Results from the NMSU NASA Marshall Space Flight Center LCROSS observational campaign

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116,, doi: /2010je003761, 2011 Results from the NMSU NASA Marshall Space Flight Center LCROSS observational campaign N. J. Chanover, 1 C. Miller, 1 R. T. Hamilton, 1 R. M. Suggs, 2 and R. McMillan 1,3 Received 14 October 2010; revised 25 April 2011; accepted 6 May 2011; published 16 August [1] We observed the Lunar Crater Observation and Sensing Satellite (LCROSS) lunar impact on 9 October 2009 using three telescope and instrument combinations in southern New Mexico: the Agile camera with a V filter on the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory (APO), a StellaCam video camera with an R filter on the New Mexico State University (NMSU) 1 m telescope at APO, and a Goodrich near IR (J and H band) video camera on the NMSU 0.6 m telescope at Tortugas Mountain Observatory. The three data sets were analyzed to search for evidence of the debris plume that rose above the Cabeus crater shortly after the LCROSS impact. Although we saw no evidence of the plume in any of our data sets, we constrained its surface brightness through analysis of our photometrically calibrated data. The minimum surface brightness that we could have detected in our Agile data was 9.69 magnitudes arc sec 2, which is 177 times fainter than the brightest part of the foreground ridge of Cabeus. In our near IR data, our minimum detectable surface brightness was 8.58 magnitudes arc sec 2, which is 370 times fainter than the brightest part of the foreground ridge in the J and H bands. The debris plume was detected by the LCROSS shepherding spacecraft and the Diviner radiometer on the Lunar Reconnaissance Orbiter. Given the plume radiance observed by LCROSS, we cannot distinguish between a conical or cylindrical plume geometry because when seen from Earth, both are below our detection thresholds. Citation: Chanover, N. J., C. Miller, R. T. Hamilton, R. M. Suggs, and R. McMillan (2011), Results from the NMSU NASA Marshall Space Flight Center LCROSS observational campaign, J. Geophys. Res., 116,, doi: /2010je Introduction [2] The NASA Lunar Crater Observation and Sensing Satellite (LCROSS) mission was designed to search for evidence of water ice in the floor of a continuously shadowed crater near the Moon s south pole. LCROSS was a piggyback experiment to the Lunar Reconnaissance Orbiter (LRO), which was launched on 18 June The LCROSS mission involved crashing the LRO upper stage into a shaded polar region of the Moon on 9 October 2009, throwing up lunar regolith high enough to be illuminated by the Sun and observed from Earth while being simultaneously observed from a shepherding spacecraft following 4 min behind the booster. [3] In 1998 the neutron spectrometer on the Lunar Prospector orbiter detected the signature of hydrogen concentrations in shaded areas of the Moon s north and south poles [Feldman et al., 1998, 2001]. This hydrogen may be locked up in hydrous minerals or water ice. More recent observations of the Moon made from Cassini [Clark, 2009], 1 Astronomy Department, New Mexico State University, Las Cruces, New Mexico, USA. 2 NASA Marshall Space Flight Center, Huntsville, Alabama, USA. 3 Apache Point Observatory, Sunspot, New Mexico, USA. Copyright 2011 by the American Geophysical Union /11/2010JE the Deep Impact spacecraft [Sunshine et al., 2009], and the Moon Mineralogy Mapper on board Chandrayaan 1[Pieters et al., 2009] also demonstrated an enhanced concentration of volatiles near the poles, along with a thin layer of water more evenly distributed on the lunar surface. Thus, there is a growing body of evidence suggesting that the Moon is not as dry as previously believed. [4] The primary scientific objective of the LCROSS mission was to detect spectroscopic evidence of water in the illuminated plume. Preliminary estimates of the plume brightness by NASA/Ames Research Center indicated that the evolving plume would be visible from Earth, thus a coordinated ground based observing campaign was organized by the LCROSS project in an effort to characterize the LCROSS plume over a longer time period and using a wider variety of instrumentation than was possible with the shepherding spacecraft and LRO [Heldmann et al., 2011]. Here we report on the results of this observing campaign. [5] Our planned observations targeted LCROSS science goal 4: Characterize the lunar regolith within a permanently shadowed crater on the Moon. Specifically, we proposed to set constraints on the ejecta mass by observing the time evolution of the expanding plume. Measurements of the ejecta plume brightness and morphology (i.e., size, shape, and altitude) as a function of time can yield information about the amount of material ejected and its velocity distribution [Korycansky et al., 2009]. This is of particular 1of15

2 Figure 1. The response curves are shown for the three instruments used for the NMSU NASA Marshall Space Flight Center LCROSS observational campaign. On the ARC 3.5 m telescope, we used Agile with a V filter (dashed line). Note that this curve does not include the neutral density filter (OD = 2.5) used for the Moon observations. On the NMSU 1 m telescope we used a Sony EX HAD video camera with an R filter (solid line). On the Tortugas Mountain Observatory 0.6 m telescope we used a Goodrich InGaAs near IR video camera with no additional filter (dash dotted line). interest to lunar spacecraft designers because ejecta from meteoroid impacts must be considered in shielding design and risk assessments. Comparisons of our measurements with laboratory impact tests and cratering models would enable the selection of the appropriate scaling laws and their extension to the smaller, kilogram sized meteoroids traveling at much higher speeds (from 20 to 70 km s 1 ). 2. Observations [6] We used a multitelescope approach to image the LCROSS plume from southern New Mexico. We observed the LCROSS impact with two telescopes at the Apache Point Observatory (APO), which is located in the Sacramento Mountains at an elevation of 2788 m. We also employed the Tortugas Mountain Observatory (TMO), at an elevation of 1505 m in Las Cruces, NM. The spectral coverage afforded by the combination of instruments used on these three telescopes spanned mm. The spectral response of each camera and filter used for this study is shown in Figure 1, and the respective image scale parameters are listed in Table 1. The data acquired with each telescope are further described below, and additional details concerning the ground based Table 1. Image Scale Parameters for the Three Telescope Instrument Combinations Used for Our LCROSS Analysis Parameter 3.5 m + Agile 1 m + StellaCam 0.6 m + Goodrich Focal length (m) a Pixel size (mm) Image scale (arc sec pixel 1 ) 0.26 b Image scale c (km pix 1 ) a Effective focal length, accounting for the 2X focal reducer used with Agile. b In 2 2 binning mode. c Based on a lunar range of km at the time of impact. 2of15

3 Figure 2. The white measuring rod is 3.5 km wide, and the colored stripes are 0.5 km thick and separated by 1 km. The colored stripes correspond to elevation ranges of km (green), km (blue), and km (violet) above the floor of the Cabeus crater. Image courtesy of the NASA Goddard Space Flight Center Scientific Visualization Studio and the LOLA Team. LCROSS observing campaign, including the participation of the three telescopes described herein, are discussed by Heldmann et al. [2011] Astrophysical Research Consortium 3.5 m Telescope [7] We used the Agile camera on the Astrophysical Research Consortium (ARC) 3.5 m telescope at APO to acquire images of the plume resulting from the LCROSS impact. The ARC 3.5 m telescope has an altitude azimuth design, with Agile permanently mounted at one of the bent Cassegrain instrument ports. Agile is a high speed time series photometer based on the Argos camera at McDonald Observatory (as described by Nather and Mukadam [2004]). It is composed of a Princeton Instruments Micromax camera and a frame transfer CCD with square 13 mm pixels. Coupled with a focal reducer on the ARC 3.5 m telescope, Agile has a field of view with an unbinned plate scale of pixel 1. Agile has a read noise of 6.62 e RMS when using the fast (1 MHz) readout rate, and a dark current of 6.8 counts s 1 pixel 1. The exposure start times and durations are slaved to GPS synchronized pulses, providing timing control accurate to within 1 ms. Agile contains a continuous frame transfer array, which results in zero latency time between exposures. With 2 2 binning the plate scale is pixel 1 and the minimum exposure time is 0.5 s. [8] We tested various filter combinations, camera gain and binning modes while observing the Moon through a range of solar illumination angles during the ten months prior to impact. Our goal was to optimize the signal to noise and dynamic range of our images while minimizing scattered light from the illuminated disk of the Moon. For the impact observations, we employed an MSSO V filter along with a neutral density filter with an optical density (OD) of 2.5, where OD is related to the filter transmission, T, by OD = log 10 (T). We operated Agile in the 2 2 binning mode with a medium gain setting, which yields 1.93 e per ADU. We implemented offset tracking velocities in the telescope tracking software in order to mitigate the challenges of observing an object moving at a nonsidereal rate. Our pointing and tracking strategies associated with observing the Moon with the ARC 3.5 m telescope are described by Heldmann et al. [2011]. [9] The LCROSS project team provided the ground based observers with projections of the height above the lunar surface to which the plume would reach (Figure 2). These renderings of the lunar surface were generated using data from the Lunar Orbiter Laser Altimeter (LOLA) on board LRO. These observational planning aids suggested that the debris plume resulting from the impact would become visible to Earth based observers once it rose above the edge of a foreground ridge at a height of roughly 2 km. [10] For the impact observations, we implemented a makeshift lunagraph to block out most of the illuminated disk of the Moon. We inserted the Agile dark slide partway into its slot, thereby creating a dark mask with a curved edge. We used a dome flat image to identify the pixel limits of the attenuation due to the dark mask. A sample image of the Moon taken with the mask in place is shown in Figure 3. The dark slide was not quite in the Agile focal plane, thus the edge of the mask is somewhat defocused. Nonetheless, this proved to be a simple and effective way to reduce scattered light from the Moon during our attempt to image the faint plume. [11] We imaged the Moon continuously during the time span 11:19:26 12:04:26 UT in order to completely characterize the Cabeus crater region before, during, and after the LCROSS impact, which occurred at 11:31: UTC [Heldmann et al., 2011]. The atmospheric seeing measured at Apache Point Observatory on the night of impact ranged between 0.8 and 1.4. Table 2 lists all of the data we acquired, including calibration data. The entire 3.5 m data set is archived in NASA s Planetary Data System and is available for download by the general public. [12] We reduced the Agile data using standard CCD data reduction techniques. The dark current corresponding to the exposure time for each science image was subtracted, which also removed the bias level for the CCD chip. We flat fielded the resultant images to remove pixel to pixel var- 3of15

4 Figure 3. The Agile dark slide was used as a mask to block out a portion of the Moon s illuminated disk within the Agile field of view. (top) The dark region with the rounded edge on the right clearly shows the dark mask. (bottom) Analysis of a dome flat image taken with the dark slide inserted showed that because the dark slide is slightly defocused, vignetting effects result in image attenuation out to the region of the vertical dashed line. We optimized our placement of the LCROSS target crater, Cabeus (circled), to be outside of the dark slide attenuation limit. iations in the CCD response using morning twilight flats acquired with the same observing parameters (filter, gain mode, binning) as the science images New Mexico State University 1 m Telescope [13] We employed a StellaCam EX (Sony HAD EX chip) video camera on the New Mexico State University (NMSU) 1 m telescope at APO. The 1 m telescope also has an altitude azimuth design, and the StellaCam was mounted at the primary instrument port. The pixel data were recorded at a cadence of 30 frames s 1, digitized to 8 bits, and recorded to a PC. Offset tracking velocities were implemented in a similar manner as for the ARC 3.5 m telescope. A Johnson Cousins R filter was installed in front of the StellaCam video camera to provide wavelength coverage complementary to that obtained with Agile on the ARC 3.5 m telescope (Figure 1). In addition to observing the Moon with this system, we recorded video of several bright G stars for calibration purposes. [14] The 2 min video segment surrounding the time of the LCROSS impact was converted to individual FITS images for our analysis. Since the camera was set to a gamma value of 0.45 (standard video) in order to maximize the dynamic range, the pixel values were linearized by the FITS conversion software using DN linear =DN 1/ NMSU 0.6 m Telescope [15] The near infrared (NIR) video camera used at NMSU s Tortugas Mountain Observatory (TMO) was a Goodrich Sensors Unlimited SU640KTSX, similar to those used on the LCROSS shepherding spacecraft [Ennico et al., 2011]. It was mounted at the Cassegrain focus of the TMO 0.6 m Boller and Chivens f/40 telescope. The pixel data were recorded at a cadence of 30 frames s 1, with exposure times of s, digitized to 12 bit data, and recorded to a Boulder Imaging Quazar video recording system with GPS time stamping of each frame. No filter was used in conjunction with the Goodrich video system; its broad spectral response includes the J and H bands (Figure 1). No offset tracking rates were used; guiding on the LCROSS target crater location was done by eye. We observed the Moon along with five calibration G stars with the NIR video system. Table 2. Description of the Three LCROSS Data Sets Acquired During the NMSU NASA Marshall Space Flight Center Observing Campaign a Image Type Target t exp (s) N img The 3.5 m Agile Imaging Data Science Moon; Cabeus crater Bias N/A Dark dark slide Flat twilight sky Calibration 68 Psc, HIP 2942, HIP 24813, standards 1 Aur, Mars, Uranus The 1 m StellaCam Video Data Science Moon; Cabeus crater b Calibration 68 Psc, 84 Psc, 30 And, 38 And, , 0.01, standards Iota Per, 14 Per, HR 157 The 0.6 m NIR Video Data Science Moon; Cabeus crater b Dark camera with lens cap on Flat twilight sky Calibration 68 Psc, 84 Psc, 38 And, c 150 standards Iota Per, 14 Per, Mars a All files are FITS images. The entire data volumes are 3.3 GB for the Agile data, approximately 2.3 GB for the StellaCam data, and 4.5 GB for the NIR video data. b These values correspond to the 2 min of video data analyzed for this study; the total number of images acquired is approximately 100,000. c The gain setting for the s images was 47 e /ADU; the gain for the s images was 72 e /ADU. 4of15

5 [16] The NIR video data were converted from Boulder Imaging metafiles to individual FITS files using custom software, and the resulting images were reduced using standard CCD data reduction techniques. The dark current corresponding to the exposure time for each image was subtracted and the resultant images were flat fielded using dawn flats Photometric Calibration [17] We photometrically calibrated the Agile data in order to derive an estimated surface brightness per pixel of our lunar images. To do this, we utilized our observations of four reference stars obtained on the night of impact. These reference objects are listed in Table 2 along with two extended sources that we observed, Mars and Uranus, which we used as an independent check of our surface brightness conversion. [18] We first used the observations of our standard stars to establish the photometric zero point of our instrument configuration. We performed aperture photometry using an aperture radius of four times the size of our typical FWHM of 3 pixels to derive stellar magnitudes of each reference. From these derived magnitudes, we obtained the following relation for our V band photometric zero point based on an average of 283 reference star exposures: V zero ¼ ð19:478 0:009Þ 2:5 log x t exp ; ð1þ where x is the total number of counts in the photometric aperture in units of DN, t exp is the exposure time in seconds, and assuming 2 2 binning and a gain of 1.93 e per ADU. [19] In order to calibrate our images of an extended object larger than several pixels, i.e., the lunar surface, we converted our zero point relation to a calculation of surface brightness. We did this by dividing the measured flux by the projected sky area subtended by a single pixel. For a small area and a square pixel, this area is simply the square of the pixel scale in arc seconds; for the case of Agile on the ARC 3.5 m telescope, this area was arc sec 2. Therefore, our surface brightness relation for the Agile data was x V SB ¼ ð19:478 0:009Þ 2:5 log ð2þ 0:0676t exp or, more simply, V SB ¼ ð15:800 0:009Þ 2:5 logðþ: x [20] We did not account for the point spread function dispersion of light between adjacent pixels. The error in equation (3) includes measurement uncertainty and the magnitude errors reported in the literature for the standard stars we observed. In order to verify our photometric calibration procedure, we applied it to our images of Uranus and compared our derived surface brightness with published values. We averaged the flux measured in the center pixel of the Uranian disk in three exposures and used equation (3) to calculate a surface brightness for Uranus. We derived a V SB = 8.41 magnitudes arc sec 2, compared to a value of 8.30 magnitudes arc sec 2 for Uranus as reported by the JPL ð3þ HORIZONS ephemerides calculator for the time of our exposures. [21] We were unable to follow the photometric calibration procedure described above for images taken with the video camera on the 1 m telescope. The reference stars we observed lack published R magnitudes, thus we conducted our analysis of the 1 m data using an instrumental magnitude calculated with an arbitrary zero point magnitude of 25. [22] To photometrically calibrate the 0.6 m near IR images, we followed a procedure similar to that used for the Agile data. We performed aperture photometry on the five observed standard stars using an aperture of three times the size of the FWHM (ranging from 2.4 to 4.3 pixels) for each image in order to determine an instrumental magnitude. Although the unfiltered passband for the Goodrich camera covers the Johnson Cousins J and H filters, we found that fitting the instrumental magnitudes to the J magnitudes provided the lowest residuals. We determined a zero point (intercept) and a slope of the instrumental versus J magnitude using a linear regression. A standard error in the linear regression of magnitudes (versus for H) indicated a good fit to the reference stars, whose J magnitudes ranged between 2.8 and 4.1, as published in the 2MASS catalog [Skrutskie et al., 2006]. The resulting fit is J pixel ¼ 22:356 2:5 log x * G=t exp ; ð4þ 0:823 where x is the pixel brightness in DN, G is the camera gain in e per ADU, t exp is the exposure time in seconds, and is the correction for the measured nonlinearity in the camera response. This nonlinearity is well determined over the range of magnitudes of the reference stars. For the camera setting of OPR 10 used for the event recording, G = 47 and t exp = , and the above relation then yields 13:859 2:5 log x J pixel ¼ ð ðþþ : ð5þ 0:823 To obtain the J magnitudes per square arc second, we used the plate scale of 0.22 arc sec pixel 1, or arc sec 2 pixel 1, to obtain our NIR surface brightness relation: 10:584 2:5 log x J SB ¼ ð ðþþ : ð6þ 0:823 Since a J magnitude for our extended source reference target, Mars, was not available, no independent check of the surface brightness calibration was possible. 3. Analysis [23] Figure 4 shows sample images from each of our three data sets at the time of impact, along with the rendering from Figure 2. We quantitatively analyzed our three sets of images to search for evidence of the LCROSS impact plume. In order to minimize the brightness variations due to image shift from seeing excursions, we applied a two step image registration algorithm to each data set using an Fast Fourier Transform technique. We first applied a crosscorrelation algorithm to shift and align the images using a relatively wide box size to include a variety of lunar terrain and brightness levels. We then narrowed the box size and 5of15

6 Figure 4. Representative images from (top left) the Agile camera on the ARC 3.5 m telescope, (top right) the StellaCam on the NMSU 1 m telescope, (bottom left) the Goodrich IR video camera on the Tortugas Mountain Observatory 0.6 m telescope, and (bottom right) the predicted location of the impact debris plume. All science images were acquired at the time of the LCROSS impact: 9 October 2009, 11:31:19 UT. Figure 4 (bottom right) is the computer generated image illustrating the predicted location of the impact debris plume, which is also shown in Figure 2. All four images were scaled to the same size and rotated to the same orientation, and cover an area approximately 90 km across centered on the Cabeus crater. focused on the Cabeus crater as our region of interest, using the shifts found in the first step as a starting point, and performed a second cross correlation using a finer grid size. We performed this alignment procedure on images that were first expanded using 5 pixel interpolation to allow for best case alignment to 0.2 pixels Agile Image Analysis [24] In order to visualize the temporal variation of the brightness of the Cabeus crater region in our Agile images, we first extracted brightness values along a given image column and displayed those values for each image in a time series, generating strip charts. A sample strip chart, spanning the time interval from 60 s prior to impact to 60 s postimpact, is shown in Figure 5. For our actual analysis we examined these strip charts for 600 s to +600 s around impact. Horizontal slices through the strip charts thus correspond to brightness cuts as a function of time for a specific (x, y) image location. Figure 6 illustrates the time variations in brightness for three locations corresponding to the top of the foreground ridge of Cabeus and elevations of 1.4 and 2.8 km above that point, identified by the horizontal lines in Figure 5. [25] We established the noise level at each of these three locations by taking the standard deviation of brightness over the entire time interval shown in Figure 6, i.e., 10 min before impact to 10 min after impact. We detected no events in which the brightness exceeded 3s for more than several seconds; transient spikes over 3s were attributed to random noise and/or seeing variations. As shown by the temporal variations in mean DN values for a specific location (Figure 6), the seeing varied substantially over the entire interval of our observations surrounding the LCROSS impact. The fact that the brightness variations behave similarly for various heights above the foreground ridge suggest that these variations are attributable to atmospheric seeing. We would have expected a plume to be manifested 6of15

7 Figure 5. (left) Sample Agile image. We extracted brightness values (DN counts) along the column of each Agile frame corresponding to the location of the impact (i.e., the pole shown in Figure 2). (right) These image slices along that column are displayed sequentially in the form of a strip chart. The strip chart shown here, made for demonstration purposes only, corresponds to the brightness along that column for 120 s centered on the impact time, i.e., 60 s before to 60 s after impact. The time of impact is marked by the short white vertical line. We analyzed brightness (DN) values along three rows of our strip charts, corresponding to the blue, red, and yellow horizontal lines, which represent the top of the foreground ridge and 1.4 and 2.8 km above that, respectively. These horizontal cuts through the strip chart are described further in Figure 6. in Figure 6 by a rise in brightness that appeared at slightly different times for the three heights shown. [26] The analysis of our Agile strip charts provided a twodimensional view (location, time) of the surface brightness evolution in an image column perpendicular to the floor of the Cabeus crater. We extended this analysis to three dimensions (x, y, time) in order to determine our timeaveraged surface brightness and limiting detection magnitude arc sec 2 of a plume at all points in the Cabeus crater. Figure 7 shows a contour map of the mean surface brightness for an area centered on Cabeus crater with a width of 29 km. We defined our limiting detection magnitude arc sec 2 as the surface brightness corresponding to the counts, in DN s 1, at the 3s level as defined by equation (3). Figure 6. Plots of Agile image brightness (in DN s 1 ) as a function of time corresponding to the three colored lines shown in Figure 5: (top) the yellow line, (middle) the red line, and (bottom) the blue line. The solid horizontal line through the middle of each panel represents the mean brightness value calculated over the 20 min of the observations we analyzed. The dotted horizontal lines represent ±3s about the mean. 7of15

8 Figure 7. Contour map of time averaged surface brightness in magnitudes arc sec 2 for our Agile V band images in a region 29 km across centered on Cabeus crater. Values are averaged over an interval from 600 s before to 600 s after impact. The vertical white strip centered at arbitrary pixel number 24 on the x axis represents the position of the white measuring rod shown in Figures 2 and 4 (bottom right). This is the region directly above the LCROSS impact site as seen from Earth. The bottom of the white strip corresponds to the top of the foreground ridge. The white strip is 1 km wide and is in the same position as the high angle plume models described in this work. The maximum surface brightness of the foreground ridge is 3.9 magnitudes arc sec 2. Figure 8 contains a contour map of those limiting surface brightnesses calculated for every pixel in Cabeus crater. The minimum surface brightness that we could have detected was 9.69 magnitudes arc sec 2, which corresponds to the maximum value in the center of Figure 8. This limiting surface brightness is 189 times fainter than the brightest part of the foreground ridge. Based on the impact location, which was within 100 m of the preimpact prediction depicted by Figure 8. Contour map of our 3 s detection limit for a plume inside Cabeus crater, in magnitudes arc sec 2, for our Agile V band images. Our detection limits range from 7.8 to 9.69 magnitudes arc sec 2 depending on the position inside the crater. This contour plot was centered at the same location and generated at the same scale as that shown in Figure 7, and the vertical white strip again designates the region directly above the LCROSS impact site as seen from Earth. The point of maximum detection sensitivity within the white strip is at arbitrary pixel number 10 on the y axis, which is 4 km above the impact point. The detection limit at that point is 9.5 magnitudes arc sec 2. 8of15

9 Figure 9. Plots of StellaCam R band image brightness (in DN s 1 ) as a function of time for elevations of 0, 1.8, 3.6 km above the foreground ridge of Cabeus. The solid horizontal line through the middle of each panel represents the mean brightness value calculated over the 2 min of the observations we analyzed. The dotted horizontal lines represent 3 s variations from the mean. the pole in Figure 4 (bottom right) [Marshall et al., 2011], the position of maximum sensitivity in our Agile data corresponds to a point 3.7 km above the crater floor and 5.5 km radially away from the impact point Video Data Analysis [27] The temporal variation of the brightness of the Cabeus crater region as seen in our R band video data acquired at the NMSU 1 m telescope is shown in Figure 9, which spans 1 min before to 1 min after impact. We examined the brightness evolution of three locations corresponding to the top of the foreground ridge of Cabeus and elevations of 1.8 and 3.6 km above that point. As for the Agile data, we see no evidence of a persistent brightening that could be attributable to the appearance of the debris plume. The R band video data were not photometrically calibrated, so Figures 10 and 11 show contour maps of the instrumental surface brightness in magnitudes arc sec 2 and the standard deviation of those instrumental surface brightness values, respectively. Thus, unlike the standard deviations shown in Figure 8, which show an absolute detection limit for the plume brightness in all locations within the Cabeus crater, Figure 11 shows a relative detection limit. This reveals a dynamic range of approximately four instrumental magnitudes from the darkest part of the crater to the bright foreground ridge. Based on the difference in instrumental magnitudes between Figures 10 and 11 we could have detected a plume in the range of times fainter than the brightest part of the foreground ridge of Cabeus. [28] We analyzed the near IR data from the TMO 0.6 m telescope using a procedure analogous to that described above for the Agile data. Figure 12 shows the time variations in the brightness of three locations in the Cabeus region from 1 min before to 1 min after impact. We examined the brightness evolution of the top of the foreground ridge of Cabeus and elevations of 1.5 and 3.1 km above that point. No evidence of a plume is seen in these brightness cuts. The contour maps of the mean surface brightness (in magnitudes arc sec 2 ) and the standard deviations, which provide a limiting detection magnitude arc sec 2, are shown in Figures 13 and 14, respectively. The minimum surface brightness that we could have detected in our near IR data was 8.58 magnitudes arc sec 2, which corresponds to the maximum value in the center of Figure 14. This limiting surface brightness is 430 times fainter than the brightest part of the foreground ridge. 4. Discussion [29] Confirmation of the LCROSS impact was provided by the shepherding spacecraft, which obtained direct observations of the debris plume from a near nadir vantage point [Colaprete et al., 2010]. The Diviner radiometer instrument on board the Lunar Reconnaissance Orbiter detected the associated thermal emission 90 s after impact, observing the impact site from an oblique angle of 48 [Hayne et al., 2010]. Diviner also saw an enhancement in the signal in its solar channel, which is attributed to scattered sunlight from the ejecta plume. The derived mass of material 9of15

10 Figure 10. Contour map of time averaged instrumental surface brightness in magnitudes arc sec 2 for the StellaCam R band video camera images in a region 29 km across centered on Cabeus crater. Values are averaged over an interval from 60 s before to 60 s after impact. The vertical white strip centered at arbitrary pixel number 29 on the x axis represents the position of the white measuring rod shown in Figures 2 and 4 (bottom right). above the crater horizon required to produce the visible light signal seen by Diviner is consistent with that derived from the LCROSS spectrometer data [Hayne et al., 2010]. We used these spacecraft observations of the plume to estimate the plume brightness as seen from Earth in an effort to explain our lack of detection of the plume in our data. [30] We based our plume brightness estimate on the maximum reported radiance of the plume as seen by the UV/Vis spectrometer aboard the LCROSS shepherding spacecraft. This peak radiance value was initially reported as 90 W m 2 sr 1 at 17 s after impact [Colaprete et al., 2010, Figure 1]; it was recently reevaluated and found to be Wm 2 sr 1 (A. Colaprete, personal communication, 2011). We employed a Kurucz model of a Vega spectrum ( kurucz.harvard.edu/stars/vega) to determine a 0 magnitude arc sec 2 reference point, and using this we calculated a peak plume surface brightness of 10.8 magnitudes arc sec 2. Figure 11. Contour map of our 3 s relative detection limits for a plume inside Cabeus crater in magnitudes arc sec 2 for our StellaCam R band video camera images. The white strip, which is 1.2 km wide, represents the position of the white measuring rod shown in Figures 2 and 4 (bottom right). This is the region directly above the LCROSS impact site as seen from Earth. The contour values shown are not photometrically calibrated and thus depict only relative detection limits within the frame. 10 of 15

11 Figure 12. Plots of near IR image brightness (in DN s 1 ) as a function of time for elevations of 0, 1.5, 3.1 km above the foreground ridge of Cabeus. The solid horizontal line through the middle of each panel represents the mean brightness value calculated over the 2 min of the observations that we analyzed. The dotted horizontal lines represent ±3s about the mean. Figure 13. Contour map of time averaged surface brightness in magnitudes arc sec 2 for the Goodrich IR video camera images in a region 29 km across centered on Cabeus crater. Values are averaged over an interval from 60 s before to 60 s after impact. The vertical white strip centered at arbitrary pixel number 29 on the x axis represents the placement of the white measuring rod shown in Figures 2 and 4 (bottom right). The bottom of the white strip corresponds to the top of the foreground ridge as seen from Earth. The maximum surface brightness of the foreground ridge is 2.0 magnitudes arc sec of 15

12 Figure 14. Contour map of our 3 s detection limits for a plume inside Cabeus crater in magnitudes arc sec 2 for our Goodrich IR video camera images. Our detection limits range from 6.0 to 8.58 magnitudes arc sec 2 depending on the position within the crater. The white strip, which is 0.8 km wide, represents the position of the white measuring rod shown in Figures 2 and 4 (bottom right). This is the region directly above the LCROSS impact site as seen from Earth. The bottom of the white strip corresponds to the top of the foreground ridge. The point of maximum 3 s detection sensitivity within the white strip is at arbitrary pixel number 10 on the y axis, which is 4 km above the impact point. The detection limit at that point is 8.4 magnitudes arc sec 2. [31] To translate the peak radiance value as seen from above to a surface brightness as seen from Earth, we assumed that the plume radiance was directly proportional to the optical depth as seen from each viewpoint. This assumed a similar particle single scattering albedo as seen from each direction. The solar phase angle of the plume particles as viewed from above was approximately 90, as the Sun was low on the lunar horizon at Cabeus crater. The solar phase angle as seen from Earth at the time of impact was 65. Given that the mean lunar particle phase function drops by approximately a factor of two between solar phase angles of 90 and 65 [Goguen et al., 2010], our plume brightness estimates may be optimistic (too bright) by as much as 0.75 magnitude arc sec 2. [32] We assumed a uniform particle density throughout the plume. The optical depth, t, can be expressed as t = Ns = ns ds, where N is the line of sight plume column density, n is the plume particle density, s is the crosssectional area of plume particles, and ds is the line of sight depth of the illuminated portion of the plume. The ratio of ds values measured from two different vantage points is a function of the assumed size and geometry of the plume. We assumed two basic plume geometries, which are further described below. To determine the ratio of average plume cross sectional areas, we derived a relation for the plume particle cross sectional area as a function of height, based on the assumption that all plume particles are ejected with the same kinetic energy. The initial velocity of a particle is therefore a function of the particle mass. We further applied the simplifying approximation that at 17 s after impact (the time of maximum radiance detected by the LCROSS UV/Vis spectrometer) the particle height was simply the product of the initial vertical velocity times 17 s (i.e., we ignored the nonlinear effects of the Moon s gravity on the particles). This yielded the following relation for particle mass as a function of height: my ð Þ ¼ m 0:833 ð0:833=yþ 2 ; ð7þ where m is the mass of a particle at km, the lowest altitude of illuminated particles, and y is altitude above the crater floor in km. After assuming that the plume particles were spherical in shape and had identical densities, we derived a relation for the cross sectional area as a function of height: ðyþ ¼ 0:833 ð0:833=yþ 4=3 ; ð8þ where s is the particle cross sectional area at a height of km. [33] Our maximum observational sensitivity inside Cabeus crater for a plume rising straight up from the impact point corresponded to the (x, y) pixel position of (24, 10) in Figure 8. This translated to a height 4 km above the crater floor. The attenuation of plume radiance at 4 km above the crater floor as seen from Earth compared to that seen from above is then given by ds earth D ¼ 2:5log ds above 4km ave above ; ð9þ where ds earth is the line of sight dimension of the plume as seen from Earth at 4 km above the crater floor, ds above is the plume dimension as viewed from above, s 4 km is the particle cross section as seen from Earth at 4 km above the 12 of 15

13 Figure 15. Diagrams depicting two model plume geometries used to estimate the plume surface brightness as seen from Earth. Both models assume a high angle plume geometry with initial plume vertical velocities as high as 2 km s 1. (left) The cone model assumes a high angle cone with a plume ejection angle of 84 with respect to the horizontal. (right) The cylinder model assumes a three component nested cylindrical plume with initial plume velocities dropping off with increasing radial distance from the impact point. Both models are shown to scale at a time 17 s after impact. The dashed horizontal line corresponds to a height of 4 km, the vertical location of our maximum V band detection sensitivity in our Agile images. crater floor, s ave_above is the average particle cross section as viewed from above, and D is in units of magnitudes arc sec 2. [34] We based our estimates of plume geometry on the published results of LCROSS visible camera (VIS) imagery and observations by the Lyman Alpha Mapping Spectrometer (LAMP) on board LRO. Schultz et al. [2010] reported that the plume became visible (above the local sun line of km) between 1.1 and 3.1 s after impact and that it measured 4 km across after 10 s and 8 km across after 20 s, as viewed from above. Spacecraft observations did not provide any information about the vertical extent of the plume. However, LAMP observations taken 45 s after impact suggested a plume velocity of at least 1.9 km s 1 [Gladstone et al., 2010]. In addition, laboratory impact studies of hollow pellets fired into a simulated lunar regolith at the LCROSS impact velocity of 2.5 km s 1 produced a plume consisting of both a low angle and a high angle component, with the high angle component having an initial velocity of >1 km s 1 [Schultz et al., 2010]. Finally, nearinfrared images taken by the shepherding spacecraft nearly 4 min after the LCROSS impact revealed a diffuse ejecta cloud that Schultz et al. [2010] interpreted as in falling plume material from a high angle plume with an initial ejection angle >80 from horizontal. We assumed that the plume radiance quoted by Colaprete et al. [2010] was due solely to a high angle plume component, as that would be the most favorable configuration for detection from Earth. [35] Given these in situ observations of the plume, we examined two possible geometries for the lunar plume, shown in Figure 15. The first shape was a cone with a plume angle of 84 to the horizontal and an initial plume particle velocity of 2 km s 1, indicated by v y0 in Figure 15. Such a cone, when viewed from above, would have a width w 10 = 4.2 km wide after 10 s, and w 20 = 8.3 km after 20 s. At 17 s after impact, the time of maximum radiance as viewed from above, this cone would be h 17 = 34.0 km high and at a height of 4 km it would be km wide. Therefore, the application of equation (9) yields that the plume surface brightness as seen from Earth was D = 1.5 magnitudes arc sec 2 dimmer than that seen from above for this cone geometry. Given the maximum plume surface brightness of 10.8 magnitudes arc sec 2 reported by Colaprete et al. [2010], we calculated a plume surface brightness of 12.3 magnitudes arc sec 2 from our vantage point on Earth. This is 2.8 magnitudes dimmer than our 3 s V band detection limit of 9.5 magnitudes arc sec 2. [36] We defined a second possible plume geometry based on images from laboratory impacts of hollow spheres shown 13 of 15

14 by Schultz et al. [2010, Figure 4]. These images indicate a high angle plume that is more cylindrical in shape than the cone model. We assumed a vertical ejecta column where the plume s initial vertical velocity drops off as a function of distance from the point of impact. Figure 15 (right) shows a three component nested cylindrical plume model, with cylinders labeled A, B, and C, corresponding to the highest plume velocities to the lowest. We assumed initial vertical velocities of 2.0, 1.0, and 0.5 km s 1 for cylinders A, B, and C, respectively. Given these initial velocities, all plume material would be visible from Earth at 17 s after impact. Based on this geometry, we calculated an attenuation of radiance as seen from Earth compared to that seen from above to be D = 2.4 magnitudes arc sec 2. Therefore, the surface brightness of the plume as seen from Earth would be 13.2 magnitudes arc sec 2 or 3.7 magnitudes dimmer than our detection limit. Therefore, we cannot distinguish between these two plume geometries based on our nondetection of the plume. Note that if the initial plume velocities were much lower than we assumed, some or most of the plume seen from above would not have risen above the foreground ridge as viewed from Earth. In that case, the plume would appear much dimmer from Earth than these estimates. [37] Working backwards from our maximum sensitivity limit of 9.5 magnitudes arc sec 2 directly above the impact point, we calculated the minimum optical depth of plume particles needed for a 3 s detection in V band. Our 9.5 magnitudes arc sec 2 detection limit translates to a radiance of W m 2 sr 1. Given an optical depth of for a downward looking radiance of W m 2 sr 1 [Colaprete et al., 2010], we calculated a minimum detectable plume optical depth as viewed from Earth of Conclusions [38] We consider two possible reasons for our lack of detection of the plume in our Earth based observations: [39] 1. The plume never reached an altitude where it was illuminated by the Sun and also above the foreground ridge of the Cabeus crater, which would have enabled Earthbased observers to detect it. According to model calculations, this corresponds to an altitude of approximately 2.5 km [Heldmann et al., 2011]. [40] 2. The debris plume did rise above the ridge of Cabeus, but it was diffuse enough that it was below our threshold of detection, which we mapped as a function of position in the crater (Figure 8). At the center of the crater, along the path of a low angle plume, the plume could not have been brighter than 9.69 V magnitudes arc sec 2 or it would have been visible in our Agile data. A high angle plume rising directly above the impact point could not have been brighter than 9.52 V magnitudes arc sec 2. Given the dynamic range of our Agile V band observations, the minimum plume brightness we could have detected for a high angle plume was times fainter than the sunlit foreground ridge of Cabeus, depending on the height of the plume. The corresponding near IR (J and H band) limiting surface brightness is 8.58 magnitudes arc sec 2 at the center of the crater, and 8.42 magnitudes arc sec 2 directly above the impact point. The minimum plume brightness in our near IR data we could have detected for a high angle plume was times fainter than the sunlit foreground ridge of Cabeus. [41] The recently revised V band radiance of the plume as measured from above by the LCROSS UV/Vis spectrometer, W m 2 sr 1, or 10.8 magnitudes arc sec 2, is itself below our detection threshold for our Agile images. Therefore, it is no surprise that we failed to detect the plume. We considered two geometries for a high angle plume and determined that the radiance of the plume as seen by Earth would be magnitudes arc sec 2 dimmer than that as seen from above. As a result, we calculate that the plume was at least magnitudes dimmer than our V band detection limits. Both of our plume geometries optimistically assumed that most of the illuminated plume particles observed by the shepherding spacecraft were above the altitude of the foreground ridge in front of Cabeus and therefore visible from Earth. If in fact a significant fraction of the plume particles were below an altitude of 2 km, the plume would have appeared even dimmer than our estimates. Given the revised LCROSS UV/Vis radiance and our detection limits, we cannot constrain the plume geometry except to note that the plume optical depth at a height of 4 km as seen from Earth could not have been greater than 3.3 times that seen from above. Both plume geometries we considered resulted in a lower optical depth when viewed from Earth compared to that viewed from above. [42] The selection of the LCROSS impact site was driven by several factors, including the association with observed hydrogen from Lunar Prospector and Lunar Reconnaissance Orbiter data, target properties such as surface roughness and slope, the geometry for solar illumination of the ejecta plume, and visibility from Earth. The ultimate choice of Cabeus was less ideal for Earth based observers due to the presence of the foreground ridge, and to our knowledge the impact debris plume was not detected by any telescopic assets on Earth. Nonetheless, deriving limiting magnitudes as a function of altitude above the impact point is significant. This information provides important constraints that can be used for future investigations of the lunar regolith properties in a permanently shadowed polar crater such as Cabeus. [43] Acknowledgments. We thank the additional members of our observing team, without whom we could not have made three successful sets of observations from three different telescopes: E. Ramesh, C. Wu, R. J. Suggs, E. Klimek, and J. Coughlin. We also thank the staff at the Apache Point Observatory for their tireless efforts in supporting the instrument modifications and the numerous observing runs required for this project. We thank P. Strycker for producing the Vega spectrum we used in our analysis of the plume brightness. We are grateful to A. Colaprete, J. Heldmann, and D. Wooden, whose commitment to involving groundbased astronomers in the LCROSS mission provided the impetus for these observations. Finally, we thank the two referees who reviewed this manuscript and provided valuable suggestions for improvement. This work was supported by contract from the Universities Space Research Association. The NASA MSFC team acknowledges partial support from the NASA Meteoroid Environment Office. 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