Earth and Planetary Science Letters 385 (2014) 68–78

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Earth and Planetary Science Letters

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The effects of water vaporization on rock fragmentation during rapiddecompression: Implications for the formation of fluidized ejectaon Mars

Audrey Hughes Rager a,b, Eugene I. Smith b,∗, Bettina Scheu c, Donald B. Dingwell c

a Materials Engineering and Research Lab (MERL), U.S. Bureau of Reclamation, Denver Federal Center, Bldg. 56, Room 1400, Denver, CO 80225, United Statesb Department of Geoscience, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154-4010, United Statesc Geo- und Umweltwissenschaften, Ludwig-Maximilians-Universität München, Theresienstrasse 41/III, 80333 München, Germany

a r t i c l e i n f o a b s t r a c t

Article history:Received 3 January 2013Received in revised form 15 October 2013Accepted 20 October 2013Available online 8 November 2013Editor: C. Sotin

Keywords:Martian rampart craterrock fragmentationshock-tube experimentsejecta fluidizationrapid decompressiongrain size analysis

Crater and ejecta morphology provide insight into the composition and structure of the target material.Fluidized ejecta surrounding Martian rampart craters are thought to result from the addition of waterto the ejecta during impact into a water-rich (ice or liquid) regolith. Here we test experimentally analternate hypothesis. We propose that the decompression of a rock–water mixture across the watervaporization curve during the excavation stage of impact cratering results in an increased proportionof fines in the ejecta. This enables the ejecta to flow with little or no liquid water present. To test thishypothesis, fragmentation experiments were conducted on sandstone (28 vol% open porosity) from thenorthern Eldorado Mountains, Nevada, using a shock-tube apparatus at the LMU Munich, Germany. Rocksamples with 0–92% of their open pore space filled with water were pressurized to 15 MPa at 177 ◦Cor 300 ◦C and rapidly decompressed. As the water vaporization curve is crossed, the water in the porespace rapidly flashes to steam causing, together with the expanding gas in the water-free pore space, thesample to fragment. The presence of water has a significant effect on the grain size distribution and grainshape of the fragmented rock samples. In comparison with (dry) control samples, samples with waterwith 15–50% open pore space exhibit much smaller grain sizes. The predominant grain shape of dry aswell as partially water-saturated samples is bladed, reflecting fracturing parallel to the decompressionfront. Samples with > 80% water in open pore space had an increase in fines and larger particles butless intermediate sized particles. Fragments from experiments with > 80% water in open pore spacealso displayed a more equant grain shape, indicating that the decompression of water caused fracturingindependent of the orientation of the decompression front. These results may provide insight into themorphology of Martian rampart craters. We propose here that even relatively low water contents in thetarget (∼16%) may be sufficient to produce a significant increased proportion of fines allowing the ejectato flow with little or no water present.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Impact crater and ejecta morphology reveal information aboutthe structure and composition of the target. This is a topic ofspecial interest on Mars where the presence of fluidized ejectamay provide clues to the history of water on the Martian surface.Knowledge about Martian rampart crater morphology and distri-bution is accumulating and, to date, two competing models havebeen generated to describe their attributes.

Imagery from the Viking Orbiter cameras, Mars Global Surveyor(MGS), Mars Orbital Camera (MOC), Mars Odyssey Thermal Emis-sion Imaging System (THEMIS), and Mars Reconnaissance Orbiter

* Corresponding author.E-mail address: gene.smith@unlv.edu (E.I. Smith).

0012-821X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.epsl.2013.10.029

(MRO) have revealed that Martian impact craters are very differentfrom the radial ejecta on the Moon and Mercury (Barlow, 2005;Carr et al., 1977). Most Martian impact craters have fluidized ejecta(89% of 10 651 cataloged craters � 5 km diameter) that hug to-pography and terminate in a distal rampart about 1.5 to 2 craterradii from the crater rim (Garvin et al., 2000, 2003; Melosh, 1989;Melosh et al., 1992; Schultz, 1992a; Schultz and Singer, 1980).Barlow (2005) classified three types of fluidized ejecta (Fig. 1 top):(1) single layer ejecta (SLE), (2) double layer ejecta (DLE), and (3)multiple layer ejecta (MLE).

The presence of secondary craters may be primarily controlledby the overall size distribution within the ejecta: a finer grainedejecta may not produce secondary craters (Schultz and Singer,1980). In the case of many rampart craters, secondary cratersare rarely observed within the fluidized ejecta blanket (Barlow,

A.H. Rager et al. / Earth and Planetary Science Letters 385 (2014) 68–78 69

2003, 2005). Later arriving ejecta deposits may have buried anysecondary craters or they may be absent if the ejecta are com-posed mainly of a fine-grained component (Schultz, 1992a; Schultzand Singer, 1980). Beyond the rampart, secondary craters extendmany crater radii beyond the edge of the blanket (Barlow, 2005).McEwen et al. (2003) for example identified a 10-km-diametercrater in the Cerberus region of Mars that has strings of sec-ondary craters extending more than 800 km from the rim butno secondary craters within the fluidized ejecta. These extensivesecondary crater strings outside of the fluidized ejecta provideimportant constraints on the cohesiveness and grain size distri-bution of the target material (Head et al., 2002; Schultz, 1992a;Schultz and Singer, 1980) and any model of rampart crater forma-tion must account for this distinctive feature.

Fluidized ejecta morphologies do not appear to correlate withelevation or terrain age (Barlow, 2005; Barlow and Bradley, 1990;Costard, 1989; Mouginis-Mark, 1979; Schultz, 1992a) but there ap-pears to be a correlation with geologic setting (Schultz, 1992a).Layered ejecta morphologies do exhibit a strong relationship withcrater diameter and geographic location (Fig. 1 bottom; Barlow,2005). In the Martian equatorial region (±30◦ latitude), SLE cratersare generally ∼5 to 20 km in diameter. However, at higher lat-itudes, SLE craters are < 1–65 km in diameter (Barlow, 2005;Barlow and Bradley, 1990; Costard, 1989; Kuzmin et al., 1988;Mouginis-Mark, 1979). DLE craters are concentrated at higherlatitudes, particularly in the northern plains. DLE are typically5–25 km in diameter. MLE craters are usually in the 20–45 kmdiameter range. MLE tend to be concentrated in lower latitudes.

According to Barlow (2005), the relative role of subsurfacevolatiles versus that of the atmosphere in the formation of layeredejecta blankets is one of the major remaining questions concerningthe geology of Mars. Barlow (2005) suggests that fluidized ejectaare produced by some combination of the atmospheric and subsur-face volatiles models. Here we build upon Schultz’s (1992a) ideathat ejecta can flow without water if the particles are fine enough.We test the hypothesis that the vaporization of water during theexcavation stage of impact cratering is the mechanism that de-creases the size of ejecta and facilitates its fluidized emplacement.As smaller particles flow more easily than larger blocks, less wa-ter might be required to produce fluidized ejecta than currentlyis thought. To test this hypothesis, we conducted rock fragmenta-tion experiments using the shock-tube apparatus at the Earth andEnvironmental Science Department of LMU Munich (Alidibirov andDingwell, 1996a, 1996b; Scheu et al., 2008, 2006; Spieler et al.,2004). We propose such an interaction between water and rock,during decompression, as describing a mechanistic bridge betweenthe atmospheric and subsurface volatile models (described in Sec-tion 1.1).

1.1. Models for fluidized ejecta formation

There are two models for the formation of fluidized ejecta onMars (Barlow, 2005; Boyce et al., 2010): the atmospheric model(Barnouin-Jha and Schultz, 1988; Barnouin-Jha et al., 1999a, 1999b;Schultz, 1992a, 1992b; Schultz and Gault, 1979a, 1979b) and thesubsurface volatile model (Baratoux et al., 2002a, 2002b; Carret al., 1977; Greeley et al., 1980a, 1982; Mouginis-Mark, 1987;Stewart et al., 2001a; Wohletz and Sheridan, 1983). In the follow-ing we summarize some of the main points of these two compet-ing models and point out differences between them.

1.1.1. The atmospheric modelThe atmospheric model for the formation of fluidized ejecta

states that the thin Martian atmosphere is the medium into whichejecta are entrained (Barlow, 2005; Barnouin-Jha and Schultz,1988; Barnouin-Jha et al., 1999a, 1999b; Schultz, 1992a, 1992b;

Fig. 1. Top: Martian rampart crater morphologies. (A) Single layer ejecta (SLE) crater(THEMIS image I02493005 located near 24◦N 101◦E); (B) Double layer ejecta (DLE)crater (THEMIS image I03350005 located near 49◦N 230.5◦E); and (C) Multiple layerejecta (MLE) crater (THEMIS image I03218002 located near 6◦N 304◦E). All scalebars are approximate. Adapted from Barlow (2005). Bottom: Geographic distribu-tion of layered ejecta morphology for Martian rampart craters. Distribution of SLEcraters (A), DLE craters (B), MLE craters (C). Base map is Mars Orbital Laser Altime-ter (MOLA) shaded relief map. Each map is centered on 0◦ longitude and coversthe region ±65◦ latitude. Crater data from Barlow Crater Database version 1 down-loaded from USGS Planetary GIS Web Server (PIGWAD). Maps created using jMARSafter Barlow (2005).

Schultz and Gault, 1979a, 1979b). Laboratory and experimentalstudies (Barnouin-Jha and Schultz 1988, 1996; Barnouin-Jha et al.,1999a, 1999b; Schultz, 1992a, 1992b; Schultz and Gault, 1979a,1979b) show that atmospheric turbulence produces a vortex ringthat entrains, transports, and deposits fine-grained ejecta. In thismodel, larger material is ballistically emplaced ahead of the vortexring. As the vortex ring passes, it may remobilize these larger clastsand pile them up in the distal rampart. Ejecta composed of fine-grained material can flow without an accompanying gas or liquidphase (Schultz, 1992a). However, for the ejecta to flow in this man-

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Fig. 2. Model of an ejecta curtain advancing through an atmosphere. The model isbased on observations at the NASA Ames vertical gun range (Barnouin-Jha et al.,1999a). The lower thicker portion is impermeable to the surrounding atmosphereand redirects the atmosphere around it. The upper more permeable portions allowatmospheric flow through the ejecta curtain, allowing flow separation to generate avortex ring. Fine-grained ejecta are decelerated out of the semipermeable portionsof the ejecta curtain and enter the vortex ring. Adapted from Barnouin-Jha et al.(1999a).

ner it is necessary that either the target material be composedof fine-grained material or the impact itself produces a sufficientamount of fine-grained material during impact excavation (Boyceand Mouginis-Mark, 2006; Schultz, 1992a).

According to Schultz (1992a), nonballistic ejecta emplacementresults from a two-stage process. First, the ejecta are aerodynami-cally decelerated to near-terminal velocity. Next, the ejecta are en-trained in atmospheric turbulence created by the outward expand-ing wall of ballistic ejecta. Conditions leading to nonballistic ejectaemplacement depend on a critical ejecta size, which depends on(1) crater size (i.e., ejection velocity), (2) ejecta size, (3) local lithol-ogy, and (4) atmospheric pressure (i.e., density). Schultz (1992a)also suggested that different ejecta emplacement styles are relatedto both crater size and the amount of finer ejecta rather than theamount of excavation of water-rich target material.

Barnouin-Jha et al. (1999a) conducted wind-tunnel experimentson the interaction of an atmosphere with an ejecta curtain. Accord-ing to Barnouin-Jha et al. (1999a), ejecta curtain width and veloc-ity, particle concentration, size distribution and motion parallel tothe curtain, as well as the density, viscosity, and compressibilityof the surrounding atmosphere, all influence the vortex circulationstrength. The circulation generated by the ejecta curtain (Fig. 2)is a function of the length (L) and outward curtain velocity (U )of the curtain at its transition from impermeable to permeable(Barnouin-Jha and Schultz, 1996). Permeability of the ejecta curtainto the surrounding atmosphere is the primary factor controllingthe circulation generated by the advancing ejecta curtain. Geomet-ric relationships between curtain porosity (φ), curtain width (w),most common curtain ejecta particle diameter (d), the velocity ofthe surrounding atmosphere impinging on the curtain (U ), and thesurrounding atmospheric density (ρ) and viscosity (μ) are shownin Fig. 2.

Circulation of the curtain-derived vortex is what ultimately con-trols nonballistic ejecta deposition (Barnouin-Jha et al., 1999a). Thecirculation of the curtain-derived vortex is controlled by the per-meability of the ejecta curtain. To estimate the initial circulationof the vortex, the time when the curtain becomes fully perme-able must be known. Experiments conducted by Barnouin-Jha etal. (1999a), show that this transition depends upon the domi-

nant grain size of the target present in the ejecta. For experi-mental impacts in coarse sand (Barnouin-Jha and Schultz, 1996;Barnouin-Jha et al., 1999b; Schultz, 1992a) this transition occursslowly. However, the transition occurs quickly for fine-grainedpumice (Barnouin-Jha et al., 1999a).

1.1.2. The subsurface volatile modelIn the subsurface volatile model, impact into a volatile-bearing

target results in a vapor cloud that deposits the entrained ejectaas a flow surrounding the crater (Baratoux et al., 2002a, 2002b;Barlow, 2005; Carr et al., 1977; Greeley et al., 1982, 1980b;Mouginis-Mark, 1987; Stewart et al., 2001b; Wohletz and Sheri-dan, 1983). In this model, ejecta interact primarily with this va-por cloud rather than the atmosphere. Support for this modelcomes from (1) correlation of rampart craters with other geomor-phic features associated with subsurface water (Carr, 1996; Costardand Kargel, 1995), (2) relationships between rampart crater di-ameter and morphology with latitude (Barlow and Bradley, 1990;Costard, 1989), (3) hydrocode simulations of impacts into mix-tures of water and rock (O’Keefe et al., 2001; Senft and Stewart,2007, 2008a; Senft and Stewart, 2008b; Stewart and Ahrens, 2003;Stewart et al., 2001a, 2003), (4) impact experiments into ice-richtargets (Stewart and Ahrens, 2005), and the (5) presence of ram-part craters on Ganymede and Europa implying that an atmo-sphere is not needed to produce fluidized ejecta (Boyce et al.,2010).

Stewart et al. (2001b) conducted experiments and modeling ofimpacts onto ice–rock mixtures to quantify the effects of subsur-face H2O on ejecta distribution, rampart and pedestal formation,and crater floor morphologies. They propose that various ejectamorphologies (SLE, DLE, and MLE) are produced by increasingamounts of ice and found that the high volatility of H2O modifiesthe crater formation process producing more vapor, higher ejectionangles, fluidized ejecta blankets, and larger crater rim uplift.

At temperatures between 150 and 273 K, ice in the Martiancrust will begin to melt after experiencing shock pressures be-tween 2.0 and 0.6 GPa, respectively (Stewart et al., 2003). Theice will melt completely after being released from shock pressuresabove 5.5 and 3.7 GPa. Stewart et al. (2003) concluded that, in thepresent climate, about half the excavated ice is melted by impactshock. About 60% of ground ice will completely melt in equatorialzones while at the poles more that 20% will be melted. Their re-sults indicate that ejecta fluidization does not require pre-existingwater near the surface because shock melting of ground ice willintroduce large quantities of liquid water into the ejecta blanket.

In addition to their central relevance to the Martian case, flu-idized ejecta are also present on Venus (Fig. 3), a planet witha very thick atmosphere but without subsurface volatiles; andon Ganymede and Europa, bodies with an icy surface but noatmosphere. These situations represent end-members of the flu-idized ejecta mechanism. Venus represents the atmospheric end-member, and Ganymede and Europa represents the subsurfacevolatile end member. With its relatively thin atmosphere andice-bearing regolith, Mars is intermediate between these two ex-tremes.

2. Methods

2.1. Fragmentation methods

The shock-tube apparatus at LMU, described in detail inAlidibirov and Dingwell (1996a) and modified by Spieler et al.(2004) and Scheu et al. (2008), was used to conduct decompres-sion experiments on rock–water mixtures. The device consists of alarge upper chamber at ambient pressure and temperature con-ditions and a lower chamber, which can be pressurized up to

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Fig. 3. Fluidized ejecta on Venus. Magellan radar image (PIA00470) of Dickisoncrater in the northeastern Atalanta Region of Venus. The image is ∼185 km wideand is centered on 74.6◦N, 177.3◦E.

40 MPa with Argon gas (Fig. 4). The lower chamber is sepa-rated from the upper chamber by a series of metal diaphragmswith varying burst conditions. When controlled rupture of the di-aphragms is initiated, a shock wave propagates into the upperchamber and a rarefaction wave travels down into the lower cham-ber, both driven from the instantaneous release of the Argon gas.When the rarefaction wave reaches the sample, the solid sam-ple part is rapidly unloaded. Overpressure builds up in the openpore space of the sample as gas escape cannot keep pace withthe unloading of the sample matrix (rock) (Fowler et al., 2010;McGuinness et al., 2012; Scheu et al., 2006). Depending on ap-plied pressure, open porosity and permeability of the sample, this

Fig. 5. Experimental pressure and temperature conditions. Samples were first pres-surized to about 10 MPa, then pressurized and heated to 15 MPa and 177 or 300 ◦Cto keep the samples above the vaporization curve for water until instantaneous de-compression.

overpressure reaches conditions for rock failure, and fragmenta-tion is initiated (Fowler et al., 2010; McGuinness et al., 2012;Mueller et al., 2008; Spieler et al., 2004). The fragmented rockparticles are ejected into the upper chamber and can be fully re-covered for grain sizes analysis.

We tested rock samples at two temperatures (177 ◦C and300 ◦C) and one applied pressure (15 MPa). In order to keep thewater in a liquid state, the samples were initially pressurized toabout 10 MPa and then slowly heated to the desired temperatureusing an external furnace (Figs. 4 and 5). Pressure was increasedincrementally approaching 15 MPa to keep the system from cross-ing the vaporization curve before failure of the diaphragms. Thefurnace was held at the experimental temperature for additional20 minutes to allow for thermal equilibration of the sample, andthen controlled failure of the diaphragms was triggered. Occasion-ally, the diaphragms failed before triggering, but in all experimen-tal runs a final applied pressure of about 15 MPa was reached.In all cases the samples were fragmented and the rock fragmentswere recovered.

Fig. 4. Shock-tube apparatus at the LMU Munich Department of Earth and Environmental Science (adapted from Kueppers et al., 2006). High pressure and temperature (HPT)autoclave is pressurized with Ar gas and heated by an external furnace. When diaphragms break under high pressure, a rarefaction wave propagates down through thesample. The rock sample fractures parallel to the rarefaction wave front due to induced internal overpressure and the fragments are ejected into the upper chamber.

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Fig. 6. A: Photomicrograph of a polished section of the volcaniclastic sandstone used in fragmentation experiments. The porous rock is composed of olivine, quartz andoccasional small rock and pumice fragments derived from mid-Miocene igneous rocks. B: Grain-size distribution of the rock sample prior to experimentation. No grain sizemode was identified; the frequency of individual phi sizes increases steadily towards larger phi sizes with a small bias towards higher frequencies for the phi classes of 2.5and 3.0.

2.2. Sample material and preparation

For these experiments a rock was chosen that is as close as rea-sonably possible to a generic regolith composition on Mars. Sedi-mentary deposits derived from basalt and andesite cover the Mar-tian surface (Bandfield et al., 2000; Barlow, 2008; Malin and Edgett,2000a, 2000b). Therefore a volcaniclastic rock from the northernEldorado Mountains of southern Nevada, USA (Anderson, 1971)was chosen for these experiments. This thinly bedded (1–3 mm)volcaniclastic sandstone is composed of rounded to subangulargrains of olivine, quartz, and occasional small (< 3 mm) rock andpumice fragments derived from mid-Miocene basalt, dacite, andrhyolite. The grain-size distribution for the rock was determinedby counting 300 grains and rock fragments on photomicrographsof polished sections (Fig. 6A). A bar graph showing the percentageof mineral grains or rock fragments in each sieve size and a cumu-lative size distribution curve (Fig. 6B) revealed a broad distributionwith a steadily increasing frequency towards larger phi sizes. Incontrast to many sandstones, no characteristic grain size mode wasdetected; only a slight deviation towards higher frequencies wasobserved for the 2.5 and 3.0 phi size range.

The rock’s uniform structure and composition make it ideal foruse in these experiments, where the physical property of the rockmust be constant for each experimental series. The rock samplesused in this investigation are not meant to be an accurate rep-resentation of the Martian regolith. However, they are similar tomany rocks on Mars, in particular those found at Meridiani Planum(Squyres et al., 2006), in that they are sedimentary rocks with vol-canic origin. They contain basalt, which is common on Mars, butalso rhyolite, which is not. As no study of this kind has ever beenconducted, it is most important to test a homogeneous materialthat permits evaluation of the effects of vaporization of varyingamounts of water. When selecting an analog material, strength andporosity may be more important than silicate composition. Thechoice of that rock must then later be evaluated by comparisonwith other lithologies in more extended studies.

Cylindrical samples 25 mm in diameter and 40 mm long weredrilled out of the rock. The samples were placed in an oven todry the open pore space. After cooling in a desiccator, each sam-ple was weighed, its exact diameter and length measured and thesample bulk volume (V calc) and bulk density calculated. A heliumpycnometer was used to determine the matrix volume (V hc) andmatrix density. We then calculated the percent of open porosity([[V calc − V hc]/V calc] × 100%). The sandstone has an average bulkdensity of 1.90 g/cm2 and average matrix density of 2.16 g/cm2,

with standard deviations of 0.005 and 0.0053, respectively. Theaverage open porosity of the samples is 27.45%, with a standarddeviation of 3.33. Samples were stored in airtight containers untilexperimental preparation.

Prior to the experiment, each rock sample was mounted into acylindrical steel crucible with an interior diameter slightly smallerthan the rock sample. This tight fit assures that the rock sampleis ejected by fragmentation alone into the upper chamber; non-fragmented samples remain in the crucible. After placement inthe crucible the sample was saturated with water to the desiredamount. Water saturation is measured by the amount of open porespace being filled with water (%OPS).

2.3. Sieving methods and grain-shape analysis

After each experiment, the rock fragments were collected andsieved between sieve sizes −4 and 4 phi at 0.5-phi intervals. Thecontents of each sieve were weighed and the weight percent ofeach sieve interval was calculated. A grain size distribution curvewas plotted on an arithmetic probability grid for each sample usingGRANPLOT, a Microsoft Excel Spreadsheet developed by Balsillieet al. (2002). Also, calculated were Inman (1952) parameters in-cluding (1) median diameter (Mdφ = φ50), the phi-size where thecumulative distribution curve crosses the 50% mark; (2) graphi-cal standard deviation (σφ = [φ84 − φ16]/2), which is a measureof sorting; and (3) first order skewness (αφ = [((φ84 + φ16) −Mdφ)]/σφ), which is a measure of asymmetry of the distribution.As an additional statistical parameter the kurtosis, a measure ofthe tightness of the distribution was calculated.

Further the short (S), intermediate (I), and long (L) axes ofeach fragment from phi sizes −1.5 and −2 for all experimentswere measured. These phi sizes were the only ones analyzed be-cause they were large enough to be measured with calipers andall samples had particles in these two combined sieve sizes (onlysample 426 had no particles in sieve size −2φ). Using the S/Lindex and form index ([L − I]/[L − S]) developed by Sneed andFolk (1958), the grain shape for each sample was determined. Theaverage S/L and form index as well as standard deviation werecalculated and plotted for each experiment.

3. Results

3.1. Grain-size distribution

The results of the grain-size analysis were grouped to five setsdepending on the percentage of water saturation (%OPS H2O) of

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Fig. 7. A–I: Individual frequency distributions and cumulative frequency curves for the nine experiments. Percent open pore space (%OPS) water and experimental temperature(in brackets) is shown for each sample below the sample number. All samples were pressurized to 15 MPa. Additional information on each sample and experiment can befound in Table 1 and Supplementary Appendix 1. K: Combined cumulative frequency distribution of the nine samples (A–I) plotted on arithmetic probability paper. Mediandiameter (Mdφ ) is the phi-size at which the sample’s curve crosses the 50% mark. Graphical standard deviation (σφ = [φ84 −φ16]/2) is calculated using the phi values at 16%(φ16) and 84% (φ84) cumulative percent values.

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Table 1Sample description and experimental conditions.

Sample Length(mm)

Diameter(mm)

Mass(g)

Calculatedvolume(ml)

Measuredvolume(ml)

Calculateddensity(g/ml)

Measureddensity(g/ml)

Openporosity(ml)

% Openporosity

Actualwateradded(ml)

% Openporositywater

Experimentaltemperature(◦C)

102 37.94 24.79 34.7696 18.3196 13.3798 1.8979 2.5987 4.9398 26.96 0 0 177108 37.32 24.89 32.7240 18.1513 12.5222 1.8028 2.6133 5.6291 31.01 1.7700 31 300109 37.88 24.84 33.4438 18.3571 12.8382 1.8218 2.6050 5.5189 30.06 2.9394 53 177110 37.24 24.82 32.8670 18.0179 12.6004 1.8241 2.6084 5.4175 30.07 0.8679 16 177112 39.20 24.93 34.2736 19.1270 13.1509 1.7919 2.6062 5.9761 31.24 5.3892 90 300113 37.83 24.85 33.5596 18.3550 12.9640 1.8284 2.5887 5.3910 29.37 4.7300 88 177114 38.28 24.85 33.4023 18.5658 12.8223 1.7991 2.6050 5.7435 30.94 3.5365 62 177422 38.11 24.93 36.3973 18.3973 13.8830 1.9574 2.6217 4.7121 25.34 0 0 300426 37.55 24.88 33.3930 18.3930 12.8442 1.8292 2.5999 5.2567 29.04 1.5780 30 177

the samples. Within these sets water saturation may vary slightly.The frequency distribution of all samples is shown in Fig. 7;the corresponding statistical parameters are listed in Supplemen-tary Appendix 1. In the following we analyze the statistical char-acteristics of the five sets in detail.

1) 0% OPS H2O: The dry control experiments (samples 102 and422) were run at 177 and 300 ◦C, respectively. Both have an overallsimilar grain size distribution as evidenced by their similar me-dian diameter (Mdφ ), graphic standard deviation (σφ ), and shapeof their frequency distribution and cumulative frequency (Figs. 7A,6B). In addition, they have similar values of skewness and kurtosis.The similarities in grain size distribution indicate that temperaturedid not play a significant role in the fragmentation behavior of thecontrol samples.

2) 15% OPS H2O: As water is added to the open pore space ofthe sample, there is a shift toward smaller grain sizes in the grainsize distribution of the fragments produced by shock decompres-sion. Sample 110 was tested at 177 ◦C with 16% of its open porespace saturated with water; it has a smaller median diameter, butsimilar graphical standard deviation (σφ = 1.76) when comparedto the control samples. In addition, its frequency distribution isless positively skewed and less peaked than the control samples(Figs. 7A–C).

3) 30% OPS H2O: Increasing the water saturation to about 30%results in a more pronounced shift toward smaller grain sizes, asseen for samples 108 at 300 ◦C and 426 at 177 ◦C (Figs. 7D and E).The graphical standard deviations for these samples are similar tothe control and 16% OPS water experiments. The products of thesetwo experiments have less peaked grain size distributions than thecontrol experiments, with kurtosis values similar to each other andsample at 16% OPS water. The grain-size distributions are also lessskewed than the control samples. The grain-size distributions forsamples with 30% OPS H2O do differ from each other in that thereis a spike at −3.5φ in the frequency distribution for sample 426(Fig. 7D) shown as a plateau on the left side of the cumulative fre-quency curves. This spike is the result of a single fragment in the3.5φ sieve size. With the exception of that spike, the experimentswith 30% OPS water have grain-size distributions that are verysimilar to each other even thought they were conducted at differ-ent temperatures. This observation may be an additional indicationthat temperature was not an important factor in the fragmentationbehavior of these samples.

4) 50–60% OPS H2O: The experiment for sample 109 with 53%OPS H2O was run at 177 ◦C (Fig. 7F). The grain size distributionfor this experiment is shifted toward much smaller grain sizeswhen compared to the control samples. However, the median grainsize is only slightly smaller than the 16% OPS water sample, andslightly larger than the samples with approximately 30% OPS wa-ter. This sample is slightly less positively skewed and peaked thanthe control sample.

Sample 114 was saturated to 62% OPS H2O and also fragmentedat 177 ◦C (Fig. 7C). It has a grain size distribution very similar tothe control samples (Figs. 7A and B). Both skewness and kurtosisvalues are similar to the 0% OPS water samples.

5) 90% OPS H2O: For samples 112 and 113, the goal was tofill the open pore space entirely with water. However, the inter-connectedness of the pore space (i.e., permeability) resulted insome of the water leaking out of the sample. Furthermore, veryfine fractures and small pores might not be filled due to cap-illary forces and adhesions resulting in incomplete filling of theopen pore space with water. Accordingly, samples 112 (300 ◦C) and113 (177 ◦C) contained 90% and 88% OPS water, respectively. Un-like the other paired experiments conducted at 177 and 300 ◦C,these water-saturated samples produced grain size distributionsthat were different from each other. The median diameter forsample 113 is much lower than the control samples, however ithas a similar graphical standard deviation, skewness, and kurtosis(Fig. 7H). Conversely, sample 112 has a median diameter similarto the control samples; however its graphical standard deviation,skewness, and kurtosis differ from the control samples (Figs. 7A,B, and I). In fact, all experiments have a similar graphical stan-dard deviation except for sample 112. The larger standard devia-tion results in a more uniform spread of the frequency distribution(Fig. 7I) and the lower slope of the cumulative frequency distribu-tion (Fig. 7K).

A plot of median diameter against graphical standard deviation(Fig. 8A) is a measure of sorting. The dry control samples havesimilar median diameters and sorting coefficients. Adding 16% OPSwater increases the median phi size (decreases the median diam-eter) to about −0.5. Adding 30% water shifts the median diameterto even smaller sizes (around 0 phi), but, as additional water isadded, the phi size decreases until it reaches sizes near the controlsample. In the case of sample 113, filling the open pore space with88% produced a median diameter larger than the control sampleafter shock decompression across the vaporization curve for water.

It appears that the maximum shift toward smaller median di-ameters in the grain size distribution is achieved with about 30%OPS water. This is best shown in Fig. 8B, which plots %OPS wa-ter against median diameter in phi-size. Note: a decrease of me-dian diameter in millimeters corresponds to increased values inphi-size. Median grain size as measured in phi-size increases fromthe control sample to the sample containing 16% OPS water. Themedian diameter peaks at samples containing 30% OPS H2O. In-creasing the amount of %OPS water to values higher than 30%decreases the median diameter until it reaches a value approach-ing or lower than the control samples.

Another way to assess fragmentation behavior is the proportionof fines produced by each experiment. Weight percent fines (> 4φ)are plotted as a function of %OPS water for each sample in Fig. 8C.There is a general trend of increasing weight proportion of finesbetween 15 and 60% OPS. Although the proportion of fines drops

A.H. Rager et al. / Earth and Planetary Science Letters 385 (2014) 68–78 75

Fig. 8. Evaluation of grain size distribution characteristics for the nine experiments.(A) Median diameter (Mdφ ) vs. graphical standard deviation (σφ ). All samples werecompressed to 15 MPa. Filled circles = 177 ◦C; open circles = 300 ◦C. Percent openpore space (%OPS) water shown in parentheses. (B) Percent open pore space (%OPS)water vs. median diameter (Mdφ ). Filled circles = 177 ◦C; open circles = 300 ◦C. Allsamples were compressed to 15 MPa. (C) Percent open pore space (%OPS) water vs.weight % fines (> 4φ). Filled circles = 177 ◦C; open circles = 300 ◦C. All sampleswere compressed to 15 MPa.

off with amounts of water > 62% OPS, the proportion of fines forthese experiments is still 73% increased for sample 112 with 90%OPS water and 95% for sample 114 with 62% OPS water. As withthe grain-size distribution, the proportion of fines for sample 113(with 88% OPS water) is about the same as for the control samples(about 2% increase).

3.2. Grain-shape analysis

Visual observation during sample recovery suggested that thefragment shape of the fully water saturated samples was more ho-mogeneous than the fragments from other experiments. To quan-tify the variations in shape, we measured the short (S), intermedi-ate (I), and long (L) axes of particles in phi sizes −1.5 and −2 forall experiments (Fig. 9). The number of particles within these twophi sizes ranged from 23 to 263 for one experiment. The mean val-ues of S/L and the form index (L − I/L − S) were plotted for each

Fig. 9. Grain shape for phi sizes −1.5 and −2.0 plotted against the grain shapefields developed by Sneed and Folk (1958).

experiment (Sneed and Folk, 1958). Ellipses around each measure-ment represent the standard deviation.

The first impression based on visual observation was affirmedby the quantitative grain-shape analysis. The fully water-saturatedsamples plot in Fig. 9 in the compact bladed field near the bound-ary of the bladed category and the standard deviation ellipses ofthe individual samples do overlap amongst each other. All othersamples plot within the bladed field and also have strongly over-lapping standard deviation ellipses. However the standard devia-tion ellipses of the fully water saturated samples show no overlapwith the group of all other samples indicating that all samples canbe grouped in two distinct groups: fully water saturated and oth-ers.

Fragmented particles under a petrographic microscope weregenerally subangular to angular with the majority of fractures oc-curring along grain boundaries for larger sieve sizes (φ < 0.0).Particles from the smaller sieve fractions include small subangu-lar individual mineral grains and small, angular rock and min-eral fragments. Fractures appear to have occurred along the grainboundaries of the former and through grain boundaries for the lat-ter. This fracturing behavior is quite typical for sedimentary rocks,and more or less pronounced depending on the strength of thecementation. One might expect that this leads to a characteristicsize distribution of the fragments, mirroring the sizes of the sandgrains. However, the volcaniclastic sandstone used in this studydoes not show a dominant grain size, but is rather composed of awide range of grain sizes (Fig. 6A and B). There was no correlationbetween the size distribution of grains composing the sandstoneand the fragment sizes determined after the experiments.

4. Discussion

Our results show a clear influence of the presence of water inthe open pore space on the fragmentation behavior of the stud-ied sandstone. However there is not a simple relationship betweenwater content in the open pore space of a sample and grain sizeafter fragmentation due to gas expansion and vaporization of wa-ter. Water vaporization does affect the grain size distribution ofthe ejecta in a more complex way. The degree of fragmentationappears to peak around 30% OPS (Figs. 7A–K). Note in sample 426

76 A.H. Rager et al. / Earth and Planetary Science Letters 385 (2014) 68–78

Fig. 10. Fragmentation models for dry to fully water saturated samples. (A) 0% open pore space (OPS) water: for dry samples fragmentation is driven purely by gas expansionduring decompression. (B) ∼15–60% OPS water: pre-existing fractures and open pore space is partially filled with water. Note: The distribution of water displayed in the figureis only schematic. Fragmentation is driven by gas expansion as well as vaporization of water due to decompression. (C) > 80% OPS water: open pores and fractures are fullywater saturated. At this step, the gas remaining in the sample is not sufficient to fragment the sample and most/all the fragmentation energy derives from decompressionand vaporization of water. For (A)–(C) in our model, closed pores are treated as additional minor weakening points of the solid rock, they do not contain gas overpressure orwater.

one large fragment in −4φ slightly masks this trend. Slightly largermedian diameter sizes were produced by the 53% OPS and 16%OPS H2O experiments. Experiments using 0%, 62%, and 90% OPSwater produced fragments with an even larger median diameter.The experiment that resulted in the largest median diameter con-tained 88% OPS water and was run at 177 ◦C. All samples havesimilar sorting coefficients as represented by the similar slope toeach curve.

Dry control samples have the lowest proportion of fines. Theweight percent of fines increases drastically at 16% OPS water andremains high until about 50% OPS water, then drops again slightlyas the sample approaches water saturation. The shape of grainsfrom fully water-saturated samples is blockier, indicating that thevaporization of water in this case may cause fracturing less de-pendent on the decompression front, but more on the orientationof pore space and eventually pre-existing fractures. Fig. 10 depictsthe proposed fragmentation mechanism from dry to fully watersaturated samples. Experiments with fully water-saturated sampleswere much louder (i.e., emitted much more audible acoustic en-ergy) than the other samples. In addition, particles were foundclinging on the lid and rim of the upper tank after the water-saturated experiments. This was not the case for the partiallysaturated and dry (control) experiments. These two qualitative ob-servations suggest that the fully water-saturated samples may befragmented more violently, eventually also by a different fracturemechanism, and the fragments may be ejected at a higher speed.Future studies may involve high-speed photography to measure thespeed of the ejection. One of the characteristics of rampart cratersis that they generally lack secondary craters within the ejecta blan-ket. If high water content results in a higher ejection velocity andforce, this could cause larger blocks to be transported further thanis typical for ballistically emplaced ejecta.

It is possible that not all of the water contained in the openpore space of each sample was driven off. Therefore the actual%OPS water probably varies from that listed in Table 1. However, allsamples were treated consistently, so the relative effects of addingwater can still be measured.

The distribution of open pore space in the sample may beanother factor affecting grain size distribution of the fragmentedsamples. If a sample’s open porosity varies too much within a sam-ple, it may cause part of a sample to fracture more than another

and that may account for some differences in grain size distribu-tion curve shape. We anticipate that future studies will addressthis problem by including measurement of the permeability andmapping of the open pore space with tomography. However, thesamples used in this study were prepared from large (20–50 cm)relatively homogeneous blocks, therefore we expect no significantinfluence of sample inhomogeneity on the results.

Fluidized ejecta are a common phenomenon not only for Mar-tian impact craters, but also for Venusian craters as well as craterson Ganymede and Europa. In laboratory experiments, Schultz(1992a) produced fluidized ejecta in a vacuum by impact into tar-gets with fine-grained particles and by impact into coarser grainedmaterials on a planetary body with high atmospheric pressuresuch as Venus. Schultz (1992a) contended that fluidized ejectaare produced at lower pressures if the ejecta particles are smallenough. Our results indicate that for samples with about 28%open porosity, the grain size distribution is significantly shiftedtoward finer grains by water vaporization. This begs the questionof whether or not water crosses the vaporization curve during im-pact cratering. Experiments and modeling conducted by Stewart etal. (2003) indicate that a substantial amount of water ice is meltedand vaporized during impact cratering. The proportion of subsur-face ice that is melted or vaporized varies with impactor size (andconsequently crater size), water content, and surface temperature(latitude). Stewart et al. (2003) found that for a 500-m diameterimpactor, about half the amount of water within the excavationzone will melt and a quarter will vaporize. This amount of wateris sufficient to drastically reduce the median diameter of the ejectaparticles.

We show that the vaporization of a small amount of water hasthe ability to increase the degree of fragmentation of the ejectaand to decrease the average ejecta size. Further, ejecta fluidiza-tion and flow is due to the fine grain size and not the presenceof water. Our model provides an alternative way of explainingejecta emplacement during the formation of Martian craters. Thisobservation is similar to that of Wada and Barnouin-Jha (2006)who, based on laboratory experiments, implied that ejecta flu-idization on Mars may be related to water absent grain flow. Flu-idization of a gas–particle mixture depends on the interaction andremixing dynamics of their components (Formisani et al., 2008;Roche et al., 2002). Abundant fine-grained material facilitates flu-

A.H. Rager et al. / Earth and Planetary Science Letters 385 (2014) 68–78 77

idization and flow of ejecta. For example Gravina et al. (2004)postulated that gas flow effectiveness in causing fluidization andflow of pyroclastic deposits is enhanced by the presence of veryfine-grained particles. In this situation the momentum exchangebetween gas and particles is enhanced, thus promoting fluidizationand rapid flow. In addition to the study of Gravina et al. (2004),the fluidization of gas–particle mixtures is a common topic in thevolcanology and multiphase flow literature (Formisani et al., 2008;Olivieri et al., 2004; Roche et al., 2002; Wilson, 1980, 1984).

The production of impact melt was not considered in our ex-periments. However, Kieffer and Simonds (1980) have addressedthe issue of the formation of impact melt for a sedimentary tar-get. They indicate that although melt is generated by impact into asedimentary target, little is preserved. Steam from pore water andpossibly CO2 expand violently during impact to finely fragmentmelted silicates. This process may contribute additional material tothe fine fraction of the ejecta. Further they suggest that any meltgenerated by impact into a sedimentary target occurs as coarsergrained glass particles in suevite-type deposits like those about theRies Crater (Sturm et al., 2013). Impact into a layered target with athin sedimentary layer covering basement rock may result in moreimpact melt and perhaps melt sheets (Kieffer and Simonds, 1980).

Scaling of laboratory experiments to larger impact events is dif-ficult because the mode of failure and fracture formation may bedifferent in larger kilometer scale cratering events. Although im-pact scaling has become very sophisticated (Collins et al., 2004;Melosh et al., 1992) extended the work of Grady and Kipp (1980,1987) to argue that physical modeling at various scales rather thanusing scaling laws is the best way understand fragmentation forlarger impact events. For smaller impacts, material strength con-trols target behavior (Fujiwara et al., 1989), but for larger impactssurface gravity may also be an important factor (Asphaug andMelosh, 1993).

5. Conclusions

Based on rapid decompression experiments we have shownhere that the vaporization of a small amount of water has theability to increase the degree of fragmentation of the ejecta andto decrease the average ejecta size. We propose that vaporizationof water during the excavation stage of impact cratering is a keymechanism for ejecta fluidization as it results in a shift towardsmaller ejecta particles allowing the ejecta to be deposited in themanner described by Schultz (1992a), Barnouin-Jha and Schultz(1996), and Barnouin-Jha et al. (1999a, 1999b). Therefore, we pro-pose that water promotes efficient target fragmentation, but ejectafluidization and flow may occur with little or no water. This hasimplications for behavior of the ejecta curtain. The permeabilityof the ejecta curtain to the surrounding atmosphere and the abil-ity of an impact to generate fine ejecta are the primary factorscontrolling the circulation generated by the advancing ejecta cur-tain (Barnouin-Jha et al., 1999a, 1999b; Schultz, 1992a). The mostcommon diameter size is a major factor affecting the length atwhich the curtain becomes permeable. Smaller ejecta diameter willresult in a shorter length at which the curtain transitions fromimpermeable to permeable. Fine-grained ejecta may be producedby a variety of mechanism on Mars. Impact into a fine-grainedtarget will yield small grain sizes (Schultz and Gault, 1979b). More-over, larger impacts will result in finer ejecta because of gravityscaling relationships that result in ejection of larger amounts offine-grained shocked material (Schultz and Gault, 1979b). Althoughthere are several mechanisms that produce small grain sizes onMars, our experiments expand the conditions that can yield fine-grained ejecta.

In addition to facilitating ejecta fluidization by creating smallerejecta particles that interact with the atmosphere, the vaporiza-

tion of water may also play a part in the dearth of secondarycraters within the ejecta blanket (Schultz and Singer, 1980). Ourresults suggest that it takes relatively small amounts of water tostrongly enhance the amount of fines in the ejecta. When a rock’spore space is nearly filled with water, larger blocks may be pro-pelled further than expected, creating smaller secondary cratersoutside the fluidized ejecta blanket. This may explain why theyare rarely found within fluidized ejecta blankets. More experimentsare needed to better constrain the effects of water vaporizationon rock fragmentation. Additional experiments to test other vari-ables including rock type, porosity, and permeability could provideinformation that can be included in numerical models of impactcratering so these models can be used to test the proposed model.

Acknowledgements

This work was supported by a NASA Nevada Space GrantFellowship, the UNLV Graduate and Student Association (bothto AHR), a Research Professorship of the Bundesexzellezinitiative(LMUexcellent), an Advanced grant of the European Research Coun-cil (247076 EVOKES) and a Collaborative project of the SeventhFramework Programme (282759 VUELCO) (to BS and DBD). Wethank Elizabeth Hausrath (UNLV), Racheal Johnsen (UNLV), RosalyLopes (JPL), Adam Simon (University of Michigan) and two anony-mous journal reviewers for comments on an earlier version of thismanuscript.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2013.10.029.

References

Alidibirov, M., Dingwell, D.B., 1996a. An experimental facility for the investigationof magma fragmentation by rapid decompression. Bull. Volcanol. 58, 411–416.

Alidibirov, M., Dingwell, D.B., 1996b. Magma fragmentation by rapid decompression.Nature (London) 380, 146–148.

Anderson, R.E., 1971. Thin skin distension in Tertiary rocks of southeastern Nevada.Geol. Soc. Am. Bull. 82, 43–58.

Asphaug, E., Melosh, H.J., 1993. The Stickney impact of Phobos: a dynamical model.Icarus 101, 144–164.

Balsillie, J.H., Donoghue, J.F., Butler, K.M., Koch, J.L., 2002. Plotting equation for Gaus-sian percentiles and a spreadsheet program for generating probability plots. J.Sediment. Res. 72, 929–932.

Bandfield, J.L., Hamilton, V.E., Christensen, P.R., 2000. A global view of Martian sur-face compositions from MGS-TES. Science 287, 1626–1630.

Baratoux, D., Delacourt, C., Allemand, P., 2002a. An instability mechanism in theformation of the Martian lobate craters and the implications for the rheology ofejecta. Geophys. Res. Lett. 29, 4.

Baratoux, D., Mangold, N., Delacourt, C., Allemand, P., 2002b. Evidence of liquid wa-ter in recent debris avalanche on Mars. Geophys. Res. Lett. 29, 4.

Barlow, N.G., 2003. Revision of the “Catalog of Large Martian Impact Craters”. In:Sixth International Conference on Mars. Abstract 3073.

Barlow, N.G., 2005. A review of Martian impact crater ejecta structures and their im-plications for target properties. In: Kenkmann, T., Hoerz, F.P., Deutsch, A. (Eds.),Large Meteorite Impacts III. In: Geol. Soc. Am. Spec. Pap., vol. 384, pp. 433–442.

Barlow, N.G., 2008. Mars: An Introduction to Its Interior, Surface and Atmosphere.Cambridge University Press, United Kingdom.

Barlow, N.G., Bradley, T.L., 1990. Martian impact craters, correlations of ejecta andinterior morphologies with diameter, latitude, and terrain. Icarus 87, 156–179.

Barnouin-Jha, O.S., Schultz, P.H., 1988. Lobateness of impact ejecta deposits fromatmospheric interactions. J. Geophys. Res. 103, 25739–25756.

Barnouin-Jha, O.S., Schultz, P.H., 1996. Ejecta entrainment by impact generated ringvortices: Theory and experiments. J. Geophys. Res. 101, 21099–21115.

Barnouin-Jha, O.S., Schultz, P.H., Lever, J.H., 1999a. Investigating the interactions be-tween an atmosphere and an ejecta curtain. J. Geophys. Res. 104, 27105–27115.

Barnouin-Jha, O.S., Schultz, P.H., Lever, J.H., 1999b. Investigating the interactionsbetween an atmosphere and an ejecta curtain; 2. Numerical experiments. J. Geo-phys. Res. 104, 27117–27131.

Boyce, J.M., Barlow, N., Mouginis-Mark, P., Stewart, S., 2010. Rampart craterson Ganymede: Their implications for fluidized ejecta emplacement. Meteorit.Planet. Sci. 45, 638–661.

78 A.H. Rager et al. / Earth and Planetary Science Letters 385 (2014) 68–78

Boyce, J.M., Mouginis-Mark, P.J., 2006. Martian craters viewed by the Thermal Emis-sion Imaging System instrument: Double-layered ejecta craters. J. Geophys. Res.Planets 111, 1–21.

Carr, M.H., 1996. Water on Mars. Oxford University Press, New York.Carr, M.H., Crumpler, L.S., Cutts, J.A., Greeley, R., Guest, J.E., Masursky, H., 1977.

Martian impact craters and emplacement of ejecta by surface flow. J. Geophys.Res. 82, 4055–4065.

Collins, G.S., Melosh, H.J., Ivanov, B.A., 2004. Modeling damage and deformation inimpact simulations. Meteorit. Planet. Sci. 39, 217–231.

Costard, F.M., 1989. The spatial distribution of volatiles in the Martian hydrolitho-sphere. Earth Moon Planets 45, 265–290.

Costard, F.M., Kargel, J.S., 1995. Outwash plains and thermokarst on Mars.Icarus 114, 93–112.

Formisani, B., Girimonte, R., Longo, T., 2008. The fluidization process of binary mix-tures of solids: Development of the approach based on the fluidization velocityinterval. Powder Technol. 185, 97–108.

Fowler, A.C., Scheu, B., Lee, W.T., McGuinness, M.G., 2010. A theoretical model of theexplosive fragmentation of vesicular magma. Proc. R. Soc. A, Math. Phys. Eng.Sci. 466, 731–752.

Fujiwara, A., Cerroni, P., Davis, D., Ryan, E., Di Martino, M., Holsapple, K., Housen,K., 1989. Experiments and scaling laws on catastrophic collisions. In: Binzel,R.P., Gehrels, T., Mathews, M.S. (Eds.), Asteroids II. University of Arizona, Tuc-son, pp. 240–265.

Garvin, J.G., Sakimoto, S.E.H., Frawley, J.J., 2003. Craters on Mars: Global geometricproperties from gridded MOLA topography. In: 6th International Conference onMars. Lunar and Planetary Institute, Houston, Texas. Abstract 3277.

Garvin, J.B., Sakimoto, S.E.H., Frawley, J.J., Schnetzler, C., 2000. North polar regioncrater forms on Mars. Geometric characteristics from the Mars Orbiter LaserAltimeter. Icarus 144, 329–352.

Grady, D.E., Kipp, M.E., 1980. Continuum modelling of explosive fracture in oil shale.Int. J. Rock Mech. Min. Sci. Geomech. Abstr. 17, 147–157.

Grady, D.E., Kipp, M.E., 1987. Dynamic Rock Fragmentation. Academic Press, London.Gravina, T., Lirer, L., Marzocchella, A., Petrosino, P., Salatino, P., 2004. Fluidization and

attrition of pyroclastic granular solids. J. Volcanol. Geotherm. Res. 138, 27–42.Greeley, R., Fink, J.H., Gault, D.E., Guest, J.E., 1982. Experimental simulation of impact

cratering on icy satellites. In: Morrison, D. (Ed.), Satellites of Jupiter. Universityof Arizona Press, Tucson, Arizona, pp. 340–378.

Greeley, R., Fink, J., Gault, D.E., Snyder, D.B., Guest, J.E., Schultz, P.H., 1980a. Impactcratering in viscous targets; laboratory experiments. In: Proceedings of the Lu-nar and Planetary Science Conference, pp. 2075–2097.

Greeley, R., Gault, D.E., Snyder, D.B., Sisson, V., Schultz, P.H., Guest, J.E., Wirth, P.,D’Alli, R., 1980b. Impact cratering in viscous targets. NASA Technical Memoran-dum. 95 pp.

Head, J.N., Melosh, H.J., Ivanov, B.A., 2002. Martian meteorite launch: High speedejecta from small craters. Science 288, 1752–1756.

Inman, D.L., 1952. Measures for describing the size distribution of sediments. J. Sed-iment. Petrol. 22, 125–145.

Kieffer, S.W., Simonds, C.H., 1980. The role of volatiles and lithology in the impactcratering process. Rev. Geophys. 18, 143–181.

Kueppers, U., Scheu, B., Spieler, O., Dingwell, D.B., 2006. Fragmentation efficiencyof explosive volcanic eruptions: A study of experimentally generated pyroclasts.In: MULTIMO: Multi-Parameter Monitoring, Modelling and Forecasting of Vol-canic Hazard Results from a European Project. J. Volcanol. Geotherm. Res. 153,125–135.

Kuzmin, R.O., Bobina, N.N., Zabalueva, E.V., Shashkina, V.P., 1988. Structural inhomo-geneities of the Martian cryosphere. Sol. Syst. Res. 22, 121–133.

Malin, M.C., Edgett, K.S., 2000a. Evidence for recent groundwater seepage and sur-face runoff on Mars. Science 288, 2330–2335.

Malin, M.C., Edgett, K.S., 2000b. Sedimentary rocks of early Mars. Science 290,1927–1937.

McEwen, A., Turtle, E., Burr, D., Milazzo, M., Lanagan, P., Christensen, P., Boyce, J.,2003. Discovery of a large rayed crater on Mars: Implications for recent volcanicand fluvial activity and the origin of Martian meteorites. In: Lunar and PlanetaryScience XXXIV. Lunar and Planetary Institute, Houston, Texas. Abstract 2040.

McGuinness, M.J., Scheu, B., Fowler, A.C., 2012. Explosive fragmentation criteria andvelocities for vesicular magma. J. Volcanol. Geotherm. Res. 237–238, 81–96.

Melosh, H.J., 1989. Impact Cratering, a Geologic Process. Oxford Monogr. Geol. Geo-phys., vol. 11. 245 pp.

Melosh, H.J., Ryan, E.V., Asphaug, E., 1992. Dynamic fragmentation in impacts,hydrocode simulation of laboratory impacts. J. Geophys. Res. Planets 97,14735–14759.

Mouginis-Mark, P.J., 1979. Martian fluidized ejecta morphology: Variations withcrater size, latitude, altitude, and target material. J. Geophys. Res. 84,8011–8022.

Mouginis-Mark, P.J., 1987. Water or ice in the Martian regolith? Clues from rampartcraters seen at very high resolution. Icarus 71, 268–286.

Mueller, S., Scheu, B., Spieler, O., Dingwell, D.B., 2008. Permeability control onmagma fragmentation. Geology 36, 339–402.

O’Keefe, J.D., Stewart, S.T., Lainhart, M.E., Ahrens, T., 2001. Damage and rock–volatilemixture effects on impact crater formation. Int. J. Impact Eng. 26, 543–553.

Olivieri, G., Marzocchella, A., Salatino, P., 2004. Segregation of fluidized binary mix-tures of granular solids. AIChE J. 50, 3095–3106.

Roche, O., Gilbertson, M., Phillips, J., Sparks, S., 2002. Experiments on deaeratinggranular flows and implications for pyroclastic flow mobility. Geophys. Res.Lett. 29, 1–4.

Scheu, B., Kueppers, U., Mueller, S., Spieler, O., Dingwell, D.B., 2008. Experimen-tal volcanology on eruptive products of Unzen volcano. J. Volcanol. Geotherm.Res. 175, 110–119.

Scheu, B., Spieler, O., Dingwell, D.B., 2006. Dynamics of explosive volcanism at Un-zen: an experimental contribution. Bull. Volcanol. 69, 175–187.

Schultz, P.H., 1992a. Atmospheric effects on ejecta emplacement. J. Geophys. Res.Planets 97, 11623–11662.

Schultz, P.H., 1992b. Atmospheric effects on ejecta emplacement and crater forma-tion on Venus from Magellan. J. Geophys. Res. Planets 97, 16183–16248.

Schultz, P.H., Gault, D.E., 1979a. Atmospheric effects on Martian ejecta emplacement.J. Geophys. Res. 84, 7669–7687.

Schultz, P.H., Gault, D.E., 1979b. Martian impact crater ejecta emplacement. In: Mars,Abstracts from the Second International Colloquium held 15–18 January, 1979at the California Institute of Technology, Pasadena, CA. In: NASA Conf. Publ.,vol. 2072. National Aeronautics and Space Administration, Scientific and Techni-cal Information Branch, Washington, DC, p. 72.

Schultz, P.H., Singer, J., 1980. A comparison of secondary craters on Moon,Mercury and Mars. In: Proc. 11th Lunar and Planetary Science Conference,pp. 2243–2259.

Senft, L.E., Stewart, S.T., 2007. Modeling impact cratering in layered surfaces. J. Geo-phys. Res. Planets 112, 11, http://dx.doi.org/10.1029/2007JE002894.

Senft, L.E., Stewart, S.T., 2008a. Impact crater formation in icy layered terrains onMars. Meteorit. Planet. Sci. 43, 1993–2013.

Senft, L.E., Stewart, S.T., 2008b. Impact crater formation in icy layered terrains onMars. In: Herrick, R., Osinski, G.R., Pierazzo, E. (Eds.), Proceedings of the Work-shop, Bridging the Gap II, Effect of Target Properties on the Impact CrateringProcess. Meteoritical Society, Fayetteville, AR, United States, pp. 1993–2013.

Sneed, E.D., Folk, R.L., 1958. Pebbles in the lower Colorado River, Texas; a study inparticle morphogenesis. J. Geol. 66, 114–150.

Spieler, O., Dingwell, D.B., Alidibirov, M., 2004. Magma fragmentation speed, an ex-perimental determination, the role of laboratory experiments in volcanology. J.Volcanol. Geotherm. Res. 129, 109–123.

Squyres, S.W., Knoll, A.H., Arvidson, R.E., Clark, B.C., Grotzinger, J.P., Jolliff, B.L.,McLennan, S.M., Tosca, N., Bell III, J.F., Calvin, W.M., Farrand, W.H., Glotch, T.D.,Golombek, M.P., Herkenhoff, K.E., Johnson, J.R., Klingelhöfer, G., McSween, H.Y.,Yen, A.S., 2006. Two years at Meridiani Planum: Results from the opportunityrover. Science 313, 1403–1407.

Stewart, S.T., Ahrens, T.J., 2003. Shock Hugoniot of H2O ice. Geophys. Res. Lett. 30, 4.Stewart, S.T., Ahrens, T.J., 2005. Shock properties of H2O ice. J. Geophys. Res. Plan-

ets 110, E03055.Stewart, S.T., Ahrens, T.J., O’Keefe, J.D., 2001a. Mars rampart crater ejecta yields re-

golith water content. In: Geological Society of America 2001 Annual Meeting.Boston, MA, United States, November 1–10, 2001. Geological Society of America,p. 433.

Stewart, S.T., Ahrens, T.J., O’Keefe, J.D., 2003. Impact-induced melting of near-surfacewater ice on Mars. In: Furnish, M.D., Gupta, Y.M., Forbes, J.W. (Eds.), Shock Com-pression of Condensed Matter, pp. 1484–1487.

Stewart, S.T., O’Keefe, J.D., Ahrens, T.J., 2001b. The relationship between rampartcrater morphologies and the amount of subsurface ice. Lunar Planet. Sci. Conf.Abstr. 32. Abstract 2092.

Sturm, S., Wulf, G., Jung, D., Kenkmann, T., 2013. The Ries impact, a double-layerrampart crater on Earth. Geology 41, 531–534.

Wada, K., Barnouin-Jha, O.S., 2006. The formation of fluidized ejecta on Mars bygranular flows. Meteorit. Planet. Sci. 41, 1551–1569.

Wilson, C.J.N., 1980. The role of fluidization in the emplacement of pyroclastic flows,an experimental approach. J. Volcanol. Geotherm. Res. 8, 231–249.

Wilson, C.J.N., 1984. The role of fluidization in the emplacement of pyroclastic flows;2. Experimental results and their interpretation. J. Volcanol. Geotherm. Res. 20,55–84.

Wohletz, K.H., Sheridan, M.F., 1983. Martian rampart crater ejecta; experiments andanalysis of melt–water interaction. Icarus 56, 15–37.