X-ray fluorescence spectra obtained by the MESSENGER spacecraft orbiting Mercury indicate that the planet's surface differs in composition from those of other terrestrial planets Relatively high Mg/Si and low Al/Si and Ca/Si ratios rule out a lunarlike feldspar-rich crust The sulfur abundance is at least 10 times higher than that of the silicate portion of Earth or the Moon, and this observation, together with a low surface Fe abundance, supports the view that Mercury formed from highly reduced precursor materials, perhaps akin to enstatite chondrite meteorites or anhydrous cometary dust particles Low Fe and Ti abundances do not support the proposal that opaque oxides of these elements contribute substantially to Mercury's low and variable surface reflectance

http://science.sciencemag.org/content/sci/333/6051/1847.full.pdf

The Major-Element Composition of Mercury’s Surface from MESSENGER X-ray Spectrometry

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on August 3, 2004, is nearing the halfway point on its voyage to become the first probe to orbit the planet Mercury. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet: (1) What planetary formational processes led to Mercury’s high ratio of metal to silicate? (2) What is the geological history of Mercury? (3) What are the nature and origin of Mercury’s magnetic field? (4) What are the structure and state of Mercury’s core? (5) What are the radar-reflective materials at Mercury’s poles? (6) What are the important volatile species and their sources and sinks near Mercury? The mission has focused to date on commissioning the spacecraft and science payload as well as planning for flyby and orbital operations. The second Venus flyby (June 2007) will complete final rehearsals for the Mercury flyby operations in January and October 2008 and September 2009. Those flybys will provide opportunities to image the hemisphere of the planet not seen by Mariner 10, obtain high-resolution spectral observations with which to map surface mineralogy and assay the exosphere, and carry out an exploration of the magnetic field and energetic particle distribution in the near-Mercury environment. The orbital phase, beginning on March 18, 2011, is a one-year-long, near-polar-orbital observational campaign that will address all mission goals. The orbital phase will complete global imaging, yield detailed surface compositional and topographic data over the northern hemisphere, determine the geometry of Mercury’s internal magnetic field and magnetosphere, ascertain the radius and physical state of Mercury’s outer core, assess the nature of Mercury’s polar deposits, and inventory exospheric neutrals and magnetospheric charged particle species over a range of dynamic conditions. Answering the questions that have guided the MESSENGER mission will expand our understanding of the formation and evolution of the terrestrial planets as a family.

/pdf/messenger-mission-overview-3x9innmxjb.pdf

MESSENGER Mission Overview

The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph (NoRH) are used to investigate coronal hard X-ray and microwave emissions in the partially disk-occulted solar flare of 2007 December 31 The STEREO mission provides EUV images of the flare site at different viewing angles, establishing a two-ribbon flare geometry and occultation heights of the RHESSI and NoRH observations of {approx}16 Mm and {approx}25 Mm, respectively Despite the occultation, intense hard X-ray emission up to {approx}80 keV occurs during the impulsive phase from a coronal source that is also seen in microwaves The hard X-ray and microwave source during the impulsive phase is located {approx}6 Mm above thermal flare loops seen later at the soft X-ray peak time, similar in location to the above-the-loop-top source in the Masuda flare A single non-thermal electron population with a power-law distribution (with spectral index of {approx}37 from {approx}16 keV up to the MeV range) radiating in both bremsstrahlung and gyrosynchrotron emission can explain the observed hard X-ray and microwave spectrum, respectively This clearly establishes the non-thermal nature of the above-the-loop-top source The large hard X-ray intensity requires a very large number (>5 x 10{sup 35} above 16 keV for themore » derived upper limit of the ambient density of {approx}8 x 10{sup 9} cm{sup -3}) of suprathermal electrons to be present in this above-the-loop-top source This is of the same order of magnitude as the number of ambient thermal electrons We show that collisional losses of these accelerated electrons would heat all ambient electrons to superhot temperatures (tens of keV) within seconds Hence, the standard scenario, with hard X-rays produced by a beam comprising the tail of a dominant thermal core plasma, does not work Instead, all electrons in the above-the-loop-top source seem to be accelerated, suggesting that the above-the-loop-top source is itself the electron acceleration region« less

https://iopscience.iop.org/article/10.1088/0004-637X/714/2/1108/pdf

Measurements of the coronal acceleration region of a solar flare

MESSENGER observations from Mercury orbit reveal that a large contiguous expanse of smooth plains covers much of Mercury’s high northern latitudes and occupies more than 6% of the planet’s surface area. These plains are smooth, embay other landforms, are distinct in color, show several flow features, and partially or completely bury impact craters, the sizes of which indicate plains thicknesses of more than 1 kilometer and multiple phases of emplacement. These characteristics, as well as associated features, interpreted to have formed by thermal erosion, indicate emplacement in a flood-basalt style, consistent with x-ray spectrometric data indicating surface compositions intermediate between those of basalts and komatiites. The plains formed after the Caloris impact basin, confirming that volcanism was a globally extensive process in Mercury’s post–heavy bombardment era.

https://science.sciencemag.org/content/sci/333/6051/1853.full.pdf

Flood volcanism in the northern high latitudes of Mercury revealed by MESSENGER.

[1] Orbital images from the MESSENGER spacecraft show that ~27% of Mercury's surface is covered by smooth plains, the majority (>65%) of which are interpreted to be volcanic in origin. Most smooth plains share the spectral characteristics of Mercury's northern smooth plains, suggesting they also share their magnesian alkali-basalt-like composition. A smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope, suggesting more ultramafic compositions, an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation. The knobby and hummocky plains surrounding the Caloris basin, known as Odin-type plains, occupy an additional 2% of Mercury's surface. The morphology of these plains and their color and stratigraphic relationships suggest that they formed as Caloris ejecta, although such an origin is in conflict with a straightforward interpretation of crater size–frequency distributions. If some fraction is volcanic, this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions. Smooth plains are widespread on Mercury, but they are more heavily concentrated in the north and in the hemisphere surrounding Caloris. No simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed. A likely volcanic origin for some older terrain on Mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large-scale volcanic deposits rather than differences in crustal formational process.

The distribution and origin of smooth plains on Mercury

The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft will provide critical measurements tracing Mercury’s origin and evolution MDIS consists of a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC) The NAC is a 15° field-of-view (FOV) off-axis reflector, coaligned with the WAC, a four-element refractor with a 105° FOV and 12-color filter wheel The focal plane electronics of each camera are identical and use a 1,024×1,024 Atmel (Thomson) TH7888A charge-coupled device detector Only one camera operates at a time, allowing them to share a common set of control electronics The NAC and the WAC are mounted on a pivoting platform that provides a 90° field-of-regard, extending 40° sunward and 50° anti-sunward from the spacecraft +Z-axis—the boresight direction of most of MESSENGER’s instruments Onboard data compression provides capabilities for pixel binning, remapping of 12-bit data into 8 bits, and lossless or lossy compression MDIS will acquire four main data sets at Mercury during three flybys and the two-Mercury-solar-day nominal mission: a monochrome global image mosaic at near-zero emission angles and moderate incidence angles, a stereo-complement map at off-nadir geometry and near-identical lighting, multicolor images at low incidence angles, and targeted high-resolution images of key surface features These data will be used to construct a global image base map, a digital terrain model, global maps of color properties, and mosaics of high-resolution image strips Analysis of these data will provide information on Mercury’s impact history, tectonic processes, the composition and emplacement history of volcanic materials, and the thickness distribution and compositional variations of crustal materials This paper summarizes MDIS’s science objectives and technical design, including the common payload design of the MDIS data processing units, as well as detailed results from ground and early flight calibrations and plans for Mercury image products to be generated from MDIS data

The Mercury Dual Imaging System on the MESSENGER Spacecraft

NASA’s MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) mission will further the understanding of the formation of the planets by examining the least studied of the terrestrial planets, Mercury. During the one-year orbital phase (beginning in 2011) and three earlier flybys (2008 and 2009), the X-Ray Spectrometer (XRS) onboard the MESSENGER spacecraft will measure the surface elemental composition. XRS will measure the characteristic X-ray emissions induced on the surface of Mercury by the incident solar flux. The Kα lines for the elements Mg, Al, Si, S, Ca, Ti, and Fe will be detected. The 12° field-of-view of the instrument will allow a spatial resolution that ranges from 42 km at periapsis to 3200 km at apoapsis due to the spacecraft’s highly elliptical orbit. XRS will provide elemental composition measurements covering the majority of Mercury’s surface, as well as potential high-spatial-resolution measurements of features of interest. This paper summarizes XRS’s science objectives, technical design, calibration, and mission observation strategy.

The X-Ray Spectrometer on the MESSENGER Spacecraft

For over three years, the MESSENGER spacecraft has been in orbit about the planet Mercury. Because of the thermal and radiation constraints imposed by MESSENGER’s proximity to the Sun, assembling weekly command loads for the payload requires tight coordination across all operational subsystems and engineering teams. In addition to close team interaction, science planning requires a thorough understanding of mission constraints, accomplished with detailed models of recorder usage, communication uplink and downlink, orbit ephemeris and spacecraft attitude. Because of the many complexities in the orbital phase of the MESSENGER mission, such as the demanding thermal environment and changes in lighting conditions, the overlapping process of these weekly command-load builds must start approximately three weeks prior to onboard execution. The mission’s planning and scheduling process is mature, having been originally designed and successfully implemented to assemble the command-load sequences for the Near Earth Asteroid Rendezvous (NEAR) orbital mission. This planning system architecture is also used on the New Horizons mission to Pluto. Refinements were periodically made during the interplanetary cruise and early orbital phases of the MESSENGER mission, and the process and code now constitute a resilient operational system as demonstrated by three very successful and productive years of Mercury orbital science operations (12+ Mercury years), with over 200,000 images captured and over 2,800 orbits completed to date. MESSENGER’s command loads are assembled through a structured process that includes guidelines for load content and data recorder management. Weekly instrument command sequences are generated to fulfill the science objectives without violating instrument and spacecraft operational constraints. The SciBox science planning software serves as a coordination tool that allows the instrument teams to plan their observations, and it uses their plans to produce a consolidated payload command load that is packaged by the mission operations team for upload to the spacecraft. Given the complex set of constraints required to safely operate MESSENGER in orbit about the planet closest to the Sun, this integrated approach ensures that potential command conflicts are resolved in a timely manner, and that the chance is minimized that an erroneous or anomalous command sequence that jeopardizes health and safety or results in data loss could ever make it through all the gates of the three-week process and then to the spacecraft.

https://arc.aiaa.org/doi/pdf/10.2514/6.2014-1915

Richard G. Shelton

Papers

The Mercury Dual Imaging System on the MESSENGER Spacecraft

The X-Ray Spectrometer on the MESSENGER Spacecraft

Getting the Message to MESSENGER: Overview of the Weekly Planning and Sequencing of MESSENGER Orbital Activities