Physics – Geophysics
Scientific paper
Jan 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003icbg.conf...15c&link_type=abstract
Impact Cratering: Bridging the Gap Between Modeling and Observations, February 7-9, 2003. LPI Contribution No. 1155. Houston,
Physics
Geophysics
Asteroids, Comets, Debris, Gravitational Effects, Meteorite Craters, Regolith, Anchors (Fasteners), Asteroid Belts, Deflection, Deposition, Ejecta, Geology, Geophysics, Imaging Techniques, Rocks
Scientific paper
Cratering and regolith processes on small bodies happen continuously as interplanetary debris rains down on asteroids, comets, and planetary satellites. Butthey are very poorly observed and not well understood. On the one hand, we have laboratory experimentation at small scales and we have examination of large impact craters (e.g. Meteor Crater on Earth and imaging of abundant craters on terrestrial planets and outer planet moons). Understanding cratering on bodies of intermediate scales, tens of meters to hundreds of km in size, involves either extrapolation from our understanding of cratering phenomena at very different scales or reliance on very preliminary, incomplete examination of the observational data we now have for a few small bodies. I review the latter information here. It has been generally understood that the role of gravity is greatly diminished for smaller bodies, so a lot of cratering phenomena studied for larger bodies is less applicable. But it would be a mistake to imagine that laboratory experiments on gravitationless rocks (usually at 1 g) are directly applicable, except perhaps to those monolithic Near Earth Asteroids (NEAs) some tens of meters in size that spin very rapidly and can be assumed to be "large bare rocks" with "negative gravity". Whereas it had once been assumed that asteroids smaller than some tens of km diameter would retain little regolith, it is increasingly apparent that regolith and megoregolith processes extend down to bodies only hundreds of meters in size, perhaps smaller. Yet these processes are very different from those that pertain to the Moon, which is our chief prototype of regolith processes. The NEAR Shoemaker spacecraft's studies of Eros provide the best evidence to date about small-body cratering processes, as well as a warning that our theoretical understanding requires anchoring by direct observations. Eros: "Ponds", Paucity of Small Craters, and Other Mysteries. Although Eros is currently largely detached from interactions with main-belt asteroids in its Earth-approaching orbit, almost all of its cratering history must have occurred in the main belt, where it almost certainly lived for a long time and where the impact rate is orders-of-magnitude greater than in its present environment. Thus NEAR Shoemaker's year-long orbital studies of Eros should be representative of asteroidal cratering processes for medium-small (tens of km) asteroids generally - with the caveat that small bodies are made of many different materials, ranging from metal to whatever comets are made of, and we already have indications from NEAR Shoemaker's flyby of Mathilde that responses to impacts on such bodies may be very different from what is observed on rocky Eros. As viewed from a distance, the saturated crater fields on Eros look similar to those on Ida and, indeed, on the Moon itself. It is at smaller scales, never before studied for asteroids, where Eros# appearance diverted dramatically from expectations based on modest extrapolations from our lunar experience. Flat, level "ponds" are common on Eros and were certainly not expected. Most striking, however, is the virtual absence of small-scale (cm to meters) craters and the dominance of rocks and boulders on the surface. Apparently many of the larger boulders were distributed about Eros by the comparatively recent impact that produced the Shoemaker crater, providing insight to ejecta processes on small bodies. But, assuming that Shoemaker didn't form practically "yesterday", the dearth of small craters is extremely puzzling. Some researchers have attempted to explain the shortage by traditional geological processes; I will explain why these fail and we are being forced to turn to explanations involving shortages of small projectiles in the asteroid belt (e.g. due to the Yarkovsky Effect). Even if projectile shortages help to explain the data, other non-lunar processes must be at work, as well. Mass-wasting processes are evident on large crater walls and the ponds reflect a still-not-understood deposition or sedimentation process. The boulder-strewn surface itself also serves to "armour" the surface against impacts. The role of seismic shaking on small bodies also must play a major role, relatively unfamiliar for larger bodies. I will summarize the observations of Eros that shed light on these various processes. Even Smaller Bodies. An interest in sub-km scale bodies has developed in the context of imagining how a potentially dangerous NEA might be diverted. Meanwhile, observational evidence concerning their general geophysical configurations has grown rapidly. A significant proportion of these bodies (approx. 20%) appear to have satellites or be binary in nature, and most of the remainder exhibit properties consistent with being "rubble piles" of one form or another. Eros, with less than a millionth the mass of the Moon, turned out to be extremely non-lunar-like in its small-scale responses to impact cratering. NEAs of the size being analyzed as prototypes for deflection are a millionth the mass of Eros. We should not expect our insights from Eros, therefore, to be directly applicable to them. And as we learn more about small asteroids and comets, we must expect to be surprised.
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