- Email: [email protected]
Dark halos and rays of young lunar craters: A new insight into interpretation
Icarus 231 (2014) 22–33
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Dark halos and rays of young lunar craters: A new insight into interpretation Vadym Kaydash a, Yuriy Shkuratov a, Gorden Videen b,⇑ a b
Astronomical Institute of Kharkov V.N. Karazin National University, Sumskaya 35, Kharkov 61022, Ukraine Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301, USA
a r t i c l e
i n f o
Article history: Received 16 August 2013 Revised 13 November 2013 Accepted 18 November 2013 Available online 24 November 2013 Keywords: Image processing Impact processes Moon, surface Photometry Regolith
a b s t r a c t Images acquired by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter allow phase-ratio imagery of young lunar craters surrounded by dark halos. Such imaging is a new optical remote-sensing technique that is sensitive to the degree of surface roughness. We apply the phase-ratio technique to LRO images of young dark-halo craters near the crater Denning and in the Balmer basin, in addition to craters created by the impacts of the Ranger-6 spacecraft and Saturn-5 sections of Apollo-13 and Apollo-17. We suggest an alternative explanation of the dark halos and rays seen near the craters at large phase angles. Phase-ratio imaging suggests that these features result from higher surface roughness. Thus, the interpretation of dark crater halos and rays as a composition/maturity variance should be used with caution. The composition and structure factors can be effectively discriminated only using images acquired in a wide range of phase angles including small angles. Published by Elsevier Inc.
1. Introduction Many young lunar craters have bright halos and rays that were formed by excavating subsurface materials at major and secondary impact locations (Oberbeck, 1971). Fresh material ejected from lunar craters is generally brighter than the surrounding region, since it consists of immature soils that are slowly altered by micrometeorite bombardment and solar wind, resulting in soil darkening over long time periods. Eventually, the reflectance of ejecta material may appear as that of the surrounding regolith. Although the excavated materials of fresh craters should be of high reflectance, darkhaloed and dark-rayed craters on the Moon have been observed, but their number is significantly lower than craters without such halos and rays (Salisbury et al., 1968). These features are considered to be deposits of low-albedo materials; however, there is no unique explanation of their origin. Some of these features, like several small craters in the old crater Alphonsus (Fig. 1a), perhaps have a volcanic origin (Salisbury et al., 1968; Head and Wilson, 1979). They are often associated with rilles and lineaments and considered as a source of pyroclastic material forming the dark halos (shown with arrows in Fig. 1a). Accordingly, the craters probably represent sites of volcanic fountaining on the Moon.
⇑ Corresponding author. E-mail addresses: (G. Videen).
[email protected],
[email protected]
0019-1035/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.icarus.2013.11.025
A feasible origin proposed for the dark ray system of the Copernican-aged crater Dionysius (Fig. 1b) implies impact melt deposits and dark primary ejecta (e.g., Schultz and Spudis, 1979; Hawke et al., 1979). Later Giguere et al. (2006) concluded that dark rays of Dionysius are dominated by mare basalts, not glassy impact melts. The crater Tycho is an example of a large crater with a dark halo of non-volcanic origin. The crater seen in Fig. 1c has a dark annulus that is considered to be caused by rock melted due to the extreme heat of impact and ejected throughout the area immediately surrounding the crater (Pieters et al., 1994; Morris et al., 2000). The dark halo of Tycho consists mostly of dark glassy materials that have pooled in the low spots surrounding the crater. There are no other large craters having such a prominent dark halo that is visible, probably because the crater Tycho is both young and large. Another type of dark halo crater can be found where a brighter ejecta blanket covers an older and darker lava flow. If a more recent impact occurs, it can pierce the layer of the bright ejecta material and excavate the darker material beneath. Examples of such craters can be found in the Schiller–Schickard region, where the mare material has been unearthed from beneath the light-colored deposits emplaced there at the Mare Orientale formation (e.g., Antonenko et al., 1997; Antonenko, 2013). Fig. 1d presents the crater Inghirami W (pointed by an arrow), located in the Schiller– Schickard region. It reveals a dark halo of excavated material. This underlying mare-type lava is called cryptomare. In principle, this mechanism of dark-halo formation is valid for all crater sizes including even those of 1–10 m. In this case dark-material layers
V. Kaydash et al. / Icarus 231 (2014) 22–33
23
Fig. 1. Different dark-haloed craters. (a) The 120-km crater Alphonsus, (b) the 18-km crater Dionysius, (c) the 82-km crater Tycho, (d) a part of the Schiller–Schickard region. The LROC WAC mosaic (643 nm) produced by NASA/GSFC/Arizona State University and available at the http://lroc.sese.asu.edu/data/pr/tiff/lrocwac643nmnearside.tif is used.
can be brought to light when the thin younger layer composed of brighter materials has been pierced. In this paper we suggest an alternative mechanism that may produce dark halos (rays) around craters, which is based on the difference of brightness phase curves of halos (rays) and their surrounding surfaces. We consider here several examples of dark halo craters whose darkness relates to the phase-angle effect. Three examples are shown of craters formed by the impacts of the Apollo 13 and Apollo 17 Saturn IVB boosters and the Ranger 6 spacecraft. The fourth and fifth we present are of young natural craters located near the old crater Denning and in the Balmer basin.
2. New mechanism and research method Each location on the lunar surface can be characterized by the dependence of its brightness (or reflectance, or apparent albedo A, or radiance factor) on phase angle a (e.g., Shkuratov et al., 2011; Hapke, 2012). At different locations this dependence is different. The brightness decreases with increasing a and the rate of this decrease can be characterized by the phase function slope. Surface roughness affects the phase function, since the shadowing effect increases with increasing roughness (e.g., Hapke, 2012). Fig. 2 shows two phase curves of absolute reflectance. One of them shows what may be considered a typical mare surface; whereas, the other one corresponds to what may be considered a typical highland surface. The data are taken from the absolute photometry of the Moon (Velikodsky et al., 2011) for small areas in Mare Tranquillitatis and near the highland crater Gylden close to
Fig. 2. Phase curves of absolute lunar reflectance. Data are taken from Velikodsky et al. (2011). Solid and dash curves correspond to typical mare and highland surfaces, respectively. The ratio of mare phase curve to highland one is in inset.
the center of the nearside. As can be seen, the phase curves cross each other. This can occur for crater halos (rays) and their surroundings. That is, at small phase angles, the halos can be brighter than their surroundings; whereas, at larger phase angles an inversion may be observed. An illustration of such an inversion is
24
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 3. Comparison of images acquired at different phase angles for the impact crater formed by the Apollo 17 Saturn IVB booster. (a) A(8°), (b) A(28°), (c) A(74°), and (d) A(80°). Here and elsewhere north is up, and west is to the right in the images.
displayed in Fig. 3, which shows the crater formed by the Apollo 17 Saturn IVB booster. The contrast of the crater rays undergoes an inversion as the phase angle increases (cf. Fig. 3a–d corresponding to increasing a). One may suppose that the effect of inversion is due to the surface roughness, since in ejecta areas (halos and rays) the surface should be rougher than the surrounding region (e.g., Shkuratov et al., 2011). We use phase-ratio imagery to study the dark halos (rays) of several craters. This technique is a prospective tool to characterize the lunar surface, especially using the space-derived photometry data acquired from orbital missions with high spatial resolution (i.e. JAXA Kaguya, ISRO Chandrayaan-1, and especially the NASA Lunar Reconnaissance Orbiter (LRO)). In this method the quotient of two coregistered images obtained at different phase angles a
are mapped (Shkuratov et al., 1994, 1999). This mapping provides information on the slope of the lunar phase function of radiance factor A(a) (Shkuratov et al., 2011) at each point of the imaged surface; thus, the slope can be characterized by the ratio A(a1)/A(a2). The phase-ratio technique provides an estimate of variations of the complexity of unresolved surface microtopography. For instance, this method allowed us to find new lunar surface formations in the southern part of Oceanus Procellarum (Shkuratov et al., 2010) with ground-based telescope data, which were suggested to be weak swirls (e.g., Starukhina and Shkuratov, 2004). The phase-ratio technique has been used to investigate the opposition effect of the Moon (Shkuratov et al., 1999; Kaydash et al., 2009a; Velikodsky et al., 2011) and many times was applied to data obtained using spacecraft (Kreslavsky et al., 2000; Kreslavsky and
25
V. Kaydash et al. / Icarus 231 (2014) 22–33
Table 1 Characteristics of the images acquired by LROC NAC for the craters formed by Apollo 13 and 17 Saturn IVB stages, Ranger 6 spacecraft, and two natural impactors. Images printed in italics are used for constructing phase-ratio images. Crater formed by
Image ID
Resolution (m/pix)
Emission angle (°)
Incidence angle (°)
Phase angle (°)
Sub solar azimuth (°)
Apollo 17 SIVB
M139985936L M111674228R M144701329L M144708115L M131731837R
0.51 0.52 0.52 0.55 0.47
1.68 1.16 13.06 24.84 1.16
6.04 29.56 49.75 48.82 81.54
7.55 28.40 36.70 73.65 80.38
151.70 185.02 182.06 183.14 180.84
Ranger 6
M139768545R M144483945R M144490730R
0.50 0.50 0.56
3.21 8.59 28.17
13.64 47.43 46.56
11.83 39.01 74.43
231.18 170.59 165.37
Apollo 13 SIVB
M160139273L M160139273R M160132488L M160132488R M140087684L M162493915R
0.52 0.51 0.52 0.53 0.48 0.49
19.1 16.26 19.98 22.83 1.68 1.16
49.95 50.03 50.87 50.95 4.27 76.9
69.03 66.26 30.93 28.17 5.88 75.74
184.15 184.06 182.95 182.95 162.95 269.09
Natural crater (Denning-Isaev)
M143676946L M143676946R M143683729R M143683729L M115367300L M115367300R
0.63 0.63 0.68 0.69 1.26 1.26
5.88 8.75 22.07 24.95 1.69 1.17
40.23 40.32 39.39 39.28 73.35 73.45
34.76 32.19 60.45 63.15 75.04 72.27
199.45 199.21 205.49 206.56 272.7 268.82
Natural crater (south to Balmer R)
M190136698R M190143847R M190150996R
0.88 0.82 0.87
22.75 1.19 19.81
44.28 43.37 42.53
24.8 42.26 60.98
201.89 203.9 209.71
Shkuratov, 2003, 2012a, 2012b; Kaydash et al., 2009b, 2011, 2012; Shkuratov et al., 2011, 2012, 2013). This method also has been exploited to analyze all the manned US Apollo landing sites (Apollo 11, 12, 14, 15, 16, and 17), and all the Soviet sample return sites (Luna 16, 20, 23, and 24) using photometric data obtained from the LRO Camera (LROC) (Kaydash et al., 2011; Kaydash and Shkuratov, 2012a, 2012b; Shkuratov et al., 2011, 2013). In particular, phase-ratio imagery has revealed photometric anomalies characterized by values of the phase-curve slope indicating a smoothing of the surface microstructure at the Apollo and Luna landing sites. This was caused by dust uplifted by the engine jets of the descent and ascent modules. Clegg et al. (2014) have confirmed this result using our phase-ratio technique for the Apollo, Surveyor, and Luna landing sites. 3. Source data We use high-resolution lunar-surface imagery data obtained by the two monochrome Narrow Angle Cameras (NACs) included in the LROC system. The spatial resolution of the NAC from the 50km orbit is 50 cm/pixel. LROC NAC images can be utilized for future landing site identification, certification, and characterization (Robinson et al., 2010). The image calibration pipeline for the LROC NAC uses a set of calibration files and converts the digital numbers in source images to radiance factor values, accounting for exposure time, dark image, flat field, non-linearity, and spectral responsivity of the detector (Robinson et al., 2010). We use calibrated LROC NAC images acquired during the nominal, science, and extended science phases of the mission. We investigate five lunar regions that include craters formed by the impacts of the Apollo 13 and Apollo 17 Saturn IVB boosters, Ranger 6 spacecraft, as well as two natural young craters. The choice of the anthropogenic craters guarantees the presence of very fresh material in their halos and rays not subjected to maturation processes. The two natural craters we studied reveal distinct dark rays in their proximal ejecta zones. Unfortunately, the number of such craters imaged by LROC NAC at similar illumination but different phase angles is rather small. The first natural crater
is located in the farside highlands between the craters Denning and Isaev, and the second one is located in the Balmer basin on the eastern limb of the nearside. Images used for phase-ratio imagery are described in Table 1. The images of the anthropogenic craters have a maximal surface resolution near 0.5 m, while those of the natural craters are imaged at 0.6-0.9 m/pixel resolution. To produce the phase-ratio mappings, we selected LROC NAC images acquired at similar illumination, i.e. similar incidence and sub-solar azimuth angles (see Table 1), thus reducing the effect of resolved large-scale topography on the phase-ratio distribution. We also selected additional images of different phase angles to track the effect of inverting the contrast of crater rays. To calculate a phase-ratio A(a1)/A(a2) image we use a rubbersheet geometric transformation, accounting for the parallax effect (Kaydash et al., 2012). The parallax data have been used not only to map the phase ratio, but also to produce anaglyphs that are images providing a stereoscopic 3D effect. The 3D phase-ratio imagery of lunar landscapes is useful to search for disturbances of surface structure, e.g., to detect areas of eroded surface, fresh slumps, accumulated material on crater walls, terraces, and floors (Kaydash et al., 2012). In this paper we use the ratio A(a1)/A(a2) when a1 > a2; whereas, in several of our previous publications (Kaydash et al., 2011; Kaydash and Shkuratov, 2012a, 2012b; Shkuratov et al., 2011) we selected a1 < a2. Darker tones on phase-ratio images in this paper correspond to lower values of the phase ratio, i.e. to steeper slopes of phase curves and thus to a rougher surface microstructure. There is a specific reason why we finally choose A(a1)/A(a2) at a1 > a2. In the intermediate range of phase angles (10–70°), a direct, almost linear correlation between A(a1)/A(a2) and albedo is observed just when a1 > a2; otherwise, the correlation is inverse and not linear. In first case, to study deviations from the regression line is a much easier task than when the line is not linear. As has been emphasized, the phase ratio A(a1)/A(a2) characterizes the phase-function slope. It is mainly dominated by the shadow effect produced by regolith particles. At phase angles larger than 40–50°, the regolith surface topography (centimeter–decimeter scales) can significantly affect the slope of the phase function as
26
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 4. Anaglyph images of the vicinity of the crater (marked by arrow) formed by the impact of the Apollo 17 Saturn IVB booster. (a) Distribution of radiance factor A(37°), (b) Distribution of phase ratio A(74°)/A(37°) (see Table 1). Darker tones correspond to a rougher surface. The images provide a stereoscopic effect. The dynamic range of the imaged parameters along with the spatial scale is shown with corresponding bars.
well. Depending on surface albedo, multiple scattering also influences the phase-function slope, since this scattering in and between the regolith particles produces additional illumination of shadows and thus decreases the slope of the phase curves. Fortunately, this plays a secondary role, if the roughness scale is sufficiently large (e.g., Shkuratov et al., 2005). At a > 70–90° the steepness of the phase function is almost independent of albedo. Thus in this case there exists a strong dependence of phase-function steepness on centimeter–decimeter regolith topography for dark halos and rays of lunar craters. We note the absence of data acquired at a > 90° for the studied areas (see Table 1), though such data has proven their usefulness for the identification of several lunar photometric anomalies (Kaydash et al., 2009b; Shkuratov et al., 2010). 4. Results and discussion 4.1. The crater formed by the impact of the Apollo 17 Saturn IVB booster After the command/service and lunar modules undocked from the stage S-IVB on December 7, 1972, the S-IVB impacted the lunar surface on December 10 at latitude 4.168°, longitude 347.670° (http://lroc.sese.asu.edu/news/index.php?/archives/
773-LROC-Coordinates-of-Robotic-Spacecraft-2013-Update.html). The impact velocity and mass of the stage were 2.56 km/s and 14,487 kg, respectively. The impact produced a crater of 30 m diameter, which was used to study the interior structure of the Moon using seismometers placed on the surface by the Apollo astronauts (Töksoz et al., 1974; Gudkova et al., 2011). Fig. 3 demonstrates the effect of contrast inversion of rays for the impact crater formed by the Apollo 17 Saturn IVB booster. At a = 8° and 28° the rays are brighter than the crater vicinity, at a = 74° the rays become darker, and at a = 80° the rays disappear as they are totally masked by the local topography. Fig. 4a and b shows anaglyphs of the radiance factor A(37°) and the phase ratio A(74°)/A(37°) of the area seen in Fig. 3 and its neighborhood. The phase ratio A(74°)/A(37°) anaglyph is constructed from the images M144708115L (a = 73.65°) and M144701329L (a = 36.70°) acquired at similar illumination conditions. Results presented in Figs. 3 and 4 demonstrate that small values of the phase ratio in the vicinity of young craters may indicate a potential inversion of brightness contrast (see Fig. 3) at some phase angle, which can be discernible as a dark halo. Thus, the crater halo and rays perhaps contain a large amount of rather small boulders and stones 60.5 m in size. A visual inspection of the images shows that the crater rays are longer in Fig. 4b than in
V. Kaydash et al. / Icarus 231 (2014) 22–33
27
Fig. 5. Surroundings of the crater (marked by arrow) formed by the impact of Ranger 6: (a–c) images acquired at phase angle 12°, 39°, and 74°, respectively (see Table 1).
Fig. 4a; i.e. phase-ratio imaging is sensitive to roughness anomalies. The next example confirms this conclusion. 4.2. The crater formed by the impact of the Ranger 6 spacecraft Ranger 6 impacted the Moon on the eastern edge of Mare Tranquillitatis on February 2, 1964. This mission was not successful and no video signal was received from Ranger 6. Nevertheless, the impact site located at 9.387°N, 21.481°E was identified from LRO images (http://lroc.sese.asu.edu/news/index.php?/archives/ 773-LROC-Coordinates-of-Robotic-Spacecraft-2013-Update.html). Fig. 5 shows the crater formed by the Ranger 6 impact: (a–c) are brightness distributions at a = 12°, 39°, and 74°, respectively. Visual inspection of these images confirms the previous observation that at small phase angles the halos can be brighter than their surroundings; whereas, at large phase angles, an inversion may be observed. Fig. 6a and b shows a brightness anaglyph of the Ranger 6 impact crater (a) and an anaglyph of the phase ratio A(74°)/A(39°) distribution (b). Darker tones in Fig. 6b correspond to rougher surface features. As in the case of the Apollo 17 Saturn IVB booster crater (Fig. 4a and b), the 3D images shown in Fig. 6a and b do not reveal noticeable relief variations in the immediate vicinity of the
Ranger 6 crater (cf. the large impact crater in the lower left part of the scene). Thus, there are no large-scale (i.e. larger than the image resolution of 0.5 m) topography variations that can be responsible for the contrast inversion. Therefore, we may conclude that the crater produced by the Ranger-6 impact reveals high values of phase-function steepness due to a large number of excavated boulders and rocks (with sizes distributed in a range smaller than the image resolution) that produce a strong shadow effect. 4.3. The crater formed by the impact of the Apollo 13 Saturn IVB booster It is well known that the Apollo 13 mission was unsuccessful. On April 14, 1970 the Apollo 13 Saturn IVB upper stage impacted the Moon north of Mare Cognitum at the location 2.555° latitude and 332.113° longitude (http://lroc.sese.asu.edu/news/ index.php?/archives/773-LROC-Coordinates-of-Robotic-Spacecraft2013-Update.html). The impact crater is roughly 30 m in diameter. In order to examine the described effect of the brightness inversion, we present in Fig. 7a sequence of images acquired at different phase angles. The low-phase-angle (a = 6°) image in Fig. 7a reveals
28
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 6. Anaglyph images of the Ranger 6 impact crater: (a and b) correspond to the brightness (39°) and phase-ratio (74°/39°) distributions, respectively. The darker tones in (b) correspond to rougher surfaces. The dynamic range of the imaged parameters along with the spatial scale is shown with corresponding bars.
Fig. 7. Comparison of images acquired at different phase angles for the impact crater formed by the Apollo 13 Saturn IVB booster: (a) A(6°), (b) A(30°), (c) A(68°), and (d) A(76°).
4.4. A natural lunar crater in the Denning-Isaev area the crater halo and ray system as the brightest objects in the scene. Then, as the phase angle increases to 30° and 68° (Fig. 7b and c) the halo and rays become equal and even lower in brightness than the surrounding region. At a = 76° (Fig. 7d) the Saturn IVB crater halo and rays disappear due to illuminated macroscopic roughness formed by the resolvable topography. Fig. 8a and b shows anaglyph images of the Apollo 13 SIVB impact crater: (a) and (b) correspond to the brightness (a = 30°) and phase ratio A(68°)/A(30°) distributions, respectively. These images provide a stereoscopic 3D effect. The darker tones of the crater rays on the phase-ratio image reveal increased phase-function steepness for the rays as compared to the surrounding surface. The actual dynamic range of the phase-ratio values is shown with the scale bar on the image. We note that the ray patterns seen in Fig. 8a and b are different. One can track rays to a greater distance from the crater with the phase-ratio image than with the moderate-phase-angle brightness image.
We found several young natural craters with dark halos at large phase angles and present here an example for which we have sufficient data to analyze. Such a crater is shown by the arrow in the Apollo 15 image AS15-M-0102 captured at a Sun elevation of 34.0° (Fig. 9, see also insert). The crater is located at 17.7°S, 144.4°E in the farside highland between the craters Denning and Isaev. Its diameter is approximately 2000 m. Fig. 10a and b shows this crater using LROC NAC data for the distributions of brightness A(33.5°) and A(74°), respectively. Lower-phase-angle image is a mosaic produced from frames M143676946L (a = 34.76°) and M143676946R (a = 32.19°). The effective phase angle for the mosaic is (34.76° + 32.19°)/2 33.5°. The A(74°) image also is a mosaic of two images acquired at phase angles 72.27° and 75.04° (see Table 1 for detail). As can be seen in the figure, the halo and rays of the crater are darker than the background in the lower-phase-angle image. At the higher phase angle, the rays are hardly distinguishable due to the influence of large-scale topography;
V. Kaydash et al. / Icarus 231 (2014) 22–33
29
Fig. 8. Anaglyph images of the Apollo 13 Saturn IVB stage impact crater: (a and b) correspond to the radiance factor at 30° and phase ratio (68°/30°) distributions, respectively. Darker tones in (b) correspond to rougher surfaces.
in this phase ratio is a mosaic of M143683729L (a = 63.15°) and M143683729R (a = 60.45°). Thus the effective phase angle for the mosaic is (63.15° + 60.45°)/2 62°. Fig. 11a and b shows, respectively, anaglyphs of the brightness A(33.5°) and phase-ratio (62°/ 33.5°) distributions reconstructed with LROC NAC data. A comparison of ray patterns in Fig. 11a and b reveals a more prominent, darker pattern for the phase-ratio image than for the albedo image. Darker tones in Fig. 11b correspond to lower phase-ratio areas that we interpret as evidence of a higher degree of surface roughness. We tried to find the halo effect in the topography by inspecting the 3D images; however, we have not found large enough fragments seen as topographical details in the halo and rays. So we conclude that the dark halo and rays seen in Fig. 11b are structural anomalies that perhaps consist of boulders and rocks whose sizes are smaller than 1 m. 4.5. A natural lunar crater in the Balmer basin
Fig. 9. The young crater located in the farside highlands between craters Denning and Isaev under study shown with the arrow. The crater is located at 17.7°S, 144.4°E. A fragment of the Apollo 15 image AS15-M-0102 is shown and the Sun elevation is 34.0°. The inset shows an enlarged image of the crater.
however, some rays are still slightly darker than the adjacent surface. To investigate the contrast change in detail, we calculated the phase ratio image A(62°)/A(33.5°). The higher-phase-angle image
Another example of a natural crater with a dark halo (rays) is the young crater located 7 km south of the crater Balmer R in the Balmer basin. The crater coordinates are 18.9°S, 69.15°E (see arrow in Fig. 12). The crater was imaged three times with the LROC NAC under similar illumination angles (see Table 1). Fig. 13 shows distributions of the radiance factor A(25°) and phase ratio A(61°)/ A(42°). The main difference between the two distributions is the prominent dark-ray pattern in the phase-ratio image at a distance less than or equal to the crater diameter. We interpret this difference as the presence of rougher ejecta material in the immediate vicinity of the crater (darker tones in Fig. 13b) and less rough ejecta material extending up to several crater diameters (brighter tones in Fig. 13b). The ejecta blanket, which extends further than the
30
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 10. The crater shown in Fig. 9 imaged with the LROC NACs: (a and b) show, respectively, radiance factor A(33.5°) and A(74°). The radiance factor range and spatial scale are shown with corresponding bars.
dark phase-ratio halo, hardly can be subdivided into classes of different roughness due to their low-contrast differences in the phase-ratio image. Using phase-ratio imaging we can estimate the slope of the phase function A(a) between two phase angles a1 and a2. Meanwhile the slope may be different in different ranges of a (see, e.g., Fig. 2). Thus, for more detailed photometry one needs to sample the phase curve in more than two points. There are only a few cases of LROC NAC lunar surface imaging more than two times with the same illumination, but different phase angles. Fortunately, LROC NAC imaged the Balmer R area three times at a = 25°, 42°, and 61°. Thus, one may approximately estimate the second derivative of the phase function A(a): @2 A Aða1 Þ þ Aða3 Þ 2Aða2 Þ. This parameter roughly @ a2 characterizes the curvature of the function in the 25–61° range. As inferred from the definition, larger values of the parameter
Fig. 11. Anaglyphs of the (a) brightness A(33.5°) and (b) phase-ratio (62°/33.5°) distributions for the crater presented in Fig. 10. Darker tones for (b) correspond to rougher surfaces.
correspond to a larger curvature of the phase function. In Fig. 14a and b, we present anaglyphs of the brightness A(42°) and second derivative for the crater shown in Fig. 13. Darker tones in Fig. 14b correspond to decreased curvature of the phase function in the 25–61° range. The distribution of the curvature reveals a dark halo of the crater. This halo pattern is different from the albedo image and phase-ratio image A(61°)/A(42°) (especially to the south of the crater). The difference may be related to the influence of roughness and multiple scattering contributions. The distribution of the curvature parameter at larger distances from the crater strongly resembles the brightness distribution (cf. Fig. 13a and b), which indicates that the effect of surface roughness on the phase function is diminished by the multiple scattering contribution that increases with albedo. Thus the dark halo in the curvature parameter may be interpreted as increasing roughness in the proximal ejecta area of the crater, which makes
V. Kaydash et al. / Icarus 231 (2014) 22–33
31
Fig. 12. The young crater located 7 km south of crater Balmer R in the Balmer basin. Crater coordinates are 18.9°S, 69.15°E (shown with arrow). Context image is LROC WAC Global Morphologic basemap (100 m/pixel) incorporated in the ACT-REACT QuickMap (http://target.lroc.asu.edu/q3/).
Fig. 13. The young crater south of Balmer R studied in the present work, as imaged with LROC NAC: (a) Radiance factor A(25°) and (b) phase-ratio (61°/42°) distribution. Radiance factor and phase-ratio range and spatial scale are shown with corresponding bars. Darker tones for (b) correspond to rougher surfaces.
the phase function steeper in the larger phase angle range (here 42–61°). The presence of an anomalously large number of boulders, blocks, and rock fragments within the halo produces roughness at scales up to spatial resolution of the data, i.e. 1 m.
5. Conclusion When craters are imaged at large phase angles, the interpretation of their dark halos (rays) as a composition anomaly should be used in lunar geological studies with caution, as phase-ratio
32
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 14. Anaglyphs of the (a) brightness A(42°) and (b) second derivative of the phase-function distributions for the crater presented in Fig. 13. Darker tones for (b) correspond to decreased curvature of the phase function, i.e. a steeper phase curve.
imaging suggests that such halos can be attributed to differences in surface roughness. The composition and structure factors can be discriminated using images acquired at different phase angles. Phase-ratio imagery of the Moon is a very useful photometric tool, as it suggests an assessment of surface roughness. This tool also can reveal details hardly visible in brightness images. It can be used for identification of areas with very recent alterations in their surface structure. These can be, for instance, anthropogenic craters. LROC NAC high resolution imagery allows for the 3D phaseratio images to be mapped. Such images are very useful for geological descriptions of regions under studies. Lunar images may reveal an inversion in the contrast for halos and rays in the vicinity of craters. This was illustrated with our investigation of the craters formed by impacts of the Ranger 6 spacecraft and Apollo 13 and Apollo 17 Saturn IVB boosters. Such an inversion suggests that the reason for these dark halos and rays primarily is due to shadowing, rather than compositional differences. Dark halos and rays of two natural young craters located in the Balmer basin and Denning-Isaev area also can be interpreted with the 3D phase-ratio mapping as structural anomalies caused by increased roughness of the ejecta material for scales under image resolution. We emphasize that all suspect dark haloed craters should be checked at higher phase angles to see if the halo persists as it would with a compositional cause.
Acknowledgments We thank the LRO project teams for their efforts in obtaining and returning the data presented here. The authors gratefully acknowledge the research support from NASA Grant NNX11AB25G Lunar Advanced Science and Exploration Research (LASER), ‘‘Imaging photometric and polarimetric remote sensing of the Moon’’.
References Antonenko, I., 2013. Re-examining the identification of dark-haloed impact craters: New criteria for modern data sets. Lunar Planet. Sci. 44, 2607. Antonenko, I., Cintala, M., Head, J., Hörz, F., 1997. Results of experimental dark halo crater studies: Implications for lunar cryptomare thickness measurements. Lunar Planet. Sci. 28, 47–48. Clegg, R., Jolliff, B., Robinson, M., Hapke, B., Plescia, J., 2014. Effects of rocket exhaust on lunar soil reflectance properties. Icarus 227, 176–194. Giguere, T.A., Hawke, B.R., Gaddis, L.R., Blewett, D.T., Gillis-Davis, J.J., Lucey, P.G., Smith, G.A., Spudis, P.D., Taylor, G.J., 2006. Remote sensing studies of the Dionysius region of the Moon. J. Geophys. Res. 111, E06009. http://dx.doi.org/ 10.1029/2005JE002639. Gudkova, T.V., Lognonné, Ph., Gagnepain-Beyneix, J., 2011. Large impacts detected by the Apollo seismometers: Impactor mass and source cutoff frequency estimations. Icarus 211, 1049–1065. Hapke, B., 2012. Theory of Reflectance and Emittance Spectroscopy. Cambridge Univ. Press, Cambridge, 513pp. Hawke, B., MacLaskey, D., McCord, T., 1979. Multispectral imaging of lunar crater deposits. Papers Presented to the Conference on the Lunar Highlands Crust, LPI Houston, pp. 50–52. Head, J., Wilson, L., 1979. Alphonsus-type dark-halo craters: Morphology, morphometry and eruption conditions. Proc. Lunar Sci. Conf. 10, 2861–2897.
V. Kaydash et al. / Icarus 231 (2014) 22–33 Kaydash, V., Shkuratov, Y., 2012a. Structural disturbances of the lunar surface caused by spacecraft. Solar Syst. Res. 46, 108–118. Kaydash, V.G., Shkuratov, Y.G., 2012b. Phase-ratio imagery identification of the surface altered in the Apollo-16 landing site. Lunar Planet. Sci. 43, #1484. Kaydash, V.G., Gerasimenko, S.Y., Shkuratov, Y.G., Opanasenko, N.V., Velikodsky, Y.I., Korokhin, V.V., 2009a. Photometric function variations observed on the near side of the Moon: Mapping. Solar Syst. Res. 43, 89–99. Kaydash, V. et al., 2009b. Photometric anomalies of the lunar surface studied with SMART-1 AMIE data. Icarus 202, 393–413. Kaydash, V., Shkuratov, Y., Korokhin, V., Videen, G., 2011. Photometric anomalies in the Apollo landing sites as seen from the Lunar Reconnaissance Orbiter. Icarus 211, 89–96. Kaydash, V., Shkuratov, Y., Videen, G., 2012. Phase-ratio imagery as a tool of lunar remote sensing. J. Quant. Spectrosc. Radiat. Trans. 113, 2601–2607. Kreslavsky, M.A., Shkuratov, Y.G., 2003. Photometric anomalies of the lunar surface: Results from Clementine data. J. Geophys. Res. 108. http://dx.doi.org/10.1029/ 2002JE001937.5015. Kreslavsky, M.A., Shkuratov, Y.G., Velikodsky, Y.I., Kaydash, V.G., Stankevich, D.G., Pieters, C.M., 2000. Photometric properties of the lunar surface derived from Clementine observations. J. Geophys. Res. 105 (E8), 20281–20296. Morris, A.R., Head, J.W., Margot, J.-L., Campbell, D.B., 2000. Impact melt distribution and emplacement on Tycho: A new look and old question. Lunar Planet. Sci. 31, 1828. Oberbeck, V., 1971. A mechanism for the production of lunar crater rays. Moon 2, 263–278. Pieters, C.M., Staid, M.I., Fischer, E.M., Tompkins, S., He, G., 1994. A sharper view of impact craters from Clementine. Science 266, 1844–1847. Robinson, M.S. et al., 2010. Lunar Reconnaissance Orbiter Camera (LROC) instrument overview. Space Sci. Rev. 150, 81–124. Salisbury, J.W., Adler, J.E.M., Smalley, V.G., 1968. Dark-haloed crater on the Moon. Mon. Not. R. Astron. Soc. 138, 245–249.
33
Schultz, P.H., Spudis, P.D., 1979. Evidence for ancient mare volcanism, Proc. Lunar Planet. Sci. Conf. 10, 2899–2918. Shkuratov, Y.G., Starukhina, L.V., Kreslavsky, M.A., Opanasenko, N.V., Stankevich, D.G., Shevchenko, V.G., 1994. Principle of undulatory invariance in photometry of atmosphereless celestial bodies. Icarus 109, 168–190. Shkuratov, Y., Kreslavsky, M., Ovcharenko, A., Stankevich, D., Zubko, E., Pieters, C., Arnold, G., 1999. Opposition effect from Clementine data and mechanisms of backscatter. Icarus 141, 132–155. Shkuratov, Y.G., Stankevich, D.G., Petrov, D.V., Pinet, P.C., Cord, A.M., Daydou, Y.H., Chevrel, S.D., 2005. Interpreting photometry of regolith-like surfaces with different topographies: Shadowing and multiple scatter. Icarus 173, 3–15. Shkuratov, Y., Kaydash, V., Gerasimenko, S., Opanasenko, N., Velikodsky, Y., Korokhin, V., Videen, G., Pieters, C., 2010. Probable swirls detected as photometric anomalies in Oceanus Procellarum. Icarus 208, 20–30. Shkuratov, Y., Kaydash, V., Korokhin, V., Velikodsky, Y., Opanasenko, N., Videen, G., 2011. Optical measurements of the Moon as a tool to study its surface. Planet. Space Sci. 59, 1326–1371. Shkuratov, Y., Kaydash, V., Videen, G., 2012. The lunar crater Giordano Bruno as seen with optical roughness imagery. Icarus 218, 525–533. Shkuratov, Y., Kaydash, V., Sysolyatina, X., Razim, A., Videen, G., 2013. Lunar surface traces of engine jets of Soviet sample return probes: The enigma of Luna-23 and Luna-24 landing sites. Planet. Space Sci. 75, 28–36. Starukhina, L.V., Shkuratov, Y.G., 2004. Swirls on the Moon and Mercury: Meteoroid swarm encounters as a formation mechanism. Icarus 167, 136–147. Töksoz, M.N., Dainty, A.M., Solomon, S.C., Anderson, K.R., 1974. Structure of the Moon. Rev. Geophys. Space Phys. 12, 539–567. Velikodsky, Y.I., Opanasenko, N.V., Akimov, L.A., Korokhin, V.V., Shkuratov, Y.G., Kaydash, V.G., Videen, G., Ehgamberdiev, S.A., Berdalieva, N.E., 2011. New Earthbased absolute photometry of the Moon. Icarus 214, 30–45.
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Dark halos and rays of young lunar craters: A new insight into interpretation Vadym Kaydash a, Yuriy Shkuratov a, Gorden Videen b,⇑ a b
Astronomical Institute of Kharkov V.N. Karazin National University, Sumskaya 35, Kharkov 61022, Ukraine Space Science Institute, 4750 Walnut St. Suite 205, Boulder, CO 80301, USA
a r t i c l e
i n f o
Article history: Received 16 August 2013 Revised 13 November 2013 Accepted 18 November 2013 Available online 24 November 2013 Keywords: Image processing Impact processes Moon, surface Photometry Regolith
a b s t r a c t Images acquired by the Narrow Angle Camera of the Lunar Reconnaissance Orbiter allow phase-ratio imagery of young lunar craters surrounded by dark halos. Such imaging is a new optical remote-sensing technique that is sensitive to the degree of surface roughness. We apply the phase-ratio technique to LRO images of young dark-halo craters near the crater Denning and in the Balmer basin, in addition to craters created by the impacts of the Ranger-6 spacecraft and Saturn-5 sections of Apollo-13 and Apollo-17. We suggest an alternative explanation of the dark halos and rays seen near the craters at large phase angles. Phase-ratio imaging suggests that these features result from higher surface roughness. Thus, the interpretation of dark crater halos and rays as a composition/maturity variance should be used with caution. The composition and structure factors can be effectively discriminated only using images acquired in a wide range of phase angles including small angles. Published by Elsevier Inc.
1. Introduction Many young lunar craters have bright halos and rays that were formed by excavating subsurface materials at major and secondary impact locations (Oberbeck, 1971). Fresh material ejected from lunar craters is generally brighter than the surrounding region, since it consists of immature soils that are slowly altered by micrometeorite bombardment and solar wind, resulting in soil darkening over long time periods. Eventually, the reflectance of ejecta material may appear as that of the surrounding regolith. Although the excavated materials of fresh craters should be of high reflectance, darkhaloed and dark-rayed craters on the Moon have been observed, but their number is significantly lower than craters without such halos and rays (Salisbury et al., 1968). These features are considered to be deposits of low-albedo materials; however, there is no unique explanation of their origin. Some of these features, like several small craters in the old crater Alphonsus (Fig. 1a), perhaps have a volcanic origin (Salisbury et al., 1968; Head and Wilson, 1979). They are often associated with rilles and lineaments and considered as a source of pyroclastic material forming the dark halos (shown with arrows in Fig. 1a). Accordingly, the craters probably represent sites of volcanic fountaining on the Moon.
⇑ Corresponding author. E-mail addresses: (G. Videen).
[email protected],
[email protected]
0019-1035/$ - see front matter Published by Elsevier Inc. http://dx.doi.org/10.1016/j.icarus.2013.11.025
A feasible origin proposed for the dark ray system of the Copernican-aged crater Dionysius (Fig. 1b) implies impact melt deposits and dark primary ejecta (e.g., Schultz and Spudis, 1979; Hawke et al., 1979). Later Giguere et al. (2006) concluded that dark rays of Dionysius are dominated by mare basalts, not glassy impact melts. The crater Tycho is an example of a large crater with a dark halo of non-volcanic origin. The crater seen in Fig. 1c has a dark annulus that is considered to be caused by rock melted due to the extreme heat of impact and ejected throughout the area immediately surrounding the crater (Pieters et al., 1994; Morris et al., 2000). The dark halo of Tycho consists mostly of dark glassy materials that have pooled in the low spots surrounding the crater. There are no other large craters having such a prominent dark halo that is visible, probably because the crater Tycho is both young and large. Another type of dark halo crater can be found where a brighter ejecta blanket covers an older and darker lava flow. If a more recent impact occurs, it can pierce the layer of the bright ejecta material and excavate the darker material beneath. Examples of such craters can be found in the Schiller–Schickard region, where the mare material has been unearthed from beneath the light-colored deposits emplaced there at the Mare Orientale formation (e.g., Antonenko et al., 1997; Antonenko, 2013). Fig. 1d presents the crater Inghirami W (pointed by an arrow), located in the Schiller– Schickard region. It reveals a dark halo of excavated material. This underlying mare-type lava is called cryptomare. In principle, this mechanism of dark-halo formation is valid for all crater sizes including even those of 1–10 m. In this case dark-material layers
V. Kaydash et al. / Icarus 231 (2014) 22–33
23
Fig. 1. Different dark-haloed craters. (a) The 120-km crater Alphonsus, (b) the 18-km crater Dionysius, (c) the 82-km crater Tycho, (d) a part of the Schiller–Schickard region. The LROC WAC mosaic (643 nm) produced by NASA/GSFC/Arizona State University and available at the http://lroc.sese.asu.edu/data/pr/tiff/lrocwac643nmnearside.tif is used.
can be brought to light when the thin younger layer composed of brighter materials has been pierced. In this paper we suggest an alternative mechanism that may produce dark halos (rays) around craters, which is based on the difference of brightness phase curves of halos (rays) and their surrounding surfaces. We consider here several examples of dark halo craters whose darkness relates to the phase-angle effect. Three examples are shown of craters formed by the impacts of the Apollo 13 and Apollo 17 Saturn IVB boosters and the Ranger 6 spacecraft. The fourth and fifth we present are of young natural craters located near the old crater Denning and in the Balmer basin.
2. New mechanism and research method Each location on the lunar surface can be characterized by the dependence of its brightness (or reflectance, or apparent albedo A, or radiance factor) on phase angle a (e.g., Shkuratov et al., 2011; Hapke, 2012). At different locations this dependence is different. The brightness decreases with increasing a and the rate of this decrease can be characterized by the phase function slope. Surface roughness affects the phase function, since the shadowing effect increases with increasing roughness (e.g., Hapke, 2012). Fig. 2 shows two phase curves of absolute reflectance. One of them shows what may be considered a typical mare surface; whereas, the other one corresponds to what may be considered a typical highland surface. The data are taken from the absolute photometry of the Moon (Velikodsky et al., 2011) for small areas in Mare Tranquillitatis and near the highland crater Gylden close to
Fig. 2. Phase curves of absolute lunar reflectance. Data are taken from Velikodsky et al. (2011). Solid and dash curves correspond to typical mare and highland surfaces, respectively. The ratio of mare phase curve to highland one is in inset.
the center of the nearside. As can be seen, the phase curves cross each other. This can occur for crater halos (rays) and their surroundings. That is, at small phase angles, the halos can be brighter than their surroundings; whereas, at larger phase angles an inversion may be observed. An illustration of such an inversion is
24
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 3. Comparison of images acquired at different phase angles for the impact crater formed by the Apollo 17 Saturn IVB booster. (a) A(8°), (b) A(28°), (c) A(74°), and (d) A(80°). Here and elsewhere north is up, and west is to the right in the images.
displayed in Fig. 3, which shows the crater formed by the Apollo 17 Saturn IVB booster. The contrast of the crater rays undergoes an inversion as the phase angle increases (cf. Fig. 3a–d corresponding to increasing a). One may suppose that the effect of inversion is due to the surface roughness, since in ejecta areas (halos and rays) the surface should be rougher than the surrounding region (e.g., Shkuratov et al., 2011). We use phase-ratio imagery to study the dark halos (rays) of several craters. This technique is a prospective tool to characterize the lunar surface, especially using the space-derived photometry data acquired from orbital missions with high spatial resolution (i.e. JAXA Kaguya, ISRO Chandrayaan-1, and especially the NASA Lunar Reconnaissance Orbiter (LRO)). In this method the quotient of two coregistered images obtained at different phase angles a
are mapped (Shkuratov et al., 1994, 1999). This mapping provides information on the slope of the lunar phase function of radiance factor A(a) (Shkuratov et al., 2011) at each point of the imaged surface; thus, the slope can be characterized by the ratio A(a1)/A(a2). The phase-ratio technique provides an estimate of variations of the complexity of unresolved surface microtopography. For instance, this method allowed us to find new lunar surface formations in the southern part of Oceanus Procellarum (Shkuratov et al., 2010) with ground-based telescope data, which were suggested to be weak swirls (e.g., Starukhina and Shkuratov, 2004). The phase-ratio technique has been used to investigate the opposition effect of the Moon (Shkuratov et al., 1999; Kaydash et al., 2009a; Velikodsky et al., 2011) and many times was applied to data obtained using spacecraft (Kreslavsky et al., 2000; Kreslavsky and
25
V. Kaydash et al. / Icarus 231 (2014) 22–33
Table 1 Characteristics of the images acquired by LROC NAC for the craters formed by Apollo 13 and 17 Saturn IVB stages, Ranger 6 spacecraft, and two natural impactors. Images printed in italics are used for constructing phase-ratio images. Crater formed by
Image ID
Resolution (m/pix)
Emission angle (°)
Incidence angle (°)
Phase angle (°)
Sub solar azimuth (°)
Apollo 17 SIVB
M139985936L M111674228R M144701329L M144708115L M131731837R
0.51 0.52 0.52 0.55 0.47
1.68 1.16 13.06 24.84 1.16
6.04 29.56 49.75 48.82 81.54
7.55 28.40 36.70 73.65 80.38
151.70 185.02 182.06 183.14 180.84
Ranger 6
M139768545R M144483945R M144490730R
0.50 0.50 0.56
3.21 8.59 28.17
13.64 47.43 46.56
11.83 39.01 74.43
231.18 170.59 165.37
Apollo 13 SIVB
M160139273L M160139273R M160132488L M160132488R M140087684L M162493915R
0.52 0.51 0.52 0.53 0.48 0.49
19.1 16.26 19.98 22.83 1.68 1.16
49.95 50.03 50.87 50.95 4.27 76.9
69.03 66.26 30.93 28.17 5.88 75.74
184.15 184.06 182.95 182.95 162.95 269.09
Natural crater (Denning-Isaev)
M143676946L M143676946R M143683729R M143683729L M115367300L M115367300R
0.63 0.63 0.68 0.69 1.26 1.26
5.88 8.75 22.07 24.95 1.69 1.17
40.23 40.32 39.39 39.28 73.35 73.45
34.76 32.19 60.45 63.15 75.04 72.27
199.45 199.21 205.49 206.56 272.7 268.82
Natural crater (south to Balmer R)
M190136698R M190143847R M190150996R
0.88 0.82 0.87
22.75 1.19 19.81
44.28 43.37 42.53
24.8 42.26 60.98
201.89 203.9 209.71
Shkuratov, 2003, 2012a, 2012b; Kaydash et al., 2009b, 2011, 2012; Shkuratov et al., 2011, 2012, 2013). This method also has been exploited to analyze all the manned US Apollo landing sites (Apollo 11, 12, 14, 15, 16, and 17), and all the Soviet sample return sites (Luna 16, 20, 23, and 24) using photometric data obtained from the LRO Camera (LROC) (Kaydash et al., 2011; Kaydash and Shkuratov, 2012a, 2012b; Shkuratov et al., 2011, 2013). In particular, phase-ratio imagery has revealed photometric anomalies characterized by values of the phase-curve slope indicating a smoothing of the surface microstructure at the Apollo and Luna landing sites. This was caused by dust uplifted by the engine jets of the descent and ascent modules. Clegg et al. (2014) have confirmed this result using our phase-ratio technique for the Apollo, Surveyor, and Luna landing sites. 3. Source data We use high-resolution lunar-surface imagery data obtained by the two monochrome Narrow Angle Cameras (NACs) included in the LROC system. The spatial resolution of the NAC from the 50km orbit is 50 cm/pixel. LROC NAC images can be utilized for future landing site identification, certification, and characterization (Robinson et al., 2010). The image calibration pipeline for the LROC NAC uses a set of calibration files and converts the digital numbers in source images to radiance factor values, accounting for exposure time, dark image, flat field, non-linearity, and spectral responsivity of the detector (Robinson et al., 2010). We use calibrated LROC NAC images acquired during the nominal, science, and extended science phases of the mission. We investigate five lunar regions that include craters formed by the impacts of the Apollo 13 and Apollo 17 Saturn IVB boosters, Ranger 6 spacecraft, as well as two natural young craters. The choice of the anthropogenic craters guarantees the presence of very fresh material in their halos and rays not subjected to maturation processes. The two natural craters we studied reveal distinct dark rays in their proximal ejecta zones. Unfortunately, the number of such craters imaged by LROC NAC at similar illumination but different phase angles is rather small. The first natural crater
is located in the farside highlands between the craters Denning and Isaev, and the second one is located in the Balmer basin on the eastern limb of the nearside. Images used for phase-ratio imagery are described in Table 1. The images of the anthropogenic craters have a maximal surface resolution near 0.5 m, while those of the natural craters are imaged at 0.6-0.9 m/pixel resolution. To produce the phase-ratio mappings, we selected LROC NAC images acquired at similar illumination, i.e. similar incidence and sub-solar azimuth angles (see Table 1), thus reducing the effect of resolved large-scale topography on the phase-ratio distribution. We also selected additional images of different phase angles to track the effect of inverting the contrast of crater rays. To calculate a phase-ratio A(a1)/A(a2) image we use a rubbersheet geometric transformation, accounting for the parallax effect (Kaydash et al., 2012). The parallax data have been used not only to map the phase ratio, but also to produce anaglyphs that are images providing a stereoscopic 3D effect. The 3D phase-ratio imagery of lunar landscapes is useful to search for disturbances of surface structure, e.g., to detect areas of eroded surface, fresh slumps, accumulated material on crater walls, terraces, and floors (Kaydash et al., 2012). In this paper we use the ratio A(a1)/A(a2) when a1 > a2; whereas, in several of our previous publications (Kaydash et al., 2011; Kaydash and Shkuratov, 2012a, 2012b; Shkuratov et al., 2011) we selected a1 < a2. Darker tones on phase-ratio images in this paper correspond to lower values of the phase ratio, i.e. to steeper slopes of phase curves and thus to a rougher surface microstructure. There is a specific reason why we finally choose A(a1)/A(a2) at a1 > a2. In the intermediate range of phase angles (10–70°), a direct, almost linear correlation between A(a1)/A(a2) and albedo is observed just when a1 > a2; otherwise, the correlation is inverse and not linear. In first case, to study deviations from the regression line is a much easier task than when the line is not linear. As has been emphasized, the phase ratio A(a1)/A(a2) characterizes the phase-function slope. It is mainly dominated by the shadow effect produced by regolith particles. At phase angles larger than 40–50°, the regolith surface topography (centimeter–decimeter scales) can significantly affect the slope of the phase function as
26
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 4. Anaglyph images of the vicinity of the crater (marked by arrow) formed by the impact of the Apollo 17 Saturn IVB booster. (a) Distribution of radiance factor A(37°), (b) Distribution of phase ratio A(74°)/A(37°) (see Table 1). Darker tones correspond to a rougher surface. The images provide a stereoscopic effect. The dynamic range of the imaged parameters along with the spatial scale is shown with corresponding bars.
well. Depending on surface albedo, multiple scattering also influences the phase-function slope, since this scattering in and between the regolith particles produces additional illumination of shadows and thus decreases the slope of the phase curves. Fortunately, this plays a secondary role, if the roughness scale is sufficiently large (e.g., Shkuratov et al., 2005). At a > 70–90° the steepness of the phase function is almost independent of albedo. Thus in this case there exists a strong dependence of phase-function steepness on centimeter–decimeter regolith topography for dark halos and rays of lunar craters. We note the absence of data acquired at a > 90° for the studied areas (see Table 1), though such data has proven their usefulness for the identification of several lunar photometric anomalies (Kaydash et al., 2009b; Shkuratov et al., 2010). 4. Results and discussion 4.1. The crater formed by the impact of the Apollo 17 Saturn IVB booster After the command/service and lunar modules undocked from the stage S-IVB on December 7, 1972, the S-IVB impacted the lunar surface on December 10 at latitude 4.168°, longitude 347.670° (http://lroc.sese.asu.edu/news/index.php?/archives/
773-LROC-Coordinates-of-Robotic-Spacecraft-2013-Update.html). The impact velocity and mass of the stage were 2.56 km/s and 14,487 kg, respectively. The impact produced a crater of 30 m diameter, which was used to study the interior structure of the Moon using seismometers placed on the surface by the Apollo astronauts (Töksoz et al., 1974; Gudkova et al., 2011). Fig. 3 demonstrates the effect of contrast inversion of rays for the impact crater formed by the Apollo 17 Saturn IVB booster. At a = 8° and 28° the rays are brighter than the crater vicinity, at a = 74° the rays become darker, and at a = 80° the rays disappear as they are totally masked by the local topography. Fig. 4a and b shows anaglyphs of the radiance factor A(37°) and the phase ratio A(74°)/A(37°) of the area seen in Fig. 3 and its neighborhood. The phase ratio A(74°)/A(37°) anaglyph is constructed from the images M144708115L (a = 73.65°) and M144701329L (a = 36.70°) acquired at similar illumination conditions. Results presented in Figs. 3 and 4 demonstrate that small values of the phase ratio in the vicinity of young craters may indicate a potential inversion of brightness contrast (see Fig. 3) at some phase angle, which can be discernible as a dark halo. Thus, the crater halo and rays perhaps contain a large amount of rather small boulders and stones 60.5 m in size. A visual inspection of the images shows that the crater rays are longer in Fig. 4b than in
V. Kaydash et al. / Icarus 231 (2014) 22–33
27
Fig. 5. Surroundings of the crater (marked by arrow) formed by the impact of Ranger 6: (a–c) images acquired at phase angle 12°, 39°, and 74°, respectively (see Table 1).
Fig. 4a; i.e. phase-ratio imaging is sensitive to roughness anomalies. The next example confirms this conclusion. 4.2. The crater formed by the impact of the Ranger 6 spacecraft Ranger 6 impacted the Moon on the eastern edge of Mare Tranquillitatis on February 2, 1964. This mission was not successful and no video signal was received from Ranger 6. Nevertheless, the impact site located at 9.387°N, 21.481°E was identified from LRO images (http://lroc.sese.asu.edu/news/index.php?/archives/ 773-LROC-Coordinates-of-Robotic-Spacecraft-2013-Update.html). Fig. 5 shows the crater formed by the Ranger 6 impact: (a–c) are brightness distributions at a = 12°, 39°, and 74°, respectively. Visual inspection of these images confirms the previous observation that at small phase angles the halos can be brighter than their surroundings; whereas, at large phase angles, an inversion may be observed. Fig. 6a and b shows a brightness anaglyph of the Ranger 6 impact crater (a) and an anaglyph of the phase ratio A(74°)/A(39°) distribution (b). Darker tones in Fig. 6b correspond to rougher surface features. As in the case of the Apollo 17 Saturn IVB booster crater (Fig. 4a and b), the 3D images shown in Fig. 6a and b do not reveal noticeable relief variations in the immediate vicinity of the
Ranger 6 crater (cf. the large impact crater in the lower left part of the scene). Thus, there are no large-scale (i.e. larger than the image resolution of 0.5 m) topography variations that can be responsible for the contrast inversion. Therefore, we may conclude that the crater produced by the Ranger-6 impact reveals high values of phase-function steepness due to a large number of excavated boulders and rocks (with sizes distributed in a range smaller than the image resolution) that produce a strong shadow effect. 4.3. The crater formed by the impact of the Apollo 13 Saturn IVB booster It is well known that the Apollo 13 mission was unsuccessful. On April 14, 1970 the Apollo 13 Saturn IVB upper stage impacted the Moon north of Mare Cognitum at the location 2.555° latitude and 332.113° longitude (http://lroc.sese.asu.edu/news/ index.php?/archives/773-LROC-Coordinates-of-Robotic-Spacecraft2013-Update.html). The impact crater is roughly 30 m in diameter. In order to examine the described effect of the brightness inversion, we present in Fig. 7a sequence of images acquired at different phase angles. The low-phase-angle (a = 6°) image in Fig. 7a reveals
28
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 6. Anaglyph images of the Ranger 6 impact crater: (a and b) correspond to the brightness (39°) and phase-ratio (74°/39°) distributions, respectively. The darker tones in (b) correspond to rougher surfaces. The dynamic range of the imaged parameters along with the spatial scale is shown with corresponding bars.
Fig. 7. Comparison of images acquired at different phase angles for the impact crater formed by the Apollo 13 Saturn IVB booster: (a) A(6°), (b) A(30°), (c) A(68°), and (d) A(76°).
4.4. A natural lunar crater in the Denning-Isaev area the crater halo and ray system as the brightest objects in the scene. Then, as the phase angle increases to 30° and 68° (Fig. 7b and c) the halo and rays become equal and even lower in brightness than the surrounding region. At a = 76° (Fig. 7d) the Saturn IVB crater halo and rays disappear due to illuminated macroscopic roughness formed by the resolvable topography. Fig. 8a and b shows anaglyph images of the Apollo 13 SIVB impact crater: (a) and (b) correspond to the brightness (a = 30°) and phase ratio A(68°)/A(30°) distributions, respectively. These images provide a stereoscopic 3D effect. The darker tones of the crater rays on the phase-ratio image reveal increased phase-function steepness for the rays as compared to the surrounding surface. The actual dynamic range of the phase-ratio values is shown with the scale bar on the image. We note that the ray patterns seen in Fig. 8a and b are different. One can track rays to a greater distance from the crater with the phase-ratio image than with the moderate-phase-angle brightness image.
We found several young natural craters with dark halos at large phase angles and present here an example for which we have sufficient data to analyze. Such a crater is shown by the arrow in the Apollo 15 image AS15-M-0102 captured at a Sun elevation of 34.0° (Fig. 9, see also insert). The crater is located at 17.7°S, 144.4°E in the farside highland between the craters Denning and Isaev. Its diameter is approximately 2000 m. Fig. 10a and b shows this crater using LROC NAC data for the distributions of brightness A(33.5°) and A(74°), respectively. Lower-phase-angle image is a mosaic produced from frames M143676946L (a = 34.76°) and M143676946R (a = 32.19°). The effective phase angle for the mosaic is (34.76° + 32.19°)/2 33.5°. The A(74°) image also is a mosaic of two images acquired at phase angles 72.27° and 75.04° (see Table 1 for detail). As can be seen in the figure, the halo and rays of the crater are darker than the background in the lower-phase-angle image. At the higher phase angle, the rays are hardly distinguishable due to the influence of large-scale topography;
V. Kaydash et al. / Icarus 231 (2014) 22–33
29
Fig. 8. Anaglyph images of the Apollo 13 Saturn IVB stage impact crater: (a and b) correspond to the radiance factor at 30° and phase ratio (68°/30°) distributions, respectively. Darker tones in (b) correspond to rougher surfaces.
in this phase ratio is a mosaic of M143683729L (a = 63.15°) and M143683729R (a = 60.45°). Thus the effective phase angle for the mosaic is (63.15° + 60.45°)/2 62°. Fig. 11a and b shows, respectively, anaglyphs of the brightness A(33.5°) and phase-ratio (62°/ 33.5°) distributions reconstructed with LROC NAC data. A comparison of ray patterns in Fig. 11a and b reveals a more prominent, darker pattern for the phase-ratio image than for the albedo image. Darker tones in Fig. 11b correspond to lower phase-ratio areas that we interpret as evidence of a higher degree of surface roughness. We tried to find the halo effect in the topography by inspecting the 3D images; however, we have not found large enough fragments seen as topographical details in the halo and rays. So we conclude that the dark halo and rays seen in Fig. 11b are structural anomalies that perhaps consist of boulders and rocks whose sizes are smaller than 1 m. 4.5. A natural lunar crater in the Balmer basin
Fig. 9. The young crater located in the farside highlands between craters Denning and Isaev under study shown with the arrow. The crater is located at 17.7°S, 144.4°E. A fragment of the Apollo 15 image AS15-M-0102 is shown and the Sun elevation is 34.0°. The inset shows an enlarged image of the crater.
however, some rays are still slightly darker than the adjacent surface. To investigate the contrast change in detail, we calculated the phase ratio image A(62°)/A(33.5°). The higher-phase-angle image
Another example of a natural crater with a dark halo (rays) is the young crater located 7 km south of the crater Balmer R in the Balmer basin. The crater coordinates are 18.9°S, 69.15°E (see arrow in Fig. 12). The crater was imaged three times with the LROC NAC under similar illumination angles (see Table 1). Fig. 13 shows distributions of the radiance factor A(25°) and phase ratio A(61°)/ A(42°). The main difference between the two distributions is the prominent dark-ray pattern in the phase-ratio image at a distance less than or equal to the crater diameter. We interpret this difference as the presence of rougher ejecta material in the immediate vicinity of the crater (darker tones in Fig. 13b) and less rough ejecta material extending up to several crater diameters (brighter tones in Fig. 13b). The ejecta blanket, which extends further than the
30
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 10. The crater shown in Fig. 9 imaged with the LROC NACs: (a and b) show, respectively, radiance factor A(33.5°) and A(74°). The radiance factor range and spatial scale are shown with corresponding bars.
dark phase-ratio halo, hardly can be subdivided into classes of different roughness due to their low-contrast differences in the phase-ratio image. Using phase-ratio imaging we can estimate the slope of the phase function A(a) between two phase angles a1 and a2. Meanwhile the slope may be different in different ranges of a (see, e.g., Fig. 2). Thus, for more detailed photometry one needs to sample the phase curve in more than two points. There are only a few cases of LROC NAC lunar surface imaging more than two times with the same illumination, but different phase angles. Fortunately, LROC NAC imaged the Balmer R area three times at a = 25°, 42°, and 61°. Thus, one may approximately estimate the second derivative of the phase function A(a): @2 A Aða1 Þ þ Aða3 Þ 2Aða2 Þ. This parameter roughly @ a2 characterizes the curvature of the function in the 25–61° range. As inferred from the definition, larger values of the parameter
Fig. 11. Anaglyphs of the (a) brightness A(33.5°) and (b) phase-ratio (62°/33.5°) distributions for the crater presented in Fig. 10. Darker tones for (b) correspond to rougher surfaces.
correspond to a larger curvature of the phase function. In Fig. 14a and b, we present anaglyphs of the brightness A(42°) and second derivative for the crater shown in Fig. 13. Darker tones in Fig. 14b correspond to decreased curvature of the phase function in the 25–61° range. The distribution of the curvature reveals a dark halo of the crater. This halo pattern is different from the albedo image and phase-ratio image A(61°)/A(42°) (especially to the south of the crater). The difference may be related to the influence of roughness and multiple scattering contributions. The distribution of the curvature parameter at larger distances from the crater strongly resembles the brightness distribution (cf. Fig. 13a and b), which indicates that the effect of surface roughness on the phase function is diminished by the multiple scattering contribution that increases with albedo. Thus the dark halo in the curvature parameter may be interpreted as increasing roughness in the proximal ejecta area of the crater, which makes
V. Kaydash et al. / Icarus 231 (2014) 22–33
31
Fig. 12. The young crater located 7 km south of crater Balmer R in the Balmer basin. Crater coordinates are 18.9°S, 69.15°E (shown with arrow). Context image is LROC WAC Global Morphologic basemap (100 m/pixel) incorporated in the ACT-REACT QuickMap (http://target.lroc.asu.edu/q3/).
Fig. 13. The young crater south of Balmer R studied in the present work, as imaged with LROC NAC: (a) Radiance factor A(25°) and (b) phase-ratio (61°/42°) distribution. Radiance factor and phase-ratio range and spatial scale are shown with corresponding bars. Darker tones for (b) correspond to rougher surfaces.
the phase function steeper in the larger phase angle range (here 42–61°). The presence of an anomalously large number of boulders, blocks, and rock fragments within the halo produces roughness at scales up to spatial resolution of the data, i.e. 1 m.
5. Conclusion When craters are imaged at large phase angles, the interpretation of their dark halos (rays) as a composition anomaly should be used in lunar geological studies with caution, as phase-ratio
32
V. Kaydash et al. / Icarus 231 (2014) 22–33
Fig. 14. Anaglyphs of the (a) brightness A(42°) and (b) second derivative of the phase-function distributions for the crater presented in Fig. 13. Darker tones for (b) correspond to decreased curvature of the phase function, i.e. a steeper phase curve.
imaging suggests that such halos can be attributed to differences in surface roughness. The composition and structure factors can be discriminated using images acquired at different phase angles. Phase-ratio imagery of the Moon is a very useful photometric tool, as it suggests an assessment of surface roughness. This tool also can reveal details hardly visible in brightness images. It can be used for identification of areas with very recent alterations in their surface structure. These can be, for instance, anthropogenic craters. LROC NAC high resolution imagery allows for the 3D phaseratio images to be mapped. Such images are very useful for geological descriptions of regions under studies. Lunar images may reveal an inversion in the contrast for halos and rays in the vicinity of craters. This was illustrated with our investigation of the craters formed by impacts of the Ranger 6 spacecraft and Apollo 13 and Apollo 17 Saturn IVB boosters. Such an inversion suggests that the reason for these dark halos and rays primarily is due to shadowing, rather than compositional differences. Dark halos and rays of two natural young craters located in the Balmer basin and Denning-Isaev area also can be interpreted with the 3D phase-ratio mapping as structural anomalies caused by increased roughness of the ejecta material for scales under image resolution. We emphasize that all suspect dark haloed craters should be checked at higher phase angles to see if the halo persists as it would with a compositional cause.
Acknowledgments We thank the LRO project teams for their efforts in obtaining and returning the data presented here. The authors gratefully acknowledge the research support from NASA Grant NNX11AB25G Lunar Advanced Science and Exploration Research (LASER), ‘‘Imaging photometric and polarimetric remote sensing of the Moon’’.
References Antonenko, I., 2013. Re-examining the identification of dark-haloed impact craters: New criteria for modern data sets. Lunar Planet. Sci. 44, 2607. Antonenko, I., Cintala, M., Head, J., Hörz, F., 1997. Results of experimental dark halo crater studies: Implications for lunar cryptomare thickness measurements. Lunar Planet. Sci. 28, 47–48. Clegg, R., Jolliff, B., Robinson, M., Hapke, B., Plescia, J., 2014. Effects of rocket exhaust on lunar soil reflectance properties. Icarus 227, 176–194. Giguere, T.A., Hawke, B.R., Gaddis, L.R., Blewett, D.T., Gillis-Davis, J.J., Lucey, P.G., Smith, G.A., Spudis, P.D., Taylor, G.J., 2006. Remote sensing studies of the Dionysius region of the Moon. J. Geophys. Res. 111, E06009. http://dx.doi.org/ 10.1029/2005JE002639. Gudkova, T.V., Lognonné, Ph., Gagnepain-Beyneix, J., 2011. Large impacts detected by the Apollo seismometers: Impactor mass and source cutoff frequency estimations. Icarus 211, 1049–1065. Hapke, B., 2012. Theory of Reflectance and Emittance Spectroscopy. Cambridge Univ. Press, Cambridge, 513pp. Hawke, B., MacLaskey, D., McCord, T., 1979. Multispectral imaging of lunar crater deposits. Papers Presented to the Conference on the Lunar Highlands Crust, LPI Houston, pp. 50–52. Head, J., Wilson, L., 1979. Alphonsus-type dark-halo craters: Morphology, morphometry and eruption conditions. Proc. Lunar Sci. Conf. 10, 2861–2897.
V. Kaydash et al. / Icarus 231 (2014) 22–33 Kaydash, V., Shkuratov, Y., 2012a. Structural disturbances of the lunar surface caused by spacecraft. Solar Syst. Res. 46, 108–118. Kaydash, V.G., Shkuratov, Y.G., 2012b. Phase-ratio imagery identification of the surface altered in the Apollo-16 landing site. Lunar Planet. Sci. 43, #1484. Kaydash, V.G., Gerasimenko, S.Y., Shkuratov, Y.G., Opanasenko, N.V., Velikodsky, Y.I., Korokhin, V.V., 2009a. Photometric function variations observed on the near side of the Moon: Mapping. Solar Syst. Res. 43, 89–99. Kaydash, V. et al., 2009b. Photometric anomalies of the lunar surface studied with SMART-1 AMIE data. Icarus 202, 393–413. Kaydash, V., Shkuratov, Y., Korokhin, V., Videen, G., 2011. Photometric anomalies in the Apollo landing sites as seen from the Lunar Reconnaissance Orbiter. Icarus 211, 89–96. Kaydash, V., Shkuratov, Y., Videen, G., 2012. Phase-ratio imagery as a tool of lunar remote sensing. J. Quant. Spectrosc. Radiat. Trans. 113, 2601–2607. Kreslavsky, M.A., Shkuratov, Y.G., 2003. Photometric anomalies of the lunar surface: Results from Clementine data. J. Geophys. Res. 108. http://dx.doi.org/10.1029/ 2002JE001937.5015. Kreslavsky, M.A., Shkuratov, Y.G., Velikodsky, Y.I., Kaydash, V.G., Stankevich, D.G., Pieters, C.M., 2000. Photometric properties of the lunar surface derived from Clementine observations. J. Geophys. Res. 105 (E8), 20281–20296. Morris, A.R., Head, J.W., Margot, J.-L., Campbell, D.B., 2000. Impact melt distribution and emplacement on Tycho: A new look and old question. Lunar Planet. Sci. 31, 1828. Oberbeck, V., 1971. A mechanism for the production of lunar crater rays. Moon 2, 263–278. Pieters, C.M., Staid, M.I., Fischer, E.M., Tompkins, S., He, G., 1994. A sharper view of impact craters from Clementine. Science 266, 1844–1847. Robinson, M.S. et al., 2010. Lunar Reconnaissance Orbiter Camera (LROC) instrument overview. Space Sci. Rev. 150, 81–124. Salisbury, J.W., Adler, J.E.M., Smalley, V.G., 1968. Dark-haloed crater on the Moon. Mon. Not. R. Astron. Soc. 138, 245–249.
33
Schultz, P.H., Spudis, P.D., 1979. Evidence for ancient mare volcanism, Proc. Lunar Planet. Sci. Conf. 10, 2899–2918. Shkuratov, Y.G., Starukhina, L.V., Kreslavsky, M.A., Opanasenko, N.V., Stankevich, D.G., Shevchenko, V.G., 1994. Principle of undulatory invariance in photometry of atmosphereless celestial bodies. Icarus 109, 168–190. Shkuratov, Y., Kreslavsky, M., Ovcharenko, A., Stankevich, D., Zubko, E., Pieters, C., Arnold, G., 1999. Opposition effect from Clementine data and mechanisms of backscatter. Icarus 141, 132–155. Shkuratov, Y.G., Stankevich, D.G., Petrov, D.V., Pinet, P.C., Cord, A.M., Daydou, Y.H., Chevrel, S.D., 2005. Interpreting photometry of regolith-like surfaces with different topographies: Shadowing and multiple scatter. Icarus 173, 3–15. Shkuratov, Y., Kaydash, V., Gerasimenko, S., Opanasenko, N., Velikodsky, Y., Korokhin, V., Videen, G., Pieters, C., 2010. Probable swirls detected as photometric anomalies in Oceanus Procellarum. Icarus 208, 20–30. Shkuratov, Y., Kaydash, V., Korokhin, V., Velikodsky, Y., Opanasenko, N., Videen, G., 2011. Optical measurements of the Moon as a tool to study its surface. Planet. Space Sci. 59, 1326–1371. Shkuratov, Y., Kaydash, V., Videen, G., 2012. The lunar crater Giordano Bruno as seen with optical roughness imagery. Icarus 218, 525–533. Shkuratov, Y., Kaydash, V., Sysolyatina, X., Razim, A., Videen, G., 2013. Lunar surface traces of engine jets of Soviet sample return probes: The enigma of Luna-23 and Luna-24 landing sites. Planet. Space Sci. 75, 28–36. Starukhina, L.V., Shkuratov, Y.G., 2004. Swirls on the Moon and Mercury: Meteoroid swarm encounters as a formation mechanism. Icarus 167, 136–147. Töksoz, M.N., Dainty, A.M., Solomon, S.C., Anderson, K.R., 1974. Structure of the Moon. Rev. Geophys. Space Phys. 12, 539–567. Velikodsky, Y.I., Opanasenko, N.V., Akimov, L.A., Korokhin, V.V., Shkuratov, Y.G., Kaydash, V.G., Videen, G., Ehgamberdiev, S.A., Berdalieva, N.E., 2011. New Earthbased absolute photometry of the Moon. Icarus 214, 30–45.