Mercury Laser Altimeter Research Articles

Precise Orbit Determination of the MESSENGER Spacecraft

The NASA MESSENGER mission explored Mercury for more than four years to investigate the properties of the planet. To safely operate in the harsh conditions around Mercury, the spacecraft was in a highly eccentric orbit with a low periapsis altitude. The radiation environment had a strong impact on the spacecraft orbit evolution because of the proximity of Mercury to the Sun. A detailed modeling of the nonconservative forces is then a key factor to enhance the precise orbit determination of the spacecraft. We present here refined models of the nonconservative forces, including thermal reradiation effects, that enabled significant improvements in the trajectory reconstruction. A crossover analysis based on the Mercury Laser Altimeter (MLA) data was carried out to cross-check the accuracy of the orbit determination results. The trajectories retrieved by using the refined spacecraft dynamical model provide reduced height misfit at crossover points, indicating a high-quality reconstruction. Our new solutions of the spacecraft orbits are then archived to be used as auxiliary information for the data analysis of other MESSENGER instruments.

Temperature-related Variations of 1064 nm Surface Reflectance on Mercury: Implications for Space Weathering

It has been predicted that high equatorial temperatures on Mercury could promote thermal annealing by Ostwald ripening, where nanophase metal particles (a product of space weathering) coalesce and grow into larger, microphase particles, resulting in lower albedo. Here, we test this prediction by studying the correlation between albedo and temperature in 1° spatial bins using newly recalibrated 1064 nm reflectance data acquired by the Mercury Laser Altimeter (MLA), low incidence angle data from the Mercury Dual Imaging System (MDIS), and newly modeled maximum surface temperatures (MSTs). Accounting for local geology and latitude, we compare the reflectance values of surfaces with MSTs >675 K (where Ostwald ripening is predicted to be most effective) to surfaces with MSTs <473 K (where ripening is predicted not to be effective). Smooth plain surfaces >675 K are 10% and 12% darker than surfaces <473 K in MLA and MDIS data, respectively, and nonsmooth plain surfaces >675 K are 8% and 7% darker than surfaces <473 K. However, open questions remain regarding the causation of this darkening; statistical tests cannot distinguish whether the reflectance differences are systematic or the result of compositional variations that happen to correlate with MST. Along Mercury’s thermal longitudes, we find that reflectance is typically lower along hot poles than along the 90°E cold pole in the low-to-midlatitudes, especially in the smooth plains, consistent with previous work identifying a decrease in optical maturity along the 90°E cold pole. Longitudinal reflectance variations correlate with temperature variations, rather than variations in micrometeoroid or solar wind fluxes.

Investigating the Stability and Distribution of Surface Ice in Mercury’s Northernmost Craters

Observations made by Earth-based radar telescopes and the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft provided compelling evidence for water ice in Mercury's polar craters. In our investigation, we constructed higher-resolution (125 m pixel−1) digital elevation models (DEMs) for four of the largest northernmost craters, Kandinsky, Tolkien, Chesterton, and Tryggvadóttir. The DEMs were leveraged to model solar illumination and the thermal environment, products that were used to identify permanently shadowed regions and simulate surface temperatures. From these models, we predicted the regions of surface stability for ice and volatile organic compounds. These predictions were then compared against the Arecibo radar, Mercury Laser Altimeter (MLA), and Mercury Dual Imaging System (MDIS) data. Our radar analysis shows that areas of high radar backscatter are correlated with areas predicted to host surface ice. Additionally, we identify radar backscatter heterogeneities within the deposits that could be associated with variations in ice purity, mantling of the ice, or ice abundances. The MDIS analysis did not reveal conclusive evidence for ice or volatiles at the surface, while MLA results support the presence of water ice at the surface in these craters. However, evidence for boundaries between the surface ice and low-reflectance volatile organic compounds, as suggested could be present by our models, was inconclusive owing to the limited MESSENGER data in these regions. BepiColombo’s upcoming orbital mission at Mercury has the opportunity to obtain new measurements of these high-latitude craters and test our predictions for the distribution of surface volatiles in these environments.

Modeling the thermal environment of Mercury’s north pole using MLA. Implications for locations of water ice

Radar observations made by the Arecibo Observatory have revealed radar-bright deposits in the north polar region of Mercury which are consistent with water ice. These deposits are generally associated with floors of deep craters in permanent shadow. In this paper, we revisit topographic data, illumination, and associated thermal conditions of the north polar region of Mercury to investigate these radar-bright deposits. We used Mercury Laser Altimeter (MLA) data from the MESSENGER mission to derive a dedicated, adjusted high-resolution Digital Terrain Model (DTM) of the north polar region at 250 m/pixel. Based on this DTM, illumination conditions and temperature were derived for the first time on the fully available MLA dataset. Generally, craters equatorward of 86 ∘N are found to be too hot to contain surface water ice due to effective self-heating. For those warm craters that do contain radar-bright deposits, temperatures within the upper 10–20 cm of regolith are cold enough for water ice to be stable. Using our improved DTM, we confirm previously MLA-modeled illumination conditions in Prokofiev crater and argue that radar-bright patches outside the PSR in the area may be explained by icy deposits in cold and shallow (within 5 cm) sub-surface layers. In the radar observations we find a strong, unphysical correlation between radar-bright pixels and the number of radar observations per pixel. Consequently, several PSRs which were classified as free of radar-bright material (mainly at longitudes 120–340 ∘E) are most likely subject to a bias in the averaging process in deriving the composite radar image and might contain water ice after all.

Surface Roughness Variation Across Polar Ice Deposit Boundaries on Mercury

AbstractWe analyze the surface roughness of five north polar craters on Mercury using 125 m/pixel Mercury Laser Altimeter data. Each crater hosts a polar deposit (PD) with a low‐reflectance surface. The PDs have distinct geologic contacts, enabling an analysis of roughness across deposit boundaries onto ice‐free portions of the host craters' floors. The low‐reflectance surfaces in Angelou, Despréz, and Ensor collocate with radar‐bright signatures consistent with the presence of several‐meters‐thick water‐ice deposits beneath their low‐reflectance surfaces. Subdued roughness in these three craters is consistent with the superposition of several meters of ice. The difference between roughness on and off their low‐reflectance surfaces is within one standard deviation (SD) of the results, but is found to be statistically significant, as indicated by K‐S and Mann‐U Whitney tests, and meaningful, as indicated by Cohen's d tests. There is no meaningful difference in Jiménez and Josetsu, two craters that lack strong radar‐bright signatures, consistent with the hypothesis that they do not host substantial water ice beneath their low‐reflectance surfaces and thus not all of Mercury's available cold traps are occupied by water ice. Analyzing the roughness of polar craters may provide insight into the presence/absence of thick volatile deposits, but higher‐resolution topography would be helpful given the high SD associated with roughness. Finally, we identify subdued roughness along three PDs that may be related to thick lag deposit margins or enhanced diffusive mixing along deposit edges. Roughness may provide new insight into surface characteristics of the ices and inform surface evolution models.

Aspects of thermal modeling using digital terrain models

Context. Our thermal model is adapted and extended in this study. Specifically the aspect of handling indirect radiation, the solar limb darkening effect, and depth profiles are addressed. Aims. Our goal is to improve the existing thermal model to handle terrain scattering and re-radiation in an adaptive way. In addition, we aim to change previously fixed and manually chosen discretization of the solar limb darkening effect and depth profile to be adaptive and applicable for various planets and purposes. Methods. The temperature was modeled based on digital terrain models (DTMs) using data of the Mercury Laser Altimeter (MLA). New implementations to handle terrain scattering and re-radiation were introduced using level-of-detail techniques. The solar disk was discretized into a variable number of rings and the depth profile was introduced as an exponential function for which the number of nodes and the maximum depth can be chosen. Results. We present results for the ideal window size and degree of level-of-detail for thermal studies of the Hermean north pole. Further we show that the previous discretization of the solar limb darkening effect proved insufficient for Mercury, and we updated the implementation accordingly. Similarly we improved the implementation for the depth profile. For the first time, we derived depth-to-ice, as well as average and maximum temperature maps based on thermal modeling of the complete north polar MLA DTM.

High-Resolution Regional Digital Elevation Models and Derived Products from MESSENGER MDIS Images

The Mercury Dual Imaging System (MDIS) on the Mercury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has provided global images of Mercury’s surface. A subset of off-nadir observations acquired at different times resulted in near-global stereo coverage and enabled the creation of local area digital elevation models (DEMs). We derived fifty-seven DEMs covering nine sites of scientific interest and tied each to a geodetic reference derived from Mercury Laser Altimeter (MLA) profiles. DEMs created as part of this study have pixel scales ranging from 78 m/px to 500 m/px, and have vertical precisions less than the DEM pixel scale. These DEMs allow detailed characterizations of key Mercurian features. We present a preliminary examination of small features called “hollows” in three DEM sites. Depth measurements from the new DEMs are consistent with previous shadow and stereo measurements.

Deriving Mercury Geodetic Parameters With Altimetric Crossovers From the Mercury Laser Altimeter (MLA)

AbstractBased on the previous applications of laser altimetry to planetary geodesy at GSFC, we use the recently developed PyXover software package to analyze altimetric crossovers from the Mercury Laser Altimeter (MLA). Using PyXover, we place new constraints on Mercury's geodetic parameters via least squares minimization of crossover discrepancies. We simultaneously solve for orbital corrections for each MLA ground track, for the geodetic parameters of the International Astronomical Union‐recommended orientation model for Mercury (pole right‐ascension and declination coordinates, prime meridian rotation rate, and librations), and for the Mercury's Love number h2. We calibrate the formal errors of our solution based on closed‐loop simulations and on the level of robustness against a priori values, data selection, and parametrization. Our solution of the Mercury's rotational parameters is consistent with published values. In particular, our new estimate for the orientation of the pole places Mercury in a Cassini state, with an obliquity ϵ = 2.031 ± 0.03 arcmin compatible with previous “surface” related measurements. Moreover, we provide a first data‐based estimate of the Love number h2 = 1.55 ± 0.65. The latter is consistent with expectations from models of Mercury's interior, although its precision does not enable their refinement.

Processing of laser altimeter time-of-flight measurements to geodetic coordinates

Laser altimeters are commonly used in planetary research for their high geodetic accuracy. A key procedure in processing of laser altimeter data is the geolocation. In this process, the time-of-flight measurements are converted to coordinates of laser pulse footprints on the surface of the target body. Here, we present a consistent and systematic formulation of three commonly used geolocation models with increasing complexity: static model, spacecraft motion model, pointing aberration model and special relativity model. We show that for small velocities of the spacecraft relative to the target the special relativity model can be reduced to the pointing aberration model without significant loss in the geolocation accuracy. We then discuss the respective accuracies of the proposed models and apply them to time-of-flight measurements from the Mars Orbiter Laser Altimeter (MOLA) onboard the Mars Global Surveyor (MGS) spacecraft and the Mercury Laser Altimeter (MLA) onboard the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). While, the archived datasets had not considered the effect of pointing aberration, we demonstrate that a correction due to pointing aberration makes insignificant improvements of 4–5 m laterally and up to ± 3 cm radially for MOLA profiles, these figures enormously increase to up to about 150 m laterally and ± 25 m radially when applied to the MLA orbital profiles.

New Illumination and Temperature Constraints of Mercury's Volatile Polar Deposits.

Images from the Mercury Dual Imaging System (MDIS) aboard the MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission reveal low-reflectance polar deposits that are interpreted to be lag deposits of organic-rich, volatile material. Interpretation of these highest-resolution images of Mercury’s polar deposits has been limited by the available topography models, so local high-resolution (125 m pixel−1) digital elevation models (DEMs) were made using a combination of data from the Mercury Laser Altimeter (MLA) and from shape-from-shading techniques using MDIS images. Local DEMs were made for eight of Mercury’s north polar craters; these DEMs were then used to create high-resolution simulated image, illumination, and thermal models. The simulated images reveal that the pixel brightness variations imaged within Mercury’s low-reflectance deposits are consistent with scattered light reflecting off of topography and do not need to be explained by volatile compositional differences as previously suggested. The illumination and thermal models show that these low-reflectance polar deposits extend beyond the permanently shadowed region, more than 1.0 km in some locations, and correspond to a maximum surface temperature of greater than 250 K but less than 350 K. The low-reflectance boundaries of all eight polar deposits studied here show a close correspondence with the surface stability boundary of coronene (C24H12). While coronene should only be viewed as a proxy for the myriad volatile compounds that may exist in Mercury’s polar deposits, coronene’s surface stability boundary supports the idea that Mercury’s low-reflectance polar deposits are composed of macromolecular organic compounds, consistent with the hypotheses of exogenous transport and in situ production.

Thick ice deposits in shallow simple craters on the Moon and Mercury

Permanently shadowed regions near the poles of Mercury and the Moon may cold-trap water ice for geologic time periods. In past studies, thick ice deposits have been detected on Mercury, but not on the Moon, despite their similar thermal environments. Here we report evidence for thick ice deposits inside permanently shadowed simple craters on both Mercury and the Moon. We measure the depth/diameter ratio of approximately 2,000 simple craters near the north pole of Mercury using Mercury Laser Altimeter data. We find that these craters become distinctly shallower at higher latitudes, where ice is known to have accumulated on their floors. This shallowing corresponds to a maximum infill of around 50 m, consistent with previous estimates. A parallel investigation of approximately 12,000 lunar craters using Lunar Reconnaissance Orbiter data reveals a similar morphological trend near the south pole of the Moon, which we conclude is also due to the presence of thick ice deposits. We find that previously detected surface ice deposits in the south polar region of the Moon are spatially correlated with shallow craters, indicating that the surface ice may be exhumed or linked to the subsurface via diffusion. The family of lunar craters that we identify are promising targets for future missions, and may also help resolve the apparent discrepancy between the abundance of frozen volatiles on Mercury and the Moon. Ice deposits up to around 50 m thick infill some craters near the Moon’s south pole and Mercury’s north pole, as inferred from the poleward shallowing of simple craters.

The thickness of radar-bright deposits in Mercury's northern hemisphere from individual Mercury Laser Altimeter tracks

The discovery of Mercury's radar-bright deposits has expanded our understanding of volatiles in the solar system. Key to deciphering the history and origin of the radar-bright deposits is an estimate of the volume of radar-bright material that in turn requires a measure of the average thickness of the deposits. In this study we investigate changes in topography across radar-bright deposits hosted in flat-floored, complex craters using individual edited Mercury Laser Altimeter (MLA) tracks. We compare the difference in heights of radar-bright regions and non-radar-bright regions of the crater floor and the difference of similarly sized and located regions in non-radar-bright craters and show that the two populations cannot be distinguished. The similarity of topography in these two sets of craters allows an upper limit of 15 m to be placed on the thickness of the radar-bright deposits.

Constraining the thickness of polar ice deposits on Mercury using the Mercury Laser Altimeter and small craters in permanently shadowed regions

Radar-bright deposits at the poles of Mercury are located in permanently shadowed regions, which provide thermally stable environments for hosting and retaining water ice on the surface or in the near subsurface for geologic timescales. While the areal distribution of these radar-bright deposits is well characterized, their thickness, and thus their total mass and volume, remain poorly constrained. Here we derive thickness estimates for selected water-ice deposits using small, simple craters visible within the permanently shadowed, radar-bright deposits. We examine two endmember scenarios: in Case I, these craters predate the emplacement of the ice, and in Case II, these craters postdate the emplacement of the ice. In Case I, we find the difference between estimated depths of the original unfilled craters and the measured depths of the craters to find the estimated infill of material. The average estimated infilled material for 9 craters assumed to be overlain with water ice is ∼41−14+30 m, where 1−σ standard error of the mean is reported as uncertainty. Reported uncertainties are for statistical errors only. Additional systematic uncertainty may stem from georeferencing the images and topographic datasets, from the radial accuracy of the altimeter measurements, or from assumptions in our models including (1) ice is flat in the bowl-shaped crater and (2) there is negligible ice at the crater rims. In Case II, we derive crater excavation depths to investigate the thickness of the ice layer that may have been penetrated by the impact. While the absence of excavated regolith associated with the small craters observed suggests that impacts generally do not penetrate through the ice deposit, the spatial resolution and complex illumination geometry of images may limit the observations. Therefore, it is not possible to conclude whether the small craters in this study penetrate through the ice deposit, and thus Case II does not provide a constraint on the ice thickness. For Mercury's polar deposits, we argue that Case I of the small craters predating the emplacement of the ice deposits is more likely, given other geologic evidence that suggests that these ice deposits are relatively young. Using the ice thickness estimates from Case I to calculate the total amount of water ice currently contained in Mercury's polar deposits results in a value of ∼1014–1015 kg. This is equivalent to ∼100–1000 km3 ice in volume. This volume of water ice is consistent with delivery via micrometeorite bombardment, Jupiter-family comets, or potentially a single impactor.

New evidence for surface water ice in small‐scale cold traps and in three large craters at the north polar region of Mercury from the Mercury Laser Altimeter

AbstractThe Mercury Laser Altimeter (MLA) measured surface reflectance, rs, at 1064 nm. On Mercury, most water‐ice deposits have anomalously low rs values indicative of an insulating layer beneath which ice is buried. Previous detections of surface water ice (without an insulating layer) were limited to seven possible craters. Here we map rs in three additional permanently shadowed craters that host radar‐bright deposits. Each crater has a mean rs value &gt;0.3, suggesting that water ice is exposed at the surface without an overlying insulating layer. We also identify small‐scale cold traps (&lt;5 km in diameter) where rs &gt;0.3 and permanent shadows have biannual maximum surface temperatures &lt;100 K. We suggest that a substantial amount of Mercury's water ice is not confined to large craters but exists within microcold traps, within rough patches and intercrater terrain.

The surface roughness of Mercury from the Mercury Laser Altimeter: Investigating the effects of volcanism, tectonism, and impact cratering

AbstractSurface roughness is a statistical measure of change in surface height over a given spatial horizontal scale after the effect of broad‐scale slope has been removed and can be used to understand how geologic processes produce and modify a planet's topographic character at different scales. The statistical measure of surface roughness employed in this study of Mercury was the root‐mean‐square deviation and was calculated from 45 to 90°N at horizontal baselines of 0.5–250 km with detrended topographic data from individual Mercury Laser Altimeter tracks. As seen in previous studies, the surface roughness of Mercury has a bimodal spatial distribution, with the cratered terrain (dominated by the intercrater plains) possessing higher‐surface roughness than the smooth plains. The measured surface roughness for both geologic units is controlled by a trade‐off between impact craters generating higher‐surface roughness values and flood‐mode volcanism decreasing surface roughness. The topography of the two terrain types has self‐affine‐like behavior at baselines from 0.5 to 1.5 km; the smooth plains collectively have a Hurst exponent of 0.88 ± 0.01, whereas the cratered terrains have a Hurst exponent of 0.95 ± 0.01. Subtle variations in the surface roughness of the smooth plains can be attributed to differences in regional differences in the spatial density of tectonic landforms. The northern rise, a 1000 km wide region of elevated topography centered at 65°N, 40°E, is not distinguishable in surface roughness measurements over baselines of 0.5–250 km.

Toward high-resolution global topography of Mercury from MESSENGER orbital stereo imaging: A prototype model for the H6 (Kuiper) quadrangle

We selected approximately 10,500 narrow-angle camera (NAC) and wide-angle camera (WAC) images of Mercury acquired from orbit by MESSENGER's Mercury Dual Imaging System (MDIS) with an average resolution of 150m/pixel to compute a digital terrain model (DTM) for the H6 (Kuiper) quadrangle, which extends from 22.5°S to 22.5°N and from 288.0°E to 360.0°E. From the images, we identified about 21,100 stereo image combinations consisting of at least three images each. We applied sparse multi-image matching to derive approximately 250,000 tie-points representing 50,000 ground points. We used the tie-points to carry out a photogrammetric block adjustment, which improves the image pointing and the accuracy of the ground point positions in three dimensions from about 850m to approximately 55m. We then applied high-density (pixel-by-pixel) multi-image matching to derive about 45 billion tie-points. Benefitting from improved image pointing data achieved through photogrammetric block adjustment, we computed about 6.3 billion surface points. By interpolation, we generated a DTM with a lateral spacing of 221.7m/pixel (192 pixels per degree) and a vertical accuracy of about 30m. The comparison of the DTM with Mercury Laser Altimeter (MLA) profiles obtained over four years of MESSENGER orbital operations reveals that the DTM is geometrically very rigid. It may be used as a reference to identify MLA outliers (e.g., when MLA operated at its ranging limit) or to map offsets of laser altimeter tracks, presumably caused by residual spacecraft orbit and attitude errors. After the relevant outlier removals and corrections, MLA profiles show excellent agreement with topographic profiles from H6, with a root mean square height difference of only 88m.

How thick are Mercury’s polar water ice deposits?

An estimate is made of the thickness of the radar-bright deposits in craters near to Mercury’s north pole. To construct an objective set of craters for this measurement, an automated crater finding algorithm is developed and applied to a digital elevation model based on data from the Mercury Laser Altimeter onboard the MESSENGER spacecraft. This produces a catalogue of 663 craters with diameters exceeding 4 km, northwards of latitude +55∘. A subset of 12 larger, well-sampled and fresh polar craters are selected to search for correlations between topography and radar same-sense backscatter cross-section. It is found that the typical excess height associated with the radar-bright regions within these fresh polar craters is (50 ± 35) m. This puts an approximate upper limit on the total polar water ice deposits on Mercury of ∼ 3 × 1015 kg.

Ames stereo pipeline-derived digital terrain models of Mercury from MESSENGER stereo imaging

In this study, 96 digital terrain models (DTMs) of Mercury were created using the Ames Stereo Pipeline, using 1456 pairs of stereo images from the Mercury Dual Imaging System instrument on MESSENGER. Although these DTMs cover only ~1% of the surface of Mercury, they enable three-dimensional characterization of landforms at horizontal resolutions of ~50–250m/pixel and vertical accuracy of tens of meters. This is valuable in regions where the more precise measurements from the Mercury Laser Altimeter (MLA) are sparse. MLA measurements nonetheless provide an important geodetic framework for the derived stereo products. These DTMs, which are publicly released in conjunction with this paper, reveal topography of features at relatively small scales, including craters, graben, hollows, pits, scarps, and wrinkle ridges. Measurements from these data indicate that: (1) hollows have a median depth of ~32m, in basic agreement with earlier shadow measurement, (2) some of the deep pits (up to ~4km deep) that are interpreted to form via volcanic processes on Mercury have surrounding rims or rises, but others do not, and (3) some pits have two or more distinct, low-lying interior minima that could represent multiple vents.

Comparison of areas in shadow from imaging and altimetry in the north polar region of Mercury and implications for polar ice deposits

Earth-based radar observations and results from the MESSENGER mission have provided strong evidence that permanently shadowed regions near Mercury's poles host deposits of water ice. MESSENGER's complete orbital image and topographic datasets enable Mercury's surface to be observed and modeled under an extensive range of illumination conditions. The shadowed regions of Mercury's north polar region from 65°N to 90°N were mapped by analyzing Mercury Dual Imaging System (MDIS) images and by modeling illumination with Mercury Laser Altimeter (MLA) topographic data. The two independent methods produced strong agreement in identifying shadowed areas. All large radar-bright deposits, those hosted within impact craters ≥6km in diameter, collocate with regions of shadow identified by both methods. However, only ∼46% of the persistently shadowed areas determined from images and ∼43% of the permanently shadowed areas derived from altimetry host radar-bright materials. Some sizable regions of shadow that do not host radar-bright deposits experience thermal conditions similar to those that do. The shadowed craters that lack radar-bright materials show a relation with longitude that is not related to the thermal environment, suggesting that the Earth-based radar observations of these locations may have been limited by viewing geometry, but it is also possible that water ice in these locations is insulated by anomalously thick lag deposits or that these shadowed regions do not host water ice.

Topographic roughness of the northern high latitudes of Mercury from MESSENGER Laser Altimeter data

AbstractWe investigated topographic roughness for the northern hemisphere (&gt;45°N) of Mercury using high‐resolution topography data acquired by the Mercury Laser Altimeter (MLA) on board the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. Our results show that there are distinct differences in the bidirectional slope and root‐mean‐square (RMS) height among smooth plains (SP), intercrater plains (ICP), and heavily cratered terrain (HCT), and that the ratios of the bidirectional slope and RMS height among the three geologic units are both about 1:2:2.4. Most of Mercury's surface exhibits fractal‐like behavior on the basis of the linearity in the deviograms, with median Hurst exponents of 0.66, 0.80, and 0.81 for SP, ICP, and HCT, respectively. The median differential slope map shows that smooth plains are smooth at kilometer scale and become rough at hectometer scale, but they are always rougher than lunar maria at the scales studied. In contrast, intercrater plains and heavily cratered terrain are rough at kilometer scale and smooth at hectometer scale, and they are rougher than lunar highlands at scale &lt;∼2 km but smoother at &gt;∼2 km. We suggest that these scale‐dependent roughness characteristics are mainly caused by the difference in density and shape of impact craters between Mercury and the Moon.