Ring-Mold Crater

  • Gro Birkefeldt Møller PedersenEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-1-4614-9213-9_318-1

Keywords

Central Plateau Habitable Zone Crater Diameter Strength Mismatch Crater Morphology 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Definition

Concentric crater, shaped like a truncated torus similar to the kitchen item (Kress and Head 2008, 2009).

Description

Ring-mold craters (RMCs) are rimless, concentric crater landforms with a circular outer moat and a variety of interior morphologies including central plateaus, pits, bowls, mounds, or multiple rings (Fig. 2), and they are named for their similarity to the cooking implement called ring mold (Kress and Head 2008). Ring-mold craters are best observed on images with a spatial resolution of 10 m/pixel or better (Pedersen and Head 2010).

Subtypes

Central plateau, central pit or bowl, multi-ring, and central mound (Fig. 2, Kress and Head 2008).

Morphometry

Ring-mold craters are small-scale craters with diameters ranging from ca. 50 m to <1 km with observed average diameter between 100 and 200 m. Ring-mold craters are typically larger than the associated bowl-shaped craters, which also are found in lineated valley fill, lobate debris aprons, and concentric crater fill (Baker et al. 2010; Kress and Head 2008; Ostrach et al. 2008).

Interpretation

Ring-mold craters have been interpreted to be a result of impact into regolith-covered glacial ice and are found in lineated valley fill, lobate debris aprons, and concentric crater fill on Mars (Fig. 1, Kress and Head 2008).
Fig. 1

Comparison of (left) a bowl-shaped crater and (right) a ring-mold crater on lobate debris apron/lineated valley fill in the Mamers Valles region. Bowl-shaped, smaller craters are interpreted not to penetrate the lag deposit layer. Bottom, diagrammatic cross-sectional model: (1) bowl-shaped crater, (2) ring-mold crater, (3) sublimation till, (4) glacial ice (Kress and Head 2008). Scale bar 200 m. MOC image E0502619 (NASA/JPL/MSSS)

Formation

Ring-mold craters are interpreted to be primary crater landforms forming as a response to impacts into an ice-rich substrate covered by an ice-poor debris till. They differ from simple bowl-shaped craters formed in silicate, regolith, and ice-cemented regolith because of the layered nature of the target and the brittle nature of the subsurface ice having a low melting point. The annular moat forms due to spallation, which thereby enlarge the crater diameter, while potential viscous flow and melting may be responsible for the interior ring-mold crater morphology. Likewise, the strength contrast in the debris-ice layered target causes shock impedance and strength mismatches also contributing to variation in interior ring-mold crater morphology. Freshly exposed ice sublimates and removes ejecta, crater rims and smoothen major features (Kress and Head 2008) (Fig. 2).
Fig. 2

Subtypes of ring-mold craters. Illumination is from the left. (a) Central pit or bowl. Scale bar 300 m. MOC R2300027 at 36.2N, 15.4E. (b) Central plateau. Scale bar 200 m. MOC E0502619 at 37.0 N, 17.5E. (c) Multi-ring. Scale bar 200 m. MOC E2300343 at 32N 18E. (d) Bowl with central mound. Scale bar 200 m (Kress and Head 2008). Modified from MOC E0502619 (NASA/JPL/MSSS)

Fig. 3

Intepreted degradational sequence for ring-mold crater (RMC) morphologies. The sun illumination is from the lower left in the image (CTX image P03_002068_2173). (a–c) Pitted deposits with characteristic RMC morphologies highlighted in boxes and enlarged to the right (numbered 1–3). Both RMCs 1 and 2 are interpreted to be in degradational stages of RMC 3. (d) Diagram illustrating the degradational sequence of the RMC 3. As deflation and sublimation of the ice-rich deposit progress (marked with black wavy arrows), the depression around RMC 3 grows, leaving a circular plateau, producing a degraded RMC like RMC 2. Further degradation of plateau sides would create a concentric pattern displaying a multi-ringed plateau, as RMC 1 (Pedersen and Head 2010; NASA/JPL/MSSS)

This formation model is based on the morphologic resemblance between ring-mold craters and resulting impact morphologies from experiments with projectiles into ice (Kato et al. 1995). The model explains why the distribution of ring-mold craters is limited to suggested ice-rich deposits such as lineated valley fill, lobate debris aprons, and concentric crater fill. Furthermore, the model explains the association with smaller bowl-shaped craters, which form when the impact energies are insufficient to excavate through the sublimation till into the underlying ice (Fig. 1, Kress and Head 2008; Ostrach et al. 2008).
Fig. 4

Crater described as “oyster shell crater” (Mangold 2003; Fig7e). Scale bar 100 m. MOC SP2-50006 (NASA/JPL/MSSS)

Degradation

Ring-mold craters can be modified either due to deformation caused by viscous flow in sloped terrain (Orgel 2010) (Fig. 5) or by degradation of the ice-rich deposit they are formed within (Baker et al. 2010; Pedersen and Head 2010). Thus, deflation of lineated valley fill, lobate debris aprons and concentric crater fill would initially increase the outer moat of the ring-mold craters leaving a circular plateau (Fig. 3 and Baker et al. 2010; Pedersen and Head 2010). Further degradation of the sides of these circular plateaus would cause a concentric pattern displaying multi-ringed plateaus. Consequently, this degradational sequence of ring-mold craters can be used for recognizing highly altered ice-rich deposits and evaluating their degree of degradation (Pedersen and Head 2010).
Fig. 5

Ring-mold crater deformed by viscous flow (Orgel 2010). Scale bar 140 m. HiRISE PSP_008834_1405 (NASA/JPL/University of Arizona)

Composition

Ring-mold craters form in ice-rich substrate covered by an ice-poor debris till (Fig. 1).

Studied Locations

Deuteronilus Mensa, Mamers Valles, Ismenius Lacus, Protonilus Mensae, Utopia Planitia, Mars.

Distribution

Found in lobate debris aprons, lineated valley fill, and concentric crater fill, which approximately are distributed in two 25° latitudinal bands centered around 40°N and 45°S (Squyres 1979; Squyres and Carr 1986).

Regional Variations

The density of ring-mold craters and associated degraded ring-mold craters varies regionally and has been interpreted to reflect different degrees of surface alteration and resurfacing of the ice-rich deposits (Baker et al. 2010; Pedersen and Head 2010).

Significance

The observation of ring-mold craters is very important because ring-mold craters can be used to detect debris-covered glacial ice and to estimate the depth to the ice table by calculating the maximum depth of the associated bowl-shaped craters (Kress and Head 2008; Baker et al. 2010; Levy et al. 2010).

If the ring-mold craters are deformed, they potentially indicate viscous flow of the ice-rich deposit after the impact of the ring-mold craters (Orgel 2010), while degraded ring-mold craters can be used for recognition and evaluation of alteration stages of ice-rich material (Pedersen and Head 2010).

Finally, the formation of ring-mold craters influences the use of crater size-frequency distributions because impacts into ice will form larger crater diameter than in, e.g., basalt, thus yielding artificially old ages (Kress and Head 2008, 2009).

Astrobiological Significance

Observations of ring-mold craters indicate buried water ice and is a climate indicator providing information on the water reservoir of Mars. Therefore, ring-mold craters can be important for mapping of potential habitable zones on Mars.

History of Investigation

Ring-mold craters were first described as oyster shell craters (Fig. 4) by Mangold (2003) who attributed the interior morphology to degradation of regular bowl-shaped craters through enhanced sublimation of ice. (McConnell et al. 2006; McConnell and Newsom 2007) also related their morphology to the presence of ice suggesting that impact into icy material would be modified by subsequent ice flow and sublimation resulting in inverted crater morphology explaining the central mounds in the interior of the ring-mold craters. Instead of associating ring-mold craters with a sublimation-controlled degradation of bowl-shaped craters, Kress and Head (2008) suggested that the morphology of ring-mold craters rather is a primary feature of impacts into relatively pure ice-rich substrate due to their resemblance with laboratory-generated impacts into ice (Kato et al. 1995).

Origin of Term

Ring-mold craters are named for their similarity to the cooking implement called ring mold (Kress and Head 2008).

See Also

References

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Copyright information

© Springer Science+Business Media New York 2014

Authors and Affiliations

  1. 1.Nordic Volcanological centerUniversity of IcelandReykjavikIceland