Clark R. Chapman SwRI, R.G. Strom Univ. Ariz.,

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MESSENGER Perspectives on Mercurys Cratering

• Clark R. Chapman (SwRI), R.G. Strom (Univ.
  Ariz.),
• S.C. Solomon (DTM, Carnegie Institution),
  J.W. Head III (Brown Univ.), and W.J. Merline
  (SwRI)

with thanks to the whole MESSENGER Team
AAS Division for Planetary Sciences Cornell
University, Ithaca NY 14 October 2008
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Mariner 10 Imaged 45 of Surface. Vivaldi
Crater Then and Now
Mariner 10 Image Shaded Relief
MESSENGER image
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Vivaldi Crater at Sunset Sunrise
M1
M2
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Long, Linear Secondary Crater Chains

• Prominence of secondary craters recognized from
  M10 images
• Long, linear chains of craters radiate from large
  peak-ring crater Eminescu
• they are obviously not SL9-like
• could they be pit craters from crustal fractures?
• how are ejecta launched in such a co-linear
  fashion?
• Note unusual orientation of some chains one
  curves!

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Long, Linear Rays (M2 Flyby)

• Rays seen by Mariner 10 now understood to be a
  global system of long, narrow rays emanating from
  a never-before-seen 110 km diameter fresh crater

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Basins on Mercury
Caloris Basin, MSGR M1 Mariner 10
New Basin, MSGR M2 M1
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Double-Ring Basin Raditladi
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Questions about Cratering on Mercury

• Relationship of Mercurys basins to lunar basins
  and the Late Heavy Bombardment
• Raditladi basin could it have formed quite
  recently?
• Early Population 1 highlands cratering is the
  relative lack of craters lt40 km diameter related
  to formation of intercrater plains? If so,
  how?
• Stratigraphy of Caloris basin (by crater
  densities, SFDs)
• Are interior plains impact melt or subsequent
  volcanics?
• Are exterior plains due to Caloris ejecta (e.g.
  Cayley plains) or the result of volcanism?
• If plains are volcanic, did interior and exterior
  volcanism end at the same time or different
  times?
• Secondary cratering on Mercury how does it
  compare with secondary cratering on the Moon?
• Absolute chronology for basin formation,
  cratering, darkening of fresh-crater rays by
  space-weathering, etc.

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• Study regions for statistics of small craters
• Smooth plains west of Caloris
• Fresh double-ring basin Raditladi
• Ejecta blanket
• Flooded floor
• Secondary crater field from several fresh, large
  primary craters
• Heavily cratered terrain
• Coverage key
• Yellow area studied
• Orange focus on small craters

Caloris
Raditladi
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Double-Ring Basin Flooded Floor and Ejecta
Blanket

• Segment above excluded region is on ejecta
  deposits
• Segment below is floor of basin
• Craters on rare non-flooded regions ex-cluded
  from analysis of floor
• Note the very fresh, crater-free terrains

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Summary Small Craters in Diverse Terrains

• Slopes of SFDs for craters lt10 km in different
  regions vary they may reflect varying
  contributions by the generally very steep SFD for
  secondaries shown in 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 the secondary crater field
• If smooth plains are post-LHB, then the
  straightforward conclusion is that Raditladi
  (both ejecta and interior plains) is lt 1 Gy in
  age, but depends on stochastics of secondaries

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Caloris Interior and Exterior Plains
MESSENGER M1

• Counts of craters gt8 km diameter within plains
  units, both inside and exterior to Caloris
• New counts from best images from Mariner 10 and
  first MESSENGER flyby

Exterior Plains
Interior Plains
Exterior Plains
Mariner 10
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Caloris Interior Plains 25 Older than Exterior
Plains
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Caloris Basin Cratering Stratigraphy
Important issue raised by these results If
exterior plains are volcanic, then interpretation
of knobby texture of Odin Formation as
Cayley-Plains-like Caloris ejecta may be wrong

• Caloris mountains on rim (measured by Caleb
  Fassett) show old, Pop. 1 signature
• Crater density much higher than on plains
• SFD shape resembles that of highlands on Moon and
  Mercury
• Hence interior plains must have volcanic origin,
  cannot be contemporaneous impact melt
• Interior and exterior plains have low density,
  and flat Pop. 2 signa-tureso they formed mainly
  after the LHB had ended

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Cratering Components

• New data consistent with M10 view Pop. 1
  (LHB), Pop. 2 (recent NEAs)
• Secondary branch upturn near 8 km (vs 2 km on
  Moon)
• Variations in R near 2 km due to proportions of
  Pop. 1, chains, clusters
• Smooth plains are 25 younger than plains on
  floor of Caloris both post-date rim

Population 1
Sample of MSGR cratered terrains more densely
cratered than Mar. 10 avg.
25 older than
Caloris plains
the smooth exterior plains
Population 2
Secondaries
Deficit w.r.t. Pop. 1 due to intercrater plains?