Clark R. Chapman (SwRI),

1

Cratering on Mercury Insights
from the MESSENGER Flybys

• Clark R. Chapman (SwRI),
• R.G. Strom, C.I. Fassett, L.M. Prockter, J.W.
  Head III, S.C. Solomon, M. E. Banks, D. Baker,
  W.J. Merline

73rd Annual Meeting of the Meteoritical
Society 5325, 1145 a.m., Tuesday, 27 July
2010 New York City, NY USA
2
Meteoritical Context for Studying Craters on
Mercury

• Chronology. Was the history of cratering on
  Mercury the same as on the Moon and Earth?
• Impactor Populations. Was the same mix of
  asteroids and comets responsible for Mercury
  cratering, or was there a later bombardment by
  vulcanoids?
• Style of Cratering on Mercury. Does the higher
  impact velocity on Mercury or other factors (e.g.
  those responsible for the prominence of large
  secondary craters on Mercury) help or hinder the
  launch of meteorites from Mercury that could
  reach the Earth?
• Composition. Does the variety of compositional
  units revealed by penetration of Mercurys crust
  by large craters provide clues about how to
  identify meteorites that might have come from
  Mercury?

• Other Issues Addressed by Mercurys Craters
• Large craters penetrate deep within the crust,
  revealing layers of different, deep geological
  units.
• Crater densities (esp. of primary craters)
  reveal stratigraphic sequences in emplacement of
  geological units expressed on the surface.
• Cratering physics why does basin morphology
  begin for smaller craters on Mercury than on
  other bodies like the Moon?

3
Mercury has Many Basins!

• Mercury has many more basins than the Moon
• Yet the spatial density of large basins per unit
  area is about the same as the Moon
• This apparent contradiction is resolved by
  realizing that basin morphology appears at a
  considerably smaller diameter on Mercury than on
  the Moon
• Data are (so far) from images with limited
  lighting and viewing geometries. Once we have
  good global coverage, and global laser altimetry,
  we can search for large quasi-circular features,
  like those found on Mars and the Moon

Rembrandt
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There are Multiple Potential Sources for
Mercurys Craters

• Primary craters from the same populations of Near
  Earth Asteroids (NEAs) and comets that crater the
  Earth, Moon, Mars, and Venus today.
• Primary craters from the same population/s that
  caused the Late Heavy Bombardment (LHB) on the
  Moon 3.9 Ga. (These may have been main-belt
  asteroids and outer solar system planetesimals,
  if the Nice Model is correct. Or other
  collisionally and dynamically processed remnants
  from accretion.)
• Vulcanoids (remnants of hypothetical population
  of planetesimals interior to Mercurys orbit,
  which would not have appreciably cratered other
  planets).
• Secondary craters from larger craters and basins.
• Endogenic craters (volcanic vents, subsidence
  craters, etc.)
• Other rare causes (e.g. tidally split SL9-like
  bodies)

5
Our Working Hypothesis

• Mercury was saturated with craters and basins
  during the Late Heavy Bombardment by the same
  population of impactors that struck the Moon.
• Mercury has since been cratered by the same
  population of NEAs that still crater other inner
  solar system bodies.
• Many-to-most craters lt10 km diameter on
  widespread plains units are secondary craters.
• Endogenic craters are a small but important
  contribution to negative relief features.

6
Interpretational Framework Cratering Components
25
7
Populations 1 and 2 (R. Strom)
Schematic diagram of possible LHB scenario

• Lunar highlands are prototype for Pop. 1 produced
  by LHB
• Young Mars plains are prototype for Pop. 2
  (current NEAs derived by size-dependent Yarkovsky
  plus resonances from main belt) plus secondaries
  at Dlt1 km
• Caloris exterior plains are dominantly Pop. 2
  (plus large secondaries Dlt10 km)

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Caloris Basin Relative Ages
Caloris is a relatively young basin, with about
half the density of superimposed craters as the
general cratered highlands of Mercury, but it was
cratered during the LHB, while the
interior/exterior plains mainly post-date the LHB.
Important result If exterior plains are even
younger than the Caloris interior plains, then
they are certainly volcanic flows. Thus Mariner
interpreta-tions of the knobby textured Odin
Formation as Cayley-Plains-like Caloris ejecta
are wrong.
Caloris Basin
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Variability of Intercrater Plains

• Bottom panel shows MESSENGER crater spatial
  densities (R values) in a part of Mercury that
  resembles the average highlands measured from
  favorably observed regions by Mariner 10. The
  distinct deficiency of craters on Mercury lt30 km
  diameter was ascribed to intercrater plains.
• New studies of other regions show considerably
  variability. Top panel shows deficiency
  extending to craters 150 km in diameter, implying
  a thick sequence of intercrater plains (i.e.
  volcanism).
• Middle panel shows a modest deficiency extending
  to 100 km diameter, but a prominent secondary
  crater branch appears at an unusually large
  diameter, 20 km.

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Twin Young Basins on Mercury
Raditladi Basin Seen on M1 Flyby
Rachmaninoff Basin Revealed on M3 Flyby

• Both basins 250-300 km diam.
• Similar inner peak rings
• Lightly cratered floors with circumferential
  extensional troughs
• Similar rim morphologies

11
A Closer Look at the Recently Discovered
Rachmaninoff 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)

12
Ejecta and Secondary Craters of Raditladi and
Rachmaninoffand a Recently Volcanically Active
Region
Raditladi Basin
Rachmaninoff Basin
Note orange color within peak ring, like other
young volcanic plains on Mercury. Also note the
proximity of Rachmaninoff to what may be a large
volcanic vent (in the very bright region
northeast of the basin).
100 km
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Relative Ages Basin Rims and Plains within
Basins
Note Very low crater densities and small areas
of counting units cause poor statisticsbut its
the best we can do!

• (A) Inner plains and annular plains of
  Rachmaninoff Inner plains are clearly younger
  than annular plains, but apparently older than
  Raditladi plains (but size distribution is not
  the same shape, confusing the comparison)
• (B) Rachmaninoff rim and ejecta suggests an
  older basin formation age than for Raditladi

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Basins and Plains Approximate Relative
Stratigraphy by Crater Density
Relative Crater Density (varies by factor gt30!)

• 1.0 Highlands craters
• 0.5 Caloris rim Rembrandt rim
• 0.35 Floor of Rembrandt
• 0.2 Interior Caloris plains (volcanic)
• 0.15 Caloris exterior plains (volcanic)
• 0.1 Rachmaninoff basin annular plains
• 0.05 Rachmaninoff inner plains
• 0.03 Floor of Raditladi rim of Raditladi (is
  floor impact melt prompt volcanism?)

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Mercurys Absolute Chronology Raditladi Example
(applying lunar chronology)

• Sequence Heavily cratered highlands ?
  Caloris basin ? Exterior plains ? Raditladi
  basin/plains
• If lunar chronology applies, then
• If exterior 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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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
Raditladi
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 Raditladi
Cratering, ray formation
Lobate scarps, crustal shortening
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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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Abstract
Introduction During its three Mercury flybys,
MESSENGER imaged most regions unseen by Mariner
10 and viewed some previously seen regions under
more favorable lighting. The surface density of
impact craters and basins on Mercury with
diameters Dgt200 km is comparable with that of the
Moon, though possibly there are fewer large
basins. The largest basin mapped from Mariner 10
(Borealis) has not been reliably recognized in
MESSENGER images. Two smaller peak-ring basins
(Raditladi and Rachmaninoff) are comparatively
young. Large craters and basins have numerous
secondary craters, which generally dominate
Mercurys crater populations at Dlt10 km.
Extensive volcanism apparently modified Mercurys
crater populations at D lt100 km, to variable
degrees in different regions, but was as
powerfully destructive of craters Dlt40 km as the
many degradation processes that affected Martian
highlands. Large Craters and Basins The
morphologies of dozens of peak-ring basins have
illuminated the transition from smaller complex
craters to basins. Caloris and Rembrandt basins
are fairly well preserved and formed during the
later part of the Late Heavy Bombardment (LHB)
craters on their rims follow the Population-1
size-frequency distribution (SFD) characteristic
of LHB cratering throughout the terrestrial
planet region (believed to be the result of
direct scattering of main-belt asteroids).
Volcanic plains formation within Caloris ended
well after the basin formed, close to the end of
the LHB its interior plains are dominated by the
later Population-2 craters typical of near-Earth
asteroids today, chiefly derived from the main
belt by size-dependent processes such as the
Yarkovsky effect. Volcanic plains formation
exterior to Caloris continued afterwards, based
on a lower density of almost purely Population-2
craters. These plains clearly postdate formation
of the Caloris basin by a substantial interval
and are not ejecta deposits like the lunar Cayley
Plains, as had been hypothesized after Mariner
10. SFDs for Mercurys craters with Dgt10 km in
various cratered regions of Mercury differ
widely, more than was appreciated from Mariner
10. In some regions, voluminous intercrater
plains obliterated all craters with Dgt100 km,
whereas elsewhere plains buried only smaller
craters so that many with Dgt40 km remain from
older eras. Intercrater plains and younger,
often more spatially restricted, smooth plains
both formed by volcanic emplacement. Small
Craters, Secondaries, and Young Plains In some
places (e.g. in regions near Raditladi) Mercurys
craters are dominated by secondaries for Dlt20 km.
In general, the upturn of the SFD at smaller
sizes occurs at Dlt8 km, a much larger diameter
than the few km typical on the Moon and Mars.
Perhaps larger secondaries are formed on Mercury
than on other bodies. The temporally sporadic
and spatially clustered nature of secondaries
hinders studies of relative ages of small and/or
recent units. Nevertheless, the extremely sparse
densities of small craters within Raditladi and
Rachmaninoff suggest that these basins are
unusually young. In the case of Rachmaninoff,
volcanism continued within its inner plains until
comparatively recently, long after basin
formation, and thus those plains cannot be impact
melt.