Impacts onto H 2 O ice: Scaling laws for melting, vaporization, excavation, and final crater size Richard G. Kraus ⇑ , Laurel E. Senft, Sarah T. Stewart Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, USA article info Article history: Received 25 May 2010 Revised 9 May 2011 Accepted 15 May 2011 Available online 24 May 2011 Keywords: Impact processes Ices Planetary formation Cratering abstract Shock-induced melting and vaporization of H 2 O ice during planetary impact events are widespread phe- nomena. Here, we investigate the mass of shock-produced liquid water remaining within impact craters for the wide range of impact conditions and target properties encountered in the Solar System. Using the CTH shock physics code and the new 5-phase model equation of state for H 2 O, we calculate the shock pressure field generated by an impact and fit scaling laws for melting and vaporization as a function of projectile mass, impact velocity, impact angle, initial temperature, and porosity. Melt production nearly scales with impact energy, and natural variations in impact parameters result in only a factor of two change in the predicted mass of melt. A fit to the p-scaling law for the transient cavity and tran- sient-to-final crater diameter scaling are determined from recent simulations of the entire cratering pro- cess in ice. Combining melt production with p-scaling and the modified Maxwell Z-model for excavation, less than half of the melt is ejected during formation of the transient crater. For impact energies less than about 2  10 20 J and impact velocities less than about 5 km s 1 , the remaining melt lines the final crater floor. However, for larger impact energies and higher impact velocities, the phenomenon of discontinu- ous excavation in H 2 O ice concentrates the impact melt into a small plug in the center of the crater floor. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Impact craters are the most common geologic feature on plan- etary surfaces. Bolides impacting at typical velocities of a few to several 10’s of km s 1 (Zahnle et al., 2003) achieve shock pressures capable of melting and vaporizing H 2 O ice (Stewart et al., 2008). In some cases, impact-generated crater lakes may persist for geologically interesting timescales (Thompson and Sagan, 1992; Artemieva and Lunine, 2005; O’Brien et al., 2005). Giant impact events, which dominated the late stages of planetary accretion, may generate oceans of melt (Tonks and Melosh, 1993). Finally, an intense period of impact events could also lead to differentia- tion of a planet or satellite (e.g., Tonks and Melosh, 1992; Tonks, W.B., Pierazzo, E., Melosh, H.J., unpublished manuscript, 1993; Monteux et al., 2009; Barr and Canup, 2010). Impact events have also shaped planetary atmospheres. The production of vapor during accretionary impacts contributes shock-released volatiles to the growth of terrestrial atmospheres (e.g., Benlow and Meadows, 1977; Lange and Ahrens, 1982). How- ever, when the mass of vaporized material becomes sufficiently large, rapid expansion may lead to partial loss of a pre-existing atmosphere (e.g., Melosh and Vickery, 1989; Ahrens, 1993; Shuvalov, 2009). At present, it is difficult to predict the mass of melted and vaporized material associated with a particular size impact crater on ice-rich surfaces. Laboratory scale experiments do not produce significant amounts of melt, and unlike cratering on rocky planets, ground truth data does not exist for planetary scale craters in ice. Observations of crater melt sheets on the icy bodies of the outer Solar System are limited (e.g., Schenk and Turtle, 2009) and com- plicated by the negative buoyancy of liquid water over ice. Recent developments in shock physics models of the equation of state (EOS) and rheology of H 2 O ice have led to simulations of full crater formation that reproduce much of the diversity of mor- phological features observed on icy bodies (Senft and Stewart, 2008, 2011). In particular, the inclusion of high-pressure solid polymorphs in the EOS leads to an unusual phenomenon during crater formation in ice called discontinuous excavation (Senft and Stewart, 2011). Discontinuous excavation causes a concentra- tion of impact melt in a small plug in the crater floor of similar dimensions to observed central pit features on the largest icy satellites. In this work, we present calculations of shock-induced melting and vaporization of H 2 O ice for the range of impact and target con- ditions found in the Solar System. Our parameter space spans low velocity accretionary impacts to the highest velocity cometary 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.05.016 ⇑ Corresponding author. E-mail addresses: rkraus@fas.harvard.edu (R.G. Kraus), laurelsenft@gmail.com (L.E. Senft), sstewart@eps.harvard.edu (S.T. Stewart). Icarus 214 (2011) 724–738 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus