Asteroids are important because they preserve information about the formation and early evolution of the solar system. Topography from the NEAR-Shoemaker Laser Rangefinder (NLR) provides precise measurements of surface heights. Topography combined with gravity, derived from measuring the Doppler shift of the radio signal used to track the NEAR-Shoemaker spacecraft, yields insight into the internal makeup of Eros. Together these data preserve the record of the collisional history of the asteroid. From analyzing Eros, we hope to learn alot about this one asteroid, but also some general things about the early solar system.
The initial results from the NLR investigation provide information on the processes that caused the asteroid to attain its present unusual shape. Asteroids have undergone a long history of collisions and one of the questions that we sought to answer is "just how battered is Eros?" One school of thought has been that asteroids are "rubble piles", collisional remnants held together by gravitational attraction. Another possibility is that, despite having undergone collisions, at least some asteroids reamin structurally competent and their shape is indicative of internal material strength. As is often the case, reality is neither black nor white. In this paper we document evidence for both gravitational and structural contributions to the shape of Eros.
Evidence for gravitational control includes the presence of bowl-shaped crater depressions, crater rims, the existence of regolith (the unconsolidated, impact-generated surface layer), and zones of mass wasting (downslope movement of regolith material). Evidence for structural competence includes the overall "twisted" appearance of Eros, clustered regions of high slopes, long, continuous structural grooves, and the presence of polygonal craters and craters that lack rims. So what does this mean? The interpretation favored by our team is that the interior of Eros is for the most part structurally competent while the shallow near-surface region is spatially heterogeneous (non-uniform) in terms of its mechanical properties. This could happen if regolith is re-distributed by collisions and/or downslope movement. (Differences in composition could also produce spatial variations in density structure, but results from the NEAR-Shoemaker XGRS instrument do not show evidence for this.)
More support our interpretation comes from combining topography with gravity to study the interior of Eros. We do not yet (and may or may not ever get) high enough resolution gravity data to study very small-scale and shallow spatial variations in the internal density of Eros. So we focus initially on the scale of the whole asteroid. The geometric center of Eros (center of figure) is offset from its center of mass by only a small amount that is consistent with a density gradient across the asteroid of about 2% the density of rock. This can be explained by a maximum change in regolith thickness across the asteroid of 100 m, or less of the interior of the asteroid is non-uniformly fractured (albeit at a low level), as is likely. The gravity field of Eros from the NEAR-Shoemaker Radio Science experiment indicates that inward attraction varies significantly over the surface, with low gravity near the ends and higher gravity elsewhere. Thus regolith material should move around with varying efficiency due to mass wasting and impacts. The internal structure of Eros is undoubtedly more complicated than the simple example given, but it is clear that this asteroid cannot be too heterogeneous in its interior.
We are interested in the general question of the collisional evolution of asteroids so one of the things that we have been trying to understand is how long Eros has been approximately its current size. We cannot tell if the craters we have studied so far formed when Eros was at its present size or while it might have been part of a larger "parent body". But we believe that the interior is sufficiently competent that Eros is not likely to have once been smaller than it currently is, i.e. accumulated from smaller asteroidal bodies.
We have just scratched the surface, so to speak, in terms of what data from the NLR can ultimately tell us about Eros and about the structure and evolution of asteroids in general. The NLR investigation depends on being able to correct observations to extremely precise levels and we know that quality of the data can still be improved significantly beyond its current level, both in terms of spatial resolution and elevation accuracy. The most important results from the NLR investigation are yet to come.
Maria Zuber
Team Leader
NEAR-Shoemaker Laser Rangefinder Science Team
Direct inquiries to: zuber@mit.edu