MErcury Surface, Space ENvironment, GEochemistry, and Ranging

MESSENGER data showed that volcanism played a critical role in shaping Mercury’s surface, where features have been formed by mechanical and thermal erosion of the planet’s surface by hot, quickly flowing lavas. There is a remarkable range in elemental abundances displayed by the volcanic deposits that make up Mercury’s surface. See: http://messenger.jhuapl.edu

This image provides a perspective view of the central portion of Carnegie Rupes, a large tectonic landform that cuts through Duccio crater (133 km diameter, centered at 58.19°N, 307.6°E; north is to the left). The image shows topographic heights as measured by the Mercury Laser Altimeter and surface features mapped by the Mercury Dual Imaging System. Variations in topography are shown in color (red: high-standing terrain; blue: low-lying terrain). Tectonic landforms such as Carnegie Rupes formed by horizontal shortening in response to cooling and contraction of the planetary interior. MESSENGER results have shown that Mercury contracted by as much as 7 km in radius, a number substantially larger than previously believed. See: http://messenger.jhuapl.edu

MESSENGER measurements have revealed that Mercury is surprisingly abundant in volatile elements that evaporate at moderately high temperatures, ruling out many of the models for its formation and early history that had been proposed before the mission. Because potassium is much more volatile than thorium, the ratio of the abundances of these two elements is a sensitive measure of thermal processes that fractionate elements by volatility. For Mercury, as seen in the first graph, this ratio is similar to that for other terrestrial planets at greater distances from the Sun but significantly higher than that for the Moon, which lost potassium during the giant impact that led to its formation. Particularly high potassium concentrations were observed by MESSENGER’s Gamma-Ray Spectrometer at high northern latitudes, as illustrated in the abundance map on the left side of the animation. Relatively high abundances of other volatile elements, including sulfur (right side of the animation), sodium, and chlorine, provide further evidence that Mercury is volatile-rich. The high sulfur contents combined with low amounts of iron on the planet’s surface additionally indicate that Mercury formed from materials with less oxygen than those that formed the other terrestrial planets, providing an important constraint on theories for the formation of all of the planets in the inner Solar System. See: (Peplowski et al. 2012, Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer), http://messenger.jhuapl.edu

Hollows are shallow, irregular depressions and are a geologic landform discovered by MESSENGER that appears to be unique to Mercury. Hollows are also some of the brightest and youngest features on Mercury’s surface. The formation of hollows remains an active area of research, but the etched nature of the features suggests that material is being lost from the surface to create the the hollows. See: http://messenger.jhuapl.edu

Observations by MESSENGER’s Magnetometer showed that Mercury’s magnetic field is offset along the planetary spin axis by about 20% of the planet’s radius. The internal magnetic field is 100 times weaker than that of Earth and barely stands off the solar wind at the subsolar point to form the magnetosphere. The interaction of the planetary field with the solar wind generates currents in the magnetosphere, which induce external magnetic fields with magnitudes similar to or larger than the planetary field in much of the magnetosphere. See: http://messenger.jhuapl.edu

Mercury’s magnetosphere is highly dynamic because of the planet’s small magnetic field and proximity to the Sun. The interaction of the solar wind with the planetary field generates waves in particles and fields, and reconnection of interplanetary and planetary magnetic field lines and circulation of magnetic flux in the magnetosphere occur 100 times faster than at Earth. The mixing of solar wind and magnetospheric plasma is just one of many processes that cause Mercury’s magnetosphere to be highly dynamic. Features of the simulation are consistent with observations by MESSENGER’s Magnetometer and Energetic Particle and Plasma Spectrometer. See: http://messenger.jhuapl.edu

A puzzle raised after brief observations by the Mariner 10 spacecraft was the nature of apparent bursts of energetic particles. Because of instrument limitations, neither the identity of these particles—ions or electrons—nor their energy was known. MESSENGER’s payload included instruments specifically designed to address these questions. Only after the MESSENGER spacecraft entered orbit about Mercury was the initial part of this puzzle solved. The energetic particles are electrons, not ions, and they have energies from several thousands of electron volts to 100 times that energy. These electrons are a quasi-permanent feature of Mercury’s magnetosphere. The sources and loss process for these energetic electrons remain under study. See: http://messenger.jhuapl.edu