(This is a sample cover image for this issue. The actual cover is not yet available at this time.) This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Icarus 221 (2012) 262–275 Contents lists available at SciVerse ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus Origin of small pits in martian impact craters ⇑ Joseph M. Boyce a, , Lionel Wilson a,b, Peter J. Mouginis-Mark a, Christopher W. Hamilton c, Livio L. Tornabene d a Hawaii Institute for Geophysics and Planetology, University of Hawaii, Honolulu, HI 96822, USA b Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK c NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA d Center for Planetary Science and Exploration, University of Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B7 article info abstract Article history: We propose a numerical model for the formation of the closely-spaced pits found in the thin, ejecta- Received 26 January 2012 related deposits superposed on the floors, interior terrace blocks, and near-rim ejecta blankets of well- Revised 6 July 2012 preserved martian impact craters. Our model predicts the explosive degassing of water from this pitted Accepted 25 July 2012 material, which is assumed to originally be water-bearing, impact melt-rich breccia at the time of depo- Available online 11 August 2012 sition. This process is analogous to what occurred in the fall-out suevite deposits at the Ries impact struc- ture in Germany. At Ries, impact heating of water-bearing target material resulted in the rapid degassing Keywords: of its water and other volatiles. The martian environment plays an important role in enhancing the effects Mars, Surface of this degassing by increasing the flow-speed of the escaping gas. The high flow-rate of gas through par- Cratering Impact processes ticulate materials, such as suevite, tends to quickly form segregation channels or vent pipes, similar to Geological processes those found in the Ries deposits. These pipes act as conduits for the efficient high-speed escape of the Terrestrial planets gas and small clasts that it entrains. Escaping gas and entrained clasts abraded and eroded the conduit walls, flaring them to form pits above a network of pipes. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction et al., 2007, 2012; Mouginis-Mark and Garbeil, 2007; Morris et al., 2010). The closely-spaced pits have been attributed to subli- Impact craters provide considerable information about the mation (Hartmann et al., 2010), or to undermining and collapse composition and structure of target materials, physical properties (McEwen et al., 2007; Tornabene et al., 2007; Mouginis-Mark and of the impactors, and impact cratering processes. For example, on Garbeil, 2007) caused by removal of water that had seeped or de- Mars, impact structures show evidence of the effects of substantial fused into these deposits following their deposition. This material water in the target material (e.g., Carr et al., 1977; Mouginis-Mark, is found in impact craters that are widely distributed over the sur- 1979). Here, we focus on a recently-identified impact ejecta and face of Mars. Consequently, if the pits require a substantial amount crater-fill facies first discovered by Mouginis-Mark et al. (2003) of water to form, then their occurrence has important implications in Mars Orbiter Camera (MOC) images. These deposits were later for the history of volatiles on Mars (Tornabene et al., 2012). mapped globally by Tornabene et al. (2012). This facies generally Here we present a numerical model of pit formation in this ejec- occurs as thin, heavily pitted deposits in relatively well-preserved ta and crater-fill facies (herein called martian pitted material) that martian impact craters (Fig. 1). The closely-spaced pits found in involves explosive degassing of the water initially contained in the these deposits are morphologically distinct from the larger, single target material. This model is summarized in Fig. 2, which is in- central pit features studied by Wood et al. (1978), Hale (1982), tended to serve as a guide to the reader in the presentation of and Barlow (2010a). In the parent craters that contain both these our model. The model is inspired by evidence that hot materials thin ejecta facies and large single central pits, the thin ejecta facies in the Ries fall-out suevite degassed immediately after deposition appears to overlie the large pits. This thin ejecta facies is thought to (Pohl et al., 1977; Newsom et al., 1986), resulting in fluidization be composed of impact-melt rich breccia impactite similar to sue- of the deposits, elutriation or separation of fine particles and for- vite at the Ries impact structure on Earth, and may have initially mation of vent pipes that carried gas from the deposit. contained substantial water (McEwen et al., 2007; Tornabene 2. Background ⇑ Corresponding author. Fax: +1 808 956 6322. E-mail addresses: [email protected], [email protected] (J.M. Boyce), [email protected] (L. Wilson), [email protected] (C.W. Hamil- The martian pitted material was first recognized in MOC images ton), [email protected] (L.L. Tornabene). by Mouginis-Mark et al. (2003) at a resolution of 3–6 m/pixel. As 0019-1035/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.icarus.2012.07.027 Author's personal copy J.M. Boyce et al. / Icarus 221 (2012) 262–275 263 Nomenclature a average radius of spherical clasts q number of tubes cutting vertically through area A A surface area of deposit through which gas escapes Q universal gas constant Aa average fractional area of void space R radius of spherical space at the center of a cube of side CD drag coefficient length (2L) E kinetic energy r typical radius of gap or tubes between the commonest f friction factor for gas flow past particles from the wall of clast in the layer the tube rmax radius of largest spherical clast resisting elutriation by Fi mass per unit time passing through any one tube gas at speed Ug Fm gas mass release rate per unit mass of suevite per unit Ta temperature of the gas time Ug flow speed of steam through a pathway at height z Fv gas generation rate above base of layer g acceleration due to gravity v void space fraction L half side length of cube used in void space calculation Z depth below the surface À1 m molecular mass of H2O, 18.0153 kg kmol z height above base of pitted material layer nw water mass fraction in the suevite deposit gg viscosity of the gas P gas pressure at a given depth j thermal diffusivity Pa Mars atmospheric pressure at the surface qg density of the gas P0 pressure at base of suevite layer at depth Z below the qr typical silicate clast density surface High Resolution Science Imaging Experiment (HiRISE) images paper we show that all of these models are inconsistent with many (25 cm/pixel) became available, workers such as McEwen et al. characteristics of the pits and pitted material. (2007), Tornabene et al. (2007, 2012), Mouginis-Mark and Garbeil McEwen et al. (2007) and Tornabene et al. (2012) listed three (2007), and Morris et al. (2010) studied this material, and con- reasons why the pitted material is impact melt-rich breccia, similar cluded that its morphology and stratigraphic position indicated to the suevite at the Ries impact structure. These reasons include that it is a facies of impact ejecta, probably impact melt-rich brec- (1) the apparent fluid flow of the material unit during emplace- cia (similar to suevite at Ries crater, Germany). They proposed that ment of the host deposit, (2) the presence of blocks embedded in the pits resulted from collapse of voids left in the pitted material by pits walls, and (3) the pitted material’s uppermost stratigraphic escape of water from pockets, or of ice from lenses, at a time well position relative to other impactite deposits. Tornabene et al. after deposition of this material. In contrast, Hartmann et al. (2010) (2012) suggested that the likelihood that the pitted material is im- suggested that the pits are produced by sublimation of ice distrib- pact melt-rich breccia implies the presence of water in the target uted more evenly throughout these deposits. However, in this materials. They based this suggestion on experimental, theoretical Fig. 1. 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. (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. Author's personal copy 264 J.M. Boyce et al. / Icarus 221 (2012) 262–275 Fig. 2. Conceptual model for the formation of pitted materials within impact structures on Mars. (a) An impactor strikes the martian surface, which includes H2O distributed in the regolith and cryosphere. (b) Ejecta from the transient crater generate an impact melt sheet and ejecta plume.