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

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

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; 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 Ida Gaspra Eros

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

Eros’ Surface from Low Orbit

“Ponds” from Low-Altitude Flyover

NEAR-Shoemaker’s 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?

Fifth Last Image (largest boulders are 3 meters across)

Eros is Covered with Rocks

Final Landing Mosaic

Closest Image of Eros

“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?

The Relative Plot (R-Plot) Shows spatial densities of craters as function of size relative to saturation

R-Plot: Eros Craters & Boulders

Eros R-Plot (annotated)

Eros is NOT Like the Moon! Eros has rocks. The Moon has craters.

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.

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