Clark R. Chapman (SwRI),

1

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
2
Origins of Craters on the Moon Mercury

• Primary impact cratering
• 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 (lt8 km diameter, basin
  secondaries)
• Endogenic craters (volcanism, etc.)

Mercurys 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 lt40 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 (lt8 km diameter) associated with large
  craters and basins

3
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
• 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)

4
Lunar Absolute Chronology. South Pole-Aitken
(oldest basin), Orientale (youngest basin)

• 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?

Apollo/Luna samples have dated some basins and
maria between 3.9 and 3.0 Ga.
5
Mercurys Geological History Determined from
Crater Record
Most visible lunar basins formed during the
latter part of the Late Heavy Bombardment (LHB)
or Cataclysm (Strom et al. 2006)

• First Goal Determine the relative stratigraphic
  history from superimposed crater densities.
• Second Goal Determine the absolute geological
  chronology.

Approach First, measure crater size-frequency
distri-butions (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).
6
Smooth Plains West of Caloris Craters, Hills
(Small Craters)

• 770 craters, green
• 190 positive relief features (PRFs), yellow

7
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 Dgt10 km, but cratered
  terrain is oldest, with order-of-magnitude more
  craters than on floor of the Raditladi basin
• Slopes of SFDs for craters lt10 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 lt a few km in the heavily cratered
  terrain and in a region rich in secondary craters
• Note extreme youth of Raditladi double-ring basin

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Interpretation Framework Impactors (Strom et
al., 2005)
Late LHB Population 1 Main-Belt Asteroids As
LHB declines, cratering by modern NEAs dominates
Population 2

• 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 Earth-crossing 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. 1
Pop. 2
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Interpretational Framework Cratering Components
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Caloris Basin Cratering Stratigraphy

• Caloris mountains on rim (measured by Caleb
  Fassett) show old, Pop. 1 signature
• 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.
  2-dominated signature so they formed mainly
  after the LHB had ended

11
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
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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

13
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 surround-ing secondary craters (next slide)

14
Ejecta and Secondary Craters of Raditladi and its
Twin Volcanically Active Region?
Raditladi Basin
Newly Seen Twin Basin
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).
100 km
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Craters on Floor of Twin Basin
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Craters on Floor of Rembrandt
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New Basin Floor Crater Data
Preliminary
Caveat! Small craters may be non-uniform
secondaries!
Cumulative craters gt D per million sq. km.
Issues D Diam. (km) Rembrandt 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 (lt40)
Near/below resolution limit, good statistics, secondaries probably dominate 2.5 X 500 1100 350
Summary Relative Density 0.3 0.01 0.02 0.007
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Basins Approx. Relative Stratigraphy
Relative Crater Density (varies by factor gt100!)

• 1.0 Highlands craters
• 0.5 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)

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Intercrater Plains (Strom, 1977)
Deficiency of smaller Mercurian craters due to
plains volcanism
20
Intercrater Plains (Strom, 2009)

• M1 approach mosaic
• Mostly intercrater plains
• Deficiency on Mercury lt30 km diam. relative to
  Moon due to flooding of smaller craters by
  plains-forming volcanism (?)

21
Thicker Intercrater Plains (Strom, 2009)

• M2 departure mosaic
• Deficiency of craters lt100 km diam. suggests
  thicker intercrater plains volcanism erased
  larger craters than in M1 approach mosaic

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Mercurys 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
• If smooth plains formed early (3.9 Ga), then
  Raditladi is 3.8 Ga (red arrows)
• If smooth plains formed 3.75 Ga then Raditladis
  age is lt1 Ga! (green arrows)

Preferred!
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Possible Role of Vulcanoids

• Zone interior to Mercurys 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 gt1 km bodies
  to impact Mercury could have taken several Gyr
  (Vokroulichy et al., 2000), cratering Mercury
  (alone) long after the LHB
• That would compress Mercurys geological
  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
  spacecrafts perihelia passages but their
  absence today wouldnt negate their possible
  earlier presence

Vulcanoid belt?
?
?

Jupiter orbit
Asteroid belt
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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
CALORIS
Bombardment, LHB, intercrater plains formation
Smooth plains volcanism
Twin plains
Cratering, rays
Lobate scarps, crustal shortening
Classical (Lunar) Chronology
Vulcanoid Chronology Example
Formation to magma ocean solidification
CALORIS
Bombardment, LHB
Vulcanoid bombardment, intercrater plains
Smooth plains volcanism Twin
Cratering, ray formation
Lobate scarps, crustal shortening
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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 Mercurys secondaries unusual? Why?
• Are basins saturated, as Mariner 10 suggested?
• Are intercrater plains simply older smooth
  plains?
• Are there independent clues about absolute
  chronology?

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Conclusion We must wait for orbital mission for
good stratigraphic studies

• Mariner 10 imaged 45 of surface? (I dont think
  so.)
• MESSENGER has almost completed coverage? Not YET
  for robust geological analysis

Mariner 10 Image Shaded Relief
MESSENGER image
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