EPSC Abstracts Vol. 13, EPSC-DPS2019-622-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019. CC Attribution 4.0 license.

Observations of the Radar Scattering (Phase) Function of Copernican Crater Ejecta on The Moon Using Mini-RF

Angela Stickle, Wes Patterson, Joshua Cahill, and the Mini-RF Team Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd., Laurel, MD , 20723, USA, ([email protected])

Abstract radar wavelengths can provide information regarding the size/frequency of scatterers emplaced as ejecta, as The Mini-RF radar is current operating in a bistatic well as unique insight into the rate of regolith configuration and examining a variety of lunar breakdown on the Moon. terrains as a function of bistatic angle. Here, we examine the ejecta blankets of Copernican aged 1.2 Bistatic Operations craters on the lunar surface in both S- and X-band to examine the scattering properties of young crater Radar observations of planetary surfaces provide ejecta. Several observed craters exhibit a clear important information on the structure (i.e., opposition effect at low bistatic (phase) angles, roughness) and dielectric properties of surface and which is consistent with optical studies of lunar soils buried materials [4-7]. These data can be acquired done in the laboratory. These observations are the using a monostatic architecture, where a single first time this effect has been measured on the Moon antenna serves as the signal transmitter and receiver, at radar wavelengths. Differences in the CPR or they can be acquired using a bistatic architecture, behaviour as a function of bistatic angle may also where a signal is transmitted from one location and provide opportunities for relative age dating between received at another. The bistatic (phase) angle is a Copernican craters. function of the angle between the incident radiation to the surface and the backscattered radiation. In 1. Introduction bistatic configuration, Mini-RF acts as the receiver for the backscattered energy from the lunar surface; The Mini-RF instrument onboard NASA’s Lunar the transmitters are the Arecibo Observatory (AO) at Reconnaissance Orbiter (LRO) is currently acquiring S-band and the DSS-13 Antenna at Goldstone in X- bistatic radar data of the lunar surface at both S-band band. This architecture maintains the hybrid dual- (12.6 cm) and X-band (4.2 cm) wavelengths in an polarimetric nature of the Mini-RF instrument [8] effort to understand the scattering properties of lunar and, therefore, allows for the calculation of the terrains as a function of bistatic angle. Previous Stokes parameters (S1, S2, S3, S4) that characterize the work, at optical wavelengths, demonstrated that the backscattered signal (and the products derived from material properties of lunar regolith can be sensitive those parameters). to variations in observation phase (bistatic) angle [1-

3]. This sensitivity gives rise to the lunar opposition effect and likely involves contributions from shadow 2. Observations hiding at low phase angles and coherent backscatter A useful product derived from the Stokes near zero phase [1]. Mini-RF bistatic data of lunar parameters is the Circular Polarization Ratio (CPR), materials indicate that such behaviour can also be (S1 − S4 ) observed for lunar materials at the wavelength scale µ = (1) of an X- and S-band radar. Here, the Circular C (S1 + S4 ) Polarization Ratios (CPRs) of the ejecta from 10 CPR information is commonly used in analyses lunar craters are characterized as a function of phase of planetary radar data [4-7], and is a representation (bistatic) angle. Work to characterize other radar of surface roughness at the wavelength scale of the products (e.g., total backscattered power, polarization € radar (i.e., surfaces that are smoother at the products) is ongoing. Observing the scattering wavelength scale will have lower CPR values and behaviour of continuous ejecta blankets at multiple Angela M. Stickle1 , G.Wes Patterson1, Josh T.S. Cahill1 , Parvathy Prem1, and the Mini-RF Team 1Johns Hopkins Applied Physics Laboratory, Laurel, MD ([email protected]).

Laboratory Analogues Laboratory analogues suggest both ice Updates were made to the Mini-RF bistatic data processing pipeline, including improvements in direct path extraction Operations and rocky material will exhibit similar, • Mini-RF is a dual-frequency radar fying onboard the Lunar Re- though distinct, Coherent Backscatter and phase rate estimate algorithms, improved calibration, adding low-resolution LOLA topography, and increased connaissance Orbiter (LRO) and is currently operating in a bi- Opposition Efects (CBOEs). resolution of processing grid, resulting in more efective looks per pixel. LRO Extended Mission 3 data had been static confguration with Arecibo Observatory in Puerto Rico It is expected that the ejecta of young reprocessed and Extended Mission 1 is in progress. The newly reprocessed data have an ~150 m azimuthal resolution, and the DSS-13 antenna at Goldstone acting as transmitters. craters will exhibit a CBOE in the CPR with a range resolution of ~100 m. Mini-RF receives the signal backscattered from the lunar sur- at low bistatic angle due to the rough face. and blocky nature of the ejecta. This Anaxagoras: Newly Reprocessed X-band obervations Comparison to older data CBOE is expected to fade as ejecta The newly reprocessed data includes new backplanes. Processor enhancements improve resolution and • DSS-13 is used for the collection of X-band (7147 MHz; 4.2 cm) blankets degrade and mature. DOY 2017-286 add structure to the curves. data (left) cartoon of expected response, S1: Total Power returned to Radar N • Arecibo Observatory is used for the collection of S-band (2380 modifed from [3,4] MHz; 12.6 cm) data CPR as a function of bistatic angle - Kepler Crater Circular Polarization Ratio (CPR) • Mini-RF receives orthogonal linear polarizations (H and V). 0 1.2 • The circular polarization ratio, CPR = (S1-S4)/(S1+S4), is one CPR The CPR values 0 1 for each pixel Mini-RF data product (right) that it is commonly used in anal- Bistatic (Phase) Angle yses of planetary radar data [1,2]. This in an annulus encompassing the 0 15.6 ratio provides an indication of surface Phase Angle roughness, as determined by the dis- 4 20 ejecta blanket are tribution of surface and buried wave- compared to the Incidence Angle length-scale scatterers (e.g., boulders). phase angle of the 54 114 respective pixels. The values for every phase angle are Emission Angle standard deviation binned (left) and 53 107 average CPR averaged (right). Only phase angle bins with >100 Height measurements are -2.9 -1.6 averaged. Range 292 348

Multi-wavelength Observations What Size Range Are We Observing? Latitude 59 90 Observations using diferent wavelengths allow us to build up a picture of how quickly Longitude regolith weathers/breaks down on the lunar -180 180 surface. These observations may be able to fnd Slope fner age diferentiations based on where ma- 0 67.6 turity efects are seen at difering wavelengths compared to single instrument analyses. Three collects were aquired for Anaxagoras in X-band, allowing a more complete phase space to be examined. Areas with overlapping phase space are averaged in the curves (above, right). The images (above) show total power returned When looking within similar wave- to the radar (S1), CPR, and the complete set of new backplanes, including: latitude, longitude, bistatic (phase) angle, lengths, diferences between signatures incidence angle, emission angle, height of the surface, range to surface, and surface slope. at craters allows relative age dating. Kepler Crater No clear opposition efect issurfaces observed thatat are rougher will have higher CPR which S-band is Observations classified as Eratosthenian) shows no S1: Total Power returned to Radar (left) Kepler Crater has a unique Hercules crater (bottom, right).values Instead,). High a CPR values can also serve as an indication of an opposition response across phase CPR signature as a function of relatively fat behavior is seenindicator as function of of the presence of water ice [9]. angle space. Continuing observations are targetingN phase (bistatic) angle in S-band phase angle. Hercules is often categorized observations. Circular Polarization Ratio (CPR) as an Eratosthenian crater, which As may part ex- of the Mini-RF bistatic observation these regions to increase the bistatic angle coverage. The ramp and level area are similar to La Condamine S and 0 1.2 plain the lack of CBOE. This campaign,may provide a CPR information for a variety lunar Additional study is ongoing to fully characterize the Schomberger A, however the baseline for examination and characteriza- CPR returns to background CPR responseBistatic (Phase) Angle with viewing geometry for these tion of Copernican v. Eratosthenianterrains craters. is being collected over a range of bistatic levels at extremely low bistatic angles. The reason for this is angles. Patterson et al. [10] analyzed the ejecta young craters. 0 15.6 under investigation. Bistatic Radar Observationsproperties of Young Lunar as a Craters function of bistatic angle for three S-band Observations of Copernicanyoung craters craters: show Byrgius multiple scattering A, Keple behaviorsr, and asBouguer. a function Bothof bistatic angle. S1: Total Power returned to Radar Some craters (Byrgius A, Aristarchus) exhibit a possible CBOE at low phase angles. Others (Schomberger A, Kepler, LaCondamine S?) exhibit a higher CPR at low phase,Kepler but it is diferentand Bygius than CBOE. A The exhibited rest show an approx.an opposition fat response as aeffect, function of phase angle (e.g., Bouguer, Hercules, Harpaluswhile), suggesting Bouguer older material did not. or diferent Patterson scattering propertieset al. [10] in the ejecta. suggest N that the radar scattering characteristics of the Circular Polarization Ratio (CPR) The divet seen in the CPR curve as a around 2 degrees appears continuous ejecta for these three craters, coupled to be due to radar processing 0 1.2 artifacts. When a subset ROI with age estimates based on crater statistics and was chosen to avoid the echo Bistatic (Phase) Angle on the lower right hand corner geologic mapping, imply a relationship between the of the crater (above), the divet opposition response of the ejecta and the age of the disappears (left). The red area 0 10.8 above shows the area where crater (i.e., Byrgius A is the youngest of the craters Phase angle = 1.5-2.5 degrees. observed and shows the strongest response). Thus, recording the CPR response as a function of bistatic @schmemela Hercules (age: • Mini-RF observations show the frst ever CBOE observed on the lunar surface angle may be a way to determine relative age Eratosthenian) is older at radar wavelengths than others examined between deposits. • A variety of scattering characteristics are seen in Copernican ejecta blankets here. The CPR response as a function of bistatic (phase) angle is relatively fat across Here, we examine the ejecta of 10 Copernican- all low phase angles • Multiple wavelength observations can help us constrain the rate of breakdown with some suggestion and Eratosthenian aged craters that range from 4 to of rocks of varying size to understand how impacts produce regolith of structure at longer phase angles. This may 69 km in diameter [13] and document CPR • Presence (or lack) of observed CBOE may provide additional methods for provide a comparison relative age dating of ejecta material (e.g., Hercules, right) of what “old” material characteristics as a function bistatic angle in order to Reprocessing the data provides a higher resolution view and more structure looks like, and allow for relative ages based in the curves. Does that represent specifc facies? Topography? Stay tuned! test that hypothesis. The spatial resolution of the data on CPR measurements. varied from one observation to another as a function of the viewing geometry, but averaged ~100 m, [1] Campbell et al. (2010), Icarus, 208, 565-573; [2] Raney et al. (2012), JGR, 117, E00H21; [3] Hapke et al. (2012), JGR, 117, E00H15, doi:10.1029/2011JE003916; [4] Hapke and Blewett (1991), Nature, 352, 46-47 providing 25 effective looks. Four of the examined craters (Byrgius A, Aristarchus, La Condamine S, and Kepler exhibit CPR characteristics suggestive of an opposition Figure 1. Observations of Kepler Crater using Mini- effect in S-band: higher CPR at lower bistatic (phase) RF. (top) Total power returned to the radar, CPR of angle. X-band observations of Anaxagoras and the crater and surrounding terrain, and bistatic (phase) Kepler also suggest an opposition surge at low angle of the observation. (bottom) CPR as a function bistatic angle, though relatively constant CPR in S- of phase for Kepler crater in S-band. band at higher bistatic angles. The increase in CPR occurs near 2–4 degrees bistatic angle. These craters References occur in both highlands and mare regions, and are all characterized as young. Three other examined craters [1] Hapke et al. (1998), Icarus, 133, 89-97; [2] (Bouguer, Harpalus, Anaxagoras) exhibit CPR that Nelson et al. (2000), Icarus, 147, 545-558; [3] Piatek remains relatively constant across bistatic angle. This et al. (2004), Icarus, 171, 531-545. [4] Campbell et al. may be for a couple reasons: 1) The craters are older (2010), Icarus, 208, 565-573; [5] Raney et al. (2012), (though most are still Copernican), and so the JGR, 117, E00H21; [6] Carter et al. (2012), JGR, 117, opposition effect will be less pronounced; or 2) E00H09; [7] Campbell (2012), JGR, 117, E06008; [8] insufficient data have been acquired to characterize Raney, R. K. et al. (2011), Proc. of the IEEE, 99, the opposition behavior of the crater ejecta. An 808-823; [9] Black et al. (2001), Icarus, 151, 167- opposition effect may be present but not yet observed. 180; [10] Patterson, et al. (2017), Icarus, 283, 2-19; Schomberger A, La Condamine S, and Kepler exhibit [11] Morota et al. (2009) Met. Planet. Sci. 44(8), scattering properties as a function of bistatic angles 1115-1120; [12] Koenig et al. (1977) Proc. LPSC, 8, that differ from the other observed crates, with areas p. 555; [13] Baldwin (1985) Icarus 61, 63-91; [14] of relatively constant CPR at various CPR values Ulrich (1969) USGS Map I-604(LAC-11);[13] (e.g., Figure 1). The oldest crater observed (Hercules, Stickle et al. (2018) LPSC Abstract #2007.