New Findings from MESSENGERS’s Low-Altitude Campaign

Figure 1.1 Evolution of the altitude at closest approach for MESSENGER's thrice-daily orbits of Mercury, from the completion of the most recent orbit-correction maneuver (OCM-12) on January 21 until the planned end of mission on April 30. Five additional OCMs are scheduled between now and late April.

Figure 2.1 Sander, an impact crater about 50 km (31 miles) in diameter, has unusual high-reflectance patches on its floor (lower left image). When the MESSENGER spacecraft entered orbit around Mercury, the bright patches were revealed to consist of irregularly shaped shallow depressions that have been named "hollows." The yellow box in the lower image covers the area of the higher-resolution main image. It is thought that hollows form when some component within the rocks is lost because of exposure to the harsh environment at Mercury's surface.

Figure 2.2a-b Scarlatti impact basin is shown in the top image. The horizontal distance from the left corner of the image to the right corner is about 216 km (134 miles). Small bright, bluish spots are seen in the lower portion of the image. The orange box shows the area of the high-resolution image in Figs. 2.3a and b. The bright speck in the orange box consists of a cluster of hollows.

Figure 2.3a This high-resolution image shows a close-up the area of the orange box in Fig. 2.2b. Small impact craters are found all around the hollows, but few if any small impacts are seen on the floor of the hollows. These relations indicate that the hollows must have formed in geologically recent times. The dimensions of the bottom image are about 4.5 km (2.8 miles) by 3.8 km (2.4 miles).

Figure 2.3b This is the same high-resolution image of the hollows in Scarlatti basin. The depths of the hollows can be calculated from the lengths of shadows and knowledge of the angle of the Sun above the horizon. Locations of several such measurements of hollow depths are shown on the image. The hollows here are a few tens of meters deep (1 meter is approximately 1.1 yards), similar to depths of hollows measured elsewhere on Mercury. Information on the depths of hollows gives clues to the composition of the material that is being lost and will help us to understand the process(es) that cause hollows to initiate and grow.

Figure 2.4a-c The image in Figs. 2.4a and 2.4b is is about 238 km (148 miles) across. In the center is a very large (50 km or 31 miles in length) volcanic vent located between the Rachmaninoff basin and Copland crater. The red box in Fig. 2.4b shows the area of the high-resolution image of Fig. 2.4c, focusing on the wall of the vent. Just as geologists on Earth can study the history of rock formation from the walls of a canyon or a road cut, the wall of this volcanic vent reveals layering within Mercury's crust. Hollows and bright-layer outcrops are exposed on the wall. These relations show that the type of material favorable for hollow formation is found at a variety of depths beneath the surface. Also of interest in this scene are gullies that have formed on the wall from landslides of loose material.

Figure 3.1 Map of Mg/Si ratio by weight on Mercury from MESSENGER X-Ray Spectrometer measurements acquired at a spatial resolution better than 100 km, compared with a published map of earlier data (Weider et al., 2015, inset), for which the best resolution is about 200 km. The higher-resolution map reveals variability on smaller scales and over a greater range of compositions. Smooth plains deposits are outlined in white.

Figure 3.2 Very-high-resolution XRS measurements of Mg/Si overlaid on enhanced color map of Mercury near Gauguin and Al-Akhtal craters. Relatively fresh material appears cyan in the enhanced-color representation, and the XRS data clearly show a distinct composition for this material compared with its surroundings, as well as other elemental variability on a few-kilometer scale.

Figure 3.3 Maps of Mercury in orthographic projection (centered on 50°N, -120°E). Low-altitude XRS measurements (Frank et al., 2015) reveal that the highest sulfur (~3 wt%) and iron concentrations (2-3 wt%) are found within the high-Mg region, where the highest concentration of Ca on Mercury, and lowest Al contents, are also found. A map of high-energy (“fast”) neutrons (Lawrence et al, 2015; bottom right) emitted by the planet indicates that the highest count rates are also from the high-Mg region. These neutrons are sensitive to Fe and thus serve as a proxy for iron abundance with better spatial coverage than direct measurements by XRS, which depend on sporadic large solar flares. The unusual composition of the high-Mg region and the relatively thin crust in this region suggest that it may be the remains of an ancient and highly degraded impact basin.

Figure 4.1 MESSENGER has confirmed that the contraction of Mercury has resulted in a global array of lobate scarps, tectonic landforms that are the surface expression of thrust faults (top right).  Lobate thrust fault scarps such as Enterprise Rupes (upper left) and Beagle Rupes (lower right) are large, often hundreds of kilometers long, and display hundreds to thousands of meters of relief.  Enterprise Rupes, about 1000 km long and with over 3 km of relief, is the largest lobate scarp on Mercury.

Figure 4.2 Images obtained after lowering MESSENGER’s altitude have revealed a population of small fault scarps (upper right, white arrows) that can be more than an order of magnitude smaller in size than their larger counterparts.  These small scarps are less than 10 km in length and have only tens of meters of relief.  They are comparable in size and morphology to small fault scarps imaged on the Moon by the Lunar Reconnaissance Orbiter.  The higher-resolution image at lower left shows a close-up of one of the scarps evident in the upper right image (upper two arrows).

Figure 4.3 Mercury’s small scarps, like small scarps on the Moon, often occur in clusters (upper left, white arrows).  These scarps frequently crosscut young impact crater with diameters less than 100 m (lower right, white arrow). Crosscutting relations with craters less than 100 m in diameter suggest that the small scarps formed no more than 800 Myr ago. The higher-resolution image at lower right shows a close-up of the scarps seen in the upper left image.

Figure 4.4 Small graben, narrow linear troughs, have been found associated with small scarps (upper left, white arrows) on Mercury and the Moon. These graben (lower right, white arrows) likely resulted from the bending and extension of the upper crust in response to scarp formation (upper right) and are only tens of meters wide. On the basis of the rate of degradation and infilling of small troughs on the Moon by continuous meteoroid bombardment, small lunar graben and their associated scarps are less than 50 Myr old. It is likely that Mercury’s small graben and their associated scarps are younger still, because the cratering rate on Mercury is a factor of ~3 greater than on the Moon.

Figure 5.1a-c Mercury’s north polar region, colored by the maximum biannual surface temperature, which ranges from >400 K (red) to 50 K (purple). As expected for the Solar System’s innermost planet, areas of Mercury’s surface that are sunlit reach high temperatures. In contrast, some craters near Mercury’s poles have regions that remain permanently in shadow, and in these regions even the maximum temperatures can be extremely low. Evidence from MESSENGER and Earth-based observations indicate that water ice deposits are present in these cold craters. The craters nearest Mercury’s poles have surface temperatures <100 K, and water ice is stable on the surface. However, many craters near but somewhat farther from Mercury’s poles have cold, permanently shadowed interiors, but the maximum temperature is too high for water ice to persist at the surface. In these craters, such as the two indicated in this figure, water ice is present but is buried beneath a thin, low-reflectance volatile layer likely consisting of organic-rich material. (Fuller crater: 27-kilometer diameter; 82.63°N, 317.35°E; unnamed crater: 18-kilometer diameter; 80.30°N, 293.47°E)

Figure 5.2a-d MESSENGER’s low-altitude campaign enabled imaging of Fuller crater by the Mercury Dual Imaging System (MDIS) in greater detail than previously possible. The top panel shows an image of Fuller, with the crater rim outlined in pink and the edge of a low-altitude broadband MDIS image in green. The second panel applies a different stretch to the MDIS broadband image in the first panel, revealing details of the shadowed surface inside the crater. In particular, as highlighted with yellow arrows in the third panel, the image reveals a region inside Fuller that is lower in reflectance. The edge of the low-reflectance region has a sharp and well-defined boundary, even when imaged at 46 m/pixel, suggesting that the low-reflectance material is sufficiently young to have preserved a sharp boundary against lateral mixing by impact cratering. The low-reflectance region agrees well with the temperature model predictions, as shown in the last panel. The model for surface and near-surface temperature within Fuller crater predicts a region that is sufficiently cold to host long-lived water ice beneath the surface but too hot to support water ice at the surface (blue and green regions). The low-reflectance region revealed in the images matches the characteristics expected for a lag deposit of volatile, organic-rich material that overlies the water ice.
(Fuller crater: 27-kilometer diameter; 82.63°N, 317.35°E)

Figure 5.3a-c MESSENGER’s low-altitude campaign enabled imaging of shadowed polar craters by the Mercury Dual Imaging System (MDIS) in greater detail than previously possible. The image shown here was acquired at 24 m/pixel, the highest resolution that has been obtained for any of Mercury’s shadowed polar craters. The first panel shows a view of an unnamed crater in Mercury’s north polar region, with the crater rim outlined in pink and the edge of the 24-meter/pixel, low-altitude broadband MDIS image in green. In the second panel, a different stretch has been applied to the MDIS broadband image in the first panel, revealing details of the shadowed surface inside the crater. In particular, as highlighted with yellow arrows in the third panel, the image reveals a region inside the crater that has a lower reflectance. The edge of the low-reflectance region has a sharp and well-defined boundary, even as imaged at this highest resolution of 24 m/pixel. The sharp boundary suggests that the low-reflectance material is sufficiently young to have preserved a sharp boundary against lateral mixing by impact craters. The sharp boundary matches the location predicted by temperature models for the stability of a surficial layer of volatile, organic-rich material tens of centimeters thick that overlies a thicker layer of water ice.
(An unnamed crater: 18-kilometer diameter; 80.30°N, 293.47°E)