Definition

Closely spaced pits found in the thin, ejecta-related deposits of well-preserved Martian and Vesta impact craters.

Description

Bowl-shaped depressions that range in size from a few meters to more than ∼3 km in diameter, but most pits are < ∼200 m in diameter. These pits occur on the floors, terraces, and exterior ejecta deposits of fresh craters (Tornabene et al. 2012; Fig. 1). The pits commonly occur in closely spaced groups or clusters whose individual pits are so close together that their rims share straight segments situated between individual pits (Fig. 2). The tendency for the pits in these closely spaced groups to be nearly circular and to share straight rim segments produces a geometry that resembles a 2-D cross section through soap froth or a honeycomb. There are isolated pits that do not exhibit shared rims, but in general pits occur within closely packed groups with rims that do not appear to be raised above the surrounding terrain. In contrast, isolated pits commonly have relatively low, raised rims.

Clusters of Small Closely Spaced Pits in Ejecta-Related Deposits, Fig. 1
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Examples of pitted material at Tooting crater, a 27 km diameter fresh Martian impact crater. (a) Location image for other sub-scenes in this figure. CTX image P01_001538_2035. (NASA/JPL-Caltech/MSSS) (b) Pits on the floor, segment of HiRISE image PSP_002158_2035. (c) Pits on the exterior rim, segment of HiRISE image PSP_002580_2035. (d) Pits on a terrace block, segment of HiRISE PSP_003569_2035 (NASA/JPL/University of Arizona)

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Clusters of Small Closely Spaced Pits in Ejecta-Related Deposits, Fig. 2
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Closely spaced pits in the floor materials of Marcia crater on Vesta. Image is from a high-resolution (∼18 m/pixel) mosaic of low-altitude mapping orbit of the Dawn mission (NASA/JPL-Caltech/UCLA/Max Planck Society/DLR/Institut für Datentechnik und Kommunikationsnetze)

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Interpretation

Indicates devolatilization of water from thin ejecta-related deposits.

Formation

A type of dewatering feature. Violent degassing of water from hot water-rich, impact melt-rich breccia through a process similar to degassing from wet pyroclastic deposits and the Ries suevite (Boyce et al. 2012) or sublimation of water/ice trapped in the pitted material after it was deposited (McEwen et al. 2007; Tornabene et al. 2007, 2012; Mouginis-Mark and Garbeil 2007; Morris et al. 2010).

Age

Found in craters of a wide variety of ages (Tornabene et al. 2012).

Surface/Structural Units

Impact ejecta and crater-fill facies that generally occur as thin heavily pitted deposits in relatively well-preserved Martian impact craters.

Prominent Examples

Tooting crater on Mars and Marcia crater on Vesta.

Distribution

Globally across Mars and a few fresh craters on Vesta.

Regional Variations

These types of pits are commonly found in most fresh single and multilayer ejecta craters on Mars. However, they are not as common in fresh double-layer ejecta craters on Mars or in fresh craters on Vesta.

Significance

The presence of these features suggests that abundant water is included in the target materials prior to crater formation.

Astrobiological Significance

The inference that abundant water in the target materials is required to form these pits suggests that the water needed for life is present in the target rock of Mars and Vesta before crater formation. Water may have come to the surface with low-velocity impacts of carbonaceous chondritic icy impactors (Young 2001).

Terrestrial Analog

Suevite deposits produced by the Ries impact structure in Germany may have eroded examples of these features.

History of Investigation

The Martian pitted material was first recognized in Mars Orbiter Camera high-resolution images by Mouginis-Mark et al. (2003). As high-resolution images became available, workers such as McEwen et al. (2007), Tornabene et al. (2007, 2012), Mouginis-Mark and Garbeil (2007), and Morris et al. (2010) studied this material and concluded that its morphology and stratigraphic position indicated that it is a facies of impact ejecta, probably impact melt-rich breccia (similar to suevite at Ries crater, Germany). These investigators proposed that the pits resulted from the collapse of voids left in the pitted material by escape of water from pockets, or of ice from lenses, at a time well after deposition of this material. In contrast, Hartmann et al. (2010) suggested that the pits are produced by sublimation of ice distributed more evenly throughout these deposits. However, Boyce et al. (2012) suggest that all of these models are inconsistent with many characteristics of the pits and pitted material and proposed that the pits are degassing features like those at the Ries impact structure and in some pyroclastic deposits. Identical features are found in thin deposits on some fresh impact craters on the asteroid Vesta (Denevi et al. 2012).

See Also

Ablation Hollow (Mars)

Pseudocrater