**4. Floor-fractured craters on Ceres**

Several impact craters on Ceres contains sets of fractures on their floors. Typical cerean FFCs reveal an irregular shaped rim, which is mostly deformed by slumping or sliding of the crater rim building wall terraces, and/or a central pit or peak structure [43, 44]. The morphology of most fractures is characterized by an irregular pattern with concentric and/or radial or polygonal shape, other fractures are almost

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**Figure 4.**

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

straight and subparallel to parallel [44]. A common fracture structure are crevices merging into various branches or narrow fissures that conjoint into straight wide fractures. These fractures bifurcate into grand fracture groups or networks which can cover nearly half of the crater floor, this is observed at Dantu and Occator crater [44] (**Figure 4A** and **B**). A special type of floor fractures is found within Yalode crater (**Figure 4C**, **C1**). The fractures appear wider and more developed than in other craters and can be divided into two generations. They show high variation in shape, width and lengths and encompass deformational features such as en echelon structures and possible strike slip faults, dilatational jogs or tilted blocks. Such

The floor-fractures are similar with floor-fractured craters (FFC) of Class 1 and 4 on the Moon (e.g., [43–45]). Depth to diameter ratios show that FFCs on Ceres are anomalously shallow similar to lunar FFCs [43]. Class 1 FFCs on Ceres includes the craters Dantu, Ezinu, Occator, Gaue, Ikapati, Azacca, Haulani, and Kupalo and shows radial and/or concentric fractures on their floors as well as central peaks or

In case of Dantu and Occator, the most prominent FFCs on Ceres, an extensive set of crosscutting fractures occur the base of their southern wall (**Figure 4A2**, **B1**). The orientation of most of these fractures is concentric to the base of the crater wall, whereas the fractures at Occator are more concentrated in the southwest corner. Other fractures are orthogonal to and crosscut the concentric ones at both crater floors [43, 44]. Both craters show fractures radial and concentric to the central peak or dome/pit structure, respectively (**Figure 4A1**, **B2**). On Ceres are several linear fractures identified which are related to faculae suggesting a cryovolcanic formation [29, 44, 46, 47]. At Occator the linear fractures are related to the lobate flow fractures of Vinalia Faculae. Dantu shows more crosscutting fractures than Occator, but there are more fractures associated with the central structure [43]. Furthermore,

*Examples of FFCs on Ceres. (A) Occator crater and (B) Dantu crater. A1 and B2 shows radial and concentric fractures around the central dome/pit structure. A2 and B1 shows crosscutting fractures at the base of the southern wall, respectively. (C) Yalode crater. C1 shows wider and more developed fractures which can be divided into two generations. (D) Azacca crater. D1 shows curvilinear fractures near the central peak.*

morphologic variations suggest different formation mechanism [44].

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

pits, similar to Class 1 FFCs on the Moon [43].

## *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*

are smaller than ring-mold craters on Ceres with diameters from ~280 to ~1,520 m with a mean of ~710 m [35]. Ponded material and lobate materials within Occator are supposed to be formed by impact melt or cryovolcanic flows (e.g., [39–41]). Furthermore, thermal modeling and gravity data suggest an extensive deep brine reservoir beneath Occator, which might have been mobilized by the heating and deep fracturing related to the Occator impact. Thus, this process would lead to a long-lived extrusion of brines and the formation of the faculae [15]. Additionally, pre-existing tectonic cracks may provide hints for deep brines migrating and dilatat-

*Ring-mold craters on Ceres. (A) shows a central pit/bowl crater. (B) shows a central plateau (bottom) and a central mound crater (top). (C) shows two central mound craters. (D) shows a ring-mold crater degraded and* 

Due to the occurrence of frozen oceanic materials rich in sodium carbonate and ammonium chloride at several locations, [26] suggest that oceanic material is frozen in the first 10s of kilometers and possibly shallower. This hypothesis is verified by [35] using the depth to diameter ratio from [42] to estimate the minimum regolith thickness on the basis of bowl-shaped craters sizes which are adjacent to ring-mold craters. The results also conclude an overlying area thickness of several tens of meters suggesting that the smaller bowl-shaped crater do not penetrate to the ice layer. Thus, subsurface ice at Occator is at a relatively shallow depth, below a thin protective layer of regolith, and those impacts hitting the subsurface ice layer form

Several impact craters on Ceres contains sets of fractures on their floors. Typical cerean FFCs reveal an irregular shaped rim, which is mostly deformed by slumping or sliding of the crater rim building wall terraces, and/or a central pit or peak structure [43, 44]. The morphology of most fractures is characterized by an irregular pattern with concentric and/or radial or polygonal shape, other fractures are almost

ing the crust creating a compositional heterogeneity [15].

*deformed by cracks. (E) and (F) show craters affected by flow features.*

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ring-mold craters [35].

**Figure 3.**

**4. Floor-fractured craters on Ceres**

straight and subparallel to parallel [44]. A common fracture structure are crevices merging into various branches or narrow fissures that conjoint into straight wide fractures. These fractures bifurcate into grand fracture groups or networks which can cover nearly half of the crater floor, this is observed at Dantu and Occator crater [44] (**Figure 4A** and **B**). A special type of floor fractures is found within Yalode crater (**Figure 4C**, **C1**). The fractures appear wider and more developed than in other craters and can be divided into two generations. They show high variation in shape, width and lengths and encompass deformational features such as en echelon structures and possible strike slip faults, dilatational jogs or tilted blocks. Such morphologic variations suggest different formation mechanism [44].

The floor-fractures are similar with floor-fractured craters (FFC) of Class 1 and 4 on the Moon (e.g., [43–45]). Depth to diameter ratios show that FFCs on Ceres are anomalously shallow similar to lunar FFCs [43]. Class 1 FFCs on Ceres includes the craters Dantu, Ezinu, Occator, Gaue, Ikapati, Azacca, Haulani, and Kupalo and shows radial and/or concentric fractures on their floors as well as central peaks or pits, similar to Class 1 FFCs on the Moon [43].

In case of Dantu and Occator, the most prominent FFCs on Ceres, an extensive set of crosscutting fractures occur the base of their southern wall (**Figure 4A2**, **B1**). The orientation of most of these fractures is concentric to the base of the crater wall, whereas the fractures at Occator are more concentrated in the southwest corner. Other fractures are orthogonal to and crosscut the concentric ones at both crater floors [43, 44]. Both craters show fractures radial and concentric to the central peak or dome/pit structure, respectively (**Figure 4A1**, **B2**). On Ceres are several linear fractures identified which are related to faculae suggesting a cryovolcanic formation [29, 44, 46, 47]. At Occator the linear fractures are related to the lobate flow fractures of Vinalia Faculae. Dantu shows more crosscutting fractures than Occator, but there are more fractures associated with the central structure [43]. Furthermore,

#### **Figure 4.**

*Examples of FFCs on Ceres. (A) Occator crater and (B) Dantu crater. A1 and B2 shows radial and concentric fractures around the central dome/pit structure. A2 and B1 shows crosscutting fractures at the base of the southern wall, respectively. (C) Yalode crater. C1 shows wider and more developed fractures which can be divided into two generations. (D) Azacca crater. D1 shows curvilinear fractures near the central peak.*

in some regions of Occator the space between fractures contains blocky fragments that seem to limit the tear faults. Other fractures are cut by slides [44]. Dantu also shows pitted fractures indicating the influence of volatile components in the subsurface [44].

Other FFCs on Ceres shows a roughly north–south trending set of curvilinear fractures and a smaller set of conjugate fractures associated with the north south fractures near the central peak in the case of Azacca (**Figure 4D**, **D1**), or contains fractures at the base of the northwestern crater wall as well as fractures orthogonal and parallel to the central crater structure in the case of Ikapati [43, 44]. Class 4 FFCs, however, are characterized by having v-shaped moats and usually hummocky floors which are more shallow than other cerean craters of their diameter. The fractures of the crater floors are less distinct. Furthermore, the craters are smaller than Class 1 FFCs [43]. Large FFCs on Ceres with diameters >50 km shows the most similarities with Class 1 lunar FFCs, while smaller FFCs are more consistent with Class 4 lunar FFCs. These results imply a similar formation of fractures due to the intrusion of a low-density material below the craters [43].

The formation of floor fractured craters (FFCs) is mainly suggested by cryomagmatic intrusion [43, 44], at which the cryomagmatic intrusion must have been trapped vertically and horizontally by weak material below the crater. Underlying reservoirs can feed such an intrusion and forming domes which uplifting and fracturing the overlying, brittle crater floor [43, 44]. Additionally, length, width and strike of the fractures vary for each crater and suggest independent formation mechanisms and imply different surface and subsurface materials. [44] also propose the following formation processes for FFCs: (1) tear-off edges in case of slumping of the crater wall, (2) cooling melting processes that lead to sinkage of the crater floor, (3) degassing, and/or (4) tectonic interactions. All four mechanism also comprises up-doming of material beneath the craters.
