- Email: [email protected]
How much material do the radar-bright craters at the Mercurian poles contain?
ARTICLE IN PRESS
Planetary and Space Science 53 (2005) 1496–1500 www.elsevier.com/locate/pss
How much material do the radar-bright craters at the Mercurian poles contain? Faith Vilasa,,1, Paul S. Cobianb,2, Nadine G. Barlowc, Susan M. Ledererb a
Planetary Astronomy Group, NASA Johnson Space Center/KR, Houston, TX 77058, USA Department of Physics, California State University, 5500 University Parkway, San Bernardino, CA 92407, USA c Department of Physics & Astronomy, Northern Arizona University, NAU Box 6010, Flagstaff, AZ 86011-6010, USA b
Received 15 December 2004; received in revised form 29 June 2005; accepted 7 July 2005 Available online 25 August 2005
Abstract The depth-to-diameter (d/D) ratios were determined for 12 craters located near the Mercurian north pole that were identified by Harmon et al. (2001, Icarus 149) as having strong depolarized radar echos. We find that the mean d/D value of these radar-bright craters is 23 the mean d/D value of the general population of non-radar-bright craters in the surrounding north polar region. Previous studies, however, show no difference between d/D values of Mercurian polar and equatorial crater populations, suggesting that no terrain softening which could modify crater structure exists at the Mercurian poles (Barlow et al., 1999, 194, Icarus 141). Thus, the change in d/D is governed by a change in crater depth, probably due to deposition of material inside the crater. The volume of infilling material, including volatiles, in the radar-bright craters is significantly greater than predicted by proposed mechanisms for the emplacement of either water ice or sulfur. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mercury; Cratering; Surfaces, planets; Radar; Terrestrial planets; Ices
1. Introduction Ground-based radar observations of the north polar region of Mercury were obtained during 1998 and 1999, when the sub-Earth latitude of Mercury favored observations of the radar-bright features near Mercury’s north pole (Harmon et al., 2001). Due to the upgrade of the NAIC Arecibo radar telescope, these radar images have spatial resolution 10 better than the pre-upgrade Arecibo images of Harmon et al. (1994). The observations reveal spatially-resolved radar-bright signals Corresponding author. Tel.: +1 281 483 5056; fax: +1 281 483 1573. E-mail address: [email protected] (F. Vilas). 1 Present address: MMT Observatory, P.O. Box 210065, University of Arizona, Tucson, AZ 85721-0065, USA. 2 Department of Chemical and Materials Engineering, California State Polytechnic University, Pomona, 3801 West Temple Avenue, Pomona, CA 91768, USA.
0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.07.003
(strong, highly depolarized echoes) across the Mercurian north pole (Harmon et al., 2001). In the north polar region imaged by Mariner 10, these bright radar signals correlate with relatively fresh impact craters (USGS grade 4, some grade 3) ranging in diameter from 90 km to o10 km (Harmon et al., 1994, 2001). The amount of surface area on the crater floors associated with the high-reflectivity signals diminishes as one moves equatorward and the amount of the floor in permanent shadow decreases. Since the bright signals are confined to the permanently-shadowed areas of polar crater floors, the presence of cold-trapped volatiles has been postulated as the origin of these signals. Both water ice and elemental sulfur have been proposed as volatiles that would produce the radar-bright signals (Slade et al., 1992; Harmon and Slade, 1992; Sprague et al., 1995). In an effort to determine whether the radar-bright signals indicate that trapped, buried caps of clean H2O ice could exist under the Mercurian polar regions,
ARTICLE IN PRESS F. Vilas et al. / Planetary and Space Science 53 (2005) 1496–1500
Barlow et al. (1999) tested the terrain near both Mercurian poles for evidence of the rapid terrain softening that would be caused by subsurface H2O. The most sensitive effect of subsurface H2O on impact craters is a decrease in crater depth/diameter (d/D) ratio, coupled with the softening over time of sharp features such as crater rims (e.g., Cintala and Mouginis-Mark, 1980; Squyres and Carr, 1986). Barlow et al. measured d/D ratios of 170 craters having fresh morphological characteristics and found on heavily cratered terrain. These craters are located in surface areas defined by four quadrangles on Mercury: the two polar regions (Borealis (H1) and Bach (H15)) where bright radar signals have been observed, and two equatorial regions (Tolstoj (H8) and Kuiper (H6)) where neither surface nor subsurface ice is expected. Within the experimental errors imposed by the quality of the Mariner 10 data, no evidence for subsurface polar ice deposits was indicated by that statistical study. The rationale and procedures behind this study described above are discussed in Barlow et al. (1999). The study by Barlow et al. was limited to craters having latitudes equatorward of roughly 7851 due to the lighting conditions imposed by location on the planet’s surface: the shadow cast by the wall of a crater located closer to either pole can cover the crater floor completely and extend up the opposing crater wall, rendering an accurate d/D measurement impossible. Barlow et al. were, however, able to measure d/D values for three of the craters with bright radar reflectivities: crater W in the Borealis Quadrangle covering the Mercurian north pole, and craters G and V in the Bach Quadrangle covering the south pole (Harmon et al., 1994). The recent, higher-resolution observations of Harmon et al. (2001) showed that original crater W and many additional craters near the north pole have radar-bright signals corresponding to the shadowed regions of the craters’ interiors. Some of these additional craters lie within the latitude region where we can use shadow measurements from Mariner 10 images, coupled with the solar illumination geometry for those images, to estimate crater depths. These new observations allow us to compare the d/D attributes of specific radar-bright craters with the statistical characteristics of non-radarbright craters in the surrounding areas. Barlow et al. (1999) have demonstrated that large, subsurface polar H2O caps do not exist on Mercury’s surface. Thus, differences in d/D for individual craters known to have radar-bright signals would suggest that the putative interior volatile deposits have affected the expected crater structure. This could provide clues to the volume and depth of the material captured in these craters, potentially leading to information about the origin of the material. To test this idea, we measured the d/D attributes of the craters visible in Mariner 10
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images identified by Harmon et al. (2001) as having bright radar signals.
2. Measurements Shadow length and rim-to-rim diameter measurements of 12 radar-bright complex craters were digitized from four Mariner 10 images (FDS 156, FDS 160, FDS 164, FDS 165, shown in Fig. 1a–d) having spatial resolutions of 1 km/pixel or better in the Borealis Quadrangle. These craters constitute a subset of the radar-bright craters identified by Harmon et al. (2001), and were selected based on the availability of Mariner 10 images having adequate lighting conditions. Shadow length and diameter were measured 10 times for each crater, and an average and error for each crater were calculated from these measurements. Following the method of Clow and Pike (in Pike, 1988), crater depths were determined using the shadow length measurements, approximate sun angles (the angle formed by the sun and a normal to the planet’s surface at a given location), and spacecraft orientation. Sun angle and spacecraft orientation were calculated using information in the scanning equalization digital radiography (SEDR) reports for Mariner 10. d/D ratios were determined for each of the 12 craters. In 8 cases, the craters were imaged twice by Mariner 10. The individual measurements of each crater were compared, and no significant differences between measurements of the radar-bright craters in different Mariner 10 images were observed.
3. Analysis: Two separate crater populations Table 1 contains the d/D results and location information for the individual craters. We compare the mean d/D values for 51 bins of latitude for both the craters measured here (Table 2), and the craters measured in Barlow et al. on cratered terrain in the Borealis quadrangle (Table 3). There is a factor of 1.5 difference in the mean d/D measurements between the radar-bright craters and the general crater population. Since, a priori, we know that the radar-bright craters have at least one different characteristic from the nonradar-bright craters, some consideration of this additional difference seems warranted. We first tested our assumption that the d/D differences are significant. We assumed that both the radarbright craters and the general Borealis Quadrangle background crater population are not necessarily identical in characteristics, but rather represent two different sample populations, and conducted a 2-sample Student’s t-test. The calculated t value is –2.78, which lies below the comparison t value of 1.796. This
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Fig. 1. Individual images of the Mercurian north polar region from Mariner 10 that were used for the depth and shadow height measurements. The measured radar-bright craters are labeled in the images with the letter designation from Harmon et al. (2001). (a) FDS 156, (b) FDS 160, (c) FDS 164, (d) FDS 165. Note the presence of some craters in more than one image.
Table 1 Depth and diameter measurements of 12 mercurian radar-bright craters Crater
F J2 K2 L2 L M2 M P2 P Q V2 W
Diameter (km)
Depth (km)
Latitude (1)
Longitude (1)
d/D ratio
26.470.3 28.570.3 32.470.2 67.970.4 20.770.4 38.970.3 35.270.1 17.670.4 23.570.3 25.970.3 18.670.2 47.070.4
1.270.12 2.170.02 1.670.02 2.770.08 1.070.02 2.470.09 1.370.02 1.670.03 1.470.02 1.470.03 1.970.04 2.870.11
87 83 84 83 84 80 85 83 82 81 80 81
116 147 145 140 47 122 30 80 40 35 59 91
0.05 0.07 0.05 0.04 0.05 0.06 0.04 0.09 0.06 0.05 0.10 0.06
indicates that the radar-bright sample is statistically different from the non-radar-bright Borealis population at the 95% confidence interval (a significance level of 0.05).
Table 2 Average d/D of radar-bright craters (this work) by latitude Quadrangle
Latitude range
Average d/D
Number of craters
Borealis Borealis
801–851N 851–901N
0.0670.02 0.0470.01
10 2
4. Physical state of radar-bright craters We next consider the implications of the population difference for the radar-bright craters. Generally, a change to a lower d/D value from a higher one signals a relaxation in the structure of both the crater walls and floor likely due to subsurface H2O; this results in a larger crater diameter and reduced crater depth, contributing to a lower overall d/D ratio. On Mars, for example, the concentration of subsurface volatiles (polar ice caps) causes a measurable change in crater d/D in the 301–551 latitude zone in both hemispheres (Squyres and Carr, 1986). Barlow et al. (1999) showed, however, that the
ARTICLE IN PRESS F. Vilas et al. / Planetary and Space Science 53 (2005) 1496–1500 Table 3 Average d/D of mercurian north polar craters (Barlow et al., 1999) by latitude Quadrangle
Latitude rangea
Average d/D
Number of craters
Borealis Borealis
751–851N 651–751N
0.0970.03 0.0970.05
11 25
a The range of 751–851 corrects a typographical error in Barlow et al. (1999) where it is wrongly listed as 751–801.
lack of any measurable d/D variation between the majority of craters in polar terrain and those craters in equatorial terrain argues against any substantial subsurface polar ice caps on Mercury. If there is no subsurface volatile cap, what constitutes the root of the observed d/D difference between radar-bright and non-radarbright craters? Expansion of the crater diameter due to relaxation of the crater structure depends largely on the properties of the target material. If changes in diameter are the cause of the observed d/D change, we should observe a difference in the d/D properties between the complex, non-radar-bright crater population near the north pole and the complex crater population near the equator. Statistically, we see no such difference. We have no observational evidence that there are major differences that would cause terrain softening in the target-material properties across the north polar region containing the mix of radar-bright and non-radar-bright craters. Thus, we conclude that the d/D differences for these large, flatfloored craters probably indicate a difference in depth. We demonstrate the significance of this change in depth by considering the mean d/D change on a 30-km diameter crater (roughly the average diameter of the radar-bright craters we examined here). Holding the diameter constant, the depth would change from 2.7 to 1.8 km, a change of 900 m of material in thickness. The volume of material that this would represent is 636 km3. No mechanism currently proposed for an exogenous source of water ice provides this volume of volatile material for Mercury (c.f., Killen et al., 1997; Moses et al., 1999). Sprague et al. (1995) have proposed elemental sulfur as the cause of the radar-bright signals, and demonstrate through modeling that sulfur deposits on the floors of craters should be stable down to 821 latitude for long periods of time in Mercury’s history. The existence of elemental sulfur at the poles is proposed to be a steady state process throughout the planet’s history, and Sprague et al. propose a deposition rate of 10–35 m/ Gyr. This leads to sulfur deposit thicknesses of tens of meters. This is considerably less than the estimated volume of material discussed above, so it does not appear that a large volume of sulfur alone could be the source of the infilling material.
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5. What is the origin of the infilling material? Examination of the results we present must consider the origin of the large total volume of material in the interiors of these craters (either volatile or non-volatile or both), coupled with apparently thin layers of volatile material that cover only the permanently-shaded areas of the crater floors. The source of the material (including volatiles) is likely exogenic, but an endogenic source cannot be ruled out. The event or events that contributed the infilling material are unknown, but must date from the Mansurian period (3.0–3.5 Gyr ago). Thus, the event(s) post-date the Caloris basin impact and the beginning of the emplacement of the Mercurian smooth plains. Harmon et al. (2001) identify craters with radar-bright signatures on smooth plains in Goethe basin and northern Borealis Planitia, indicating the deposition of infilling material after the emplacement of smooth plains. Our measurements include two of these craters (P, Q); no significant d/D difference exists between these craters and other radar bright craters that we measured. There is no requirement for all of the infilling material to be volatile in origin; the only requirement here is for the last event to fill these craters with material containing largely volatiles, since we see the radar-bright signatures suggesting volatiles on the crater floor surfaces. We cannot offer any more insight into the origin of the large quantities of material of any sort in the crater interiors. Harmon et al. (2001) suggest that protective mantling could facilitate an increase in the ice layer, if one or more major impact episodes provided thick, icy deposits, followed by accumulation of a layer of material that insulates these volatiles from sublimation. Building on this idea, We note that Vasavada et al. (1999) proposed that sublimation of an ice/dust mixture could have continued until a crust of dark, non-volatile residue left over from surface evaporation of the ice component self-sealed the ice deposit in the crater. In an effort to understand how large quantities of mixed volatile and non-volatile material could be retained inside a polar crater, we consider the case where volatiles and dust resulting from a comet impact are trapped in a crater near one of Mercury’s poles. Sublimation will then occur across the entire crater floor, although perhaps unevenly between the portions of the crater than are illuminated and those portions that are permanently shadowed. A cap of the dark material contained in the comet collapses and seals the top of the volatiles across the sunlit portion of the crater’s interior. A volatile layer is, however, preserved in the shadowed portion of the crater. The amount of material preserved in the crater’s interior could vary in depth across the crater’s floor (although we are not able to determine if this is the case, since we cannot measure depth of material in the permanently-shadowed part of
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the crater). We suggest that the remnant caps of material covering the surfaces of the volatiles, and exposed in the unshadowed portions of the craters, could represent very primitive, outer solar system material. Assuming that more than one event is required to explain the overall large volume of infilling material present in the radar-bright craters, layers of volatiles with partial dark material caps could exist in the radarbright craters.
6. Mercurian radar-bright smaller craters Harmon et al. (2001) observed bright radar signals in the interiors of several small (diameter o10 km) Mercurian craters. Pike (1988) determined that the diameter breakpoint of 10.3 km separated ‘‘small, simple’’ from ‘‘large, complex’’ craters on the surface of Mercury, based on occurrence of morphological features (e.g., flat vs. bowl-shaped floors, central peaks). Theoretical studies (e.g., Paige et al., 1992) suggest that the internal radiation reflections from simple, bowlshaped craters would cause larger amounts of ice to be lost to sublimation. The large volume of material we observe through the d/D measurements could explain the presence of the radar-bright material in the smaller, bowl-shaped craters: if a much larger volume of volatiles was initially present in these simple craters, the sublimation could still be an ongoing process, although proportionately less of the volatiles would remain in the simple craters than in the large, complex craters. Alternatively, sublimation could have continued until a crust of residue left over from surface evaporation selfsealed the ice deposit in the crater, as proposed by Vasavada et al. (1999). There is support in the d/D measurements for the idea that material could be lost more rapidly from smaller-diameter craters until a layer of residue became thick enough to stop the sublimation: the two smallest craters that we measured in this study had diameters of 17.6 and 18.6 km, and d/D ratios of 0.09 and 0.10, respectively. These are greater than the ratios we measured for the other, larger radar-bright craters in this study, and equivalent to the mean d/D values Barlow et al. (1999) measured for the general cratered terrain in the Borealis quadrangle. (These values are included in the calculated mean d/D ratio for the radar-bright craters.) We suggest that, proportionately, more material has been lost from these smaller craters than from the larger, flat-floored craters resulting in the larger d/D values. We predict that future measurements of the characteristics of the smaller, bowl-shaped radar-bright craters closer to the poles will
show that a smaller relative amount of the volatiles will be present than in the larger craters.
7. Summary The d/D measurements of 12 radar-bright craters identified near the north pole of Mercury are statistically lower than the d/D measurements of craters on the surrounding cratered terrain. This suggests that significant infilling has occurred in the recent history of these craters, and requires that explanations for the processes that trap volatiles in the radar-bright craters must also include a mechanism that provides a large volume of material to these craters.
Acknowledgements This work was conducted while P.S.C. and S.M.L. were in residence at NASA’s Johnson Space Center under the support of the 2003 NASA/ASEE Summer Faculty Fellowship program. References Barlow, N.G., Allen, R.A., Vilas, F., 1999. Mercurian impact craters: implications for polar ground ice. Icarus 141, 194–204. Cintala, M.J., Mouginis-Mark, P.J., 1980. Martian fresh crater depths: more evidence for subsurface volatiles? Geophys. Res. Lett. 7, 329–332. Harmon, J.K., Slade, M.A., 1992. Radar mapping of Mercury: fulldisk images and polar anomalies. Science 258, 640–643. Harmon, J.K., Slade, M.A., Velez, R.A., Crespo, A., Dryer, M.J., Johnson, J.M., 1994. Radar mapping of Mercury’s polar anomalies. Nature 369, 213–215. Harmon, J.K., Perillat, P.J., Slade, M.A., 2001. High resolution radar imaging of Mercury’s north pole. Icarus 149, 1–15. Killen, R.M., Benkhoff, J., Morgan, T.H., 1997. Mercury’s polar caps and the generation of an OH exosphere. Icarus 125, 195–211. Moses, J.I., Rawlins, K., Zahnle, K., Dones, L., 1999. External sources of water for Mercury’s putative ice deposits. Icarus 137, 197–221. Paige, D.A., Wood, S.E., Vasavada, A.R., 1992. The thermal stability of water ice at the poles of Mercury. Science 258, 643–646. Pike, R.J., 1988. Geomorphology of impact craters on Mercury. In: Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.), Mercury. University of Arizona Press. Slade, M.A., Butler, B.J., Muhleman, D.O., 1992. Mercury radar imaging: evidence for polar ice. Science 258, 635–640. Sprague, A.L., Hunten, D.M., Lodders, K., 1995. Sulfur at Mercury, elemental at the poles and sulfides in the regolith. Icarus 118, 211–215. Squyres, S.W., Carr, M.H., 1986. Geomorphic evidence for the distribution of ground ice on Mars. Science 231, 249–252. Vasavada, A.R., Paige, D.A., Wood, S.E., 1999. Near-surface temperatures on mercury and the moon and the stability of polar ice deposits. Icarus 141, 179–193.
Planetary and Space Science 53 (2005) 1496–1500 www.elsevier.com/locate/pss
How much material do the radar-bright craters at the Mercurian poles contain? Faith Vilasa,,1, Paul S. Cobianb,2, Nadine G. Barlowc, Susan M. Ledererb a
Planetary Astronomy Group, NASA Johnson Space Center/KR, Houston, TX 77058, USA Department of Physics, California State University, 5500 University Parkway, San Bernardino, CA 92407, USA c Department of Physics & Astronomy, Northern Arizona University, NAU Box 6010, Flagstaff, AZ 86011-6010, USA b
Received 15 December 2004; received in revised form 29 June 2005; accepted 7 July 2005 Available online 25 August 2005
Abstract The depth-to-diameter (d/D) ratios were determined for 12 craters located near the Mercurian north pole that were identified by Harmon et al. (2001, Icarus 149) as having strong depolarized radar echos. We find that the mean d/D value of these radar-bright craters is 23 the mean d/D value of the general population of non-radar-bright craters in the surrounding north polar region. Previous studies, however, show no difference between d/D values of Mercurian polar and equatorial crater populations, suggesting that no terrain softening which could modify crater structure exists at the Mercurian poles (Barlow et al., 1999, 194, Icarus 141). Thus, the change in d/D is governed by a change in crater depth, probably due to deposition of material inside the crater. The volume of infilling material, including volatiles, in the radar-bright craters is significantly greater than predicted by proposed mechanisms for the emplacement of either water ice or sulfur. r 2005 Elsevier Ltd. All rights reserved. Keywords: Mercury; Cratering; Surfaces, planets; Radar; Terrestrial planets; Ices
1. Introduction Ground-based radar observations of the north polar region of Mercury were obtained during 1998 and 1999, when the sub-Earth latitude of Mercury favored observations of the radar-bright features near Mercury’s north pole (Harmon et al., 2001). Due to the upgrade of the NAIC Arecibo radar telescope, these radar images have spatial resolution 10 better than the pre-upgrade Arecibo images of Harmon et al. (1994). The observations reveal spatially-resolved radar-bright signals Corresponding author. Tel.: +1 281 483 5056; fax: +1 281 483 1573. E-mail address: [email protected] (F. Vilas). 1 Present address: MMT Observatory, P.O. Box 210065, University of Arizona, Tucson, AZ 85721-0065, USA. 2 Department of Chemical and Materials Engineering, California State Polytechnic University, Pomona, 3801 West Temple Avenue, Pomona, CA 91768, USA.
0032-0633/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2005.07.003
(strong, highly depolarized echoes) across the Mercurian north pole (Harmon et al., 2001). In the north polar region imaged by Mariner 10, these bright radar signals correlate with relatively fresh impact craters (USGS grade 4, some grade 3) ranging in diameter from 90 km to o10 km (Harmon et al., 1994, 2001). The amount of surface area on the crater floors associated with the high-reflectivity signals diminishes as one moves equatorward and the amount of the floor in permanent shadow decreases. Since the bright signals are confined to the permanently-shadowed areas of polar crater floors, the presence of cold-trapped volatiles has been postulated as the origin of these signals. Both water ice and elemental sulfur have been proposed as volatiles that would produce the radar-bright signals (Slade et al., 1992; Harmon and Slade, 1992; Sprague et al., 1995). In an effort to determine whether the radar-bright signals indicate that trapped, buried caps of clean H2O ice could exist under the Mercurian polar regions,
ARTICLE IN PRESS F. Vilas et al. / Planetary and Space Science 53 (2005) 1496–1500
Barlow et al. (1999) tested the terrain near both Mercurian poles for evidence of the rapid terrain softening that would be caused by subsurface H2O. The most sensitive effect of subsurface H2O on impact craters is a decrease in crater depth/diameter (d/D) ratio, coupled with the softening over time of sharp features such as crater rims (e.g., Cintala and Mouginis-Mark, 1980; Squyres and Carr, 1986). Barlow et al. measured d/D ratios of 170 craters having fresh morphological characteristics and found on heavily cratered terrain. These craters are located in surface areas defined by four quadrangles on Mercury: the two polar regions (Borealis (H1) and Bach (H15)) where bright radar signals have been observed, and two equatorial regions (Tolstoj (H8) and Kuiper (H6)) where neither surface nor subsurface ice is expected. Within the experimental errors imposed by the quality of the Mariner 10 data, no evidence for subsurface polar ice deposits was indicated by that statistical study. The rationale and procedures behind this study described above are discussed in Barlow et al. (1999). The study by Barlow et al. was limited to craters having latitudes equatorward of roughly 7851 due to the lighting conditions imposed by location on the planet’s surface: the shadow cast by the wall of a crater located closer to either pole can cover the crater floor completely and extend up the opposing crater wall, rendering an accurate d/D measurement impossible. Barlow et al. were, however, able to measure d/D values for three of the craters with bright radar reflectivities: crater W in the Borealis Quadrangle covering the Mercurian north pole, and craters G and V in the Bach Quadrangle covering the south pole (Harmon et al., 1994). The recent, higher-resolution observations of Harmon et al. (2001) showed that original crater W and many additional craters near the north pole have radar-bright signals corresponding to the shadowed regions of the craters’ interiors. Some of these additional craters lie within the latitude region where we can use shadow measurements from Mariner 10 images, coupled with the solar illumination geometry for those images, to estimate crater depths. These new observations allow us to compare the d/D attributes of specific radar-bright craters with the statistical characteristics of non-radarbright craters in the surrounding areas. Barlow et al. (1999) have demonstrated that large, subsurface polar H2O caps do not exist on Mercury’s surface. Thus, differences in d/D for individual craters known to have radar-bright signals would suggest that the putative interior volatile deposits have affected the expected crater structure. This could provide clues to the volume and depth of the material captured in these craters, potentially leading to information about the origin of the material. To test this idea, we measured the d/D attributes of the craters visible in Mariner 10
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images identified by Harmon et al. (2001) as having bright radar signals.
2. Measurements Shadow length and rim-to-rim diameter measurements of 12 radar-bright complex craters were digitized from four Mariner 10 images (FDS 156, FDS 160, FDS 164, FDS 165, shown in Fig. 1a–d) having spatial resolutions of 1 km/pixel or better in the Borealis Quadrangle. These craters constitute a subset of the radar-bright craters identified by Harmon et al. (2001), and were selected based on the availability of Mariner 10 images having adequate lighting conditions. Shadow length and diameter were measured 10 times for each crater, and an average and error for each crater were calculated from these measurements. Following the method of Clow and Pike (in Pike, 1988), crater depths were determined using the shadow length measurements, approximate sun angles (the angle formed by the sun and a normal to the planet’s surface at a given location), and spacecraft orientation. Sun angle and spacecraft orientation were calculated using information in the scanning equalization digital radiography (SEDR) reports for Mariner 10. d/D ratios were determined for each of the 12 craters. In 8 cases, the craters were imaged twice by Mariner 10. The individual measurements of each crater were compared, and no significant differences between measurements of the radar-bright craters in different Mariner 10 images were observed.
3. Analysis: Two separate crater populations Table 1 contains the d/D results and location information for the individual craters. We compare the mean d/D values for 51 bins of latitude for both the craters measured here (Table 2), and the craters measured in Barlow et al. on cratered terrain in the Borealis quadrangle (Table 3). There is a factor of 1.5 difference in the mean d/D measurements between the radar-bright craters and the general crater population. Since, a priori, we know that the radar-bright craters have at least one different characteristic from the nonradar-bright craters, some consideration of this additional difference seems warranted. We first tested our assumption that the d/D differences are significant. We assumed that both the radarbright craters and the general Borealis Quadrangle background crater population are not necessarily identical in characteristics, but rather represent two different sample populations, and conducted a 2-sample Student’s t-test. The calculated t value is –2.78, which lies below the comparison t value of 1.796. This
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Fig. 1. Individual images of the Mercurian north polar region from Mariner 10 that were used for the depth and shadow height measurements. The measured radar-bright craters are labeled in the images with the letter designation from Harmon et al. (2001). (a) FDS 156, (b) FDS 160, (c) FDS 164, (d) FDS 165. Note the presence of some craters in more than one image.
Table 1 Depth and diameter measurements of 12 mercurian radar-bright craters Crater
F J2 K2 L2 L M2 M P2 P Q V2 W
Diameter (km)
Depth (km)
Latitude (1)
Longitude (1)
d/D ratio
26.470.3 28.570.3 32.470.2 67.970.4 20.770.4 38.970.3 35.270.1 17.670.4 23.570.3 25.970.3 18.670.2 47.070.4
1.270.12 2.170.02 1.670.02 2.770.08 1.070.02 2.470.09 1.370.02 1.670.03 1.470.02 1.470.03 1.970.04 2.870.11
87 83 84 83 84 80 85 83 82 81 80 81
116 147 145 140 47 122 30 80 40 35 59 91
0.05 0.07 0.05 0.04 0.05 0.06 0.04 0.09 0.06 0.05 0.10 0.06
indicates that the radar-bright sample is statistically different from the non-radar-bright Borealis population at the 95% confidence interval (a significance level of 0.05).
Table 2 Average d/D of radar-bright craters (this work) by latitude Quadrangle
Latitude range
Average d/D
Number of craters
Borealis Borealis
801–851N 851–901N
0.0670.02 0.0470.01
10 2
4. Physical state of radar-bright craters We next consider the implications of the population difference for the radar-bright craters. Generally, a change to a lower d/D value from a higher one signals a relaxation in the structure of both the crater walls and floor likely due to subsurface H2O; this results in a larger crater diameter and reduced crater depth, contributing to a lower overall d/D ratio. On Mars, for example, the concentration of subsurface volatiles (polar ice caps) causes a measurable change in crater d/D in the 301–551 latitude zone in both hemispheres (Squyres and Carr, 1986). Barlow et al. (1999) showed, however, that the
ARTICLE IN PRESS F. Vilas et al. / Planetary and Space Science 53 (2005) 1496–1500 Table 3 Average d/D of mercurian north polar craters (Barlow et al., 1999) by latitude Quadrangle
Latitude rangea
Average d/D
Number of craters
Borealis Borealis
751–851N 651–751N
0.0970.03 0.0970.05
11 25
a The range of 751–851 corrects a typographical error in Barlow et al. (1999) where it is wrongly listed as 751–801.
lack of any measurable d/D variation between the majority of craters in polar terrain and those craters in equatorial terrain argues against any substantial subsurface polar ice caps on Mercury. If there is no subsurface volatile cap, what constitutes the root of the observed d/D difference between radar-bright and non-radarbright craters? Expansion of the crater diameter due to relaxation of the crater structure depends largely on the properties of the target material. If changes in diameter are the cause of the observed d/D change, we should observe a difference in the d/D properties between the complex, non-radar-bright crater population near the north pole and the complex crater population near the equator. Statistically, we see no such difference. We have no observational evidence that there are major differences that would cause terrain softening in the target-material properties across the north polar region containing the mix of radar-bright and non-radar-bright craters. Thus, we conclude that the d/D differences for these large, flatfloored craters probably indicate a difference in depth. We demonstrate the significance of this change in depth by considering the mean d/D change on a 30-km diameter crater (roughly the average diameter of the radar-bright craters we examined here). Holding the diameter constant, the depth would change from 2.7 to 1.8 km, a change of 900 m of material in thickness. The volume of material that this would represent is 636 km3. No mechanism currently proposed for an exogenous source of water ice provides this volume of volatile material for Mercury (c.f., Killen et al., 1997; Moses et al., 1999). Sprague et al. (1995) have proposed elemental sulfur as the cause of the radar-bright signals, and demonstrate through modeling that sulfur deposits on the floors of craters should be stable down to 821 latitude for long periods of time in Mercury’s history. The existence of elemental sulfur at the poles is proposed to be a steady state process throughout the planet’s history, and Sprague et al. propose a deposition rate of 10–35 m/ Gyr. This leads to sulfur deposit thicknesses of tens of meters. This is considerably less than the estimated volume of material discussed above, so it does not appear that a large volume of sulfur alone could be the source of the infilling material.
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5. What is the origin of the infilling material? Examination of the results we present must consider the origin of the large total volume of material in the interiors of these craters (either volatile or non-volatile or both), coupled with apparently thin layers of volatile material that cover only the permanently-shaded areas of the crater floors. The source of the material (including volatiles) is likely exogenic, but an endogenic source cannot be ruled out. The event or events that contributed the infilling material are unknown, but must date from the Mansurian period (3.0–3.5 Gyr ago). Thus, the event(s) post-date the Caloris basin impact and the beginning of the emplacement of the Mercurian smooth plains. Harmon et al. (2001) identify craters with radar-bright signatures on smooth plains in Goethe basin and northern Borealis Planitia, indicating the deposition of infilling material after the emplacement of smooth plains. Our measurements include two of these craters (P, Q); no significant d/D difference exists between these craters and other radar bright craters that we measured. There is no requirement for all of the infilling material to be volatile in origin; the only requirement here is for the last event to fill these craters with material containing largely volatiles, since we see the radar-bright signatures suggesting volatiles on the crater floor surfaces. We cannot offer any more insight into the origin of the large quantities of material of any sort in the crater interiors. Harmon et al. (2001) suggest that protective mantling could facilitate an increase in the ice layer, if one or more major impact episodes provided thick, icy deposits, followed by accumulation of a layer of material that insulates these volatiles from sublimation. Building on this idea, We note that Vasavada et al. (1999) proposed that sublimation of an ice/dust mixture could have continued until a crust of dark, non-volatile residue left over from surface evaporation of the ice component self-sealed the ice deposit in the crater. In an effort to understand how large quantities of mixed volatile and non-volatile material could be retained inside a polar crater, we consider the case where volatiles and dust resulting from a comet impact are trapped in a crater near one of Mercury’s poles. Sublimation will then occur across the entire crater floor, although perhaps unevenly between the portions of the crater than are illuminated and those portions that are permanently shadowed. A cap of the dark material contained in the comet collapses and seals the top of the volatiles across the sunlit portion of the crater’s interior. A volatile layer is, however, preserved in the shadowed portion of the crater. The amount of material preserved in the crater’s interior could vary in depth across the crater’s floor (although we are not able to determine if this is the case, since we cannot measure depth of material in the permanently-shadowed part of
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the crater). We suggest that the remnant caps of material covering the surfaces of the volatiles, and exposed in the unshadowed portions of the craters, could represent very primitive, outer solar system material. Assuming that more than one event is required to explain the overall large volume of infilling material present in the radar-bright craters, layers of volatiles with partial dark material caps could exist in the radarbright craters.
6. Mercurian radar-bright smaller craters Harmon et al. (2001) observed bright radar signals in the interiors of several small (diameter o10 km) Mercurian craters. Pike (1988) determined that the diameter breakpoint of 10.3 km separated ‘‘small, simple’’ from ‘‘large, complex’’ craters on the surface of Mercury, based on occurrence of morphological features (e.g., flat vs. bowl-shaped floors, central peaks). Theoretical studies (e.g., Paige et al., 1992) suggest that the internal radiation reflections from simple, bowlshaped craters would cause larger amounts of ice to be lost to sublimation. The large volume of material we observe through the d/D measurements could explain the presence of the radar-bright material in the smaller, bowl-shaped craters: if a much larger volume of volatiles was initially present in these simple craters, the sublimation could still be an ongoing process, although proportionately less of the volatiles would remain in the simple craters than in the large, complex craters. Alternatively, sublimation could have continued until a crust of residue left over from surface evaporation selfsealed the ice deposit in the crater, as proposed by Vasavada et al. (1999). There is support in the d/D measurements for the idea that material could be lost more rapidly from smaller-diameter craters until a layer of residue became thick enough to stop the sublimation: the two smallest craters that we measured in this study had diameters of 17.6 and 18.6 km, and d/D ratios of 0.09 and 0.10, respectively. These are greater than the ratios we measured for the other, larger radar-bright craters in this study, and equivalent to the mean d/D values Barlow et al. (1999) measured for the general cratered terrain in the Borealis quadrangle. (These values are included in the calculated mean d/D ratio for the radar-bright craters.) We suggest that, proportionately, more material has been lost from these smaller craters than from the larger, flat-floored craters resulting in the larger d/D values. We predict that future measurements of the characteristics of the smaller, bowl-shaped radar-bright craters closer to the poles will
show that a smaller relative amount of the volatiles will be present than in the larger craters.
7. Summary The d/D measurements of 12 radar-bright craters identified near the north pole of Mercury are statistically lower than the d/D measurements of craters on the surrounding cratered terrain. This suggests that significant infilling has occurred in the recent history of these craters, and requires that explanations for the processes that trap volatiles in the radar-bright craters must also include a mechanism that provides a large volume of material to these craters.
Acknowledgements This work was conducted while P.S.C. and S.M.L. were in residence at NASA’s Johnson Space Center under the support of the 2003 NASA/ASEE Summer Faculty Fellowship program. References Barlow, N.G., Allen, R.A., Vilas, F., 1999. Mercurian impact craters: implications for polar ground ice. Icarus 141, 194–204. Cintala, M.J., Mouginis-Mark, P.J., 1980. Martian fresh crater depths: more evidence for subsurface volatiles? Geophys. Res. Lett. 7, 329–332. Harmon, J.K., Slade, M.A., 1992. Radar mapping of Mercury: fulldisk images and polar anomalies. Science 258, 640–643. Harmon, J.K., Slade, M.A., Velez, R.A., Crespo, A., Dryer, M.J., Johnson, J.M., 1994. Radar mapping of Mercury’s polar anomalies. Nature 369, 213–215. Harmon, J.K., Perillat, P.J., Slade, M.A., 2001. High resolution radar imaging of Mercury’s north pole. Icarus 149, 1–15. Killen, R.M., Benkhoff, J., Morgan, T.H., 1997. Mercury’s polar caps and the generation of an OH exosphere. Icarus 125, 195–211. Moses, J.I., Rawlins, K., Zahnle, K., Dones, L., 1999. External sources of water for Mercury’s putative ice deposits. Icarus 137, 197–221. Paige, D.A., Wood, S.E., Vasavada, A.R., 1992. The thermal stability of water ice at the poles of Mercury. Science 258, 643–646. Pike, R.J., 1988. Geomorphology of impact craters on Mercury. In: Vilas, F., Chapman, C.R., Matthews, M.S. (Eds.), Mercury. University of Arizona Press. Slade, M.A., Butler, B.J., Muhleman, D.O., 1992. Mercury radar imaging: evidence for polar ice. Science 258, 635–640. Sprague, A.L., Hunten, D.M., Lodders, K., 1995. Sulfur at Mercury, elemental at the poles and sulfides in the regolith. Icarus 118, 211–215. Squyres, S.W., Carr, M.H., 1986. Geomorphic evidence for the distribution of ground ice on Mars. Science 231, 249–252. Vasavada, A.R., Paige, D.A., Wood, S.E., 1999. Near-surface temperatures on mercury and the moon and the stability of polar ice deposits. Icarus 141, 179–193.