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RESEARCH ARTICLE Analysis of MESSENGER high-resolution images 10.1002/2016JE005070 of 's and implications Key Points: for hollow formation • Average depth of 2518 hollows in 552 high-resolution images of Mercury is T. Blewett1, Amanda C. Stadermann2, Hannah C. Susorney3, Carolyn M. Ernst1, Zhiyong Xiao4,5, 24 ± 16 m 1 1 1 6 1 • Hollows enlarge by scarp retreat and Nancy L. Chabot , Brett W. Denevi , Scott L. Murchie , Francis M. McCubbin , Mallory J. Kinczyk , 7 8,9 cease to deepen when a devolatilized Jeffrey J. Gillis-Davis , and Sean C. Solomon lag becomes thick enough to prevent further volatile loss 1Planetary Exploration Group, The Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA, • Carbon could be lost from Mercury's 2Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri, USA, 3Department of Earth and surface via ion sputtering or Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland, USA, 4China University of Geosciences, Wuhan, conversion of graphite to methane by 5 6 proton bombardment China, Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, Norway, NASA Johnson Space Center, Houston, Texas, USA, 7Hawaii Institute of Geophysics and Planetology, University of Hawaii, Honolulu, Hawaii, USA, 8Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York, USA, 9Department of Terrestrial Magnetism, Carnegie Supporting Information: Institution of Washington, Washington, District of Columbia, USA • Supporting Information S1 • Data Set S1 • Data Set S2 Abstract High-resolution images from MESSENGER provide morphological information on the nature and

Correspondence to: origin of Mercury's hollows, small depressions that likely formed when a volatile constituent was lost from the D. T. Blewett, surface. Because graphite may be a component of the low-reflectance material that hosts hollows, we [email protected] suggest that loss of carbon by ion sputtering or conversion to methane by proton irradiation could contribute to hollows formation. Measurements of widespread hollows in 565 images with pixel scales <20 m indicate Citation: that the average depth of hollows is 24 ± 16 m. We propose that hollows cease to increase in depth when a Blewett, D. T., et al. (2016), Analysis of volatile-depleted lag deposit becomes sufficiently thick to protect the underlying surface. The difficulty of MESSENGER high-resolution images of Mercury's hollows and implications for developing a lag on steep topography may account for the common occurrence of hollows on crater central hollow formation, J. Geophys. Res. peaks and walls. Disruption of the lag, e.g., by secondary cratering, could restart growth of hollows in a Planets, 121, 1798–1813, doi:10.1002/ location that had been dormant. Images at extremely high resolution (~3 m/pixel) show that the edges of 2016JE005070. hollows are straight, as expected if the margins formed by scarp retreat. These highest-resolution images

Received 5 MAY 2016 reveal no superposed impact craters, implying that hollows are very young. The width of hollows within rayed Accepted 10 AUG 2016 crater suggests that the maximum time for lateral growth by 1 cm is ~10,000 yr. A process other Accepted article online 16 AUG 2016 than entrainment of dust by gases evolved in a steady-state sublimation-like process is likely required to Published online 30 SEP 2016 explain the high-reflectance haloes that surround many hollows.

1. Introduction The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, the first spacecraft to conduct a protracted investigation of Mercury, revolutionized knowledge of the 's geol- ogy, geochemistry, interior structure, and space environment [e.g., Solomon, 2011]. One of the key results from the mission was the discovery of surface features called hollows [Blewett et al., 2011, 2013]. Hollows are small depressions that likely formed as a result of the loss of a volatile-bearing phase that is present in surface rocks. MESSENGER geochemical remote sensing has shown that Mercury has a relatively high abun- dance of the moderately volatile elements sulfur [Nittler et al., 2011; Weider et al., 2015] and potassium [Peplowski et al., 2012], the volatile elements sodium [Peplowski et al., 2014] and chlorine [Evans et al., 2015], and carbon [Peplowski et al., 2015, 2016; Murchie et al., 2015]. Therefore, hollows are interpreted to be a geomorphological consequence of the planet's high volatile content. Hollows are found most commonly in association with impact craters and basins. At lower spatial resolution, hollows are conspicuous as high-reflectance patches that have a relatively low (shallow) increase in spectral reflectance with wavelength in the visible and near-infrared compared with the global average for Mercury [Robinson et al., 2008; Blewett et al., 2009, 2011, 2013], imparting a bluish hue in color-composite images (Figure 1) and principal-component renderings. Prior work [Blewett et al., 2011, 2013; Xiao et al., 2013; Thomas et al., 2014] indicates that hollows form predominantly in terrains that have reflectances lower than that fl fl ©2016. American Geophysical Union. of the global average: the low-re ectance material (LRM) and low-re ectance blue plains (LBP) [Denevi et al., All Rights Reserved. 2009; Klima et al., 2016]. LRM is exposed in the interior of crater, for instance, as shown in Figure 1.

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Images of this crater with higher spatial resolution (Figure 2) show the character- istic morphology of hollows. They con- sist of shallow, rimless depressions with irregular, rounded outlines. The floors tend to be flat rather than having bowl- or V-shaped bottoms. The depres- sions have high-reflectance interiors and often are surrounded by high- reflectance haloes. Morphological simi- larities with features on icy surfaces (i.e., the south polar cap of Mars, icy satellites) suggest that hollows form in a manner similar to the “sublimation degradation” [Moore et al., 1996, 1999] thought to operate on those bodies. That is, loss of one or more volatile phases causes the remaining material to weaken and crumble, leading to ground collapse and lateral scarp retreat.

Figure 1. Crater de Graft, which is 68 km in diameter and centered near The specific process or processes driv- 22.0°N, 2.0°E. The color composite illustrates the high reflectance and ing the volatile loss on Mercury is not relatively bluish color of the hollows occupying the crater's floor. Hollows known. However, the environment at fl are also found on the central peak, which is composed of low-re ectance the planet's surface offers a number of material (LRM). The ray crossing the image originated at . Images EW1047699599I, EW1047699591G, and EW1047699595F as red- possibilities, including sublimation dri- green-blue; 308 m/pixel. ven by solar heating or by contact with impact melt or volcanic eruptive pro- ducts; destruction by solar ultraviolet flux, solar wind ions, or magnetospheric ions; and heating and vapor- ization by micrometeoroid bombardment. On the basis of the high surface abundance of sulfur, sulfide minerals have been considered as leading candidates for the component subject to loss [Blewett et al., 2011, 2013; Vaughan et al., 2012; Helbert et al., 2013; Bennett et al., 2016]. If this is the case, then the forma- tion of hollows on Mercury could repre- sent a greatly magnified version of the highly surficial sulfur loss proposed to have occurred on Eros in response to space weathering [Nittler et al., 2001; Killen, 2003; Kracher and Sears, 2005; Loeffler et al., 2008]. A spec- tral feature similar to that seen in spec- tra of powders of MgS or CaS has been found in hollows at crater [Vilas et al., 2016]. In this paper we propose an alternative loss mechanism, involving destruction of graphite by proton bombardment, that may contribute to formation of hollows. We also make use of high-spa- Figure 2. Portion of de Graft's interior, seen in targeted monochrome tial-resolution images, some collected image EN0250851946M, 45 m/pixel. Illumination is from the east at an during low-altitude operations late in angle of 77° from the vertical, emphasizing the texture of the hollows: shallow, irregular, and rounded depressions with flat floors. the MESSENGER mission, to obtain

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new information on the morphology of hollows and hence gain additional insights into the nature of hollows and the processes by which they form. In particular, we report the results of a study of the depths of hollows and our interpretation of the findings. In addition, we present estimates of the rates at which hollows are forming, and we describe an analysis of one proposed mechanism for the origin of the high-reflectance haloes that surround many hollows.

2. Possible Formation of Hollows by Loss of Graphite Evidence for a relatively high carbon content in Mercury's surface materials has recently been presented [Peplowski et al., 2015, 2016; Murchie et al., 2015; Bruck Syal et al., 2015]. Petrologic experiments conducted by Vander Kaaden and McCubbin [2015] indicate that because of the low FeO content of Mercury's silicate portion, graphite is the only mineral that would float in melts of mantle composition. Thus, during a hypothe- sized magma ocean Mercury may have had a primary graphite flotation crust, which, although thin, would have been mixed with erupted volcanic materials as a result of later magmatism and extensive impact reworking [Vander Kaaden and McCubbin, 2015; Peplowski et al., 2016]. Graphite is a candidate darkening agent contributing to the generally low reflectance of Mercury [Denevi et al., 2009; Murchie et al., 2015], and in greater abundances graphite may be responsible for the low reflectance of the LRM [Murchie et al., 2015; Peplowski et al., 2016]. As discussed above, destruction of sulfide minerals and subsequent loss of sulfur is considered to be one can- didate process for the formation of hollows. Here we suggest another possibility, involving loss of graphite. Although graphite is highly refractory, bombardment by ions from the solar wind or magnetosphere could cause sputtering of carbon and subsequent loss to space. Also, proton irradiation of graphite-bearing rocks

could convert graphite to methane (CH4) [e.g., Chen et al., 1999, 2001; Kanai et al., 2001] via the reaction þ CðÞgraphite þ 4H ðÞsolar wind ←→CH4gasðÞ: (1) Although at temperatures below 1500 K reactions between graphite and molecular hydrogen are kinetically sluggish (i.e., methane formation rates are in the range 10 16 to 10 12 mol g 1 s 1), graphite exposed to atomic hydrogen, such as that dominating the solar wind, reacts rapidly even at room temperature [Wood and Wise, 1969]. Furthermore, Haasz and Davis [2004] noted that combined irradiation of graphite by protons and oxygen ions creates hydrocarbons, carbon oxides, and water, although the formation of carbon dioxide and water could be prohibited by the highly reducing nature of Mercury's surface materials [McCubbin et al., 2012; Zolotov et al., 2013]. These graphite breakdown products could be a source for the low- and high- reflectance volatile deposits found in permanently shadowed polar craters [e.g., et al., 2013; Chabot et al., 2014], though additional work is needed to evaluate if such a source could produce deposits that match the purity and the abundance of those near Mercury's poles. In addition to destruction by ion bombardment, heating and mixing of graphite with solar-wind-saturated material (as in impact melting) may also lead to production of methane or other species that are readily desorbed or lost. Destruction of gra- phite by these mechanisms might initiate and sustain the growth of hollows via collapse of the remaining matrix, and removal of the darkening agent could account for the high reflectance of hollows.

3. MESSENGER Image Data for Hollows MESSENGER's systematic mapping of Mercury's surface commenced after the spacecraft was inserted into orbit about the planet in 2011. The Mercury Dual Imaging System (MDIS) [Hawkins et al., 2007, 2009] carried out multiple campaigns [Chabot et al., 2016] to map the surface under a variety of lightning conditions with the narrow-angle camera (NAC) and through combinations of narrowband filters on the wide-angle camera (WAC) during the orbital mission that lasted just over four Earth years. Throughout orbital operations, the spacecraft's highly eccentric orbit took it closer to the surface over the northern hemisphere, enabling collec- tion of images at higher spatial resolution of this portion of the planet through targeted observations. In addi- tion, during MESSENGER's final year in orbit, after the completion of multiple global base maps, an emphasis was placed on acquiring higher-resolution images of the many terrain types in Mercury's northern hemisphere. To sample the varied terrains at high resolution, images were acquired with the NAC in a “ride-along” mode when the spacecraft's altitude was lower than 350 km to obtain images of whatever portion of the planet's

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Figure 3. An example measurement of the depth of a hollow from shadow length. The main image (EN1058851700M) shows a small cluster of hollows near 43.0°N, 115.4°E. These hollows are within LRM exposed on the highly degraded rim of a crater ~115 km in diameter that adjoins the basin. The image pixel dimension is 9.1 m, and the solar incidence angle at the center of the image is 51.6°. The arrow points to the shadow area shown enlarged in the inset. The inset image is a screen capture from the ISIS qview tool. The user marks the sunward edge of the shadow to be measured, and the tool automatically draws a line in the anti-sunward direction. The user marks the end of the shadow, and the tool reports the length of the shadow and the height of the object casting it. In this case, the shadow length (yellow line segment) is 69.5 m, and the depth is 55.0 m. Depth measurements were made at five other locations in this image (see supporting information).

surface was sunlit and at nadir, as constrained by the available spacecraft data recorder volume. Some of these NAC ride-along images are of very high spatial resolution and show important details of hollows morphology. Images were processed with the Integrated Software for Imagers and Spectrometers (ISIS) [Anderson et al., 2004], which is maintained by the U.S. Geological Survey's Astrogeology Science Center. Spacecraft pointing information was used to place each image in a map projection. All images in this paper are presented in equirectangular projection with north toward the top. The WAC color composite images shown here were created by displaying an image taken through the 997-nm filter in the red channel, the 749-nm filter in the green channel, and the 433-nm filter in the blue channel.

4. Measurements of the Depths of Hollows Hollows are relatively small features, ranging in size from a few tens of meters to several kilometers in the long- est dimension, although in places many individual hollows have coalesced to form extensive fields. Previous measurements of the depths of hollows were reported by Blewett et al. [2011], Vaughan et al. [2012], and Thomas et al. [2014], but those results were limited by the resolution of images available at the times of those studies. Here we present a large number of additional depth measurements, enabled by high-resolution images acquired during MESSENGER's final year of operations. Our measurements, like those reported previously, are estimates derived from the length of the shadow cast by the edge of an individual hollow. The measurements were made on map-projected images using the interactive shadow-length tool in the ISIS “qview” image display application. As illustrated in Figure 3, the user marks the sunlit edge of the object casting a shadow and the end- point of the shadow in the anti-sunward direction. With knowledge of the image pixel scale and the Sun's ele- vation and azimuth at the time the image was collected, the program then computes the height of the object casting the shadow. In our case, this height corresponds to the depth of the hollow. Hollows appear to have predominantly straight walls and flat floors, as illustrated by the examples in Figure 3 and in section 5 below.

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Figure 4. Map of the locations of hollows found in high-resolution images in this study. The central longitude is 0°E. Blue dots mark the location of each image that contains hollows and for which shadow-length depth measurements were made. Red dots indicate images in which hollows were found but that were not suitable for shadow-length depth measurements.

Therefore, some of the issues associated with the determination from shadow lengths of the depths of simple impact craters (which can have parabolic cross sections) or volcanic pits (which can have conical or cylindrical cross sections) discussed, e.g., by Chappelow and Sharpton [2002], Chappelow [2013], and Lopez et al. [2012], are not of major concern when measuring the depths of hollows. We screened all MDIS images with pixel dimensions <20 m and solar incidence angles <85° for the presence of hollows, a total of more than 51,000 images. Small incidence angles result in very short or no shadows, so some images that contain hollows are not suitable for shadow-length measurements. Hollows were found in 882 images; the average pixel dimension of these 882 images was 13.3 m, with a standard deviation of 4.9 m. Of these, 565 images were suitable for making depth measurements. The map in Figure 4 presents the locations of the two sets of images that contain hollows. The hollows are globally distrib- uted in longitude. The images contain- ing hollows are mostly found at middle latitudes in the northern hemisphere, where MDIS acquired the majority of high-resolution images. Depth measurements were made at 2518 individual locations. The average inci- dence angle was 67.3°, with a standard deviation of 9.4°. We estimate the uncer- tainty in measuring the length of the sha- dow to be ±0.5 pixel on each end, for a total uncertainty (Δl)of1pixel.Thecorre- sponding uncertainty in the depth mea- Δ Figure 5. Contour plot of the estimated uncertainty in the shadow-length surement ( d,inm)dependsonthe depth measurements as a function of image pixel scale and solar image pixel scale (s,m/pixel)andthe incidence angle for an assumed shadow-length uncertainty of 1 pixel. incidence angle (i): Δd = sΔltan(90°–i). Contours of the depth uncertainty are labeled in meters. For example, in Here we ignore the uncertainty in the the yellow band the uncertainty is between 9 and 11 m. The vertical incidence angle and the uncertainty dashed line is at the mean image pixel scale value for the data set, 13 m. fl The horizontal dashed line is at the mean incidence angle, 67°. The value of related to a sloping oor. The contour pixel scale and incidence angle of each image used for depth plot in Figure 5 shows the depth uncer- measurement is marked with a black dot. tainty for combinations of image pixel

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Figure 6. Depictions of the depth measurements made on 2518 hollows locations. (a) Histogram of measured depths, with the data divided into 15 m bins (except for the last bin, 90–120 m). (b) Box and whisker plot shows the mean (vertical line), first and third quartiles (left and right ends of the box, respectively), and the minimum and maximum values in the data set (left and right whiskers, respectively).

scale and incidence angle. As indicated by the dashed lines in the figure, the depth uncertainty is ~ ±5 m for the mean pixel scale and the mean incidence angle of our data set. The mean value of the 2518 depth measurements is 24 m, with a standard deviation of 16 m, though the actual distribution is skewed with a tail that includes a few larger depths. The distribution of the depth data is illustrated in the histogram and box-and-whisker plot in Figure 6. The supporting information provides the file name for each of the 882 images, along with latitude, longitude, incidence angle, and pixel scale, and the 2518 depth measurements. The Mercury Laser Altimeter (MLA) on MESSENGER was capable of resolving surface height differences of < ~1 m [Zuber et al., 2012]. Figure 7 shows the location of footprints along an MLA track that crosses the extensive floor hollows in crater. In this data set, the patch of ground illuminated by the laser was

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~60 m in diameter, with the spots spaced along track at a distance of ~500 m. The spacing of the spots in rela- tion to the lateral size of hollows, imper- fect registration between the MDIS image and the MLA footprints, and topographic variations not associated with hollows limit the ability to deter- mine the depth of specific individual hollows, but the elevation profile allows for an approximate assessment of depths. The elevation difference between adjacent points in the profile ranges from 0.7 to 33 m. The MDIS image in Figure 6 has a pixel scale of 24 m and so was not included in the sur- vey of images with pixel size <20 m described above. The solar illumination angle in this image is 49°, casting short fi Figure 7. MLA elevation data for the floor of Sander crater. Locations of shadows that are somewhat dif cult to laser altimeter footprints are shown as circles, and the elevations relative measure. However, a measurement of to a sphere of radius 2440 km are shown in the plot on the right. Dots are the shadow indicated by the arrow in alternately colored white and yellow to facilitate locating them in the Figure 7 indicates a depth of 41 m. plot on the right. The diameter of the dots is ~120 m, approximately twice Given the incidence angle and pixel the diameter of the area illuminated by each laser range measurement. MLA profile mlascirdr1104050305. The MDIS image is a portion of scale, the depth uncertainty is ±21 m. EN1040458509M centered at ~42.2°N, 154.7°E, 24 m/pixel, with illumina- This 41-m depth is in line with the range tion from the lower right. A shadow-length measurement at the tip of the of MLA footprint elevation differences arrow indicates a hollow depth at that location of 41 ± 21 m. and expectations from our extensive survey of shadow-length measurements from MDIS images. Blewett et al. [2011] made shadow-length measurements of the depths of hollows on the floor of the , reporting a mean depth value of 44 m. Vaughan et al. [2012] found depths of ~30 m for hollows in the floor of Kertesz crater. Thomas et al. [2014] made 108 depth measurements within 27 clusters of hollows distributed around the planet and found a mean depth of 47 m with a standard deviation of 21 m. The mean value determined here, 24 m, is less than that of the prior studies, in line with the expectation that using higher-resolution images permitted measurements to be made on smaller hollows that presumably are shallower than their larger counterparts. Nonetheless, the mean depth from the present work confirms earlier reports that the typical depth of hollows is on the order of several tens of meters. The data on hollows depths provide clues to the factors that control the depths to which hollows grow. Blewett et al. [2013] discussed two causes that could limit depth growth [see Blewett et al., 2013, Figure 20]. First, if the volatile-bearing phase that is susceptible to loss is present in a specific layer near the surface, per- haps because the volatiles had been concentrated by some process, then the depth of individual hollows would be limited by the thickness of that layer. Alternatively, if the volatiles exist as a pervasive constituent throughout the host lithology, then it could be that depth growth is arrested when build-up of remnant volatile-depleted material (i.e., a lag deposit) reaches sufficient thickness to protect the underlying volatile- bearing material from further loss. Hollows form almost exclusively in the LRM and LBP units. The LRM and LBP are globally distributed and of variable thickness [Denevi et al., 2009; Ernst et al., 2010, 2015; Klima et al., 2016], but in general, the units are much thicker than several tens of meters. For example, Ernst et al. [2015] concluded that the LRM beneath the Caloris basin is ~8 km thick. Therefore, the growth of hollows in LRM and LBP is unlikely, in general, to be lim- ited by exhaustion of a “layer” of volatile-bearing material, although there are some situations where volatiles may have been concentrated into a layer, e.g., via differentiation of impact melt [Vaughan et al., 2012] or by volcanic processes [Blewett et al., 2011, 2013; Helbert et al., 2013].

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Figure 8. (a) Color-composite image of the western portion of the basin. Hollows appear as the high-reflectance spots with a bluish cast on and around the inner basin ring. The arrow marks the hollows shown at higher resolution in (b) and Figure 9. Portions of images EW1008569498I, EW1008569494G, and EW1008569492F; 317 m/pixel. The view is centered at ~40.6°N, 257.9°E. (b) Targeted NAC image EN0231906135M (30 m/pixel) of the southwestern portion of the inner ring. The arrow points to the cluster of hollows indicated in (a) and shown at higher resolution in Figure 9.

We suggest instead that the factor that controls the depths of hollows in most cases is the development of an armoring lag deposit. The lag material would provide thermal insulation from solar heating and protection from ultraviolet, micrometeoroid, and ion fluxes. The difficulty of developing a lag on steep topography may account for the common occurrence of hollows on crater central peaks and walls. That is, on steep slopes, mass wasting prevents a lag from developing and exposes fresh material, so loss of volatile compo- nents is continuous. On flat terrain, in contrast, loss only occurs from the active scarps that bound the depres- sions, with loss from the substrate on the flat floor having been shut off by development of the lag deposit.

5. The Highest-Resolution Views of Hollows Hollows are remarkable for their fresh morphological appearance and lack of superposed impact craters [Blewett et al., 2011, 2013], making hollows among the youngest non-impact features on Mercury's surface [see also Xiao et al., 2013; Watters et al., 2015]. Blewett et al. [2013] pointed out locations at the foot of the central peak mountains of the Raditladi basin where down-slope movement of material related to the forma- tion of hollows has partly buried impact craters ~100–200 m diameter. This observation stresses the youth of hollows and suggests that some hollows are actively forming at present. The low-altitude phase of the MESSENGER mission [Solomon et al., 2014] returned images of especially high spatial resolution that permit further assessment of the small-scale morphology of hollows. In this section we discuss images from three locations that are among the highest-resolution and best-quality views of hollows in the entire MESSENGER data set. These images have pixel dimensions of ~3 m. Scarlatti is an impact basin 132 km in diameter and Calorian in age [Kinczyk et al., 2016]. LRM is exposed on the inner ring, and hollows have formed in this material (Figure 8a). Fortunately, views of the hollows at several resolutions are available. Figures 8b and 9 present targeted views at 30 and 3.8 m/pixel, respectively. The image at 30 m/pixel clearly reveals that the bright, bluish spots seen at lower resolution are rimless depres- sions with the typical hollows morphology. At 3.8 m/pixel (Figure 9), the flat floors can be seen, though the area interior to the main cluster is occupied by numerous small hills that are likely to be partially devolatilized remnant material or lumps that happened to have a lower volatile content than the surrounding material. The high-reflectance halo around the main cluster is perceptible even with illumination and viewing

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conditions (60.3° incidence, 89.9° phase) that mute differences in reflectance. Shadow-length depth measurements were made at 16 locations in this image (refer to the supporting information for latitudes, longitudes, and depths). The mean depth is 30 m, the minimum depth is 13 m, and the maximum depth is 55 m. is a basin of 189 km diameter that has been classified as Mansurian in age on the basis of its degradation state [Kinczyk et al., 2016]. As in Scarlatti, Strindberg's inner ring exposes low- reflectance material that hosts hollows (Figure 10a). A serendipitous NAC ride- along image with pixel scale of just 2.7 m includes a small cluster of these hollows (Figure 10b). Much of the scene Figure 9. Targeted NAC image EN1051805374M (3.8 m/pixel) is a close- is hidden in shadow as a consequence up of the cluster of hollows indicated by the arrow in Figures 8a and 8b. of the 75.8° solar incidence angle, but The image center is at 39.9°N, 258.2°E. important details of the hollows can be perceived. In particular, the floors of the hollows are flat and appear to be quite smooth where not interrupted by small hills of remnant mate- rial. The outlines of the hollows are curved, and the profiles of the walls (from the brink to the floor) are straight rather than concave or convex. The combination of a large incidence angle and small pixel scale provides an opportunity to make high-precision measurements of particularly small hollows. Six shadow- length depth measurements made on this image yielded a mean depth of 6 m. The smallest and largest depths measured were 4 and 10 m. These depths are at the low end of the distribution measured in this work (Figure 6). The basin is 196 km in diameter and assigned to the Calorian period [Kinczyk et al., 2016]. Little if anything remains of the basin's inner ring. The eastern wall experienced extensive collapse; some of these materials are of low reflectance (Figure 11a). Scattered hollows are present and are often found on small knobs and hills. A ride-along NAC image at 3.1 m/pixel provides an exceptionally clear view of one of the clusters (Figure 11b). The morphology here is very crisp. The floors are flat and smooth at the scale of the image. The wall profiles are straight. Shadow-length depth measurements were made at 10 locations in this image. The mean depth is 21 m, with minimum and maximum depths of 6 m and 42 m, respectively.

As discussed by Melosh [2011, his section 8.1.2 and Figure 8.6], straight slope profiles are characteristic of for- mation by landsliding (mass wasting). The straight wall profiles evident in the MDIS images for hollows are dissimilar to the lobate (convex upward) shapes determined for the “smooth deposits” within lunar irregular mare patches (IMPs) [Braden et al., 2014]. The Moon's IMPs bear a superficial resemblance to hollows on Mercury, but important contrasts in the morphology, spectral character, and geological setting imply that the two types of features are fundamentally different [Blewett et al., 2013]. Braden et al. [2014] interpreted the lobate margins of IMPs to be indicative of formation by a constructional process (flow of lava), whereas the available evidence suggests that hollows on Mercury form by an erosive process involving loss of volatile-bearing material, collapse, and scarp retreat. The three high-spatial-resolution images in Figures 9, 10b, and 11b are of particular interest because they are capable of revealing small impact craters. None of the hollows shown in Figures 9, 10b, and 11b have craters visible within their interiors, even though craters are present on the surrounding surface. In these three images, the smallest identifiable craters on the surroundings are ~3 to 5 pixels across (~8 to 20 m). The lack of superposed craters of this size range indicates that these hollows must be very young.

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Figure 10. Hollows on the inner ring of Strindberg. (a) Color-composite image shows that hollows have formed in LRM exposed on the inner ring. The arrow points to the cluster of hollows seen in Figure 10b. Portions of images EW0260070966I, EW0260070958G, and EW0260070962F; 103 m/pixel. (b) NAC ride-along image EN1052326136M, centered at 52.6°N, 222.2°E, 2.7 m/pixel, shows the hollows at the arrow in Figure 10a.

6. Constraining the Formation Rate of Hollows Hollows have a fresh appearance and lack superposed small impact craters, indicating that they are among the youngest non-impact features on the planet. An estimate of the minimum rate at which hollows grow can be made if the age of an that hosts hollows can be determined. Blewett et al. [2011] made an estimate of the age of hollows on the floor of Raditladi basin, constrained by the 1-Gyr age of Raditladi obtained by

Figure 11. Hollows in Sholem Aleichem basin. (a) Color composite image consisting of portions of EW0241878939I, EW0241878931G, EW0241878935F; 149 m/pixel. The arrow marks the small cluster of hollows shown in (b). (b) NAC ride-along image EN1051631967M, 3.1 m/pixel, centered at 52.0°N, 272.0°E. The solar incidence angle is 62.9°, with illumination from the lower right.

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Figure 12. Views of the western floor and wall of Balanchine crater, in the same map projection at 28 m/pixel and centered at ~38.6°N, 175.3°E. (a) High-Sun image (40° incidence angle) emphasizes the high-reflectance interiors and haloes of hollows. Portion of targeted NAC image EN1024844936M. (b) Low-Sun image (81° incidence angle) illustrates the texture of hollows, their approximately constant depths, and the characteristic lateral size. The hollows along the edge of the wall (arrows) are ~300 m wide. Portion of targeted NAC image EN1022827014M.

Stromet al. [2008]. If horizontal growth of the hollows occurred at a constant rate and began immediately following basin formation, the typical lateral size of the hollows on the floor of Raditladi suggests that the hollows enlarged at a rate of 1 cm per 70,000 yr. Additional images from later portions of the MESSENGER mission permit us to make similar estimates for hollows hosted by impact structures younger than Raditladi. We analyzed hollows in the rayed crater Balanchine (41 km in diameter) (Figure 12). The size-frequency distribution of Mercury rayed (Kuiperian period) craters >7 km in diameter, along with crater scaling laws and a crater production function, suggests that the base of the Kuiperian is at ~300 Ma [Banks et al., 2016]. As shown in Figure 12b, on the western margin of the floor, hollows appear to have formed by scarp retreat from the crater wall over a distance of ~300 m. Thus, if the hollows began to form at 300 Ma, then the average rate of enlargement was 1 cm per 10,000 Earth years. This rate is a lower limit, because Balanchine may be younger than 300 Myr, because it is not known if the hollows began to grow immediately after the Balanchine-forming impact, because the growth rate may not have been constant, and because the hollows may have grown by radial enlargement rather than from the crater wall toward the center. For context, it has been estimated that kilogram-sized rocks on the surface of the Moon are eroded by micrometeoroid bom- bardment at a rate of ~1 cm per 107 yr [Ashworth, 1977]. Malin [1987] reported average aeolian abrasion rates of different rock types in Antarctica to be between 1 cm per 667 yr and 1 cm per 100 yr. Thus, a process mod- ifying Mercury's terrain much faster than lunar micrometeoroid erosion but perhaps more slowly than terres- trial aeolian abrasion can account for why hollows appear to be much younger than the impact structures in which they are found. The evidence that hollows are young gives rise to a question about the triggering mechanism. Why are not all exposed LRM or LBP surfaces covered with hollows? We envision a scenario in which the growth of a new hollow is initiated when the right combination of chemical and physical conditions exists. These factors may include the presence of a threshold concentration of the volatile-bearing phase and sufficiently high

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temperatures (the latter determined by latitude, longitude, and slope aspect). If ion bombardment (sputter- ing and/or implantation) plays a necessary role in loss of the volatile phase, then especially energetic ion pre- cipitation events [cf. Slavin et al., 2010] could set off growth of a new hollow. In the past, if Mercury's internally generated magnetic field were stronger and better able to stand off the solar wind, hollows may have been less common or absent compared with today. We argue in section 4 that hollows likely cease to grow when a lag deposit of devolatilized material develops to a thickness sufficient to protect the underlying surface. Therefore, disruption of the lag deposit would re-expose volatile-bearing material to the conditions needed for hollows to grow. Small primary impacts and secondary craters, which are especially plentiful on Mercury [Strom et al., 2011; Xiao et al., 2014], provide one means to remove a lag deposit and restart formation of hollows on a surface that had been dormant. Areas of steep topographic slope, on which mass wasting would expose fresh material on a more regular basis and thus inhibit development of a lag deposit, may host hollows that are longer lived than those on flatter areas if the subsurface volatile-bearing lithology is sufficiently abundant. This topographic effect may account for why hollows are common on steep surfaces such as crater and basin walls, central peaks, and peak rings [Blewett et al., 2011, 2013; Thomas et al., 2014].

7. Origin of High-Reflectance Haloes How do the high-reflectance haloes that are associated with hollows form? A previous suggestion [Blewett et al., 2013] was that haloes consist of dust entrained by gases generated in the process of volatile loss (e.g., by sublimation), analogous to the process that takes place to raise dust from the nucleus of a comet. Here we consider the physics of dust lofting to evaluate this hypothesis. In order to estimate the maximum particle size that is capable of being lofted by gases generated by the process of volatile loss, we assess the balance between the weight of the dust particle and the upward lift provided by gas drag on the particle [e.g., Whipple, 1951]. We derive the critical grain radius (above which particles cannot be lofted) as follows. The gravitational force on a spherical particle (its weight) is 3 Fg =(4π/3)ρpa gM, where ρp is the particle's density, a is the particle's radius, and gM is the surface gravitational acceleration on Mercury. The gas-drag force, Fd, on a particle is related to the speed (v)of the particle relative to the gas, the gas density (ρg), the particle's cross-sectional area, and the gas-drag 2 2 coefficient (CD): Fd ≈ (CDρgπa v )/2. At lift-off these two forces balance, Fg = Fd. Equating the two expressions and rearranging for the critical particle radius (acr) above which the gas flux will not provide sufficient 2 force to lift the particle gives (3CDρgv )/(8ρpgM). Recognize that the quantity vρg is the gas flux, m′,in kg/m2/s. Thus, ÀÁ ¼ ′ = ρ acr 3CDm v 8 pgM (2)

3 We assign as constants the typical silicate particle density of ρp = 3000 kg/m and Mercury's surface gravitational acceleration of 3.7 m/s2. We take the gas speed to be the mean thermal speed of the gas 23 molecules, v = √[3kT/(μMH)], where k is the Boltzmann constant (1.38 × 10 J/K), T is the absolute tempera- 27 ture, μ is the mean molecular weight of the gas, and MH is the mass of a hydrogen atom (1.67 × 10 kg). Let T = 700 K (representative of Mercury's maximum dayside temperature). The drag coefficient for a spherical

particle under the conditions considered here is often taken as CD =2 [e.g., Gombosi et al., 1986; Tenishev et al., 2011]. We thus have a relationship between the critical particle size and the mass flux. Figure 13a is a plot

of the gas mass flux for methane (CH4; μ = 16), atomic sulfur (S; μ = 32), and molecular sulfur (S2; μ = 64) as a function of the critical particle size as computed from equation (2). The gas outflow speeds for the assumed

700 K temperature are 1040, 740, and 520 m/s for CH4, S, and S2,respectively. Next, consider the rates of material loss that are implied by the gas fluxes determined above. For the case in which the gas originates from destruction of graphite in surface rocks, the “depth” loss rate of surface mate- rial (d′, in units of m/s) may then be estimated as the gas mass flux (m′) divided by the density of graphite (i.e., we assume that the graphite is totally lost): d' = m′/(2266 kg/m3). For the case of sulfur lost from sulfides, the

sulfur is assumed to escape as S or S2, with the more refractory calcium or magnesium retained on the surface [Helbert et al., 2013]. Thus, we use the bulk density of native sulfur (2000 kg/m3) to estimate the material loss rate: d′ = m′/(2000 kg/m3).

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Figure 13b is a plot of the loss rates (in more easily comprehended units of cm/yr) needed to support lofting of par- ticles of different sizes. Figure 13b also shows the enlargement rate estimated in section 6 for the hollows in Balanchine crater (1 cm per 10,000 yr). The Balanchine hollows have enlarged at a rate that appears to be several orders of magnitude lower than the rate necessary to support gas lofting of even the very tiniest dust particles, those with a radius of 0.01 μm. Note that the particular identity of the gaseous spe- cies (the molecular weight) does not change this conclusion. Therefore, we infer that deposition of dust particles lofted by gases produced in a steady- state sublimation-like process is not likely to have been responsible for formation of the bright haloes that surround Mercury's hollows. A caveat is that the enlargement rate for the Balanchine hollows is a lower limit, as discussed in section 6. Thus, if the hol- lows are substantially younger than the adopted 300 Myr age of Balanchine, and/or if the hollows formed at a rapid, non-constant rate, then it is possible fl ′ Figure 13. (a) Plot of the gas mass ux (m ) needed to lift a dust grain of that very fine dust was entrained by radius a , according to equation (2). The gas species was assumed to cr evolved gases and deposited to form have molecular weights of 16, 32, and 64 (CH4, S, and S2, respectively); other assumptions in the calculation are discussed in section 7. (b) The gas the high-reflectance haloes. mass flux in Figure 13a translated to a linear loss (enlargement) rate, The photometric behavior of the bright under the assumption that the gas is supplied via destruction of graphite or sulfides in the rocks. The horizontal dashed line indicates the loss rate haloes in the basin hollows estimated in section 6 for the hollows in Balanchine crater. contrasts with that of the nearby non- hollows regolith [Blewett et al., 2014]. The difference is consistent with the presence in the bright haloes of material with finer particle sizes. Therefore, dust could be the cause of the high reflectance, although the contribution of other factors that influence scattering properties, such as reflectance and the characteristics of individual particles, cannot be ruled out. Hence, if the haloes are composed of dust, the analysis above suggests that the dust was likely to have been dispersed by some process other than simple gas lift during volatile loss. Conceivable alternatives for dispersal of dust include energetic thermal decomposition of volatiles, build-up and explosive release of gas pressure (perhaps methane) [see also Xiao et al., 2013], or electrostatic dust levitation [e.g., Colwell et al., 2007], although these processes have not been numerically modeled and applied to the formation of Mercury's hollows. Other processes that might contribute to modification of particle sizes or scattering behavior are re-condensation of sublimated material and physical modification or chemical alteration of the surface by re-deposited sublimation products.

8. Discussion and Conclusions Images of Mercury returned by the MESSENGER spacecraft during low-altitude operations offer views of sur- face features with an unprecedented level of detail. In this paper we use high-resolution images to obtain measurements of depths of hollows and to make inferences on their nature and origin. We also consider

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the process driving the loss of volatiles that is responsible for formation of hollows and the origin of the bright haloes. We summarize our conclusions as follows.

1. On the basis of recent evidence that carbon in the form of graphite is likely to be an important com- ponent of Mercury's crust [Peplowski et al., 2015, 2016; Murchie et al., 2015; Vander Kaaden and McCubbin, 2015], we propose a process that could be responsible for the loss of material involved in formation of hollows: loss of carbon caused by ion sputtering of graphite or conversion of the graphite to methane or another gaseous species in response to ion bombardment. This set of processes also has the attraction that partial or total removal of the low-reflectance graphite phase would leave behind higher-reflectance remnant material, thus providing an explanation for the high reflectance of hollows, although consideration of color trends within hollows may point to the volatile phase having a redder spectrum than graphite [Thomas et al., 2016]. The loss of graphite might be in addition to loss of sul- fides [Blewett et al., 2011, 2013; Vaughan et al., 2012; Helbert et al., 2013; Vilas et al., 2016; Bennett et al., 2016]. 2. Shadow-length depth measurements were performed at 2518 hollows locations within 565 high- resolution (pixel size <20 m) images. The mean depth is 24 m, with a standard deviation of 16 m. These findings are consistent with prior reports derived from fewer measurements made from shadow lengths on lower-resolution images. Hollows form predominantly in a unit (the low-reflectance material, LRM) that is generally much thicker than the typical tens-of-meters depth of hollows. Therefore, we interpret the approximately constant depth to which hollows grow as indicating that depth is generally not controlled by the thickness of a volatile-bearing layer. Instead, we suggest that volatile-depleted remnant material builds up within the depressions as hollows enlarge. When this lag deposit is sufficiently thick to shield the subjacent material from further loss, hollows cease to increase in depth [Blewett et al., 2013]. 3. If development of a lag deposit slows and stops the growth of hollows, then disruption of the lag deposit could expose volatile-bearing material to the conditions needed to re-start growth. Small primary or sec- ondary cratering events are possible triggers. The common occurrence of hollows on steep topography (e.g., crater walls and central peaks) can be explained by the difficulty of developing a lag deposit in the presence of mass wasting. 4. MESSENGER returned a small number of images of hollows at very high spatial resolution (~3 m/pixel). These high-resolution images reveal that the walls of hollows (i.e., the scarps that bound the flat- floored depressions) have straight profiles from brink to floor. Straight wall profiles are expected from for- mation by scarp retreat (an erosional process). Straight profiles differ from the lobate (convex upward) margins of lunar irregular mare patches, which have been compared with hollows but are likely to be con- structional in origin. We conclude that hollows form in a manner analogous to the “sublimation degrada- tion” process operating on some icy surfaces [Moore et al., 1996, 1999]. 5. The highest-resolution images show no impact craters within the hollows. The lack of craters > ~8–20 m in diameter indicates that the hollows must be very young relative to the rest of Mercury's surface. 6. The occurrence of some hollows within rayed impact craters also implies that hollows are quite young and allows an estimate of their youth. We estimated the growth rate of hollows in the rayed crater Balanchine. We take Balanchine's age to be the base of the Kuiperian (300 Ma). If the hollows on Balanchine's floor began enlargement at a constant rate to their present horizontal size of ~300 m immediately after the crater formed, then the growth rate would have been 1 cm per 10,000 yr. This estimate is a lower limit, because it is possible that the hollows were not initiated immediately after the impact event, Balanchine could have formed more recently than 300 Ma, or growth may not have been at a con- stant rate. 7. Analysis of the physics of comet-style dust lofting indicates that lift from gases evolved during the process of volatile loss is not expected to be sufficiently vigorous to entrain dust grains on the surface of Mercury. Therefore, the bright haloes that are found around many hollows are likely to have formed by some means other than deposition of dust lofted by gases produced in a steady-state sublimation-like process. Dust generated by hollow-forming activity could be dispersed into haloes by electrostatic levitation, by explosive release of gas, or by energetic decomposition of volatiles, though such processes remain to be modeled in detail for Mercury. Formation of the bright haloes by other means such as re-condensation of sublimated material, or by physical modification or chemical alteration of the surface by re-deposited sublimation products, also remain possibilities.

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