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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

Abstract

Vesta is a large differentiated rocky body in the main asteroid belt that accreted within the first few million years after the formation of the earliest solar system solids. The Dawn spacecraft extensively imaged Vesta’s surface, revealing a collision-dominated history. Results show that Vesta’s cratering record has a strong north-south dichotomy. Vesta’s northern heavily cratered terrains retain much of their earliest history. The southern hemisphere was reset, however, by two major collisions in more recent times. We estimate that the youngest of these impact structures, about 500 kilometers across, formed about 1 billion years ago, in agreement with estimates of Vesta asteroid family age based on dynamical and collisional constraints, supporting the notion that the Vesta asteroid family was formed during this event.

Asteroid 4 Vesta is the second most massive body in the main asteroid belt, and, according to models (15), its early evolution occurred in an environment where collisions with other asteroids were much more frequent than they are today. One notable feature emerging from early observations of the Dawn mission (6) is that the surface of Vesta is dominated at all scales by impact craters. Dawn’s framing camera extensively imaged Vesta during its survey phase, at an altitude of ~2700 km. These data have been used to build a global visual image mosaic with a resolution of 260 meters per pixel, covering ~ 80% of Vesta’s surface and leaving only a portion of the northern hemisphere unseen. We used the global mosaic and digital terrain models (DTMs) (7) to map craters larger than 4 km in diameter (Fig. 1). Numerous large, shallow topographic depressions are inferred to be degraded impact structures (Fig. 1B).

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.

The shape of Vesta has been noticeably modified by a ~500 ± 25–km–diameter impact structure, named Rheasilvia, located near the south pole (8, 9). The formation of this structure obliterated the older cratering record of Vesta’s southern hemisphere, including nearly half of a ~400-km-diameter basin, named Veneneia, whose rim remnants are visible in the DTMs (Fig. 1B) (9). The volume of material excavated by the impact that formed Rheasilvia (9) is an order of magnitude larger than the estimated volume of asteroidal members of Vesta’s dynamical family (10), the so-called Vestoids. Vestoids are thought to be the immediate parent bodies of howardite, eucrite, and diogenite (HED) meteorites (1115).

The cratering record shows that the northern hemisphere escaped such violent resetting, with the density of craters increasing from the Rheasilvia rim to northern latitudes. The crater density has a distinctive pattern, reaching maximum values in two heavily cratered terrains (HCTs) centered at about 348°E, 17°N and 110°E, 17°N (Fig. 2). The overall crater density in the northern hemisphere and the extent of the HCTs appear to have been shaped by the formation of large craters that obliterated preexisting craters. The best examples are two craters with diameters of ~50 and 60 km and located at 195°E, 10°N in the Marcia quadrangle (6). These craters produced ejecta blankets that, along with impact-induced seismic shaking, apparently obliterated the previous cratering record up to a distance of about one crater radius from their rims (16). The northward extension of the HCTs is presently unmapped, so it is possible that the highest crater density has yet to be detected.

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.

Interestingly, some local topography seems to be correlated with crater areal density. The highest-relief terrains (HRTs) of Vesta correspond to the high-standing flanks immediately proximal to the rim of Rheasilvia and extend in northward direction (outlined in Fig. 2) (7, 9). These areas generally have crater densities intermediate between HCTs and the low crater density seen on Rheasilvia’s floor (Fig. 2).

The cumulative crater size-frequency distributions of Rheasilvia’s floor, HRTs, and HCTs are shown in Fig. 3A. The clear separation in terms of crater density indicates that HCTs are much older than Rheasilvia. Although the crater retention age of Rheasilvia may not necessarily correspond to its formation age, it is concluded that most mass-wasting on Rheasilvia’s floor occurred shortly after its formation as a consequence of complex crater-floor collapse (9). The plot also shows the curve corresponding to the 10% level of the geometric crater saturation curve (17, 18). Although the empirical lunar saturation may well not apply to Vesta, given the different impactor populations and crater sizes investigated, the fact that lunar terrains saturate at the 3 to 5% level of geometric saturation (17) suggests that the heavily cratered terrains on Vesta may be close to saturation. Note that the HCT’s size-frequency distributions are characterized by a slope transition (19) at about crater size D = 10 km. Craters in the 4- to 10-km size range have a cumulative slope of –1.9 ± 0.1, which is close to the geometric saturation slope –2, whereas the cumulative slope is –2.9 ± 0.2 for craters in the 10- to 20-km size range. These observations are consistent with the overall behavior of saturated terrains (17), in which the shallower branch of the distribution is controlled by saturation, and the steeper branch would represent the crater production curve (16). Even if the crater distribution on the HCTs has not reached a fully saturated state, the overall crater density is high enough that crater obliteration by repeated impacts may have still played an important role. These observations, along with large uncertainties in the impactor flux over time, make the process of cratering age determination for HCTs uncertain.

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).

The HRT crater size-frequency distribution shows a slightly higher crater density (by a factor of ~1.6 at D = 4 km) than that of Rheasilvia. The reduced crater densities associated with the HRTs proximal to the Rheasilvia rim are most simply explained by coverage with a thick ejecta blanket from Rheasilvia, in rough agreement with recent modeling (20), whereas Rheasilvia secondaries may be responsible for the slightly higher crater density. Similar conclusions also apply to the subequatorial region (21) between 0° and 100°E longitude close to a sharp segment of Rheasilvia’s rim, named Matronalia Rupes (Fig. 2). Although Vesta’s capability of forming secondary craters requires detailed analysis, a recent model (22) shows that secondary craters can be important on small saturnian satellites whose gravity is comparable with Vesta’s (0.22 m/s2).

The crater size-frequency distribution of large degraded craters (LDCs) is reported in Fig. 3B. Its overall cumulative slope (–0.8 ± 0.2) is considerably shallower than that of HCTs, although, for craters larger than 100 km, its slope is close to that estimated using the model main-belt population (2) and the Pi-group crater scaling law (23). On the other hand, the crater size-frequency distribution of the younger Rheasilvia floor matches the model main-belt crater production function quite well (Fig. 3B) (16). Although preliminary, these results show that the current main-belt crater production does not bridge the overall crater distribution from small craters on HCTs to large degraded craters, the latter being in excess by a factor of ~4 with respect to small craters, according to the present main belt (Fig. 3B). This result suggests either that the overall HCT-LDC distribution is saturated or that HCTs have been reset by a recent event, whereas the LDCs record an older population. The latter hypothesis is not supported by the distribution of recognized large craters (Fig. 1B). The first scenario, however, is at odds with the observation that the HCT crater size-frequency distribution for craters in the size range 10 to 20 km is steeper than the main-belt crater production function (Fig. 3B). This result supports the idea that the steep slope may be due to secondary craters from Rheasilvia or other LDCs, arguing against the saturation of the overall HCT-LDC distribution. Thus, it appears that the overall HCT-LDC cumulative slope may indicate a much shallower main-belt size-frequency distribution in early solar system history.

The Rheasilvia model best fit presented in Fig. 3B can also be used to derive a crater retention age (24, 25). The present intrinsic collisional probability of other main-belt asteroids with Vesta is ~2.8 × 10−18 km−2 year−1 (26, 27), which yields a very young crater retention age of ~1.0 ± 0.2 billion years (Gy), assuming a constant impact rate over that time span, consistent with the results of geological mapping and independent crater counting (9). The estimated probability for such an impact to occur over the past 1 Gy is ~25% (28). Rheasilvia’s young cratering age agrees with its undegraded morphology (9) and with the steep Vestoid size-frequency distribution (10, 16, 29). According to collisional evolution models (29), if the Vesta family were older than ~1 Gy, its size-frequency distribution would have evolved by collisional grinding to match the background population of main-belt asteroids. This is also the minimum time scale required for “fugitive” V-type asteroids (spectrally related but dynamically distinct from the Vesta family) to migrate away from the family under the action of secular perturbations (10). The fact that our Rheasilvia crater retention age is in agreement with the time scales for age and evolution of the Vesta family provides strong support for the Rheasilvia event being the primary source of the Vesta family, although a minor contribution from older family-populating impact events cannot be ruled out.

Unraveling the convoluted collisional evolution of Vesta is required for a global understanding of Dawn data and for interpreting HED meteorites. In this respect, the north-south cratering dichotomy is responsible for much of the compositional diversity across Vesta’s imaged surface. The occurrence of superposed craters having very different sizes in the northern hemisphere (Fig. 1) indicates that they excavated through different depths of Vesta’s crust and produced extensive local mixing, as inferred from HED meteorites. Similar processes likely occurred in the southern hemisphere before the terrains were excavated by the formation of the south pole basins, whose ejected material may be sampled by HED meteorites. The Visible and Infrared Spectrometer data indicate that most of Vesta’s surface resembles howardites (30), composed of admixed eucrite and diogenite components, confirming the mixing inferred from crater imagery. Moreover, the observation that the overall composition of the equator and low-latitude northern hemisphere is compatible with eucrite-enriched howardites (30) may suggest that the large impact structures in the northern hemisphere did not expose substantial portions of the diogenitic crust, unlike the Rheasilvia basin, which contains diogenite-enriched howardite. The DTM-based depths of the largest craters in the northern hemisphere (Fig. 4) are <15 km, which is less than the average estimated crustal depth of ~20 km, assuming that Vesta’s bulk composition is chondritic (13). A few small regions with less eucritic-enriched howardite composition have been identified beyond the floor of Rheasilvia. One is found on Rheasilvia’s smooth ejecta units (9), possibly indicating diogenite-enriched ejecta.

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.

The early Dawn observations of Vesta’s surface reveal a collision-dominated world. From very large basins to HCTs, most of Vesta’s surface properties have been affected by impacts. These observations suggest that the HCTs may have reached an equilibrium level for craters with sizes ranging from 4 to 10 km. In addition, the slope of the overall size-frequency distribution from ~4-km craters to craters hundreds of kilometers in size is considerably shallower than predicted by the present main-belt population, possibly revealing that the primordial main belt had a greater proportion of large asteroids, as envisaged by recent dynamical models (31). One of the most intriguing results from our work is the young crater retention age of the Rheasilvia basin, estimated to have formed ~ 1 Gy ago. This major impact event dramatically altered the inner main belt by producing the massive Vesta asteroid family and may have generated a cascade of events that we are just beginning to fully understand.

Supplementary Materials

www.sciencemag.org/cgi/content/full/336/6082/690/DC1

Materials and Methods

Figs. S1 to S3

References (3243)

References and Notes

  1. It is also possible that a part of the Vestoids and HED meteorites have been originated by the older 400-km basin underneath Rheasilvia.
  2. Methods and additional data are available as supplementary materials on Science Online.
  3. The slope of a cumulative crater size-frequency distribution is defined by Δlog(N)/Δlog(D), where N is the cumulative number of craters and D is the crater size.
  4. Note that this region excludes the floor of the large basin underneath Rheasilvia, and it extends for about 10° to 15° degrees northward starting from Rheasilvia’s rim.
  5. Intrinsic collisional probabilities among asteroids are computed by analytical means using the orbital distribution of asteroids >10 km (25). The main source of uncertainty in the age-estimation process, which is considered in the final error estimate, is due to the lack of detailed knowledge of the impactor size-frequency distribution.
  6. The estimated size of the impactor that produced Rheasilvia is ~50 km or larger [e.g., (20)], resulting in an impact every ~16 Gy, according to the present main-belt impact rate. The knowledge that the event did happen in 4.5 Gy implies a 25% probability of occurrence in the past 1 Gy.
  7. Acknowledgments: We thank the Dawn Science, Instrument, and Operations Teams for support. This work was supported by the Italian Space Agency and NASA’s Dawn at Vesta Participating Scientists Program. A portion of this work was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. S. Marchi thanks Istituto Nazionale d’Astrofisica for the support in carrying on this work.
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