www.sciencemag.org/cgi/content/full/330/6003/468/DC1

Supporting Online Material for

The LCROSS Cratering Experiment

Peter H. Schultz,* Brendan Hermalyn, Anthony Colaprete, Kimberly Ennico, Mark Shirley, William S. Marshall

*To whom correspondence should be addressed. E-mail: peter_schultz@brown.edu

Published 22 October 2010, Science 330, 468 (2010)

DOI: 10.1126/science.1187454

This PDF file includes:

SOM Text

Figs. S1 to S3

Table S1

References

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Online Supplementary Material

1. Estimates for ejected mass: Based on pre-encounter expectations (1-3), the Centaur

impact was predicted to generate a multi-component ejecta sequence including: (i.)

heated high-speed, low-angle, ejecta; (ii.) high-angle, high-speed ejecta plume; (iii.)

nominal outward-expanding ejecta curtain; and (iv.) expanding vapor released by solar

irradiation. The following discussion first reviews the cratering sequence.

Crater-scaling relations (e.g., 1,2) and computational models (2,3) provided estimates for

the excavated mass and crater diameter for planning the encounter. Extrapolations from

laboratory experiments to the scale of the LCROSS impact experiment were based on

dimensionless scaling relations (4). Cratering efficiency measures the amount of mass

displaced (driven downward and ejected) by the impact relative to the projectile mass.

The controlling independent variable scales the inertial force created by the impact to

either target strength or gravitational resisting forces. The former case is illustrated by an

impact into an ice-welded regolith or cohesive lunar bedrock. The latter case describes

crater growth in a loose particulate (or shock-weakened) target where gravity prevents

ejecta from exceeding the crater rim.

The impact experiment during the Deep Impact mission (5) provided one end-member

value for the dimensionless gravity-controlled scaling parameter: high speed, small

projectile mass, low impact angle (relative to surface). The LCROSS Centaur impact,

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however, represents the other extreme: low speed, high projectile mass, and high impact

angle.

For a projectile mass of 2300kg and effective density (total mass/volume) of less than

0.03 g cm-3, the predicted total ejected mass approached 2.5x105 kg with a rim-to-rim

crater diameter of 21m for the impact speed of 2.5 km/s and near-vertical trajectory into a

gravity-controlled target (1). Other extrapolations yielded larger craters with ejected

masses ranging from 6x106 to more than 1x107 kg (2). For example, computational

models using a smooth particle hydrodynamic code (SPH) typically yield more than 106

kg for a variety of assumptions (2). The final size of the LCROSS crater, however, is

consistent with expectations based on craters produced by artificial impacts in the 1960’s

and 1970’s (e.g., Ranger and Apollo 13 and 14 missions).

Accurate predictions for the LCROSS impacts were ultimately limited by targeting

uncertainty, nature of the regolith (e.g., ice-welded regolith), unseen landscapes in deep

shadow, the very low effective density of the fuel-emptied Centaur (in addition to its

heterogeneous mass distribution), and the orientation of the tumbling Centaur at the

moment of impact (see 2).

The primary goal for LCROSS, however, was to assess the presence/absence of water ice

possibly stored below the surface. Since the impact occurred in a permanently shadowed

region (PSR), ejected material would not become visible until it reached the sunlight

horizon, 833m above the impact surface. Solar illumination would then heat the ejecta,

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thereby causing it to release attached or entrained volatiles (6). Consequently, the total

excavated mass (or crater diameter) is not as important as determining the amount of

high-speed ejecta reaching the sunlight horizon and entering the fields of view (FOV) of

the Shepherding Spacecraft (SSc) instruments is more relevant than the total excavated

mass (or crater diameter), the vast majority of which never would reach sunlight.

Dimensionless scaling relations typically provide a valid strategy to estimate the amount

of ejected mass launched above a given velocity for near-vertical impacts (7). Such

scaling relations, however, break down for high ejection speeds (8) or for oblique impacts

(8, 10). This prevents application of such scaling relations to the higher speed ballistic

ejecta (>200m/s), the component capable of reaching the sunlight horizon at an altitude of

0.833km above the Centaur impact point and remaining aloft for much more than a

minute.

For LCROSS, most of the Centaur mass is concentrated in the spherical oxygen tank and

rocket motors at one end with a less massive cylinder (hydrogen) at the other. A thin-

walled cylinder (~10m long, ~3m in diameter) supports the entire assembly.

Consequently, the impacting Centaur can be approximated by its greatest mass

concentration, i.e., the hollow spherical oxygen tank. Two or more mass concentrations,

however, also can result in high-angle rays (e.g., 2). Experiments using hollow

projectiles with buried tracers indicate that the initial upward-directed ejecta comes from

a shallow depth (less than a projectile diameter), but incorporates deeper levels with time.

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New laboratory impact experiments at the NASA Ames Vertical Gun Range focused on

the early-stage ejecta-velocity distribution (9,11) and the consequences of both low-

density projectiles (hollow aluminum spheres) and a compressible particulate target (e.g.,

air-fall pumice). Several specific conclusions could be made from these studies.

First, the earliest stages of ejection (for both sand and fine-grained air-fall pumice targets)

were characterized by much lower low-ejection angles (100m/s in laboratory experiments for 2.5km/s

vertical impacts) emerge when late-stage ejecta scaling relations do not hold.

Consequently, the earliest, low-angle ejecta represent only a few projectile masses and

originate from a relatively shallow depth. This would limit the amount of ejecta reaching

sunlight.

Second, experiments revealed that impacts by hollow spheres produce a distinctive high-

speed (>1km/s) near-vertical plume in experiments, starting soon after impact and

persisting until near the end of excavation (11). This component fills in the expanding

annulus of main-stage ejecta, normally associated with impacts into porous targets (e.g.,

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8). A high-angle plume also develops from higher speed impacts into an under-dense and

compressible silicate targets but is due to a fundamentally different process (5).

The high-angle ejecta component is attributed to cavitation during the penetration stage

of cratering. At low impact speeds (

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smaller Shepherding Spacecraft would not be expected to generate a significant high-

angle plume.

2. UV/Visible Spectrometer (VSP): Figure S-1 provides closer views of spectra from

the VSP instrument. Figure S-1 provides closer views of spectra from the VSP

instrument. The instrumental error in radiance was measured for the instrument during

calibration. These errors decrease with increasing wavelength: the average error in

radiance was 1.06x10-3± 6.3x10-4 (one standard deviation) for 265nm to 300nm; 6.55

x10-4 ± 1.9x10-4  (300nm to 400nm); 4.83x10-4 ± 3.2x10-5  (400nm to 550nm); and 4.61 ±

0.44x10-4 (550nm to 650nm).

Subtraction of successive 2-second exposures prior to impact (during which the VSP

FOV covers only the shadowed floor of Cabeus) provides a different measure of

observational uncertainty with standard deviations from zero as ± 3.4x10-3  (265-

400nm), ± 2.1x10-3  (300nm-400nm), ± 1.9x10-3 (400-550nm), and ± 2.6Ex10-3 (550nm-

650nm). These variations, which are greater than the instrumental error, demonstrate the

robustness of a particular identified species since they are much smaller than identified

lines (2 sigma uncertainty in Figure S-1, lower right).

For identification of a spectral features, it must (a) exceed two standard deviations from

the subtracted pre-impact exposures, (b) persist in subsequent post-impact integrations,

and (c) are not attributable to features in the solar spectrum are interpreted as impact-

related emission lines (in the same ratio as an adjacent line). In several cases, the

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radiance difference grows with time, even though the pre-impact spectra show a negative

feature.

Many spectral features closely correlate with both solar and pre-impact spectra even at

the 1-sigma. This further demonstrates that the observed lines from the VSP are not

simply due to noise down to a spectral resolution of

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overall brightness relative to the pre-impact spectra, emission lines emerge with the most

prominent lines at the following wavelengths: 266nm, 271nm, 283nm, 289nm, 297nm,

306nm, 313nm, and (as well as possible lines at 351nm, 458nm, 560nm, 580nm, 596nm,

604nm, 609m, 611nm, 613nm, and 620nm, 641nm, and 645nm). The emission lines at

289nm, 340nm, and 351nm are consistent with CO2+.

Analyses of spectral data from the LAMP instrument derived significant quantities of CO

along with Ca, Mg, Hg, and H2 (12). In the VSP, strong solar absorptions from scattered

light mask emission lines from Ca and Mg; hence, they are not easily identified without

proper modeling of the particle-size distribution, which is beyond the scope of this

contribution. Slight increases in radiance at 475nm-481nm and 490nm-499nm after

16.3sec coincide with diffuse CO bands, and a line at 426nm coincides with CO+. Such

identifications remain tentative but are noted for their relevance to the nature of the

regolith. Photo-ionization of CO2 requires much longer exposure times to solar UV than

those experienced by ejecta lofted by the impact. Consequently, the presence of CO2+

would require weak bonding to regolith grains. While possible lines at 313nm, 365nm,

405nm and 456nm occur at the Hg wavelengths, they appeared at different times and not

in the correct sequence for excitation temperatures. For example, an emission line at

313nm coinciding with Hg emerged during the first 0.8sec. Other Hg lines, however, did

not appear until the following 2-second exposure (365nm) or later (after 3.4sec at

405nm), and a Hg line at 456nm line was not evident. Hence, the LAMP observations

appear to provide a view of impact-liberated species that is very different from (and

complementary to) the observations from the VSP spectrometer on SSc. These

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distinctions reflect differences in spectral coverage, fields of view, duration in sunlight,

and depth of origin (abundances).

The next 2-second exposure from the VSP (ending ~3.1sec after impact) exhibits

elevated brightness that persists over the next 234 seconds (see 13). Emission lines in the

near-UV (313nm and 351nm) faded while three strong emission lines at 328nm and

338nm emerged. The line pair at 328nm and 338nm is consistent with the position of Ag

excitation. These two lines have nearly equivalent transition probabilities, but their

relative heights depend on the level of excitation. At high energies, the 328nm is

stronger than the 338nm but at lower energies, the reverse is true. The faint impact flash,

delayed emission lines (e.g., Na), emergence of the line pair attributed to Ag, and delayed

thermal response in the NSP1 provide important clues for the sequence of crater

excavation as discussed in the text.

Ejecta continued to reach sunlight, increasing the relative brightness captured in the VSP-

difference spectra ~3-5 sec after impact (Fig. S-2). The emission lines near 328nm and

338 nm strengthened, along with numerous other emission lines and bands, which

became more apparent in the following exposure. Specifically, Figure S-3 reveals many

other emission lines at longer wavelengths. The Na line (D2) at 589nm overcame the

strong solar absorption from scattered sunlight and remained strong until the exposure

starting at 16.3sec. Lines attributed to oxygen, H2O, and NH2 appeared at these longer

wavelengths after 3.4sec.

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The lines at 374nm and 601nm are attributed to cosmic ray hits. The CCD device for the

VSP has 1044x64 pixels, but does a co-addition along the spatial direction, resulting in a

1044x1 pixel spectrum (1024 spectral pixels, 20 dark level pixels). Consequently,

information on the spatial location of the cosmic rays is lost. From a total energy

perspective, 'bleed over' (e.g., the effect of a saturated pixel on its neigbors) will occur for

the largest cosmic ray impacts on a single pixel. A cosmic ray (of any energy), however,

could impact the chip obliquely and affect the DN values diagonally across columns, and

therefore “bleed” energy into adjacent spectral lines without saturation. In order to assess

a cosmic ray hit, we assume that a very large emission line appearing in only one

exposure is due to a cosmic ray. This assessment is based on comparison of other cosmic

ray hits during lunar flyby calibrations.

Hydroxyl band strength (near 310nm) increased up to about 100 seconds after impact and

then decreased until about 220 seconds (13). Absorptions due to water molecules (and

other H-species) captured at longer wavelengths by the NSP1 lasted nearly the entire 4

minutes of approach. By about 15sec, the ejecta plume filled the VSP field of view (Fig.

1c and d in main text), thereafter sub-sampling only a portion of the cloud as the SSc

continued its trajectory to the surface. The spectrometers detected volatiles for almost 4

minutes after impact, even though the ejecta remained in the visible cameras for less than

42 seconds after impact.

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Figure S1

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Figure S2

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Figure S3

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Figure S-1: Difference spectra (from 250nm to 400nm) from the VSP instrument of the

impact flash and evolving ejecta that reached sunlight. A reference spectrum (labeled

“pre-impact”) was a 2-second exposure taken prior to impact and included only scattered

light within the shadows of Cabeus (not to same scale as differences). A solar spectrum

(from above Earth’s atmosphere) is also shown for comparison. Instrumental errors are

shown for the first 2-second exposure. The subtraction of additional 2-second pre-impact

exposures provides another measure of the noise in the signal with the 2-sigma deviation

between pre-impact spectra shown at the lower right. Small dots identify likely features

that can be attributed to scattered sunlight off the ejecta cloud mirrored by features in the

pre-impact and solar spectra. Each spectrum was subtracted from this difference to

reveal contributions from the impact flash and scattered light from sunlit ejecta. The

first exposure ended 0.8 seconds after the moment of impact (red) and exhibited emission

lines due to impact-heated volatiles. The second exposure (1.1-3.1sec) reveals an

increased radiance due to scattered light off sunlit ejecta. The peaks of the line pair and

hot pixel (cosmic ray) have been truncated (see Fig. 1c,d).

Figure S-2: Difference spectra (from 400nm to 550nm) from the VSP instrument of the

impact flash, evolving ejecta reaching sunlight, and comparison with solar spectrum as

labeled in Figure S-2. Increasing radiance at longer wavelengths relates to the particle

size distribution within the ejecta cloud. After 15sec, the field of view of the VSP was

smaller than the ejecta cloud; consequently, the increased radiance was no longer

proportional to the total amount of ejecta in sunlight.

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Figure S-3: Difference spectra (from 550nm to 650nm) from the VSP instrument of the

impact flash, evolving ejecta reaching sunlight, and comparison with solar spectrum as

labeled in Figure S-1. Because of the large differences in radiance levels over this

spectral range, spectra have been shifted closer together (see Fig. 1d in the manuscript).

The strong emission line at 601nm (3.4 to 5.4sec after impact) is interpreted as a cosmic

ray due to its strong and unique (only one exposure) appearance.

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Table S-1 Identified emission lines are referenced to their first appearance (after subtraction of background), i.e., time after impact (Time AI), and identified according to persistence (through successive spectra) and heights relative to a 2 second pre-impact exposure showing scattered sunlight corresponding to the solar spectra. Each identified line exceeded a 2-sigma variation measured from subtraction of successive 2-second pre-impact exposures. Time After Impact (AI) corresponds to the beginning of different exposure shown in Figure S-1. Spectral line (nm) Time AI (sec) Possible source 266 0.8 CS 271 0.8 279 0.8 289 0.8 (CO2+) 290 3.1 OH 297 3.1 305 5.4 308 3.1 OH 313 0.8 (8.1) OH “prompt emission” water 328.3 3.1 Ag 338.4 3.1 Ag 335 3.1 340.3 3.1 NH and/or CO2+ 351 0.8 CO2+ 366 3.1 405.3 5.4 416 0.8 CN 426.8 5.4 CO+ 457.5 0.8 NH 461.4 16.3 H2S 472-482 16.3 (CO) 489-499 16.3 (CO) 529.4 0.8 NH2 542.8 0.8 NH2 552 5.4 557 3.1 O 574 3.1 NH2 577 16.3 580.6 0.8 589 3.1 Na 596.7 0.8 611 0.8

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613 0.8 619 16.3 H2O+ 621 0.8 627 0.8 628 0.8 634.5 3.1 641 0.8 642 0.8

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12. Gladstone et al., Science, this volume, (2010).

13. A. Colaprete et al., Science, this volume, (2010).