Origin of small pits in martian impact craters

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2012-09, Vol.221 (1), p.262-275
• Main Authors: Boyce, Joseph M., Wilson, Lionel, Mouginis-Mark, Peter J., Hamilton, Christopher W., Tornabene, Livio L.
• Format: Article
• Language: English
• Subjects: Astronomy; Conduits; Cratering; Craters; Degassing; Deposition; Earth, ocean, space; Entrainment; Exact sciences and technology; Geological processes; Impact processes; Mars, Surface; Mathematical models; Pipe; Pits; Solar system; Terrestrial planets
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2012.07.027

► A model for the formation of closely-spaced pits in martian impact craters is proposed. ► The mechanism relies on explosively degassed of steam from the pitted material. ► The high-speed escape of this steam is mainly through numerous vent pipes. ► The escaping steam and entrained clasts erodes the pipes to form pits at the surface. ► The low atmospheric pressure and gravity of Mars enhances the gas escape speed. We propose a numerical model for the formation of the closely-spaced pits found in the thin, ejecta-related deposits superposed on the floors, interior terrace blocks, and near-rim ejecta blankets of well-preserved martian impact craters. Our model predicts the explosive degassing of water from this pitted material, which is assumed to originally be water-bearing, impact melt-rich breccia at the time of deposition. This process is analogous to what occurred in the fall-out suevite deposits at the Ries impact structure in Germany. At Ries, impact heating of water-bearing target material resulted in the rapid degassing of its water and other volatiles. The martian environment plays an important role in enhancing the effects of this degassing by increasing the flow-speed of the escaping gas. The high flow-rate of gas through particulate materials, such as suevite, tends to quickly form segregation channels or vent pipes, similar to those found in the Ries deposits. These pipes act as conduits for the efficient high-speed escape of the 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.