Near-Earth Objects 297
FIGURE 14 A montage of images showing structural features on 433 Eros. Shallow troughs are partially filled-in linear structures; pit
chains are crater chains. A ridge winding almost around the entire asteroid is called Rahe Dorsum; fractures are at the end of Rahe
Dorsum, and grooves are evenly spaced and may have raised rims. (Photo credit: NASA/JHU/APL.)volatilization, and subsequent sputtering, though it could
also represent a different starting composition for Eros as
compared to ordinary chondrites, or partial melting.
The gamma ray spectrometer (GRS) produced observ-
able and interpretable signal from the landing site of the
NEARspacecraft (Fig. 15). Five elements were detected:
K, Mg, Si, O, and Fe. The abundance of potassium and
the Mg/Si and Si/O ratios are chondritic. These results
agree with the XRS findings and are consistent with the
imaging and near-infrared spectrometer’s findings. With the
depleted S/Si ratio found with XRS, the abundance of potas-
sium, a relatively volatile element, was expected to be low
as well, but it isn’t at the Eros landing site. Eros’ Fe/Si and
Fe/O ratios are low compared to chondritic values, and low
compared to the XRS results, too. They fall within the region
of differentiated and partially differentiated meteorites and
above the values observed for meteorites. The most likely
explanation for low elemental abundances is for a regolith
process where the iron migrates within the regolith. The
GRS samples to greater depths than the XRS, so the dif-
ferent results between the two instruments is explained by
real differences in the top 100μm of the regolith compared
to the 10s of centimeters depth sampled by GRS. Spatially
resolved spectra from theNEARmission indicate that the
surface composition of Eros is uniform, showing very little
compositional variation, except in some interior regions of
craters where fresh material is exposed.6.2 Sample Return Mission
TheHayabusamission to asteroid Itokawa included in situ
observations over a 3 month tour. This S-type asteroid, with
a 12.1 hour rotation period, was observed with 4 instru-
ments: an imaging camera with 8 filters, a near-infrared