**2. Asymmetric craters on Vesta and Ceres**

A special type of craters occurs von Vesta. These craters show an asymmetric interior morphology and ejecta distribution and are formed on slopes. The main characteristics are a well-formed semi-circular sharp rim on the uphill side and a smooth rim on the downhill side [2, 7]. The downhill rim is covered by a local accumulation of material, whereas ejected material around the uphill rim is only sporadically distributed in thin layers (**Figure 1**). Mass wasting material is observed on the upslope inner crater walls of most craters. The majority of asymmetric craters have relatively steep inner slope angles of ~24° to 28° on the uphill side and a shallower slope angle of about 13° to 16° on the downhill side [7]. In many cases a straight line occurs on the crater floor between the oblique and the shallow side of the crater at which mass wasting material from the uphill crater wall meets the downhill crater rim material (**Figure 1**). The morphology of asymmetric craters comprises the main type described above (**Figure 1A**), as well as crater with an elongated shape in uphill direction (**Figure 1B**), v-shaped craters with one extended wall (**Figure 1C**) and craters with a lateral elongated form (**Figure 1D**) [7].

Asymmetric craters on Vesta ranging in diameter from 0.3 km to 43 km and are globally distributed. Most craters were formed on slope angles between 10° and 20°.

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**Figure 1.**

*perspective view of the repective craters.*

*Special Crater Types on Vesta and Ceres as Revealed by Dawn*

The authors of [7] shows that the topography is the main cause for the asymmetries observed in these craters on Vesta. Numerical simulations demonstrate that the asymmetric form of these craters can be produced by an oblique impact into a slope. Additionally, the deposition of ejected material in uphill direction is prevented by the slopes, in particular by slopes >20° and results in a larger accumulation of ejecta within the crater and on the downhill crater rim. Post-impact processes are not likely because of comparable ages of crater floors and continuous ejecta [7].

*Examples of asymmetric craters on Vesta. A1 Antonia crater shows the classic type with a smooth downhill rim and a sharp uphill rim which is separated by a straight boundary. B1 oblique elongated crater at 6°S, 299°E. the ejecta is distributed only on the downhill rim. C1 V-shaped crater at 48°S, 129°E. D1 lateral elongated crater at 50°S, long 266°Ewith a downhill ejecta distribution at 501S, long 2661E. A2, B2, C2, D2 show the* 

*DOI: http://dx.doi.org/10.5772/intechopen.96671*

*Special Crater Types on Vesta and Ceres as Revealed by Dawn DOI: http://dx.doi.org/10.5772/intechopen.96671*

*Solar System Planets and Exoplanets*

**2. Asymmetric craters on Vesta and Ceres**

A special type of craters occurs von Vesta. These craters show an asymmetric interior morphology and ejecta distribution and are formed on slopes. The main characteristics are a well-formed semi-circular sharp rim on the uphill side and a smooth rim on the downhill side [2, 7]. The downhill rim is covered by a local accumulation of material, whereas ejected material around the uphill rim is only sporadically distributed in thin layers (**Figure 1**). Mass wasting material is observed on the upslope inner crater walls of most craters. The majority of asymmetric craters have relatively steep inner slope angles of ~24° to 28° on the uphill side and a shallower slope angle of about 13° to 16° on the downhill side [7]. In many cases a straight line occurs on the crater floor between the oblique and the shallow side of the crater at which mass wasting material from the uphill crater wall meets the downhill crater rim material (**Figure 1**). The morphology of asymmetric craters comprises the main type described above (**Figure 1A**), as well as crater with an elongated shape in uphill direction (**Figure 1B**), v-shaped craters with one extended

wall (**Figure 1C**) and craters with a lateral elongated form (**Figure 1D**) [7].

Asymmetric craters on Vesta ranging in diameter from 0.3 km to 43 km and are globally distributed. Most craters were formed on slope angles between 10° and 20°.

the only dwarf planet in the inner Solar System, which is supposed to be a relict ocean world [14, 15]. Recent observations by Dawn suggest that Ceres is a weakly differentiated body with a 40 km thick volatile-dominated crust and a rocky mantle down to a depth of 100 km comprising remnants of brines and hydrated rocks such as clays [16]. The crust is thought to be dominated by a mixture of ammoniated phyllosilicates, carbonates, salts, clathrate hydrates and no more than 30–40% water ice [17–20]. This volatile-rich outer layer is suggested to have an average thickness of 41.0 km [21–23]. The brines within the mantle of Ceres could be related to residual liquid from the freezing of a global ocean, as already proposed prior to the Dawn mission [24]. Several locations on Ceres's crust are enriched in salt compounds such as carbonates and ammonium chlorides [25]. A very large amount of the water could exist in the form of clathrate hydrates, which is conforming to geophysical conclusions for the abundance of water in Ceres's crust [17, 20, 26]. Since Ceres's globally homogenous surface is supposed to be made of material formed deep inside, a large-scale formation mechanism is suggested for that scenario. However, local heterogeneities associated with impact craters and landslides containing sodium carbonate and other salts suggest that those components are available in the shallow subsurface [26]. Sodium carbonates are found in brines of two remarkable emplacements on Ceres: Ahuna Mons [27, 28] and the bright (faculae) material in Occator crater (e.g., [19, 29]). Recent emplacement of bright deposits sourced from brines confirms that Ceres is a persistently geologically active world [19]. Generally, sodium carbonates are related to large impacts that can source deep material [26]. The most distinctive features found on both bodies are impact craters. Cratering processes on planetary bodies happen continuously and cause the formation of a large variety of impact crater morphologies. Images from the Dawn Spacecraft have revealed a diversity of impact craters, including craters with an individual appearance. The shape of an impact crater, and mainly its ejecta distribution, is the effect of a multifaceted interaction of topographic setting [30]. The majority of impact craters are more or less symmetrical and circular in shape. They display a classical circular bowl-shaped form with crater rims on the same elevation level at every azimuth and approximately parabolic interior profiles. Special topographic and subsurface conditions on both bodies have led to the development of special crater types. This chapter covers these special crater types found on Vesta and Ceres.

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#### **Figure 1.**

*Examples of asymmetric craters on Vesta. A1 Antonia crater shows the classic type with a smooth downhill rim and a sharp uphill rim which is separated by a straight boundary. B1 oblique elongated crater at 6°S, 299°E. the ejecta is distributed only on the downhill rim. C1 V-shaped crater at 48°S, 129°E. D1 lateral elongated crater at 50°S, long 266°Ewith a downhill ejecta distribution at 501S, long 2661E. A2, B2, C2, D2 show the perspective view of the repective craters.*

The authors of [7] shows that the topography is the main cause for the asymmetries observed in these craters on Vesta. Numerical simulations demonstrate that the asymmetric form of these craters can be produced by an oblique impact into a slope. Additionally, the deposition of ejected material in uphill direction is prevented by the slopes, in particular by slopes >20° and results in a larger accumulation of ejecta within the crater and on the downhill crater rim. Post-impact processes are not likely because of comparable ages of crater floors and continuous ejecta [7].

Asymmetric craters are also found on Ceres. The analysis of high resolution data reveals craters similar to those on Vesta, however, with diameters from 0.30 to 4.2 km and a mean of 0.98 km the craters are much smaller [31]. The morphology of those craters shows an asymmetric crater interior with an oblique and a shallower side, as well as an asymmetric ejecta distribution. The crater reveals a semi-circular sharp and well-formed rim on the uphill side, as well as a smooth rim on the downhill side (**Figure 2**). The latter is not clearly detectable because of local accumulation of material covering the downhill crater rim [31].

#### **Figure 2.**

*Examples of asymmetric craters on Ceres. A1 crater at 245.96° E and 12.57° N and B1 crater at 218.18° E and 9.55° S show a sharp uphill rim and a smooth downhill rim, covered withmass wasting material from the crater flanks. C1 crater at 245.16° E and 12.63° N shows a more elongated shape in the uphill direction than the other craters. A2, B2, C2 show the perspective view of the repective craters.*

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*Special Crater Types on Vesta and Ceres as Revealed by Dawn*

Further similarities to Vesta's asymmetric craters comprise sporadically detected thin layers of ejecta on the uphill rim and mass wasting features on the uphill inner crater wall on most craters. Additionally, a relative straight border in the lower third through the crater, separating the oblique from the shallower crater floor [31]. Asymmetric craters are more or less circular in shape (**Figure 2A**), nevertheless there are craters showing a slightly elongated shape in uphill direction (**Figure 2C**). This crater rim seems to merge with the slope and the downhill rim is less elevated than of the typical asymmetric craters, but with the same ejecta and mass wasting material distribution. The crater floor appears wider than the others. The morphology of these craters indicating a formation on a slope crest [31]. The craters are only visible in high resolution data of the second extended mission of Dawn, and therefore, the study area is limited, although the craters are more or less homogeneous distributed over the study area, spanning around 60° N to 60° S latitude and 197° E to 265° E longitude. Most asymmetric craters are formed on slope angles between 10 and 20 degrees. [31] suggests, that the uphill and downhill material of ejecta were deposited simultaneously, and thus, not influenced by post-emplacement

Furthermore, the shape and the ejecta distribution as well as the formation on slopes are quite similar to those on Vesta. Although, the crater sizes on Ceres are much smaller for asymmetric craters, the topography is suggested to be the main cause for the asymmetries [31]. Vesta's extreme topographic differences with a total relief of ~41 km [2] have caused many craters to be formed on slopes. Moreover, the topographic differences with a total relief of ~15 on Ceres [23] is less distinctive and not as steep as on Vesta. Thus, the low variations of the topography cause lower slope angles and could have limited the crater size formation on the slopes [31].

Ring- mold craters (RMC) are common on lineated and lobated debris aprons, filling valleys, and concentric crater fills on Mars [32–34]. They are interpreted as impacts into ice covered by a thin layer of regolith. Ring-mold craters have diameters between 167 and 697 m and are generally surrounded by a rimless, circular moat. Furthermore, ring-mold craters show a variety of complex interior features. [33] found four morphological types of ring-mold craters: (1) a central pit or bowl;

On Ceres ring-mold craters are located on the southern crater floor of Occator [35]. They predominantly appear on the lobate smooth material; a few craters are found on the terrace material as defined on the geologic maps of [36, 37]. Some ring-mold craters are located on or near the tectonic structure in the southern part of the Occator floor. Ring-mold craters show an almost circular shape seem to be subsiding into the surface, causing less elevated crater rims (**Figure 3**). Numerous ring-mold craters are degraded (**Figure 3**) and contains cracks (**Figure 3D**) or lobate material (**Figure 3E** and **F**). [35] found three classes of ring-mold craters: (1) central pit or bowl craters (**Figure 3A**); (2) central mound craters (**Figure 3B** and **C**); and (3) central plateau craters (**Figure 3B**). They show, that ring-mold craters on Ceres are comparable to those on Mars (e.g., [32, 33, 38]). Both bodies show nearly rimless craters with a circular outer moat and similar interior morphologies, like central pits or bowls, plateaus, and mounds. Moreover, ring-mold craters on both bodies are associated with flow features, lineated valley fills and lobate debris aprons on Mars and lobate materials within Occator on Ceres. The similarities of morphology and location indicate a similar formation process [35]. Although, Martian ring-mold craters with diameters between 697 (mean 225 m; [32]) and 750 m (mean 102 m; [33])

(2) a central plateau; (3) a multiring; and (4) central mound craters.

*DOI: http://dx.doi.org/10.5772/intechopen.96671*

modifications.

**3. Ring-mold craters on Ceres**

#### *Special Crater Types on Vesta and Ceres as Revealed by Dawn DOI: http://dx.doi.org/10.5772/intechopen.96671*

*Solar System Planets and Exoplanets*

tion of material covering the downhill crater rim [31].

Asymmetric craters are also found on Ceres. The analysis of high resolution data reveals craters similar to those on Vesta, however, with diameters from 0.30 to 4.2 km and a mean of 0.98 km the craters are much smaller [31]. The morphology of those craters shows an asymmetric crater interior with an oblique and a shallower side, as well as an asymmetric ejecta distribution. The crater reveals a semi-circular sharp and well-formed rim on the uphill side, as well as a smooth rim on the downhill side (**Figure 2**). The latter is not clearly detectable because of local accumula-

*Examples of asymmetric craters on Ceres. A1 crater at 245.96° E and 12.57° N and B1 crater at 218.18° E and 9.55° S show a sharp uphill rim and a smooth downhill rim, covered withmass wasting material from the crater flanks. C1 crater at 245.16° E and 12.63° N shows a more elongated shape in the uphill direction than the other* 

*craters. A2, B2, C2 show the perspective view of the repective craters.*

**214**

**Figure 2.**

Further similarities to Vesta's asymmetric craters comprise sporadically detected thin layers of ejecta on the uphill rim and mass wasting features on the uphill inner crater wall on most craters. Additionally, a relative straight border in the lower third through the crater, separating the oblique from the shallower crater floor [31]. Asymmetric craters are more or less circular in shape (**Figure 2A**), nevertheless there are craters showing a slightly elongated shape in uphill direction (**Figure 2C**). This crater rim seems to merge with the slope and the downhill rim is less elevated than of the typical asymmetric craters, but with the same ejecta and mass wasting material distribution. The crater floor appears wider than the others. The morphology of these craters indicating a formation on a slope crest [31]. The craters are only visible in high resolution data of the second extended mission of Dawn, and therefore, the study area is limited, although the craters are more or less homogeneous distributed over the study area, spanning around 60° N to 60° S latitude and 197° E to 265° E longitude. Most asymmetric craters are formed on slope angles between 10 and 20 degrees. [31] suggests, that the uphill and downhill material of ejecta were deposited simultaneously, and thus, not influenced by post-emplacement modifications.

Furthermore, the shape and the ejecta distribution as well as the formation on slopes are quite similar to those on Vesta. Although, the crater sizes on Ceres are much smaller for asymmetric craters, the topography is suggested to be the main cause for the asymmetries [31]. Vesta's extreme topographic differences with a total relief of ~41 km [2] have caused many craters to be formed on slopes. Moreover, the topographic differences with a total relief of ~15 on Ceres [23] is less distinctive and not as steep as on Vesta. Thus, the low variations of the topography cause lower slope angles and could have limited the crater size formation on the slopes [31].
