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Cratering on Small Bodies: Lessons from Eros
Cratering on Small Bodies: Lessons from Eros
Clark R. ChapmanClark R. Chapman
Southwest Research Institute Boulder, Colorado, USA
Impact Cratering: Bridging the Gap
between Modeling and Observations
Lunar & Planetary Inst., Houston, 9 Feb. 2003
Goals of Studying Cratering on Eros
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
Planetary craters provide an historical record. We must understand not only their formation but also their degradation and ancillary processes (e.g. secondary cratering).
Planetary craters provide an historical record. We must understand not only their formation but also their degradation and ancillary processes (e.g. secondary cratering).
Eros in Context of Asteroids Imaged by Spacecraft
Eros is typical in size, though an Earth-approacher
All are S-types, except C-type Mathilde; Mathilde’s 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
MathildeMathilde
GaspraGaspra
IdaIda
ErosEros
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
NEAR-Shoemaker’s Landing Spot on Eros
How typical is the edge of Himeros of Eros?How typical is the edge of Himeros of Eros?
How typical is Eros of other asteroids?How typical is Eros of other asteroids?
Inset shows Himeros
Estimated positions of last images end within a 50 meter diameter crater
“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 don’t lunar ponds exist?
“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 don’t lunar ponds exist?
The Relative Plot (R-Plot)
Shows spatial densities of craters as function of size relative to saturation
Summary of Population Estimates
0
1
2
3
4
5
6
7
8
9
10
1015202530
0.01 0.1 1 10-1
0
1
2
3
4
5
6
7
8
Harris (this paper)D'Abramo et al. (LINEAR 00-01)Discovered as of July 2002Werner et al. 2001 (p
V = 0.25)
Werner et al. 2001 (pV = 0.11)
Constant power lawBottke et al. 2001Stuart 2001 (LINEAR)Rabinowitz et al. 2000 (NEAT)Rabinowitz et al. 2000 (SW)
file: d:\nea\ast3\fig3h.plt
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file: d:\nea\ast3\fig3h.pltfile: d:\nea\ast3\fig3h.pltfile: d:\nea\ast3\pop.plt
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file: d:\nea\ast3\pop.pltfile: d:\nea\ast3\pop.plt
file: d:\nea\ast3\pop.pltfile: d:\nea\ast3\pop.pltfile: d:\nea\ast3\pop.outfile: d:\nea\ast3\pop.pltfile: d:\nea\ast3\werner.pltfile: d:\nea\ast3\werner.pltfile: d:\nea\ast3\werner.bakfile: d:\nea\ast3\werner.dat
Absolute Magnitude H
Lo
g[N
(<H
)]
Lo
g[I
mp
act
in
terv
al,
ye
ars
]
Diameter, km
Summary of NEA Population Estimates (A. W. Harris, 2002)
Why is Eros so Different from the Moon at Small Scales?
Covering-up by mass-wasting, seismic shaking, ejecta blanketing -- doesn’t 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 3…we need orders of magnitude (note: few craters in ponds).
Yarkovsky Effect (meteorite-sized bodies depleted from asteroid belt, some delivered to Earth) -- hasn’t worked quantitatively, yet.
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