Cratering on Small Bodies: Lessons from Eros

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Cratering on Small Bodies Lessons from Eros
Clark R. Chapman
Southwest Research Institute Boulder, Colorado,
USA
Impact Cratering Bridging the Gap between
Modeling and Observations Lunar Planetary
Inst., Houston, 9 Feb. 2003
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Goals of Studying Cratering on Eros
Planetary craters provide an historical record.
We must understand not only their formation but
also their degradation and ancillary processes
(e.g. secondary cratering).

• Chief goal of cratering specialists study
  moderate-scale cratering on a nearly
  gravitationless body
• Some of my goals
• Determine projectile population size-distribution
  in main asteroid belt (where Eros lived most of
  its life)
• Determine cratering age of Eros
• How old Eros is since its creation as an
  independent body or since its last global
  resurfacing event
• Evidence (at high res) indicating its duration in
  near-Earth orbit
• Understand ejecta/secondary cratering processes
• Understand regolith evolution on a small body
• Learn (from surface expression) about interior of
  Eros

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Eros in Context of Asteroids Imaged by Spacecraft
Mathilde

• Eros is typical in size, though an
  Earth-approacher
• All are S-types, except C-type Mathilde
  Mathildes unique giant craters are probably due
  to its high porosity/low density
• Angular Gaspra has low crater density, perhaps
  due to metallic composition
• Craters similar on Eros Ida

Ida
Gaspra
Eros
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Some Aspects of the Larger Craters on Eros

• Two of largest craters (Himeros and Psyche) are
  large relative to the width of Eros
• Compressive ridge extends around to
  other side of Eros
• Bowl-shaped Psyche has markedly different
  shape from youngest large crater
  (Shoemaker, not shown here)
• Bright/dark interior slopes indicate downslope
  slippage, unusual space weathering

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Eros Surface from Low Orbit
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Ponds from Low-Altitude Flyover
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NEAR-Shoemakers Landing Spot on Eros

• Inset shows Himeros
• Estimated positions of last images end within a
  50 meter diameter crater

• How typical is the edge of Himeros of Eros?
• How typical is Eros of other asteroids?

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Fifth Last Image (largest boulders are 3
meters across)
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Eros is Covered with Rocks
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Final Landing Mosaic
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Closest Image of Eros
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Ponds and Beaches?

• Ponds are flat, level, and are sharply bounded
• Beaches (not always seen) surround some ponds
  and are relatively lacking in either craters or
  boulders
• Although stratigraphically younger, ponds may
  have more small craters than typical terrains,
  suggesting that boulders may armor crater
  production
• How are they formed? Electrostatic levitation,
  seismic shaking? If mass-wasting, why dont
  lunar ponds exist?

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The Relative Plot (R-Plot)

• Shows spatial densities of craters as function
  of size relative to saturation

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R-Plot Eros Craters Boulders
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Eros R-Plot (annotated)
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Eros is NOT Like the Moon!
Eros has rocks.
The Moon has craters.
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Summary of NEA Population Estimates (A. W.
Harris, 2002)
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Why is Eros so Different from the Moon at Small
Scales?
???

• Covering-up by mass-wasting, seismic shaking,
  ejecta blanketing -- doesnt work boulders would
  be covered, too.
• Unless...Shoemaker crater formed yesterday!
• Armoring by boulders impactors strike but few
  craters are formed -- probably explains factor of
  3we need orders of magnitude (note few craters
  in ponds).
• Yarkovsky Effect (meteorite-sized bodies depleted
  from asteroid belt, some delivered to Earth) --
  hasnt worked quantitatively, yet.

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Final Comment...
Cratering on asteroids is unexpectedly weird and
varied