The Violent Collisional History of Asteroid 4 Vesta

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Science  11 May 2012:
Vol. 336, Issue 6082, pp. 690-694
DOI: 10.1126/science.1218757

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  1. Fig. 1

    Vesta’s global crater distribution. (A) Location of the 1872 identified craters ≥4 km superposed on the survey phase global mosaic (260 meters per pixel). Illumination conditions are not uniform over the mosaic: On average, the northern quadrants have low incidence angles, whereas equatorial quadrants have high incidence angles. Consequently, the detectability of craters is not uniform across the surface. Despite these limitations, the quality of the mosaic allows a uniform detection limit for craters ≥4 km. (B) Recognized craters ≥50 km (yellow solid lines) superposed on DTM topography referred to the reference 285-by-285-by-229–km ellipsoid. Minimum and maximum elevations are about –22 km and 19 km, respectively. The large crater Rheasilvia appears to be the youngest structure among the largest craters (except for the two 50- to 60-km craters in Marcia quadrangle visible at the left-hand side of the map). The number of observed large craters is probably a lower limit. It is possible that other large irregular depressions in the northern hemisphere are impact features (red dashed lines), although their identification is presently unclear.

  2. Fig. 2

    Global crater areal density (D ≥ 4 km). Crater density (in number of craters per 104 km2) was derived by averaging over a radius of 80 km. Black solid lines indicate the contour level for the HRTs, as derived by DTM topography, and correspond to elevations of 12, 13.5, 15, and 16.5 km (referred to the reference ellipsoid). This region corresponds to the southern part of the larger high-elevation terrains, named Vestalia Terra. The same contour levels also define Rheasilvia’s central mound (300°E, 70°S). The vertical gradient in the crater density for latitudes ≥30°N (yellow dotted line) is an artifact due to the lack of data at high latitudes.

  3. Fig. 3

    Crater size-frequency distributions of key units on Vesta. (A) Cumulative crater size-frequency distributions of Rheasilvia’s (RS) floor, HCTs (composite of HCTs 1 and 2) (Fig. 2), and HRTs. The 10% level of geometric crater saturation (17, 18) is reported, as well as the crater size-frequency distribution of the oldest unit observed on 21 Lutetia (Achaia region), the second-largest asteroid imaged by a spacecraft (25). Vesta’s HCT crater size-frequency distributions appear steeper than Lutetia’s, although the overall crater densities are rather similar. Moreover, none of Vesta’s crater size-frequency distributions exhibit the characteristic kink shown by Lutetia crater size-frequency distribution at D = 5 to 8 km, which is interpreted to result from inhomogeneous properties (25). (B) Vesta’s HCT crater size-frequency distribution compared with that of large craters observed on the imaged surface. The main-belt crater production functions for three different levels of crater density are given for comparison (red dashed curves).

  4. Fig. 4

    Close-up view of one of the largest impact structures detected in the northern hemisphere. This structure is ~180 km across and located at the border of the Bellicia and Gegania quadrangles (Fig. 1B). (A) Color-coded topography. (B) Color-coded crater areal density (number of craters per 104 km2) superposed on topographic contour levels for elevations of –13, –9, –6, –1, and 0.5 km with respect to the reference ellipsoid. The formation of these large craters locally reset the previous crater record and is responsible for the observed pattern in the overall crater density. These large impact structures have shallow concavities due to erosion by numerous superposed impact craters, reaching maximum relative depths with respect to surrounding terrains of ~10 to 15 km. A similar conclusion also applies to other large degraded craters in the northern hemisphere.