Spectral probing of impact-generated vapor in laboratory ...Spectral probing of impact-generated...

Icarus 248 (2015) 448–462

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Icarus

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Spectral probing of impact-generated vapor in laboratory experiments

http://dx.doi.org/10.1016/j.icarus.2014.10.0410019-1035/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail addresses: [email protected] (P.H. Schultz), [email protected]

(C.A. Eberhardy).

Peter H. Schultz ⇑, Clara A. EberhardyDepartment of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street, Providence, RI 02912, United States

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

Article history:Received 23 April 2014Revised 14 October 2014Accepted 23 October 2014Available online 31 October 2014

Keywords:Impact processesExperimental techniquesSpectroscopy

High-speed spectra of hypervelocity impacts at the NASA Ames Vertical Gun Range (AVGR) captured therapidly evolving conditions of impact-generated vapor as a function of impact angle, viewpoint, and time(within the first 50 ls). Impact speeds possible at the AVGR (<7 km/s) are insufficient to induce signifi-cant vaporization in silicates, other than the high-temperature (but low-mass) jetting component createdat first contact. Consequently, this study used powdered dolomite as a proxy for surveying the evolutionand distribution of chemical constituents within much longer lasting vapor. Seven separate telescopesfocused on different portions of the impact vapor plume and were connected through quartz fibers totwo 0.35 cm monochromaters. Quarter-space experiments reduced the thermal background and opaquephases due to condensing particles and heated projectile fragments while different exposure times iso-lated components passing through different the fields of view, both above and below the surface withinthe growing transient cavity. At early times (<5 ls), atomic emission lines dominate the spectra. At latertimes, molecular emission lines dominate the composition of the vapor plume along a given direction.Layered targets and target mixtures isolated the source and reveal that much of the vaporization comesfrom the uppermost surface. Collisions by projectile fragments downrange also make significant contri-butions for impacts below 60� (from the horizontal). Further, impacts into mixtures of silicates with pow-dered dolomite reveal that frictional heating must play a role in vapor production. Such results haveimplications for processes controlling vaporization on planetary surfaces including volatile release, atmo-spheric evolution (formation and erosion), vapor generated by the Deep Impact collision, and the possibleconsequences of the Chicxulub impact.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

Hypervelocity impacts generate a sudden and intense flash.Early studies using microscopic projectiles recognized multiplecomponents of the flash (Gehring and Warnica, 1963; Jean andRollins, 1970; Eicchorn, 1976; Tsemblis et al., 2008). High-speedcameras recording macro-scale (cm) experimental impacts, how-ever, reveal that the flash represents a complexly evolving plumeof cooling plasma (Schultz, 1996; Crawford and Schultz, 1999),early plasmas and jetting phases (Kondo and Ahrens, 1983; Ang,1990; Yang and Ahrens, 1995; Sugita et al., 1998; Sugita andSchultz, 1999), later expanding vapor (Schultz, 1996; Kadono andFujiwara, 1996; Sugita et al., 1998; Schultz et al., 2006; BruckSyal et al., 2012; Mihaly et al., 2013), and molten/heated ejecta(Ernst and Schultz, 2004, 2007; Tsemblis et al., 2008; Ernst et al.,2011). More recent studies characterize plasmas generated byimpacts into plates using time-resolved spectroscopy at speeds

from 8 km/s (e.g., Heunoske et al., 2013) to 25 km/s (Reinhart etal., 2006).

The NASA Ames Vertical Gun Range (AVGR) permits impacts atdifferent angles into flat-laying unconsolidated half-space targetsand water. The large size of the impact chamber (2.5 m) allowsimpact-generated vapor to not only expand freely but also separateinto different components without interference. Rather than a radi-ating point source at the point of first contact, the high-speed imag-ing reveals that the ‘‘flash’’ from a 0.635 cm diameter projectile isactually a large (10s of cm in scale), self-luminous vapor plume(Schultz, 1996). The transient cavity briefly restrains part of thevapor, just as in early models of vertical impacts (e.g., O’Keefe andAhrens, 1977), after which the vapor rapidly evolves into a plumeexpanding spherically above the surface before eventually engulf-ing the surface. For oblique impacts, however, the plume exhibitsa more complex evolution. An early-stage, high-speed jetting phasemoves downrange at speeds that are three times the initial impactspeed (e.g., Walsh et al., 1953; Jean and Rollins, 1970; Gault andHeitowit, 1963; Kieffer, 1977; Sugita and Schultz, 1999), followedby a later downrange-moving and expanding vapor plume(Schultz, 1996). Consequently, oblique impacts result in both hot

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and cool phases evolving in different directions (Schultz, 1996;Pierazzo and Melosh, 2000; Schultz et al., 2006).

Hypervelocity impacts generate vapor when material undergoesirreversible heating unloading behind a high-enough shock. As aresult, peak shock pressure is commonly used to predict vaporiza-tion, both analytically and in numerical models. Equations of state(EoS) are needed in order to determine the amount of vaporizationwith values that are unique for a given material (e.g., Pierazzo andMelosh, 2000). The EoS describes the thermodynamics of a systemover a wide range of pressures, temperatures, and specific volumes.At the high speeds characterizing planetary impacts, the EoS is bestunderstood along the Hugoniot, i.e., the locus of possible end statesobtained across a single shock wave from an initial state.

Gupta et al. (2002) determined that complete devolatization ofsolid carbonate requires a peak pressure of 110 ± 10 GPa. Althoughconsistent with prior measurements by Yang and Ahrens (1996),such high pressures seemed at odds with results of other impactexperiments (Schultz, 1996). More recently, Ohno et al. (2008)found that the shock pressure for complete vaporization of solid cal-cite would need to be only 25 GPa. Porous targets require evenlower peak pressures, with the onset speed for vaporization of por-ous carbonate targets as low as 1–1.5 km/s (Shen et al., 2003). Forreference, a 2D model calculates that a 5 km/s (Pyrex) verticalimpact into porous (40%) carbonate, such as dolomite (CaMg(CO3)2),produces a peak pressure of �24 GPa. For oblique impacts, peakpressures decrease as (sinh)2 for impact angles (h) referenced tothe horizontal (Gault and Wedekind, 1978). A 30� impact angle,therefore, should yield a peak pressure of only 6 GPa, equivalentto a 1.25 km/s vertical impact. Although such pressures may be con-sistent with onset conditions for vaporization following Shen et al.(2008), the effect of impact angle remains enigmatic. Specifically,an impact at 15� generates more than two orders of magnitude morevapor than does a vertical impact (Schultz, 1996), in contrast withnumerical models for silicates at the much higher speed of 20 km/s (Pierazzo and Melosh, 2000). Hence, heating from scouring andfrictional shear by the failed projectile impacting downrange mustplay a role, at least for volatile-rich targets.

Documenting the speciation and distribution of the vapor prod-ucts (e.g., atomic versus molecular components) provides a betterunderstanding of the impact-vaporization process and chemicalprocessing within the plume. Such an understanding contributesto understanding volatile delivery to the Moon and Mercury, theconsequences of the Chicxulub impact on Earth 65 myr ago, andinterpretations of the Deep Impact mission results. Because muchhigher speeds are necessary for impact-generated vapor from sili-cate targets, the present study uses dolomite as a proxy target inorder to assess the evolution of (and composition within) the vaporplume and the underlying processes.

Sugita et al. (1998) previously captured time-resolved spectro-scopic observations at the AVGR as viewed from above the impactpoint. The ratio of spectral emissions with different excitation tem-peratures established the effect of impact speed and angle on theearliest components for solid dolomite targets. Moreover, the ini-tial stages (first few microseconds) were found to depend on thevertical component of velocity, consistent with expectations forpeak pressures. Derived temperatures ranged from near 6000 K(vertical impacts) to 4000 K (30� impact) for speeds near 5 km/s.Since the jetting phase dominates the earliest spectral emissions(mostly atomic), time-resolved spectroscopy could be used torefine the theory of jetting, including the relative contribution ofthe projectile and target (Sugita and Schultz, 1999). The totalintensity of optical emission over visible wavelengths was foundto depend on impact velocity with a power law exponent of 5 withdifferent emission lines exhibiting different power laws dependingon the specific emitting species (Sugita et al., 2003). Hence,multiple processes may contribute to the observed vapor.

Considerable spectral content, however, remains well after thejetting phase, especially for porous targets (Schultz et al., 2007).Consequently, the present study surveys the evolution of spectralemission lines from different viewpoints for impacts into poroustargets. The specific objectives are to: (a) assess the evolving spec-tral content as a function of time and location; (b) place first-orderconstraints on the source (depth) of vaporization; (c) examine someof the controlling processes at the impact velocities available; and(d) consider the implications for Solar System exploration. Ourresults serve as a guide for follow-on quantitative studies (e.g., tem-peratures and abundances) and benchmarked numerical models forfreely expanding impacts into half-space targets.

2. Laboratory experiments

The NASA Ames Vertical Gun Range (AVGR) is designed for awide range of projectiles and targets with impact speeds up to7 km/s at different impact angles into gravity-affected targets(sands, water, etc.). View windows from seven different directionsallow documentation of the evolving impact-generated vapor withhigh-speed imaging and spectrometers. The following study usestargets composed of target materials previously observed to vapor-ize experimentally, such as carbonates (e.g., powdered dolomite),dry ice, or polycarbonates.

For the present study, four separate telescopes were connectedto one of two McPherson Model 2035 Monochromaters (Czerny-Turner type, 0.35 m focal length with 300 lines/mm or 600 lines/mm gratings) through quartz fibers connected to each of two OrielInstaSpecV-ICCD detectors (Model 77193-5). Consequently, a totalof seven spectral observations were possible. In general, the spectralrange covered typically 440–600 nm (in some cases to 640 nm).Four narrow-field telescopes (�2.5 cm field of view, FOV) linkedto each monochromater simultaneously captured spectra in multi-ple regions of the expanding vapor plume (Fig. 1A). Exposure timesvaried, depending on the intensity of the source region and experi-mental objectives. Intensity calibrations were made using both a fil-ament light source (Tungsten halogen lamp, Oriel Corporation,Model 63355) and an extended light source (Labsphere, ModelUSS-600), following the procedures and measurement errorsdescribed in Sugita et al. (2003). As noted there, relative intensitywithin the same spectrum has an error of only 3%. For some exper-iments, a Princeton Instruments PI-MAX: 512-HQ thermal camera(512 � 512) simultaneously captured short exposure (50 ns)exposures at certain times with sensitivity from the visible to nearinfrared (flat response over from �540 nm to �850 nm).

Full-space experiments refer to energy release below the sur-face within a semi-infinite target, whereas half-space experimentsrefer to a target with a free surface, e.g., by an impacting projectile.A quarter-space experiment, then, splits the target in half again byusing a transparent sheet: specifically a thick (3/400 thick) acrylicsheet placed along the trajectory and perpendicular to the detector(Fig. 1B). Such a strategy provides a window into the cratering pro-cess, such as crater growth (e.g., Piekutowski et al., 1977; Schultz,2013). Here, quarter-space experiments are used to look inside theevolving vapor plume, both above and below the surface (e.g.,Eberhardy and Schultz, 2003, 2004).

Careful positioning of the target allows the projectile to pass byan acrylic sheet within a centimeter or less. First contact by theprojectile too close to the sheet fractures and disrupts the window;contact too far, obscures the initial stages. Optimally, the projectileshould be within about a projectile radius (3 mm) away from thesheet. After the shock wave propagates away from the sheet, theinitial penetration of the projectile creates a gap next to the sheet,thereby coupling little energy to the Plexiglas even at hyperveloc-ities (5–6 km/s). In successful quarter-space experiments, a smallzone in the sheet exhibits light scouring. This zone occurs just

A

B

Vapor plume

Fig. 1. Spectrometers set up for observing the vapor plumes generated byhypervelocity impacts (A). Seven telescopes (in the window) are coupled to twospectrometers by quartz fibers. Also shown are four high-speed cameras forcapturing the evolving vapor plume. The launch assembly is in the upper right,positioned for an impact at 30� from the horizontal. The large impact chamberallows free expansion of impact-generated vapor. (B) Schematic showing a quarter-space experimental set up (inside the impact chamber) where the projectile justmisses a vertical sheet of Plexiglass, thereby allowing detectors to probe inside thetransient crater and vapor plume.

450 P.H. Schultz, C.A. Eberhardy / Icarus 248 (2015) 448–462

below the surface with a depth (depending on impact angle) andcorresponds to the zone of maximum energy transfer, i.e., the‘‘effective depth of burst.’’ Here we compare spectra from the sameseries of experiments in order to minimize effects of variations inexperimental setup and pointing. All experiments were done undernear-vacuum conditions (pressure less than 0.7 mmHg). Any resid-ual atmosphere has little to no effect on either heating the vapor orslowing expansion because of the amount of vapor produced. Thiscan be demonstrated by observations of the jetting phase and latervapor that are not decelerated, as well as the absence of any spec-tral signature from the atmosphere (e.g., Schultz, 1996).

Sugita et al. (1998) provides a review of the basics of spectralobservations as applied to impact-generated vapor plumes. Verybriefly, atomic emission lines are generated by electronic transi-tions from an excited state back to a ground or less-excited state.The energy level, Einstein-A coefficient, and number of atoms inthe excited state control the intensity and wavelength of a specificemission line. The spectral range chosen for the study here coversthe visible range, generally between 390 nm and 620 nm. Thischoice of both spectral range and resolution is based on the

sensitivity of the available ICCD cameras, the desired exposuretimes, and available spectral transparency of the view ports. Ongo-ing and future studies (e.g., Heunoske et al., 2013; Bruck Syal andSchultz, 2014) will quantify conditions (temperature, abundances,plasma components) within the vapor plume using greater resolu-tions and shorter wavelengths allowed by the next-generationICCD cameras.

The following survey results from 33 different impact experi-ments. First, we contrast the evolution of the vapor plume as deter-mined from viewpoints above and below in order to place thespectral observations in context. Next, a general comparison ismade for different target types in images taken from above andbelow at different times in order to place later spectral observa-tions in context. These results illustrate the need for looking insidethe vapor plume by the use of quarter-space experiments (aboveand within the transient crater at different times and for differentimpact angles). We then explore the process and source of vapor-ization by examining the effect of surface layers and mixtures onthe vaporization process. We finally consider possible implications,including the release of carbon dioxide from carbonate targets.

3. Vapor plume evolution viewed from above (half space)

Different portions of the moving and expanding plume passthrough the fields of view of each spectrometer during differentexposures. Consequently, it is important to understand what isactually being recorded in the resulting spectra. Fig. 2 illustratesthe evolution of vapor phases viewed from above (1/2 space, 30�impact, Fig. 2A) and from the side (1/4 space, 60� impact, Fig. 2B)as captured in short exposures (5 ns) by a thermal imaging cameraat different times. Views from above reveal the rapid downrangemotion of the expanding vapor with a well-defined serrated front,likely the result of Kelvin-Helmholtz instabilities created at firstcontact by the spherical projectile. Laterally, the front initiallyexpands at very high speeds (>10 km/s) but slows after approxi-mately 5 ls to a value representing the initial non-isentropic gasexpansion speed. Downrange, the front travels at a speed corre-sponding to this expansion rate added to the horizontal velocitycomponent of the impactor (Schultz et al., 2007). The leading edgeshown here does not represent the jetting phase, which continuesdownrange at speeds three times the initial impact speed until hit-ting the witness plate attached to the wall of the chamber. Thiscomponent composed of hot vapor is captured in other imagingsystems (e.g., Schultz, 1996) but not detected in the thermal imag-ing system.

For oblique impacts, sheared off portions of the projectile(decapitation fragments) decouple from the impact the soon afterfirst contact but strike the surface downrange. As a result, thebrightest (hottest) regions occur downrange from the impact soonafter first contact (Fig. 2A). In views from above, an opaque compo-nent partly obscures the region just downrange, likely the result ofthe disrupted projectile along with early-time ejecta debris. Sideviews using the 1/4-space configuration (Fig. 2B), however, allowlooking inside the vapor plume at different locations and reducethermal contributions from hot debris. In this case, the subsurfaceis blocked-off from view in order to reduce scattered light fromwithin the cavity. The vapor plume expands not from the centerof the final crater but from a point offset downrange from theimpact point.

Previous experiments used copper projectiles impacting soliddolomite in order to increase the peak pressures and optimizeatomic emission spectra (e.g., Sugita et al., 1998). Here we concen-trate instead on vapor generated from impacts into porous (pow-dered) dolomite targets, which results in a plume smaller thanthat from a solid target (over the first 5 ls after impact, Fig. 3).In both cases, the expanding plume exhibits a serrated leading

1µs 2µs 5µs 8µs 1cm

A

B

Fig. 2. (A) Half-space experiment: evolution of plume (view from above) for 0.635 cm-diameter Pyrex spheres impacting into a mixture of dolomite and sand at 30� fordifferent times after impact (microseconds). Bright areas downrange from the impact point result from heating by the failed projectile impacting downrange. (B) Quarter-space experiments: evolution for 60� impact angle (from the horizontal) that exposes the interior of the vapor plume. Non-volatile ejecta emerge laterally by 5 ls (right) asopaque phases. Circles indicate the impact point. Each image represents separate experiments at speeds close to ~5.8 km/s (5.9 km/s; 5.98 km/s; 5.72 km/s; 5.93 km/s). Thejetting phase is absent in this short-exposure thermal image (and in (A)) since it is composed largely of hot gas phases.

Fig. 3. Comparing solid (left) vs. powdered dolomite (right) targets viewed from above 5 ls after impact by a 0.635 cm Pyrex sphere (30�) at 5.9 km/s and 5.8 km/s,respectively for half-space experiments. The very bright (hot) regions downrange correspond to downrange impacts by decapitated fragments of the projectile. As in Fig. 2,the hot jetting plasma (leading the plume boundary) is absent in this thermal image. Dark stringers downrange from the impact represent cooler debris in the foreground(largely projectile fragments). Bar-scale corresponds to 1 cm; cross indicates impact point.


edge along with a strong thermal source (bright in these thermalimages) downrange. Although at about the same distance fromthe impact point, these ‘‘hot’’ zones differ downrange: the impactinto the solid dolomite has a narrow linear zone perpendicular tothe trajectory while the impact into the powdered dolomite resultsin two bright spots. These zones result from impacts by the decap-itated projectile impacting at low angles downrange (see Schultz,1996). The opaque stringers downrange from the impact pointlikely indicate cooler fragments sheared (and spalled) off the topof the projectile in the foreground (Schultz and Gault, 1990). Theleading edge of the vapor from the solid dolomite outpaced thatfrom the powdered dolomite (at 5 ls) due to the contrast in bothcoupling time at first contact and containment of the vapor beforefree expansion.

Target porosity affects not only the evolution of the expandingvapor plume but also the spectral content. Over the first 50 ls,

impacts into the dolomite block (Fig. 4, image B) and dolomite pow-der (Fig. 4, image C) both exhibit a strong thermal background(increasing intensity toward the red). They differ in that atomicand molecular emission lines characterize the impact into the soliddolomite, in contrast with the weaker atomic emissions but stron-ger molecular emission lines from the powdered dolomite targets.The molecular components (e.g., CO, CaO, MgO, and C2) representeither products of thermal decomposition at lower temperatures(e.g., from later stages of coupling) or back reactions (i.e., atomicand molecular species recombining within the plume). In the lattercase, formula balancing requires the presence of emission lines of Caand Mg in the plume as well. Consequently, the latter case shouldhave detected these atomic emissions. Their absence may indicatean optically thick plume of both gas and dust in the foreground.

Covering the dolomite with a layer of sugar with a thicknessequal to a projectile radius significantly suppressed the emergence

C

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0

0.1

0.2

0.3

400 430 460 490

0.4A

µµ

Fig. 5. (A) Spectra (600 line grating) captured during the first 50 ls at differentdistances (right panel) from the impact (30�, 5.89 km/s, Pyrex sphere) that revealthe evolving composition of the expanding vapor plume. Close to the impact,atomic emission lines dominate; farther away along the same radial line from theimpact point, molecular bands of CO, C2, and MgO dominate. Since the same packetof vapor passed by the downrange field of view over 50 ls, such compounds arisefrom either a cooling plume undergoing fast back-reactions among components orthe effects of late-emerging low temperature gas (or both). For other fields of view,see Schultz et al. (2007). (B) Evolving spectral content (600 line grating) revealed bythe leading edge of the vapor plume passing by the field of view (filled circle atright) over two different times as viewed from above for 5.9 km/s (30�) impacts intoa mixture of sand and dolomite. Grey-filled circles indicate fields of view consideredin (A). Over the first 50 ls, all parts of the plume pass through the field of view (bluecurve) and exhibit both strong elemental and molecular emission lines superim-posed on a thermal continuum. Delaying the exposure (12–50 ls, red) excludes theleading edge (including the un-imaged jetting phase). The thermal image at rightshows the approximate location of the leading edge of the plume after 12 ls. Thisspectrum captured the late-stage, evolving vapor (not just jetting phase). It appearsthat ~75% of the molecular signal comes from the later arriving plume (after 12 ls).(C) Comparison between spectra viewed from above (half-space experiment) andfrom the side (quarter-space experiment) for hypervelocity impacts into dolomiteat 30� exposed from 0 to 50 ls after impact. The view from the above captures awide range of components superimposed on a thermal (black body) background.The 1=4-space experiment viewed from the side (downrange), however, minimizesthe thermal component. View from above (blue): 0.635 cm Pyrex sphere at 5.8 km/s. Side view (1=4-space): 0.635 cm Pyrex sphere at 5.6 km/s.

Fig. 4. Effect of porous surface layers on evolving spectra focused (from above).Spectrum A: Thin (3 mm) layer of sugar over dolomite powder with a sprinkling(~0.3 mm layer) of graphite on top (5.94 km/s). This spectrum reveals effectivenessof a thin layer in suppressing (or masking) emission lines from underlying dolomitepowder. The thin graphite veneer results in a strong thermal component due to theheated carbon droplets expanding in the foreground, above the vapor components.Spectrum B: The greatest spectral content comes from an impact (5.89 km/s) into asolid dolomite block and reveals dissociation products of dolomite (Ca, Mg, CaO,Mg, and C2). Spectrum C: Impact into dolomite powder (5.8 km/s) with moresuppressed expression of atomic emission lines. Spectrum D: Impact (5.75 km/s)into dolomite with 3 mm layer of sugar over dolomite powder. Except for Case D, allspectral lines are superimposed on an elevated thermal background. All experi-ments used 0.635 cm diameter Pyrex spheres (30�); spectra exposed from 0 to50 ls. The noise in these (and following) spectra is not instrumental but resultsfrom scattering by small particles. Location of the field of view is shown Fig. 5B.


of atomic and molecular emissions from the dolomite below andresulted in a very weak blackbody component (Fig. 4, spectrumD). Sprinkling graphite on top of this layer of sugar, however, gen-erated atomic and molecular emissions (Ca, Mg) superimposed ona much stronger blackbody background (Fig. 4, spectrum A). Thecontrast in spectral content between spectra A and D can be attrib-uted to contributions from hot, condensed carbon particles in A.The oblique impact into the thin graphite layer on top of the sugar(spectrum A) generated carbon condensates within 5 ls (time ofthe accompanied exposed image A) but then mixed with the sugarand dolomite below during penetration. The impact into just thelayer of sugar (spectrum D), however, fully vaporized the sugar.Much later imaging for the same experimental set up revealed adelayed emergence of the strong blackbody farther downrange,out of the view captured here (see Schultz et al., 2007). Thissequence indicates delayed condensation of carbon droplets outof the plume of vaporized sugar since such delay only occurs whensugar is added to the target material. In all cases, the spectral‘‘noise’’ is not due to poor signal to noise ratio but to Mie scatteringfrom condensed droplets of different sizes.

The composition of impact-generated vapor evolves with time,as documented in views from above. One strategy for capturing thisevolution is to observe the plume passing by two different fields ofview radial from the impact point (Fig. 5A); another is to sample thevapor plume at the same locations at different times (Fig. 5B). InFig. 5A, the change in composition along a radial from the impactpoint corresponds to temporal changes during a 50 ls exposurefrom the moment of impact. Close to the impact, atomic emissionlines are strong (Mg, Ca); farther away, atomic emissions fade whilemolecular lines increase. The one exception is the 423 nm Ca line,which remains strong in both locations due to its low excitationenergy level (see Sugita et al., 1998). Since both fields of view cap-tured the same parcel of vapor (both hot and cool) passing by dur-ing the 50 ls exposure, we conclude that the vapor plumechemistry must be rapidly evolving with back reactions, i.e., therecombination of Ca, Mg, CO in the plume that lead to MgO and CO.

Another strategy captures the passage of the plume at the samelocation at two different times directly downrange from the impact(Fig. 5B). In this case, however, the target is a 1:1 mix (by weight)

A

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Target plane

Fig. 6. (A) Quarter-space experiments comparing the evolution of the vapor plumefrom a vertical (left, 90�) at 5.78 km/s with the plume from an oblique impact (atright, 60�) at 5.69 km/s into dolomite powder, both 5 ls after impact. Images havebeen logarithmically stretched and contrast enhanced to bring out detail. Bar-scalecorresponds to 1 cm. Note plume with a bright, serrated edge that leads the brightercentral portion. (B) Locations and fields of view (FOV) for different spectra for aseries of 60� impacts in dolomite superimposed on the thermal image in (A)(providing reference for illustrations to follow). Numbers indicate specific telescopeFOV (about 2.5 cm in diameter in grey) linked to the spectrometer by quartz fibersto two different spectrographs: 1 and 5 (25� above the surface from the impactpoint); 2 and 4 (45�); 3 and 6 (60�); 7 (90�). Image is the same as shown in Fig. 2B,showing a side view of an oblique impact into dolomite at 5.69 km/s (exposed from0 to 5 ls after impact).

400 425 450 475 500 525 550 575 600 625

400 425 450 475 500 525 550 575 600 625

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Fig. 7. Comparison of spectral content within vapor plume in different FOV’suprange (colored dots, Fig. 6) during the first 5 ls from an impact by a Pyrex sphere(0.635 cm) impact at 5.45 km/s (60�) downrange (top, (A)) and uprange (below, (B)).Note that the spectral content in the plume is non-uniform. The strongest atomicemissions come from FOV’s directed downrange, even for this relatively high angleimpact (FOV-4 and -5). Nevertheless, emission lines viewed above the impact point(FOV-6 and -7) are weaker than those observed uprange (FOV-1 and -2). Theprojectile (Pyrex) contributes to the sodium line, which can be seen uprange (FOV-1, -2). The absence of spectra beyond 575 nm in (A) is due to a different center pointfor the monochromater.

Fig. 8. Comparison between early and late stages below the impact point for a 60�impact into dolomite powder (filled circle in inset). Vaporized dolomite products fillthe cavity and produce strong atomic emission lines with evolving compositions. Athermal component (hot debris) raises the overall level. Sodium line results fromvaporization of portions of the projectile and appears to reduce in strength withtime as it escapes the transient cavity. Later, molecular species from the targetbecome more important. The 0–2 ls exposure captured emissions from a 5.68 km/s,0.635 cm Pyrex sphere, whereas emission lines from the 2 to 20 ls exposureresulted from a 5.73 km/s, 0.635 cm, Pyrex sphere.


of sand and dolomite. The long exposure (0–50 ls) samples theearliest and hottest part of the vapor as well as the later coolercomponents. Delaying the exposure by 12 ls, however, eliminatescontributions from the hottest portion of the vapor. For example,the jetting component would have traveled 18 cm by this time,well beyond the FOV for this viewpoint. The composition of thislater arriving vapor resembles the component captured in theslightly more distant FOV (0–50 ls) in Fig. 5A but differs in theblackbody component. The delayed exposure (12–50 ls, Fig. 5B)captures a cooler background (lower slope of the blackbodybackground) directly downrange, perhaps because of the lesseramount of condensed species from dolomite (due to the sandmixture).

All spectra viewed from above (excepting Case D from a layer ofsugar in Fig. 4) exhibit a significant thermal component. Fig. 5Ccontrasts a view from above (1/2 space) with a view from the side(1/4 space) exposed over the first 50 ls. Surprisingly, the side viewresults in very little contribution from a thermal component, alongwith a much cleaner spectrum due to reduced scattering from con-densed droplets. While spectra taken from above contain spectrallines from both molecular and atomic emissions, the 1/4-spaceexperiment exhibits no contributions from molecular species.

Consequently, the view from above appears to be capturing emis-sions from multiple phases with contrasting temperatures passingthrough the field of view during the exposure.


Consequently, Figs. 4 and 5 demonstrate the need to: (a) isolatedifferent components; (b) reduce the contribution from the ther-mal component; and (c) use different exposure times in order tounderstand the evolving vapor plume. In addition, it appears thatthe source of the vapor component comes from the uppermost sur-face. These three issues are considered next.

4. Vapor plume evolution viewed from side (quarter space)

In half-space experiments, views from above capturemulti-phase components (projectile fragments, melt, and differ-ent-temperature vapor components) passing through the field ofview during long exposures (Fig. 5C). Views from the side, on theother hand, are dominated by contributions from the leading edgeexpanding in the foreground. The quarter-space arrangement,however, eliminates contributions from foreground while differentexposures at different times isolate contributions from various ele-ments of the plume passing through the FOV.

Before comparing spectra, we first contrast the evolution ofexpanding impact-generated vapor from a vertical impact and anoblique impact (Fig. 6A). This difference illustrates the degree oftemporary containment within the transient crater before freelyexpanding above the surface. In vertical impacts, the transient cra-ter temporarily restricts and redirects expanding gas. Obliqueimpacts, however, result in a vapor plume that expands upwardas it moves rapidly downrange, thereby exposing different stagesof the process more clearly.

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Fig. 9. Comparison of the evolving vapor plume as viewed above and below thetarget surface. Elemental emission lines dominate early-time spectra from a5.68 km/s impact. (A) molecular emissions dominate later from a 5.73 km/s impact.(B) Vapor fills the transient cavity, even later (2–20 ls). Above the impact point, thereversal in relative intensities of 424 nm and 432 nm calcium lines at differenttimes indicate decreasing temperature of the gas phase. These emission lines aresuperimposed on a background continuum. Sodium line from the projectile(0.635 cm Pyrex sphere) is strong above the impact but weak within the cavityafter 2 ls.

4.1. Above and below the impact

Each light-gathering telescope focused on different regions cap-ture different portions of the vapor plume (Fig. 6B). The positioningclose to the impact was necessary in order to use short exposuretimes for some spectra. The strongest spectral emission (greatestamount of vapor and/or highest temperature) occurs 2.5 cm down-range, only about 1 cm above the surface at FOV-5 (Low Down-range = Low-DR, Fig. 7A). Even for impact angles as high as 60�,asymmetry in the spectral content is evident here, with the leastsignal in the direction of the incoming projectile (no. 6, Fig. 7B).Directly uprange (FOV-1, UR-Low), the inferred vapor mass (basedon the overall spectral intensity) exceeds that directly above theimpact point (no. 7, above). Note that the sodium line likely comesfrom the Pyrex projectile (2.8% by weight) since dolomite has less(<0.3%). This component is not caught in the thermal imaging cam-era but can be identified in high-speed imaging.

Impact vapor also fills the transient crater directly below theimpact point during the first 2 ls (Fig. 8). For these experiments,a mask placed in front of the acrylic sheet blocks out scatteringfrom sources above the impact point. The observed vapor couldnot be related to jetting, which is created at the surface betweenthe projectile and target (see Sugita and Schultz, 1999), althoughit is possible that this component is redirected below the surfaceduring penetration and mixed turbulently in porous targets. Notethat the sodium-D couplet (589.29 nm) is strongest during the ear-liest exposure due to vaporization of the projectile at first contact.

Later (between 2 ls and 20 ls, Fig. 8), vapor continues to fill thegrowing cavity, but strong CaO and CO emission lines replace dis-association products (Ca, C2, and Mg) after 2 ls. As before (e.g.,Fig. 5A), the increased abundance of molecular bands reflectseither back reactions within the cavity or lower temperaturebyproducts. In addition, the reduced intensity of atomic emissionsis accompanied by a slight increase in the blackbody (thermal com-ponent). This means that either heated particulates (projectile andtarget) are emerging from the cavity or that the vapor phases arecondensing.

Above the point of first contact, spectral emission lines remainmuch stronger than emission below the surface, whether duringthe initial 2 ls (Fig. 9A) or later (Fig. 9B). During the first 2 ls,the continuum is relatively flat from 400 nm to 600 nm with weakmolecular bands (CaO, MgO, CO) but very strong lines of Ca, Mg,and C2, both above and below the impact point. In addition, theNa line (from the projectile) appears strongest above the surfacein an absolute sense, but not relative to other lines below the sur-face. Later (2–20 ls), the molecular bands of CaO, MgO, and CaOdominate the spectrum above the impact. Also note the reducedchange in the contribution from Mg (near 519 nm) relative to Ca(528 nm).

The greater intensity of the Ca line near 430 nm (characterizedby higher excitation temperature) compared to the line near445 nm confirms expectations for the higher temperature at earliertimes, yet the stronger 422.7 nm line above the impact point at latetime seems inconsistent with this interpretation. This inconsis-tency results from self-absorption at 422.7 nm due to its low exci-tation temperature, which decreases its intensity under strongemission. Greater spectral resolution can establish actual vaporphase temperatures through the Boltzmann equation (followingSugita et al., 1998; Bruck Syal and Schultz, 2014), but this is notthe focus of this general survey.

Melted ejecta and condensates entering the FOV contribute astronger blackbody above the surface between 2 ls and 20 ls(Fig. 9B). There, different components of the vapor plume (e.g., jet-ting, shock heated ejecta, and thermal disassociation products)

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Fig. 11. Evolution of spectral content in different locations within the transientcrater at two different times (60� impact, 5.61 km/s). (A) Immediately after impact(2.5 ls), the transient cavity is filled with high-temperature vapor, which isdominated by atomic emission lines from disassociation products of dolomite (Ca,C2, and Mg). Strongest emission lines occur below the impact point. (B) Molecularspecies (CaO, MgO) dominate spectral content later (2.5–20 ls). This figure revealsthat the vapor is not uniform within the transient cavity: the highest temperaturevapor phases occur directly below the impact point but fill the cavity in everydirection before escaping. Different spectral limits between (A) and (B) reflectdifferent ICCD cameras with different band centers in the monochromater. Theinsets in the upper right are two superimposed images (50 ns exposure, 5 ls afterimpact) of a 60� impact (quarter space) isolating the vapor plume expanding aboveand filling the transient crater below the surface.


freely expand and move rapidly downrange as they pass throughthe FOV. Higher temperature components observed earlier nearthe point of first contact superimpose other contributions. Thetransient cavity still retains some of vapor beneath the surface atthis time.

Longer exposures (�50 ls) capture vapor emerging justuprange from the point of impact with emission lines muchweaker than those from within the transient cavity (Fig. 10). Theupper states of electronic transitions labeled in Fig. 10 (fromSugita et al., 1998) reveal that the relative intensity of Ca lines near445 nm and 430 nm is a sensitive indicator of temperature in thevapor plume. At 3000 K, the 430 nm line is weaker than the445 nm line, but the relative intensities reverse at 6000 K. Thisreversal in strength reflects different energy levels of the upperstates of electronic transitions (divided by the Boltzmann con-stant): �5430 K for the 445 nm line, versus 5540 K for the430 nm line. As expected, the relative strength of these two linesindicates that the earlier exposure captures higher temperaturevapor. The intensity of the emission line near 616 nm, however,is extremely strong (as shown below, this line is not superposedon a molecular band). Because the other Ca lines are consistentlyweak, the 616 nm line with its low excitation temperature indi-cates a much lower temperature in the vapor component directeduprange.

Shorter exposures at different times reveal a vapor compositionthat evolves within the transient crater (Fig. 11). Strong emissionlines occur below the impact point (uprange inside the transientcrater, Fig. 11A) within the first 2.5 ls. Away from the sub-impactpoint (DR Below, Fig. 11A), emission line intensities (and the back-ground) decrease. The intensity of the 445 nm emission line rela-tive to the 430 nm line again provides a qualitative comparisonof temperature based on electronic transitions. Directly belowthe impact point, the 445 nm emission line is stronger than the430 nm line. Farther away (DR Below), the reverse is true. Hence,vapor filling the cavity during the first 2.5 ls below the surface ishotter farther from the impact than the vapor just below it. Thisindicates that the vapor moves downrange within the transientcavity, just as it does above the surface.

The FOV’s beneath the impact point captures both hot andcooler vapor components that rapidly escape the cavity in differentdirections. A higher temperature component expands and fills thegrowing cavity below (DR Below, Fig. 11A), while a cooler compo-nent dominates above (Fig. 10). Later (2.5–20 ls), the 430–445 nmline intensity ratio above the surface (Fig. 11B) resembles the ratioobserved earlier, below-impact spectrum (Fig. 11A), consistent

Fig. 10. Comparison of above and below the target surface for an exposure from0.25 ls to 50 ls after impact by a Pyrex sphere (0.635 cm diameter) at 5.72 km/s.Values in parentheses indicate the energy levels of the upper states of electronictransitions (divided by the Boltzmann constant) in 1000K (see Sugita et al., 1998).Lower values (<50 indicated in blue) represent transitions occurring at lowertemperatures; higher values (>60 in red) transition at higher temperatures, withintermediate values in black. The absence of data for the subsurface FOV is due to adifferent wavelength center for this monochromater.

with a lower temperature gas escaping the cavity and expandingabove. Within the transient cavity farther away (DR-Below,Fig. 11A), a cooler component now remains (DR-Below, Fig. 11B).

Overall, line strengths for Ca and Mg lines (above the contin-uum) are reduced by at least half at later time (Fig. 11B), in spiteof a sevenfold increase in exposure for all three spectra. Thehigh-temperature component has disappeared while much stron-ger MgO, CO, and CaO bands indicate the added contributions fromlower temperature vapor still residing within the cavity after2.5 ls.

4.2. Effect of impact angle

Impact angle affects spectral content and temperature, bothabove and below the impact point (Fig. 12). All experiments for thisseries took place during the same run at the AVGR with the iden-tical configuration and exposure (0–50 ls) in order to minimizeissues related to calibration or pointing. In a vertical impact, nearlythe entire spectral content occurs above the impact point and hasthe richest and strongest emission lines of all the impact angles,with only a weak signal coming from within the transient crater(Fig. 12A). In addition, the vertical impact generated vapor having

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Fig. 12. Effect of impact angle on spectral composition above and below (exposure of 0–50 ls after impact). (A) Impact angle of 90� results in strong spectral content abovethe impact but absent elsewhere (5.74 km/s). (B) Impact angle of 60� (5.72 km/s) results in the lowest thermal background. The 616 nm calcium emission line (circled)dramatically changes in intensity uprange from the impact point, decreasing in strength with decreasing impact angle. (C) Impact angle of 45� (5.4 km/s) results in thestrongest emission lines; downrange, a thermal component raises the base level, relative to other directions. (D) Impact angle of 30� (5.57 km/s) results in lower thermalcomponent, likely due to fragments rapidly leaving the field of view. All spectra use a 300-line grating.

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Fig. 13. Thin layers of calcium carbonate significantly suppress vaporization from underlying material. (A) Impact by a 6.35 mm-diameter aluminum sphere (30� at 4.7 km/s)into a copper block covered with a 1 mm later of CaCO3 results in strong aluminum oxide lines (from the reaction between aluminum and the volatized carbonate) but only asingle line from the substrate. (B) Impact by a 3.18 mm-diameter copper sphere (60� at 5.37 km/s) into an aluminum block covered with a 0.1 mm layer of CaCO3 results invery weak aluminum oxide lines from reactions between the vaporized carbonate and the substrate but clear emission lines from the copper projectile. These spectraillustrate the effectiveness of even a thin layer on interactions with the substrate. The overlapping spectral lines near 525 nm result from two different (but simultaneous)spectral measurements (600 line grating). Both (A) and (B) cover 20 ls after first contact.


the highest temperatures, based on the 430 nm-to-445 nm lineratio and the emergence of lines with high excitation temperatures(e.g., near 535 nm, 487 nm, 459 nm, and 510 nm lines). The

presence of strong 555 nm and 610 nm CaO bands could indicateadditional contributions from a cooler component emerging laterover the long exposure (0–50 ls). Note that very little blackbody

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emission exists over the sampled wavelength, in spite of the longexposures (compare with spectrum C in Fig. 4).

For the 60� impact, the strongest emission lines also occurdirectly above the impact point (Fig. 12B). In contrast with the ver-tical impact, other regions contain spectral content as well. The‘‘downrange-low’’ FOV captured strong molecular emissions fromCaO and CaO, while atomic emissions below the surface indicatea high-temperature vapor filling the transient crater. The Ca emis-sion line near 616 nm (indicative of a lower temperature vapor)exhibits the highest peak intensity (above or below the cavity)but occurs above the surface in the uprange FOV, as noted above(e.g., Fig. 10).

The 45� impact appears to generate a spectrum with the great-est overall intensity (DR-low FOV), but this impression is largelydue to a greater contribution from blackbody emission (Fig. 12C).The relative intensities of Ca and Mg emission lines (above the con-tinuum) from a 45� impact are not that different from lines from a60� impact, but both are weaker than those generated from thevertical impact. The very strong molecular lines from CaO, CO,and MgO at 45� represent cooler vapor presumably trailing behindthe fast-moving gas downrange during the 50 ls exposure.

Condensates and hot projectile fragments (see Figs. 4–6) con-tribute to a significant blackbody in the DR-low FOV at 45�(Fig. 12C), but to lesser levels in the adjacent FOV above (DR-high),which exhibits obvious molecular (CaO and CO) lines but weakatomic emission. Hot, high-speed vapor escapes from the transientcrater and rapidly moves downrange (DR-low). The second stron-gest atomic Ca emission lines, however, occur directly above theimpact. High-speed imaging reveals that this component likelycomes from vapor rapidly emerging directly above the initial tran-sient cavity (Schultz et al., 2007; Schultz, 2009). As observed forthe 60� impact, the weakest spectral content occurs uprange,excepting the 616 nm line indicative of cooler vapor. While stillstrong, it is weaker than the emission from the 60� impact.

For the 30� impact, Na emission from the projectile and molec-ular emission lines (CaO and CO) dominate close to the surfacedownrange (DR-low FOV, Fig. 12D). Above (DR-high FOV), molecu-lar emissions (primarily CaO) are much reduced but still present.The strongest Ca and Mg emission lines, however, occur abovethe impact point while very little emission occurs below the target.Only the 616 nm Ca line persists uprange.

Most of the vapor from the 30� impact expands freely above thesurface while moving downrange. This plume must hug the surfacedownrange since it is not captured as well in the DR-High FOV.Another component (dominated by atomic species) expands abovethe impact point and the transient cavity. Nevertheless, a coolervapor component (strong 616 nm Ca line) again emerges uprange.This component does not contain the sodium line from the projec-tile, which is strongest downrange. In contrast to the 45� impact,the thermal background is very low, even though projectile frag-ments should be in the field of view. The most likely explanationis that the lower impact angle resulted in reduced heating of thedecapitated projectile fragments (e.g., see Schultz and Gault,1990) that decoupled and impacted farther downrange, beyondthe fields of view covered here.

Fig. 14. Experiments designed to isolate the source of vaporization and vaporcomponents. Different fields of view combined with different exposure and delaytimes serve to isolate contributions to the vaporization by an impact (angle of 30�).(A) shows the isolation of vaporization resulting from first contact (above) with a100 lm layer of dolomite on one half of a milled copper block. Another experimentisolates vapor due to the downrange hypervelocity projectile fragments impacting a100 lm layer of dolomite. (B) shows the fields of view (2 cm diameter) at differentlocations downrange from the impact. Modified from Schultz et al. (2006).

5. Source regions and vaporization process

5.1. Source regions

Layered targets allow isolating the source depth for impact-generated vaporization. Even a thin layer of sugar or graphite ontop of particulate target dramatically alters the spectral contentand masks contributions from below (see Fig. 4). Here we considerthis process further, first with the effect of thin layers on top of

solid blocks (Fig. 13), then with layers placed at the point of firstcontact or downrange.

Prior studies presented evidence that frictional shear heatingmust play an important role in impact vaporization of carbonatesand that multiple impacts downrange by the failed projectile con-tribute to this process (Schultz, 1996; Schultz et al., 2006). This canbe demonstrated by isolating contributions from first contact bythe projectile and impacts by projectile fragments (Fig. 14). Inone case, the projectile makes first contact with a thin layer(100 lm) of dolomite (Fig. 14A, above), called Case A here. In theother, the projectile makes first contact with the copper such thatthe only contribution from the projectile comes from impactingprojectile fragments downrange (Fig. 14A, below), called Case B.Here, telescopes cover four different fields of view downrange fromthe impact point in order to isolate the different vapor components(Fig. 14B).

In Fig. 15A, the FOV just downrange from the impact pointmisses the signal from the thin layer since the exposure wasdelayed 2.8 ls after impact. The only spectral line comes fromsodium in absorption (rather than emission) as a result of vaporexpanding in front of a blackbody source (from the heated target).Farther downrange (Fig. 15B), projectile fragments impacting thedolomite layer downrange (Case B, Fig. 14) generate emission linesthat are actually stronger than lines from vapor generated by theprojectile making first contact (Case A, Fig. 14). The Ca linestrengths, however, appear nearly identical. The exposure timeand distance from the impact point samples vapor traveling down-range at a speed of at least 1.8 km/s, including the jetting phase.Consequently, both processes contribute atomic emission linesfrom jetting, but the isolated fragments impacting downrange gen-erate greater amounts of CaO and CO.

Still farther downrange (FOV-C), the vapor component capturedby the spectrometer requires speeds greater than 4.9 km/s

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Fig. 15. (A) Spectra were captured for experiment where the projectile made first contact at an angle of 30� with carbonate layer (‘‘isolated first contact’’) contrasted withexperiment where the projectile first contacted the clean copper surface before fragments impacted the carbonate layer downrange (‘‘isolated downrange impacts’’) as shownFig. 14. Exposures were 20 ls (2.8–22.8 ls and 5.2–25.2 ls, respectively), which captured packets of vapor traveling between 0.3 and 2.1 km/s. (B) Farther downrange, strongemission lines are superimposed on top of the background continuum (from the exposed heated copper target and projectile fragments). The sodium emission representsvapor from the projectile and is strongest in the experiment where the projectile made first contact with a clean copper surface (‘‘isolated first contact’’). (C) Fartherdownrange, field of view C includes little background continuum (off the target). This field of view captures ejecta traveling at speeds greater than ~4.9 km/s, which includesboth the jetting phase and trailing high-speed downrange-moving vapor. The pronounced sodium is stronger for the case where the projectile first hit the clean coppersurface before hitting the carbonate (‘‘isolated downrange impacts’’) and is likely part of the jetting phase. (D) Far from the impact point, spectra include both the jettingphase and trailing vapor with speeds greater than 7.2 km/s. Vapor from the impact directly into the carbonate layer (‘‘isolated first contact’’) exhibits much more CaO but littleCO, in contrast to fragments impacting the downrange carbonate later (‘‘isolated downrange impacts’’). These isolated views demonstrate the significant contribution of vaporgenerated by decapitation fragments impacting downrange. Modified from Schultz et al. (2006).


(Fig. 15C). Here, isolated fragments impacting at very low anglesdownrange (Case B) generate vapor with CO, C2, and Na emissionlines that are stronger than those from the first contact (Case A)but with about the same CaO. Again, the strong Na line comes fromthe Pyrex projectile. The emission lines from the target (Ca) indi-cate higher temperatures (e.g., see Fig. 9A) resulting from the frag-ments impacting carbonate downrange (Case B), rather than at thefirst-contact. Impacts by a large number of isolated downrangefragments at very low angles would increase the number of contactsurfaces, thereby generating a stronger Na emission than that fromthe intact projectile (isolated first contact).

Farther downrange (FOV-D), the solid projectile making firstcontact with the carbonate layer (Case A) generates much strongerCaO and Ca emissions than did the isolated fragments (Fig. 15D).The reversal in intensities most likely indicates contrasting speedsof the downrange vapor plume for the two cases. Isolated down-range impacts by numerous projectile fragments (Case B) resultin significant interactions and instabilities, thereby slowing thedownrange component relative to the vapor generated at first con-tact. Multiple impacts by these fragments also generate a muchstronger sodium line.

Consequently, Fig. 15 demonstrates that both the first contactby the projectile and second contact by decapitated fragmentsdownrange contribute to the spectral content in an oblique impact.

These fragments impact downrange at lower angles than the initialtrajectory of 30� (Schultz and Gault, 1990). Because peak shockpressures depend on the vertical component of velocity, the low-angle downrange fragments must generate high-temperature fric-tional shear heating that contributes to vaporization even at pres-sures much lower than those generated at first contact.

The same processes must occur for impacts into particulate tar-gets. As a first example, dolomite powder was sieved lightly on topof a target of powdered pumice to a level where the pumice couldnot be seen (less than a millimeter thick). The thermal imagingcamera reveals that this thin layer resulted in a plume resemblingthat of dolomite powder alone but with slower expansion (Fig. 16).Consequently, upper surface layers may contribute a significantamount of the spectral content for oblique impacts into both solidand particulate substrates.

5.2. The vaporization process

Frictional shear heating between grains appears to contribute tovaporization. Such a process can be illustrated by examining theeffects of mixing the volatile component (dolomite) with a non-volatile component (no. 20-30 sieved sand). This experiment(Fig. 17) resulted in an increased thermal background (andenhanced CaO and CO bands) due to the added contribution of

Fig. 16. Sequence illustrating the source of self-luminous vaporization from near-surface materials revealed by layered targets. All experiments were for 30� impact angles(from the top); view from above is offset slightly to one side. (A) (left): An impact into pumice powder about 20 ls after impact. Incandescent silicates line the oblongtransient cavity. (B) (middle): Similar impact conditions except with a thin layer (<1 mm) of dolomite (0.25–5 ls exposure after impact at 5.65 km/s). Bright oval uprangefrom the impact point corresponds to an uprange-directed plume. (C) (right): An impact into dolomite powder (0.25–5 ls exposure after impact at 5.8 km/s). Bar-scalecorresponds to 1 cm; cross indicates impact point.

Fig. 17. Comparing spectra resulting from impacts into a mixture of sand anddolomite powder, a dolomite block, and just dolomite powder (all viewed fromabove and downrange, see FOV, Fig. 5B). Mixing no. 20–30 sand with dolomiteraises the blackbody above the level for just powdered dolomite due to contribu-tions from heated sand grains. The even greater blackbody for the dolomite blockresults from fragments and melt droplets from the disrupted Pyrex projectile. Allimpacts used 0.635 cm Pyrex, 30� impacting at 5.85 km/s (± 0.08 km/s) with anexposure of 0–50 ls.

Fig. 18. Consequences of volatile layer (dolomite powder) over a non-volatile layer(sand), red line, versus the reverse sequence, blue line. In both cases, the vaporphase must come from dolomite. The experiment with the sand layer on top ofdolomite (blue line) had little effect. If anything, the sand layer on top appears togenerate more CaO and CO. This result is likely due to thermal decomposition of theunderlying dolomite generated from interactions with shocked sand grains. Thesmall circle indicates the location of the spectrometer field of view (quarter-spaceexperiments). Both spectra were exposed over the first 50 ls. Impacts were oblique(60�) at 5.7 km/s (dolomite over sand) and 5.8 km/s (sand over dolomite).


heated sand grains for comparable impact speeds (5.8–5.93 km/sat 60�). As shown in Figs. 13 and 14, upper surface layers contrib-ute most (if not all) of the spectral content. Introducing a non-vol-atile surface layer, therefore, should suppress emission lines fromthe substrate (Fig. 4, spectrum A). Fig. 18, however, reveals that athin (�2 mm) layer of sand over dolomite produces the same, ifnot stronger, emission lines (above the thermal background) fornearly identical impact speeds (e.g., CO and MgO). The one excep-tion is the 425 nm emission line, which is affected by self-absorp-tion. Consequently, interactions with the heated sand grains asthey are driven downward into the dolomite must contribute tovaporization from thermal decomposition, rather than shock.

6. Discussion

Spectral probing of impact-generated vapor generated fromporous targets reveals a complex evolution, separate and well afterthe early-time jetting phase. Above-impact views can become dif-ficult to interpret because multiple components contribute to thespectra, including mixed phases (e.g., solid ejecta, melt droplets,and vapor from both target and projectile) passing through thefields of view. Quarter-space experiments, however, partly isolateconditions inside the plume (as well as the transient cavity). Such

a strategy helps to minimize contributions from not only conden-sation droplets but also fragmented projectile debris, both produc-ing a strong thermal component (except for views directlydownrange from an oblique impact). Beneath the surface, this per-spective reveals that the hot vapor from oblique impacts initiallyfills the transient crater for at least 20–50 ls after impact. Ironi-cally, vertical impacts result in vapor phases rapidly escaping(within first 2 ls) with little signal detected below the surface.

Different components pass by the field of view at differenttimes. At early times, atomic emission lines (Ca, Mg, and C2) dom-inate and indicate high-temperature disassociation products, asinferred from the relative intensity of key emission lines with dif-ferent excitation levels. Molecular species (CaO, MgO, and CO)develop later as a result of both thermal disassociation at lowertemperatures (farther from the source) and recombination (backreactions within the plume). At later times, the blackbody back-ground increases due to condensation of small carbon droplets.Powdered dolomite targets contain small amounts of graphite onthe crater floor, consistent with carbon calabashes generated fromimpacts into solid dolomite targets (Rietmejier et al., 2003).Uprange, a lower temperature component also emerges from obli-que impacts, especially for a 60� impact angle. Because the shockwave in an oblique impact is directed downrange (Schultz and


Anderson, 1996; Dahl and Schultz, 1998, 2001; Pierazzo andMelosh, 2000), shock pressures should be less uprange andreleased vapor should be cooler. As the projectile continues to pen-etrate the target, a gap is created between the less-shockeduprange transient crater wall and the downrange-moving projec-tile. This gap allows vapor to escape uprange without furtherinteractions.

Experimental design further reveals that most of the vaporiza-tion (at these impact speeds) comes from the uppermost surfacelayer, whether for solid or particulate targets. In oblique impacts,the failed projectile fragments impact the target downrange andcontribute a significant additional vapor component. The largeamount of vaporized mass observed for oblique impacts, then, alsomust require vapor generated from an extended area downrangerelated to impacts by projectile fragments (Schultz, 1996). Becausethese fragments impact at very low angles, shock levels arereduced; consequently, waste heat behind the shock front doesnot appear to play a major role. Instead, the source of vaporizationmust come from thermal decomposition at low pressures due tothe high temperatures generated by frictional shear heating frommultiple sources.

The combination of non-volatile silicate sand (no. 20-30 sieve)with dolomite powder provides further insight. If intimatelymixed, the thermal blackbody increases due to particle–particleinteractions behind the shock and within the flow field. Typicallya thin, non-volatile layer over a solid volatile target acts as a flakjacket, reducing the amount of vapor generated by the impact, con-sistent with experiments investigating the effect on peak pressures(Stickle and Schultz, 2011). From the results here, spectral observa-tions indicate that the shock- and shear-heated sand grains in thesurface layer generate secondary thermal release of volatiles fromdolomite powder below.

A revised shock Hugoniot for carbonates indicates that hyperve-locity impacts should not vaporize easily (Ivanov and Deutsch,2002). Spectral data seem to indicate otherwise. This contradictioncan be reconciled by considering the increased contribution of fric-tional shear heating that result in high temperatures (5000 K) butlow pressures (less than 0.1 GPa), along with free expansion. Con-sequently, estimates for vaporization based on peak pressure alonelikely underestimate the amount of vaporization and melting,especially for both lower impact speeds (<10 km/s) and obliqueimpacts, which generate reduced peak pressures.

Numerical models indicate that oblique impacts do not resultin increased vaporization at lower impact angles (Pierazzo andMelosh, 2000), in contrast with experimental results (Schultz,1996). Moreover, some models may not accurately capture therange of temperatures within the vapor plume, especially forlower speeds (Quintana et al., 2013) and impact angles. Such dif-ferences between experiments and models indicate that numericalmodels should be benchmarked with spectral observations of theplume under different impact angles and conditions (e.g., mix-tures) and should be designed to isolate various contributions tothe vaporization process, including granular interactions, sourcedepth, frictional shear heating, and projectile fragments impactingdownrange.

7. Summary and implications

7.1. Summary

Hypervelocity impact experiments into volatile-rich particulatetargets reveal that the generated vapor exhibits considerable vari-ability in its distribution, composition, and temperature in bothtime and space. Several specific conclusions are reached (with ref-erence to relevant figures illustrating each point):

1. Half-space experiments viewed from above capture multiplecomponents passing through the field of view at the same time(e.g., condensation droplets, heated/melted projectile frag-ments, and contrasting plume conditions), which can compli-cate interpretations of conditions. Figs. 4 and 5A, B.

2. Quarter-space experiments of particulate targets viewed fromthe side allow probing within the vapor plume not only abovethe surface but also inside the transient crater. This strategyminimizes contributions from multiple components whenviewed from above. Figs. 5C and 7.

3. Both the projectile at first contact and downrange impacts byprojectile fragments contribute to the vaporization process foroblique impacts less than 45�. Figs. 4 (insets A and D) and 15.

4. Impact-generated vapor fills the transient cavity and evolves incomposition as it expands above. Figs. 5 and 8–13.

5. The composition of vapor phases within the transient crater(below the surface) evolves from atomic to molecular emis-sions. The molecular emissions are due to less heated thermaldecomposition products emerging later, as well as recombina-tion products. Figs. 8 and 9.

6. In oblique impacts, a low-temperature vapor componentemerges uprange, for angles as high as 60�. Figs. 7 and 12.

7. The source of vaporization generally comes from the uppermostlayers for solid targets (at these speeds), even for high-angleimpacts (e.g., 60�). Figs. 4 and 13–15. Conversely, even a thinrefractory layer over a solid target can suppress or mask vapor-ization. Fig. 4.

8. Vaporization is not just due to peak pressures at first-contact; italso depends on the temperature generated by frictional shearheating, i.e., high temperatures under low pressures. Figs. 13–18.

9. Both direct (i.e., at contact) and indirect (interactions withheated projectile fragments or heated particulates in the target)contribute to vaporization, especially for oblique impacts(<45�). Fig. 18.

7.2. Implications

Examples of three implications from this study include: (a) theamount of vapor generated by the Chicxulub impact; (b) the deliv-ery of volatiles to planetary surfaces; (c) spectra of the Deep Impactcollision.

First, the amount of vaporization from oblique impacts may beunderestimated if based on peak pressure alone or the peak-pres-sure footprint, i.e., an ellipse dependent on the angle. For example,an oblique trajectory for the Chicxulub asteroid impacting into thethick carbonate platform on the present-day Yucatan could havereleased much more CO2 than some prior estimates based onlyon pressure (e.g., Ivanov et al., 1996; Pierazzo et al., 1998). If theChicxulub impact had occurred at an impact angle of 30� with avelocity of 25 km/s, then �26 projectile masses could have beenvaporized, based on extrapolations of experimental results from6 km/s to 23 km/s (Schultz, 1996). With the assumption that only25% of the vaporized sediment cover had been composed of car-bonate, then 164,000 GT would have been vaporized, therebyyielding 57,000 GT of CO2. From the results presented here, signif-icant vaporization comes from frictional shear created by thefailed/melted projectile after first contact and is derived from sur-face materials less than a projectile radius in depth and covering afan-shaped area with a width and length equal to the cotangent ofthe projectile diameter (see Schultz, 1996). So, another strategyconsiders the amount of vapor liberated from a downrangefan-shaped zone about 15 km wide and 15 km with a 3 km thicksedimentary cover, again with the assumption that carbonatescomprise 25% of the sediment layer above basement and only35% of this mass could actually yield CO2. In this case, about


74,000 GT of CO2 would have been released. Such estimates areconsistent with the highest amounts from early studies (O’Keefeand Ahrens, 1989) but well exceed later determinations (Takateand Ahrens, 1994). These values dwarf the abundance of CO2

thought to have been present (9000 GT) in the late Cretaceousatmosphere (Berner, 1994), even if much of this vaporized massescaped the Earth or the CO2 recombined into other forms. Suchestimates serve only to emphasize the need for (and implicationsof) more complete modeling of oblique impact process.

Second, the experiments demonstrate that a component ofimpact-generated vapor fills the transient cavity. This result hastwo implications. First, vaporized products from both the projectileand target can condense out on regolith grains within the cratercavity, thereby contributing to space weathering (Noble et al.,2001). Impact experiments using iron-bearing minerals alreadyhave demonstrated vaporization of iron at speeds achievable inthe AVGR (Adams et al., 1997; Bruck Syal and Schultz, 2014). Sec-ond, cometary particles contribute a significant fraction of the fluxon the Moon for sizes less about 10 cm (Nesvorny et al., 2010).Vaporized portions from small objects should be injected into theregolith and/or plated out on grains, especially during the coldlunar (or mercurian) night. Thermal recycling during a lunationcould then release volatile components later with speeds muchlower than those during the impact itself. This process would pro-vide a daily supply of volatiles to cold traps at the poles (e.g., Stern,1999; Schultz, 2011). Moreover, not all impact-generated vaporphases emerge at high temperatures: low-temperature vapor alsoescapes uprange in oblique impacts. Such vapor has lower expan-sion speeds, which would increase the likelihood of contributingvolatiles to polar cold traps. Future studies at higher resolutionspectra along matched by hydrocode models should establishnew constraints on vapor temperatures and chemistries for morerealistic planetary target materials.

Third, the Deep Impact (DI) collision resulted in multiple stagesof ejection visible for more than 14 min (e.g., A’Hearn et al., 2005).The laboratory experiments here, however, only capture the earli-est stages including: the rapidly moving (but evolving) downrangeplume; an uprange-directed plume (followed by a high-angleplume); and the hemispherical plume above the crater. Resultsindicate that spectra of the initial high-speed (distal) componentsfrom the DI impact came from the uppermost surface materialsbut included other contributions depending on the field of view(direction of observation). Downrange, the high-speed (10 km/s),highest temperature plume (>6000 K) passed through the slit at10 km/s within about 10–20 ms. While imaging captured this com-ponent, the relatively long exposure time for the IR spectrometer(>700 ms) recorded multiple components passing by, includinghydrocarbons, H2O, and CO2 at lower temperatures (1000–2000 K), as previously described (A’Hearn et al., 2005). Furtheranalyses concluded that this mix evolved after �2 s into multiplecomponents: first, a downrange zone composed of water ice; sec-ond, lateral ejecta of dust and ice (Sunshine et al., 2007). Impactsinto layered targets (e.g., Fig. 4, inset D) demonstrate that a surfacelayer significantly suppresses contributions from below. Conse-quently, the Deep Impact IR spectra captured either a trailingplume downrange derived from just below a more refractory sur-face layer or a high-angle plume in the foreground from deeperbelow. Finally, the vapor filling the transient crater in laboratoryexperiments (Figs. 11 and 12D) establishes a process for initiatingboth the reverse and high-angle plumes of snowy particulates fromdepth (Schultz, 2009).

Although these experiments use impactor speeds and sizes wellbelow those at larger scales (e.g., Chicxulub), they neverthelessreveal fundamental processes that should occur even at largescales. For example, models not including the constitutive and fail-ure models at sufficient resolution will not capture contributions

by the failed impactor impacting downrange (e.g., Stickle andSchultz, 2012) nor the contributions of frictional shear heating(Quintana et al., 2013; Crawford and Schultz, 2013). Additionally,models indicate that silicates should not vaporize at speeds of only6 km/s, yet spectral observations document dissociation products,such as iron and magnesium emission lines (e.g., Adams et al.,1997; Bruck Syal and Schultz, 2014). Finally, surface expressionsof impactor failure on the Moon and Mars demonstrate that therole of impactor failure observed in laboratory experiments alsooccurs at much larger scales (e.g., Schultz and Lutz-Garihan,1982; Schultz and Stickle, 2011; Schultz et al., 2012; Schultz andCrawford, 2014). The initial peak pressures experienced by theChicxulub asteroid impacting at a 20� angle should be reducedby 75%, making a 23 km/s collision equivalent to a 7.9 km/s verticalimpact, comparable to laboratory impact speeds where the projec-tiles is observed to survive (Daly and Schultz, 2013). If numericalmodels cannot match processes actually observed in the laboratoryexperiments, then they may not fully capture the vaporization pro-cesses on the planets and asteroids.

8. Concluding remarks

High-speed spectroscopy reveals unexpected processes thatcould focus hydrocode modeling and not just be a technique forbenchmarking. Future experiments will detail conditions (temper-ature) within the late-stage vapor at higher spectral and temporalresolution, use more realistic planetary materials or proxies (e.g.,hydrous silicates), explore the effect of a greater range of targetporosities, and reverse the roles of target and projectile for vola-tile-bearing materials.

Acknowledgments

This contribution would not have been possible without theoutstanding support from personnel at the NASA Ames VerticalGun Range including C. Cornelison, D. Holt, D. Bowling, R. Smythe,and J.P. Wiens, along with the unfailing support of George Raiche,Branch Chief in the Thermophysics Facilities Branch (Code TSF) atNASA Ames. Michael Wilder in the Aerothermodynamics Branch(Code TSA) at NASA Ames kindly provided use of a thermal imagingcamera. We also benefitted greatly from the advice and assistanceof Seiji Sugita during the initial stages of the effort and C.M. Ernstfor her unfailing assistance during the experiments. NASA grantsNASA-NNX13AB75G (along with prior awards) and NASA-NNX07AG18G supported different aspects of this study. One ofus (CAE) also acknowledges partial support from the NASA RhodeIsland Space Grant program during the analyses. We thank twoanonymous reviewers for their thorough and constructive reviews.Finally, we gratefully acknowledge the Paduano Foundation fortheir generous support through a development grant to BrownUniversity.

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