Newly Seen Basin Revealed on M3 Flyby
 Both basins ~260 km diam.
 Similar inner peak rings
 Lightly cratered floors with
circumferential extensional
troughs
 Similar rim morphologies
A Closer Look at the Newly
Seen “Twin” Basin
 Compare very
low crater
density inside
peak ring with
slightly higher
crater density
between peak
ring and rim
 Lighter colored
interior floor
has breached
peak ring on the
bottom
 Both basins
have fairly
young ejecta
blankets and
many surrounding secondary
craters (next slide)
Ejecta and Secondary
Craters of Raditladi and
its “Twin”; Volcanically
Active Region?
Raditladi Basin
Newly Seen “Twin” Basin
100 km
Note “orange” color within peak
ring, like other young volcanic
plains on Mercury. Also note
the proximity of “Twin” basin to
what may be a large volcanic
vent (in the very bright region
northeast of the basin).
Craters on Floor of “Twin” Basin
Craters on Floor of Rembrandt
New Basin Floor Crater Data
Preliminary
Caveat! Small craters may be non-uniform secondaries!
Cumulative # craters > D per million sq. km.
Issues
D
Diam. Rembrandt
(km)
Raditladi
floor
“Twin”
outer
floor
“Twin”
inner
floor
No secondaries, poor
statistics
8
170
(40)
70
(0)
Better statistics, possible
secondary contamination
5
4500
(40)
140
(<40)
2.5
X
500
1100
350
0.3
0.01
0.02
0.007
Near/below resolution limit,
good statistics, secondaries
probably dominate
Summary:
Relative Density
Basins: Approx. Relative Stratigraphy
Relative Crater Density (varies by factor >100!)
 1.0:
 0.5:
Highlands craters
Caloris rim = Rembrandt rim
[note poor statistics: same to within 50%]
 0.3:
Floor of Rembrandt
 0.1:
Floor of Caloris (volcanic)
 0.08: Caloris exterior plains (volcanic)
 0.02: Outer floor of “Twin”
 0.01: Floor of Raditladi = rim of
Raditladi (is floor recent volcanism or impact melt?)
 0.007: Inner floor of “Twin” (unexpectedly
recent volcanism)
Intercrater Plains (Strom, 1977)
Deficiency of smaller Mercurian craters due to plains volcanism
Intercrater Plains (Strom, 2009)
 M1 approach mosaic
 Mostly intercrater plains
 Deficiency on Mercury
<30 km diam. relative to
Moon due to “flooding” of
smaller craters by plainsforming volcanism (?)
Thicker Intercrater Plains (Strom, 2009)
 M2 departure mosaic
 Deficiency of craters <100 km diam. suggests thicker
intercrater plains volcanism erased larger craters than in
M1 approach mosaic
Mercury’s Absolute Chronology:
Raditladi Example (applying lunar chronology)
 Sequence: Heavily cratered highlands
→ Intercrater plains → Caloris basin
→ Smooth plains → Raditladi
basin/plains → “Twin” interior floor
 If lunar chronology applies, then
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Preferred!
If smooth plains formed early (3.9
Ga), then Raditladi is 3.8 Ga (red
arrows)
If smooth plains formed ~3.75 Ga then
Raditladi’s age is <1 Ga! (green
arrows)
Possible Role of Vulcanoids
Vulcanoid belt?
♂
 Zone interior to Mercury’s orbit is dynamically
stable (like asteroid belt, Trojans, Kuiper Belt)
 If planetesimals originally accreted there, mutual
collisions may (or may not) have destroyed them
♀
 If they survived, Yarkovsky drift of >1 km bodies to
impact Mercury could have taken several Gyr
(Vokroulichy et al., 2000), cratering Mercury (alone) long
after the LHB
☼
 That would compress Mercury’s geological
Jupiter orbit
Asteroid belt
chronology toward the present (e.g. thrust-faulting
might be still ongoing)
 Telescopic searches during last 25 years have not
yet set stringent limits on current population of
vulcanoids [MESSENGER is looking during
spacecraft’s perihelia passages]; but their absence
today wouldn’t negate their possible earlier
presence
Two Chronologies for Mercury
Age before present, Ga
4.5
4
3.5
3
2.5
2
1.5
1
0.5
NOW
Formation to magma ocean/crustal solidification
C
A Bombardment, LHB, intercrater plains formation
L
O
R
I
S
Smooth plains volcanism
“Twin” plains
Cratering, rays
Lobate scarps, crustal shortening
Classical (Lunar) Chronology
Vulcanoid Chronology Example
Formation to magma ocean solidification
C
A
L
Vulcanoid bombardment, intercrater plains O
“Twin”…
R Smooth plains volcanism
I
Cratering, ray formation
S
Bombardment, LHB
Lobate scarps, crustal shortening
Some Important Cratering Issues
 Are current production functions (and those in the past)
the same on Mercury and the Moon?
 What are relationships between “Class 1” fresh craters,
rayed craters, and straigraphically young craters?
 Are Mercury’s secondaries unusual? Why?
 Are basins saturated, as Mariner 10 suggested?
 Are intercrater plains simply older smooth plains?
 Are there independent clues about absolute
chronology?
Conclusion: We must wait for orbital
mission for good stratigraphic studies
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Mariner 10 imaged 45% of surface? (I don’t think so.)
MESSENGER has almost completed coverage? Not YET for robust geological analysis
Mariner 10 Image & Shaded Relief
MESSENGER image