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The Science Of Eros
Laurel - June 1, 2000 - NEAR Shoemaker continues its detailed exploration of Eros from 50 km orbit, a process which resembles in some ways an archaeological expedition. We have to map our site globally, pick through it carefully to find any interesting specimens, study some of these in detail back in the laboratory, and then attempt a synthesis and a placing of results in context. To date, NEAR Shoemaker has completed about one quarter of its planned orbital operations, including our Northern Hemisphere mapping campaigns with the imager and the near infrared spectrometer, and our primary global mapping campaign with the laser altimeter. However, our Southern Hemisphere mapping campaign with the imager is scheduled to start in September of this year, once this region emerges into sunlight, and the global mapping campaigns with the x-ray and gamma ray spectrometers have just begun. So, our exploration of Eros is in progress, but it is far from complete. We have many fascinating specimens in hand, but are still picking our way through the site. Some of our findings have been featured in recent images of the day. Three images, those for the days May 16, 22 and 23, have shown areas with numerous boulders that protrude above the surrounding surface (in NEAR images, features protruding above the surface are lit on the right side and shadowed on the left, whereas the opposite is true for features that are depressions, such as craters). The boulders are mysterious in many ways. Some appear angular (May 22), and others appear rounded (May 16 and May 23), suggesting various origins or histories;
Several images of the day display linear features on Eros, which are the grooves and ridges that we have previously described as comprising a global fabric. This fabric has turned out to be quite complex, suggesting a complicated and interesting geologic history. Several different styles of grooves - elongated, linear depressions - have been identified on Eros. Some look like chains of pits or craters - we don't know which - as seen on May 19. Some appear as elongated craters or furrows, although we have to be careful because even a perfectly round crater may appear as an oval when viewed obliquely. The grooves often appear in the form of nearly parallel systems, cutting across one another and even cutting into and out of craters, as on May 26. In a few places, such as the 'saddle' region (May 9, and 25) and the 'paw' crater (May 23), the grooves are wider and deeper and based on laser altimetry; the alternating light and dark bands in these images can result from topography and/or albedo variations. Also the density of craters (meaning the number of craters of a given size within a given area) is much lower, suggesting a younger surface and/or a process that has smoothed out craters, such as motion of regolith. Boulders are ubiquitous. Ridges are likewise ubiquitous and are often associated with nearly parallel systems of grooves as on May 26. One particularly interesting ridge is the one shown on May 24, which is several kilometers long and is crosscut by another system of grooves. On Earth, deformation can produce ridges and associated fractures or faults. Many of us are asking if the grooves and ridges on Eros indicate the presence of underlying fractures, and if so, which of these may have resulted from processes (such as impacts) on Eros in roughly its present form, and which were pre-existing and therefore may have formed while Eros was part of a much larger parent body. To answer these questions, as well as others about the boulders and about features that we haven't discussed today (such as, the bright areas in the crater walls), we have to complete our survey of the site and finish picking through it. We have to construct detailed maps of the site, and specifically in our case, map the distributions of boulders, craters, and linear features. We also have to map the orientations of the grooves and ridges and study the crosscutting relations (which are a clue to relative ages). We can then begin to sort out the history of Eros. Until then, we will not even know how sensible our questions are.
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