Elsevier

Icarus

Volume 193, Issue 1, January 2008, Pages 39-52
Icarus

433 Eros lineaments: Global mapping and analysis

https://doi.org/10.1016/j.icarus.2007.06.028Get rights and content

Abstract

Using images and laser ranging data from the NEAR-Shoemaker mission, we map lineaments on the surface of Eros in order to investigate the relationship between surface morphology and interior structure. Several sets of lineations are clearly related to visible impact craters, while others suggest that different parts of the asteroid may have undergone different stress histories. Some of these sets infer internal structure, at least on a local level. This structure may derive from Eros' parent body and suggest, although largely coherent, Eros' interior may have portions that have not undergone a common history.

Introduction

The Near-Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft orbited the S-type Asteroid 433 Eros for a year from 2000–2001. The NEAR Multi-Spectral Imager (MSI) collected tens of thousands of high-resolution images and as a result Eros is the most comprehensively studied asteroid in the Solar System. A number of different types of linear structural features—including grooves, fractures, troughs and ridges—were observed on Eros's surface in the MSI images (e.g., Prockter et al., 2002). Determining how these features formed yields information about the nature and history of the asteroid.

Decades ago, grooves were discovered on the martian moon Phobos in Viking orbiter imagery. Thomas and Veverka (1979) suggest that the grooves on Phobos are most likely the result of the Stickney crater impact. They also predicted that similar grooves would be observed on other small cratered bodies, a reasonable prediction given the natural tendency for craters to possess radial fractures in the laboratory (e.g., Fuijwara and Asada, 1983) and on the planets (e.g., Reimold et al., 1998). This prediction was realized when images of S-type Asteroids Ida and Gaspra were returned by the Galileo spacecraft in 1993.

Numerical calculations of impacts on asteroids also indicate that impacts could be responsible for the formation of asteroid fractures. Axi-symmetric calculations of an impact which would generate a Stickney-sized impact in a Phobos-like ellipsoid predicts sizes of spall that compare favorably with the spacing of grooves and fractures seen on Phobos (Asphaug and Melosh, 1993). In another effort, where impacts into Ida are considered, Asphaug et al. (1996) indicate that fractures can be generated far from the impact. Indeed, a 3-D simulation of the formation of a large crater at one elongate end of Ida shows fracturing as far away its antipode, where grooves have been observed on the asteroid (Veverka et al., 1994). In this last study calculations also indicate that impacts into the flat portion of an elongated ellipsoid generate circumferential fractures around the edge of the asteroid perpendicular to the impact normal; impacts on the curved ends of the asteroid result in fracturing mainly at the antipode. The main simplification of these calculations is that the modeled “asteroids” are physically homogeneous.

Eros is an S-type asteroid composed of silica-rich rock that most likely behaves in a brittle manner for most of the impact pressures responsible for the observed surface structures. The observed presence of grooves on Eros can be interpreted as being due to dynamic fracturing that is typically a function of strain rate (e.g., Asphaug and Melosh, 1993) and is a consequence of the transmission of impact shock waves through a brittle material (Prockter et al., 2002). However, two large-scale lineations on Eros—the Rahe Dorsum ridge and the shallow troughs of Calisto Fossae—were found by Thomas et al. (2002) to describe a plane parallel to a large flat region (the southern “facet”) on one end of the asteroid (Fig. 1). They interpreted these observations as indicative of a pre-existing structure throughout most of the asteroid, consistent with a fabric inherited from a parent body.

Whether the linear features observed on Eros are due to impact events or are the surface expression of a pre-existing internal fabric, the presence of large-scale structures on Eros seems to indicate that the asteroid is, for the most part, a coherent body. However, the existence of planar structure through the asteroid suggests that Eros is likely a fragment of a parent body large enough to have formed joints (Thomas et al., 2002), while the formation of grooves through impact does not require this interpretation.

An alternate hypothesis for the formation of grooves on Phobos was rolling boulders (e.g., Head and Cintala, 1979, Wilson and Head, 1989). However, Prockter et al. (2002) found only a few grooves on Eros that are associated with boulders, mostly in the interior of craters. They thus determined that downslope scouring could not be the primary cause of the globally distributed lineaments.

It has also been suggested that some of the observed lineaments on Eros could be the result of thermal stresses (Dombard and Freed, 2002). The thermal stress model for lineament formation invokes the changes in temperature the surface of Eros would experience as it first moved from the asteroid belt into the inner Solar System, and then wandered around the near-Earth region (Dombard and Freed, 2002). Expected expansion and subsequent contraction of the asteroid could lead to observed features whose orientations significantly depend on the shape of Eros, but in general are predicted to trend E–W at the poles and N–S at the lower latitudes. An in-depth investigation of whether or not thermally induced stresses make up some the lineaments mapped will be the topic of a future study.

In this study, we are creating a global database of all Eros lineaments to better understand the global distribution of these features and the interior structure of the asteroid. We investigate whether Eros' regional and local-scale grooves formed around impacts on the asteroid's surface or whether they represent areas of weakness present before the current Eros was formed that were subsequently reactivated when a shock wave from an impact swept across them. We identify types of lineaments across the surface using MSI images, and classify them according to region, including areas suggestive of thicker regolith. We compare lineament orientation to impact crater location to determine if there is a causal relationship between cratering events and lineament formation. We perform a statistical analysis on similarly oriented lineations to determine if they could represent a pre-existing planar structure through the asteroid.

Section snippets

Mapping

It is particularly challenging to map lineament orientations on a non-spherical body (Eros is the shape of a yam, measuring 34 km on the long axis). Re-projecting Eros onto a flat surface, as is typically done with spherical bodies, results in severe distortions of surface features, making it difficult, if not impossible, to interpret them in any meaningful way.

Another challenge of mapping lineations on Eros is that many lineaments may not be visible on the surface under certain lighting or

Observations

We have mapped 2141 lineations, ranging up to tens of kilometers in length, on 180 high resolution (5–11 m/pixel) images of Eros, creating a global lineation map of the asteroid (Fig. 5). No strong global planar fabric becomes evident by plotting the poles of all of the individual lineations; the poles are randomly distributed (Figs. 5g and 5h). However, there are local peaks in the pole orientation that may indicate the presence of localized internal structure.

Although there are some regions

Implications for Eros interior

We observe two sets of lineations that are clearly related to impact cratering: lineations radial to craters and lineations circumferential to craters (set 1). However, we also observe at least one lineation set (set 2) that has no obvious correlation to impact events and may be related to internal structure within Eros.

There are four proposed states of asteroid structural modification (Wilkison et al., 2002): (1) completely coherent, (2) coherent but fractured, (3) heavily fractured (e.g.,

Conclusions

Eros lineaments appear to have several different origins. Several craters have radial fractures, but many lineaments have no obvious relationship to impact craters. The lineations in set 1 and oriented radial to large impact craters were most likely formed due to impact events. Set 2 lineations are inferred to represent a planar fabric in the tail of Eros. Set 3 may encompass more than one similarly oriented planes of weakness, including a small subset that may be due to impact, but do not have

Acknowledgements

We thank Peter Thomas and Karl Mueller for their thoughtful and helpful reviews. This work was funded by the NASA Discovery Data Analysis Program, grant number NAG512621, PI Louise Prockter and grant number NNG05GL34G, PI Olivier Barnouin-Jha.

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