MESSENGER's Mercury Research Articles

A hybrid geological map of Sibelius Crater on Mercury, and its associated ejecta and impact melt deposits

Abstract Using MESSENGER Mercury Dual Imaging System data, we produced three new maps of Sibelius Crater, Mercury. Geomorphological and spectral maps were combined into a single hybrid map containing units associated with ejecta deposits, crater floor landforms and impact melt ponding. Spatial measurement of these units shows that ∼50% of the mapped melt pond area lies within a large, degraded impact crater (crater B), beyond the significantly lower northern Sibelius rim, with a potential melt flow to a smaller, degraded impact structure further north (crater C). Freshly processed spectral data from the eight-colour Map Projected Multispectral Reduced Data Record data highlight the emplacement of multiple uplifted ejecta units with distinct spectral properties. A new, high-resolution digital elevation model was created to help define and analyse crater floor uplift features and disrupted crater rims and to create detailed cross-sections. These illustrate a proposed location of B's central uplift structure exposed in the northern wall slopes of Sibelius. Small features at the limit of visibility, such as a groove possibly associated with a rolling or sliding mega-boulder and lobate melt flow on the crater floor with accompanying channel opening, are highlighted for future investigations by BepiColombo's instruments once it reaches orbit.

Extending MESSENGER's Mercury dual imager's eight-color photometric standardization to cover all eleven filters

The photometric standardization model derived from the Mercury Dual Imaging System's (MDIS) eight-color photometric observations has been extrapolated to provide photometric parameters for the remaining three colors, such that images acquired through each of the eleven narrow-band filters can be photometrically standardized using a consistent model. The resulting photometric standardization parameters for the three filters not included in the original eight-color analysis display spectral variations commensurate with those observed within the original eight-color photometry. Some caution should be exercised on spectral interpretations based strongly on the behavior in the 698.8-nm filter.

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.

Rembrandt impact basin: Distinguishing between volcanic and impact-produced plains on Mercury

The surface of Mercury has been heavily modified by impact cratering and volcanic plains since the initial formation of its lithosphere and crust. As in the case of the Moon, the origin of early plains deposits has been difficult to determine because ponded impact basin ejecta and impact melt deposits can mimic the topography and texture of extrusive volcanic plains. In order to understand better the role that impact basins play in resurfacing on Mercury, we used MESSENGER Mercury Dual Imaging System (MDIS) data to map the distribution of plains deposits within and around the relatively young Rembrandt impact basin (∼720km in diameter). There are two different Rembrandt plains units: (1) high-reflectance interior and exterior plains, and (2) low-reflectance exterior plains. The morphology, crater size–frequency distribution, and N(20) crater density values were analyzed for each deposit. Observations of the two high-reflectance plains deposits are consistent with previous studies of volcanically produced smooth plains on Mercury. The low-reflectance exterior plains are older than the other high-reflectance plains and comparable in age to the Rembrandt basin impact event. On the basis of age, areal distribution, and albedo relationships with basin materials, the low-reflectance plains are interpreted as basin impact melt deposits. The large areal extent of the low-reflectance plains suggests that basin impact melt deposits may have played an important role in resurfacing Mercury, especially during the early geologic history of the planet when impact rates were higher.

Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron-stimulated desorption

To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed and density but contrasting interplanetary magnetic field (IMF) directions. During the first flyby the IMF had a northward component, while during the second flyby the IMF was southward. Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some solar wind electrons follow complex trajectories at or near where dayside reconnection occurs and enter the magnetosphere at these locations. The entry locations depend on the IMF orientation (north or south). As the electrons move through the entry regions they can be energized as they execute non-adiabatic (demagnetized) motion. Some electrons become magnetically trapped and drift around the planet with energies on the order of 1–10 keV. The highest energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence of high-energy (>35 keV) electrons observed during either MESSENGER flyby. Once within the magnetosphere, a fraction of the electrons precipitates at the planetary surface with fluxes on the order of 10 9 cm −2 s −1 and with energies of hundreds of eV. This finding has important implications for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the formation of the exosphere and heavy ion cloud around Mercury. From laboratory estimates of ESD ion yields, a calculated ion production rate due to ESD at Mercury is found to be on par with ion sputtering yields.

Messenger's mercury flight reveals water source

Messenger's mercury flight reveals water source