What we Know (and Don’t Know) about Asteroid Surfaces

1. What we Know (and Don’t Know) about Asteroid Surfaces Clark R. Chapman SouthwestResearchInstitute Boulder, Colorado, USA Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids Arlington VA Rosslyn Hyatt, 3 Sept. 2002

2. Surfaces: Aspects Relevant for Mitigation Issues • There is much of scientific interest about asteroid surfaces, but here I concentrate on aspects relevant to mitigation and other human-scale interactions with asteroids. • Are surfaces solid so we could anchor to them? • Are regoliths deep or thin,patchy or ubiquitous? • Are surfaces covered with huge boulders? Dusty? • How reliably do remote-sensing observations and pictures tell us about interior properties? • Are asteroid surfaces likely to be similar or the same, or must we study the particular body to be deflected? • In particular, is our knowledge from Eros relevant to NEOs maybe a million times less massive?

3. Deflection Options • Attach a thrusting device: e.g. powerful rocket, ion engine, solar sail, mass driver • Practical issues: e.g. how to you get through regolith to “bedrock”? • Is there real bedrock, so that if you try to drag or push the asteroid while anchored at one place, the whole thing will move? (What is internal structure?) • Make the object thrust itself: e.g. stand-off neutron bomb evaporates surface, vaporize ice within a comet so it develops a strong, “nongravitational” thrust, Yarkovsky effect • What is surface layer like? How will it respond? • What is structure of surface/interior layers that will shape the body’s own thrust?

4. Sources of Information about Asteroid Surfaces • Telescopes. Hemispherically averaged, ambiguous info on particle size, mineralogy, and not much else • Radar. Roughness on a human-scale and reflectivity (ice, metal, rock…), especially for NEAs that pass close • Inferences from Meteorites. Better than nothing, but meteorites are not regolith samples • Spacecraft. Only a few studied to date; best data by far for Eros (NEAR)

5. “Ponds” from Low-Altitude Flyover

6. “Ponds”, “Beaches” & Debris Flows • “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 ponds formed? Electrostaticlevitation, seismicshaking? If mass-wasting, why don’t lunar ponds exist? Upper smooth area may be shaped by “debris flow” mechanics (Cheng et al., Aug. 2002 MAPS)

7. NEAR-Shoemaker’s Landing Spot on Eros Inset shows Himeros Estimated positions of last images end within a 50 meter diameter crater, which may have a “pond” on its floor • How typical is the edge of Himeros of Eros? • How typical is Eros of other asteroids?

8. Fifth Last Image (largest boulders are 3 meters across)

9. Measuring (Big) Craters and (Small) Boulders • Sparse craters, tens to hundreds of meters across, are measured in whole image • Boulders, mostly less than 15 meters across, are more-than-well sampled in one-quarter of image • This image is from NEAR-Shoemaker Low Altitude Flyover (10/00)

10. Eros is Covered with Rocks

11. Final Landing Mosaic

12. Closest Image of Eros

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

14. R Plot: Eros Craters & Boulders

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

16. Eros/MoonComparisons The Surface of Eros is NOT like the Lunar Regolith! • Ejecta is very widespread on Eros, much lost to space, few generations of churning • Lunar ejecta is repeatedly churned in situ, becomes very mature • Rocks (ejecta blocks from far-away large impacts and exhumed from below) remain in place, cover the surface of Eros • Lunar rocks are fragmented and eroded; surface is covered by craters • Flat, pond-like deposits (of fines) common in depressions -- few rocks or craters • Electrostatically levitated dust on Moon does not form ponds, at least not commonly

17. Other Asteroids and Inexact Asteroid Analogs (Mars Moons) Ida Mathilde Gaspra (Eros) Phobos and Deimos

18. Mathilde and Its Huge Craters C-type asteroids may be very different places, at all scales, compared with what we have found at Eros

19. “Steep”, Undersaturated Size Distribution for Gaspra Craters

20. Ida Craters Saturated < 1 km Diameter

21. Ida Looks Much Like the Moon (and Eros)... …at scales of 100’s of meters and larger, but it may be just like Eros (and not like the Moon) at human scales

22. Dactyl: Best Analog for an NEO we May Want to Deflect? False color image • Dactyl: 1.6 km diam. • 1st asteroidal moon • Sub-spherical shape: broke up, reaccreted? • Up to 29 craters: satur-ated? Crater chain? We can’t tell what Dactyl’s surface is really like, even if it were relevant!

23. Asteroid Surfaces: Comparisons • GASPRA Big craters absent (except “facets”?); small craters undersaturated. Young and/or made of strong metal, not rock. • IDA Saturated with large craters. Old, lunar-like megaregolith (2-chunk rubble pile?); small-scale surface like Eros. Anchor to what? • MATHILDE Supersaturated by giant craters (small scales unknown). Low-density materials and/or voids, perhaps compressible or loosely bound. Analogs: mud, sand, styrofoam? • EROS Shattered shard, only source of data at hi-res scales. Amazing! Surface character still not understood. Consolidated within?

24. Conclusions: We Need to Learn More About Asteroids! • Are surfaces solid so we could anchor to them? Not for those we’ve seen, but small NEAs could well be “bare rocks”. • Are regoliths deep or thin,patchy or ubiquitous? Interiors could be megaregoliths; surface regoliths (if they exist) could be rocky or dusty, but lunar regolith is a poor analog. • Are surfaces covered with huge boulders? Dusty? Probably. • How reliably do remote-sensing observations and pictures tell us about interior properties? Better than nothing, but very poorly. • Are asteroid surfaces likely to be similar or the same, or must we study the particular body to be deflected? Great diversity; studying them helps us prepare for the unexpected, but we must study the particular body, if at all possible. • In particular, is our knowledge from Eros relevant to NEOs maybe a million times less massive? Not very. NEOs are smaller than Eros just as much as Eros is smaller than the Moon. We must study: (a) diversity of surfaces; (b) interiors