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International Journal of Impact Engineering 33 (2006) 771–780

www.elsevier.com/locate/ijimpeng

The role of ricochet impacts on impact vaporization

P.H. Schultza,�, S. Sugitab, C.A. Eberhardya, C.M. Ernsta

aDepartment of Geological Sciences, Brown University, Providence, RI 02912, USAbDepartment of Complexity Science and Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Available online 13 November 2006

Abstract

Vaporization of carbonate targets by hypervelocity impact increases with decreasing impact angle (from the horizontal),

in contrast with expectations based only on peak shock pressures. Experiments at the NASA Ames Vertical Gun Range

were designed to allow isolating the underlying controlling processes and probing the vapor composition using high-speed

spectroscopy. Vaporization associated with the maximum peak pressures (first contact) was separated from vaporization

generated only by downrange ricochet impacts through the use of split targets. Four telescopes isolated the vapor with

different velocities and revealed that grazing ricochet debris downrange contributed a significant fraction to the overall

vaporization process. These results can be understood by the high temperatures and low pressures created by high strain-

rate shear.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Vaporization; Ricochet; Spectra; Temperatures; Oblique impact

1. Introduction

Previous experimental studies document multiple components of impact vaporization reflecting the effectsof jetting, shock, projectile ricochet, and shear [1,2]. As a result, separate components of vapor can beobserved and measured by imaging [1], electrostatics [3], and spectroscopy [1,2,4–7]. Even though peak shockpressures decrease with decreasing impact angle measured from the horizontal (e.g. [8]), the degree of impactvaporization of carbonate targets appears to increase by three orders of magnitude from 901 (vertical) to 301(from the horizontal) based on free-expansion of the vapor cloud [1]. This observation contrasted with resultsfrom other experiments using closed containers (e.g. [9]), perhaps as a result of fast back reactions (e.g. [10]) orthe difference between closed and open (freely expanding) systems [1,11]. Nevertheless, such enhanceddecomposition of carbonates is not confirmed by computational codes (e.g. [12]).

A possible explanation for the disparate experimental results was provided by revised phase diagramsshowing that carbonates do not thermally decompose under high pressures, even at very high temperatures[11]. But the discrepancy between experimental and theoretical results for oblique impacts also could representdifferent controlling processes for vaporization and could have significant effects on impact-induced releaseof greenhouse (CO2) or toxic (CO) gases following the end-Cretaceous Chicxulub impact (e.g. [13]).

ee front matter r 2006 Elsevier Ltd. All rights reserved.

mpeng.2006.09.005

ing author. Tel.: +401 863 2417; fax: +401 863 3978.

ess: peter_schultz@brown.edu (P.H. Schultz).

ARTICLE IN PRESSP.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780772

Consequently, a new series of experiments were performed in order to understand the possible underlyingprocesses responsible for the contrasting results.

Hypervelocity experiments reveal a multi-component evolution of the vapor cloud as more fully described in[1]. Four different vapor components can be identified from high-speed imaging of oblique impacts intoparticulate carbonate targets. The first component represents the jetting phase that travels three times theinitial impact speed, i.e., �19 km/s for a 5.5 km/s impact. The second component is a downrange vapor cloudthat expands while moving downrange at a velocity comparable to the horizontal component of the initialimpact velocity. The third component is centered near the point of impact and expands hemi-spherically abovethe target slightly downrange. A fourth component is controlled by the initial penetration cavity and is theresult of cavitation within the transient cavity. This vapor phase exits the cavity as a directed plume (e.g. [1]).

The jetting phase has been studied spectroscopically in detail [4]. The primary focus here, however, is toisolate this jetting phase from both the shock and ricochet-induced vaporization. Ricochet-inducedvaporization results from hypervelocity projectile fragments impacting downrange. As impact angle decreases,the first-contact peak pressure in the projectile (and target) decreases. Once the peak pressure exceeds thetensile strength, spall fragments decouple from the projectile at low impact angles with a velocity vectordetermined by the spall velocity (related to the peak stress) and the initial impactor velocity [14]. For highlyoblique impacts, ricocheted spall fragments (‘‘siblings’’) strike the target downrange at very low angles, wellaway from the impact point.

Because the downrange velocity component of spall fragments is only slightly less than the initial impactvelocity, ricochet ‘‘siblings’’ should be distinguishable from slower ejecta originating from the target. The sizeof the individual spall fragments approach 10% the mass of the initial impactor [14]. This process isparticularly apparent at laboratory-scale impact velocities (5–7 km/s) where the initial peak pressure for a 301impact angle (from the horizontal) will be reduced by a factor of 4 and for a 151 angle, by a factor of 15. Thesize distribution of the siblings is dependent on strain rate, impactor tensile strength, and impedance contrastwith the target [14].

Fig. 1a shows a double exposure for a 0.635 cm Pyrex sphere 5.4 km/s impacting powdered dolomite at 301.At impact, spectral and thermal emission from the first contact and downrange ricochet result in self-illumination, whereas a later pulsed laser sheet captures high-speed ejecta �10 ms later above the target.A narrow band-pass filter was used to reduce the effect of the initial flash. Different components of the vaporcan be traced back to source regions representing both the first-contact and the sibling impacts [15]. This offsetappearance of illuminated ejecta and impactor is due to the laser sheet situated above the surface and off-axis

Fig. 1. (a) Double-exposed image (from above) of a 151 impact (from horizontal) into dolomite powder at 5.4 km/s. Both the point of first

contact, (I) and downrange ricochet components (R) are self-illuminated (captured in one camera) while a pulsed laser sheet illuminates

high-speed ejecta components (captured in a second camera). The original projectile size (0.635 cm) corresponds to the width of the first

contact at ‘‘I’’. (b) Close view of the Pyrex sphere viewed from above showing the self-illuminated projectile produced by a 5 km/s into

dolomite block.

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perspective of the camera. In Fig. 1b, the moment of first contact is captured because the projectile istranslucent and the high-temperatures developed between the target and impactor last for only a fewmicroseconds.

Consequently, vaporization and melting cannot be viewed simply as a shock process but a combination ofprocesses including shock, shear, and multiple downrange impacts by impactor siblings that all contribute tothe observed enhanced vaporization by oblique impacts into carbonate targets [1]. The goal of the presentstudy is to isolate the effects of ricochet-generated vaporization from first-contact vaporization processes(e.g., jetting) and to characterize resulting conditions and products.

2. Experimental approach

Experiments were performed at the NASA Ames Vertical Gun Range at the NASA Ames Research Center,a national facility open to qualified users. The large impact chamber allows the vapor phase to freely expandwithout interference with the chamber walls. Two different sets of experiments were performed, all at 301impact angles.

The first set of experiments assessed spectral signatures from vapor exposed over 20 ms in a single field ofview (5 cm in diameter; positioned 2.5 cm downrange from the point of impact) for three different projectiletypes (quartz; copper; and aluminum) into carbonate-veneered targets of copper and aluminum. The purposewas to determine the role of different veneer thicknesses on vaporization and the generation of vapor from theunderlying substrate. The second set of experiments used multiple fields of view in order to isolate differentprocesses. Pyrex projectiles (0.635 cm in diameter) were launched at 301 into carbonate-veneered (100 mm)copper blocks (Fig. 2) at 5.4 km/s. Three-different configurations were used (see Fig. 2a) in order to isolate thevaporization processes. Fully coupled impact (FCI): The impactor couples with the carbonate veneer by boththe first contact and the ricochet components. Consequently, vaporization results from all processes (jetting,shock, and shear).

Partially coupled impact (PCI): First contact occurs only in the uprange carbonate half. This strategyprecludes vapor generated by the downrange ricochet fragments, which impact the exposed milled coppersurface. Ricochet-coupled impact (RCI): First contact occurs in the copper surface. As a result, any carbonatevapor results only from the ricochet debris striking the downrange carbonate veneer.

High-speed 0.35m spectrometers were coupled to two narrow-field telescopes through quartz fibers(see [2,7]) while a six-channel photodiode simultaneously recorded the evolution of the blackbody temperature(see [18]). The two telescopes isolated areas on the target from above, each covering a ‘‘field of view’’ (FOV)2 cm across and focusing just slightly downrange from the point of impact (FOV-A) and isolating areas atprogressively downrange locations (FOV-B through D).

Fig. 2. Experimental design showing the strategy used to the isolate first-contact vaporization versus ricochet-generated vaporization

processes. (a) Different configurations as described in the text. One-half of each copper block was milled for PCI and RCI in order to have

a flat target surface with the veneer. (b) Locations of the fields of view simultaneously captured by the four spectrometers.

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3. Results

The first set of experiments contrasted the effect of carbonate veneer thickness on vapor passing thespectrometer field of view just downrange (Fig. 3). Atomic (Ca, Na) and molecular (CaO) emission lines wereproduced by a 0.635-cm quartz sphere impacting into a 100-mm layer of carbonate powder veneer over a solidcopper block at 5.16 km/s (Fig. 3a). Fig. 3b illustrates spectra from a 0.635-cm-diameter aluminum sphere intoa thicker (1mm) layer covering a copper block at 4.69 km/s. For contrast, Fig. 3c shows spectra from a 0.318-cm-diameter copper projectile (�5 km/s) into a 100 mm thick layer over an aluminum block. Last, Fig. 3dreveals the blackbody continuum superimposed by atomic copper (Cu) lines for the impact by a 0.635 cmquartz sphere (5.25 km/s) into just the copper block. Color temperature from the blackbody curve in Fig. 3dcorresponds to �3300K.

The intensities of both the Ca atomic line at 526-nm and the CaO molecular bands near 550 and 620 nmproduced from the 1mm veneer (0.16 projectile diameters, Figs. 3b and c) are only about 30% greater inradiance than for the impact by the quartz projectile into the 100 mm layer (0.016 projectile diameters, Fig. 3a).After correcting for differences in impact velocities, v (for a v2 dependence) and impedance contrast, the vaporyield is very similar relative to the factor of 10 in carbonate thickness.

Significant chemical reactions between the aluminum impactor and byproducts from carbonatevaporization are clearly evident in the aluminum oxide bands. In both cases, spectral emission lines(511 and 578 nm) from the copper substrate were not detected (see Fig. 3d). Consequently, the first set ofexperiments showed that a thin carbonate veneer shielded the impactor from the copper substrate, i.e., theseimpacts did not produce the large blackbody signal or copper emission lines.

The second experiment set isolated the processes responsible for vaporization using the strategy illustratedin Fig. 2. Fig. 4 shows the resulting craters for 0.635 cm Pyrex spheres impacting at 301 (�5.4 km/s). Althoughthe carbonate veneer was quite thin, it did affect crater morphology and coupling of the ricochet to the copperin the partially coupled (PC) impact (Fig. 4a). Projectile fragments entrained in the vapor produced at firstcontact reduced the degree of scouring in the exposed downrange copper surface but can be seen as faint

4x104

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2x104

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1x104

50000

450 500 550 600 650

450 500 550 600 650 450 500 550 600 650

450 500 550 600 650Wavelength (nm)

Wavelength (nm) Wavelength (nm)

Wavelength (nm)

CaO

CaO

Na

8x104

7x104

6x104

5x104

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3x104

2x104

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0

Spe

ctra

l Rad

ianc

e

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nce

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ianc

e

AIOCaO

CaO

1.2x106

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8x104

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0

Cu

Cu

5x104

4x104

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2x104

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Cu

Cu

(a) (b)

(c) (d)

Ca

Fig. 3. The effect of projectile type, veneer thickness, and substrate on impact-generated vaporization by 301 impacts (�5 km/s) into

different targets. (a) Impact into carbonate veneer over copper block. (b) Impact into dolomite veneer (MgCaCO3 powder) over a copper

block. (c) Impact by 0.318 cm copper sphere into CaCO3 powder veneer over an aluminum block. (d) Impact into just a copper block by a

0.635 cm quartz sphere.

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Fig. 4. (a) Impact into carbonate-veneered half (right) of a copper block (Partially Coupled Impact, PCI) resulting in only first-contact

vaporization processes. (b) Impact into the exposed copper half of the target (Ricochet-coupled impact, RCI), thereby isolating

vaporization from only the ricochet fragments.

Table 1

Velocity range (km/s) captured in different fields of view (A, B, C, D) downrange (dr)

Configurationa A B C D

FCI 1.2–5.8 2.8–14 45.4 47.5

PCI 0.3–2.1 1.8–15 44.9 47.2

RCI 0.3–2.1 1.8–15 44.9 47.2

Type Crater+vapor dr vapor dr vapor+jetting Jetting

aConfigurations correspond to: fully coupled impact (FCI), partially coupled impact (PCI), ricochet-coupled impact (RCI)

P.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780 775

streaks. In Fig. 4b, ricochet fragments interacted with the downrange carbonate veneer leaving carbonate meltadhered to the downrange copper surface in the RCI. As shown in Fig. 2, FOV-A recorded the residualthermal signature inside the crater. FOV-B recorded downrange vapor phases moving at least 1.8 km/s (PCIand RCI) or 2.8 km/s (FCI). FOV-C captured both the jetting (15–18 km/s) and non-jetting vapor componentstraveling downrange, but FOV-D (farthest downrange) could record only the jetting phase (see Table 1). Pyrexprojectiles ensured catastrophic failure and minimized contributions to the spectra. Exposure times of 20 msfurther allow temporally resolving different components by subtraction.

Exposure times (5–25 ms) for the FCI experiment isolated the highest speed component of the vapor phasetraveling downrange in the view area farthest downrange (FOV-D), i.e., the jetting component traveling fasterthan 7.5 km/s (see Table 1 and Fig. 5a). FOV-C captured both the jetting phases and any moderate-velocitydownrange vapor (45.4 km/s). In FOV-B, the jetting phase had already passed the FOV before the exposurebegan but did record moderate-velocity vapor phases and projectile debris (2.8–14 km/s). While the atomicemission line of Na and Ca can be readily identified, the prominent CaO and weak (diffuse) CO molecularbands were absent. Lastly, FOV-A largely recorded cooler vapor and target heating (blackbody) with speedsless than 5.8 km/s. FOV-A captured spectra in absorption (Na, Ca) as these cooler gases expanded abovethe residual blackbody inside the crater. The intensity of FOV-C far exceeded D because it combined both thehigh-speed jetting phase and any downrange vapor components as they passed through the FOV at nearly theinitial speed of the impact (�5.4 km/s).

The PCI configuration (Fig. 5b) produced jetting from the carbonate veneer at first contact but excludedvapor from the ricochet component over the 2.8–22.8 ms exposure times. As a result, FOV-C and -D hadnearly identical spectra (both from first-contact processes). FOV-B exhibited a slightly reduced blackbodysignal (relative to the FCI), but the earlier starting time for the exposure (2.8 ms after impact) also allowedcapturing a portion of the initial jetting phase expressed by emission lines superimposed on the thermalcomponent (not above the target as in A or in FCI).

The experiment designed to isolate the RCI component revealed dramatic differences from both the FCIand the PCI experiments (Fig. 5c). The downrange view area (FOV-D) captured the jetting phase due only tothe ricochet impacts due to scouring of the downrange calcium-carbonate layer. In contrast with the FCI andPCI configurations, the RCI-jetting component exhibited not only CaO but also three additional broad bandsbetween 465and 530 nm from CO molecules with minimal contributions from blackbody sources. These bandsare inconsistent with AlO (see Fig. 3b) and are identified as superimposed diffuse molecular bands of CO (theAngstrom series) and the C2 Swann bands. This is more evident in FOV-C, which recorded vapor components

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Ricochet Coupled Impact (RCI)

0

0.2

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420 440 460 480 500 520 540 560 580 600 620 640

wavelength (nm)

D (> 7.2 km/s)C (> 4.9 km/s)B (1.8-15 km/s)A (0.3-2.1 km/s)

CaCa

Na

CaOCaOCO

B

C

A

DCO

Fully Coupled Impact (FCI)(3-23µs after impact)

(2.8-22.8µs after impact)

(5.2-25µs after impact)

0

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wavelength (nm)

D (>7.5 km/s)C (> 5.4 km/s)B (2.8-14 km/s)A (1.2-65.8 km/s)Ca

Ca

NaCaO

CaO

CO

B

C

A

DCO

Partially Coupled Impact (PCI)

0

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1

420 440 460 480 500 520 540 560 580 600 620 640

wavelength (nm)

D (> 7.2 km/s)C (> 4.9 km/s)B (1.8-15 km/s)A (0.3-2.1 km/s)

Ca

NaCaO

CaO

CO

B

C

A

D

COCa

Fig. 5. Comparison of spectra for different experimental configurations (Fig. 2a) and for different fields of view shown in (Fig. 4b). Major

atomic and emission lines from the target are shown. Spectra have been offset (vertically) for clarity. The ordinate is in arbitrary units.

P.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780776

traveling downrange with speeds greater than 4.9 km/s. Because the relative intensities in FOV-C far exceededD, the component in FOV-C must include significant vapor phases with speeds between 4.9 and 7.2 km/s.Consequently, vapor produced in the RC experiment is uniquely characterized by co-existence of Ca, CaO andCO in FOV-B, -C, and -D whether separated in time (passing the FOVs at different times) or space (expandingdifferentially).

Fig. 6 allows the direct comparison of vapor products from the first-contact impact (PCI) with ricochet-generated phases (RCI) more directly for each field of view. Fig. 6a shows that the thermal blackbody in A forboth configurations is quite similar. Such a similarity most likely indicates residual thermal sources (meltlining the crater cavity) within the target since all other components would have left the FOV over theduration of the exposures. FOV-B (Fig. 6b) reveals that the downrange vapor phases created by the projectilesiblings impacting the carbonate veneer (RCI) was much more important than the contribution from the first-contact vapor (PCI). Additionally, CO molecular emissions (near 565 and 595 nm) occurred in RCI but not inPCI. The FOV at B for PCI and RCI can capture only a trailing portion of the jetting phase since thedownrange FOV edge requires velocities of at least 18 km/s (due to delay time from first contact and distancefrom impact). The much greater signal in FOV-C (jetting and downrange vapor) relative to D (jetting only)supports this conclusion. By contrast, the PC impact resulted in nearly equivalent radiance levels for FOV-C

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(C)

0

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wavelength (nm)

Ca

Na

CaO

CaO

CO

Vapor + Jetting

(> 4.9 km/s)

PCI

RCI

CO

CaCu

COC2

C2

COCu2

COCa

(D)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

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420 440 460 480 500 520 540 560 580 600 620 640wavelength (nm)

Ca

Ca

Na

CaO

CaO

Jetting (> 7.2 km/s)

PCI

RCI

CO

CO

CaC2

CO

Cu2

CO

C2

(B)

0

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wavelength (nm)

CaCa

Na

CaO

_ CaO _

COV apor + Ricochet

( 1.8 - 15 km/s)

PCI

RCICO

Cu

CuO

CaCa

Ca

(A)

0

0.05

0.1

0.15

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wavelength (nm)

Na

Ricochet (0.3 - 2.1 km/s)

PCI

RCI

CO

CO

Fig. 6. Comparison of spectra produced from the carbonate first-contact (PCI) and the ricochet-contact experiments (RCI) for the

different fields of view (A, B, C, D) are shown in Fig. 2b. The vertical axes have been stretched in order to reveal other molecular and

atomic species. As discussed in the text, the RCI experiment generated significant vapor with different compositions in B, C, and D due to

different conditions generating thermal decomposition. The ordinate is shown in arbitrary units (au) for this particular set of experiments.

P.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780 777

and -D, thereby indicating that vapor in FOV-C included the jetting phase passing the FOV. It should benoted that the slight offset in band position for CO in FOV-D for the RCI and PCI results from beingsuperimposed on adjacent lines (CaO in PCI and Na in RCI).

Temperature measurements using a six-channel photodiode system [18] revealed that the spatially integrated(wide field of view) color temperature for the RCI exceeded 6500K (see Fig. 7), whereas the PCI and FCIexhibited lower temperatures (5000 and 3300K, respectively). Such differences reflect, in part, the contrast inimpedance at first contact for the two configurations. The highest color temperature resulted from the RCI,whereas the lowest temperature resulted from the FCI. The microsecond temperature spike is attributed to thefirst contact (see Fig. 3). The rise in the thermal component between 5 and 25 ms correlates with the ricochetdebris striking the downrange target. Spectral measurements demonstrate that the thermal (blackbody)component exists only near the point of impact. As underscored by the contrast with results shown in Fig. 3d,the color temperatures (Fig. 7) illustrate that even a very thin carbonate veneer (�0.1% of the projectile

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Fig. 7. Evolution of the color temperature for the different experiments using a wide-field six-channel photodiode system. The maximum

lateral distance traveled by the ricochet (�5 km/s) and the jetting phase (415 km/s) is shown at the top. For comparison, a 301 impact by

Pyrex into silicate powder (pumice) resulted in a gradual decrease in temperature [16].

P.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780778

diameter) shielded the copper substrate, as also shown for crater scaling [17]. For reference, the duration of thespectral measurements is inserted. The downrange chamber wall is about 180 cm from the point of impact;consequently, the signals are not the result of interactions with the chamber.

4. Discussion

Experimentally isolating the ricochet-generated vapor provides fundamental clues into the vaporizationprocess. In Fig. 7, the highest peak color temperature result from the RCI (6500K) owing to the largeimpedance contrast between the impactor and the copper target at first. Conversely, the lowest peak colortemperature result from the FCI (�3400K around 30 ms after impact). The lower temperature is not only dueto the effective role of the thin carbonate layer in shielding the underlying copper substrate at first contact butalso due to energy deposited into molecular and atomic emissions, rather than a thermal component. Evenafter 150 ms, all emitting sources are in the FOV of the photodiode system. Neither the jetting components norricochet debris had yet collided with the chamber walls. Consequently, the higher temperatures at 100 ms forthe PCI (relative to the RCI) reflect the added contribution to the thermal signal by projectile debris strikingthe exposed copper surface downrange (e.g. see Fig. 3d). The low temperatures at 100 ms for the RCI resultfrom reduced thermal heating by impactor sibling fragments striking the carbonate veneer at very shallow(o101) angles.

While the top of the projectile does not initially couple to the target (see [14,18]), the lower half (or more) ofthe projectile fully couples and results in a brief (microsecond) high temperature spike (Fig. 7). This spikeresults from the projectile/target interface (including initiation of jetting) being ‘‘imaged’’ on the backside ofthe frosted surface of the translucent Pyrex projectile (clearly shown in high-speed imaging). The subsequenttemperature rise (5–20 ms) from the FC and PC impacts is less than the RCI because the FC and PC ricochetcomponents skim across the target surface.

Low-angle impacts result in low peak pressures below the impact point; therefore, the observedvaporization of the carbonate veneer in the spectral data must be primarily due to high strain-rate shear. Therevised carbonate phase diagram [10] indicates that vapor only can form under high temperatures (42500K)and at very low pressures (o 0.5GPa). Thermal measurements (Fig. 7) document the required hightemperatures, while the low impact angles (o 121) by the ricochet ensure low peak pressures. Such a process

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would then account for thermal decomposition into a gas (CaO, CO, 2O) in the present experiments. Freeexpansion of the vapor phases downrange during an oblique impact further reduces the rapid recombinationof the constituent gasses as observed in previous studies [1]. The wavelengths used here, however, do not allowassessing neutral or ionized oxygen or carbon.

Ricochet-generated vapor, therefore, not only allows CO and C2 to form but it also contributes to the freelyexpanding vapor cloud downrange (FOV-B, -C) and the jetting component (FOV-D). Hypervelocity impactsinto dolomite targets [2-4] yielded only the diffuse molecular CO band near 600 nm (superposed on the CaOband), most likely due to the single FOV focused just downrange from the point of impact (between A and Bin the present study). Future strategies will use different timing and configurations (including quarter-spaceexperiments, see [7,19]) in order to probe the rapidly evolving conditions within the vapor plume.

5. Implications

Previous studies concluded that hypervelocity impacts should not generate significant thermal decomposi-tion of carbonate targets due to the high pressures and temperatures [11]. Consequently, it has been suggestedthat release of CO or CO2 could not have contributed to climate change or ecological collapse following amajor impact at the end of the Cretaceous. Even during post-shock waste heating during decompression, somestudies argue that rapid back reactions preclude the release of decomposition products such CO and CO2 [10].The laboratory-scale experiments described here, however, demonstrate that high strain-rate shear by thericochet component not only generates high temperatures under low pressures but also releases the vaporphases freely due to the rapid decoupling from the target behind the hypervelocity ricochet impacts.

Although small in scale, the experimental results still should apply to large-scale events provided thatimpact velocities (440 km/s) and angles are not too high (4401 from the horizontal). Much higher impactvelocities result in greater fractions of the projectile vaporizing, whereas higher impact angles reduce thedownrange dispersal. Direct measurements of peak pressures [8], crater scaling [17], and three-dimensionalhydrocodes [12] indicate that peak pressures below the impact point decrease as sin2y (for impact angle ymeasured from the horizontal). Impactor failure (including strain-rate effects) also yields a similar dependence[14]. Consequently, a 151 impact by a 25 km/s impact generates an initial peak pressure in the projectile that isreduced by a factor of 15. For illustration, a peak pressure of 220GPa was calculated at one projectilediameter for vertical impact by a gabbroic anorthosite projectile into gabbroic anorthosite target at 25 km/s[12]. Consequently, a 151 impact would result in a peak pressure of �15GPa. Because the downrange ricochetstrikes the surface downrange at less than one-half the initial impact angle, peak pressures at first contact bythe ricochet will be less than 1GPa, then rapidly decompressing and freely expanding due to the largetranslational motion. Consequently, the prerequisite for vapor release (e.g., pressures less than 0.1GPa at5000K and free expansion) suggested by the revised phase diagram [11] is met.

Decoupling occurs only after the shock reflects off the top of the impactor with spall velocities alsodependent on the vertical component of velocity. For vertical impacts, the projectile penetrates the target toincreasing depths with increasing velocities before the shock reaches the back (top) of the projectile. But foroblique impacts, the vertical component of velocity reduces projectile penetration while the shock stillpropagates into the projectile rapidly at the moment of first contact. Consequently, the upper portions of theprojectile should disrupt and disperse downrange at planetary scales as well. Such a process is evident in highlyoblique impacts on the planets (e.g., [14,20,21]). Thermal decomposition and free vapor expansion due to highstrain-rate shear by the downrange ricochet, therefore, very well may apply at much larger scales.

6. Conclusions

Experimental design has allowed isolating the ricochet contribution to the vaporization process. Whilejetting dominates the impact ‘‘flash’’ [4], impactor ricochet plays the controlling role for impact-generatedvapor. The carbonate vapor comprising the jetting phase is dominated by ionized gas [22], whereas thericochet vapor contains both atomic (Ca, Na) and molecular (CaO, CO) emissions. Conclusions previouslydrawn about reduced melting/vaporization at lower impact angles [12] may inadequately account for the fateof the impactor and its contributing role in the impact process. This role includes the creation of multiple shear

ARTICLE IN PRESSP.H. Schultz et al. / International Journal of Impact Engineering 33 (2006) 771–780780

planes (resulting in bulk heating) in the fragmented projectile, the contribution from the downrange ricochet,and free expansion of the vapor.

Acknowledgments

The authors wish to acknowledge the assistance of the technical crew at the NASA Ames Vertical GunRange (Don Holt, Don Bowling, Rick Smythe, and Chuck Cornelison). This research was performed underNASA Grant NNG05G137G.

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